WO2004113974A1 - Polarizer and polarization separation element - Google Patents

Abstract

A polarization separation element comprising alternating multi-layer films of high-refractive-index medium layers and low-refractive-index medium layers having cyclic irregular shapes to separate by transmitting and reflecting a light incident vertically or obliquely to a plane into a polarization component parallel to the irregular shapes and a polarization component vertical to the irregular shapes, wherein a laminate cycle is changed within a range that provides a transmission region common to one polarization and a shielding region to at least part of the other polarization to thereby expand an operating band, whereby providing a polarizer and a polarization separating element that are thin, high in power resistance, and permit the setting of a wide operating wavelength range from ultraviolet to near infrared.

Description

 Description Polarizer and polarization separation element

 INDUSTRIAL APPLICABILITY The present invention is used in an optical device using polarization control of light, and provides a function of transmitting only a linearly polarized component of incident light in a specific direction, and a polarizer that converts incident light into two orthogonal linearly polarized light components. The present invention relates to a polarization splitting element that provides a function of separating light into light. Background art

 A polarizer is an element for converting unpolarized or elliptically polarized light whose electromagnetic field vibrates in an unspecified direction into linearly polarized light by transmitting only a vibration component in a certain direction. This is one of the most basic optical elements, and is widely used in optical communication devices, optical disk pickups, liquid crystal projectors, liquid crystal displays, and optical applied measurement. Polarizers are roughly classified into two types, depending on the operation mode: (1) those that absorb unnecessary polarized waves, and (2) those that separate two orthogonal polarization components incident on the same optical path into separate optical paths. ② is also used as a polarization separation element. Here, both are collectively called a polarizer.

 Polarizers that are currently in practical use and that perform the above operation (2) include polymer films containing dichroic molecules such as iodine, and glass with needle-like metal particles arranged in one direction. . On the other hand, as the polarization splitting element that performs the operation (2), there are an element (so-called PBS) using a Prüster angle by forming a multilayer film on the slope of the prism, and a polarizing prism made of birefringent crystal such as calcite. .

 Recently, in the field of optical recording and displays, a polarizer operating in the visible region has been emphasized. In particular, in a liquid crystal projector, linearly polarized light is incident on each of the RGB liquid crystal elements, but light with a high power density is often used for high brightness> miniaturization. To be absorbed, -f ¾thermal power> '. This is a key issue that degrades the reliability of the I-J and is a problem. In type (2), PBS is not usually used because it is not suitable for miniaturization because it has a cubic shape, and the birefringent crystal prism is expensive.

 Meanwhile, as a new polarization separation element belonging to (1), a photonic crystal polarization separation element according to Patent Document 1 (Japanese Patent Application Laid-Open No. 2001-83331) has been proposed. It consists of a multilayer structure of wavy thin films, which can be separated into a polarization component parallel to the groove direction and a polarization component perpendicular to the groove direction. Since it uses a lossless material, there is no absorption inside and there is no problem of heat generation even for light with a high light density.

¾Si and S i in one signal wavelength range. _It can be made difficult by using and to construct. For example, non-patent; ¾t 1 (T. Kawa shima, eta 1., Photonic crys ta lo 1 arizati on beam split ters and the ir appl i cat i ons, "Opt i ca 1 Fiber Commun i cat i on Conference (OFC2003), At l anta,

According to USA, March 2003, paper Th I 2.), around 0.2 wavelength, Si loss is less than 0.2dB and ¾extinction non-40dB¾ is obtained.

On the other hand, Si has an absorption at a wavelength shorter than about 1 wavelength, and is not suitable for use. This

One Una Instead of the wavelength region S i, the refractive transparent dielectrics constant high material T a ₂ 0 _5, Nb ₂ 〇 _{5, T i 0 2, Z} r 0 2 , etc. may be used to. However, the refractive index of these materials is 2 to 2.4, which is not as large as 3.5 of Si. Therefore, there is a problem that the cutoff wavelength range in the multilayer film combined with the low refractive index material becomes narrow, and the operating band of the polarizer becomes narrow. For example, cover the R (red), G (green), and B (blue) wavelength bands (for example, 400 nm to 500 nm, 500 nm to 600 nm, and 600 nm to 700 nm) commonly used in display devices. Is difficult. Further, when light is obliquely incident on the surface of the polarizer, the band is further narrowed, which is not suitable for practical use. Disclosure of the invention

 The present invention is to solve the above-mentioned problem of broadening the band, and an object of the present invention is to increase the band by changing the thickness of the laminated film or providing a plurality of regions having different periods. is there. At the same time, the objective is to increase the manufacturing tolerance by suppressing the band shift due to the change in the film thickness, and to increase the tolerance of the incident angle.

 First, a photonic crystal polarizer will be briefly described. On a transparent material substrate 101 having a periodic groove array as shown in FIG. 1, a transparent 髙 refractive index medium 102 and a low refractive index medium 103 are alternately laminated while preserving the shape of the interface. . Each layer has periodicity in the X direction, but may be uniform in the y direction, or may have a periodic or aperiodic structure having a length sufficiently larger than the X axis direction. It may be periodic in the z-axis direction, but as shown in FIG. 1, as in the idea of the present patent, the lamination cycle may change gradually or a combination of a plurality of cycles changing stepwise.

The formation of such a periodic structure ί¾1 is called 麵 self-cloning 〖ί described in Patent Document 2 (Japanese Patent Application Laid-Open No. Hei 10-335758, published by β), and is called 〖ίproperty, uniformity ”. This is an excellent method for producing fine periodic jobs (photonic crystals).

 In the periodic structure obtained in this way, i, ^ perpendicular to the X y plane are incident on non-polarized light or elliptically polarized light from oblique directions.> Polarization parallel to the groove array, i.e., j polarization> TE mode and TM mode light are excited inside the periodic structure for the orthogonal X polarization. In general, a multilayer film has a wavelength region where light can propagate and a wavelength region where light is reflected and cut off. In the case of having an irregular period in the plane as shown in Fig. 1, the cutoff region can have polarization dependence. For example, it can be designed so that a TM wave is transmitted and a TE wave is reflected.

As a low-refractive index medium, a material mainly composed of SiO ₂ is most common, has a wide transparent wavelength range, is chemically, thermally, and mechanically stable, and can be easily formed. . However, other optical glass may be used, and a material having a lower refractive index such as MgF ₂ may be used. As the high refractive index material, oxidation of the S i, semiconductors and the like Ge, etc. Ta ₂ 〇 _{5, T i 0 2, Nb} 2 0 5, H f 〇 _2, A l ₂ 〇 _3, S i ₃ N ₄ , Nitrides, or mixtures thereof can be used, have a wide transparent wavelength range, and can be used in the visible light region. On the other hand, semiconductors are limited to the near infrared region, but have the advantage of a large refractive index.

 FIG. 2A is an example of a dispersion curve used in the design of a polarizer. The vertical axis is the value obtained by normalizing the reciprocal of the wavelength with the lamination period, and the horizontal axis is the value obtained by normalizing the phase change amount during one cycle with π.

Two You. Open circles indicate TE waves and black circles indicate TM waves. Here, assuming that the in-plane period is L x and the lamination period is L _z , L _Z ZL _X = 1 in (a). In the hatched band, the TE wave is reflected as a band gap, and the TM wave is transmitted because it is in the propagation region, and thus operates as a polarization splitting element. Parameter Isseki for controlling the operating wavelength range, the refractive index of the material constituting, fill factor, and the like period L _x, the period in the stacking direction L _z of the groove array. Among, L _z is as possible out be arbitrarily changed during lamination. For example, when Lz is increased, the wavelength band that blocks the TE wave shifts to the longer wavelength side. Figure 3 illustrates the concept. The solid line 301 indicates the transmittance of a TE wave having a small L z in a periodic structure, and transmission is blocked in the wavelength range indicated by A. On the other hand, the broken line 302 is the transmittance of the TE wave of the periodic structure with a large Lz, which is cut off in the region B on the slightly longer wavelength side. The solid line 303 shows the TM wave transmittance of both structures. Therefore, in the wavelength range indicated by C, the TM wave becomes the propagation region in the entire stacking direction, and the TE wave is at least partially included in the cutoff region. By changing the lamination period in this way, it becomes possible to expand the wavelength region that operates as a polarizer. FIG. 1 is a diagram showing the structure of a polarization splitting element.

FIG. 2 is a diagram illustrating a dispersion relation of light propagating through the periodic structure.

FIG. 3 is a diagram showing the concept of enlarging the cutoff area by multiplexing the stacking periods.

FIG. 4 is a diagram showing a cross-sectional structure of the polarization beam splitter.

FIG. 5 is a diagram showing a transmission spectrum of a polarizer.

FIG. 6 is a diagram showing a cross-sectional structure of the polarization beam splitter.

FIG. 7 is a diagram showing one of the examples.

FIG. 8 is a diagram showing one of the examples.

Embodiment of the Invention

FIG. 4 is a diagram showing the structure of the embodiment of the present invention. In this figure. Reference numeral 4 0 1 is a layer of Amoru Fass T a ₂ 〇 _5, reference numeral 4 0 2 is a layer of S 0 _2. Each layer has a wavy shape, and the period L x in the X-axis direction is 0.20 m. The layers are alternately stacked in the z-axis direction. Immediately after the substrate, the layers are stacked at a period of 0.20 m, and thereafter, the period is gradually increased, and the period is set to 0.22 xm in the latter half of the multilayer film. By combining multilayer films with different periods, it is possible to combine the wavelength regions covered by one period while shifting them little by little, so that the operating wavelength region of the entire device can be broadened.

The fabrication method is described below. First, periodic grooves as shown in FIG. 4 are formed on the substrate 101 by electron beam lithography and dry etching. Other photolithographic interference exposure and a stamping technique using a mold may be used. The cross-sectional shape of the groove is rectangular, but may be other shapes such as a triangle. Here, quartz glass was used for the substrate. The width of the groove is 0.1 lim and the depth of the groove is 0.1 m. An anti-reflection film 403 is provided to prevent reflection at the boundary surface. On this substrate, using a T a ₂ 〇 ₅ and S i 0 ₂ targets, stacked alternately multilayered film in combination of sputter deposition dictionary emissions and bias sputtering. Then, X of each layer

Three It is important to set the bias conditions appropriately so that the periodic uneven shape is preserved in the axial direction. The conditions were as follows. Gas pressure 2mTo rr, target applied high frequency power 300W, Ta ₂ 0 ₅ layer deposition, target applied high frequency power 300W, Si 0 ₂ layer deposition, gas pressure 6mTo rr, target applied high frequency power 300W, sputter etching S i 0 _{After the two} layers are formed, the gas pressure is 2mTo r i-, and the high frequency power applied to the substrate is 90W. The thickness ratio of ta ₂ 0 ₅ layer and S i 0 ₂ layer 43: was 57.

 The period in the stacking direction is changed to extend the operating band of the polarizer. Specifically, three cycles are stacked immediately after the substrate with a cycle of 0.20, and then the cycle is gradually increased by 2% to 0.22 μπι at the center of the multilayer film. Three cycles are stacked at this cycle.

Figures 2 (a) and (b) show the band structure of a structure with a simple repetition period in the stacking direction (L _x = 0.2 m, L _z = 0.22 m). The calculation used the FDTD method. The operating range where the TE wave (polarization parallel to the groove) becomes the stop band and the TM wave (polarization perpendicular to the groove) becomes the pass band is 384 nm to 408 nm in (a) (L _X ZA = 0.52). ~ 0. 49), (b) in a _{39 3~433 nm (L x / a} = 0. 5 1~0. 462). By combining these, the operating range of the polarizer can be from 383 nm to 433 nm. The purpose of this design was to extend the operating wavelength range, and as shown in Fig. 4, the stacking cycle was linearly varied from 200 nm to 220 nm. Figure 5 shows the transmission spectrum of each of the TE and TM waves calculated by the FDTD method. The propagation mode around the wavelength of 390 nm is reduced, and the substantial band is widened.

 Depending on the parameters such as the refractive index of the constituent materials, the operating band, the incident angle, and the like, the ratio of the change in the lamination period and the width of the change may be other values. For example, the same effect can be realized by gradually reducing the film thickness according to the lamination. Also, as shown in FIG. 6, the lamination cycle may be made thicker, and may be made thinner in the middle. Also, the period may be changed to be thick, thin and thick as shown in FIG. 7 (a), or the cycle may be changed stepwise as shown in FIG. 7 (b).

 Fabrication was performed with a lamination cycle of 23 (4 mm square, lamination thickness 5 / im or less). The transmission spectrum of the polarizer measured by a spectrophotometer operates as a polarizer because the TE wave is blocked from a wavelength of 350 nm to 418 nm. The insertion loss at the flat transmission region excluding Fresnel reflection at the substrate boundary was about 0.1 dB, and the extinction ratio of the TE wave measured with a laser beam of 400 nm was 40 dB or more. .

(Example)

 FIG. 8 shows another embodiment of the present invention. As an example of a polarizer that operates at oblique incidence, the case where the incident angle of light is 45 degrees with respect to the substrate plane is shown. The direction of the groove formed on the substrate 101 is a direction perpendicular to the paper surface. In this case, the reflected light becomes S wave and the transmitted light becomes P wave. On such a substrate, a photonic crystal polarizer 801 composed of a multilayer film is formed by a self-cloning method. The method of film formation is the same as in the above embodiment.

 The thickness of the laminated film needs to be designed according to the incident angle. In this structure, the operating band shifts to longer wavelengths than in the case of normal incidence. For example, in order to operate at a wavelength of 490 nm to 520 nm, the pitch of the unevenness of the substrate must be 0.2 / zm, and the lamination period must be chopped from 0.20 m to 0.22 zm. In addition to the above examples, the operating wavelength range is the range where the material is transparent.

Four You can choose arbitrarily. Set the pitch of the irregularities on the substrate and the lamination period according to the wavelength to be operated. For example, for a wavelength of 1064 nm, an example of a design that modulates the uneven pitch of the substrate to 560 nm and modulates the lamination period from 0.3011 to 0.34 im.

 It is also possible to design the direction of the groove parallel to the paper. In this case, the reflected light becomes P wave and the transmitted light becomes S wave. In that case, the calculation shows that the bandwidth becomes narrower when the angle of incidence is changed, but the shift of the center wavelength is small.

 With oblique incidence as in the present embodiment, the reflected component can be greatly shifted from the axis of the incident light. Therefore, unnecessary polarization components can be separated and removed by an absorber installed separately. Alternatively, it is also possible to separate into two polarizations and use both polarization components.

Industrial applicability

 The polarized light separating element having the structure of the present invention is made of a thermally and chemically stable inorganic material thin film, and can correspond to a visible ultraviolet region to a near infrared region. In particular, since it operates in the visible range, it is useful for optical components such as liquid crystal projectors, optical pickups, and laser printers that handle output light. In addition, this structure is suitable for mass production by a technology based on a bias sputtering method called a self-cloning method, and can replace conventional elements.

Claims

The scope of the claims 1. In the three-dimensional orthogonal coordinates x, y, and z, a high-refractive-index medium layer made of a transparent, high-refractive-index medium and a low-refractive-index medium layer made of a transparent, low-refractive-index medium alternate in the z direction. A multi-layer structure laminated on The shape of each of the medium layers has a periodic uneven structure not more than the wavelength of light used in the X-axis direction, a uniform structure in the y-axis direction, or a period longer than the X-axis direction. Having a regular or aperiodic concave-convex structure, The stacking cycle of the alternating layers in the z direction differs in at least a part of the stacking direction. For light incident perpendicularly or obliquely to the y plane, the electric field has a polarization orthogonal to the y axis or a polarization orthogonal to the x axis. One of the periods in the X-axis direction is such that one of them is included in the propagation region in all regions having different periods in the stacking direction, and the other is included in the cutoff region in at least a part of the region in the stacking direction. A polarizer or a polarization separation element, wherein the period of the periodic concavo-convex structure and the lamination period in the z-axis direction are selected. 2. the high refractive index medium layer S 1 or T i 0 ₂ or Ding & ₂ 0 ₅ or? ^ 13 ₂ 0 ₅ or 2 r 0 ₂ or H f 〇 ₂ or A 1 ₂ 〇 ₃ or mixtures thereof _2. The polarizer or polarization separation element according to claim 1, wherein the low refractive index medium layer is made of SiO ₂ or MgF ₂ . 3. The polarizer or polarized light separating element according to claim 1, wherein the polarizer or the polarized light separating element operates in a partial wavelength range from a wavelength of 200 nm to 2000 nm. 4. Polarized light or polarized light according to claim 1, 2 or 3, characterized in that it operates in at least one of the wavelength ranges of 430 nm to 490 nm, 510 nm to 570 nm, and 590 nm to 650 nm. Separation child. 5. The polarizer or polarized light separating element according to claim 1, 2, 3, or 4, wherein an incident angle of light is 45 degrees or less with respect to an xy plane.