JP4848496B2 - Asymmetric Polarizability Distribution Periodic Array Optical Second Harmonic Generator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical second harmonic generation device using an optical waveguide having an asymmetric polarizability distribution periodic array.
[0002]
[Prior art]
An optical second harmonic generation (SHG) device, for example, converts laser light having a half wavelength of incident laser light, such as converting infrared laser light into blue laser light or blue laser light into ultraviolet laser light. It is a device that outputs. Laser light with a wavelength that cannot be obtained directly from existing lasers can be obtained. For chemical reaction control that requires laser light with a specific wavelength, various film forming devices, or medical devices that use lasers, etc. It is an indispensable device.
Conventionally, optical second harmonics are generated in electro-optic crystals that exist in nature, in which the crystal structure has no inversion symmetry, such as LiNbO ₃ single crystal, KTP (KTiOPO ₄ ) single crystal, etc. Has been done.
However, in these single crystals, since the origin of optical nonlinearity is derived from the crystal structure, there is a problem that the target is extremely limited when searching for a higher performance material.
[0003]
In addition, in the optical second harmonic generation using these single crystals, the excitation light and the second harmonic propagate through the crystal due to the difference in phase velocity between the incident laser light, that is, the excitation light and the second harmonic. As a result, there is a problem that the phase is not matched and the second harmonic output is reduced. Therefore, in these apparatuses, means for matching the phases of the excitation light and the second harmonic, that is, phase matching means is indispensable. Examples of the phase matching means include those using the birefringence of the electro-optic crystal used, those using Cherenkov radiation, and quasi phase matching using domain inversion.
However, these phase matching means have a problem in that the types of single crystals that can be used are further limited, or a complicated apparatus configuration is required, and the characteristics are deteriorated due to optical damage of the crystals. .
[0004]
Under these circumstances, semiconductor quantum structures can be produced at will according to recent advances in crystal growth technology and semiconductor microfabrication technology. Along with this, research on the interaction between radiation fields and matter systems using photonic crystals and semiconductor microresonators has been actively conducted. The results of these studies not only contribute to the understanding of basic physics, but can also be applied to high-performance optical devices.
[0005]
[Problems to be solved by the invention]
In view of the above problems, the present invention provides an asymmetric polarizability distribution periodic array type optical second harmonic generator capable of generating a second harmonic without selecting a substance to be used and a laser wavelength to be used. For the purpose.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the first asymmetric polarizability distribution periodic array type optical second harmonic generator according to the present invention is an asymmetric structure in which a polarizability distribution asymmetric in the electric field direction of pumping light is periodically arrayed in an optical waveguide. An asymmetrical polarizability distributed periodic optical waveguide has an asymmetrical relief made of a transparent material having a first polarizability and an asymmetrical relief made of a transparent material having the first polarizability. An asymmetric polarizability distribution period comprising excitation light having a predetermined wavelength and a propagation mode of TM or TE , which is composed of a transparent material for confining light having a different polarizability from the first polarizability laminated on the top It is made to enter into an arrangement | sequence optical waveguide, and an optical 2nd harmonic is generated.
A second asymmetric polarizability distribution periodic array type optical second harmonic generator of the present invention includes an asymmetric polarizability distribution periodic array optical waveguide in which an asymmetric polarizability distribution in the electric field direction of excitation light is periodically arrayed in the optical waveguide. The asymmetric polarizability distribution periodic array optical waveguide is composed of a symmetric relief made of a transparent material having a first polarizability, and a transparent material having a second polarizability laminated asymmetrically on the symmetric relief, Excitation light having a predetermined wavelength and TM or TE propagation mode is incident on an asymmetric polarizability distribution periodic array optical waveguide to generate an optical second harmonic.
A third asymmetric polarizability distribution periodic array type optical second harmonic generator according to the present invention comprises an asymmetric polarizability distribution periodic array optical waveguide in which an asymmetric polarization distribution in the direction of the electric field of the excitation light is periodically arrayed in the optical waveguide. has an asymmetric polarization distribution periodic array optical waveguide, and asymmetric relief made of a transparent material having a first polarization, is composed of a transparent material having a second polarization that is laminated on the asymmetrical relief, predetermined Excitation light having a wavelength and TM or TE propagation mode is incident on an asymmetric polarizability distribution periodic array optical waveguide to generate an optical second harmonic.
In the above configuration, the surface of the asymmetric polarizability distribution periodic array optical waveguide is preferably covered with an optical confinement transparent material having a third polarizability.
With the above configuration, the second harmonic can be generated without selecting the substance to be used and without selecting the laser wavelength to be used. Since the surface of the asymmetric polarizability distribution periodic array optical waveguide is covered with a transparent material for optical confinement, the confinement of the excitation light and the second harmonic in the asymmetric polarizability distribution periodic array optical waveguide increases, and the second Increases the efficiency of harmonic generation.
[0007]
Further, the period Λ of the periodic array is characterized by being shorter than the coherence length of the excitation light.
With this configuration, electrons in the polarizability distribution that is asymmetric in the direction of the electric field of the excitation light can feel an asymmetric potential V (x: position coordinate), and thus generate a second harmonic component.
[0010]
In addition, the period Λ of the periodic array has a wavelength λ ₀ of the excitation wave in vacuum, a refractive index n ₁ in the optical waveguide of the excitation wave, a refractive index n ₂ in the optical waveguide of the second harmonic, and m as positive integers. ,
Λ = m · λ ₀ / (2n ₂ −n ₁ ).
With this configuration, the second harmonics generated in each asymmetric polarizability distribution are phase-matched and output with increased light intensity.
[0011]
Further, the surface of the asymmetric polarization distribution, characterized in that it cracks covered with a transparent material having a third polarization.
With this configuration, the confinement of the excitation light and the second harmonic into the asymmetric polarizability distribution periodic array optical waveguide is increased, and the efficiency of second harmonic generation is increased.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to FIGS. Note that substantially the same members will be described using the same reference numerals.
FIG. 1 is a schematic diagram showing the cross-sectional shape of an asymmetric polarizability distribution periodic array type optical second harmonic generator of the present invention used when the propagation mode of excitation light is TM.
In FIG. 1, the coordinate axis is defined as the x axis in the traveling direction of the excitation light 1, the direction perpendicular to the x axis and perpendicular to the cross section as the y axis, and the direction perpendicular to the x axis and parallel to the cross section as the z axis.
FIG. 1A is a schematic cross-sectional view of an asymmetric polarizability distribution periodic array optical waveguide formed of an object having an asymmetric shape. In FIG. 1A, an object having a shape that is not symmetrical in the electric field direction of the excitation light, that is, an asymmetric shape object 2 is periodically arranged with a length Λ in the x-axis direction and the same cross section in the y-axis direction. Appropriate length spreads in shape. On the surface of the asymmetrically shaped object 2, a transparent material 5 having a high refractive index for confining light having different polarizabilities is laminated.
The asymmetrically shaped object 2 has, for example, a right triangle cross-sectional shape, and the polarizability distribution of the unit periodic structure composed of the asymmetrically shaped object 2 and the transparent material 5 is the electric field direction of the TM mode excitation light x The section 4 perpendicular to the axis cannot be overlapped as a mirror surface, that is, it does not have symmetry in the electric field direction of the excitation light.
[0013]
The asymmetrically shaped object 2 is obtained by processing the surface of a substrate 3 which is an amorphous transparent material resistant to light damage, such as glass or polymer, into a sawtooth cross-sectional shape using, for example, a photolithography process and an etching process. Form.
[0014]
FIG. 1B is a schematic cross-sectional view of an asymmetric polarizability distribution periodic array optical waveguide configured by asymmetrically stacking objects having different polarizabilities. In FIG. 1B, an asymmetric laminated object 6 formed by asymmetrically laminating objects having different polarizabilities is an object having a shape having symmetry in the electric field direction of the excitation light 1, that is, a symmetrical object 7 and its upper surface. And a transparent material 8 laminated on the right side surface. The polarizabilities of the transparent material 8 and the symmetrical object 7 are different from each other. The asymmetric laminated body 6 is periodically arranged with a length Λ in the x-axis direction and has an appropriate length extending in the same cross-sectional shape in the y-axis direction. On the surface of the asymmetric laminated object 6, a transparent material 5 having a high refractive index for confinement of light having a different polarizability from the symmetrical object 7 and the transparent material 8 is laminated.
The polarizability distribution of the unit periodic structure composed of the asymmetric laminated body 6 and the transparent material 5 cannot be overlapped with the cross section 4 perpendicular to the x-axis, which is the electric field direction of the TM mode excitation light, as a mirror surface, There is no symmetry in the electric field direction of the excitation light.
The asymmetrical laminated body 6 is formed by processing the surface of the substrate 3 which is an amorphous transparent material resistant to light damage such as glass or polymer into a rectangular cross section using, for example, a photolithography process and an etching process. On this surface, for example, the transparent material 8 is formed by oblique deposition.
[0015]
FIG. 1C is a schematic cross-sectional view of an asymmetric polarizability periodic array optical waveguide configured by stacking an asymmetric object and objects having different polarizabilities on the asymmetric object.
In FIG. 1 (c), the asymmetrically shaped laminated object 9 includes an asymmetrically shaped object 2 and a transparent material 10 laminated on a part of the asymmetrically shaped object 2. The asymmetrical stacked body 9 is periodically arranged with a length Λ in the x-axis direction, and has an appropriate length extending in the same cross-sectional shape in the y-axis direction. On the surface of the asymmetrically shaped laminated body 9, a transparent material 5 having a high refractive index for confinement of light having a different polarizability from that of the asymmetrically shaped object 2 and the transparent material 10 is laminated.
The polarizability distribution of the unit periodic structure composed of the asymmetrically shaped laminated body 9 and the transparent material 5 cannot be overlapped with the cross section 4 perpendicular to the x-axis, which is the electric field direction of the TM mode excitation light, as a mirror surface. It does not have symmetry in the electric field direction of the excitation light.
The asymmetrically shaped laminated body 9 has a refractive index different from that of the asymmetrically shaped object 2 in a part of the serrated groove of the asymmetrically shaped object 2 manufactured by the same method as the asymmetric polarizability distribution periodic array optical waveguide shown in FIG. The transparent materials 10 having different sizes are embedded by, for example, spin coating.
[0016]
FIG. 2 is a schematic cross-sectional view of an asymmetric polarizability distribution periodic array optical waveguide in which an asymmetric polarizability distribution is formed by ion implantation.
FIG. 2A is a schematic cross-sectional view of an asymmetric polarizability distribution periodic array optical waveguide formed on the surface of the substrate 3 by controlling the ion implantation depth of atoms asymmetrically.
In FIG. 2A, the ion implantation layer 11 is formed by controlling atoms having different polarizabilities from those of the substrate 3 and the transparent material 5 while controlling the ion implantation depth asymmetrically. Have different polarizabilities. The transparent material 5 is laminated after the ion implantation layer 11 is formed.
The polarizability distribution of this configuration does not have symmetry in the electric field direction of the excitation light as in the case of FIG.
[0017]
FIG. 2B shows an asymmetric polarizability distribution periodic array optical waveguide formed by ion-implanting first and second atoms having different polarizabilities from the surface layer 12 and the transparent material 5 into the surface layer 12 of the substrate 3. FIG. The ion implantation layer 13 into which the first atoms are ion implanted and the ion implantation layer 14 into which the second atoms are ion implanted have different polarizabilities and lengths. After forming the ion implantation layers 13 and 14, the transparent material 5 is laminated.
The polarizability distribution of this configuration does not have symmetry in the electric field direction of the excitation light as in the case of FIG.
[0018]
FIG. 3 is a schematic diagram showing a cross-sectional shape of an asymmetric polarizability distribution periodic array type optical second harmonic generator of the present invention used when the propagation mode of excitation light is TE.
FIG. 3A is a schematic plan view of an asymmetric shape-type asymmetric polarizability distribution periodic array optical waveguide, and FIG. 3B is a schematic cross-sectional view thereof. In FIG. 3A, the asymmetrically shaped object 15 has a shape that gradually becomes thinner in the positive region of the y axis, for example, and a cross section 16 perpendicular to the y axis that is the electric field direction of the TE mode excitation light. Cannot be superimposed as a mirror surface, that is, they do not have symmetry in the electric field direction of the excitation light.
The asymmetrically shaped object 15 is made of a material having a different polarizability from the substrate 3 and the transparent material 5 with high refractive index for optical confinement, and is periodically arranged with a length Λ in the x axis direction.
Although not shown, a polarizability distribution having this configuration may be formed by ion implantation as shown in FIG.
[0019]
Next, the period Λ of the asymmetric polarizability distribution periodic array type optical second harmonic generator of the present invention will be described.
The period Λ of the arrangement of the present invention is formed to a length that matches the phase of the excitation light 1 and the second harmonic. That is, the period lambda, the wavelength lambda ₀ in the vacuum of the excitation light 1, the refractive index n ₁ of the optical waveguide of the excitation light 1, the refractive index n ₂ of the optical waveguide of the second harmonic, and m is a positive integer , Λ = m · λ ₀ / (2n ₂ −n ₁ ).
[0020]
Next, the operation of the asymmetric polarizability distribution periodic array optical second harmonic generator of the present invention will be described.
FIG. 4 is a diagram showing a polarization wave caused by electrons in a substance, that is, a waveform of scattered light.
FIG. 4A shows a waveform of scattered light in a substance having inversion symmetry, and FIG. 4B shows a waveform of scattered light in a substance having no inversion symmetry. . α and β represent the electric field strength of the excitation light and the electric field strength of the scattered light, respectively.
Scattered light p (t) by a substance is generated by displacement of valence electrons weakly bonded to the nucleus of the outer shell of the substance atom in accordance with the electric field of the excitation light, that is, polarization. When the displacement of the electrons from the parallel position is x, the electron density is N, the time is t, and the electron charge is e, the scattered light p (t) is
p (t) = − eNx (t) (1) Polarizability is defined as p (t) _max per unit electric field.
In a substance having inversion symmetry, the potential energy V (x) for this electron is V (x) = V (−x), so that A and B are constants.
V (x) = Ax ² + Bx ⁴ +... Only the even power term of x is included as shown in equation (2). The restoring force F acting on the electrons is
F = −dV / dx = −2Ax−4Bx ³ −... (D: differential symbol) (3), and the electron has a restoring force having the same magnitude in both the positive displacement x and the negative displacement x. Receive. Therefore, as shown in FIG. 4A, the scattered light p (t) has a waveform in which + displacement and-displacement are symmetrical.
[0021]
On the other hand, in a substance having no inversion symmetry, the condition of V (x) = V (−x) does not hold, V (x) includes an odd-numbered term, and C is a constant.
V (x) = Ax ² + Cx ³ + ·· (4) The restoring force F (x) acting on the electrons is ignored if the higher order terms of x are ignored.
F (x) = − dV / dx = − (2Ax + 3Cx ² ) (5) Therefore, the restoring force with respect to the + displacement x and the restoring force with respect to the − displacement x are different in magnitude, and the scattered light p (t) has a + displacement and a − displacement as shown in FIG. Has an asymmetric waveform.
[0022]
FIG. 5 shows the result of Fourier analysis of asymmetric scattered light.
FIG. 5A shows the asymmetric scattered light generated in the material having no inversion symmetry shown in FIG. 4B, and FIG. 5B shows the scattering having the same frequency component as the excitation light. FIG. 5C shows the second harmonic component of the scattered light having a frequency twice the frequency of the excitation light. Thus, the asymmetric scattered light has a second harmonic component.
As described above, the scattered light excited by the excitation light in the material having no inversion symmetry has an asymmetric waveform, and thus generates a second harmonic component.
[0023]
Electrons in the polarizability distribution that is asymmetric in the direction of the electric field of the excitation light have an asymmetric polarizability distribution shape if the width of the polarizability distribution, that is, the size of the polarizability distribution is smaller than the coherence length of the excitation light. An asymmetric potential V (x) reflecting the above can be felt.
That is, pump light 1 having a coherence length longer than the period Λ and a propagation mode of TM can be incident on the asymmetric polarizability distribution periodic array optical second harmonic generator shown in FIGS. For example, a second harmonic component is generated.
If the pump light 1 having a coherence length longer than the period Λ and the propagation mode TE is incident on the asymmetric polarizability distribution periodic array optical second harmonic generator shown in FIG. Generates a second harmonic component.
[0024]
Next, the action of the period Λ of the asymmetric polarizability distribution periodic array type optical second harmonic generator will be described. The array period Λ of the asymmetric polarizability distribution shown in FIGS. 1, 2, and 3 is formed so as to have a phase matching between the pumping light 1 and the second harmonic.
In order for the second harmonics generated by each asymmetric polarizability distribution to intensify each other and be extracted as a large output, when the excitation light 1 and the second harmonic propagate through adjacent periodic structures separated by a distance Λ The phase difference between the excitation light 1 and the second harmonic may be an integer multiple of 2π. That is, assuming that the propagation constant, the wavelength in the periodic structure, and the frequency of the excitation light 1 and the second harmonic are k ₁ , k ₂ , λ ₁ , λ ₂ , and ν ₁ , 2ν ₁ , respectively.
k ₂ Λ−k ₁ Λ = 2πm (m: positive integer) Equation (6)
1 / λ ₂ −1 / λ ₁ = m / Λ (7) Since λ ₁ = c / n ₁ ν ₁ and λ ₂ = c / n ₂ 2ν ₁ , the relationship of equation (7) is
n ₂ 2ν ₁ / c−n ₁ v ₁ / c = m / Λ (8) From this relationship, Λ is
If Λ = mc / ν ₁ (2n ₂ −n ₁ ) = mλ ₀ / (2n ₂ −n ₁ ) (9), the adjacent periodic structure in which the excitation light 1 and the second harmonic are separated by a distance Λ The phase difference between the pumping light 1 and the second harmonic when propagating becomes an integer multiple of 2π, and the second harmonics generated by the periodic structure intensify each other and can be extracted as a large output.
That is, the second harmonics generated in the asymmetric polarizability distribution periodic array optical waveguide shown in FIG. 1, FIG. 2, and FIG.
[0025]
【The invention's effect】
As can be understood from the above description, in the asymmetric polarizability distribution periodic array optical second harmonic generator of the present invention, the asymmetric polarizability distribution is periodically arrayed in the optical waveguide to generate the second harmonic, Since the phase matching between the excitation light and the second harmonic is performed according to the period of the periodic array, the second harmonic can be generated without selecting the wavelength of the substance and the incident laser light.
[Brief description of the drawings]
FIG. 1 is a schematic view showing a cross-sectional shape of an asymmetric polarizability distribution periodic array type optical second harmonic generator of the present invention used when the propagation mode of pumping light is TM.
FIG. 2 is a schematic cross-sectional view of an asymmetric polarizability distribution periodic array optical waveguide in which an asymmetric polarizability distribution is formed by ion implantation.
FIG. 3 is a schematic diagram showing a cross-sectional shape of an asymmetric polarizability distribution periodic array optical second harmonic generator of the present invention used when the propagation mode of pumping light is TE.
FIG. 4 is a diagram showing a waveform of scattered light by electrons in a substance.
FIG. 5 is a waveform diagram showing the result of Fourier analysis of asymmetric scattered light.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Excitation light 2 Asymmetrically shaped object 3 Substrate 4 Mirror surface 5 High refractive index transparent material 6 Asymmetric laminated object 7 Symmetric shaped object 8 Transparent material with different polarizability 9 Asymmetrically shaped laminated object 10 Transparent material 11 with different polarizability 11 Ion implantation layer 12 Surface layer 13 Ion implantation layer 14 Ion implantation layer Λ Period

Claims (6)

An asymmetric polarizability distribution periodic array optical waveguide in which an asymmetric polarizability distribution in the electric field direction of the excitation light is periodically arranged in the optical waveguide,
The asymmetric polarizability distribution periodic array optical waveguide includes an asymmetric relief made of a transparent material having a first polarizability, and a first polarizability laminated on the asymmetric relief made of a transparent material having the first polarizability. Composed of transparent materials for optical confinement having different polarizabilities,
An asymmetric polarizability characterized by causing the excitation light having a predetermined wavelength and a TM or TE propagation mode to enter the asymmetric polarizability distribution periodic array optical waveguide to generate an optical second harmonic. Distributed periodic array optical second harmonic generator. An asymmetric polarizability distribution periodic array optical waveguide in which an asymmetric polarizability distribution in the electric field direction of the excitation light is periodically arranged in the optical waveguide,
The asymmetric polarizability distribution periodic array optical waveguide is composed of a symmetrical relief made of a transparent material having a first polarizability and a transparent material having a second polarizability laminated asymmetrically on the symmetrical relief,
An asymmetric polarizability characterized by causing the excitation light having a predetermined wavelength and a TM or TE propagation mode to enter the asymmetric polarizability distribution periodic array optical waveguide to generate an optical second harmonic. Distributed periodic array optical second harmonic generator. An asymmetric polarizability distribution periodic array optical waveguide in which an asymmetric polarizability distribution in the electric field direction of the excitation light is periodically arranged in the optical waveguide,
The asymmetric polarizability distribution periodic array optical waveguide is composed of an asymmetric relief made of a transparent material having a first polarizability, and a transparent material having a second polarizability laminated on the asymmetric relief,
An asymmetric polarizability characterized by causing the excitation light having a predetermined wavelength and a TM or TE propagation mode to enter the asymmetric polarizability distribution periodic array optical waveguide to generate an optical second harmonic. Distributed periodic array optical second harmonic generator.   4. The asymmetric polarizability distribution periodic array optical second harmonic generation device according to claim 1, wherein a period of the periodic array is shorter than a coherence length of the excitation light. The period of the periodic array is such that the period is Λ, the wavelength of the excitation light in vacuum is λ ₀ , the refractive index of the excitation light in the asymmetric polarizability distribution periodic array optical waveguide is n ₁ , and the second harmonic The refractive index in the optical waveguide is n ₂ and m is a positive integer.
Λ = mλ _{0 /} wherein the (2n 2 _{-n 1)} is asymmetric polarizability distributed periodically arranged optical second harmonic generating device according to any one of claims 1 to 3.   4. The asymmetric polarizability distribution period according to claim 2, wherein a surface of the asymmetric polarizability periodic array optical waveguide is covered with a transparent material for confining light having a third polarizability. 5. Array type optical second harmonic generator.

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