RU2682605C1 - Circular dichroism dichrographs calibration device - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: invention relates to the optical devices, simulating having circular dichroism (CD) substance, with possibility to control the specified effect value in a wide range of values at the selected wavelength, which preserves the light beam movement strictly along the optical axis during the calibration process. Circular dichroism dichrographs calibration device contains linear polarizer, phase plate, providing the path difference between ordinary and extraordinary rays (2m+1)⋅λ/4). As the polarizer, two isotropic transparent dielectric plates of equal thickness with equal fixed angles of inclination are used α and -α relative to the optical axis and with possibility of their consistent rotation about this axis.

EFFECT: development of the circular dichroism dichrographs calibration device, which maintains the system alignment strictly along the optical axis during the calibration process.

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Description

The invention relates to optical devices that simulate a substance with circular dichroism (CD), used to calibrate circular dichroism dichrographs.

Circular dichroism is one of the effects of optical anisotropy, manifested in the difference in the absorption coefficients of light polarized along the right and left circles. CD spectra, as a rule, contain narrow, well-resolved bands that are individual for each substance; therefore, CD measurements are used quite widely in various fields of science, especially chemistry, medicine, and biophysics. 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 several fractions of a percent of the absorption coefficient in unpolarized light. Establishing a relationship between the readings of instruments and the absolute magnitudes of the effects, the demand for experimenters measuring CD.

In the vast majority of cases, calibration of CD dichrographs is carried out using an optically active substance - a standard, the value of which is known at a certain wavelength. For example, in the application [RF No. 2013123106, IPC G01N 21/00, publ. November 27, 2014] it is proposed to use a polymer optically active material, which is a gel, in which particles of double-stranded nucleic acid molecules are distributed and immobilized, which have a characteristic abnormal circular dichroism with a signal of a certain magnitude when irradiated with circularly polarized radiation at a discrete wavelength in UV spectral range and retaining this characteristic for several months after its manufacture.

Note that the use of reference substances has a number of significant drawbacks. First of all, this is the instability of a given signal value in time and under the influence of various factors (temperature, pressure, humidity, etc.). Secondly, each reference substance is characterized by a limited number of CD peaks and there are no substances with a sufficient number of peaks in a wide spectral range, which leads to the need to have a set of reference substances with characteristic features at different wavelengths. In addition, the magnitude of the effect of the reference substance should be close to the magnitude of the effect of the measured substance, and this is a range of 10 ^-6 -10 ^-1 . Taking into account the instability of chemical solutions, it becomes clear the difficulties that researchers encounter when calibrating CD dichrographs and searching for reference substances.

In the work [Kostyuk, G.K. A device for calibrating a dichrogrof in a wide spectral region / G.K. Kostyuk, E.K. Galanov, M.V. Leikin // Optical-mechanical industry. - 1976. - No. 5. - S. 28-31.] Described an optical device that sets the value of dichroism in a wide range of wavelengths and does not require a specific chemical compound. The device is a combination of a quarter-wave plate and a linear polarizer. The disadvantage of this device is the almost 100% linear polarization of the beam at the output, since when setting small CD values, the light becomes elliptically polarized with a large axis ratio, which introduces distortions in the measurement results, since there are elements sensitive to linear in the general scheme of spectrometers for measuring CD polarization. This limits the wide practical use of this device.

In the patent [RF No. 2590344, IPC G01N 21/19, G01M 11/02, publ. 07/10/2016.], A device for calibrating dichrographs, which imitates a substance with circular dichroism, with the ability to control the magnitude of the specified effect in a wide range of values at a selected wavelength, is described. This device contains an isotropic transparent dielectric plate and a phase plate, thickness d = ((2m + 1) λ / 4) / (n _o -n _e ), where n _o , n _e are the refractive indices of an ordinary and extraordinary wave, m is the order plates, λ is the wavelength. An isotropic transparent dielectric plate has the ability to rotate 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 described device allows you to simulate a substance with a CD in a wide range of values without the use of real optically active substances, and with a complete absence of linear polarization of light at the exit of the device. The disadvantages of this device include, firstly, the quadratic dependence of the obtained CD signal on the angle of rotation of the isotropic plate, and secondly, the displacement of the beam from the optical axis due to the laws of refraction when light passes through the inclined dielectric plate, which is extremely undesirable in any optical scheme.

The difficulties described above for the use of reference substances and optical devices that change state or cause a shift in the light beam make it necessary to create devices for calibrating CD dichrographs that specify the exact value of the CD signal with a precisely time-stable, without the indicated drawbacks.

The closest in technical solution to the proposed device is an optical device for calibrating dichrographs of circular dichroism, patent [RF No. 2629660, IPC G01N 21/01, publ. 08/30/2017.].

The device contains a phase plate providing a travel 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. The partial linear polarization of light obtained by the passage of light through an isotropic dielectric plate is converted to a partial circular polarization when a phase plate passes, which is identical to the passage of light through an optically active substance with CD. The dependence of the KD value on the rotation angle of the isotropic dielectric plate in the working range of values is linear. In this device, when a beam passes through an isotropic transparent dielectric plate with a fixed angle of inclination relative to the direction of light propagation, due to Snell's law

where α is the angle of incidence, β is the angle of reflection, n ₁ and n ₂ are the refractive indices of the medium from which the beam falls on the interface and into which, accordingly, the beam deviates (Δx) from the optical axis (Fig. 1). Knowing the refractive index and the thickness of the dielectric plate, from simple geometric considerations, we can estimate the deflection of the beam by the following formula:

. Так, в случае использования изотропной прозрачной пластины из плавленого кварца, для которой на длине волны λ=550 нм, показатель преломления n=1.46, при фиксированном угле наклона относительно оптической оси, равном углу Брюстера, α=α_Б=55.6°, и толщине h=1 мм, рассчитанный угол β равен 34.41°, а отклонение составляет 0.44 мм. Таким образом, при вращении пластины, центр светового луча будет описывать окружность радиусом, равным отклонению, вокруг оптической оси, (фиг. 2). Описанное смещение луча от оптической оси крайне нежелательно в любой оптической схеме, особенно если размеры поперечного сечения луча сопоставимы с размерами рабочих элементов других устройств, располагающихся на оптической оси, например, щелью монохроматора.where h is the width of the inclined plate, and the angle β according to formula (1) will be equal to

. So, in the case of using an isotropic transparent fused silica plate, for which at a wavelength of λ = 550 nm, the refractive index is n = 1.46, with a fixed angle of inclination relative to the optical axis equal to the Brewster angle, α = α _B = 55.6 °, and the thickness h = 1 mm, the calculated angle β is 34.41 °, and the deviation is 0.44 mm. Thus, when the plate rotates, the center of the light beam will describe a circle with a radius equal to the deviation around the optical axis (Fig. 2). The described beam displacement from the optical axis is extremely undesirable in any optical scheme, especially if the dimensions of the beam cross section are comparable with the sizes of the working elements of other devices located on the optical axis, for example, a slit of a monochromator.

The disadvantage of the prototype is the deviation of the light beam from the optical axis as a result of the passage of the inclined isotropic dielectric plate, leading to the displacement of the light beam from the optical axis. To eliminate this drawback, we offer a new device.

The technical result of the invention is to provide a device for calibrating dichrographs of circular dichroism, preserving the alignment of the system strictly along the optical axis during the calibration process.

The technical result is achieved by the fact that in the device for calibrating circular dichroism dichrographs containing a linear polarizer, a phase plate providing a path difference between the ordinary and extraordinary rays (2m + 1) ⋅λ / 4), the new fact is that two polarizers are used isotropic transparent dielectric plates of equal thicknesses with equal fixed angles of inclination α and -α relative to the optical axis and with the possibility of their consistent rotation about this axis.

The differences between the claimed device and the prototype are that in the claimed invention, as a linear polarizer, a combination of two isotropic transparent dielectric plates of equal thickness with equal fixed angles of inclination α and -α relative to the optical axis and with the possibility of their consistent rotation about this axis is used.

The above signs allow us to conclude that the proposed technical solution meets the criterion of "novelty."

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

In FIG. 1 shows a diagram of the passage of a light beam in a prototype through an isotropic dielectric plate located at an angle to the direction of light propagation. In FIG. Figure 2 shows the shift of the transverse profile of the intensity of the light wave (for example, a Gaussian distribution) relative to the optical axis. In FIG. Figure 3 shows a diagram of a device for setting circular dichroism, in which the alignment of the light beam and the optical axis is maintained during the calibration of the dichrograph. In FIG. Figure 4 shows the dependence of the reflection coefficients of s and p waves on the angle of incidence for silica glass for λ = 550 nm.

The device (Fig. 3) contains a pair of isotropic transparent dielectric plates (1) and (2) of equal thickness with equal fixed angles of inclination relative to the optical axis and a phase plate (3) cut from a uniaxial crystal parallel to its optical axis, for which the condition ( n _o -n _e ) d = (2m + 1) λ / 4, where m is any integer or zero, n _o and n _{e are} the refractive indices of the rays whose electrical vibrations occur along the optical axis of the crystal (ordinary beam) and perpendicular to the axis (extraordinary ray), d - plate thickness. The optical axes lie in the plane of the phase plate, perpendicular to the direction of light propagation, with zero azimuth. The passage of the phase plate introduces a phase difference for two linear polarizations. The first isotropic transparent dielectric plate in the path of the beam is located at a fixed angle (angle α) relative to the direction of light propagation, the second isotropic transparent dielectric plate in the path of the beam is located at a fixed angle (angle -α) relative to the direction light propagation, and this pair of plates, has the ability to rotate about the optical axis (angle ϕ). The same fixed angle of inclination, equal thickness of isotropic plates and their simultaneous rotation ensures that the light beam travels strictly along the optical axis at the output of the device, while providing a certain partial linear polarization of the transmitted beam. After inclined plates, already partially linearly polarized light enters the phase plate, and if the rotation angle of the plates is ϕ = 0 °, then the angle between the plane of linear polarization of light and the main directions of the phase plate is 0 ° and 90 °. The linearly polarized component of light, in this case, will pass through the phase plate without change and the output will receive exactly the same partially linearly polarized light. If the angle ϕ is nonzero, then the linearly polarized component of the light incident on the phase plate will have a projection onto the two optical axes of the crystal of the phase plate, which will provide an additional phase difference of linear polarizations and, accordingly, the ellipticity of the considered component at the exit from it. Thus, the partial linear polarization of light obtained by the passage of light through a system of two matched inclined isotropic dielectric plates will be converted into partial circular polarization at the output of the device, which is identical to the passage of light through an optically active substance with CD, while the proposed scheme will not have a bias output beam relative to the incident.

The device operates as follows:

Natural (non-polarized) light can be represented as the sum of two linearly polarized waves of equal intensity, in which the oscillations occur, respectively, in 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 normal to the surface. With normal incidence of monochromatic light 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 in the transmitted s-wave, which leads to a partial linear polarization of the transmitted and reflected waves.

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

коэффициент r_p будет равен нулю и, соответственно, степень поляризации преломленного и отраженного лучей будет максимальна (фиг. 4). Это условие выполняется когда (α+β)=π/2.where r _S is the reflection coefficient of the s-wave; r _p is the reflection coefficient of the p-wave; α is the angle of incidence of the light wave, in our case it is the angle of inclination of an isotropic glass plate; n is the refractive index; ΔK is the degree of polarization of the transmitted beam. At the angle of incidence of Brewster, the so-called angle of total polarization:

the coefficient r _p will be zero and, accordingly, the degree of polarization of the refracted and reflected rays will be maximum (Fig. 4). This condition is satisfied when (α + β) = π / 2.

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

Then, falling on the second plate of the same thickness as the first one, located at an angle (-α) to the optical axis, again due to Snell's law, the light beam will be refracted and the beam will shift in exactly the opposite direction, as a result of which the light beam will return on the optical axis (Fig. 3). 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 path of the light beam strictly along the optical axis during the calibration of the dichrograph. Since light, when passing through such a polarizer, crosses four inclined faces of isotropic transparent plates, the degree of linear polarization of the transmitted light should be calculated by the formula

After light passes through a matched pair of transparent isotropic dielectric plates, partially linearly polarized light enters the phase plate. At an angle 0 <ϕ <90, the light splits into two components, the electric vibrations of which occur along the optical axis of the crystal (ordinary beam) and perpendicular to the axis (extraordinary beam), and a travel difference appears, defined for the quarter wave plate by the ratio (n _o - n _e ) d = (2m + 1) λ / 4, where m is any integer or zero, n _o and n _{e are} the refractive indices of the ordinary and extraordinary rays. When partially linearly polarized light passes through the phase plate, the non-polarized component of the beam will not change, and the linearly polarized component will be converted to elliptical. By changing the angle ϕ from zero to ϕ = 45 °, it is possible to set the ellipticity (or CD) from zero (at ϕ = 0 °) to a certain maximum (at ϕ = 45 °) determined by the refractive index and the angle of inclination of the dielectric plate (at angle α = α _B , the maximum possible value is obtained).

Thus, as a result of the passage of light through the device, a signal is obtained that is identical to the signal after the light passes through an optically active substance with a CD, while the coordinated arrangement and rotation of two plates of an isotropic dielectric of the same thickness preserves the light beam strictly along the optical axis during the calibration of the dichrograph.

To confirm the identity of the circular polarization of light created by the proposed device, and circular polarization arising in a real optically active medium, we will describe the behavior of light using Mueller matrices [Sherkliff, U. Polarized light / U. Sherkliff. // - Moscow: Publishing house Mir per. from English., 1965. - 264 p.].

In the Müller matrix representation, a luminous flux of any polarization can be associated with a single Stokes vector column:

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

The expressions describing any optical device (polarizer, phase plate, etc.) are a 4 × 4 Mueller matrix. Concrete matrices characterize not only the device itself, but also its orientation (azimuth). To obtain the Stokes vector characterizing the luminous 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 with the following conditions: the vector representing the incident light is written from the right, and the matrices corresponding to various devices are arranged sequentially from right to left.

We write the Muller matrices describing the passage of natural light through a substance with CD and the passage of light through the proposed device, consisting of an inclined isotropic transparent dielectric plate with an arbitrary azimuth and a phase quarter of a wave plate with zero azimuth.

Case 1. Natural light passes through a substance with CD

- коэффициенты пропускания+, и - круговых волн.Where

- transmittance +, and - circular waves.

I - Stokes vector of incident unpolarized light of unit intensity

II - A substance with circular dichroism (the concept of azimuth does not make sense)

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

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

- коэффициенты пропускания+, и - круговых волн, n=cos2ϕ, m=sin2ϕ, ϕ - угол вращения однородных пластин диэлектрика.Where

are the transmission coefficients of +, and - of circular waves, n = cos2ϕ, m = sin2ϕ, ϕ is the rotation angle of homogeneous dielectric plates.

IV - Forward rotation matrix with arbitrary azimuth

V - Inclined isotropic plate with an azimuth of 0 ° relative to the horizon (device with linear dichroism)

VI - The matrix of the reverse rotation with an arbitrary azimuth

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

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

The following is a sequential multiplication of matrices describing the proposed device.

Comparing the results obtained after the passage of light through a substance with 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 in both cases are identical. A comparison of the matrix elements responsible for circular polarization shows that Δ = mb, and since m = sin2ϕ, and b = K ₊ - K_, this Δ element depends on the sine of the rotation angle of the dielectric plate.

значит зависимость величины КД, получаемой прохождением света через предлагаемое устройство, будет описываться формулой синуса угла поворота изотропных пластин.By definition, circular dichroism is

means the dependence of the magnitude 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 an inclined isotropic transparent plate, we take a fused silica plate, for which the refractive index for the wavelength λ = 550 nm is n = 1.46. When the angle of inclination of the plate equal to the Brewster angle (for fused silica (α _B = 55.6 °), the coefficient r _p will be zero and, accordingly, the degree of linear polarization of the transmitted beam calculated by formula (3) will reach a maximum and will be equal to ≈7% and after passing through four faces of two isotropic plates (formula (6)) ≈ 25%. The use of two plates of equal thickness with an equal fixed angle of inclination not only preserves the alignment of the light beam and the optical axis, but also allows you to increase the range of the set values of dichroism. when fixed Given the angle of inclination of the fused silica plates equal to the Brewster angle, by rotating a matched pair of plates relative to the optical axis (by changing the angle ϕ), the value of “pseudodichroism” can be set in the range from 0 to 0.25.

For a wavelength of λ = 550 nm, the minimum thickness of a phase plate made of crystalline quartz will be 15.3 μm (since n _o = 1.545 n _e = 1.554 and (n _o -n _e ) d = ((2m + 1) λ / 4 ). The working region of the rotation angles φ is determined from formula (7). The actual values of the CD of most substances are in the range Δ≤10 ^-3 . Given that the value of the angle of inclination of the plates is fixed at the Brewster angle, the value is b = 0.25. Accordingly,

2ϕ≤0.2292

ϕ≤0.114rad. (6.57 °).

The working region of the angles of rotation of a matched pair of isotropic does not exceed 0.114 rad (~ 6.5 °). The difference from linearity in this range of angles does not exceed 1%.

Claims (1)

A device for calibrating circular dichroism dichrographs containing a linear polarizer, a phase plate providing a path difference between ordinary and extraordinary rays (2m + 1) ⋅λ / 4), characterized in that two isotropic transparent dielectric plates of equal thickness with equal thicknesses are used fixed angles of inclination α and -α relative to the optical axis and with the possibility of their consistent rotation about this axis.

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