AU2018233022B2 - Counterfeit prevention structure and counterfeit prevention medium - Google Patents

Abstract

One object is to determine with a high accuracy authenticity of a counterfeit prevention medium having a counterfeit prevention structure. The counterfeit prevention structure provided on the medium to determine the authenticity of the medium includes a hybrid area in which a plurality of types of split ring resonators is formed in a mixed state in a predetermined ratio. Each split ring resonator includes an open part. The direction of the open part of each type of the split ring resonators is different from each other. Most Illustrative Drawing: FIG. 1 1/13 FIG-1 rl ............................... ............................... ............................... ............................... ............................... ............................... ............................... ............................... ............................... ............................... ............................... ............................... ............................... ............................... 0 0 0 0 ............................... x ............................j 2 0 2 1 .............................. ............................... 'LY ............................ 000000 23 .............................. .............................. ............................... ............................... ............................... ............................... ............................... 000000 .............................. .............................. 000000000 22 ............................... ............................... ............................... ............................... ............................... .............................. ............................... ............................... .............................. ............................. . ............................... ................... 0000000000 ................... ........ . .............................. ........................... ... .......................... .... .......................... .... ......................... ..... ......................... ..... 0000000000 .............................. ........................ ..... ............................... ............................... ....................... ....... ............................... ...................... ........ 0000000000 ...................... ........ ............................... ............................... ..................... ........ ..................... ....... ....................... ....... ........................ ...... ........................ ...... .................... .......... 00000000 ........................ ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 ................... .... ...... ........................ ...... .................... .. ....... 00000000 ..................... . ....... ..................... ........ ....................... ...... ........................ ..... .......................... ............................ .............................. ............................... ID O O O O ............................... ............................... ............................... ............................... ........... 20a 20 21 22 23 21a 23a 0 00 0 22a

Description

1/13

FIG-1 x ...... rl ............................... ............. ............................... ............................... ............................... ............................... ............................... .........j 20 2 1 ............................... ............................... ............................... ............................... ............................... ............................... ............................... ............................... 0000 ............................... .............................. ............................... 'LY ............................ 000000 23 .............................. .............................. ............................... ............................... ............................... ............................... ............................... .............................. 000000 .............................. ............................... ............................... 000000000 22 ............................... ............................... ............................... .............................. ............................... ............................... .............................. ............................. ............................... ...................

. ................... .............................. ........ . . 0000000000 ........................... .......................... .... . .......................... ......................... ......................... .... ..... ..... 0000000000 .............................. ........................ ............................... ..... ............................... ....................... ............................... ....... ...................... ...................... ............................... ............................... ..................... ........ ........ ........ 0000000000 ..................... ....................... ........................ ....... ....... ...... .................................. .................... ...... 00000000 ......................... .................... . . . . ....... . . . . . . . . . . . . . . . . . . .... .. . . ...... 16 ........................ .................... ..................... ..................... ....................... .. . ...... ....... ....... ........ ...... 00000000 ........................ .......................... ............................ .............................. ..... ............................... ............................... ............................... ........... ID O O O O

20a 20 21 22 23

23a

21a 0 0 22a

I COUNTERFEIT PREVENTION STRUCTURE AND COUNTERFEIT PREVENTION MEDIUM BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to a counterfeit

prevention structure capable of preventing counterfeiting and a

counterfeit prevention medium including the counterfeit prevention

structure.

2. Description of the Related Art

[0002] Conventionally, a counterfeit prevention structure

that prevents counterfeiting is arranged in a sheet-like valuable

medium such as a banknote (paper currency), a stock certificate,

abond, acheck, acoupon, and the like. Forexample, Japanese Patent

Application Laid-Open No. 2016-000498 discloses a technique ofusing

aconductive layer, inwhichsplit ringresonators (SRRs) are formed,

as the counterfeit prevention structure. The SRRs, each of which

an outer diameter is around severalhundredmicrometers, have effect

on terahertz electromagnetic waves. Metamaterial formed by the

minute SRRs is used for counterfeit prevention.

[0003] Specifically, a conductive layer, in which SRRs of a

predetermined shape are arranged in a matrix layout at regular

intervals, is formed such that transmittance ofapredeterminedvalue

is obtained when the SRRs are irradiated with terahertz

electromagnetic waves of a specific frequency. Such a conductive

layer is arranged inside or on a medium as the counterfeit prevention

structure. The counterfeit prevention structure is irradiated with the terahertz electromagnetic waves, and the authenticity of the medium can be determined based on the obtained value of the transmittance.

[0004] The transmittance of the terahertz electromagnetic

waves penetrating the conductive layer changes depending on the

relation between a polarization direction of the terahertz

electromagnetic waves and directions of open parts of the SRRs. The

conductive layer is divided in a plurality of areas, and the open

part directions of the SRRs in each area are different. By this

structure, a counterfeit prevention structure in which the

transmittance in each area is different can be obtained. The

transmittance is measured while scanning each of the areas of the

counterfeit prevention structure with the terahertz electromagnetic

waves, and the authenticity of the medium is determined based on

whether the change in the measured transmittance matches with a

transmittance and a scan width of each of the areas.

[00051 In the conventional technique, however, the

authenticity of a medium in which the counterfeit prevention

structure is arranged may not be determined with a high accuracy.

For example, positions of a transmitting unit that transmits the

terahertz electromagnetic waves and a receiving unit that receives

the terahertz electromagnetic waves are fixed in an apparatus, and

when measuring the transmittance of the terahertz electromagnetic

waves, themediumis transportedsuch that the counterfeitprevention

structure thereof passes between the transmitting unit and the

receiving unit. As the medium is transported, when the counterfeit

prevention structure made by the SRRs blocks the terahertz electromagnetic waves between the transmitting unit and the receiving unit, different transmittance is obtained depending on the directions of the open parts of the SRRs. At this time, if the transported medium inclines (skewed transport state), an angle between the polarization direction of the terahertz electromagnetic waves and the directions of the open parts changes, and the transmittance also changes. For example, in a certain counterfeit prevention structure that is designed such that the angle between the polarization direction of the terahertz electromagnetic waves and the directions of the open parts are 60 degrees, the value of the transmittance changes between 30% and 60% when the medium is inclined by between -15 degrees and 15 degrees. The authenticity is determined by comparing the value of the transmittance with a threshold value. However, if the threshold value is set so as to permit such a huge change in the transmittance, the authenticity cannot be determined with a high accuracy.

[00061 A range of variation of the transmittance due to the

inclination of the medium varies according to the directions of the

open parts of SRRs. In the conventional technique, the counterfeit

prevention structure is divided in a plurality of areas, and the

directions of the openparts in eachof the areas are setin different

directions. In this case, when the medium is inclined, the

transmittance of each of the areas changes in a different range of

variation depending on the directions of the open parts. Therefore,

the change in the transmittance obtained by scanning the counterfeit

prevention structure is different from the original change so that

the authenticity may not be determined with a high accuracy.

SUMMARY OF THE INVENTION

[0007] It would be desirable to solve the problems in the conventional

technique. For example, it would be desirable to provide a counterfeit

prevention structure and a counterfeit prevention medium that allow

highly accurate determination of the authenticity.

[0008] According to one aspect of the present invention there is

provided a counterfeit prevention structure provided on a medium to

determine authenticity of the medium, the counterfeit prevention

structure comprising: a hybrid area in which a plurality of types of

split ring resonators is formed in a mixed state in a predetermined

ratio, wherein each split ring resonator includes an open part, an

opening direction of the open part of each type of the split ring

resonators is different from each other, the hybrid area has a

transmittance of a constant value when irradiated with a terahertz

electromagnetic wave having a predetermined frequency and a

predetermined polarization direction, and the hybrid area includes at

least two types of split ring resonators of which respective opening

directions are different to suppress variation in the transmittance due

to an inclination of the counterfeit prevention structure.

[0009] A counterfeit prevention medium according to another aspect of

the present invention is a counterfeit prevention medium including the

above counterfeit prevention structure.

[0010]The above and other objects, features, advantages and technical

and industrial significance of this invention will be better understood

by reading the following detailed description of presently preferred

embodiments of the invention, when considered in connection with the

accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0011]

FIG. 1 is a view indicating one embodiment of a counterfeit

prevention structure.

FIG. 2 is a view for explaining a shape of a split ring

resonator.

FIGS. 3Ato 3C are views indicatingexamples ofpatterns formed

by using a plurality of types of the split ring resonators.

FIG. 4 is a view indicating an example of frequency

characteristics of transmittance obtained when an area in which the

split ring resonators are arranged is irradiated with terahertz

electromagnetic waves.

FIG. 5 is a view indicating an example of another pattern formed

by using a plurality of types of the split ring resonators.

FIG. 6 is a view for explaining an example of the transmittance

of the counterfeit prevention structure.

FIG. 7 is a view of an example of the counterfeit prevention

structure in which a plurality of types of the patterns is combined.

FIG. 8 is a view indicating another pattern obtained by

rotating one of the patterns shown in FIGS. 3A to 3C.

FIG. 9 is a view for explaining a change in the transmittance

of the terahertz electromagnetic waves observed in a counterfeit

prevention medium provided with the counterfeit prevention

structure shown in FIG. 7.

FIG. 10 is a view for explaining change in the transmittance

obtained when a secondary resonance frequency is used.

FIG. 11is a schematic diagram indicating a schematicinternal

configuration of an authenticity determination apparatus seen from

a side thereof.

FIGS. 12A and 12B are schematic diagrams of the configuration

shown in FIG. 11 when seen from above.

FIG. 13 is a block diagram indicating a schematic functional

configuration of the authenticity determination apparatus.

FIG. 14 is a schematic cross section indicating another

structural example of the counterfeit prevention structure.

FIG. 15 is a view indicating an example of a counterfeit

prevention structure having split ring resonators in which

directions of open parts are different.

FIG. 16 is a view indicating another example of the counterfeit

prevention structure that is divided in a plurality of areas.

EMBODIMENTS

[0012] Exemplary embodiments of a counterfeit prevention

structure and a counterfeit prevention medium according to the

present invention are explained below in detail by referring to the

accompanying drawings. One feature of the present invention is that

a plurality of types of split ring resonators (SRRs) is used to obtain

a predetermined value of transmittance when terahertz

electromagneticwaves penetrate a counterfeit prevention structure.

[0013] The SRRhas a ring-like shape and has an open part (Split).

For example, the shape of the SRR can be substantially like the

English character C in which an open part is provided in a circular

ring shape. Alternatively, the shape of the SRR can be rectangular

in which an open part is provided in a rectangular ring shape. For

example, SRRs eachhavingan openpartin aring-like shape are formed

with conductive material on a sheet of insulating material. When irradiating the SRRs with a terahertz electromagnetic wave, depending on the frequency and the polarization direction of the terahertz electromagnetic wave, the transmittance of the terahertz electromagnetic wave changes. Specifically, the transmittance of the terahertz electromagnetic wave that resonates with the SRRs is lower than the transmittance of the terahertz electromagnetic wave that does not resonate with the SRRs.

[0014] Alternatively, for example, an SRR can be formed by

carving a sheet of conductive material into a ring-like shape having

an open part. Particularly, an SRR formed by carving conductive

material is called a complementary split ring resonator (CSRR).

Even in a complementary split ring resonator, when irradiating the

SRR with the terahertz electromagnetic wave, depending on the

frequency and the polarization direction of the terahertz

electromagnetic wave, the transmittance of the terahertz

electromagnetic wave penetrating the sheet changes. Specifically,

the transmittance of the terahertz electromagnetic wave that

resonates with the SRR is higher than the transmittance of the

terahertz electromagnetic wave that does not resonate with the SRR.

[0015] By arranging a large number of SRRs in an area, the

transmittance of the terahertz electromagnetic wave of a specific

frequency in this area can be controlled. For example, the large

number of SRRs are arranged in a matrix layout in which the SRRs

are arranged in a longitudinal direction and a lateral direction

at regular intervals. Alternatively, the SRRs are arranged in a

checkered pattern layout or in a honeycomb pattern layout.

[0016] One method of forming an area having a predetermined transmittance is to form the ring-like SRR by using conductive material on a sheet of insulating material. Another method is to form the SRR by carving a sheet of conductive material into a ring-like shape. An area, in which the transmittance of terahertz electromagneticwave is a predetermined value, can be formed by using anyofthe abovemethods; however, acase offorming the SRRbycarving a sheet of conductive material is explained as an example in the present embodiment.

[0017] The counterfeit prevention structure according to the

present embodiment includes a conductive layer. When the prevention

structure is irradiated with the terahertz electromagneticwave that

has a predetermined frequency and a predetermined polarization

direction, the conductive layer shows a transmittance of a

predetermined value. At least two types of the SRRs of which the

directions of the open parts differing by 90 degrees are arranged

in the conductive layer. Coordinate axes are shown in each of the

drawings to facilitate understanding of the correspondence of the

polarization direction of the terahertz electromagnetic waves used

for measuring the transmittance, the direction of the open part of

the SRR, and the like. The predetermined direction used in the

context of the terahertz electromagnetic waves is a direction

selected as the polarization direction of the terahertz

electromagnetic waves used in the measurement of the transmittance.

The predetermined frequency in the context of the terahertz

electromagnetic waves is a frequency (resonance frequency) at which

the terahertz electromagnetic waves resonate with the SRRs, and it

is the frequency selected as the frequency of the terahertz electromagnetic waves used in the measurement of the transmittance.

To detect a difference in the transmittance depending on the types

of the SRRs, it is desirable that the predetermined frequency of

the terahertz electromagnetic wave is the frequency at which the

transmittance greatly changes when the direction of the open part

of the SRR is changed with respect to the predetermined direction

(polarization direction). Specifically, it is desirable that the

terahertz electromagnetic wave has a frequency band including the

central frequency corresponding with the peak frequency at which

the transmittance has a peak value. If the peak frequency is stable

in the counterfeit prevention structures, the terahertz

electromagnetic wave having a single frequency can be also used.

If it is possible to accommodate the variation in the transmittance,

the terahertz electromagnetic wave can have the predetermined

frequency that is not the peak frequency.

[0018] FIG. 1 is a view indicating one embodiment of a

counterfeit prevention structure 10. Aplan view of the counterfeit

prevention structure 10 is shown in the upper left part of FIG. 1

and a partially enlarged view of a partial area of the counterfeit

prevention structure 10 is shown in the upper right part of FIG.

1. Moreover, a plurality of types of SRRs 20 to 23 included in the

counterfeit prevention structure 10 is shown in the bottom part of

FIG. 1. This counterfeit prevention structure 10 is arranged in a

counterfeit prevention medium (hereinafter, "medium") to prevent

counterfeiting of the medium. The medium is, for example, a sheet

valuable medium. Such a mediumincludes a banknote (paper currency),

a stock certificate, a bond, a check, and a coupon.

[0019] FIG. 1 shows an example of the counterfeit prevention

structure 10 in which the plurality of types of the SRRs 20 to 23

are arranged in a matrix layout. The SRRs have open parts in

different directions from each other. The plurality of types of the

SRRs 20 to 23 is mixed in a predetermined ratio. In such a structure,

transmittance of the terahertz electromagnetic wave of a specific

frequency that penetrates the counterfeit prevention structure 10

can be maintained to a predetermined value.

[0020] The counterfeit prevention structure 10 includes a

conductive layer 16 in which the plurality of types of the SRRs 20

to 23 are formed in the matrix layout at regular intervals. Each

of the SRRs 20 to 23 has a shape substantially like the English

character C in which a part of a circular ring is cut to form open

part 20a to 23a. As shown in FIG. 1, when seen from a center of the

ring part, the SRR 20 has the open part 20a in the positive X-axis

direction. When seen from the center of the ring part, the SRR 21

has the open part 21a in the positive Y-axis direction. When seen

from the center of the ring part, the SRR 22 has the open part 22a

in the negative X-axis direction. When seen from the center of the

ring part, the SRR 23 has the open part 23a in the negative Y-axis

direction. The shape of the SRR 20 matches with the shape of the

SRR 21 when the SRR 20 is rotated clockwise by 90 degrees, the shape

of the SRR 21 matches with the shape of the SRR 22 when the SRR 21

is rotated clockwise by 90 degrees, and the shape of the SRR 22 matches

with the shape of the SRR 23 when the SRR 22 is rotated clockwise

by 90 degrees. That is, the directions of the open parts of the SRRs

20 to 23 vary by 90 degrees from each other. The direction of the open part mentioned in the present embodiment is the direction when see from the center of the ring part of the SRR having the open part.

[0021] As shown in the partially enlarged view in the upper

right part of FIG. 1, the four types of the SRRs 20 to 23 are arranged

at regularintervals and formapredeterminedpattern. Specifically,

a basic pattern of a two-by-two matrix is formed by arranging the

four SRRs 20 to 23 in two rows and two columns. In the basic pattern,

the SRR 21 is arranged on the right side (in the positive Y-axis

direction) ofthe SRR20, the SRR23is arrangedbelow (in the negative

X-axis direction) the SRR 20, and the SRR 22 is arranged on the right

side of the SRR 23. The four types of the SRRs 20 to 23 are arranged

at regular intervals by repeating this basic pattern. The details

about the basic pattern formed by using the four SRRs 20 to 23 will

be explained later.

[0022] The SRRs 20 to 23 are formed by carving the conductive

layer 16 made of conductive material into a shape substantially like

the English character C. The four SRRs 20 to 23 have the same

structure except that the directions (position in the ring part)

of the open parts 20a to 23a thereof are different. The SRRs 21 to

23 can be obtained by rotating the SRR 20 and therefore the specific

structure of the SRRs 20 to 23 will be explained below by using the

SRR 20 as an example.

[0023] FIG. 2 is a view for explaining the shape of the SRR 20.

A plan view of the SRR 20 is shown in the upper part of FIG. 2 and

a cross-section along a line AA shown in the plan view is shown in

the lower part of FIG. 2. The counterfeit prevention structure 10

includes a base member 17 made of insulating material and a thin conductive layer 16 formed on a surface of the base member 17. The base member 17 is made of insulating material, such as paper and resin, through which the terahertz electromagnetic waves can penetrate. On the other hand, the conductive layer 16 is made of conductive material, such as Al, Fe, Au, Cu, Ag, Mg, Zn, and Sn, that blocks the terahertz electromagnetic waves.

[0024] The SRR 20 is formedbycarving fromthe conductive layer

16, which is formed on the base member 17, an area having the shape

substantially like the English character C. Specifically, the SRR

20 is formed by carving the ring-shaped conductive layer 16 having

a predetermined width in the diameter direction while leaving behind

only the open part 20a. The area corresponding to the ring part

having the shape substantially like the English character C is a

groove and a surface of the base member 17 is exposed at the bottom

of the groove. In an area other than the ring part, including the

open part 20a, the surface of the base member 17 is covered with

the conductive layer 16. Each of other SRRs 21 to 23 can be formed

by changing the area that is left behind as the open part 21a to

23a when forming the groove having the shape substantially like the

English character C. The machining method to form the SRR in the

conductive layer, the functions ofthe SRR, and the like are disclosed,

for example, in Japanese Patent Application Laid-Open No.

2016-000498.

[0025] A length and a width of the sheet-like counterfeit

prevention structure 10 are, for example, 20 mm. An inner diameter

d of the SRR 20, shown in the upper part of FIG. 2, is a few hundred

pm and a width g of the open part 20a is a few ten pm. A width W of the SRR 20, shown in the lower part of FIG. 2, in a diameter direction thereof is a few ten pm. Other SRRs 21 to 23 are formed with the same size as that of the SRR 20. In the counterfeit prevention structure 10, the SRRs 20 to 23 are arranged vertically and horizontally at regular intervals and form the matrix layout.

A distance between adjacent SRRs 20 to 23 is a few ten pm. For example,

in 10 mm long, several tens of SRRs 20 to 23 is arranged at regular

intervals. The shape of each SRRs 20 to 23 and the layout of the

SRRs 20 to 23 are determined such that, resonance occurs when the

SRRs 20 to 23 are irradiated with the terahertz electromagnetic wave

of the predetermined frequency, and the terahertz electromagnetic

wave penetrate the SRRs 20 to 23 at predetermined transmittance.

The frequency of the terahertz electromagnetic wave is set, for

example, between 0.1 THz and 1 THz. A dimension of an area of the

conductive layer 16 irradiated with the terahertz electromagnetic

wave is determined based on the SRRs 20 to 23 as the target for

irradiation andis about1mm to about 5 mmin a diameter in ahalf-band

width.

[0026] A minimal configuration of the counterfeit prevention

structure 10 is shown in FIG. 2. As long as the properties of the

conductive layer 16 with respect to the terahertz electromagnetic

wave are not affected, another layer maybe provided on the conductive

layer 16 and/or below the base member 17. Moreover, another layer

may be provided between the conductive layer 16 and the base member

17.

[0027] The thin counterfeit prevention structure 10 can be

embedded in the medium, such as the coupon, that is the target of counterfeit prevention. Alternatively, the counterfeit prevention structure 10 can be affixed to the medium. For example, as the counterfeit prevention structure 10, both the conductive layer 16 and the base member 17 are newly arranged in the medium such as the coupon. For another example, the medium, such as the coupon, itself is used as the base member 17, and the conductive layer 16 is formed on the medium.

[0028] FIGS. 3A to 3C are views indicating examples of patterns

formed by using the SRRs 20 to 23. A pattern that functions as a

basic unit is shown in the left part of FIGS. 3A to 3C. A partial

area of the counterfeit prevention structure 10 formedby repeatedly

arranging these basic pattern in a matrix layout is shown in the

right part of FIGS. 3A to 3C. Each pattern constitutes a hybrid area

in which the plurality of types of the SRRs 20 to 23 are mixed in

a predetermined ratio.

[0029] Afirst pattern 31shownin FIG. 3Ais a two-by-twomatrix

pattern. In the first pattern 31, the SRR20 is arrangedin the upper

left corner, the two SRRs 22 are arranged on the right side of and

below the SRR 20, and the other SRR 20 is arranged on the right side

of the lower SRR 22. The first pattern 31 is the hybrid area in which

two types of the SRRs, that is, the SRRs 20 and 22, are mixed in

the predetermined ratio. The first pattern 31is constitutedby only

the SRRs 20 and 22 that have the open parts 20a and 22a in the X-axis

direction.

[0030] When the first pattern 31 is irradiated with the

terahertz electromagnetic wave of the predetermined frequency

(primary resonance frequency), which has the polarization direction in the X-axis direction, the transmittance of the SRRs 20 and 22, of which the open parts 20a and 22a are arranged in the X-axis direction that is the polarization direction, will be maximum.

Therefore, when the counterfeit prevention structure 10 constituted

by the first pattern 31 is irradiated with the terahertz

electromagneticwave having the polarization direction in the X-axis

direction, the transmittance will be maximum.

[0031] About the order of the resonance frequency will be

explained next. FIG. 4 is a view indicating an example of frequency

characteristics of transmittance obtained when the area in which

the SRRs are arranged is irradiated with the terahertz

electromagnetic wave. The frequency characteristics shown in FIG.

4 is obtained when, as shown in FIGS. 3A to 3C, a large number of

the SRRs having the open parts are arranged at regular intervals

in a sufficiently wider area than the area irradiated with the

terahertz electromagnetic wave.

[0032] When the polarizationdirection ofthe emitted terahertz

electromagnetic wave and the directions of the open parts of the

SRRs formedin the areairradiatedwith the terahertzelectromagnetic

wave match, that is, when both the directions are parallel, the

frequency characteristics shown with a solid line in FIG. 4 is

obtained. On the other hand, when the polarization direction of the

emitted terahertz electromagnetic wave and the directions of the

open parts of the SRRs formed in the area irradiated with the

terahertz electromagnetic wave are orthogonal, the frequency

characteristics shown with a dotted line in FIG. 4 is obtained.

Specifically, for example, when the directions of the open parts of the SRRs are in the X-axis direction, the frequency characteristics as shown with the solid line will be obtained if the polarization direction of the terahertz electromagnetic wave matches with the X-axis direction, and the frequency characteristics shown with the dotted line will be obtained if the polarization direction of the terahertz electromagnetic wave matches with the

Y-axis direction.

[00331 As shown with the solid line in FIG. 4, two peaks P1 and

P2 are clearly observed when the directions of the open parts of

the SRRs and the polarization direction of the terahertz

electromagnetic wave match. On the other hand, as shown with the

dotted line in FIG. 4, one peak V1 is clearly observed when the

directions of the open parts of the SRRs and the polarization

direction of the terahertz electromagneticwave are orthogonal. The

frequencies at which the peaks are obtained are referred to as P1,

V1, and P2 sequentially from the low frequency side.

[0034] As has been mentioned above, it is desirable that the

frequency (predetermined frequency) of the emitted terahertz

electromagnetic wave is the resonance frequency at which the

transmittance greatly changes when the directions of the open parts

of the SRRs are changed with respect to the polarization direction

(predetermined direction) of the emitted terahertz electromagnetic

wave. When comparing a ratio of the dotted line showing

transmittance with respect to Y-polarization to the solid line

showing transmittance with respect to X-polarization at each peak

P1, Vi, and P2, the ratios at P1 and Vi are bigger than the ration

at P2. To compare the difference in each of the transmittance obtained when the SRRs are irradiated with the terahertz electromagnetic waves having different polarization directions, it is preferable to adopt the peak P1 and the peak V1. Accordingly, the present embodiment is explained by taking the frequency of the peak P1 as a primary resonance frequency and by taking the frequency of the peak V1 as a secondary resonance frequency. As has been mentioned above, it is allowable to take the frequency bandincluding the frequency of the peak P1 as the primary resonance frequency, and take the frequency band including the frequency of the peak V1 as the secondary resonance frequency.

[00351 A second pattern 32 shown in FIG. 3B is a two-by-two

matrix pattern. In the second pattern 32, the SRR 20 is arranged

in the upper left corner, the SRR 21 is arranged on the right side

of the SRR 20, the SRR 22 is arranged below the SRR 20, and the other

SRR 20 is arranged on the right side of the lower SRR 22. The second

pattern 32 is obtained by replacing the upper right SRR 22 of the

first pattern 31with the SRR 21. The second pattern 32 is the hybrid

area in which three types of the SRRs, that is, the SRRs 20 to 22,

are mixed in the predetermined ratio. The second pattern 32 is

constituted by three SRRs 20 and 22 having the open parts 20a and

22a in the X-axis direction and one SRR 21 having the open part 21a

in the Y-axis direction. A ratio of the number of the SRRs 20 and

22 having the open parts 20a and 22a thereof parallel to the X-axis

direction and the number of the SRR 21 having the direction of the

open part 21a thereof orthogonal to the X-axis direction is 3:1.

When four SRRs arranged in the two-by-two matrix pattern are selected

from the area in which the SRRs of the second pattern 32 are arranged successively as shown in the right part of FIG. 3B, a ratio of the number of the SRRs having the open parts parallel to the X-axis direction and the number of the SRRs having the open parts orthogonal to the X-axis direction is 3:1. That is, when a desired area having the same shape as the second pattern 32 is selected, the ratio of the number of the SRRs 20 and 22 and the number of the SRRs 21 will always be the same.

[00361 When the SRRs are irradiated with the terahertz

electromagnetic wave of the predetermined frequency (primary

resonance frequency) having the polarization direction in the X-axis

direction, the transmittance of the SRRs 20 and 22 having the open

parts 20a and 22a parallel to the polarization direction (X-axis

direction) will be maximum. On the other hand, the transmittance

of the SRRs 21 and 23 having the open parts 21a and 23a orthogonal

to the polarization direction (X-axis direction) will be minimum.

[0037] When the counterfeit prevention structure 10 having the

plurality of types of the SRRs, of which the open parts have different

directions, is irradiated with the terahertz electromagnetic wave,

the transmittance will be a value between a transmittance Tx and

Ty. The value Tx is a transmittance obtained when all the SRRs have

the open parts in a direction parallel to the polarization direction

of the terahertz electromagnetic wave. The value Ty is a

transmittance obtained when all the SRRs have the open parts in a

direction orthogonal to the polarization direction of the terahertz

electromagnetic wave.

[00381 In the counterfeit prevention structure 10 according to

the second pattern 32, a ratio of the number of the SRRs having the open parts parallel to the polarization direction (X-axis direction) and the number of the SRRs having the open parts orthogonal to the polarization direction (X-axis direction) is 3:1. Therefore, when the counterfeit prevention structure 10 of the second pattern 32 is irradiated with the terahertz electromagnetic wave of the predetermined frequency (primary resonance frequency) having the polarization direction in the X-axis direction, the transmittance will be a value near (3xTx+Ty)/4. The above-mentioned dimension of the area irradiated with the terahertz electromagnetic wave is determined so as to be at least larger than the area occupied by the SRRs arranged in the two-by-two matrix pattern.

[00391 Athirdpattern 33 shownin FIG. 3Cis a two-by-twomatrix

pattern. In the thirdpattern 33, the SRR20is arrangedin the upper

left corner, the SRR 21 is arranged on the right side of the SRR

20, the SRR 23 is arranged below the SRR 20, and the SRR 22 is arranged

on the right side of the SRR 23. The third pattern 33 is obtained

by replacing the lower left SRR 22 of the second pattern 32 with

the SRR 23 and replacing the lower right SRR 20 of the second pattern

32 with the SRR 22. The third pattern 33 is the hybrid area in which

four types of the SRRs, that is, the SRRs 20 to 23, are mixed in

the predetermined ratio. The third pattern 33 is constituted by two

SRRs20 and22having the openparts20aand22ain the X-axis direction

and two SRRs 21 and 23 having the open parts 21a and 23a in the Y-axis

direction. A ratio of the number of the SRRs 20 and 22 having the

openparts 20aand22aparallelto the X-axis direction and the number

of the SRRs 21 and 23 having the open parts 21a and 23a orthogonal

to the X-axis direction is 1:1. When four SRRs arranged in the two-by-two matrix pattern are selected from the area in which the

SRRs of the third pattern 33 are arranged successively as shown the

right part of FIG. 3C, a ratio of the number of the SRRs having the

open parts parallel to the X-axis direction and the number of the

SRRs having the open parts orthogonal to the X-axis direction is

1:1. That is, when a desired area having the same shape as the third

pattern 33 is selected, the ratio of the number of the SRRs 20 and

22 and the number of the SRRs 21 and 23 will always be the same.

The third pattern 33 shown in FIG. 3C is used in the counterfeit

prevention structure 10 shown in FIG. 1.

[0040] When the counterfeit prevention structure 10 of the

third pattern 33 is irradiated with the terahertz electromagnetic

wave of the predetermined frequency (primary resonance frequency)

having the polarization direction in the X-axis direction, the

transmittance will be a value between the transmittance Tx obtained

when all the SRRs having the open parts in a direction parallel to

the polarization direction (X-axis direction) and the transmittance

Ty obtained when all the SRRs having the open parts in a direction

orthogonalto the polarization direction (X-axis direction). In the

third pattern 33, a ratio of the number of the SRRs having the open

parts parallel to the polarization direction (X-axis direction) and

the number of the SRRs having the open parts orthogonal to the

polarization direction (X-axis direction) is 1:1. Therefore, the

transmittance will be a value near (Tx+Ty)/2. The above-mentioned

dimension of the area irradiated with the terahertz electromagnetic

wave is determined so as to be at least larger than the area occupied

by the two-by-two matrix pattern SRRs.

[0041] The pattern formed with the four types of the SRRs 20

to 23 is not limited to the two-by-two matrix pattern. FIG. 5 is

a view indicating an example of another pattern formed by the SRRs

20 to 23. FIG. 5 shows, in the left part, a fourth pattern 34 that

functions as a basic unit. A partial area of the counterfeit

prevention structure 10 formed by repeatedly arranging the fourth

pattern 34 in the matrix layout is shown in the right part of FIG.

5.

[0042] In the fourth pattern 34 shown in FIG. 5, nine SRRs 20

to 23 are arranged in a three-by-three matrix pattern. The SRR 22

is arranged at the center, the two SRRs 20 arranged adjacent to the

central SRR 22 in the diagonal direction are in the upper left and

the lower left of the SRR 22, and the two SRRs 22 arranged adjacent

to the central SRR 22 in the diagonal direction are in the upper

right and the lower right of the SRR 22. The two SRRs 23 are arranged

on the left side and the right side of the central SRR 22, and the

two SRRs 21 are arranged above and below the central SRR 22. The

fourth pattern 34 is the hybrid area in which four types of the SRRs,

that is, the SRRs 20 to 23, are mixed in the predetermined ratio.

The fourth pattern 34 is constituted by five SRRs 20 and 22 having

the open parts 20a and 22a in the X-axis direction and the four SRRs

21 and 23 having the open parts 21a and 23a in the Y-axis direction.

A ratio of the number of the SRRs 20 and 22 having the open parts

20a and 22a parallel to the X-axis direction and the number of the

SRRs 21 and 23 having the open parts 21a and 23a orthogonal to the

X-axis direction is 5:4. When SRRs forming a three-by-three matrix

are selected from the area in which the SRRs of the fourth pattern

34 are arranged successively, a ratio of the number of the SRRs having

the open parts parallel to the X-axis direction and the number of

the SRRs having the open parts orthogonal to the X-axis direction

is 5:4. That is, when a desired area having the same shape as the

fourth pattern 34 is selected, the ratio of the number of the SRRs

20 and 22 and the number of the SRRs 21 and 23 will always be the

same.

[0043] When the counterfeit prevention structure 10 of the

fourth pattern 34 is irradiated with the terahertz electromagnetic

wave of the predetermined frequency (primary resonance frequency)

having the polarization direction in the X-axis direction, the

transmittance will be a value between the transmittance Tx obtained

when all the SRRs having the open parts in a direction parallel to

the polarization direction (X-axis direction) and the transmittance

Ty obtained when all the SRRs having the open parts in a direction

orthogonalto the polarization direction (X-axis direction). In the

fourth pattern 34, a ratio of the number of the SRRs having the open

parts parallel to the polarization direction (X-axis direction) and

the number of the SRRs having the open parts orthogonal to the

polarization direction (X-axis direction) is 5:4. Therefore, the

transmittance will be a value near (5xTx+4xTy)/9. The

above-mentioned dimension of the area irradiated with the terahertz

electromagnetic wave is determined so as to be at least larger than

the area occupied by the SRRs arranged in the three-by-three matrix

pattern.

[0044] In this manner, a basic patter is formed by selecting

the SRRs from the four types of the SRRs 20 to 23 having the open parts 20a to 23a parallel to or orthogonal to the X-axis direction.

The transmittance of the terahertz electromagnetic wave can be

changed by changing the type and the number of the SRRs. By using

this technique, the first pattern 31 to the fourth pattern 34 are

formed such that each gives a different transmittance. Moreover,

the basic pattern set by using the SRRs 20 to 23 is successively

arranged in a matrix layout thereby forming the counterfeit

prevention structure 10. By this structure, the variation in the

transmittance due to an inclination of the counterfeit prevention

structure 10 can be suppressed.

[0045] FIG. 6 is a view for explaining an example of the

transmittance of the counterfeit prevention structure 10. The

frequency characteristics shown in the lower part of FIG. 6 is a

schematic representation of the change in the transmittance when

the counterfeit prevention structure 10 of the first pattern 31 to

the fourth pattern 34 is irradiated with the terahertz

electromagneticwave having the polarization direction in the X-axis

direction. The horizontal axis represents the frequency of the

emitted terahertz electromagnetic wave and the vertical axis

represents the transmittance value. As shown in the upper part of

FIG. 6, an angle of an inclination of the counterfeit prevention

structure 10 is taken as a. The frequency characteristics of the

transmittance shown with a dotted line in the lower part of FIG.

6 is obtained when the counterfeit prevention structure 10 is not

inclined (a=0 degree). On the other hand, the frequency

characteristics of the transmittance shown with a solid line is

obtained when the counterfeit prevention structure 10 is inclined by 15 degrees (a=15 degrees). A range of variation of the transmittance of the primary resonance frequency fl (THz) that can occur when the counterfeit prevention structure 10 is inclined is shown with "r" in FIG. 6. , Even when the counterfeit prevention structure 10 is inclined, the range r of variation of the transmittance is very smallandit is only fewpercent of the absolute value of the transmittance.

[0046] Specifically, when the counterfeit prevention

structure 10 that is not inclined (a=0 degree) is irradiated with

the terahertz electromagnetic wave of the primary resonance

frequency fl (THz) having the polarization direction in the X-axis

direction, the transmittance of the counterfeit prevention

structure 10 of the first pattern 31 is about 40%. Moreover, the

transmittance of the counterfeit prevention structure 10 of the

second pattern 32 is about 35%, the transmittance of the counterfeit

prevention structure 10 of the third pattern 33 is about 30%, and

the transmittance of the counterfeit prevention structure 10 of the

fourth pattern 34 is about 30%. On the other hand, when a

conventional counterfeit prevention structure, in which the angle

between the polarization direction of the terahertz electromagnetic

wave and the directions ofthe openparts ofallthe SRRsis 60degrees,

is similarly irradiated with the terahertz electromagnetic wave,

the transmittance is about 30%.

[0047] When the counterfeit prevention structure 10 of the

first pattern 31 inclines in a range of -15 degrees to 15 degrees

(-15 degrees a 15 degrees) while irradiating with the terahertz

electromagnetic wave, the transmittance varies between about 40% and about 38%. The range of variation of the transmittance is about

2% in the counterfeit prevention structure 10 of the first pattern

31. Similarly, the range of variation of the transmittance is about

1% in the counterfeit prevention structure 10 of the second pattern

32, the range of variation of the transmittance is almost 0% in the

counterfeit prevention structure 10 of the third pattern 33, and

the range of variation of the transmittance is about 0.3% in the

counterfeit prevention structure 10 of the fourth pattern 34. In

a case where the conventional counterfeit prevention structure, in

which the angle between the polarization direction of the terahertz

electromagnetic wave and the directions of the open parts are 60

degrees, inclines in a range of -15 degrees to 15 degrees, the angle

variesinarange of45 degrees to75 degrees andthe range ofvariation

of the transmittance is about 20%.

[0048] When the counterfeit prevention structure 10 is not

inclined, the transmittance in the counterfeit prevention structure

10 of the third pattern 33, the counterfeit prevention structure

of the fourth pattern 34, and the conventional counterfeit

prevention structure is almost the same and it is about 30%. On the

other hand, when the counterfeit prevention structure is inclined

in the range of-15 degrees to 15 degrees, while the range ofvariation

of the transmittance of the conventional counterfeit prevention

structure is about 20%, the range of variation of the transmittance

of the counterfeit prevention structure 10 of the third pattern 33

and the counterfeit prevention structure 10 of the fourth pattern

34 remains less than 1%. This means that, in the counterfeit

prevention structure 10 according to the present embodiment, the range of variation of the transmittance with respect to the inclination thereof can be suppressed in comparison with the conventional structure.

[0049] The reason why the range of variation of the

transmittance is suppressed in the counterfeit prevention structure

10 of the first pattern 31 is because the range of variation arising

from the inclination is less when the directions of the open parts

of the SRRs are parallel to the polarization direction of the

terahertz electromagnetic wave.

[00501 The reason why the range of variation of the

transmittance is suppressed in the counterfeit prevention structure

10 of the second pattern 32 to the fourth pattern 34 is because of

the use of a mixture of a plurality of types of the SRRs 20 to 23

in which the directions of the open parts differ by 90 degrees unit.

Specifically, for example, if the SRRs are inclined while the

terahertz electromagnetic wave of the primary resonance frequency

is emitted, the transmittance of the SRRs having the open parts

parallel to the polarization direction of the terahertz

electromagnetic wave decreases, but the transmittance of the SRRs

having the open parts orthogonal to the polarization direction

increases. Therefore, the increase and the decrease in the

transmittance are offset and the range of variation of the

transmittance can be suppressed.

[0051] There is a mixture of the SRRs for which the

transmittance increases and the SRRs for which the transmittance

decreases when the counterfeit prevention structure 10 is inclined

with respect to the polarization direction of the terahertz electromagneticwave, an effect ofsuppressing the range ofvariation of the transmittance due to the inclination can be achieved. The types of the SRRs used to make the counterfeit prevention structure

10 are not limited to the SRRs in which the directions of the open

parts are different by 90 degrees. However, by using the SRRs in

which the directions of the open parts are different by 90 degrees,

irrespective of the polarization direction of the terahertz

electromagnetic wave, there will be a mixture of the SRRs for which

the transmittance increases and the SRRs for which the transmittance

decreases when the counterfeit prevention structure 10 is inclined.

Accordingly, an effect of suppressing the range of variation of the

transmittance due to the inclination of the counterfeit prevention

structure 10 can be achieved irrespective of the polarization

direction of the terahertz electromagnetic wave.

[0052] In FIGS. 3A, 3B, 3C, and 5 is shown an example of the

counterfeit prevention structure 10 formed by successively

arranging in a matrix layout one pattern of the SRRs selected from

the four types of the SRRs 20 to 23; however, the counterfeit

prevention structure can be obtained by combining a plurality of

types of the patterns.

[00531 FIG. 7 is aview of an example of a counterfeit prevention

structure 50 in which a plurality of types of the patterns is combined.

In the left part of FIG. 7 is shown a plan view of the counterfeit

prevention structure 50 including a first area 11 (11a and 11b),

a second area 12, and a third area 13 (13a and 13b), and in the right

part is shown an enlarged view of a partial area 15 containing these

three areas 11 to 13. The first area 11 has the shape substantially like the English character L. The third area 13 has a shape obtained by rotating the first area 11 by 180 degrees. An area surrounded by the first area 11 and the third area 13 is the second area 12.

The sheet-like counterfeit prevention structure 50 is, for example,

a square of a length and a width 20 mm. The second area 12 arranged

at the center of the counterfeit prevention structure 50 is a square

of a length and a width 10 mm.

[0054] As shown in the partially enlarged view in the right part

of FIG. 7, the first pattern 31 shown in FIG. 3A is successively

arranged into a matrix layout in the first area 11. In the third

area 13, the third pattern 33 shown in FIG. 3C is successively

arranged into a matrix layout.

[00551 The second area 12 is constituted by a fifth pattern 35

obtained by rotating the first pattern 31 in the counterclockwise

direction by 90 degrees. FIG. 8 indicates a configuration of the

fifth pattern 35. The fifth pattern 35 that functions as a basic

unit is shown in the left part of FIG. 8. A part of the second area

12 formed by repeatedly arranging the fifth pattern 35 in a matrix

layout is shown in the right part of FIG. 8. The fifth pattern 35

is a two-by-two matrix pattern in which the SRR 21 is arranged in

the upper left corner, the SRRs 23 are arranged on the right side

of and below the SRR 21, and the SRR 21 is arranged on the right

side of the lower SRR 23. The fifth pattern 35 is constituted by

only the SRRs 21 and 23 that have the open parts 21a and 23a in the

Y-axis direction.

[00561 FIG. 9 is a view for explaining a change in the

transmittance of the terahertz electromagnetic wave observed in a medium 100 on which the counterfeit prevention structure 50 shown in FIG. 7 is provided. A plan view of the medium 100 having the counterfeit prevention structure 50 is shown in the upper part of

FIG. 9. The counterfeit prevention structure 50 of the square shape

is arranged such that each edge thereof is parallel to the

corresponding edge of the rectangular medium 100. A scanning

position and a scanning direction when the counterfeit prevention

structure 50 is scanned with the terahertz electromagnetic wave are

shown with an arrow 200 in the central part of FIG. 9. A waveform

of the transmittance of the terahertz electromagnetic wave obtained

at the scanning position is shown in the lower part of FIG. 9. This

transmittance waveform is a schematic representation of the change

in the transmittance of the counterfeit prevention structure 50 when

scanned with the terahertz electromagnetic wave of the primary

resonance frequency.

[0057] A substantially central part in the X-axis direction of

the counterfeit prevention structure 50 is scanned in the direction

shown with the arrow 200 with the terahertz electromagnetic wave

of the predetermined frequency having the polarization direction

in the X-axis direction. The medium 100 is scanned in a where that

the medium 100 namely the counterfeit prevention structure 50 is

not inclined. The transmittance in the first area 11 constituted

by the first pattern 31, the transmittance in the second area 12

constituted by the fifth pattern 35, and the transmittance in the

third area 13 constituted by the third pattern 33 show different

values corresponding to the respective pattern.

[00581 For example, for the primary resonance frequency, the transmittance in the first area 11 shows a high value (about 40%), and the transmittance in the second area 12 shows a very low value

(about 2%). The transmittance in the third area 13 shows a value

(about 20%) between the transmittance in the first area 11 and the

transmittancein the secondarea12. Therefore, as shownin the lower

part of FIG. 9, after a waveform 71 indicating a substantially

constant and high transmittance is obtained in the first area 11

on the right side of FIG. 9, the transmittance decreases in the

central second area 12. After a waveform 72 indicating a

substantially constant transmittance is obtained in the second area

12, the transmittance increases again in the third area 13 on the

left side of FIG. 9. A waveform 73 indicating a substantially

constant transmittance lower than the waveform 71 is obtained in

the third area 13. In this manner, if the counterfeit prevention

structure 50 is constituted by a plurality of areas each of which

indicates a different transmittance when irradiated with the

predetermined terahertz electromagnetic wave, a characteristic

waveform in which the transmittance changes while scanning the

counterfeit prevention structure 50 can be obtained. The

authenticity of the medium 100 can be determined based on the feature

of the obtained transmittance waveform.

[00591 Even when the counterfeit prevention structure 50 is

inclined by 15 degrees, the range of variation of the transmittance

of the first area 11 constituted by the first pattern 31 and the

range of variation of the transmittance of the second area 12 remain

as low as 2%. Moreover, the transmittance of the third area 13

constituted by the third pattern 33 almost does not vary. Therefore, even when the counterfeit prevention structure 50 is inclined, as shown in the lower part of FIG. 9, a stepped waveform in which the transmittance decreases from the waveform 71 to the waveform 72 and the transmittance increases from the waveform 72 to the waveform

73 is obtained. Moreover, the relation of the magnitudes of the

waveform 71, the waveform 72, and the waveform 73 does not change

even when the counterfeit prevention structure 50 is inclined.

Therefore, even when the inclined medium 100 namely the inclined

counterfeiting prevention structure 50 is scanned for measuring the

transmittance, a characteristic waveform in which the transmittance

changes in three phases can be obtained. The authenticity of the

medium 100 can be determined based on the feature of the obtained

transmittance waveform.

[00601 A case of irradiating the counterfeit prevention

structures 10 and 50 with the terahertz electromagnetic wave of the

primary resonance frequency (P1 of FIG. 4) having the polarization

directionin the X-axis directionismainlyexplainedabove; however,

different transmission characteristics will be obtained with the

secondary resonance frequency (V1 of FIG. 4) . FIG. 10 is a view for

explaining a change in the transmittance obtained when the secondary

resonance frequency is used. A plan view of the medium 100, which

is the same as that shown in FIG. 9, having the counterfeit prevention

structure 50 is shown in the upper part of FIG. 10, and a scanning

position and a scanning direction when the counterfeit prevention

structure 50 is scanned with the terahertz electromagnetic wave are

shown with the arrow 200 in the central part. A transmittance

waveform obtained when the counterfeit prevention structure 50 is scanned with the terahertz electromagnetic wave of the secondary resonance frequency at the scanning position is shown in the lower part of FIG. 10.

[0061] A substantially central part in the X-axis direction of

the counterfeit prevention structure 50 is scanned in the direction

shown with the arrow 200 with the terahertz electromagnetic wave

of the predetermined frequency having the polarization direction

in the X-axis direction. The medium 100 is scanned in a state where

the medium 100 namely the counterfeit prevention structure 50 shown

in FIG. 7 is not inclined. The transmittance waveform shown in the

lower part of FIG. 9 is obtained when the scanning is performed by

using the terahertz electromagnetic wave of the primary resonance

frequency, and the transmittance waveform shown in the lower part

of FIG. 10 is obtained when the scanning is performed by using the

terahertz electromagneticwave of the secondary resonance frequency.

The relation between the polarization direction of the terahertz

electromagnetic wave, the directions of the open parts of the SRRs

20 to 23 and the transmittance value varies depending on a resonance

mode. For the primary resonance frequency, the transmittance

becomes maximum when the directions of the open parts of the SRRs

are parallel to the polarization direction of the terahertz

electromagnetic wave. On the other hand, for the secondary

resonance frequency, the transmittance becomes maximum when the

directions of the open parts of the SRRs are orthogonal to the

polarization direction of the terahertz electromagnetic wave.

[0062] Even for the secondary resonance frequency, different

transmittance can be obtained in each of the first area 11 to the third area 13; however, the transmittance in the first area 11 shows a very low value of a few percent and the transmittance in the second area 12 shows a high value. The transmittance in the third area 13 shows a value between the transmittance in the second area 12 and the transmittance in the first area 11. Therefore, as shown in the lower part of FIG. 10, after a waveform 81indicating a substantially constant and low transmittance is obtained in the first area 11 on the right side of FIG. 10, the transmittance increases in the central second area 12. After a waveform 82 indicating a substantially constant transmittance is obtained in the second area 12, the transmittance decreases again in the third area 13 on the left side of FIG. 10. A waveform 83 indicating a substantially constant transmittance higher than the waveform 81 is obtained in the third area 13. The authenticity of the medium 100 can be determined based on the feature of the obtained transmittance waveform.

[00631 Even when the counterfeit prevention structure 50 is

inclined by 15 degrees, like in the case of the primary resonance

frequency, each of the range of variation of the transmittance of

the first area 11, the range of variation of the transmittance of

the second area 12, and the range of variation of the transmittance

of the third area 13 remains as low as 4%. Therefore, even when the

counterfeit prevention structure 50 is inclined, a stepped waveform

in which the transmittance increases from the waveform 81 to the

waveform 82 is obtained as shown in the lower part of FIG. 10. There

is a difference of about 15% between the transmittance of the second

area 12 shown with the waveform 82 and the transmittance of the third

area 13 shown with the waveform 83. Even when the counterfeit prevention structure 50 is inclined by 15 degrees, the range of variation of the transmittance of the second area 12 remains as low as about 4%, and the transmittance of the third area 13 almost does notvary. Therefore, evenwhen the counterfeitprevention structure is inclined, a stepped waveform in which the transmittance decreases from the waveform 82 to the waveform 83 is obtained as shown in the lower part of FIG. 10. Moreover, the relation of the magnitudes of the waveform 81, the waveform 82, and the waveform

83 does not change even when the counterfeit prevention structure

50 is inclined. Even when measuring the transmittance by scanning

the inclined medium 100 namely the inclined counterfeit prevention

structure 50, a characteristic waveform in which the transmittance

changes in three phases can be obtained. The authenticity of the

medium 100 can be determined based on the feature of the obtained

transmittance waveform.

[0064] As explainedby referring FIGS. 9 and10, the counterfeit

prevention structure 50 provided in the medium 100 is scanned with

the terahertz electromagnetic wave. Such a scanning can be

implemented by an authenticity determination apparatus. In the

authenticity determination apparatus, a position from which the

terahertz electromagnetic wave is emitted and a position at which

the emitted terahertz electromagnetic wave is received are fixed.

The medium 100 is scanned by being transported inside the

authenticity determination apparatus. Such an authenticity

determination apparatus is explained below by taking the

measurements corresponding to FIG. 9 as an example.

[00651 FIG. 11 is a schematic diagram indicating a schematic internal configuration of the authenticity determination apparatus seen from a side thereof. A transport unit 63 transports the medium

100 in a direction shown with an arrow 201. A terahertz

electromagnetic wave transmitting unit 61 is arranged above the

transport unit 63. A terahertz electromagnetic wave receiving unit

62 is arranged below the transport unit 63. The terahertz

electromagnetic wave transmitting unit 61 transmits the terahertz

electromagnetic wave of the predetermined frequency having the

polarization direction in the X-axis direction in a lower direction

as shown with an arrow 202. The counterfeit prevention structure

50 provided on the medium 100 being transported by the transport

unit 63 is irradiated with the terahertz electromagnetic wave. The

terahertz electromagnetic wave receiving unit 62 receives the

terahertz electromagnetic wave that penetrates the counterfeit

prevention structure 50. A position from which the terahertz

electromagneticwave is emitted and aposition at which the terahertz

electromagnetic wave is received are fixed. The terahertz

electromagnetic wave receiving unit 62 detects intensity of the

received terahertz electromagnetic wave, and obtains a

transmittance from the detected intensity. The transmittance is a

ratio of the detected intensity to intensity of the terahertz

electromagnetic wave that is detected in a state where there is no

medium 100 being transported by the transport unit 63. As shown in

FIG. 11, the medium 100 is transported by the transport unit 63 in

the direction shownwiththe arrow201. Themedium100passes through

the position at which the terahertz electromagnetic wave is emitted

and received. While the medium 100 passes through the position, the counterfeitprevention structure 50is scannedin the direction shown with the arrow 200 and the waveform of the transmittance is obtained as shown in FIG. 9. Instead of calculating the transmittance in the terahertz electromagneticwave receiving unit 62, the transmittance can be calculated by a control unit 64. In this case, the terahertz electromagnetic wave receiving unit 62 outputs the intensity of the received terahertz electromagnetic wave to the control unit 64, and the control unit 64 calculates the transmittance.

[00661 FIGS. 12A and 12B are schematic diagrams of the

configuration shown in FIG. 11 when seen from above. FIG. 12A shows

a case in which the medium 100 is transported without inclining.

FIG. 12B shows a case in which the medium 100 is transported while

the medium 100 is inclined by an angle a. The transmittance of the

terahertz electromagnetic wave penetrating the counterfeit

prevention structure 50 is different for the state shown in FIG.

12A and the state shown in FIG. 12B, however, the range of variation

of the transmittance is small. Therefore, the authenticity of the

medium 100 can be determined with a high accuracy based on the value

of the transmittance, the waveforms of the transmittance obtained

by scanning the counterfeit prevention structure 50, and the like.

[0067] FIG. 13 is a block diagram indicating a schematic

functionalconfiguration ofan authenticity determination apparatus

1. The authenticity determination apparatus 1 includes the control

unit 64 and a memory 65 in addition to the configuration shown in

FIG. 11. The memory 65 is a nonvolatile storage device constituted

by a semiconductor memory and the like. In the memory 65, reference

data is previously prepared. The reference data includes the values of the transmittance, the waveforms of the transmittance, the characteristic features of the waveforms, and the like to be obtained by irradiating the counterfeit prevention structure 50 with the predetermined terahertz electromagnetic wave.

[00681 The control unit 64 controls the transport of the medium

100 by the transport unit 63, the transmission of the terahertz

electromagnetic wave by the terahertz electromagnetic wave

transmittingunit 61, the receiving of the terahertz electromagnetic

wave by the terahertz electromagnetic wave receiving unit 62, and

the like. Moreover, the control unit 64 acquires the values of the

transmittance of the terahertz electromagnetic wave that penetrates

the counterfeit prevention structure 50, the waveforms of the

transmittance, and the like. The control unit 64 determines the

authenticity of the medium 100 by comparing with the reference data

prepared previously in the memory 65 at least one among the values

of transmittance, the waveforms of the transmittance, the

characteristic features of the waveforms, and the like. The control

unit 64 outputs the determination result of the authenticity to a

not-shown external apparatus. For example, the determination

result of the authenticity is output to a display apparatus to be

displayed and alarmed.

[00691 The present embodiment explained an example in which the

counterfeit prevention structures 10 and 50 includes the base member

17 and the conductive layer 16 in which the SRRs 20 to 23 are formed;

however, the structure of the counterfeit prevention structures 10

and 50 is not limited to this. FIG. 14 is a schematic cross section

indicating another structuralexample of the counterfeit prevention structures 10 and 50. In the counterfeit prevention structures 10 and 50 shown in FIG. 14, the conductive layer 16 shown in FIGS. 1 and 7 is adhered to a surface of the medium 100 via an adhesive layer

41. A hologram layer 42 is arranged on the conductive layer 16, and

a release layer 43 is arranged on the hologram layer 42. For example,

after the release layer 43, the hologram layer 42, the conductive

layer 16, and the adhesive 41 are sequentially formed on a

predetermined base material, the layers above and including the

release layer 43 are separated from the base material, the top and

bottom of the separated structure is reversed, and the configuration

shown in FIG. 14 is obtained by sticking the released structure to

the medium 100 via the adhesive 41. The release layer 43 is made

ofmaterialsuchas transparent resin. Under the visible light, when

the counterfeit prevention structures 10 and 50 shown in FIG. 14

is seen from above, a three-dimensional image recorded in the

hologram layer 42 can be seen. The SRRs 20 to 23 each having the

shape substantially like the English character C are minute

structures formed in the thin conductive layer 16 having a thickness

of few pm and therefore, it is difficult to see with the naked eyes.

Moreover, because the predetermined design recorded in the hologram

layer and the like arranged on the conductive layer 16 is seen, it

becomes more difficult to notice the SRRs 20 to 23 whereby the effect

of the counterfeit prevention is enhanced.

[0070] The present embodiment explained an example in which the

directions of the open parts of the SRRs are either parallel or

orthogonal to the polarization direction of the terahertz

electromagnetic wave; however, the direction of open part is not limited to this. FIG. 15 is a view indicating an example of the counterfeit prevention structure 10 having SRRs 120 to 123 of which the openparts arein differentdirections. The SRRs120 to 123 shown in FIG. 15 have a shape obtained by rotating each of the SRRs 20 to 23 shown in FIG. 1 in the clockwise direction by 45 degrees. The directions of open parts 120a to 123a of the SRRs 120 to 123 make an angle of 45 degrees to the X-axis direction and the Y-axis direction. Even when the SRRs 20 to 23 constituting the firstpattern

31 to the fifth pattern 35 are replaced with the SRRs 120 to 123

showninFIG. 15, respectively, an area in which the transmittance

of the terahertz electromagnetic wave shows a predetermined value

can be realized as explained above.

[0071] In the example explained with reference to FIGS. 9 and

10, themedium100providedwith the counterfeitprevention structure

50 is rectangle, and the directions of the open parts of the SRRs

and the orientations of the edges of the medium 100 are parallel

or orthogonal; however, the angle between the directions of the open

parts and the edges ofthe medium100 canbe 45 degrees. Specifically,

for example, the medium 100 shown in FIG. 9 can be kept as it is

and only the counterfeit prevention structure 50 can be rotated in

the clockwise direction by 45 degrees. Alternatively, for example,

the SRRs of the counterfeit prevention structure 50 can be replaced

with the SRRs 120 to 123 shown in FIG. 15. Even in this case, a

waveform having different transmittance of the terahertz

electromagnetic wave can be obtained in the first area 11 to the

third area 13 as explained above.

[0072] Moreover, a structure different from that shown in FIG.

7 can be adopted as the structure in which the transmittance changes

in various areas while scanning the counterfeit prevention structure

50 of the medium 100 being transported. FIG. 16 is a view indicating

another example of a counterfeit prevention structure 150 that is

divided in a plurality of areas. The counterfeit prevention

structure 150 having a square shape shown in FIG. 16 is divided into

eight areas at regular intervals in the diagonal direction. These

eight areas make an angle of 45 degrees to the edges of the counterfeit

prevention structure 150. The areas are constituted by two types

of areas, a first area 111 and a second area 112, and the two types

of areas are arranged alternately. For example, a desired one of

the first pattern 31 to the fifth pattern 35 can be selected for

each of the first area 111 and the second area 112 thereby obtaining

areas having different patterns. Moreover, for example, the first

area 111 can be an area made of insulating material through which

the terahertz electromagnetic wave can penetrate or can be made of

conductive material that blocks the terahertz electromagneticwave,

and that does not include the SRRs. In this example, the second area

112 can be an area constituted by a pattern selected among the first

pattern 31 to the fifth pattern 35, and that includes the SRRs. With

this configuration, when a substantially central in the X-axis

direction of the counterfeit prevention structure 150 is scanned

in the Y-axis direction with the predetermined terahertz

electromagnetic wave, the transmittance changes in the first area

111 and the second area 112. The authenticity determination can be

performed based on the transmission characteristics of the

counterfeit prevention structure 150.

[0073] The present embodiment explained an example in which

each area of the counterfeit prevention structure 50 shown in FIG.

7 is constituted by one of the first pattern 31 and the third pattern

33 shown in FIGS. 3A and 3C and the fifth pattern 35 shown in FIG.

8; however, the patterns used to form the areas are not particularly

limited. For example, the second pattern 32 and the fourth pattern

34 can be used. Moreover, the counterfeit prevention structure 50

can be divided in two areas or can be divided in four or more areas.

Moreover, it is explained to use the fifth pattern 35 obtained by

rotating the first pattern 31 by 90 degrees as the basic pattern;

however, it is allowable to use a pattern obtained by rotating the

second pattern 32, the third pattern 33, and the fourth pattern 34

by 90 degrees.

[0074] The present embodiment explained an example in which the

first pattern 31 to the fifth pattern 35 are taken as the basic

pattern; however, the basic patterns are not limited to this. The

shape of the basic pattern, and the type, the number, the layout,

and the like of the SRRs that constitute the basic pattern are not

particularly limited as long as, in an area which has the same shape

as the basic pattern and is selected from a desired position of an

area in which the basic patterns are repeatedly arranged in a matrix

layout, the ratio of the number of the SRRs having the open parts

parallel to the polarization direction of the terahertz

electromagnetic wave to the number of the SRRs having the open parts

orthogonal to the polarization direction of the terahertz

electromagnetic wave is the same as that of the basic pattern.

Specifically, for example, regarding the layout of the SRRs in the basic pattern, other than the matrix layout, in which the SRRs are arranged repeatedly vertically and horizontally, the SRRs can be arranged in the checkered pattern layout or in the honeycomb pattern layout. Regarding the layout of the basic patterns in each area, other than arranging the basic patterns in the matrix layout, a block pattern layout, a honeycomb pattern layout, or a layout in which the basic patterns are repeated as desired can be used. The shape of the SRRs is also not limited as long as the desired transmittance can be obtained when the terahertz electromagnetic wave of predetermined frequency is emitted. For example, the ring part can have a rectangle shape. Moreover, as long as the resonance frequency is the same, it is not necessary that all of the types of the SRRs are ofthe same shape. Itis allowable that the SRRs havingdifferent shapes, such as the rectangle shape, the circular shape, are mixed.

[0075] The present embodiment explained an example in which the

polarization direction of the terahertz electromagnetic wave used

for the authenticity determination is mainly along the X-axis

direction; however, the terahertz electromagnetic wave having the

polarization direction in the Y-axis direction can be used. The

transmittance in each of the basic patterns changes when the

polarization direction of the terahertz electromagnetic wave

changes, however the authenticity determination can be performed

as explained above by previously acquiring the transmittance

corresponding to the polarization direction.

[0076] The present embodiment explained an example in which the

transmittance of the terahertz electromagnetic wave is used for the

authenticity determination of the counterfeit prevention structure; however, it is allowable to use reflectivity of the terahertz electromagnetic wave. The transmittance and the reflectivity of terahertz electromagnetic wave have such a relation that when one of them increases the other decreases. For example, the terahertz electromagnetic wave transmitting unit 61 and the terahertz electromagnetic wave receiving unit 62 are arranged across the transported medium 100 in FIG. 11; however, these two units can be arranged on the same side of the medium 100. The reflectivity can be measured by receiving the terahertz electromagnetic wave, which is emitted by the terahertz electromagnetic wave transmitting unit

61 and reflected from the medium 100, in the terahertz

electromagnetic wave receiving unit 62. Accordingly, the

characteristics of the counterfeit prevention structure can be

obtained based on the reflectivity of the terahertz electromagnetic

wave and the authenticity determination of the counterfeit

prevention structure can be performed by using the transmittance

in the same manner as explained above.

[0077] As explained above, when the authenticity determination

apparatus according to the present embodiment is used, it is possible

to irradiate the counterfeit prevention medium, such as the banknote

or the coupon, provided with the counterfeit prevention structure

with the terahertz electromagnetic wave and determine the

authenticity of the counterfeit prevention medium based on the

transmission characteristics, such as the frequency and the

transmittance, of the emitted terahertz electromagnetic wave.

[0078] To allow determination of the authenticity, the

plurality of types of the split ring resonators that constitute the counterfeit prevention structure include, for example, the open parts in the direction that is parallel or orthogonal to the polarization direction of the emitted terahertz electromagnetic wave. By adjusting the ratio of the number of the split ring resonators having the open parts parallel to and the number of the split ring resonators having the open parts orthogonal to the polarization direction, the counterfeit prevention structure through which the terahertz electromagnetic wave of the predetermined frequency penetrates at the predetermined transmittance can be realized. Moreover, by using the split ring resonators having the open parts parallel and orthogonal to the polarization direction of the terahertz electromagnetic wave, the variation in the transmittance of the counterfeit prevention structure when the counterfeit prevention structure is inclined with respect to the polarization direction of the terahertz electromagnetic wave can be suppressed. Therefore, the authenticity determination can be performed with a high accuracy by using the counterfeit prevention structure.

[0079] The counterfeit prevention structure according to one

aspect ofthe presentinventionis acounterfeitprevention structure

provided on a medium to determine authenticity of the medium. The

counterfeit prevention structure includes a hybrid area in which

a plurality of types of split ring resonators is formed in a mixed

state in a predetermined ratio. Each split ring resonator includes

an open part. A direction of an open part of each type of the split

ring resonators is different from each other.

[00801 In the above counterfeit prevention structure, the plurality of types of the split ring resonators resonates with a terahertz electromagnetic wave having the same frequency.

[0081] In the above counterfeit prevention structure, the

hybrid area is formed by repeatedly arranging a basic pattern that

includes at least two types of the split ring resonators.

[0082] In the above counterfeit prevention structure, when the

counterfeit prevention structure is irradiated with a terahertz

electromagnetic wave of a predetermined frequency having a

polarization direction in a predetermined direction, a

transmittance of the terahertz electromagnetic wave in the hybrid

area indicates a value depending on a ratio of mixed types of the

split ring resonators.

[0083] In the above counterfeit prevention structure, the

plurality of types of the split ring resonators includes at least

two types of the split ring resonators having the open parts of which

respective opening directions are different by 90 degrees.

[0084] The above counterfeit prevention structure includes a

plurality of types of areas each of which indicates a different

transmittance when irradiated with a terahertz electromagneticwave

of a predetermined frequency having a polarization direction in a

predetermined direction. At least one of the plurality of types of

the areas is the hybrid area.

[0085] In the above counterfeit prevention structure, the

plurality of types of the areas includes a plurality of types of

the hybrid areas. Each of the plurality of types of the hybrid areas

has a different mixed ratio of the plurality of types of the split

ring resonators and a different transmittance.

[00861 The above counterfeit prevention structure includes a

hologram layer operable to generate a predetermined design under

visible light.

[0087] The above counterfeit prevention structure is formed on

a banknote.

[00881 A counterfeit prevention medium according to another

aspect of the present invention is a counterfeit prevention medium

including the above counterfeit prevention structure.

[00891 In the counterfeitprevention structure according to the

present invention, the range of variation of the transmittance due

to the inclination thereof can be suppressed and the authenticity

determination can be performed with a high accuracy in comparison

with the counterfeit prevention structure in which all the sprit

ring resonators have the same directions of the open parts.

[00901 As explained above, the counterfeit prevention

structure and the counterfeit prevention medium according to the

present invention are useful in determining the authenticity of the

counterfeit prevention medium provided with the counterfeit

prevention structure with a high accuracy.

Although the invention has been explained with respect to

specific embodiments for a complete and clear disclosure, the

appended claims are not to be thus limited but are to be construed

as embodying all modifications and alternative constructions that

may occur to one skilled in the art that fairly fall within the basic

teaching of the claims.

Claims (11)

We claim:

1. A counterfeit prevention structure provided on a medium to

determine authenticity of the medium, the counterfeit prevention

structure comprising:

a hybrid area in which a plurality of types of split ring

resonators is formed in a mixed state in a predetermined ratio,

wherein

each split ring resonator includes an open part,

an opening direction of the open part of each type of the split

ring resonators is different from each other,

the hybrid area has a transmittance of a constant value when

irradiated with a terahertz electromagnetic wave having a predetermined

frequency and a predetermined polarization direction, and

the hybrid area includes at least two types of split ring

resonators of which respective opening directions are different to

suppress variation in the transmittance due to an inclination of the

counterfeit prevention structure.

2. The counterfeit prevention structure as claimed in claim 1,

wherein the hybrid area includes at least two types of split ring

resonators of which the opening directions are different by 180 degrees.

3. The counterfeit prevention structure as claimed in claim 1 or 2,

wherein the hybrid area includes at least two types of split ring

resonators of which the opening directions are different by 90 degrees.

4. The counterfeit prevention structure as claimed in any one of claims 1 to 3, wherein the plurality of types of the split ring resonators resonates with a terahertz electromagnetic wave having a same frequency.

5. The counterfeit prevention structure as claimed in any one of

claims 1 to 4, wherein the hybrid area is formed by repeatedly arranging

a basic pattern that includes at least two types of the split ring

resonators.

6. The counterfeit prevention structure as claimed in any one of

claims 1 to 5, wherein when the counterfeit prevention structure is

irradiated with the terahertz electromagnetic wave, the transmittance

of the terahertz electromagnetic wave in the hybrid area indicates a

value depending on a ratio of mixed types of the split ring resonators.

7. The counterfeit prevention structure as claimed in any one of

claims 1 to 6, wherein

the counterfeit prevention structure includes a plurality of

types of areas each of which indicates a different transmittance when

irradiated with the terahertz electromagnetic wave, and

at least one of the plurality of types of the areas is the hybrid

area.

8. The counterfeit prevention structure as claimed in claim 7,

wherein

the plurality of types of the areas includes a plurality of types

of the hybrid areas, and

each of the plurality of types of the hybrid areas has a different mixed ratio of the plurality of types of split ring resonators and indicates a different transmittance.

9. The counterfeit prevention structure as claimed in any one of

claims 1 to 8, further comprising a hologram layer operable to generate

a predetermined design under visible light.

10. The counterfeit prevention structure as claimed in any one of

claims 1 to 9, wherein the counterfeit prevention structure is formed

on a banknote.

11. A counterfeit prevention medium comprising the counterfeit

prevention structure as claimed in any one of claims 1 to 9.