RU2442142C2 - Method and device for refraction factor measurement - Google Patents

Abstract

FIELD: optic measurements.

SUBSTANCE: invention is used for refraction factor measurement of the studied medium on the boundary with an optically transparent solid body with an additional capability of monitoring the width of the adsorption layer on the said boundary. The refraction factor is measured through the angle of total internal light reflection from the said border, and the solid body is at least partially composed of layers with alternating refraction factors, i.e. a photonic crystal is used as the solid body. It allows to drastically reduce the reflection ratio from the said boundary at angles less than the angle of total reflection, to increase the rate of refraction factor change near the angle of total reflection and to improve the signal-to-noise ratio.

EFFECT: measurement accuracy of the total reflection angle is increased.

6 cl, 5 dwg

Description

Technical field

The invention relates to the field of measuring concentrations of gas and liquid media through registration of their optical refractive index at the interface of this medium with a photonic crystal with the additional possibility of recording the thickness of the adsorption layer at this interface.

State of the art

Measurement of the refractive indices of liquids and gases is widely used to determine the concentration of impurities in these media, in chromatographic detectors and other sensors. The advantage of concentration sensors based on the measurement of the refractive index is their versatility, since they do not require the analyte to have any specific property, for example, fluorescence, absorption, or electrochemical activity.

One of the first and still popular methods for measuring the refractive index is the Abbe method, in which the refractive index of a medium is determined by measuring the critical angle of total internal reflection (TIR) from a given medium. The desired refractive index is calculated by the formula

where n ₀ is the refractive index of the prism in which the critical air defense angle θ ₀ is measured. If the optical beam falls on the interface between the prism and the medium under study at an angle greater than the critical angle, and n ₀ > n _e , then air defense occurs from the external medium under study and the reflection coefficient is unity: R (θ) = 1. If the optical beam falls on the interface between the prism and the medium under study at an angle less than the critical, then part of the light is refracted into the external medium and the reflection coefficient becomes less than unity. In the idealized case, considering media without absorption and a plane optical wave without angular divergence (i.e., an infinite wave incident on an infinite boundary), we can expect that the derivative ∂R (θ) / ∂θ will be equal to infinity for the critical air defense angle θ ≡θ ₀ and, therefore, the critical angle of air defense and the refractive index of the studied external environment n _e can be determined with absolute accuracy. However, under real conditions, the natural divergence of optical beams (with a transverse diameter D and wavelength λ) is greater than or equal to λ / D ~ 10 ^-4 -10 ^-3 radians and, in addition, all media have a nonzero imaginary part of the refractive index. All this leads to the fact that the derivative ∂R (θ) / ∂θ at the point of air defense is finite (and not too large).

The small value of this derivative at the point of air defense leads to limited accuracy in measuring the critical angle of air defense and, as a result, to limited accuracy in determining the refractive index of the medium under study. This is a disadvantage of all known methods of recording a refractive index based on the Abbe method (see, for example, [1]). Therefore, there is a need to develop a more sensitive method for detecting the critical angle of air defense and, as a consequence, the refractive index.

SUMMARY OF THE INVENTION

The objective of the invention is to increase the sensitivity of the measurement of the refractive index. A specific technical result of the present invention is to increase the accuracy of measuring the critical angle of air defense θ ₀ and, as a result, to increase the accuracy of determining the refractive index in accordance with formula (1).

The problem is solved due to the fact that in the classical Abbe method [1], which includes finding and measuring the critical angle when light is reflected from the interface between the medium under investigation and a transparent prism, the following differences are provided:

A) layers with periodically alternating refractive indices are deposited on the prism face adjacent to the medium under study, i.e. a photonic crystal (FC) is formed.

A causal relationship between the presence of FC near a given interface and an increase in sensitivity is that:

the presence of a photonic crystal near the boundary makes it possible to sharply reduce the reflection coefficient R (θ) at angles θ that are close to the critical angle of the air defense, but smaller (i.e., at θ≤θ ₀ ), while at θ> θ ₀ the reflection coefficient is still equal to 1. Therefore, we get an increase in the steepness of the change in R (θ) near θ ₀ (see Figure 2 below).

This distinctive essential feature is sufficient to achieve the technical result provided by the invention, since an increase in the slope of the reflection coefficient R (θ) near the critical air defense angle (i.e., an increase in the derivative ∂R (θ) / ∂θ at the point θ ₀ ) improves the signal ratio -noise and increases the accuracy of determining the critical angle of air defense and, as a result, increases the accuracy of determining the refractive index of the investigated medium.

In another embodiment of the invention, we also obtain an additional result: we determine the layer thickness at a given interface adsorbed from the medium under study (if such an effect — adsorption from the medium — takes place). To do this, we:

B) we choose the thicknesses of the PC layers so that the PC has at least one waveguide mode at the interface between the PC and the medium under study.

In this embodiment of the invention, a causal relationship between these essential features (A, B) and the achieved technical result is that:

the propagation parameters of optical radiation along a given waveguide near a given boundary depend both on the refractive index of the medium under study and on the thickness of the adsorbed layer at a given interface. Therefore, by registering two parameters — the critical angle and the angle of excitation of the waveguide mode — we can determine two quantities: the refractive index and the thickness of the adsorbate.

That is, in this embodiment of the invention, in addition to the technical result common to the whole invention, to increase the sensitivity of measuring the refractive index (due to an increase in ∂R (θ) / ∂θ), we also obtain an additional result — the layer thickness at the interface adsorbed from the studied substances. This additional result can be used both to further increase the accuracy of determining the refractive index (by introducing appropriate corrections for the presence of an adsorbed layer), and by itself if the thickness of the adsorption layer is of independent interest.

List of drawings

The invention and examples, confirming the possibility of its implementation, are explained below using the drawings, which schematically depict:

Figure 1 - Diagram of a device for measuring the refractive index.

Figure 2 - The calculated values of the reflection coefficients near the critical angle θ ₀ for a prism with FC on the surface (9 - p-polarization) and for a prism without coating (10 - p-polarization; 11 - s-polarization).

Figure 3 - The experimentally observed change in the refractive index of a glucose solution with a change in glucose concentration.

Figure 4 - The experimentally observed change in the refractive index of a liquid upon injection of streptavidin into the liquid.

Figure 5 - The experimentally observed change in the thickness of the surface layer during the deposition of streptavidin on the surface.

Information confirming the possibility of carrying out the invention

For a more complete understanding of the invention and for the purpose of illustrating it, an example of its experimental implementation is given below. However, it should be understood that its various modifications are possible, obvious to a person skilled in the art, not changing the essence of the invention and not going beyond the scope of the invention defined by the attached claims.

To experimentally demonstrate the feasibility of the invention, we applied the following structure to a prism with periodically alternating refractive indices:

prism / (LH) ³ L '/ water, where the layer thicknesses L (consisting of SiO ₂ ) with a low refractive index n ₁ = 1.49 are equal to d ₁ = 154.0 nm, the layer thicknesses H (consisting of Ta ₂ O ₅ ) with a high index the refractions n ₂ = 2.12 are d ₂ = 89.4 nm and the thickness of the last layer L '(consisting of SiO ₂ ) is d ₃ = 638.5 nm.

Such a 7-layer SiO ₂ / Ta ₂ O ₅ FC structure was deposited on a prism made of VK-7 glass with a refractive index n ₀ = 1.52, and the critical angle of air defense was measured in aqueous solutions with a refractive index of the order of n _e = 1.335.

Figure 1 shows a diagram of a device for measuring the refractive index. The optical radiation from the laser 1 is transmitted through the optical fiber 2 (to improve the quality of the beam), and then this optical radiation 3 is focused by the lens 4 onto the base of the transparent prism 5. Then this radiation is reflected from the multilayer structure (one-dimensional photonic crystal) 6 adjacent to the medium 7 , and falls on the optical radiation receiver - diode array 8, which registers the angular distribution of the intensity 9 of the reflected beam.

Theoretically calculated angular dependences of the reflection coefficients are presented in figure 2. Solid curve 9 is the angular distribution of intensity when reflected from a prism, on the base of which a photonic crystal with the parameters indicated above is applied, and dashed curve 10 and dotted curve 11 are the angular dependences of reflection coefficients for p- and s-polarized light from a prism without coating (i.e., as in the classical Abbe method).

As a test demonstration of the sensitivity of our method, we present (Figure 3) a change in the refractive index of a solution of glucose in water at various concentrations. Glucose concentrations are indicated in the drawing. The achieved sensitivity (refractive index noise) is 9 × 10 ^-8 RIU, which corresponds (in concentration units) to 10 ^-4 % (= 10 ^-4 Brix) or 0.76 / µg / mL.

As already mentioned above, an embodiment of the invention is possible in which, in addition to the critical angle of air defense, we also register the excitation angle of one waveguide mode propagating along the surface of the multilayer structure. For the FC parameters indicated above, such a waveguide mode exists near the PC surface and the excitation angle of this mode can be recorded on the same diode array.

Figure 4 and Figure 5 presents the results demonstrating the feasibility of this variant of the invention. The arrow in the figures indicates the moment of injection of streptavidin with a concentration of c _str = 12 µg / mL in an aqueous phosphate-saline buffer solution bordering the surface of the FC, which was previously biotinylated (and thus prepared for binding of streptavidin to the surface). The different kinetics of the change in the refractive index of the solution and the change in the thickness of the adsorbed streptavidin layer on the surface indicate a real separation of volume and surface changes through different recording channels. This additional result (the thickness of the adsorption layer) can be of independent interest, and can be used to further increase the accuracy of determining the refractive index if the test solution is prone to adsorption on the surface, and this correction for adsorption should be taken into account when determining the refractive index of the solution.

Literature

1. G.Meeten. Refractive index errors in the critical-angle and the Brewster-angle methods applied to absorbing and heterogeneous materials. Meas. Sci. Technol., Vol. 8, p. 728-733, 1997.

Claims (6)

1. The method of measuring the optical refractive index of the test medium, which consists in sending optical radiation through a optically transparent solid body having a common boundary between this solid and this test medium, determine the critical angle of incidence of this optical radiation at a given boundary , such that at angles of incidence greater than a given critical angle, this optical radiation is completely reflected back into a given solid, while at angles smaller of a given critical angle, a part of this optical radiation is refracted into a given test medium, the desired refractive index of a given test medium is obtained by multiplying the refractive index of a given solid by the sine of a given critical angle, characterized in that at least a part of a given solid near this interface is from layers with periodically alternating refractive indices, the thickness of these layers is chosen so that a waveguide mode exists at this interface , this optical radiation is used both to determine a given critical angle and to determine a given angle of excitation of a given mode, the values of a given critical angle and a given angle of excitation of a given mode are used to determine both the desired refractive index of a given test medium and the thickness of an adsorption layer on a given interface 2. The method according to claim 1, characterized in that the thickness of these layers with periodically alternating refractive indices is chosen so that a waveguide mode exists at this interface, and the angle of excitation of this mode by this optical radiation is more than an angle of critical angle from this critical angle than 1/500 rad. 3. The method according to claim 1 or 2, characterized in that the optical radiation that is used to determine a given critical angle is polarized orthogonally to the optical radiation that is used to determine a given excitation angle of this mode. 4. A device for measuring the optical refractive index of a test medium, containing an optically transparent solid adjacent to the test medium, an optical radiation source that irradiates the interface between this test medium and this solid, at least one optical radiation detector that records the intensities of this optical radiation at angles of incidence greater than or less than the critical angle of incidence, given optical radiation at a given boundary, such that at angles of incidence I, large of a given critical angle, this optical radiation is completely reflected back into a given solid, while at angles smaller than a given critical angle, part of this optical radiation is refracted into this studied medium, characterized in that at least part of this solid bodies near a given interface consists of layers with periodically alternating refractive indices, these layers with periodically alternating refractive indices have such thicknesses that at this interface waveguide mode exists, at least one active receiver detects the angle of the excitation of the fashion. 5. The device according to claim 4, characterized in that these layers with periodically alternating refractive indices have such thicknesses that there is a waveguide mode at this interface, and the angle of excitation of this mode by this optical radiation is more than an angle from the given value of the critical angle 1/500 rad. 6. The device according to claim 4 or 5, characterized in that the given source of optical radiation is used to produce two beams of optical radiation with mutually orthogonal polarization, the optical beam used to irradiate a given interface between a given test medium and a given solid to determine this critical angle, polarized orthogonally to the optical beam, which irradiate this interface to determine a given angle of excitation of a given mode.

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