RU2377542C1 - Device for determining optical absorption losses in thin films - Google Patents

Abstract

FIELD: physics; optics.

SUBSTANCE: invention relates to measurement techniques. The device includes a laser resonator with an active element, laser pumping and control apparatus, apparatus for measuring energy or power of the laser radiation and a plane-parallel plate which is placed in the resonator with possibility of displacement perpendicular its axis relative radiation which is incident on its inclined face with an angle θ₀ to the base. On the rear surface of the plane-parallel plate the test thin film is deposited in form of a wedge with vertex angle β, with minimum d and maximum 3d thickness.

EFFECT: simple design and increased accuracy of determining absorption losses in thin films.

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Description

The invention relates to measuring technique, namely to the study of materials using optical means, and can be used for accurate measurement of optical absorption losses in thin films.

A device is known that contains a radiation source, a mirror system, a movable reflector, at two positions of which a measuring and reference channels, a sample holder and a radiation receiver are formed (Bukhstab MM Measurement of small optical losses. L .: Energoatomizdat, 1988, p.32 [ one]). A disadvantage of the known device is the low measurement accuracy and bulkiness of the device. The light beam first falls on the auxiliary mirror and, only reflected from it, hits the sample, therefore, when probing twice, a large number of mirrors are required, and therefore, the adjustment of the device becomes more complicated and its dimensions increase.

A device is known for measuring spectral transmittance and reflection, which includes a radiation source, a mirror system, movable reflectors forming the reference and measuring channels, sample holder, lens and radiation receiver. By measuring the ratio of the luminous flux during single or multiple sounding of the sample to the luminous flux incident on the sample, the value (square, cube) of the absolute reflection and transmission coefficient is determined (SU 1229661 [2]). A disadvantage of the known device is the impossibility of high-precision measurement of the reflection and transmission coefficients of samples with a small area during multiple sounding, the presence of a large number of mirrors, the difficulty in ensuring equal losses due to reflection in the corresponding channels, the large dimensions of the device and the complexity of its adjustment.

Closest to the technical nature of the proposed device is a device for measuring spectral transmittance and reflection, known from RU 2018112 [3]. The device comprises a sample holder and optically coupled radiation source, a mirror system, a movable reflector, a lens and a radiation receiver. The mirror system is made in the form of a straight hexagonal prism with three reflecting side faces, two of which are located at an angle to each other, and the third face, enclosed between them from the side of the corner apex, is perpendicular to two transparent faces, parallel to the plane of the sample and the movable reflector, consisting of two or more mirrors mounted rotatably 180 ° relative to an axis lying in the plane of installation of the sample. In this case, the prism is symmetric with respect to the plane perpendicular to the third reflective face and is mounted with the possibility of translational movement perpendicular to the plane of installation of the sample. In addition, a compensator is introduced into the device in the form of a rotating plane-parallel plate with an axis of rotation passing through the center of symmetry of the plate and perpendicular to the plane of incidence of the light beam onto the sample mounted along the light beam between the prism and the lens.

A disadvantage of the known device is the presence of a large number of mirrors, the difficulty in ensuring equal losses due to reflection in the corresponding channels, the large dimensions of the device and the complexity of its adjustment.

The inventive device is aimed at simplifying the design and increasing absorption losses in thin films.

This result is achieved by the fact that the device for determining optical absorption losses in thin films includes a laser resonator with an active element, means for pumping and controlling it, means for measuring the energy or power of laser radiation and installed in the resonator with the possibility of its movement perpendicular to its axis relative to the incident axis its inclined beam end plane-parallel plate with the angle θ ₀ at the base, wherein a portion of the back surface of a plane-parallel plate is applied and pytuemaya thin film in the form of a wedge with an angle at its apex β, the minimum and maximum thickness d 3d, when this value θ _0, d, and β are given by:

θ ₀ + arcsin ((cos θ ₀ ) / n ₁ ) ≥arcsin (1 / n ₁ ),

Where

n ₁ is the refractive index of the plate;

n ₂ is the refractive index of the test film;

λ is the laser wavelength, m;

θ ₀ is the angle between the plane of the plane-parallel plate and its inclined end, rad;

θ is the angle of incidence of radiation on the film, rad;

a is the diameter of the laser beam, m;

β is the angle at the top of the wedge of the test film, rad;

d is the thickness of the test film, m

The technical result of the proposed device is the simplification of its design, achieved due to the fact that it uses a simple, easily manufactured measuring element (plane-parallel plate), on which, using almost any of the known methods of film deposition (thermal, electron beam, etc. .), you can apply a measured film. In addition to the named element, the device does not use any other auxiliary elements (standards, plates, mirrors, etc.), and, therefore, do not perform additional measurements on these elements. The use of the proposed device for determining the absorption of thin films of a laser resonator eliminates the use of complex signal amplification and detection systems in the device. This is achieved due to the fact that the level of the signal emerging from the laser resonator after repeated reflections from the film remains high due to the high brightness of the radiation and high quality factor of the resonator. Since the divergence of the laser radiation is small, there is also no need for complex collecting and focusing optics. These features of the proposed device simplify its design.

Improving the accuracy of measuring the small absorption of thin films in the proposed device is achieved due to the fact that the measured film is located in the laser cavity, it is deposited on a part of the face of the total internal reflection of a plane-parallel plate. Such features of the proposed device provide when measuring the absorption of a thin film to achieve maximum absorption of laser radiation in the film. This is because when the resonator is tuned into resonance, the beam will be reflected many times and pass through the film (due to the phenomenon of impaired total internal reflection), and this provides a multiple increase in the total path along which the radiation is absorbed by the film and, therefore, an increase in the total absorption of radiation by the film. When measuring the energy or power of laser radiation, large relative changes in energy or power will be recorded during measurements, which will increase the accuracy of their measurement and, as a result (see the formula for calculating the absorption of the film below), increase the accuracy of measuring low absorption of the film. In addition, during measurements, the high quality factor of the laser resonator is maintained, since the film is deposited on a part of the face of the total internal reflection of the plate, and, therefore, when a ray hits a portion of this face without a film, no losses are introduced into the resonator during measurements (in fact, at this moment measurement of losses from the reference). But even when the beam hits the film, when reflected from its second surface, calculations show that total internal reflection also occurs, and, therefore, in addition to losses in the film, other losses are not introduced into the resonator during measurements. The film introduces into the laser cavity two types of losses: small absorption losses in the film (fundamental for the film material, defects in it, etc.) and losses caused by the violation of the total internal reflection by the film. The application of a thin film with a refractive index of n2 (n ₁ ≠ n ₂ ) violates the total internal reflection at the "plate - film" boundary. In this case, the energy reflection, transmission, and absorption coefficients R, T, and A can be calculated from the following relations (see M. Born, E. Wolf. Fundamentals of Optics. M., 1970)

Where

r _{i + 1} and t _{i + 1} - Fresnel coefficients at the interfaces between the media with refractive indices n _i and n _{i + 1} ;

n _i is the refractive index of the material of the medium or film;

d _i is the film thickness;

θ _i is the angle of incidence or refraction on / in the film;

for i = 2

n ₂ is the refractive index of the film.

-ik₂, и k₂ - искомый показатель поглощения материала пленки может быть вычислен из (1) - (8).Since the film absorbs, then n ₂ =

-ik ₂ , and k ₂ - the desired absorption coefficient of the film material can be calculated from (1) - (8).

The system of equations (1) - (8) allows us to determine the absorption characteristics of the film from a number of measured values of R, or T, or A. So, they can be used to determine the absorption of a film of a given thickness or the absorption coefficient of the film material. But the exact solutions for this system in general are extremely difficult to find. As a rule, the curves of the dependences of absorption losses on the film thickness are preliminarily plotted for a certain range of absorption coefficient values. Then, by comparing the measured and calculated values of the absorption loss using numerical methods, the absorption characteristics are determined: either the absorption of the film of a given thickness, or the absorption coefficient of the film material. An example of the dependence of absorption losses on the film thickness for an absorbing TiO ₂ film (n ₂ = 2,3) and a glass air defense prism (n ₁ = 1,5) for radiation of a helium-neon gas laser (λ = 0.632 μm) is shown in FIG. 2.

It is seen that these dependences are characterized by the presence of maxima of the total absorption (A _a ) of the film for a number of film thicknesses. From the theory (see M. Born, E. Wolf. "Fundamentals of Optics", M., 1970) it is known that the presence of such maxima, their position and magnitude are explained by the interference of the rays repeatedly reflected and refracted from the film boundaries into which the incident radiation to the film, provided that the output boundary of the film is the surface of total internal reflection. It is clear that a high accuracy in determining the absorption loss in the film will be observed for maximums on the dependence of the absorption loss for a given range of film thicknesses, since in this case the largest relative changes in the measured energy or radiation power coming out of the resonator will be recorded. In the proposed method, the maxima depending on the absorption loss on the film thickness for film thicknesses not exceeding λ can be detected by scanning a wedge-shaped film from one end of the wedge (film thickness equal to d) to the other (film thickness equal to 3d).

The minimum (d) and maximum (3d) film thickness were selected based on the numerical analysis of equations (6) - (13). It was found that for small film thicknesses (d <λ) and small film absorption levels (k <10 ^-5 ), the absorption maximum lies in the thickness range determined by the condition 0.25≥δ _i ≤0.75 (see 13). From this condition, the minimum film wedge thickness d is determined as follows:

то естьThe choice of angle β is due to the need to limit the maximum phase incursion on the diameter of the laser beam when it is inclined to fall onto the film by a value equal to

i.e

A numerical simulation of equations (6) - (13) shows that this requirement ensures the insignificant effect of phase incursion on the diameter of the laser beam when it is inclined to fall onto the film on the accuracy of measuring the small absorption of the film by the proposed method.

The essence of the claimed invention is illustrated by an example of its implementation and drawings. Figure 1 presents a schematic diagram of a device; figure 2 is an example of the dependence of the loss of absorption on the film thickness for an absorbing film TiO ₂ ; figure 3 is an example of the dependence of the pump power on the absorption loss in the film.

A device for determining optical absorption losses in thin films includes a laser resonator with an active element 1, a blind mirror 2 and a translucent mirror 3. The laser contains means for pumping and controlling it (not shown due to their popularity). The device is equipped with means for measuring the energy or power of laser radiation, which can be selected from among the known (not shown in the drawing). A plane-parallel plate 4 with an angle at the base θ _{0 is} installed in the resonator with the possibility of its movement perpendicular to the radiation beam incident on its inclined end face. A test thin film 5 is applied on a part of the surface of a plane-parallel plate in the form of a wedge with an angle β at its apex, with a minimum thickness d and a maximum 3d.

The device operates as follows.

First option. The means for pumping the active element 1 are turned on and, using the controls, they achieve stable generation of radiation through a translucent mirror 3. After that, a plane-parallel plate 4 is placed on the surface of the laser radiation inside the cavity formed by mirrors 2 and 3, on which surface 5 a wedge. First, the plate 4 is installed so that after the laser beam enters it, it would be reflected from its surface on which the film is applied, but from its part free of the film. After that, stable generation is again achieved with the minimum electric power supplied to the pumping means. Then, the plate 4 is displaced perpendicular to the beam of radiation incident on its inclined end (shown in the drawing by arrows), so that the radiation falls inside the plate at one of the edges deposited on the outside of the film, and determine the minimum electric power supplied to the pumping means necessary for sustainable generation. And thus, the procedure is repeated so that the reflection of the beam inside the plate occurs at points with different film thicknesses at the reflection point, and the maximum of the difference required for pumping power in case of reflection from the free surface and at one of the points with the deposited film is fixed. Since the magnitude of the losses in the resonator is directly related to the energy or power supplied to the pumping means to ensure stable generation, the change in the supplied energy can also be used to judge the change in the magnitude of the losses, and hence the magnitude of the absorption losses in thin films. In the general case, the absorption coefficient of a film cannot be expressed explicitly in terms of measured values. Therefore, it is necessary to first plot the dependences of the pump power on film losses. An example of such a dependence is shown in figure 3. Then, by comparing the measured and calculated values of the pump power, the values of the loss coefficient in the film material are determined using numerical methods.

The second option. They turn on the pumping means of the active element 1 and, using the controls, achieve stable generation of radiation through a translucent mirror 3. After that, using known means, for example IMO-2H (Russia) or OPHIR-32 (USA), measure the energy or power of laser radiation. Then, a plane-parallel plate 4 is placed inside the resonator formed by mirrors 2 and 3, on one of the surfaces of which the test film 5 is applied in the form of a wedge. First, the plate 4 is installed so that after the laser beam enters it, it would be reflected from its surface on which the film is applied, but from its part free of the film. Measure the energy or power of the laser radiation and record the measured values.

Then, the plate 4 is displaced perpendicular to the beam of radiation incident on its inclined end (shown in the drawing by arrows) so that the radiation enters the plate at one of the edges of the film deposited on the outside, and the laser radiation power or energy is determined. And thus, the procedure is repeated so that the reflection of the beam inside the plate occurs at points with different film thicknesses at the reflection point, and the maximum difference between the measured values of energy or power is recorded. Since a decrease in the energy or power of the laser radiation at the exit from the resonator indicates losses introduced into it, then the magnitude of the maximum difference between the measured power or energy of radiation at the exit of the resonator for surface areas of a plane-parallel plate with a film and without it can be used to judge the value of losses absorption in thin films. For this, the dependences of the absorption losses on the film thickness for a certain range of absorption coefficients are preliminarily constructed using equations (6) - (13). The obtained dependences are normalized for use in the resonator used according to a number of loss standards, measuring the radiation power or energy at the exit from the resonator when a laser beam is incident on a surface portion of a plane-parallel plate without a film. For example, pure plane-parallel glass plates can be used as loss standards. Then, by comparing the measured and previously calculated values of the absorption loss, the absorption loss values for the film of a given thickness or absorption coefficient of the film material are determined.

Claims (1)

A device for determining optical absorption losses in thin films, including a laser resonator with an active element, means for pumping and controlling it, means for measuring the energy or power of laser radiation and installed in the resonator with the ability to move it perpendicular to its axis relative to the incident radiation beam incident on its end face a plane-parallel plate with the angle θ ₀ at the base, wherein a portion of the back surface of a plane-parallel plate is applied under test thin film in the form of a wedge glom at its apex β, the minimum and maximum thickness d 3d, when this value θ _0, d, and β are given by:
θ ₀ + arcsin ((cos θ ₀ ) / n ₁ ) ≥arcsin (1 / n ₁ ),

where n ₁ is the refractive index of the plate;
n ₂ is the refractive index of the test film;
λ is the laser wavelength, m;
θ ₀ is the angle between the plane of the plane-parallel plate and its inclined end, rad;
θ ₂ - angle of incidence of radiation on the film, rad;
a is the diameter of the laser beam, m;
d is the thickness of the test film, m

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