JP2759115B2 - Measurement device for third-order nonlinear optical characteristics - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring a third-order nonlinear optical characteristic of a substance, in particular, a third-order susceptibility.

[Conventional technology]

Conventionally, optical non-linear characteristics of materials, especially third-order susceptibility χ ⁽³⁾
The third harmonic generation method (THG method) that irradiates a substance with a high-intensity light beam such as a laser beam to measure the intensity of a third harmonic (TH wave) resulting therefrom is used. I have. In the case of such a THG method, a substance whose tertiary susceptibility χ ⁽³⁾ is already known is used as a standard sample, and the intensity of the TH wave generated from the standard sample and the measurement sample composed of the substance to be measured are used. From the intensity of the TH wave generated from the
⁽³⁾ Calculate the value. Hereinafter, a conventional measuring device using the THG method (THG measuring device) will be described with reference to the drawings.

Figure 5 is a diagram for explaining a conventional example of a THG measuring apparatus, reference numeral 1 is the measured sample of the target substance chi ⁽³⁾ measure, is fixed on a rotating sample stage 2. The measurement sample 1 may be in any state of a solid (amorphous, single crystal, vapor-deposited film, etc.), liquid (solution, etc.) and gas, and may have any shape as long as the thickness is constant. However, in the case of liquids and gases, a container for containing the liquid is generally used, but a glass container having a sufficiently small χ ⁽³⁾ is used to avoid the influence of the TH wave from the container.

Reference numeral 4 denotes a high intensity light beam 3 having an angular frequency ω.
Is a high output laser that oscillates. This high power laser 4
A condenser lens 5 is provided between the sample stage 2 and the sample stage 2, and the light beam 3 generated by the high-power laser 4 is converged to the position of the sample 1 by the condenser lens 5. I have. The focal length of the condenser lens 5 is usually set to about 100 mm to 50 mm. When the light beam 3 irradiates the sample 1, a third light having an angular frequency of 3ω is obtained.
(A TH wave) 7 is emitted.

On the emission side of the sample 1, a filter 6 and a photodetector 8 are arranged. The filter 6 has a third harmonic (TH) having an angular frequency of 3ω emitted when passing through the sample 1.
Wave) and a light beam having an angular frequency of ω are incident, absorb a light beam having an angular frequency of ω, and obtain a TH wave having an angular frequency of 3ω.
Through. The TH wave 7 is applied to the photodetector 8
Is to be detected.

Next, FIG. 6 shows the result of measuring the TH wave by the apparatus shown in FIG. This figure shows a change in the TH wave when the sample stage 2 is rotated to change the incident angle θ at which the incident light beam 3 enters the sample 1. (A) in FIG.
The results are obtained when a measurement sample having a sufficient thickness of about 100 μm is used, and the pattern obtained here is called a maker fringe pattern. Although the details are omitted, the coherent length Lc of the sample is obtained from this pattern, and the intensity I of the TH wave is obtained from the peak value of the envelope of the pattern. (B)
Is a result when a thin measurement sample having a thickness of about 1 μm is used, and a pattern called a single fringe pattern is obtained. From this pattern, the intensity I of the TH wave is obtained.
(C) is a result when a quartz plate having a thickness of 1 mm, which is a standard sample, is used. From this result, the coherent length Lc, s and intensity Is of the standard sample are obtained. Quartz plates are frequently used as standard samples because of their stable material and good data reproduction.

The value of χ ⁽³⁾ of sample 1 is a sample that produces a maker fringe pattern as shown in FIG.
For a single crystal, liquid or gas sample of about 1 mm from
； ⁽³⁾ = Cχ ^{(3) A} sample which can be obtained from s (Lc, s / Lc) (I / Is) ^1/2 and produces a single fringe pattern as shown in FIG. 6 (b). For example, if the thickness d is
For a thin film sample such as a deposited film of about 0.1 μm to 10 μm,
； ⁽³⁾ = Cχ ⁽³⁾ s (Lc, s / d) (I / Is) ^1/2 (2 / π) Here, C is a constant, which is determined by the refractive indexes of the standard sample and the measurement sample. Χ ⁽³⁾ s is known as the χ ⁽³⁾ value of the standard sample.

In the above example, an isotropic substance is usually used as the measurement sample, but a crystal sample having optical anisotropy may be used as the sample. Many crystals have a major optical axis and this anisotropy needs to be considered.
The third order susceptibility χ ⁽³⁾ has 81 components in the tensor. Among them, ３ ⁽³⁾ ₁₁₁₁ along the optical axis and χ ⁽³⁾ ₂₂₂₂ in a direction perpendicular to the optical axis are the main third-order nonlinear optical characteristics.

Next, a method of obtaining the tensor component of such an anisotropic crystal using the apparatus shown in FIG. 5 will be described.
(WKBurns and N. Bloembergen, Physical Review B
Journal, Vol. 4, p. 3437, 1971).

In this case, first, the crystal is processed so that the optical axis corresponding to χ ⁽³⁾ ₁₁₁₁ is parallel to the crystal surface, and a measurement sample is prepared. Further, a light beam having linear polarization is used as the incident light beam 3 in FIG. 5, and the polarization direction is set so as to be parallel to the optical axis, and the rotation axis of the rotating sample stage is set to be parallel to the polarization direction. I do. That is, the light is incident in the s-wave arrangement. The value obtained at this time is the first χ ⁽³⁾ value, that is, χ ⁽³⁾
₁₁₁₁ .

Next, with the s-wave incident, the crystal is rotated by 90 degrees about the axis of the light beam, and the TH wave is measured to obtain χ ⁽³⁾ ₂₂₂₂ value.

[Problems to be solved by the invention]

According to the above method, in principle, the crystal axis arrangement is different.
An arbitrary tensor component can be obtained using one crystal sample.

However, a crystal to be measured is required.
Processing must be performed with high precision so that the tensor axis of ⁽³⁾ is included in parallel with the sample surface, which requires not only advanced technology but also difficulties in the case of molecular crystals, etc.
There was a major problem that processing was often impossible.

SUMMARY OF THE INVENTION The present invention solves the above-described problems, and provides a simple, low-cost, and accurate measurement of a third-order nonlinear optical characteristic that can evaluate a plurality of components of the third-order susceptibility χ ⁽³⁾ of an anisotropic crystal. It is intended to provide a device.

[Means for solving the problem]

In the present invention, a light source that emits a light beam having an angular frequency ω,
It has a condenser lens for converging a light beam oscillated from a light source on a sample, a sample stage for fixing the sample, and a photodetector for measuring a third harmonic having an angular frequency of 3ω emitted from the sample. In the tertiary nonlinear optical characteristic measuring apparatus, a polarization converter is provided between the light source and the condenser lens to convert a polarization state of a light beam from the light source into a linear polarization state in two directions orthogonal to each other. Alternatively, the solution is that the sample stage is a rotating sample stage having two orthogonal rotation axes perpendicular to each other.

[Action]

According to the apparatus having the configuration including the polarization converter, the polarization state of the light beam from the light source is converted into the linear polarization state in two directions orthogonal to each other. By measuring the wave and comparing the results of the two, it is easy and accurate to determine the optically anisotropic crystal material 3
The tensor component of the next susceptibility in any direction can be obtained.

Similarly, in an apparatus in which the sample stage is a rotary sample stage having two orthogonal rotation axes,
A tensor component in any direction of the third-order susceptibility of the optically anisotropic crystal material can be obtained. Therefore, in such an apparatus, unlike a conventional apparatus, a crystal having optical anisotropy is not processed in advance, and a third-order nonlinear optical characteristic can be measured while maintaining a naturally grown surface. It has great advantages.

 Hereinafter, the present invention will be described in detail with reference to examples.

〔Example〕

FIG. 1 shows an example of a THG measuring apparatus as an example of the measuring apparatus of the third-order nonlinear optical characteristic of the present invention.

In the THG measuring apparatus of this example, a laser emitting a vertical linearly polarized light is used as the light source 4, and the linearly polarized light of the light beam 3 is disposed between the laser 4 and the condenser lens 5. A polarization converter 9 for rotating the polarization direction by 90 degrees is provided. The rotation axis of the sample stage 2 is set to be vertical as in the case of the conventional apparatus. That is, the relationship between the polarization direction and the rotation axis is two kinds, parallel and perpendicular, and corresponds to the s-wave arrangement and the p-wave arrangement, respectively.

FIG. 2 shows an example of a quartz glass prism used as the polarization converter 9. This quartz glass prism is equivalent to a combination of three small prisms,
By rotating the polarization direction of linearly polarized light by 90 degrees without depending on the wavelength of the incident light at all, it is possible to convert longitudinally polarized waves into laterally polarized waves or horizontally polarized waves into longitudinally polarized waves.

 Next, a second embodiment of the present invention will be described.

The device of this example does not include the polarization converter 9 such as a quartz glass prism as in the device of the first embodiment described above.
Instead, the sample stage 2 is a rotating sample stage having rotation axes in two directions orthogonal to each other.

FIG. 3 shows a diagram for explaining the positional relationship between the rotation axis of the rotating sample stage 2 and the polarization direction of the incident light beam 3. Reference numeral 1 in the figure denotes a sample fixed to a rotating sample stage. Assuming that the incident direction of the incident light beam 3 incident on the sample is Z, the rotation axis of the rotating sample stage 2 is parallel (X-axis direction) and perpendicular (X-axis direction) to the polarization direction of the incident light beam 3. (Y-axis direction). The case where the X axis is the rotation axis is the s-wave arrangement, and the case where the Y axis is the rotation axis is the p-wave arrangement.

Hereinafter, a method of measuring the third-order nonlinear optical characteristics using the optically anisotropic crystal material as a measurement sample using the apparatus of the second embodiment will be described with reference to FIG.

As the measurement sample 1, a single crystal of diethylaminonitrostilbene (DEANS) having a molecular crystal and anisotropy (thickness: 60 μm) was used. As in Figure 3, a crystal is uniaxial anisotropic crystal, the direction of the orientation axis of the molecule in the crystal is not parallel to the crystal surface S, is inclined at an orientation angle theta _0. In the case of such a sample, the value that needs to be evaluated is the 次⁽³⁾ component χ ⁽³⁾ ₁₁₁₁ (= χ ⁽³⁾ along the orientation axis, which is the third-order nonlinear optical constant together with the orientation angle θ _0. ‖) And its vertical component χ ⁽³⁾ ₂₂₂₂ (= χ ⁽³⁾ ⊥).

 The method for obtaining these values is described below.

First, the standard sample and the measurement sample are rotated around the X axis in an s-wave arrangement, and a manufacturer fringe pattern as shown in FIG. 6 is measured. In this embodiment, the orientation axis of the molecule is in a virtual plane A in the crystal. FIG. 4 (a) shows the result of the measurement sample 1, and a fringe pattern similar to the conventional example of FIG. 6 is obtained.

Next, the sample is rotated around the Y axis in a p-wave arrangement, and the fringe pattern is measured. FIG. 4 (b) shows the result of the measurement sample 1, and it can be seen that an asymmetric peculiar fringe pattern is obtained. Based on the results obtained above, the θ
₀ , χ ⁽³⁾ χ, and χ ⁽³⁾ ⊥ are determined by the following analysis method.

The shape of the fringe pattern in the s-wave arrangement can be expressed by the following equation as a function Vs using the incident angle θ as a variable.

_{Vs = Cs (T 1, s} ) 3 T 3, s Ds 2 (χ (3) s) 2 (Lc, s) in ² F (theta) where Cs is a constant, the angular frequency of the beam omega, refraction materials Rate, light beam diameter, and intensity of incident light.
T ₁ s and T ₃ s are the transmittances of the incident wave and the TH wave on the sample surface, and use a normal Fresnel reflection formula. Ds is a coefficient by which the light beam diameter changes between the outside and the inside of the sample, and is given by the following equation.

Ds = cos θ / cos θin Here, θ is the above-mentioned incident angle, and θin is the corresponding incident angle inside the crystal. χ ⁽³⁾ s is effective χ ⁽³⁾
A value, chi ⁽³⁾ ‖ and it includes a vertical chi ⁽³⁾ the contribution of both ⊥, it is approximately expressed by the following equation.

χ ⁽³⁾ s = χ ⁽³⁾ ‖ cos ³ θ ₀ + χ ⁽³⁾ ⊥ sin ³ θ ₀ Lc, s is also the same as in ⁽³⁾ s, the coherent length Lc (Lc の) And Lc (Lc⊥) in the vertical direction, and is expressed by the following equation.

^{Lc, s = {Lc‖ 2 cos} 2 θ 0 + Lc⊥ 2 sin 2 θ 0} 1/2 χ (3) s and Lc, the value of s is constant even the incident angle theta change bad. F (θ) is an expression representing a fine periodic structure of a commonly used maker fringe pattern, and is determined by the incident angle of light, the coherent length, and the thickness of the crystal.

The shape function Vp of the fringe pattern in the p-wave arrangement can be expressed by the following equation, as in the case of the s-wave.

Vp = Cp (T ₁ , p) ³ T ₃ , p Dp ² (χ ⁽³⁾ p) ² (Lc, p) ² F (θ) As in the case of the s-wave, Cp is a constant, T ₁ , p and T ₃ , p is the transmittance of the incident wave and the TH wave on the sample surface, and Dp is a coefficient and is given by the following equation.

Dp = cos θ / cos θin Further, χ ⁽³⁾ p is expressed by the following equation.

χ ⁽³⁾ p = χ ⁽³⁾ ‖ cos ³ θr + χ ⁽³⁾ ⊥ sin ³ θr where θr is the angle between the polarization direction of the light beam and the molecular orientation axis inside the crystal, and the incident angle θin And the orientation angle θ ₀ is expressed by the following equation.

θr = | θin−θ ₀ | Lc, p is also represented by the following equation, similarly to χ ⁽³⁾ p.

Lc, unlike in the case of ^{p = {Lc‖ 2 cos 2 θr} + Lc⊥ 2 sin 2 θr} 1/2 s -wave, chi ⁽³⁾ p and Lc, the value of p is changed by the value of the angle of incidence theta. F (θ) is an expression representing the fine structure, but can be ignored since it is sufficient to analyze the shape of the envelope of the fringe pattern.

The fitting was performed so that the calculated values of the shape functions Vs and Vp of the fringe pattern described above coincide with the measurement results of FIG. From the results in FIG. 4 (a), χ ⁽³⁾ s is 3.3 × 1
0 ^-12 esu and Lc, s were determined to be 1.4 μm. Further, using these values, FIG. 4 (b) shows that the orientation angle θ ₀ is 45 degrees, χ
⁽³⁾ ‖ was 7.0 × 10 ^-12 esu and χ ⁽³⁾を 得 obtained 1/10 of this result. The calculation result of the fitting is as shown in FIG. 4 (b), which agrees well with the envelope of the measurement result.

As described above, the present invention has been specifically described based on the embodiments. However, it is needless to say that the present invention is not limited to the above-described embodiments and can be variously modified without departing from the gist thereof. In this embodiment, an example is shown in which a uniaxial crystal is used, in which the optical axis is inclined with respect to the sample crystal surface.However, more generally, for example, a biaxial crystal is used as a sample. The present invention is applicable. Furthermore, an illegal thin film can be used as a sample. In the conventional apparatus, in order to obtain these values, the crystal had to be processed so as to have a plane parallel to the molecular orientation axis, but in the present invention, the measurement was performed with the crystal in a naturally grown shape. It has the great advantage that it can be done.

In the first embodiment, an example is shown in which a quartz glass prism is used as the polarization converter 9 for realizing both the s-wave and p-wave arrangements. good. Further, the light source 4 is not limited to a high-power laser, but may be, for example, a semiconductor superluminescent diode.

In addition, an optical lens having a focal length of 70 mm or less can be used as the condenser lens 5. According to this, the influence of the TH wave due to air can be minimized, and the tertiary susceptibility of the substance in air can be reduced. χ ⁽³⁾ can be measured accurately.

〔The invention's effect〕

As described above, the present invention provides a light source that emits a light beam having an angular frequency ω, a condenser lens for converging a light beam oscillated from the light source on a sample, and a sample stage for fixing the sample, An apparatus for measuring a third-order nonlinear optical characteristic having a photodetector for measuring a third harmonic having an angular frequency of 3ω emitted from a sample, comprising:
A polarization converter for converting a polarization state of a light beam from a light source into a linear polarization state in two directions orthogonal to each other, or a rotating sample stage having a rotation axis in two directions orthogonal to each other; Therefore, the direction of change of the incident light beam is equivalently rotated by 90 degrees, and the incident light beam is incident on the measurement sample in the arrangement of the s-wave and the p-wave. By comparing the TH wave patterns in both cases, the optical difference is obtained. A plurality of χ ⁽³⁾ components can be measured without processing a crystalline material having anisotropy and maintaining a naturally grown surface. For this reason, it is possible to measure even a substance that cannot be processed, and it is possible to measure the susceptibility of the third order simply, at low cost and with high accuracy.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example of a third-order nonlinear optical device according to the present invention. FIG. 2 is a diagram showing an example of a polarization converter for rotating the polarization direction of a light beam used in the present invention.
FIG. 4 is a diagram showing a positional relationship between a rotation axis of a rotating sample stage used in the present invention and a polarization direction of an incident light beam, and FIG. 4 is a diagram showing a sample of a crystalline material having optical anisotropy in an embodiment of the present invention. FIG. 5 is a diagram showing a fringe pattern of a TH wave when used as a device, FIG. 5 is a configuration diagram showing an example of a conventional THG measurement device, and FIG. 6 is a diagram showing measurement results of a measurement sample and a standard sample according to a conventional example. It is. 1 ... sample, 2 ... sample stage, 3 ... incident light beam of angular frequency ω, 4 ... light source, 5 ... condensing lens, 7 ... third harmonic (TH wave) of angular frequency 3ω, 8: photodetector, 9: polarization converter.

Continuation of the front page (72) Inventor Toshikuni Kaino 1-6-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-63-98547 (JP, A) JP-A-3-115595 (JP, A)

Claims (1)

(57) [Claims] 1. A light source for emitting a light beam of each frequency ω, a condenser lens for converging a light beam oscillated from the light source on a sample, a sample stage for fixing the sample, and an angle emitted from the sample. In a characteristic device having a third-order nonlinear optical characteristic having a photodetector for measuring a third harmonic having a frequency of 3ω, a polarization state of a light beam from a light source is orthogonal to that between the light source and a condenser lens. Provide a polarization converter for converting to a two-way linear polarization state, or,
An apparatus for measuring tertiary nonlinear optical characteristics, wherein the apparatus is a rotating sample stage having rotation axes in two directions orthogonal to each other.