Electro-optical characteristic evaluation method for nonlinear optical substance and device thereof

Abstract

To provide a method for locally quantitatively evaluating electro-optical characteristics of a nonlinear optical substance using an electric field modulation imaging method, and a device therefor.SOLUTION: An electro-optical characteristic evaluation method for a nonlinear optical substance guides transmitted light in a thickness direction of a sample to an imaging element to obtain a difference image of intensities before and after application of a modulated electric field, and then measures an electro-optic coefficient. The method includes a thickness measurement step that continuously changes phase difference with a variable wavelength plate to give a thickness, using an optical system including: a polarizer that guides light from a monochromatic light source to the sample as linearly polarized light; an analyzer that gives a phase change of the transmitted light from the sample as an intensity change; and a variable wavelength plate that controls the phase difference due to birefringence. A device for the electro-optical characteristic evaluation method also includes thickness measurement means that continuously changes the phase difference with the variable wavelength plate to give a thickness at a position of giving the difference image.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method and an apparatus for locally and quantitatively evaluating the electro-optical properties of a nonlinear optical material using an electric field modulation imaging method. [Background technology]

[0002] When an electric field is applied to a ferroelectric material in the opposite direction to its polarization, a polarization domain with the same direction as the electric field appears inside the ferroelectric material above a certain threshold. Methods for visualizing and observing these polarization domains have been proposed using various microscopes, such as a scanning electron microscope (SEM), a piezoelectric response microscope (PFM), a second harmonic generation (SHG) microscope, and a terahertz emission microscope. Here, optical microelectro-field modulation spectroscopy aims to visualize the polarization domains by observing the slight optical changes in the optical transmittance / reflectance caused by applying an electric field to a ferroelectric material using an optical microscope.

[0003] For example, Patent Document 1 discloses a method (electric field modulation imaging method) in which positive and negative electric fields of a magnitude that does not invert the polarization domains in a ferroelectric thin film are alternately formed with a period T, and in synchronization with this, the ferroelectric thin film is irradiated with monochromatic light, and the transmitted or reflected light is optically imaged using a two-dimensional photodetector. Contrast is provided by obtaining an integrated image from multiple difference images of a pair of images captured in a positive electric field and a negative electric field, and noise in the difference images can be removed by shifting the period T to the shorter time side. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2018-036088 A Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, in order to quantitatively evaluate the electro-optical properties of a material that produces an electro-optical effect, such as a ferroelectric, it is necessary to know the optical path length for the physical property values ​​measured optically. In a thin film or bulk crystal sample with a uniform thickness, such as the thin film sample in Patent Document 1 mentioned above, the electro-optical properties can be quantitatively evaluated using the thickness for the physical property values ​​measured in the thickness direction. On the other hand, in a bulk crystal having a thickness distribution, it is necessary to measure the electro-optical properties locally and also to measure the thickness of the bulk crystal at the measurement point.

[0006] The present invention has been made in consideration of the above-mentioned circumstances, and has an object to provide a method and an apparatus for locally quantitatively evaluating the electro-optical properties of a nonlinear optical material using an electric field modulation imaging method. [Means for solving the problem]

[0007] The electro-optical characteristic evaluation method according to the present invention is an electro-optical characteristic evaluation method in which transmitted light passing through a sample made of a nonlinear optical material in the thickness direction is guided to an imaging element to obtain a difference image of the intensity before and after application of a modulated electric field to the sample, and then the electro-optical coefficient is measured, and is characterized in that it includes a thickness measurement step using an optical system including a polarizer provided in an optical path between a light source and the sample so as to guide light from the monochromatic light source to the sample as linearly polarized light, an analyzer provided in an optical path between the sample and the imaging element so as to provide the phase change of the transmitted light from the sample as an intensity change, and a variable wavelength plate provided in the optical path between the sample and the analyzer for controlling the phase difference due to birefringence, and which continuously changes the phase difference with the variable wavelength plate to provide the thickness at a position to provide the difference image.

[0008] According to this feature, the thickness of the sample can be easily measured by utilizing the birefringence change in the electro-optic imaging method, and the electro-optic coefficient in the difference image can be locally evaluated.

[0009] In the above-mentioned invention, the thickness measuring step may be characterized in that the phase difference is continuously changed between 0 and 2π by the variable wavelength plate to measure maximum and minimum values ​​of the intensity of the transmitted light, and the thickness is given by determining the intensity of the transmitted light that makes the phase difference 0. According to this feature, the thickness in the difference image can be easily and accurately measured, and the electro-optic coefficient can be locally evaluated.

[0010] In the above-mentioned invention, the polarizer may be arranged so as to give a polarization direction inclined at 45 degrees from the principal axis of the refractive index ellipsoid of the sample in a plane normal to the optical axis of the transmitted light passing through the sample, and the analyzer may be arranged so as to give a polarization direction parallel or perpendicular to the polarizer. With this feature, the thickness in the difference image can be accurately measured, and the electro-optic coefficient can be locally evaluated.

[0011] In the above-mentioned invention, the light source may be characterized in that it emits incoherent light and has a half-width of a finite wavelength. With this feature, a difference image can be obtained well and the electro-optic coefficient can be locally evaluated.

[0012] In the above-mentioned invention, the difference image may be obtained by applying a rectangular pulse voltage to the sample to alternately form a binary electric field and capturing an image with the imaging element in synchronization with the application of the voltage, and by integrating a plurality of images of a set of images captured with each of the binary electric fields, and the electro-optic coefficient is obtained from the contrast in the integrated image and the thickness provided in the thickness measurement step. According to this feature, the thickness in the difference image can be easily and accurately measured, and the electro-optic coefficient can be locally evaluated.

[0013] In addition, an electro-optical characteristic evaluation device according to the present invention is an electro-optical characteristic evaluation device that guides transmitted light passing through a sample made of a nonlinear optical material in the thickness direction to an imaging element, obtains a difference image of the intensity before and after application of a modulated electric field to the sample, and measures the electro-optical coefficient, and is characterized in that the device includes an optical system including a polarizer provided in an optical path between a light source and the sample so as to guide light from the monochromatic light source to the sample as linearly polarized light, an analyzer provided in the optical path between the sample and the imaging element so as to provide the phase change of the transmitted light from the sample as an intensity change, and a variable wavelength plate provided in the optical path between the sample and the analyzer for controlling the phase difference due to birefringence, and further includes a thickness measurement means that provides the thickness at a position where the difference image is provided by continuously changing the phase difference with the variable wavelength plate.

[0014] According to this feature, the thickness of the sample can be easily measured by utilizing the birefringence change in electro-modulation imaging, and can provide a local estimate of the electro-optic coefficient in the difference image.

[0015] In the above-mentioned invention, the thickness measurement means may be characterized in that it measures maximum and minimum values ​​of the intensity of the transmitted light by continuously changing the phase difference between 0 and 2π using the variable wavelength plate, and determines the intensity of the transmitted light that makes the phase difference 0 to give the thickness. According to this feature, it is possible to provide a simple and accurate measurement of the thickness in the difference image, and to provide a local evaluation of the electro-optic coefficient.

[0016] In the above-mentioned invention, the polarizer may be arranged to give a polarization direction of the light transmitted through the sample in a plane normal to the optical axis of the light, the polarization direction of the light being inclined at 45 degrees from the principal axis of the refractive index ellipsoid of the sample, and the analyzer may be arranged to give a polarization direction parallel or perpendicular to the polarizer. This feature allows for an accurate measurement of thickness in the difference image, thereby allowing for a local evaluation of the electro-optic coefficient.

[0017] In the above-mentioned invention, the light source may be characterized in that it emits incoherent light and has a finite half-width wavelength, which allows a good differential image to be obtained and provides a local evaluation of the electro-optic coefficient.

[0018] The above-mentioned invention may further include an imaging means for applying a rectangular pulse voltage to the sample to alternately form a binary electric field and capturing an image with the imaging element in synchronization with the application of the pulse voltage, and acquiring the difference image by an integrated image of a set of images captured with each of the binary electric fields, and a processing means for providing the electro-optic coefficient from the contrast in the integrated image and the thickness provided in the thickness measurement step. According to this feature, it is possible to provide a simple and accurate measurement of the thickness in the difference image, and to provide a local evaluation of the electro-optic coefficient. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a block diagram of an electro-optical characteristic evaluation apparatus in one embodiment according to the present invention. [Diagram 2] 13 is a graph showing the intensity of transmitted light when the phase difference of the transmitted light is continuously changed by a variable wavelength plate. [Diagram 3] 1 is a graph showing deviations of electro-optic coefficients from literature values ​​with respect to distribution Δφ of the phase of transmitted light. [Figure 4] These are transmission and difference images in a crossed Nicol configuration using LiTaO3 as a sample. [Diagram 5] 1 is a graph showing transmitted light intensity and differential intensity versus phase φ of transmitted light. [Figure 6] 1 is a graph plotting measured values ​​of electro-optic coefficients versus wavelength λ of transmitted light, and literature values ​​for comparison. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] First, an electro-optical characteristic evaluation apparatus according to one embodiment of the present invention will be described.

[0021] As shown in FIG. 1, an electro-optical property evaluation apparatus 10 includes a sample stage 2 on which a sample 1 made of a nonlinear optical material that produces an electro-optical effect, such as a ferroelectric, is placed, a light source 3 that irradiates light onto the sample 1 from below, and a light receiving unit 5 that receives the transmitted light that has passed through the sample 1 via an objective lens 4.

[0022] The light receiving unit 5 is capable of measuring the transmitted light intensity of the entire surface of the sample in a lump and spatially divided manner. Therefore, the light receiving unit 5 uses a two-dimensional detector in which image pickup elements are arranged on a plane, and acquires the transmitted light intensity distribution as two-dimensional data divided for each pixel. For example, a CMOS camera can be suitably used as the light receiving unit 5. As described above, in this embodiment, since the imaging method is used instead of a single-point measurement, an LED or a lamp capable of irradiating incoherent light so as not to cause speckle scattering is used as the light source 3. However, as described later, a bandpass filter is also used in order to obtain monochromatic light having a certain level of wavelength purity.

[0023] The electro-optical characteristic evaluation apparatus 10 also includes, as optical systems on the optical path from the light source 3 to the light receiving unit 5, a polarizer 11 between the light source 3 and the sample 1, an analyzer 12 between the sample 1 and the light receiving unit 5, and a variable wavelength plate 13 between the sample 1 and the analyzer 12. Here, the variable wavelength plate 13 is disposed closer to the sample 1 than the objective lens 4.

[0024] In these optical systems, the polarizer 11 is for converting the incident light from the light source 3 into linearly polarized light. The polarizer 11 is arranged so that the polarization direction is inclined at 45 degrees between the principal axes of the index ellipsoid of the sample 1 in a plane normal to the optical axis of the transmitted light. That is, the polarizer 11 is arranged so that ordinary light and extraordinary light, which are polarized in two directions with different refractive indices, are contained in equal proportions. The analyzer 12 is a polarizing plate used to obtain the phase change of the transmitted light due to birefringence as an intensity change. In order to improve the measurement accuracy of the electro-optical coefficient by the electro-optical characteristic evaluation device 10, the polarizer 11 and the analyzer 12 can be measured in both a parallel arrangement in which the polarization axes are parallel to each other, and a crossed Nicol arrangement in which the polarization axes are perpendicular to each other.

[0025] On the other hand, the variable wavelength plate 13 is made of, for example, a liquid crystal material having birefringence, and the birefringence can be controlled arbitrarily from the outside. That is, the variable wavelength plate 13 can control the phase difference caused by the birefringence.

[0026] The electro-optical property evaluation device 10 further includes a calculation unit 6 such as a personal computer (PC) that captures images from the light receiving unit 5 and processes the images, a voltage amplifier 7 that applies an electric field to the sample 1, and a function generator 8 for controlling the electric field. The function generator 8 is connected to the light receiving unit 5 and the light source 3, and can synchronize the timing of imaging by the light receiving unit 5 and the timing of light emission by the light source 3 with the timing of applying the electric field to the sample 1. For example, it is also preferable to apply a rectangular pulse voltage that switches between 0 and positive or negative, or between positive and negative potentials, to the sample 1 by the voltage amplifier 7, thereby forming an electric field with two values ​​alternately.

[0027] A method for evaluating the sample 1 using the electro-optical characteristic evaluation device 10 will be described. When measuring the electro-optical coefficient of a nonlinear optical material that produces an electro-optical effect, including a ferroelectric material, the thickness of the sample 1 at the measurement point is also measured. First, a method for measuring the thickness will be described.

[0028] In the zero electric field state before the electric field is applied, the ordinary and extraordinary light passing through sample 1 have different speeds due to the birefringence of the material, so a phase difference occurs between them. As a result, the light passing through sample 1 becomes elliptically polarized instead of linearly polarized. The phase difference at this time is determined by the anisotropy of the refractive index n aniso = n (extraordinary light) - n (ordinary light) and the thickness d of the sample 1. The elliptically polarized transmitted light is passed through an analyzer 12, and the change in the polarization plane is observed as a change in the intensity of the transmitted light. In this embodiment, this birefringence is used to measure the local thickness of the sample 1.

[0029] Therefore, in the electro-optical characteristic evaluation device 10, the variable wave plate 13 is disposed between the sample 1 and the objective lens 4 as described above. As described above, the birefringence of the variable wave plate 13 can be controlled arbitrarily from the outside. That is, the light transmitted through the sample 1 is subjected to birefringence from the variable wave plate 12 in addition to birefringence from the sample 1. Therefore, the phase difference of the transmitted light at the light receiving unit 5 is φ=φ 0 +φ ext where φ 0 is the phase difference due to the birefringence of sample 1, and φ 0 =2πdn aniso / λ (Equation 1), where λ is the central wavelength of the light emitted from the light source 3. φ ext is the phase difference due to the birefringence of the variable wavelength plate 13, and 0<φ ext The intensity of the light transmitted through the analyzer 12 can be arbitrarily controlled within the range of cos 2 It shows a φ dependence according to (φ / 2).

[0030] That is, as shown in FIG. 2, the variable wavelength plate 13 produces a phase difference φ ext By continuously changing φ 0 For example, φ ext Obtain the maximum and minimum values ​​of the transmitted light intensity while changing φ ext By comparing the transmitted light intensity at φ = 0, the phase difference φ due to birefringence of sample 1 is 0 can be sought.

[0031] From that value, n aniso Using λ, the local thickness d is determined from the above (Equation 1). To be precise, there is an uncertainty of 2π, so λ / n aniso It is advisable to check the thickness with an optical microscope up to the order of 10 μm, and then use the above method to determine the thickness accuracy below that.

[0032] Next, the measurement of the electro-optic coefficient will be described.

[0033] After determining the local thickness, the local electro-optic coefficient is determined using the difference image of the transmitted light intensity before and after the application of the modulated electric field and a theoretical formula showing the change in the transmitted light intensity due to the application of the electric field. The order of the change rate is usually 10 -3 Therefore, it is preferable to perform imaging measurement with high sensitivity. In this embodiment, as in Patent Document 1, an electric field is repeatedly applied at a modulation frequency of 15 Hz to 1 kHz, and the noise is removed by accumulating and acquiring the difference image. In addition, in order to increase the accuracy of the measurement, the phase φ ext The phase difference images were obtained at five different phases by changing the polarizer 11 arrangement, and measurements were made in two arrangements, a parallel arrangement and a crossed Nicol arrangement, and the electro-optic coefficient was measured by fitting a theoretical equation to all of these results using the least squares method.

[0034] In the electro-optical property evaluation device 10, the electro-optical properties of the entire surface of the sample 1 can be measured in a lump and in any local area by spatially resolving them, by using a CMOS camera as the light receiving unit 5. Since the pixel interval of the imaging image is narrower than the diffraction limit, in principle, it is possible to measure the electro-optical coefficient in a small area of ​​the length scale of the diffraction limit. Therefore, even if the sample has a distribution in thickness, it can be measured if it can be considered to be uniform on the length scale of the diffraction limit. Furthermore, even in the case of a sample (multi-domain structure) in which the polarity direction is not controlled, the domain size is generally larger than 10 μm, so that it can be sufficiently considered to be a single domain structure on the length scale of the diffraction limit, and therefore measurement is possible. Naturally, by changing the wavelength of the LED used as the light source 3, it is also possible to measure the electro-optical coefficient in a wide wavelength range. Depending on the wavelength characteristics of the light receiving unit 5, it is also preferable to use a CMOS camera in the visible light range and an InGaAs camera in the infrared range. In the case of samples with low sensitivity (materials with a small electro-optic coefficient or a sample shape to which a large electric field cannot be applied), it is possible to improve sensitivity with almost no changes to the measurement system by attaching an imaging intensifier.

[0035] In this embodiment, a spatially resolvable imaging method is used for local evaluation of the electro-optic coefficient. As a light source for imaging, a laser light with high coherence used in the ellipsometry method is not suitable, and it is preferable to use a light source that emits incoherent light (e.g., an LED). LEDs generally have a lower wavelength purity than laser light, and output light with a finite wavelength width. In this embodiment, the measurement accuracy of the electro-optic coefficient when a light source with a finite wavelength width is used is organized by the index Δφ shown below, and the conditions required for measuring the local electro-optic coefficient by the imaging method for samples with various characteristics are successfully found individually.

[0036] In order to clarify the relationship between the evaluation accuracy of the electro-optic coefficient and the wavelength purity of the light source, the electro-optic coefficient r 33 When the wavelength width is finite, the phase of the transmitted light is φ+Δφ=2πdn aniso / (λ+Δλ), where Δφ is the phase distribution of the transmitted light, and Δλ is the effective wavelength width of the light irradiated from the light source 3. aniso is a material-specific value, and the thickness d depends on the shape of the sample, while Δλ is a quantity that can be controlled by using bandpass filters with different half-widths.

[0037] As shown in FIG. 3, the electro-optic coefficient r of a sample made of a known material measured by an electro-optical property evaluation device 10 33exp and the literature value of the electro-optic coefficient r of the same material 33 The horizontal axis is the distribution of the phase of the transmitted light Δφ, and the vertical axis is the deviation of the electro-optic coefficient Δr 33 =r 33exp -r 33 It was confirmed that the electro-optic coefficient is underestimated compared to the literature value when Δφ is large. In other words, the low wavelength purity increases the phase uncertainty, and the evaluation accuracy is low. In other words, the n anisoFor d and the measurement wavelength λ, there is an upper limit for Δλ required for accurate evaluation of the electro-optic coefficient. Therefore, by utilizing the relationship in the figure and selecting an appropriate bandpass filter (wavelength purity) for each sample, it may be possible to accurately measure the electro-optic coefficient for samples of a wide variety of shapes.

[0038] [Measurement test] Using the electro-optical property evaluation device 10, a known electro-optical material, LiTaO 3 The electro-optic coefficients were measured for LiTaO and mNA (meta-nitroaniline). The central wavelength of the light emitted by the LED used as the light source was 100 nm. 3 For the , λ was set to 800 nm, and for the mNA, λ was set to 671 nm, 561 nm. A CMOS camera with sensitivity from 300 to 1100 nm, a maximum frame rate of 100 fps, a data bit of 16 bits, and a pixel count of 4 million pixels was used as the two-dimensional detector, which is the light receiving unit 5. The measurement system also included a function generator 8 for applying voltage, a voltage amplifier 7, a high-speed image calculation device that integrates the image difference, and a PC 6 for control.

[0039] The average thickness of the sample, previously estimated by optical microscopy, was LiTaO 3 The diameter was 270 μm and the mNA was 40 μm. Gold wires were attached to the sides of the sample 1 with silver paste as terminals for applying voltage. The voltage was applied parallel to the anisotropic axis of the sample. The sample 1 was arranged on the sample stage 2 so that the anisotropic axis was tilted 45 degrees with respect to the polarization axis of the polarizer 11. A variable wavelength plate 13 for phase compensation of the transmitted light was inserted between the sample 1 and the objective lens 4, and the phase delay axis of the variable wavelength plate 13 was parallel to the anisotropic axis of the sample. An analyzer 12 was arranged between the objective lens 4 and the light receiving unit 5, and the polarization axis of the analyzer was measured in two arrangements: parallel arrangement with the polarization axis of the polarizer 11 and crossed Nicol arrangement. Voltages of +100V and -100V were repeatedly applied to the sample 1 as modulation voltages with a modulation frequency of 45 Hz. Note that when such a voltage is applied, a value twice the difference between 0V and +100V is obtained, thereby increasing the sensitivity. The total number of shots was 32,768, and the shooting time was approximately six minutes.

[0040] The above method was used to 3 The electro-optic coefficients of the NA and mNA were measured. To perform accurate measurements, the bandpass filter was LiTaO 3 The half-width was set to 10 nm for the measurement of mNA (λ = 800 nm) and 2.6 nm for the measurement of mNA (λ = 671 nm, λ = 561 nm). 3 For the measurements, shorter wavelengths (λ = 730 nm, 636 nm, 550 nm) were also used for the same bandpass filter with a half-width of 10 nm. Furthermore, for the measurements of mNA, wider (half-width 10 nm) bandpass filters for λ = 636 nm and 550 nm were also used.

[0041] Figure 4 shows the LiTaO 3 (λ = 800 nm, bandpass filter 10 nm) is shown. ⊥ ), we can see that the brightness changes depending on the phase of the transmitted light φ. Following this, the difference image (ΔT ⊥ / T ⊥ ) have switched signs.

[0042] Figure 5 shows a graph in which the transmitted light intensity and differential intensity in a local area of ​​the sample are plotted against φ. LiTaO 3 The average value within a range of 100 pixels × 100 pixels is used for mNA, and 10 pixels × 10 pixels for mNA. The dashed line is a fitting curve using an expression for the change in transmitted light intensity due to the change in refractive index, and it was confirmed that it matches the experimental value very well.

[0043] As shown in Fig. 6(a), the electro-optic coefficient was measured using this fitting, and the results showed that LiTaO 3 For the thickness d = 270 μm, the refractive index anisotropy n aniso When the half-width of the bandpass filter is 10 nm, the electro-optic coefficient r33 = 28 pm / V (λ = 800 nm). This is the literature value r 33 = 30.3 pm / V (λ = 980 nm), which is comparable to the value of the electro-optic coefficient. On the other hand, it was also confirmed that the electro-optic coefficient is underestimated when measured at short wavelengths (when Δφ is large).

[0044] As shown in FIG. 1(b), the same measurement was performed on mNA, and the thickness was d=40 μm, n aniso = -0.050 (λ = 671 nm), -0.056 (λ = 561 nm), by using a bandpass filter with a half-width of 2.6 nm, r 33 = 16.0 pm / V (λ = 671 nm) and 16.6 pm / V (λ = 561 nm). The literature value of the electro-optic coefficient of mNA is r 33 = 16.7 pm / V (λ = 632.8 nm), which also shows good agreement. On the other hand, it was also confirmed that the electro-optic coefficient was underestimated when a bandpass filter with a wide half-width was used (condition where Δφ is large).

[0045] These results demonstrate that it is possible to measure the electro-optic coefficient locally with high precision by using an appropriate bandpass filter according to the characteristics of the sample to be measured. In this way, it is now possible to measure the electro-optic coefficient of a diverse group of substances, including samples with thickness distribution and materials whose polarity cannot be controlled, which were previously difficult to measure. This will promote the search for new electro-optic materials, and is believed to be a technology that will dramatically expand the scope of electro-optic materials.

[0046] Although the embodiments of the present invention and modifications based thereon have been described above, the present invention is not necessarily limited thereto, and a person skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims. [Explanation of symbols]

[0047] 1. Sample 3 light source 4 Objective Lenses 5 Light receiving section 7 Voltage Amplifier 10. Electro-optical property evaluation equipment 11 Polarizer 12 Analyzer 13 Variable Wave Plate

Claims

1. A method for evaluating electro-optical characteristics, comprising: guiding transmitted light passing through a sample made of a nonlinear optical material in a thickness direction to an imaging element; obtaining an intensity difference image before and after application of a modulated electric field to the sample; and measuring an electro-optic coefficient, a polarizer provided in an optical path between a monochromatic light source and the sample to direct light from the light source as linearly polarized light to the sample; an analyzer provided in an optical path between the sample and the image sensor to provide a phase change of the transmitted light from the sample as an intensity change; an optical system including a variable wavelength plate that is located in an optical path between the sample and the analyzer and controls a phase difference due to birefringence; A method for evaluating the electro-optical characteristics of a nonlinear optical material, comprising a thickness measurement step of continuously varying the phase difference with the variable wavelength plate to obtain the thickness at a position that provides the differential image.

2. 2. The method for evaluating the electro-optical characteristics of a nonlinear optical material according to claim 1, wherein the thickness measurement step comprises measuring the maximum and minimum values ​​of the intensity of the transmitted light by continuously changing the phase difference between 0 and 2π using the variable wavelength plate, and determining the intensity of the transmitted light for which the phase difference is 0 to provide the thickness.

3. 3. A method for evaluating the electro-optical properties of nonlinear optical materials as described in claim 2, characterized in that the polarizer is positioned so as to give a polarization direction inclined 45 degrees from the principal axis of the refractive index ellipsoid of the sample within a plane normal to the optical axis of the transmitted light passing through the sample, and the analyzer is positioned so as to give a polarization direction parallel or perpendicular to that of the polarizer.

4. 4. A method for evaluating the electro-optical characteristics of a nonlinear optical material according to claim 1, wherein the light source emits incoherent light and has a half-width of a finite wavelength.

5. The method for evaluating the electro-optical properties of a nonlinear optical material described in claim 4, characterized in that the differential image is obtained by applying a rectangular pulse voltage to the sample to alternately form binary electric fields and capturing images with the imaging element in synchronization with the formation of the binary electric fields, and the electro-optic coefficient is obtained from the contrast in the accumulated image and the thickness provided in the thickness measurement process.

6. An electro-optical characteristic evaluation device for measuring an electro-optical coefficient by guiding transmitted light passing through a sample made of a nonlinear optical material in a thickness direction to an imaging element to obtain an intensity difference image before and after application of a modulated electric field to the sample, comprising: a polarizer provided in an optical path between a monochromatic light source and the sample to direct light from the light source as linearly polarized light to the sample; an analyzer provided in an optical path between the sample and the image sensor to provide a phase change of the transmitted light from the sample as an intensity change; a variable wavelength plate that is located in an optical path between the sample and the analyzer and controls a phase difference due to birefringence, an electro-optical characteristic evaluation device for evaluating the electro-optical characteristics of a non-linear optical material, comprising a thickness measurement means for continuously varying the phase difference by the variable wavelength plate to provide the thickness at a position providing the differential image.

7. 7. The apparatus for evaluating the electro-optical properties of a nonlinear optical material according to claim 6, wherein the thickness measuring means measures the maximum and minimum values ​​of the intensity of the transmitted light by continuously changing the phase difference between 0 and 2π using the variable wavelength plate, and obtains the intensity of the transmitted light that makes the phase difference 0 to provide the thickness.

8. The electro-optical characteristic evaluation device for nonlinear optical materials described in claim 7, characterized in that the polarizer is arranged to give a polarization direction inclined 45 degrees from the principal axis of the refractive index ellipsoid of the sample within a plane normal to the optical axis of the transmitted light passing through the sample, and the analyzer is arranged to give a polarization direction parallel or perpendicular to that of the polarizer.

9. 9. An apparatus for evaluating the electro-optical characteristics of a nonlinear optical material according to claim 6, wherein said light source emits incoherent light and has a half-width of a finite wavelength.

10. The electro-optical characteristic evaluation device of claim 9, further comprising an imaging means for applying a rectangular pulse voltage to the sample to alternately form a binary electric field and capturing an image with the imaging element in synchronization therewith, and acquiring the differential image by integrating images from a set of images captured with each of the binary electric fields, and further comprising a processing means for obtaining the electro-optical coefficient from the contrast in the integrated image and the thickness provided in the thickness measurement process.

Images