A single-pulse broadband circular dichroism absorption measurement apparatus for femtosecond time-resolved spectroscopy

Abstract

The application discloses a kind of single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy, it is related to optical measuring instrument technical field, including: femtosecond laser, output light is divided into pump light and probe light two parts;Pump light path module, including optical parametric amplifier or nonlinear frequency doubling module, polarization modulation device and chopper, for adjusting pump light to circularly polarized light and its frequency is reduced to one half of femtosecond laser frequency;The single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy, before sample probe light is vertical direction or horizontal direction linearly polarized light, in the propagation process will not be influenced by the reflectivity of optical element to vertical polarization light and horizontal polarization light different.The two beams of light separated can realize self-reference and improve signal-to-noise ratio, and avoid using beam splitting optical element and affect the accuracy of polarization state. Avoid using active optical modulation element to reduce the damage risk of equipment and device.

Description

Technical Field

[0001] This invention relates to the field of optical measurement instrument technology, specifically to a single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy. Background Technology

[0002] Circular dichroism spectroscopy is an optical rotation spectrum used to infer the configuration and conformation of asymmetric molecules. It is defined as the difference in absorption of left-handed and right-handed polarized light by a sample. Time-resolved circular dichroism spectroscopy analyzes the dynamics of molecular configuration by measuring the change in circular dichroism absorption over time after photoexcitation.

[0003] The key to time-resolved circular dichroism spectroscopy lies in circular dichroism measurement techniques, which mainly consist of two methods:

[0004] The first method involves measuring and subtracting the absorption of left- and right-handed circularly polarized light separately. To improve measurement efficiency, researchers introduced photoelectric modulators or photoelastic modulators to rapidly switch polarization states. This requires measuring the four spectra before and after pump light excitation (left-hand and right-handed) and calculating the transient circular dichroism absorption. This method has two main problems: first, it is difficult to adapt the modulator to the femtosecond laser; photoelectric modulators have a low damage threshold and are easily damaged, while photoelastic modulators have frequency synchronization issues; second, to eliminate probe light fluctuations, a beam splitter is needed for reference light correction, but the beam splitter has different reflectivities for vertical and horizontal polarized light, causing circularly polarized light to become elliptically polarized light, leading to measurement errors.

[0005] The second method utilizes an ellipsometer to analyze the probe light transmitted through the sample. The simplified setup consists of a quarter-wave plate and a polarizing beam-splitter placed behind the sample. The linearly polarized probe light can be considered as a superposition of left- and right-hand circularly polarized light. After passing through the sample, it is converted into horizontally and vertically linearly polarized light by the quarter-wave plate. This polarizing beam-splitter separates the light, and dual detectors measure the circular dichroism absorption. The advantages of this approach are that it eliminates the need for active modulation devices, avoids synchronization issues, uses a self-referenced design that eliminates the need for beam splitters, and can obtain circular dichroism absorption with a single pulse. However, its drawbacks include the asymmetry in reflection and transmission efficiency of ordinary polarizing cube pairs, leading to unbalanced outputs from the dual detectors when no sample is present, limited contrast, and low linear polarization degree after separation, all of which introduce measurement errors into the system. Summary of the Invention

[0006] The purpose of this invention is to provide a single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy, so as to overcome the shortcomings of the prior art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy, comprising: a femtosecond laser, the output light of which is divided into pump light and probe light; a pump light path module, including an optical parametric amplifier or nonlinear frequency doubling module, a polarization modulation device, and a chopper, for modulating the pump light into circularly polarized light and reducing its frequency to half the frequency of the femtosecond laser; and a probe light path module, including a mechanical optical delay line and a nonlinear white light crystal, wherein the mechanical optical delay line is used to control the time difference between the probe light pulse and the pump light pulse arriving at the sample, and the nonlinear white light crystal... A white light crystal is used to convert the probe light into supercontinuous white light, which is linearly polarized light in the horizontal or vertical direction. A circular dichroism absorption measurement module, located after the sample, is used to obtain circular dichroism absorption information from a single probe light pulse. The circular dichroism absorption measurement module includes, in sequence along the optical path: an achromatic quarter-wave plate, an achromatic Wollaston prism, an achromatic lens, and two fiber-coupled spectrometers. The achromatic Wollaston prism consists of a four-piece, two-set prism structure. The first set of two prisms is used to separate the horizontally and vertically polarized light, and the second set of two prisms is used to compensate for the separation angular dispersion generated by the first set of prisms.

[0008] Preferably, the first and second sets of prisms of the achromatic Wollaston prism are made of materials with complementary birefringence coefficient dispersion relations, one set of which is a positive birefringent crystal and the other set of which is a negative birefringent crystal.

[0009] Preferably, the positive birefringent crystal is yttrium vanadate, and the negative birefringent crystal is barium borate or Iceland spar.

[0010] Preferably, the apex angle and thickness of each prism in the achromatic Wollaston prism are determined by ray tracing optimization to ensure that the emission angle of beams of different wavelengths is consistent.

[0011] Preferably, the achromatic quarter-wave plate is calibrated at an angle before use to ensure that the intensity of the broadband white light reaching the two fiber-coupled spectrometers is equal when there is no sample.

[0012] Preferably, the two fiber-coupled spectrometers are used to receive the horizontally and vertically polarized light separated by the achromatic Wollaston prism, and to measure its spectral intensity.

[0013] Preferably, the polarization modulation device in the pump optical path module is a single-wavelength quarter-wave plate, an achromatic quarter-wave plate, or an ultra-wideband arbitrary phase delay system.

[0014] Preferably, the nonlinear white light crystal is an undoped sapphire crystal used to generate visible white light in the 420-780nm wavelength range.

[0015] Preferably, the measurement method includes the following steps: the femtosecond laser outputs laser light, which is divided into pump light and probe light; the pump light is modulated in wavelength by an optical parametric amplifier or a nonlinear frequency doubling module, and then adjusted to circularly polarized light by a polarization modulation device. After the frequency is reduced to half the frequency of the femtosecond laser by a chopper, it is used to illuminate the sample; the probe light is delayed by a mechanical-optical delay line and then focused on a nonlinear white light crystal to generate supercontinuous white light, which is used to illuminate the sample; the probe light passing through the sample passes sequentially through an achromatic quarter-wave plate, an achromatic Wollaston prism, and an achromatic lens, and is separated into horizontal and vertical polarized light, which are then coupled into two fiber-coupled spectrometers; the transient circular dichroism absorption spectrum is calculated.

[0016] Preferably, the probe light transmitted through the sample is linearly polarized light, which can be regarded as a superposition of left-handed and right-handed circularly polarized light. After interacting with the sample in different ways, it is converted into linearly polarized light in the horizontal and vertical directions by an achromatic quarter-wave plate.

[0017] In the above technical solution, the present invention provides a single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy. The probe light in front of the sample is linearly polarized light in either the vertical or horizontal direction, and its propagation is not affected by the different reflectivities of the optical elements for vertically and horizontally polarized light. The separated two beams can achieve self-reference, thereby improving the signal-to-noise ratio, and avoid the use of beam-splitting optical elements that could affect the accuracy of the polarization state. The use of active optical modulation elements is avoided, thus reducing the risk of damage to the equipment and devices.

[0018] Meanwhile, the advantages of this invention lie in its detection scheme, which eliminates chromatic aberration and allows the use of broadband probe light, enabling the acquisition of time-resolved circular dichroism absorption spectra within the 400-800 nm wavelength range in a single measurement. The use of an achromatic Wollaston prism instead of a conventional polarizing cubic crystal significantly reduces system measurement errors. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0020] Figure 1 This is a schematic diagram of the structure provided for an embodiment of the present invention;

[0021] Figure 2 This is a schematic diagram of the structure provided for an embodiment of the present invention;

[0022] Figure 3 This is a schematic diagram of the structure provided for an embodiment of the present invention;

[0023] Figure 4This is a schematic diagram of the structure provided for an embodiment of the present invention;

[0024] Figure 5 This is a schematic diagram of the structure provided for an embodiment of the present invention;

[0025] Figure 6 This is a schematic diagram of the structure provided for an embodiment of the present invention;

[0026] Figure 7 The pulse timing diagram of the pump light and probe light provided in the embodiments of the present invention is shown. Detailed Implementation

[0027] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0028] Circular dichroism spectroscopy is an optical rotation spectroscopy used to infer the configuration and conformation of asymmetric molecules. Many optically active substances, especially optically active biomolecules, do not interact equally with left-handed and right-handed polarized light. If the absorption spectra of a sample for left-handed and right-handed polarized light are measured separately... and Subtract the results to obtain This is known as circular dichroism spectroscopy. For some photoactive substances, photoexcitation alters the molecular configuration, inducing different absorptions for left- and right-handed rotation—circular dichroism absorption. After the excitation light disappears, the molecular configuration gradually returns to a more stable ground state, and the circular dichroism absorption also disappears. Therefore, the dynamic properties of molecular configuration can be analyzed by studying the change in circular dichroism absorption over time after photoexcitation. This method is called time-resolved circular dichroism spectroscopy. Mathematically, it is expressed as:

[0029]

[0030] Time-resolved circular dichroism (CD) absorption spectroscopy is a spectral measurement technique developed based on pump-probe transient absorption spectroscopy. It generally includes the following modules: a femtosecond laser as the light source, a pump light modulation device (such as a chopper), a time delay device to modify the delay in the formula, a probe light generation device, and a CD measurement module. The first four components are essentially the same as in a time-resolved transient absorption spectrometer, while the key difference lies in the CD measurement technique. There are two main methods for measuring CD:

[0031] Measure the absorption spectrum of the sample under left-handed polarized light and the absorption spectrum of the sample under right-handed polarized light, respectively. Subtract the two spectra after obtaining them.

[0032] In initial attempts, some researchers first measured the absorption spectrum of a sample using left-handed circularly polarized probe light, then measured it using right-handed circularly polarized probe light, and finally subtracted the two to obtain the circular dichroism absorption spectrum. Because the pulsed laser used may exhibit instability due to changes in the measurement environment, fluctuations in the probe light can introduce measurement bias when the measurement time interval between left-handed and right-handed polarization conditions is long. Unfortunately, circular dichroism spectral signals are often very small, and transient circular dichroism spectral signals can be less than one-hundredth of the steady-state circular dichroism spectral signal. Such fluctuations in the probe light can completely drown out meaningful signals.

[0033] To increase the switching frequency of left- and right-handed polarized light, photoelectric modulators or photoelastic modulators are introduced to periodically change the left- and right-handed polarization characteristics of each pulse at high speed. This allows left- and right-handed measurements to be obtained under almost identical conditions, reducing noise caused by laser fluctuations. This technique has been introduced into time-resolved circular dichroism spectroscopy. The output light of the femtosecond laser is split into pump and probe beams, and the polarization modulator is placed before the probe beam generation device. Left- and right-handed polarized light is output from the modulator. The left- and right-handed polarized light then enters the probe beam generation device, exciting a nonlinear crystal to generate circularly polarized broadband white light for detection. The overall structure is as follows: Figure 1 As shown.

[0034] To obtain the change in circular dichroism absorption induced by photoexcitation, this method requires sequentially measuring four types of spectra: the left-handed polarization spectrum after pump light excitation, the right-handed polarization spectrum after pump light excitation, the left-handed polarization spectrum before pump light excitation, and the right-handed polarization spectrum before pump light excitation. The final transient circular dichroism absorption spectrum is then calculated using the following formula.

[0035]

[0036] The time-resolved circular dichroism spectroscopy measurement technique faces significant feasibility issues. Modulators capable of rapidly altering beam polarization are not well-suited for femtosecond lasers. Furthermore, the optically induced changes in circular dichroism absorption spectra are often weak. A reference beam technique is needed to eliminate probe light fluctuations and improve the signal-to-noise ratio. This technique requires splitting the generated circularly polarized white light into two beams using a beam splitter or other beam-splitting device, and simultaneously measuring the intensity of both beams to compensate for potential white light intensity and spectral fluctuations. During this process, the polarization state of the circularly polarized light is affected, thus interfering with the measurement results.

[0037] The second method utilizes an ellipsometer or polarization analyzer to analyze the probe light transmitted through the sample. In non-time-resolved circular dichroism spectroscopy, the circular dichroism absorption can be calculated by measuring and comparing the polarization state and light intensity before and after passing through the sample using a rotating polarization measuring instrument. In time-resolved circular dichroism spectroscopy, since the input light intensity before the sample can be canceled out in the calculation, the device is simplified to a combination of a fixed glass slide and a polarizing prism. This method can obtain circular dichroism absorption spectrum information in a single femtosecond laser pulse.

[0038] The measuring device is positioned behind the sample to be measured. It includes the following modules: a quarter-wave plate, a polarizing beam splitter prism, and a detector; the specific structure is as follows: Figure 2 As shown.

[0039] For vertically or horizontally polarized light passing through a sample, it can be considered as a superposition of left-handed and right-handed polarized light of equal intensity. The left-handed and right-handed polarized light are absorbed to different degrees after passing through the sample, resulting in a certain phase delay. The probe light then passes through a quarter-wave plate. The linearly polarized light is converted into circularly polarized light. This process is equivalent to converting the superimposed left-handed and right-handed polarized light into horizontally and vertically polarized light of equal intensity but different phases. When circular dichroism absorption or phase deflection exists, elliptically polarized light is obtained. This elliptically polarized light can be considered as a superposition of horizontally and vertically polarized light of different intensities and phases. This process is as follows: Figure 3 As shown.

[0040] Based on the above, this invention provides a single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy, comprising: a femtosecond laser whose output light is split into pump light and probe light; a pump light path module, including an optical parametric amplifier or nonlinear frequency doubling module, a polarization modulation device, and a chopper, for modulating the pump light into circularly polarized light and reducing its frequency to half the frequency of the femtosecond laser; and a probe light path module, including a mechanical optical delay line and a nonlinear white light crystal, wherein the mechanical optical delay line is used to control the time difference between the probe light pulse and the pump light pulse arriving at the sample, and the nonlinear white light crystal... A crystal is used to convert the probe light into supercontinuous white light, which is linearly polarized light in the horizontal or vertical direction. A circular dichroism absorption measurement module, located after the sample, is used to obtain circular dichroism absorption information from a single probe light pulse. The circular dichroism absorption measurement module includes, in sequence along the optical path: an achromatic quarter-wave plate, an achromatic Wollaston prism, an achromatic lens, and two fiber-coupled spectrometers. The achromatic Wollaston prism consists of a four-piece, two-set prism structure. The first set of two prisms is used to separate the horizontally and vertically polarized light, and the second set of two prisms is used to compensate for the separation angular dispersion generated by the first set of prisms.

[0041] The achromatic Wollaston prism comprises two sets of prisms, the first and second sets, made of materials with complementary birefringence coefficients and dispersion relations. One set uses a positive birefringent crystal, and the other uses a negative birefringent crystal. Based on the above, it should be noted that for a single Wollaston prism, under perpendicular incidence, the separation angle between the vertically polarized and horizontally polarized light of different wavelength beams is: Where b is the birefringence coefficient of the material: ,and The apex angle of the prism.

[0042] Therefore, the change in separation angle caused by the change in wavelength is:

[0043]

[0044] Therefore, if two materials with positive and negative Δb values ​​can be introduced, it may be possible to cancel out the angular dispersion relationship, allowing beams of the same polarization direction at different wavelengths to exit the Wollaston prism in the same direction. Combined with a lens to focus the parallel light, the polarized light can be focused onto the fiber optic inlet of a fiber-coupled spectrometer, thus achieving broadband detection. Ideally, the birefringence dispersion induced by the first material can be compensated for by the second material at all wavelengths, and the following differential equation should hold true at all wavelengths:

[0045] .

[0046] Among them, such as Figure 4 As shown, the positive birefringent crystal is yttrium vanadate (YVO4), and the negative birefringent crystal is barium borate (BBO) or Iceland spar (CaCO3).

[0047] The apex angle and thickness of each prism in the achromatic Wollaston prism are determined by ray tracing optimization to ensure that the emission angle of beams of different wavelengths is consistent.

[0048] The achromatic quarter-wave plate is calibrated at an angle before use to ensure that the intensity of the broadband white light reaching the two fiber-coupled spectrometers is equal when there is no sample.

[0049] A schematic diagram of the post-sample measurement module based on an ellipsometer is shown below. Figure 5 As shown in the figure. The optical components in the figure, from left to right, are: achromatic quarter-wave plate, achromatic Wollaston prism, achromatic lens, and optical fiber.

[0050] The two fiber-coupled spectrometers are used to receive horizontally and vertically polarized light separated by an achromatic Wollaston prism, and to measure its spectral intensity.

[0051] The polarization modulation device in the pump optical path module is a single-wavelength quarter-wave plate, an achromatic quarter-wave plate, or an ultra-wideband arbitrary phase delay system.

[0052] The nonlinear white light crystal is an undoped sapphire crystal used to generate white light in the visible wavelength range of 420-780nm.

[0053] In this process, the output light of the femtosecond laser is split into two parts: most of the energy is used as pump light, and a small portion is used as probe light. The pump light is modulated by an optical parametric amplifier and an arbitrary phase delayer to become circularly polarized light, inducing transient circular dichroism absorption. The pump light is then modulated by a chopper to reduce its frequency to half that of the femtosecond laser. The probe light first passes through a mechano-optical delay line to control the time difference between the probe light pulse and the pump light pulse arriving at the final sample. The probe light is then focused into a nonlinear white light crystal to generate supercontinuous white light for detection. This white light is linearly polarized in either the horizontal or vertical direction. The linearly polarized probe light can be considered as a superposition of left- and right-handedly polarized light, which can interact differently with the transient circular dichroism absorption induced by the pump light. The superimposed left- and right-handedly polarized light is converted into two linearly polarized lights in the horizontal and vertical directions by an achromatic broadband quarter-wave plate. The two linearly polarized lights are separated by an achromatic Wollaston prism invented in this patent, and then focused by an achromatic lens onto the fiber input ends of two fiber-coupled grating spectrometers. The process is as follows Figure 3 As shown. Figure 7 The timing of the pump and probe pulses is shown. When both the pump and probe pulses are present, the sample exhibits photoexcitation-induced transient circular dichroism absorption. However, when the pump pulse is blocked by the chopper, the sample does not exhibit transient circular dichroism absorption but only steady-state circular dichroism absorption.

[0054] The time-resolved transient circular dichroism spectrum of the signals output from the two fiber-coupled detectors is calculated using the following formula:

[0055] The subscripts "pumped" and "unpumped" indicate whether pump light simultaneously reaches the sample for the measurement of the probe pulse.

[0056] The probe light transmitted through the sample is linearly polarized light, which can be regarded as the superposition of left-handed and right-handed circularly polarized light. After interacting with the sample in different ways, it is converted into linearly polarized light in the horizontal and vertical directions by an achromatic quarter-wave plate.

[0057] Based on the above, this single-pulse broadband circular dichroism absorption spectroscopy measurement device is applied to a femtosecond time-resolved circular dichroism absorption spectrometer system, and the overall structure is as follows: Figure 6 As shown.

[0058] Figure 7 The timing of the pump and probe pulses is shown. When both pulses are present, the sample exhibits photoexcitation-induced transient circular dichroism absorption. However, when the pump pulse is blocked by a chopper, the sample does not exhibit transient circular dichroism absorption but only steady-state circular dichroism absorption.

[0059] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy, characterized in that, include: A femtosecond laser's output light is divided into two parts: pump light and probe light. The pump optical path module, including an optical parametric amplifier or nonlinear frequency doubling module, a polarization modulation device and a chopper, is used to modulate the pump light into circularly polarized light and reduce its frequency to half the frequency of the femtosecond laser. The probe optical path module includes a mechanical-optical delay line and a nonlinear white light crystal. The mechanical-optical delay line is used to control the time difference between the arrival of the probe light pulse and the pump light pulse at the sample. The nonlinear white light crystal is used to convert the probe light into supercontinuous white light, which is linearly polarized light in the horizontal or vertical direction. A circular dichroism absorption measurement module, located after the sample, is used to obtain circular dichroism absorption information from a single probe light pulse. The circular dichroism absorption measurement module includes, in sequence along the optical path: an achromatic quarter-wave plate, an achromatic Wollaston prism, an achromatic lens, and two fiber-coupled spectrometers. The achromatic Wollaston prism consists of four prisms in two sets. The first set of two prisms is used to separate horizontal and vertical polarized light, and the second set of two prisms is used to compensate for the separation angular dispersion caused by the first set of prisms.

2. The single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy according to claim 1, characterized in that, The first and second sets of prisms of the achromatic Wollaston prism are made of materials with complementary birefringence coefficient dispersion relations, one set of which uses a positive birefringent crystal and the other set of which uses a negative birefringent crystal.

3. The single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy according to claim 2, characterized in that, The positive birefringent crystal is yttrium vanadate, and the negative birefringent crystal is barium borate or Iceland spar.

4. The single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy according to claim 1, characterized in that, The apex angle and thickness of each prism in the achromatic Wollaston prism are determined by ray tracing optimization to ensure that the emission angle of beams of different wavelengths is consistent.

5. The single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy according to claim 1, characterized in that, The achromatic quarter-wave plate is calibrated at an angle before use to ensure that the intensity of broadband white light reaching the two fiber-coupled spectrometers is equal when there is no sample.

6. The single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy according to claim 1, characterized in that, The two fiber-coupled spectrometers are used to receive horizontally and vertically polarized light separated by an achromatic Wollaston prism, and to measure its spectral intensity.

7. The single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy according to claim 1, characterized in that, The polarization modulation device in the pump optical path module is a single-wavelength quarter-wave plate, an achromatic quarter-wave plate, or an ultra-wideband arbitrary phase delay system.

8. The single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy according to claim 1, characterized in that, The nonlinear white light crystal is an undoped sapphire crystal used to generate white light in the visible wavelength range of 420-780nm.

9. A single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy according to any one of claims 1-8, characterized in that, The measurement methods and steps include the following: Femtosecond lasers output laser light, which consists of pump light and probe light. The pump light is modulated by an optical parametric amplifier or a nonlinear frequency doubling module, then converted into circularly polarized light by a polarization modulation device, and finally its frequency is reduced to half the frequency of the femtosecond laser by a chopper before irradiating the sample. After the probe light is delayed by the mechanical-optical delay line, it is focused onto the nonlinear white light crystal to generate supercontinuous white light, which then illuminates the sample. The probe light passing through the sample passes sequentially through an achromatic quarter-wave plate, an achromatic Wollaston prism, and an achromatic lens, and is separated into horizontally and vertically polarized light, which is then coupled into two fiber-coupled spectrometers. Calculate the transient circular dichroism absorption spectrum.

10. A single-pulse broadband circular dichroism absorption measurement device for femtosecond time-resolved spectroscopy according to claim 9, characterized in that, The probe light transmitted through the sample is linearly polarized light, which can be regarded as the superposition of left-handed and right-handed circularly polarized light. After interacting with the sample in different ways, it is converted into linearly polarized light in the horizontal and vertical directions by an achromatic quarter-wave plate.

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