SU1702259A1 - Interference method of metering the monocrystal refractive indices - Google Patents

Abstract

The invention relates to instrumentation technology and be used as an interference method for measuring the refractive indices of single crystals. The purpose of the invention is to increase the productivity and information content of the method, as well as to simplify its implementation without reducing the measurement accuracy. The goal is achieved by using a new principle of forming an interference pattern of light scattered by a crystal, which is based on the effect of spatial modulation of scattered light in birefringent single crystals, realized, in particular, when the sample of a single crystal is monitored with a monochromatic polarized beam of light alternately along each of the the three main axes of the optical indicatrix and the registration of interference patterns from the sides of the crystal, orthogonal to bisectors of the angles between in two other principal axes of the optical indicatrix Numerical values of refractive index is calculated by rezultatamizmereni periods of the interference fringes of the interference patterns corresponding to the scattered light from the crystal system of three equations based on the known value of the wavelength of the irradiating light from the single crystal C

Description

The invention relates to instrumentation engineering, in particular, to interference methods for measuring the refractive indices of anisotropic media, and can be used to measure the main refractive indices of birefringent single crystals.

A known interference method for measuring the refractive indices is to illuminate the anisotropic substance under investigation with monochromatic light, to form and record the interference pattern in transmitted and / or reflected light with the subsequent measurement of the geometrical parameters of its topology and calculation

The corresponding conversion factor.

To measure the values of all (two or three) main refractive indices of single crystals (respectively, for optical uniaxial or biaxial crystals), it is necessary to carry out the described measurement operations at least at least two or three times.

The closest to the invention is the interference method of measuring the refractive indices of single crystals, which consists in the fact that a sample of a single crystal, provided with a flat face pre-printed on its surface with several

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sections of two-layer AqCI-Ag films of various thicknesses, are irradiated with a monochromatic polarized light beam along the normals to the specified face, fixing the corresponding interference patterns of scattered light in the region of the AgCI-Ag films of different thicknesses, measure the periods of the interference fringes of the corresponding interference patterns and the results of these measurements are calculated refractive index values.

The disadvantage of this method is low productivity and complexity of implementation, which is caused by the need to use auxiliary procedures for the formation and fixation of the corresponding interference patterns of scattered light, the realization of which, as well as the measurement of the geometrical parameters of their topology, requires very complex methodological, instrumental and technological support. A disadvantage is also the limited information content, which is due to the possibility of measuring the value of the refractive index using this sample of a single crystal for only one given direction.

The aim of the invention is to increase productivity and simplify the process without reducing the measurement accuracy.

The goal is achieved by the fact that in a known method of measuring the refractive indices of single crystals, namely, that a sample of a single crystal is irradiated with a monochromatic polarized light beam, an interference pattern of the scattered crystal is recorded, followed by measuring the period of interference fringes and calculating the values of the indicators from the results of these measurements refraction of a single crystal, a sample of a single crystal is alternately irradiated along each one of the three main axes of the optical indicatr The spectra register the interference pattern of the light scattered by the crystal from the face orthogonal to the bisector of the angle between the respective two other main axes of the optical indicatrix, and using the results of measuring the periods of the interference fringes of the corresponding interference patterns, calculate the main refractive indices of the single crystal from the system of the following equations:

I

P2-P1-.

And pz-n2;

pz-ni

BUT.

d2

where u, pa, pz are the values of the main refractive indices of the single crystal, corresponding to the directions of the three main axes of its optical indicatrix;

di, da, d3 are the periods of interference fringes in the interference patterns of scattered light when the sample of a single crystal is irradiated along the main axes of the optical indicatrix, m;

A is the wavelength of light irradiating a sample of a single crystal, m.

The method is based on the effect of spatial modulation of scattered light in birefringent crystals.

The essence of this effect is as follows. A polarized light wave entering a birefringent crystal splits into two partial coherent waves propagating at different speeds and mutually orthogonal directions of oscillations of the electric induction vector D (along one or another of the two main axes of the optical indicatrix). The phase difference of these waves b depends on the distance I traveled by them in a given direction in the crystal, and the difference in refractive indices (n - n)

light in the same direction: d p -t- (g / n) -l, where A is the wavelength of light. Each of these partial waves is scattered by micro-inhomogeneities, which always occur in a real crystal, and if the characteristic dimensions of these micro-inhomogeneities are not more than 0.1 L, then the scattered light turns out to be completely linearly polarized with the orientation of the plane of oscillations of the vector and perpendicular to the plane in which the falling lum and the line of observation lie.

Thus, for certain orientations of the observation line, the scattered light oscillations are orthogonal to both the observation line and the light beam in the crystal. As a result, in the direction of the crystal face orthogonal to such an observation line, only certain components of the two scattered partial light waves pass, the intensities of which are added up taking into account their phase difference dp. Due to the fact that the magnitude b varies along the direction of light propagation in the crystal, the intensity of the scattered light transmitted in the direction of the crystal face, orthogonal to the specified observation line, appears to be spatially moduloAA

with a period of L-7-3 ,. which the

corresponds to a change in dp by 2 liters, i.e. on the side of the indicated crystal face, an interference pattern is observed in the pattern of scattered light in the form of a periodic system of alternating light and dark bands. The most contrasting interference pattern occurs when the line of observation is parallel to the bisector of the angle between those two axes of the optical indicator, which correspond to the corresponding orthogonal directions of oscillations of the vector D of two partial coherent light waves, into which the original light wave splits into a split wavelength - A crystal. At the same time, the spatial distribution of the observed interference pattern of scattered light along the direction of propagation of the light beam in

6 (p

6 (p

crystal obeys the law cos -x

sin

2 dip

2

Each direction of light propagation in a crystal corresponds to its value (n1-n) and, accordingly, its value of the period L of the spatial modulation of the scattered light, and hence its value of the period d of the contrast interference fringes observed from the side of the crystal face orotgonal angular bisector of the angle between the respective main axes of the optical indictrix.

This is what determines the possibility of irradiating a crystal, for example, in two different directions (i, j) in one or another of the irradiation planes and measuring the periods di and dj of the corresponding interference patterns, as well as the wavelength A of the irradiating crystal, to determine the numerical values of the difference of the refractive indices (nu n) j and (p-n) j in this direction in the single crystal, and made a series of similar measurements for different directions of propagation of the irradiating beam of light in the crystal, identify the numerical values and the values themselves by azateley refractive index for the respective directions in the crystal.

To obtain complete information about the values of the refractive indices of light in any direction in the crystal, it is necessary and sufficient to know the values of the main refractive indices corresponding to the directions of the main axes of the optical indicatrix of the crystal. In turn, to measure the values of the principal

5 10 15 20

25

thirty

35

40

45

50

55

crystal refractive indices using the described effect of spatial modulation of scattered light and taking into account what has been said about choosing the facet of observing the interference pattern of scattered light, it is also necessary to determine such a set of directions of irradiation of the crystal such that the measured values (n1 - n) j, jk, l would allow to calculate the main values of u, P2 and pz.

The results of theoretical and experimental studies have shown that in order to make it possible to determine the full set of values of the main refractive indices n, P2, P3 using the effect of spatial modulation of the scattered light in single-crystal single crystals, the single crystal sample should be ps. Irradiate alternately with a monochromatic polarized beam of light along each of the three main axes of the optical indicatrix, while registering the interference pattern of scattered light from the side of the crystal orthogonal to the bisset of the angle of the two main axes of the optical indicatrix that are orthogonal to its main axis along which it crystal irradiation. In this case, the periods di, j, k of the three alternately observed interference patterns are determined by the following relations:

A a

di ---; d2

d3

PZ -P2 I

PZ - P1

P2 - Hz

where ni, P2, pz are the main refractive indices of the crystal. In turn, the system of these three equations allows us to find a unique solution for the values of u, P2, pz.

The method is carried out as follows.

A sample of a single crystal is alternately irradiated with a monochromatic polarized beam of light along each one of the three main axes of the optical indicatrix. In this case, each time, the interference pattern of the light scattered by the crystal from the side of the crystal face orthogonal to the bisector of the angle between the two other main axes of the optical indicatrix is recorded. The values of the periods di, d2, ds of the interference fringes of the corresponding interference patterns in each irradiation cycle are measured and, based on the results of these measurements, the magnitude of the wavelength I of the irradiating light beam is calculated, the values of the main refractive indices t, P2, pz of the single crystal are calculated from the system of the following equations:

I ;

BUT .

PZ - P2

BUT

CJ1

(here the indices 1, 2, 3 correspond to the numbers of the main axes of the optical indicatrix of the single crystal).

It should be noted that the proposed method is maximally adapted to optically biaxial crystals, for which the optical indicatrix is a triaxial ellipsoid. Indeed, in the case of optically uniaxial crystals, in which, as is well known, the optical indicatrix is an ellipsoid of rotation, and only two main indexes of refraction, cs and nx2 are numerically different, direct application of the proposed method makes it possible to measure only the difference between the two main refractive indices pz and p2. Moreover, for this purpose it is sufficient to carry out only one operation of measuring the period di d2 of the interference pattern when the crystal is irradiated along the small main axis of the optical indicatrix. If it is necessary to numerically estimate the values of pz and nc, it is sufficient to carry out just one operation of measuring di d2 when the crystal is irradiated in a direction that differs from the small main axis of the optical indicatrix at a certain angle a. It can be shown that when

this ,,., | r g ptt

j + 5in fth - -n,

it I nЈI

where / 3 is the angle between the small principal axis of the optical indicatrix and the direction of propagation of the light beam in the crystal, which is related to the angle a of the entrance of the beam into the crystal by the Fresnel equation.

This equation, together with the previously obtained d2 --- is sufficient for

identification of the numerical values of pz and gi of an optically uniaxial crystal.

Thus, the proposed method is suitable for single crystal of any type, however, in terms of increasing the information content and performance of measurements, as well as simplifying the implementation, it is most adapted to optically biaxial single crystals.

In the example of the practical implementation of the method, the values of the main refractive indices of single crystals of moladbdata gadolinium Gd2 (MoCM) 3-G # Q, related to mm point symmetry and optically biaxial, were measured.

Measurements u,. pz were performed on just one sample of a single GMO crystal, equipped with two systems of three orthogonal faces (100), (010). (001) and

(011), (101), (110), the first of which was used to alternately irradiate the crystal along the normals to these faces with a monochromatic polarized light beam (from an He – Ne laser with a wavelength

A 0.6328 µm), and the second, for sequential recording of the corresponding interference patterns scattered on micro-heterogeneity of a single crystal of light. The periods of the corresponding interference Corgins, recorded from the sides (011), P01) and (110), were measured using an electronic unit containing a photosensitive matrix of CCD elements and was measured by measuring the number of interference fringes on a fixed length of a sample portion of a single crystal with using a scanning CCD photodetector. The measurement results gave the following values of the di, da periods. da: dicr (12.2 ± 0.1) µm; - d24l2.1 ± 0.1) μm; d32 (1528 ± 2) µm. Calculated from measured values of di, d2, da and the known value of I, 0.6328 µm, the magnitudes of the pavilous ps of the index of refraction, u, P2. pz single GMO tallies totaled: pz -1,900; (P2 - gi) 4 and P2 - n i 1, a- {8. The found values of n, P2, PZ are in full accordance with the known reference data for this single crystal.

Claims (1)

The invention The interference method of measuring the refractive indices of monocrystal allah, which consists in the fact that the sample monocrystals are irradiated with a monochromatic polarized light beam, the interference pattern of the light scattered by the sample is recorded, followed by measuring the period of interference fringes: and the results of these measurements are used to calculate the values of the single crystal refractive indices in order to improve performance and simplify the method without reducing 0 measuring accuracy, the sample of the single crystal is alternately irradiated along each of the three main axes of the optical indicatrix, the interference pattern of light scattered by the sample from the side is recorded, 5 orthogonal bisector of the angle between the respective two other main axes of the optical indicatrix, and using the results of measuring the periods of the interference fringes of the corresponding interference patterns, the values of the main refractive indices of the single crystal are calculated from a system of the following equations: pa-ni A / da; pz - P2 A / di; pz-ni A / d2, adi ni, 02, pz - the values of the main refractive indices of the single crystal, I answer the directions of the three main axes of its optical indicatrix; di, d2, az are the periods of interference fringes in the interference patterns of light scattered by a sample when the sample of a single crystal is irradiated along the main axes of the optical indicatrix; A is the wavelength of light irradiating a sample of a single crystal.