A method for measuring two-dimensional distribution of optical activity and circular dichroism based on circularly polarized light wave digital holography

Abstract

The application discloses a method for simultaneously and dynamically measuring two-dimensional distribution of optical activity and circular dichroism of a chiral sample by using circularly polarized object light wave digital holography. The method utilizes the modulation characteristics of left and right circularly polarized light amplitude and phase of the chiral sample, and the full-field, non-contact and high-precision dynamic measurement advantages of digital holography on the complex amplitude distribution of the light field. Left and right circularly polarized light is taken as object light wave respectively, and the simultaneous and dynamic measurement of the two-dimensional distribution of optical activity and circular dichroism of the chiral sample can be realized by using double-exposure digital holography.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of chiral measurement, in particular to the field of simultaneously measuring the two-dimensional distribution of optical activity and circular dichroism of a chiral sample. BACKGROUND

[0002] Chirality refers to the property that an object cannot coincide with its mirror image no matter how the object is transformed or rotated. Chiral substances exist widely in nature, such as sugars, amino acids, DNA, drugs, etc. have chirality, and the research on chiral substances plays an important role in the fields of industry, chemistry, medicine, etc. The interaction of chiral substances with circularly polarized light of different chirality will produce circular birefringence and circular dichroism. When chiral substances interact with linearly polarized light, the circular birefringence effect will exhibit optical rotation effect, and the chiral characteristics that can produce circular birefringence / optical rotation effect are also called optical activity. Measuring the optical activity and circular dichroism of chiral substances at different wavelengths can obtain optical rotation dispersion and circular dichroism spectrum respectively, which can provide rich internal structural information of chiral substances. As an important optical means for characterizing and detecting chiral substances, commercial instruments for measuring optical rotation dispersion and circular dichroism spectrum have developed rapidly and have been widely used in the fields of biomedicine, pharmacy, material science, etc.

[0003] In order to obtain the spatial distribution information of the chiral signal of the sample, wide-field imaging, confocal imaging, second harmonic generation imaging, two-photon luminescence imaging and near-field scanning imaging based on circular dichroism detection have been proposed. However, these chiral distribution imaging methods are limited by weak light-chiral substance interaction, resulting in low imaging contrast, and only the circular dichroism distribution information of the chiral sample can be obtained. It should be noted that although the optical activity of chiral substances can be converted from the measured circular dichroism by K-K relationship, this conversion is based on the conversion between spectral information. Therefore, the two-dimensional distribution of optical activity and circular dichroism at a single wavelength must be measured separately. So far, there are few reports on simultaneous dynamic measurement of the two-dimensional distribution of optical activity and circular dichroism of chiral samples.

[0004] In theory, whether it is optical activity or circular dichroism, it ultimately reflects the modulation of chiral substances on the amplitude, phase and / or polarization state of light waves. As long as the complex amplitude and polarization distribution of the object light wave modulated by different chiral parameters can be quantitatively measured, the simultaneous demodulation of the entire chiral signal distribution of the chiral sample can be realized. Digital holography can realize full-field, non-contact, high-precision and dynamic measurement of the complex amplitude distribution of light field, so the present application proposes a method for measuring chiral parameter distribution using digital holography. SUMMARY

[0005] Technical problems to be solved

[0006] In order to solve the problem that the optical activity and the circular dichroism two-dimensional distribution of a chiral sample cannot be measured simultaneously, the application provides a method for simultaneously and dynamically measuring the optical activity and the circular dichroism two-dimensional distribution of a chiral sample based on digital holography of circularly polarized object light waves.

[0007] Technical scheme

[0008] The application is based on the idea that the optical activity and the circular dichroism of a chiral sample are reflected on the difference between the real part and the imaginary part of the refractive index of left and right circularly polarized light, and are expressed as the optical path and the absorption of two circularly polarized light passing through the chiral sample. The left and right circularly polarized light passes through the chiral sample in turn and is used as the object light wave, and is interfered with the same reference light wave. The phase difference and the intensity difference of the object light wave under the irradiation of the two circularly polarized light are measured by using the double-exposure digital holographic interference method, so that the optical activity and the circular dichroism two-dimensional distribution of the chiral sample can be measured quantitatively and simultaneously.

[0009] An optical activity and circular dichroism two-dimensional distribution measurement method based on digital holography of circularly polarized object light waves, characterized in that the method comprises the following steps:

[0010] Step 1: a left circularly polarized parallel light passes through a chiral sample in a perpendicular irradiation mode, and the emergent light is used as an object light wave to reach a photoelectric imaging device target surface;

[0011] The parallel light is visible light with a wavelength of λ, the geometric length of the chiral sample is L(x, y), the refractive index distribution of the chiral sample to the left circularly polarized light is n LCPL (x, y), and the absorption rate distribution is A LCPL (x, y); (x, y) represents the two-dimensional spatial coordinates of the sample;

[0012] Step 2: another parallel light coherent with the object light wave in step 1 is used as a reference light wave, and is overlapped with the object light wave at a certain angle on the photoelectric imaging device target surface to interfere, and a digital hologram H1 is recorded by the photoelectric imaging device;

[0013] The certain angle is 2-3 degrees;

[0014] Step 3: the object light wave in step 1 is changed into right circularly polarized light by using a circularly polarized light generating unit, and a digital hologram H2 is recorded by the photoelectric imaging device;

[0015] The circularly polarized light generating unit is a quarter-wave plate or an electrically controlled variable delay wave plate;

[0016] The refractive index distribution of the chiral sample to the right circularly polarized light is n RCPL (x, y), and the absorption rate distribution is A RCPL (x, y);

[0017] Step 4: According to the scalar diffraction theory, the computer numerical algorithm is used to respectively perform numerical reconstruction on the holograms H1 and H2, and the complex amplitude distribution of the left and right circularly polarized object light waves is obtained, further, the phase distribution of the object light wave reconstructed from H2 is subtracted from the phase distribution of the object light wave reconstructed from H1, and the phase difference distribution Δφ(x, y) of the object light wave is obtained; the intensity distribution of the object light wave reconstructed from H2 is subtracted from the intensity distribution of the object light wave reconstructed from H1, and the intensity difference distribution ΔI(x, y) of the object light wave is obtained;

[0018] Step 5: The phase difference distribution Δφ(x, y) = 2π[n LCPL (x, y) - n RCPL (x, y)]L(x, y) / λ and the intensity difference distribution ΔI(x, y) = A RCPL (x, y) - A LCPL (x, y) obtained in Step 4 are obtained.

[0019] The n LCPL (x, y) - n RCPL (x, y), A LCPL (x, y) - A RCPL (x, y) respectively reflect the optical activity and the circular dichroism two-dimensional distribution information of the chiral sample.

[0020] Advantageous effects

[0021] The method for simultaneously measuring the optical activity and the circular dichroism two-dimensional distribution of the chiral sample by using the circularly polarized object light wave digital holography provided by the application utilizes the modulation characteristics of the chiral sample on the amplitude and phase of the circularly polarized light, and the full-field, non-contact and high-precision dynamic measurement advantages of the digital holography on the complex amplitude distribution of the light field, and the simple digital holography light path is adopted, so that the simultaneous measurement problem of the optical activity and the circular dichroism of the chiral sample can be solved. The method is simple, and the dynamic measurement of the two-dimensional distribution of the two chiral parameters can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 : is the circularly polarized object light wave digital holography light path for simultaneously measuring the optical activity and the circular dichroism two-dimensional distribution of the chiral sample involved in the application;

[0023] In the figure: 1 is a depolarization beam splitter prism, 2 is a mirror, 3 is a mirror, 4 is a circularly polarized light generating unit, 5 is a chiral sample to be measured, 6 is a depolarization beam splitter prism, 7 is a polarizer, and 8 is a photoelectric imaging device. DETAILED DESCRIPTION

[0024] The application will be further described in combination with embodiments and the drawings:

[0025] The experimental optical path of the optical activity and two-dimensional distribution measurement method of circular dichroism based on digital holography of circularly polarized light waves, which relates to the present invention, is shown in the attached figure. Figure 1 As shown, it includes: a depolarizing beam splitter 1, a reflector 2, a reflector 3, a circularly polarized light generating unit 4, a chiral sample to be tested 5, a depolarizing beam splitter 6, a polarizer 7, and a photoelectric imaging device 8.

[0026] The workflow of the method for measuring the optical activity and two-dimensional distribution of circular dichroism based on digital holography of circularly polarized light waves is as follows:

[0027] Linearly polarized light (wavelength λ) transmitted through depolarizing beam splitter 1 is reflected by mirror 2 and depolarizing beam splitter 2 again, and used as a reference light wave. The light wave reflected by depolarizing beam splitter 1 is reflected by mirror 1, passes through the circularly polarized light generating unit, and then passes through the chiral sample to be tested (geometric length L(x,y), the refractive index distribution of the chiral sample for left-handed and right-handed circularly polarized light is n...). LCPL (x,y),n RCPL (x,y), the absorptivity distributions are A LCPL (x,y), A RCPL The object light wave (x, y) is combined with the reference light wave by a depolarizing beam splitter and then passed through a linear polarizer. Finally, an off-axis digital hologram is formed on the target surface of the photoelectric imaging device. The linear polarizer placed in front of the photoelectric imaging device serves two purposes: ensuring that the reference light wave and the object light wave interfere with each other with the same polarization state; and controlling the intensity ratio of the reference light wave and the object light wave to achieve the highest contrast of the interference fringes. The circularly polarized light generating unit can be a quarter-wave plate or a electrically controlled variable liquid crystal phase delay device, and the optimal choice must be made based on the specific experimental measurement requirements. For example, for dynamic process measurement requirements, an electrically controlled control unit is needed to increase the switching speed between the two types of circularly polarized light, and LabVIEW programming is used to simultaneously control the circularly polarized light generating unit and the photoelectric imaging device.

[0028] Left- and right-hand circularly polarized light are generated using a circularly polarized light generation unit, and digital holograms H1 and H2 are recorded by a photoelectric imaging device, respectively. Off-axis digital holography numerical algorithms are used to numerically reconstruct holograms H1 and H2 to obtain the complex amplitude distributions of the left- and right-hand circularly polarized object light waves. Then, the phase difference distribution Δφ(x,y) is obtained by subtracting the phase distribution of the object light wave reconstructed from H2 from that reconstructed from H1; the intensity difference distribution ΔΙ(x,y) is obtained by subtracting the intensity distribution of the object light wave reconstructed from H2 from that reconstructed from H1. Where: Δφ(x,y)=2π[n LCPL (x,y)-n RCPL (x,y)]L(x,y) / λ, ΔΙ(x,y)=A RCPL(x,y)-A LCPL (x,y), whereby both the optical activity and the circular dichroism two-dimensional distribution information of the chiral sample are obtained simultaneously.

Claims

1. A method for measuring two-dimensional distribution of optical activity and circular dichroism based on circularly polarized light wave digital holography, characterized in that The method comprises the following steps: Step 1: a bundle of left circularly polarized parallel light passes through a chiral sample in a perpendicular irradiation mode, and the emergent light reaches a target surface of a photoelectric imaging device as an object light wave; The parallel light is visible light with a wavelength of λ, the geometric length of the chiral sample is L(x, y), and the refractive index distribution of the chiral sample to left circularly polarized light is n LCPL (x, y), and the absorption rate distribution is A LCPL (x, y); wherein (x, y) represents the two-dimensional spatial coordinates of the sample. Step 2: another bundle of parallel light as a reference light wave is overlapped with the object light wave at a certain angle on the target surface of the photoelectric imaging device and interferes with the object light wave, and a digital hologram H1 is recorded by the photoelectric imaging device; The certain angle is 2-3 degrees; Step 3: the object light wave in step 1 is changed into right circularly polarized light by using a circularly polarized light generating unit, and a digital hologram H2 is recorded by the photoelectric imaging device; The circularly polarized light generating unit is a quarter-wave plate or an electrically controlled variable retardation wave plate; The distribution of the refractive index of the chiral sample for right circularly polarized light is n RCPL (x,y), the distribution of the absorption is A RCPL (x,y); Step 4: according to the scalar diffraction theory, the holograms H1 and H2 are respectively numerically reconstructed by using a computer numerical algorithm, the complex amplitude distributions of the left and right circularly polarized object light waves are obtained, further, the phase distribution of the object light wave reconstructed from H2 is subtracted from the phase distribution of the object light wave reconstructed from H1 to obtain a phase difference distribution Δφ(x, y) of the object light wave, and the intensity distribution of the object light wave reconstructed from H2 is subtracted from the intensity distribution of the object light wave reconstructed from H1 to obtain an intensity difference distribution ΔI(x, y) of the object light wave. Step 5: The phase difference distribution Δφ(x, y) = 2π[n LCPL (x, y) - n RCPL (x, y)]L(x, y) / λ, the intensity difference distribution ΔΙ(x, y) = A RCPL (x, y) - A LCPL (x, y); The n LCPL (x,y)-n RCPL (x,y), A LCPL (x,y)-A RCPL (x,y) respectively reflect the optical activity and circular dichroism two-dimensional distribution information of the chiral sample.

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