JP2759114B2 - 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 this case, the tertiary susceptibility χ ⁽³⁾ is determined by using a substance whose value of χ ⁽³⁾ is already known as the standard sample, and by using the intensity of the TH wave generated by the standard sample and the substance to be measured. It is performed by calculation from the ratio with the intensity of the generated TH wave. Hereinafter, a conventionally used measuring device based on the THG method (THG measuring device) will be described with reference to the drawings.

Figure 1 is a diagram for explaining a conventional example of a THG measuring apparatus, reference numeral 1 denotes a measurement sample comprising a target substance chi ⁽³⁾ measure, it is fixed on the sample stage 2. The sample stage 2 can rotate. The measurement sample 1 may be in a solid (amorphous, single crystal, vapor-deposited film or the like), liquid (solution or the like) or any expected state, and may have any shape as long as the thickness is constant. However, in the case of a solution or gas, a container for containing the solution is generally required, but in order to avoid the effects of TH waves from the container, a glass material whose substance ⁽³⁾ of the container is sufficiently small is used. A container is used.

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 500 mm. By irradiating the sample 1 with the light beam 3, a third harmonic (TH wave) having an angular frequency 3ω is emitted.

On the emission side of the sample 1, a filter 6 and a photodetector 8 are arranged. This filter 6 is used for the sample 1
Third harmonic of angular frequency 3ω (TH
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. 3 shows the result of measuring the TH wave 7 by the apparatus shown in FIG. This figure shows the change of the TH wave 7 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.
Is a result 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 and the TH wave 7
Is obtained. (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 7 is obtained. (C) shows the results when a 1 mm thick quartz plate as a standard sample was used. From this result, the coherent length Lc, s and intensity Is of the standard sample were obtained.
Is found.

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⁾ = χ ^{(3) A} sample that can be obtained from s (Lc, s / Lc) (I / Is) ^1/2 and that produces a single fringe pattern as shown in FIG. 3 (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⁾ = χ ⁽³⁾ It can be obtained from s (Lc, s / d) (I / Is) ^1/2 (2 / π). Here, χ ⁽³⁾ s is the χ ⁽³⁾ value of the standard sample, and is a known value. Lc, s, and Lc are the coherent lengths, and I and Is are the measured intensities. Quartz glass, whose material is stable and has good data reproducibility, is used very frequently as a standard sample.
^{(3) The} value is 2.8 × 10 ^-14 esu.

[Problems to be solved by the invention]

However, the TH wave output from the standard sample and the measurement sample is actually superimposed with a TH wave caused by air existing on the path of the light beam, and the output light intensity is changed due to a difference in a relative positional relationship between the waves. Changes. Therefore, there is a problem that accurate measurement cannot be performed when the intensity of the TH wave from the sample is weak and is substantially equal to the intensity of the TH wave from the air. In particular, quartz, which is a standard sample, is easily affected by air because the nonlinear optical constant is small and the intensity of the TH wave is weak. This measurement error was a major problem in determining χ ^{(3) of the} substance.

To solve this problem, an improved measuring device has been proposed (F. Kajzar and J. Messier, Nonlinear Optical
Properties of Organic Molecules and Crystals, Acade
mic, Press, Vol. 2, Chapter III-2). This improved measuring device is to put a sample whose χ ^{(3) is} to be measured in a vacuum vessel, shield the sample from surrounding air, and reduce the influence of TH waves due to air. However, in this improved measuring device, it is necessary to connect an exhaust device in order to achieve a vacuum state in the vacuum vessel, and there is a problem that the device is large and complicated, and a sample is required for each measurement. Had to be replaced with a vacuum container. further,
When a sample whose shape changes due to sublimation or the like in a vacuum is used, it is necessary to newly prepare a container for preventing the shape change.

By the way, in the THG method, in order to obtain accurate TH wave intensity,
It is necessary to determine the fringe pattern (a pattern as shown in FIGS. 2A and 2B). In this case, the first
The incident angle θ of the incident light beam shown in the drawing must be changed in a sufficiently wide range. For this purpose, the sample stage in the vacuum vessel can be rotated at a wide angle, or the sample to be measured is processed into a wedge shape,
Means such as performing measurement while moving this in a direction perpendicular to the light beam is required.

However, when the former means is adopted, there is a problem that the vacuum vessel becomes very large in the improved measuring device. Further, in the latter means, there is a problem that the material must be processed with high accuracy, and furthermore, this processing cannot be performed depending on the type of the material.

Furthermore, in the improved measuring device, since various devices surrounding the measurement sample are required as described above, it is necessary to use a lens having a long focal length as a condenser lens,
Therefore, there is a problem that the spot diameter of the light beam becomes large and the resulting TH wave intensity becomes weak (the TH wave intensity is inversely proportional to the fourth power of the spot diameter when the power of the light beam is the same). .

Under such circumstances, the technology proposed so far has not been able to realize a simple, low-cost, and highly accurate device for measuring the amount of a substance ⁽³⁾ by the THG method.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and to provide a third-order nonlinear optical characteristic measuring device that can easily, accurately, and accurately evaluate a third-order susceptibility χ ⁽³⁾ of a substance. And

[Means for solving the problem]

In the present invention, a light source that emits a monochromatic light beam having an angular frequency ω, a condenser lens for converging a light beam oscillated from the light source onto a sample, and a third harmonic having an angular frequency of 3ω emitted from the sample are measured. In a measuring device for a third-order nonlinear optical characteristic having a photodetector, a solution is to use an optical lens having a focal length of 70 mm or less as the condensing lens.

[Action]

By using the short focus lens of 70 mm or less in this way, the influence of the TH wave due to air is minimized, and it is possible to accurately measure the substance ⁽³⁾ even in air.

 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. The configuration of this THG measuring apparatus is different from the above-described conventional example of the THG measuring apparatus in that an optical lens having a condensing distance of 70 mm or less is used as the condensing lens 5, and other configurations are the same. Because there is
Detailed description is omitted.

FIG. 2 shows the influence of air when the TH wave of a quartz plate as a standard sample was measured using lenses having a focal length f of 50 mm and 70 mm in this embodiment, respectively. This figure shows how the intensity of the TH wave from the quartz plate changes when the air pressure in the vacuum vessel containing the quartz plate changes. The result when f is 80 mm is an example used for comparison and is outside the scope of the present invention. It has already been confirmed that no signal is detected when only the vacuum vessel is used. The figure shows the result when the wavelength λ corresponding to the angular frequency ω of the incident light beam is 1.95 μm.
The same result was obtained in the wavelength range of 2.5 μm.

As is clear from this figure, f of the condenser lens 5 is 50 mm.
In this case, the intensity of the TH wave hardly changes even if the air pressure in the container is changed. However, when f is set to a long focal length of 70 or 80 mm, the signal strength decreases as the air pressure increases. Since the focal length of the condensing lens in the conventional device was 100 mm or more, it is easily inferred from this figure that the influence of air was extremely large. χ ⁽³⁾ The measurement accuracy of the value is required to be about ± 15%, but since χ ⁽³⁾ is proportional to the half power of the TH wave intensity, the measurement accuracy of the TH wave intensity must be within ± 30%. It becomes the accuracy of a cigarette. In other words, if a lens with a focal length f shorter than 70 mm is used,
The required measurement accuracy can be achieved.

Such a THG measuring apparatus is a small measuring apparatus which does not require a large vacuum vessel or an exhaust device as in the above-mentioned improved measuring apparatus, and can exchange a sample extremely easily.

Further, in this THG measuring apparatus, since the focal length of the condenser lens 5 is short, the focal point, that is, the beam spot diameter in the substance is small, and the light intensity is large, so that the intensity of the generated TH wave is large. That is, as compared with the conventional measuring apparatus, the measuring apparatus of the present invention has an advantage that even a substance having a small value of χ ⁽³⁾ can be measured even when laser light having the same intensity is used.

Incidentally, the measuring device of the third-order nonlinear optical characteristic of the present invention is not limited to the above embodiment, as long as the wavelength of the incident light is other than the embodiment, It goes without saying that various changes can be made, such as when a semiconductor superluminescent diode is used instead of a laser as a light source.

〔The invention's effect〕

As described above, since the third-order nonlinear optical characteristic measuring apparatus of the present invention uses an optical lens having a focal length of 70 mm or less as a condensing lens, it minimizes the influence of TH waves due to air. Even in the air, the third-order susceptibility 物質⁽³⁾ of the substance can be accurately measured.

In addition, the device of the present invention does not require a large device around the measurement sample, and can be downsized.In addition, the sample can be prepared without requiring processing of the target substance, and the handling of the sample is simple. Therefore, quick evaluation of various substances is possible.

Furthermore, since the short focus lens is used, the intensity of the irradiation light is high, and as a result, the intensity of the generated TH wave is high, and the range of the substance to be measured can be widened.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example of the present invention and an example of a conventional measuring device for third-order nonlinear optical characteristics. FIG. 2 is a diagram showing an embodiment of the present invention in which the focal length of a condenser lens is changed by air. FIG. 3 is a diagram showing the effect of the third harmonic on the signal strength, and FIG. 3 is a diagram showing the measurement results of a measurement sample and a standard sample according to a conventional example. 1 ... Sample, 3 ... Incident light beam with angular frequency ω, 4 ... Laser (light source), 5 ... Condenser lens, 7 ... Third harmonic (TH wave) with angular frequency 3ω, 8 ... Light detector.

Claims (1)

(57) [Claims] 1. A light source for emitting a monochromatic 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 third harmonic having an angular frequency of 3ω emitted from the sample. An apparatus for measuring third-order nonlinear optical characteristics, comprising a photodetector for measurement, wherein an optical lens having a focal length of 70 mm or less is used as the condenser lens.