RU2801066C1 - Device for calibrating circular dichroism dichrographs - Google Patents

Abstract

FIELD: optical instrumentation.

SUBSTANCE: invention relates to a device for calibrating circular dichroism dichrographs. The device includes a partially linear polarizer consisting of a pair of isotropic dielectric plates of equal thicknesses with fixed tilt angles ofϕ and ϕ+90° relative to the optical axis and a phase plate that provides a path difference between the ordinary and extraordinary beams (2m+1)⋅λ/4. In addition, the device additionally contains a second partially linear polarizer located between the first partially linear polarizer and the phase plate, and consisting of a pair of isotropic dielectric plates of equal thicknesses with fixed tilt angles of ϕ and ϕ+90° relative to the optical axis. The first polarizer in the path of the beam has the ability to rotate about the optical axis, and the second polarizer is stationary. The fast axis of the phase plate is oriented at an angle of 45° to the plane of partial linear polarization of the light beam of the isotropic plates of the second partially linear polarizer.

EFFECT: providing a device for calibrating circular dichroism dichrographs in which there is no partial linear polarization of light introduced by the elements of the device.

1 cl, 6 dwg

Description

The invention relates to optical devices simulating a substance having circular dichroism (CD) used for calibrating circular dichroism dichrographs.

Circular dichroism is one of the effects of optical anisotropy, which manifests itself in the difference between the absorption coefficients of light polarized in the right and left circles. As a rule, CD spectra contain narrow, well-resolvable bands that are individual for each substance; therefore, CD measurements are used quite widely in various fields of science, especially chemistry, medicine, biophysics, and physics. The CD method is a highly sensitive method for studying various substances, and an important aspect of the measurement is the accuracy of signal calibration, since the magnitude of the effect usually does not exceed a few fractions of a percent of the value of the absorption coefficient in unpolarized light. Establishing an exact unambiguous relationship between instrument readings and the absolute magnitudes of effects is a highly demanded necessity for experimenters who measure CD.

The use of reference optically active substances for the calibration of CD dichrographs, the values of which are known, has encountered a number of difficulties. First, a certain substance is characterized by a limited number of CD peaks, so it is necessary to have a set of reference substances with characteristic features at different wavelengths. Secondly, the instability of the CA value of solutions of chemical substances, due to long-term storage and the influence of various external factors (temperature, pressure, humidity, etc.).

In the application [RF No. 2013123106, IPC G01N21/00, publ. November 27, 2014] as a reference substance, it is proposed to use a polymer gel with particles of double-stranded nucleic acid molecules, which, when irradiated with circularly polarized radiation at a discrete wavelength in the UV spectral range, has a certain CD value, but retains this characteristic only for several months after its manufacturing. In [Gusev, V.M. DNA nanostructures for testing and calibration of circular dichroism spectrometers / V.M. Gusev, O.N. Kompanets, M.A. Pavlov, D.P. Chulkov, Yu.M. Evdokimov, S.G. Skuridin // Science-intensive technologies. - 2013. - No. 4. – P. 68-75.] the possibility of using a polymer material based on DNA nanostructures included in polyethylene oxide hydrogels as a standard for calibrating CD dichrographs in a wide wavelength range is considered, but the proposed material retains its characteristics only for a year.

The first constructive device (not a substance) for calibration in a wide range of wavelengths with the ability to control the magnitude and sign of the effect was proposed in [Kostyuk, G.K. A device for calibrating a dichrograph in a wide range of the spectrum / G.K. Kostyuk, E.K. Galanov, M.V. Leikin // Optical-mechanical industry. - 1976. - No. 5. - S.28-31.]. The device is a combination of a quarter-wave plate and a linear polarizer. The disadvantage of the device is almost 100% linear polarization of the beam at the output, since when small values of the CD are set, the light becomes elliptically polarized with a large ratio of axes, which introduces distortions into the measurement results, since in the general scheme of spectrometers for measuring CD there are elements that are sensitive to linear polarization (lenses, prisms, receiver, etc.). This limits the wide practical use of this device.

Since then, a number of other devices have been proposed that simulate a substance with circular dichroism, with the ability to control the magnitude of the desired effect over a wide range of values at a selected wavelength. The proposed devices differ in the presence, relative location and rotation of optical elements in order to minimize the shortcomings of the original design.

A device for calibrating dichrographs is known [RF patent No. 2590344, IPC G01N21/19, G01M11/02, publ. 07/10/2016], simulating a substance with circular dichroism. This device contains an isotropic transparent dielectric plate and a phase plate with a thickness d = ((2m+1)λ/4)/( n _o - n _e ), where n _o , n _e are the refractive indices of the ordinary and extraordinary waves, m is the order plates, λ is the wavelength. An isotropic transparent dielectric plate can be rotated about an axis perpendicular to the direction of light propagation and making an angle of 45° with the main directions of the phase plate. The disadvantages of this device include the quadratic dependence of the received CD signal on the angle of rotation of the isotropic plate, and some displacement of the beam path from the optical axis due to the laws of refraction, due to the passage of light through an inclined dielectric plate.

A device is known [RF patent No. 2629660, IPC G01N21/01, G01N21/19, publ. 08/30/2017], containing a phase plate that provides a path difference between ordinary and extraordinary rays, a multiple of λ / 4, and an isotropic transparent dielectric plate with a fixed angle of inclination relative to the direction of light propagation and the possibility of rotation relative to this direction. Partial linear polarization of light obtained by passing light through an isotropic dielectric plate is converted into partial circular polarization when passing through a phase plate, which is identical to the passage of light through an optically active substance with a CD. In this case, the dependence of the CD value on the angle of rotation of the isotropic dielectric plate in the working range of values is linear, but the beam remains shifted from the optical axis, and the partial linear polarization of the beam remains. When the plate rotates, the center of the light beam describes a circle around the optical axis, with a radius equal to the deviation, due to the law of refraction, which is highly undesirable in any optical scheme, especially if the dimensions of the beam cross section are comparable to the dimensions of the working elements of other devices located on the optical axis, for example, the slit of a monochromator.

In one of the variants of the devices proposed in the application [WO2016/057464 A1, G01N21/19, G01N21/21, publ. 04/14/2016], contains two isotropic plates located at the same, but with a different sign of angles to the optical axis, and a phase plate, but the phase plate is located on the path of the light beam to the isotropic plates, and therefore converts circularly polarized light into linearly polarized, and in the listed variety of options for the operation of such a device, the coordinated rotation of these isotropic plates is not provided, but there is only the possibility of changing the angle of their inclination relative to the optical axis and rotation of the phase plate. The operation of such a device as a calibrator is possible only in a certain type of dichrograph, where all the main elements of the dichrograph (monochromator, etc.) are located before the sample. This device, according to its principle of operation, is close to the already mentioned device described in [Kostyuk, G.K. A device for calibrating a dichrograph in a wide range of the spectrum / G. K. Kostyuk, E. K. Galanov, M. V. Leikin // Optical-mechanical industry. - 1976. - No. 5. - P.28–31.], with a common significant drawback - almost 100% linear polarization of the light beam at the output of the device.

A device for calibrating circular dichroism dichrographs is known, adopted as a prototype [RF patent No. 2682605, IPC G01N21/01, G01N21/19, publ. 03/19/2019], containing a linear polarizer and a phase plate that provides a path difference between ordinary and extraordinary rays (2m+1)λ/4, and a combination of two isotropic transparent dielectric plates of equal thickness with fixed angles α and –α is proposed as a linear polarizer inclination relative to the optical axis with the possibility of their coordinated rotation relative to this axis. This device removed the shift of the light beam, its adjustment occurs strictly along the optical axis during the calibration process, and the dependence of the CD value in the working range of values is linear, but there remains a partial linear polarization of the beam at the output of the device.

Note that in the devices listed above (RF patents No. 2590344, No. 2629660, No. 2682605), the value of dichroism is minimized, i.e. it is comparable to the CD value of real substances, and there is no 100% linear polarization of the light beam after passing through all the elements of the devices. The output signal is practically unpolarized light, only a small fraction of which is circularly polarized, which is almost analogous to the passage of light through a real dichroic substance.

The technical result of the invention is the creation of a device for calibrating circular dichroism dichrographs, which is an analogue of a dichroic substance in which there is no partial linear polarization of light introduced by the elements of the device.

The technical result is achieved by the fact that in a device for calibrating circular dichroism dichrographs, containing a partially linear polarizer, consisting of a pair of isotropic dielectric plates of equal thickness transparent in the optical range with fixed tilt angles φ and φ+90° relative to the optical axis, and a phase plate that provides the path difference between the ordinary and extraordinary beams is (2m+1)⋅λ/4, what is new is that the device additionally contains a second partially linear polarizer located between the first partially linear polarizer and the phase plate, and consisting of a pair of isotropic plates transparent in the optical range dielectric of equal thicknesses with fixed tilt angles φ and φ + 90° relative to the optical axis, while the first partially linear polarizer in the path of the beam can rotate relative to the optical axis, the second partially linear polarizer is fixed, and the fast axis of the phase plate is oriented at an angle of 45° to the plane of partial linear polarization of the light beam of the isotropic plates of the second partially linear polarizer.

The difference between the proposed device and the prototype lies in the fact that the device contains two partially linear polarizers, the first partially linear polarizer in the path of the beam can rotate about the optical axis, the second is stationary, and the fast axis of the phase plate is oriented at an angle of 45° to the plane of partial linear polarization of the light beam of the second pair of isotropic plates. The above features allow us to conclude that the proposed technical solution meets the criterion of "novelty".

When studying other known technical solutions in this field of technology, the features that distinguish the claimed invention from the prototype have not been identified, and therefore they provide the claimed technical solution with the criterion of "inventive step".

In FIG. Figure 1 shows a diagram of a device for calibration, containing: 1 and 2 - a partially linear polarizer, each of which consists of a pair of isotropic dielectric plates, 3 - a phase plate. The location of the polarizers corresponds to the maximum linear polarization of the beam at the output and, accordingly, the maximum value of the specified CD. In FIG. 2 shows a diagram of the device, the arrangement of elements in which the same polarization is created at the output of the device, as in the diagram in FIG. 1. In FIG. 3 shows the spatial arrangement of the elements of the device shown in FIG. 1 and FIG. 2. In FIG. 4 shows a diagram of a device for calibration in which the arrangement of elements corresponds to zero dichroism. In FIG. 5 shows the spatial arrangement of the elements of the device shown in FIG. 4. In FIG. 6 shows the dependence of the reflection coefficients of s- and p-waves on the angle of incidence for quartz glass for λ=550 nm.

The device (Fig. 1) contains two partially linear polarizers (1) and (2), each of which consists of a pair of isotropic transparent dielectric plates of equal thickness with fixed tilt angles relative to the optical axis φ and φ+90° and a phase plate (3) , providing a phase shift (2m +1)λ/4, where m is any integer or zero. The optical axes (fast and slow) of the phase plate lie in the plane of the phase plate, perpendicular to the direction of light propagation, and at an angle of 45° to the transmission plane of the second partial polarizer. The first partial linear polarizer on the path of the light beam has the ability to rotate about the axis of light propagation. The location of the elements in Fig. 1 and FIG. 2 are completely identical for the passage of a light wave and provide maximum polarization at the output of the device. The arrangement of elements shown in Fig. 3 shows another extreme case, where the partial linear polarization of the second pair compensates for the partial polarization formed after the passage of the first pair of plates. The adjustable amount of decompensation (rotation of the first partial polarizer in the path of the light beam) will set a partial linear polarization, which, after passing through the phase plate, will turn it into a circular one and create dichroism.

Thus, the passage of light through the proposed device is identical to the passage of light through an optically active substance with a CD, while in the proposed scheme the alignment of the beam strictly along the optical axis will be preserved, and the light beam will not be additionally linearly polarized, that is, it will retain its original state of polarization.

The device works as follows:

Natural (unpolarized) light can be represented as the sum of two linearly polarized waves of equal intensity, in which oscillations occur, respectively, parallel (p-polarization) and perpendicular to the plane of incidence (s-polarization) of light. The plane of incidence is the plane containing the beam and the normal to the surface. When monochromatic light is normally incident on a dielectric, the light remains unpolarized. With oblique incidence of light on an isotropic transparent dielectric plate, the intensity of the p-wave decreases in the reflected beam, and the intensity of the s-wave decreases in the transmitted beam, which leads to partial linear polarization of the transmitted and reflected waves.

The degree of linear polarization ΔK of the transmitted beam depends on the angle of incidence of light on an isotropic plate and its refractive index, and is determined using the Fresnel formulas [Lansberg, G.S. Optics / G.S. Lansberg. - Moscow: Iz-vo Nauka, 1976. - 928 p.]

Where - s-wave reflection coefficient; - p-wave reflection coefficient; α is the angle of incidence of the light wave, in our case it is the angle (90° - φ); n is the refractive index; ΔK is the degree of polarization of the transmitted beam. At Brewster's angle of incidence, the so-called angle of full polarization: coefficient will be equal to zero and, accordingly, the degree of polarization of the refracted and reflected beams will be maximum (Fig. 6). This condition is satisfied when the angle of incidence and the angle of refraction add up to π/2.

When light passes through the first inclined isotropic transparent dielectric plate, the beam will shift relative to the optical axis, due to the Snell law, which can be calculated using formulas (1) and (2).

Further, falling on the second plate of the same thickness as the first, located at an angle to the optical axis (φ + 90 °), again due to Snell's law, the light beam will experience refraction, and exactly the same beam will shift in the opposite direction, as a result of which the beam light will return to the optical axis. Thus, the use of a pair of isotropic transparent dielectric plates of equal thicknesses with equal fixed angles of inclination relative to the optical axis as a polarizer preserves the course of the light beam strictly along the optical axis during the calibration of the dichrograph. Since the light, when passing through such a polarizer, will cross four inclined faces of isotropic transparent plates, the degree of linear polarization of light ( b ) passing through one pair of plates should be calculated by the formula:

In our device, there are two such pairs of plates, that is, a beam of light will cross 8 faces, and its value will be given by the formula:

After that, the light falls on the phase plate, and, after the passage of partially linearly polarized light through the phase plate, the unpolarized component of the beam will not change, and the linearly polarized component will be converted to circular polarization. By changing the angle of rotation of the first pair of plates, it is possible to set the CD value from zero (at 90° between the planes of partial linear polarization of the first and second pairs of plates in Fig. 4) to the maximum value (at 0° between the planes of partial linear polarization of the first and second pairs of plates in Fig. 1 and 2).

Thus, as a result of the passage of light through the device, a signal is obtained that is identical to the signal after the passage of light through an optically active substance with a CD, while there is no partial linear polarization of light introduced by the elements of the device.

To confirm the identity of the circular polarization of light created by the proposed device, and the circular polarization that occurs in a real optically active medium, we will describe the behavior of light using Muller matrices [Shercliffe, W. Polarized light / W. Shercliffe. // - Moscow: Publishing House Mir per. from English, 1965. - 264 p.].

A light flux of any polarization in the Muller matrix representation can be associated with a single column-Stokes vector:

,

four parameters, which corresponds to the time-averaged intensity. The first parameter I is called intensity. The parameters M, C and S are called, respectively, the parameters of the predominant horizontal polarization, the predominant polarization at an angle of 45° and the predominant right-handed circular polarization. The negative value of the parameter corresponds to the predominant orthogonal form of polarization.

The expressions describing any optical device (polarizer, phase plate, etc.) is a 4 x 4 Muller matrix. Specific matrices characterize not only the device itself, but also its orientation (azimuth). To obtain the Stokes vector characterizing the light flux that has passed through a set of devices, it is necessary to multiply the corresponding matrices according to the usual rules of matrix algebra, subject to the following conditions: the vector representing the incident light is written on the right, and the matrices corresponding to different devices are arranged sequentially from right to left.

Let us write the Muller matrices that describe the passage of natural light through a substance with a CD and the passage of light through the proposed device, which consists of an inclined isotropic transparent dielectric plate with an arbitrary azimuth and a phase quarter-wave plate with zero azimuth.

Case 1. Natural light passing through a substance with a CD

III II I

where S = , Δ= , U ⋅= , , - transmission coefficients +, and - circular waves.

I- Stokes vector of incident unpolarized light of unit intensity

II- Muller matrix corresponding to a substance with circular dichroism (the concept of azimuth does not make sense for it)

III- The result of the passage of light through a substance with a CD

Case 2. Natural light passes through an inclined isotropic plate with an arbitrary rotation azimuth (matrices VI, V, IV) and then through a phase quarter-wave plate with zero azimuth (matrix VII)

VII VI V - IV V IV I

where a = , b = , c⋅ = , , - transmission coefficients +, and - circular waves, n = cos2β, m =sin2β, β - angle of rotation of a pair of plates relative to the optical axis Angle β=0 corresponds to the crossed position of the planes of partial polarization of pairs of isotropic plates (Fig.4).

IV- Direct turn matrix with arbitrary azimuth

-IV-Reverse rotation matrix with arbitrary azimuth

V- Pair of inclined isotropic plates with azimuth 0° relative to the horizon (linear dichroism device)

VI- Phase plate, creating a path difference between the ordinary and extraordinary rays of a quarter wavelength (azimuth = 0 °)

VII- The result of the passage of light through the described device.

The Stokes vector of the result obtained after successive multiplication of matrices describing the proposed device (multiplication is performed from right to left, in accordance with the rules of matrix multiplication).

Comparing the results obtained after the passage of light through a substance with a CD and after the passage of light through the proposed device, it can be argued that the elements responsible for the intensity and circular polarization of the wave are identical in both cases.

The first element of the Stokes vector ( ). In him she is negligible equals 1, at the Brewster angle. n =cos2β=1 at β=0, and then the component (the second element of the Stokes vector) will be equal to zero. Thus, the resulting Stokes vector coincides with the expression for the CD in a dichroic substance ( case 1 ).

A comparison of the matrix elements responsible for circular polarization shows that = , and since m =sin2β and b = , ≈1 then the element depends on the sine of the angle of rotation of the dielectric plate.

= b sin2β (8)

By definition, circular dichroism is , it means that the dependence of the value of the CD obtained by the passage of light through the proposed device will be described by the formula for the sine of the angle of rotation of the isotropic plates.

As inclined isotropic transparent plates, plates of fused quartz were taken, in which, for λ=550 nm, the refractive index is n =1.46. At the plate inclination angle equal to the Brewster angle (for fused quartz α _B =55.6°), the coefficient will be equal to zero and, accordingly, the degree of linear polarization of the passing beam, calculated by formula (3), will reach a maximum and will be equal to ≈7%, and after passing eight faces of two pairs of isotropic plates (formula (7) ≈ 44%.

Thus, at a fixed angle of inclination of the fused quartz plates equal to the Brewster angle, rotating the first pair of plates relative to the optical axis, one can set the value of "pseudo-dichroism" in the range from 0 to 0.44.

The working area of rotation angles β is determined from formula (8). The actual values of the CD of most substances are in the range ≤ 10 ^-3 . Considering that the value of the angle of inclination of the plates is fixed at the Brewster angle, the value of b = 0.25. Respectively,

0.44 sin2φ ≤ 0.001, sin2φ≤ , sin2φ≤0.00227, 2φ≤arcsin0.00227

2φ≤0.13 rad.

φ≤0.065 rad. (3.705°).

The working range of rotation angles of a matched isotropic pair does not exceed 0.065 rad (̴ 3.7°). The difference from linearity in this range of angles is much less than 1%.

Claims (1)

A device for calibrating circular dichroism dichrographs, containing a partially linear polarizer, consisting of a pair of isotropic dielectric plates of equal thickness transparent in the optical range with fixed tilt angles ϕ and ϕ+90° relative to the optical axis, and a phase plate providing a path difference between ordinary and extraordinary beams (2m+1)⋅λ/4, characterized in that the device additionally contains a second partially linear polarizer located between the first partially linear polarizer and the phase plate, and consisting of a pair of isotropic dielectric plates transparent in the optical range of equal thicknesses with fixed tilt angles ϕ and ϕ + 90° relative to the optical axis, while the first partially linear polarizer in the beam path can rotate relative to the optical axis, the second partially linear polarizer is stationary, and the fast axis of the phase plate is oriented at an angle of 45° to the plane of partial linear polarization of the light beam of isotropic plates second partially linear polarizer.