JP4182655B2 - Method and apparatus for generating light or electromagnetic wave - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to light or electromagnetic wave generation methods and generators capable of generating electromagnetic waves such as terahertz light.
[0002]
[Prior art]
Conventionally, there has been no effective coherent light source so far, particularly for light in the terahertz region (submillimeter wave). If light (submillimeter wave) in the terahertz region can be easily generated, it can be used as a new optical component and a special light source for an inspection apparatus. Terahertz light is safer than X-rays and ultraviolet rays, and can be applied to medical diagnosis and the like. Thus, terahertz light is expected to be widely used in various industries as a safety light source to replace X-rays, and it is considered that a new application field is developed. However, until now, coherent light in this wavelength region could not be generated easily.
[0003]
Here, an electromagnetic wave having a frequency of 1 to 10 THz is referred to as a terahertz electromagnetic wave or terahertz light. This frequency region corresponds to a wavelength of 30 to 300 microns. It is difficult to generate a coherent electromagnetic wave in this region, and it is desired to realize a high-quality light source (electromagnetic wave source) that can easily oscillate.
[0004]
As already described, the light source in the terahertz region is expected to be applied to the medical field as a new safety light source that replaces X-rays. Researches such as skin condition observation and dental tomographic diagnosis are being conducted by irradiating terahertz light. In addition, many applications are considered such as concrete fault diagnosis, visualization of banknote watermarks, moisture content measurement of packaged foods, and internal fluoroscopy of floppy disks.
[0005]
As a device for generating an electromagnetic wave in this region, there has conventionally been a large-scale device using a free electron laser, but the scale is too large to be suitable for practical use. Research has also been conducted to generate terahertz light by using a carbon dioxide laser with a wavelength of 10.6 microns as an excitation light source, irradiating it with gas molecules, and using vibration and rotation levels of the gas molecules at that time. However, since the rotation and vibration levels of the gas are fixed, there is a problem that the wavelength variable region is narrow and the apparatus is too large, and it has not been put into practical use.
[0006]
Also, a method has been announced in which a parametric oscillator is configured using lithium niobate (LiNbO ₃ ), which is a nonlinear optical element, and light in the THz region is extracted using an NdYAG laser as an excitation light source. When LiNbO ₃ is irradiated with an NdYAG laser (1.06 μm), it is decomposed into light having a wavelength almost equal to excitation light called idler light and terahertz light in a specific direction. At this time, the K vector of the incident light and the K vector of the two emitted lights are stored (O plus E, Vol. 22, No. 1, p69 (January 2000)).
[0007]
Furthermore, a method for generating terahertz light using an optical switch has been studied. In this method, a minute metal dipole antenna is provided on a gallium arsenide substrate, and a DC bias voltage is applied to the antenna. At this time, when a femtosecond laser pulse is irradiated between the parallel lines of the antenna, an electron hole is generated. At this time, since an instantaneous current flows due to the generation of the electron hole, terahertz light that is dipole radiation is generated ( O plus E, Vol. 22, No. 1, p41 (January 2000)).
[0008]
[Problems to be solved by the invention]
However, in the method of causing parametric oscillation by irradiating the LiNbO ₃ with the above-described LiNbO ₃ , it is necessary to use a large nonlinear optical single crystal in order to obtain high generation efficiency. There is a problem that it is difficult. Further, the method using the dipole antenna has a problem that the generation efficiency of terahertz light is low due to the pulse oscillation and the electron hole screening effect.
[0009]
Accordingly, an object of the present invention is to provide a light or electromagnetic wave generation method and generator capable of efficiently generating an electromagnetic wave such as terahertz light.
[0010]
[Means for Solving the Problems]
The purpose of the present invention is to generate light or electromagnetic waves that generate a plurality of light or electromagnetic waves different from the incident light or electromagnetic waves by making light or electromagnetic waves incident on a photonic crystal having a nonlinear optical effect and a periodic dielectric constant distribution. This is achieved by the generation method. Here, light or electromagnetic waves can be incident on the photonic crystal from specific controlled directions (including the same direction and different directions). The incident light or electromagnetic wave can be laser light. Further, at least one of the plurality of light or electromagnetic waves generated corresponds to a state where the inclination of the photonic band of the photonic crystal is small or a region where the group velocity of light is very small compared to the velocity of light in vacuum. It can be. Furthermore, light having the same wavelength as one of a plurality of generated light or electromagnetic waves can be incident on the photonic crystal from a specific direction. One wavelength of the generated plurality of light or electromagnetic waves is, for example, in the range of 30 μm to 300 μm, and the frequency can be a so-called terahertz region.
In the method for generating light or electromagnetic waves according to the present invention, a plurality of light or electromagnetic waves different from the generated light or electromagnetic waves are generated by generating light or electromagnetic waves inside a photonic crystal having a nonlinear optical effect and a periodic permittivity distribution. It is also possible to generate light or electromagnetic waves.
[0011]
The light or electromagnetic wave generator according to the present invention includes a photonic crystal having a nonlinear optical effect and a periodic permittivity distribution, and light or electromagnetic waves from a specific direction (including the same direction and different directions) in the photonic crystal. And a light or electromagnetic wave incident device. Here, the light or electromagnetic wave incident device can include a laser device that emits laser light, and a branching member that branches the laser light and transmits it to the photonic crystal. In this case, the laser device can be a wavelength tunable laser.
By comprising in this way, electromagnetic waves, such as terahertz light, can be generated efficiently.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Before describing embodiments of the present invention, the principle of the present invention will be described first.
[0013]
FIGS. 1A and 1B are diagrams showing examples of photonic crystals. FIG. 4A shows a spherical photonic crystal 11, and FIG. 4B shows a lattice-like photonic crystal 12. A photonic crystal is a structure having a periodic refractive index distribution comparable to the wavelength of artificially produced light. For example, CuCl, LiNbO ₃ , GaAs or the like is used as the material. The present invention utilizes the strong nonlinear optical effect of the photonic crystal, and efficiently oscillates electromagnetic waves such as terahertz light having a wavelength significantly different from that of the incident wave.
[0014]
A photonic crystal is an artificial medium that provides a periodic field of refractive index or dielectric constant, but the light placed in it is similar to that of an electron placed in a periodic Coulomb potential in the case of a semiconductor. Behave. That is, the characteristics of light can be described using the Brillouin zone.
[0015]
This will be described in more detail. Light is an electromagnetic wave, and it is well known to follow four equations related to Maxwell's electric field. Under the artificially created periodic dielectric constant photonic crystal, Maxwell's equation for the electric field can be written as: Note that r, a, k, and u in the following formulas and descriptions indicate vectors.
[1 / ε (r)] ∇ × [∇ × E (r)] = [ω ² / c ² ] E (r)
In this equation, the dielectric constant satisfies the periodicity ε (r) = ε (r + a). Here, a is a basic translation vector of the photonic crystal. A similar equation holds for the magnetic field.
[0016]
The above equation is invariant with respect to the translational symmetry operation, and it can be proved that the Bloch's theorem holds for the electric field vector E (r, t) [or magnetic field] of the light as well as the electron in the semiconductor (Phys. Rev. B52, 7982). (1995)).
Therefore, the state of light in the photonic crystal is the same as the state in the electron crystal,
E (r, t) = u _nk (r) exp (ikr)
It is known that can be written. n represents a band. Here, u _nk (r) satisfies the following relationship.
u _nk (r) = u _nk (r + a)
Here, a is the period of the dielectric constant of the photonic crystal and corresponds to the lattice constant of the photonic crystal.
[0017]
These indicate that a band structure appears for light or electromagnetic fields. In addition, an energy band where light cannot exist in a photonic crystal having a certain symmetry appears. This region where light cannot exist is called a photonic band gap.
[0018]
FIG. 2 is a diagram illustrating an example of a band structure of a photonic crystal. As can be seen from this figure, the light has a very large wave vector in the photonic crystal compared to when it is in a normal dielectric or vacuum.
Further, from the light band calculation, it can be seen that there is a region where the band is flat as shown by B in FIG. 2 in the photonic crystal. Reflecting this, the refractive index (n = ck / ω) increases in the energy region. It can also be seen that the group velocity of light corresponding to the band inclination dω / dk is significantly reduced.
[0019]
Next, the nonlinear effect of the photonic crystal will be described. Consider a case where the dielectric constant of a material constituting the photonic crystal having a periodic dielectric constant has nonlinearity. That is, assuming that the photonic crystal has a second-order nonlinear susceptibility χ ⁽²⁾ (r), the incident light (ω is the angular frequency of the light) is combined and the second harmonic of 2ω (SHG) Has also been studied (Phys., Rev. B, Vol. 54, p5732 (1996)).
[0020]
FIG. 3 is a diagram for explaining the conservation law of the wave vector in the nonlinear photonic crystal. In the present invention, two types of light having a specific direction (including the same direction and different directions) are introduced into the nonlinear photonic crystal, and the sum of the combined K vectors is made small as shown in the figure. think of. At this time, the energy of the two types of incident light may be different or the same. To simplify the explanation, consider laser beams with the same energy. Further, it is assumed that the incident light is in the first branch of the photonic band. At this time, for example, the incident laser light is incident from different directions. In this way, the combined K vector of the two laser beams can be reduced as described below.
[0021]
When incident laser beams are incident from different directions, the light beams are coupled to each other through the nonlinear susceptibility due to the nonlinear optical effect. The composite K vector is
K = K ₁ (ω ₀ ) + K ₂ (ω ₀ )
It is expressed.
Here, K ₁ (ω ₀ ) represents that the traveling direction of the laser light is K ₁ , and the energy of the light at that time is (h / 2π) ω ₀ (h is Planck's constant). Similarly, K ₂ (ω ₀ ) represents that the traveling direction of the laser beam is K ₂ , and the light energy at that time is (h / 2π) ω ₀ .
[0022]
Now, at this time, it can be seen that a nonlinear effect occurs and there exists a solution satisfying the following K vector and energy conservation.
K = K ₃ (2ω ₀ −Δ) + K ₄ (Δ)
Assuming that Δ is smaller than ω ₀ , K ₃ (2ω ₀ −Δ) has energy substantially equal to the second harmonic, and corresponds to the light in the photonic band B shown in FIG. Since this light has a gentle band inclination, its group velocity is very small compared to the speed of light in the vacuum at the same frequency, so that light having this K ₃ (2ω ₀ −Δ) is strong. It is thought to be excited. The enhancement effect due to the group velocity anomaly is one of the basic nonlinear optical characteristics in the photonic crystal.
[0023]
On the other hand, K ₄ (Δ) is light that appears on the low energy side, and corresponds to the terahertz light in the present invention. In this way, terahertz light and other desired wavelength light can be obtained.
By changing the angle formed by the two incident lights, the value of the combined K vector can be selected. By changing the magnitude of the combined K vector, K ₄ (Δ) that satisfies the conservation law of K vector and energy changes greatly. Therefore, since the angular frequency Δ can be freely selected, it goes without saying that this light is not limited to terahertz light.
[0024]
Further, when light having the same angular frequency as that of idler light is incident from the outside, the light has K ₃ (2ω ₀ −Δ) inside the photonic crystal. At this time, the probability of the stimulated scattering process of FIG. 3 is increased, and the spectrum of the light (electromagnetic wave) of K ₄ (Δ) can be narrowed. That is, when light of K ₃ (2ω ₀ −Δ) is introduced from the outside, a stimulated scattering effect due to the light occurs, and the light mixing process is increased, so that not only the light of K ₃ (2ω ₀ −Δ) but also the signal Terahertz light, which is light, is also strongly observed.
[0025]
In the above description, it was assumed that a laser beam was incident from the outside of the photonic crystal. However, it is obvious that the same effect can be obtained even if the beam is generated inside the photonic crystal. That is, a structure capable of laser oscillation may be produced in a plurality of photonic crystals, and the light may be synthesized using them.
[0026]
As can be seen from the above description, the electromagnetic wave obtained by the present invention is not limited to terahertz light. That is, the present invention is used as a means for making two types of light incident on a photonic crystal, decomposing the incident light using its nonlinearity, and generating two types of light or electromagnetic waves having desired wavelengths different from the incident light. Because it can.
[0027]
This can be expressed in terms of energy as follows.
E (K ₁ ) + E (K ₂ ) = E (K ₃ ) + E (K ₄ )
Here, K ₁ and K ₂ are wave number vectors of incident light, K ₃ and K ₄ are wave number vectors of decomposed and emitted light (or electromagnetic waves), and E (Ki) is light having wave numbers Ki (or electromagnetic waves). ) Energy.
[0028]
As described above, by using the above-described method of irradiating the photonic crystal with light, the energy of light of two incident light beams can be efficiently decomposed into energy of two different lights.
Hereinafter, based on an Example, this invention is demonstrated concretely.
[0029]
(Example 1)
FIG. 4 is a diagram showing an embodiment of the light or electromagnetic wave generator according to the present invention. As shown in the figure, the present embodiment includes a wavelength tunable laser 41, a branching member 42, reflecting members 43 to 47, and a photonic crystal 48. Here, the wavelength tunable laser 41 is excited using, for example, a dye laser (polymethylene dye) (1.1 μm band) excited by Chisso laser or Ar laser. If the optimum wavelength is known by the dye laser, a wavelength-fixed semiconductor laser (1.3 μm) may be used thereafter. The branch member 42 is, for example, a half mirror, and the reflection members 43 to 47 are, for example, mirrors. In the present embodiment, the wavelength tunable laser 41, the branching member 42, and the reflecting members 43 to 47 constitute a light or electromagnetic wave incident device.
[0030]
The photonic crystal 48 is produced as follows. First, GaP (gallium phosphide) is deposited on a transparent glass substrate (not shown) by a vacuum deposition method for 1 micron. Next, SiO ₂ is similarly deposited by 1 micron. By repeating this, a one-dimensional photonic crystal 48 can be produced. GaP is a strong nonlinear optical material having a relative dielectric constant of 8.4. The relative dielectric constant is large compared to other materials. On the other hand, SiO ₂ has a relative dielectric constant (about 2.1), and its nonlinearity is small. The difference between the dielectric constants of the two types of materials is large, so that a photonic band structure appears.
[0031]
When vacuum deposition is performed, each layer does not become a complete single crystal but is in a polycrystalline state, but the columnar directions are roughly aligned. Therefore, the macroscopic crystal orientation appears although it is not as clear as the single crystal. This vacuum deposition is alternately repeated 10 times to produce a one-dimensional photonic crystal 48 having a thickness of about 20 microns.
[0032]
Next, the operation will be described. First, laser light (wavelength λ) is emitted from the wavelength tunable laser 41. This laser light is branched by the reflecting member 42, reaches both sides of the one-dimensional photonic crystal 48 via the reflecting members 43 to 45 and 46 to 47, and is irradiated at a certain angle θ. In this example, laser light is irradiated at an angle θ1 from the upper surface of the photonic crystal 48 and at an angle θ2 from the lower surface.
[0033]
Here, when the wavelength of the laser light emitted from the wavelength tunable laser 41 is changed, light having an energy of ch / (λ / 2) −Δ and light having a long wavelength having an energy Δ at a certain wavelength λ. Observed. When this light is dispersed, it can be seen that electromagnetic waves such as terahertz light are generated on the long wavelength side.
[0034]
The energy E (K ₄ ) of the terahertz light generated at this time is E (K ₄ ) = E (K ₁ ) + E (K ₂ ) −E (K ₃ )
Since this relationship is strictly established from the conservation law of energy, the wavelength (or frequency) on the long wavelength side emitted can be accurately known by observing the light of E (K ₃ ).
In this embodiment, the wavelength of incident light is 1.1 μm, and the light generated thereby is approximately ½ of 0.55 μm wavelength light and 0.3 mm long wavelength light (electromagnetic wave). Separation is easy because each light has a significantly different direction and energy.
[0035]
(Example 2)
FIG. 5 is a diagram showing another embodiment of the light or electromagnetic wave generator according to the present invention. As shown in the figure, the present embodiment includes a laser light source 51, a branching member 52, reflecting members 53 to 56, and a photonic crystal 57. Here, as the laser light source 51, for example, a semiconductor laser having a wavelength of 1.3 μm (1.05 eV) can be used. The branch member 52 is, for example, a half mirror, and the reflection members 53 to 56 are, for example, mirrors. In the present embodiment, the laser light source 51, the branching member 52, and the reflecting members 53 to 56 constitute a light or electromagnetic wave incident device.
[0036]
Next, a method for producing the photonic crystal 57 will be described. First, an indium phosphide InP substrate having a nonlinear optical effect is processed to a thickness of 0.5 μm, and a stripe structure is formed using an electron beam (The 48th Applied Physics Related Conference, Proceedings of Proceedings (2001) (March, 29p-ZH-7) These layers are stacked to obtain a photonic crystal structure, specifically, a 2-inch InP substrate is used as a substrate, and InGaAs and InP are deposited on the InP substrate by MOCVD. The sacrificial layer of InGaAs is deposited in a thickness of 0.8 μm, the InP layer is deposited in a thickness of 0.5 μm, a resist is applied, and a stripe pattern of the resist is formed by an electron beam lithography system. The width between the stripes was 0.5 μm and the period between the stripes was 1 μm, and then the InP and InGaAs layers were formed by ICP (inductively coupled plasma) method. Etching at the same time Etching and removing the InGaAs layer as a sacrificial layer to create an air bridge structure, and then removing the 0.5 μm InP layer suspended in the air by the micromanipulation method. A three-dimensional photonic crystal can be fabricated, and is a diagram showing an example of a state in which two InP layers are stacked, in which an InP layer 62 crosses at a right angle on an InP layer 61. Are stacked.
[0037]
In order to remove the sacrificial layer by etching, a sulfuric acid-based etchant was used as an etchant. Lamination is performed 100 times to produce a three-dimensional photonic crystal 57 having a thickness of about 50 μm. The length of one side of the photonic crystal 57 was 2 mm.
[0038]
Next, the operation will be described. First, laser light having a wavelength of 1.3 μm (1.05 eV) is emitted from the laser light source (semiconductor laser) 51 of FIG. This laser beam is branched by the reflecting member 52 and irradiated before and after the three-dimensional photonic crystal 57 through the reflecting members 53 to 55 and 56, respectively. At this time, since a photonic band of an energy level corresponding to about 2 eV is excited, light (2ω ₀ −Δ) having energy about twice that of incident light,
E (K ₄ ) = E (K ₁ ) + E (K ₂ ) −E (K ₃ )
Terahertz light (Δ), which is a long-wavelength electromagnetic wave satisfying the above, is generated. Also in this case, since the direction and energy in which each light is generated are greatly different, the separation is easy.
[0039]
As described above, in the present invention, the two types of incident light ω1 and ω2 are efficiently decomposed into two types of light having two different wavelengths or electromagnetic waves ω3 and ω4 by amplifying the nonlinear effect of the photonic crystal. Can do. Usually, this type of experiment is often performed at a low temperature in order to suppress the influence of light scattering by phonons. However, since phonons do not exist uniquely in photonic crystals, the enhancement effect is noticeable even at room temperature. There are features.
[0040]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the generation method and generator of light or electromagnetic waves which can generate | occur | produce efficiently electromagnetic waves, such as terahertz light, can be obtained.
[Brief description of the drawings]
FIGS. 1A and 1B are diagrams showing examples of photonic crystals, respectively.
FIG. 2 is a diagram showing an example of a band structure of a photonic crystal.
FIG. 3 is a diagram for explaining a conservation law of a wave vector in a nonlinear photonic crystal.
FIG. 4 is a diagram showing an embodiment of a light or electromagnetic wave generator according to the present invention.
FIG. 5 is a diagram showing another embodiment of the light or electromagnetic wave generator according to the present invention.
FIG. 6 is a diagram illustrating an example of a state in which two InP layers are stacked.
[Explanation of symbols]
41 Tunable lasers 42 to 47 Reflecting member 48 Photonic crystal

Claims (7)

By injecting two kinds of light or electromagnetic waves from different directions into a photonic crystal having a nonlinear optical effect having a second-order nonlinear susceptibility and a periodic permittivity distribution, through the nonlinear susceptibility due to the nonlinear optical effect, The incident light or electromagnetic wave causes coupling to each other, and the energy of the two types of combined light or electromagnetic wave is decomposed into two types of light or electromagnetic wave energy different from the incident two types of light or electromagnetic wave energy. Two types of light or electromagnetic waves different from the incident light or electromagnetic waves are generated, and two types of light or one of the two types of decomposed light or electromagnetic waves incident on the photonic crystal or A method for generating light or electromagnetic waves, characterized by having an energy substantially equal to the second harmonic with respect to one of the energy of electromagnetic waves . Light or electromagnetic wave method occurrence of claim 1, wherein the light or electromagnetic radiation is incident is a laser beam. The method for generating light or electromagnetic waves according to claim 1 or 2, wherein one wavelength of the plurality of generated light or electromagnetic waves is in the range of 30 µm to 300 µm. By generating two kinds of light or electromagnetic waves in different directions inside a photonic crystal having a nonlinear optical effect having a second-order nonlinear susceptibility and a periodic permittivity distribution, the nonlinear optic effect causes the nonlinear susceptibility to pass through the nonlinear susceptibility. The generated light or electromagnetic wave is coupled to each other, and the energy of the two types of combined light or electromagnetic wave is decomposed into two types of light or electromagnetic wave energy different from the generated two types of light or electromagnetic wave energy. Two types of light or electromagnetic waves that are different from the generated light or electromagnetic waves are generated, and one of the two types of decomposed light or electromagnetic waves is generated inside the photonic crystal. light or generating side of the electromagnetic waves, characterized in that it has to have an energy approximately equal to the second harmonic wave with respect to one of the optical or electromagnetic energy . A light or electromagnetic wave generator for use in the light or electromagnetic wave generation method according to any one of claims 1 to 3, wherein the photo has a nonlinear optical effect having a second-order nonlinear susceptibility and a periodic dielectric constant distribution. A light or electromagnetic wave generating device comprising: a nick crystal; and a light or electromagnetic wave incident device that makes two types of light or electromagnetic waves incident on the photonic crystal from different directions. 6. The light or electromagnetic wave generation device according to claim 5 , wherein the light or electromagnetic wave incident device includes a laser device that emits laser light and a branching member that branches the laser light and transmits it to the photonic crystal. apparatus. 7. The light or electromagnetic wave generator according to claim 6, wherein the laser device is a wavelength tunable laser.

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