JPH0552740A - Nonlinear optical constant evaluating method - Google Patents

Abstract

PURPOSE:To evaluate nonlinear optical constants by totally reflecting a fundamental wave with a prism, arranging a powder sample, using the evanescent wave caused by the total reflection, generating the high-order high frequency, and measuring the generating efficiency. CONSTITUTION:The output light of a laser 11 as a fundamental wave undergoes linear polarization. The light is cast into a total reflection prism 16 by using a rectangular prism 15. When the incident angle is made to be 80 degrees based on the refractive index of a sample 23, the intruding length becomes about 0.1mum. The length can be made sufficiently smaller in comparison with the average particle size and the coherence length. Then, the generated second high frequency is condensed 20. The fundamental wave is cut 17. The light is received with a photomultiplier tube 18 and integrated 19. A part of the fundamental wave is split 14 in order to correct the fluctuation of the intensity of the fundamental wave and cast into the quartz of a reference sample 21. The second high frequency is monitored and made to be the reference signal. The signal from the sample is divided by the reference signal. The powder sample 23, which is loaded in the central part of a sample stage 22, is obtained by grinding down the powder crystal of 2-methyl-4-nitroaniline, and the average particle size is made to be 10mum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention evaluates a nonlinear optical material used for optical communication, optical computer, optical recording, optical measurement, etc. by performing wavelength conversion of light, and evaluation of nonlinear optical constants for searching. Regarding the method.

[0002]

2. Description of the Related Art Conventional methods for evaluating second-order or third-order non-linear optical constants include, for example, Shinsuke Umegaki, Organic Non-linear Optical Materials (published by Bushin Publishing Co., January 31, 1990), pages 60 to 69. As described in, the powder method of irradiating a polycrystalline powder sample with a fundamental wave laser beam and detecting the generated harmonics, and the method of irradiating a single crystal sample with a fundamental wave laser beam and rotating it etc. There is a maker fringe method in which the harmonic intensity is measured by changing the typical crystal length.

[0003]

Conventionally, a powder method, which is easy to prepare a sample, has often been used. However, in the powder method, the relative size of the average crystal grain size with respect to the coherence length of the sample causes In some cases, correct evaluation could not be performed because the harmonic generation efficiency changed. On the other hand, the maker fringe method does not have such ambiguity and enables a correct evaluation, but requires the sample to be made into a relatively large single crystal. When searching for a material with a large nonlinear optical constant from many materials, making a single crystal sample is
It takes a lot of time and effort. Therefore, an evaluation method that can obtain a correct result using a powder sample has been desired. An object of the present invention is to provide a simple evaluation method in which the result is not easily affected by the relative size of the average particle size to the coherence length even when using a powder sample.

[0004]

By using an evanescent wave due to total internal reflection as the fundamental wave, it is possible to use a powder sample and perform evaluation independent of the particle size. Since the intensity of the evanescent wave is low, high-sensitivity measurement is required to detect high-order harmonics, but it can be easily realized by using a photomultiplier tube and combining it with a photon counting method. further,
If a metal layer is provided on the total reflection surface, high-order harmonic intensity can be increased by exciting surface plasmons,
It is possible to facilitate detection.

[0005]

The harmonic intensity I generated from the substance having the nonlinear optical constant d is given by the equation 1.

[0006]

[Equation 1]

Here, Lc is a coherence length defined by the equation 2, L is an interaction length, and in the case of the conventional powder method and maker fringe method, it corresponds to the grain size or crystal length.

[0008]

[Equation 2]

The harmonic intensity I changes with the square of the sine function depending on the magnitude of the interaction length L with respect to the coherence length Lc. Therefore, in the powder method, the particle size dependency of I becomes a problem, and correct evaluation cannot be performed unless the particle size is properly controlled. Therefore, this problem can be solved by using an evanescent wave due to total reflection as the fundamental wave. When light having a wavelength λ is totally reflected by a prism having a refractive index n1 and enters a medium having a refractive index n2, the distance ξ at which the evanescent wave penetrates into the medium is given by the above-described equation 2 when the incident angle is θ. In the present case, this is the interaction length L for generating higher harmonics. Here, the incident angle θ means an angle formed by the optical axis and the normal line of the total reflection surface of the prism. If the distance ξ is sufficiently smaller than the particle size of the sample or the coherence length Lc, the harmonic intensity I does not depend on the particle size. FIG. 6 shows the relationship between the relative magnitude of the distance ξ and the coherence length Lc and the second harmonic intensity. From this, the distance ξ is 1 of the coherence length Lc of the sample.
It is desirable to be / 10 or less. Also, the error is 20%
If it is allowed up to 1/3, it may be about 1/3. actually,
Even if the coherence length Lc is difficult to achieve phase matching,
Since it is often about 1 μm or more, ξ may be set to about 0.1 to 0.3 μm.

In the above description, the density of the sample was not taken into consideration. When performing measurements on various samples, the volume into which the fundamental wave penetrates, that is, the effective volume for generating higher harmonics, changes depending on the particle size of the sample. Therefore, the signal strength also depends on the particle size. Therefore, it is effective to make the particle diameter uniform by using a filter or the like so that the effective volume does not depend on the sample. As a result, even if powder is used, it is possible to evaluate the efficiency of high-order harmonic generation or the nonlinear optical constant d, regardless of the coherence length Lc of the sample. Equation 2 is for the s-wave, but it is essentially the same for the p-wave.

If a metal layer is provided on the total reflection surface, the intensity of the evanescent wave can be increased by exciting the surface plasmons, and the intensity of higher harmonics can be increased. This makes it possible to easily detect the harmonic intensity I.

Since the sample is a powder, if the particle size becomes small, scattering may occur at the total reflection surface of the prism, and harmonics due to the fundamental wave that is not totally reflected may occur. This is because the effective refractive index is not spatially uniform because air exists between the samples or in the gap between the samples and the prism. To prevent this, the gap between the sample and the gap between the sample and the prism should be filled with a liquid having a refractive index that is the same as or close to that of the sample. Since the liquid has the inversion symmetry and does not generate the second harmonic, there is no problem in evaluating the second-order nonlinear optical constant d. In the case of the third order, it is necessary to use a material having a nonlinear optical constant sufficiently smaller than that of the sample.

[0013]

【Example】

[Embodiment 1] An embodiment of the present invention will be described with reference to FIG. As a fundamental wave, a Q-switched Nd: YAG laser 11
Output light (λ = 1.064 μm) is linearly polarized through the infrared transmission filter 12 and the polarizer 13, and is incident on the total reflection prism 16 using the rectangular prism 15. As the total reflection prism 16, a rutile processed into a semi-cylindrical shape (cylindrical prism) as shown in FIG. 2 was used. Since the refractive index of rutile is 2.44, the critical angle for MNA (2-methyl-4-nitroaniline) is 46.8 degrees. MNA
Has a refractive index of 1.78. Therefore, the incident angle θ needs to be larger than this. As a sample, MNA (2-methyl-
Grind powder crystals of 4-nitroaniline in a mortar,
A sieve having an average particle size of 10 μm was used.
The coherence length of MNA is about 0.7 μm when the nonlinear optical constant d is 11. FIG. 3 shows the relationship between the incident angle and the penetration length of the evanescent wave when the rutile prism is used. This is because n1 = 2.44 and n2 =
It is calculated as 1.78. In reality, since the sample is a powder, the air in the gap affects it, so that the refractive index is different. However, if the angle with which the refractive index of the sample can be used is used, the condition of total reflection is always satisfied in the air portion, so the above condition is used here. Than this,
When the incident angle is 80 degrees, the penetration length, that is, L is about 0.
It becomes 1 μm, which can be made sufficiently smaller than the average particle size and the coherence length. Second harmonic generated (λ = 0.532μ
m) is condensed by the lens 20, and the infrared cut filter 17
The fundamental wave is cut by, received by the photomultiplier tube 18, and integrated by the boxcar integrator 19. In order to correct the intensity fluctuation of the fundamental wave, a part of the fundamental wave is branched by the beam splitter 14 and is incident on the quartz of the reference sample 21, and the second harmonic thereof is monitored and used as a reference signal. Is used to divide the signal from the sample by the reference signal. The infrared transmission filter 12 is a Q switch Nd: YA
This is for cutting the excitation light of the G laser. Figure 2
Figure 2 shows a detailed view of the total reflection prism and the sample. Here, the powder sample 2 loaded in the central portion of the prism 16 and the sample table 22
Although 3 is closely contacted with each other, a gap having a penetration length may be provided therebetween. However, in that case, the critical angle, penetration depth, etc. are different, so care must be taken.

[Embodiment 2] Another embodiment of the present invention will be described with reference to FIG. An overall structure similar to that of Example 1 was used in which the silver vapor deposition film 31 (vacuum vapor deposition) was provided on the plane portion of the prism 16 as shown in FIG. The film thickness was about 500Å. When the incident angle is changed while observing the reflected light intensity, the reflected light intensity suddenly decreases at a certain angle. At this time,
Surface plasmons are excited. In this state, the second
Harmonic intensity is about 5 times. As a result, it becomes possible to perform measurement with higher sensitivity than when the metal layer is not provided. In this embodiment, silver is used as the metal layer, but gold or aluminum may be used, and surface plasmons can be excited even if the prism surface has a periodic structure such as a grating.

Here, the second-order nonlinear susceptibility due to the generation of the second harmonic is evaluated as an example.
It is also possible to measure harmonics to evaluate the third-order nonlinear susceptibility.

[Embodiment 3] Another embodiment of the present invention will be described with reference to FIG. The configuration of the measurement system is almost the same as in Fig. 1,
The direction of collecting the second harmonic is different. In the configuration of FIG.
Since the reflected light of the fundamental wave is directly incident on the photomultiplier tube,
Although it is convenient to observe the excitation of surface plasmons by using a prism as shown in Example 2, on the other hand, the infrared cut filter 17 must be combined with several interference filters and colored glass filters to obtain the fundamental wave. Cannot be completely removed. With the configuration of the third embodiment shown in FIG. 5, since the scattered light enters the photomultiplier tube, the fundamental wave can be easily removed. In the embodiment of the present invention, rutile was used as the total reflection prism, but since it has a larger refractive index than the sample and is transparent to the fundamental wave and higher harmonics, high refractive index glass or sapphire can also be used.

In Examples 1 to 3, the reproducibility of the measurement was improved by putting the refractive index matching oil (n = 1.8) and the powder sample in the sample stage. Refractive index 1.75
Similar results could be obtained by using the above.

[0018]

According to the present invention, the nonlinear susceptibility can be easily evaluated using a powdery sample without performing crystal growth, and the efficiency of material search and selection is improved.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a measurement system that is a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of a sample stage and a total reflection prism according to the first embodiment of the present invention.

FIG. 3 is a characteristic diagram showing a relationship between an incident angle at a rutile prism and a vacuum interface and a seeping length of an evanescent wave.

FIG. 4 is an explanatory diagram of a sample stage and a total reflection prism provided with a metal layer, which is a second embodiment of the present invention.

FIG. 5 is a configuration diagram of a measurement system that is a third embodiment of the present invention.

FIG. 6 Relative magnitude of penetration depth and coherence length and second
Characteristic diagram showing the relationship of harmonic intensity

[Explanation of symbols]

11 ... Q-switch Nd: YAG laser, 14 ... Beam splitter, 16 ... Total reflection prism, 18 ... Photomultiplier tube, 19 ... Boxcar integrator, 21 ... Reference sample, 22
… Sample stand, 23… Powder sample.

Front page continuation (72) Inventor, Akio Taniguchi, 2520, Akanuma, Hatoyama-cho, Hiki-gun, Saitama, Ltd., Hitachi Research Laboratories, Inc. Basic Research Center

Claims (3)

[Claims] 1. A fundamental wave is totally reflected by a prism, and a sample is brought into close contact with the prism or arranged with a minute gap, and an evanescent wave by total reflection is used to generate a high-order harmonic wave. A non-linear optical constant evaluation method characterized by measuring efficiency. 2. The nonlinear optical constant evaluation method according to claim 1, wherein a metal thin film layer is provided on the reflecting surface of the prism. 3. The nonlinear optical constant evaluation method according to claim 1, wherein the gap between the sample and the gap between the sample and the prism or the metal thin film layer is the same as that of the sample,
A method for evaluating a nonlinear optical constant, characterized by filling with a liquid having a refractive index of a close value.