CN112285029B - Terahertz microstructure polarization sensing system for liquid chiral sample and detection method thereof - Google Patents
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Abstract
The invention discloses a terahertz microstructure polarization sensing system for a liquid chiral sample and a detection method thereof. The reflective system is adopted to realize the sensing detection of the liquid sample, and the signal intensity and the system signal to noise ratio are obviously improved. Polarizing plates are respectively arranged in front of and behind the reflection system, the complete polarization state of an emergent wave signal can be obtained by rotating the directions of the polarizing plates, a reflection type time domain polarization spectrum (RTDPS) system is formed, and the detection sensitivity is improved, and meanwhile, the detection of the polarization state and the chiral information of a sample is realized. By using the single-layer spiral microstructure as a sensor, the optical response of a natural chiral sample can be greatly enhanced by the local resonance and artificial chiral response of the microstructure under the oblique incidence condition of electromagnetic waves. Taking an amino acid sample as an example, the concentration detection precision of the system is 10 ‑5 In the order of g/mL and by significant differences in the polarization spectra at characteristic frequency positions, the D-and L-form chiral enantiomers of amino acids can be identified.
Description
Technical Field
The invention belongs to the technical field of terahertz science, and particularly relates to a high-sensitivity reflective microstructure polarization sensing system for a liquid chiral sample and a specific sensing method thereof.
Background
Terahertz (THz) waves generally mean that the frequency is in the range of 0.1 to 10THz (1thz = 10) 12 Hz), has received wide attention in sensing and detecting due to its excellent properties such as small photon energy, non-invasion and non-ionization, and its applications cover various fields such as safety, medical treatment, biology, etc. Conventional THz sensing is mainly based on Time Domain Spectroscopy (TDS) systems, i.e. characterized by the transmission or absorption spectra of the sample. Although the TDS system can realize the spectral measurement in a broadband range, the method has strict requirements on the state of the sample, cannot detect a liquid (especially an aqueous solution) sample, cannot detect polarization information of the sample, and has the problems of low sensing precision and limited sensing information, and the applicability of the TDS system is greatly reduced due to the defects.
General liquid samples, especially aqueous solution samples, have strong absorption loss to THz waves, so that detection signals are weakened, and the signal-to-noise ratio is reduced. Researchers have proposed methods such as Analytical Sciences,25, 457 (2009) and the use of microfluidic chips (Infrared, millimeter-Wave, and Terahertz Technologies IV, (2016), but these methods all have the problems of complex operation and low detection accuracy. At present, a THz sensing detection method with high sensitivity and high precision aiming at a liquid sample does not exist.
The THz sensor has been spotlighted as a core device in a sensing and detecting system. Currently, the research of the existing THz sensor is mainly based on the application of a metamaterial isoplanar structure. The super-surface is an artificial electromagnetic structure consisting of sub-wavelength plane units arranged according to a certain rule, and can control the amplitude, the phase and the polarization of electromagnetic waves in time and space. The optical response of the sample can be enhanced by the strong local resonance effect of the super surface, and the detection sensitivity can be obviously improved by the super surface serving as a sensor. For example, sensing of silicon nanosphere/ethanol solutions is achieved using a symmetrical open resonator ring (Applied Physics Letters,91, 062511 (2007)), and using a crossed dipole super-surface as the sensor, sample sensing can be performed at normal and oblique incidence, respectively (Applied Physics Letters,108, 111104 (2016)). By utilizing the characteristic that the super surface can flexibly control electromagnetic waves, the unit structure can be designed to have optical chirality. Left-hand circular polarization (LCP) light and right-hand circular polarization (RCP) can be viewed as "chiral states" of light, while the optical chirality of a material or structure manifests itself as different propagation constants for LCP and RCP. Generally, three-dimensional chiral structures have strong intrinsic chiral optical responses, such as three-dimensional helices and multi-layer chiral coupled structures. However, when the anisotropic structure of the single layer is incident obliquely to the electromagnetic wave, the chiral optical response is also remarkable. In recent years, many researches on artificial chiral structures have been carried out, and few reports have been made on the work of utilizing a chiral super surface as a terahertz sensor to carry out sensing detection on a chiral sample.
In addition, in addition to the detection of a sample by transmission or absorption spectroscopy, the polarization state of the emergent wave may also change with the sample. Conventional THz sensing ignores the detected phase and polarization information and therefore the sample characteristic information that can be detected is very limited. The detection of the polarization state change of the emergent signal can reflect the type and size of a detected sample, can provide structural, kinetic and thermodynamic information of molecules, is a powerful tool for researching different types and sizes of chiral molecules in the fields of biology, medicine, chemistry, physics and the like, and particularly for analyzing the secondary structure and conformation of macromolecules (Science Advances 3.
In summary, on one hand, the rapid development of the terahertz technology and the urgent needs in the fields of chemistry, biology and the like put forward higher performance requirements on the THz sensing detection system; on the other hand, the existing traditional THz sensing system has the defects of low sensitivity, limited detection information, poor applicability and the like. Therefore, the method has important scientific research value and significance for related researches facing liquid samples, high sensitivity, polarization detection and novel characterization means.
Disclosure of Invention
The invention aims to provide a terahertz microstructure polarization sensing system facing a liquid chiral sample and a sensing detection method thereof. The system can be used for sensing and detecting the liquid chiral sample, can obtain the polarization characteristic spectrum and the chiral response spectrum of the sample, and provides more sensing information of the sample while improving the sensitivity of the system.
In order to achieve the purpose, the device and the structure comprise a front terahertz polaroid (1), a front terahertz lens (2), a metal reflecting prism (3), a chiral microstructure sensor (4), a sample cell (5), a rear terahertz lens (6) and a rear terahertz polaroid (7), wherein the chiral microstructure sensor (4) is used as the bottom of the sample cell (5), and the sample cell (5) is positioned above the metal reflecting prism (3). The chiral microstructure sensor (4) is composed of a quartz glass substrate (8) and a chiral spiral metal microstructure (9): the chiral spiral metal microstructure (9) is periodically attached to the front surface of the quartz glass substrate (8); the quartz glass substrate (8) has a thickness of 300 to 500 [ mu ] m; the chiral spiral metal microstructure (9) is formed by etching on a gold film with the thickness of 200 nm; the period along the x-axis and the y-axis is 200 mu m, the line width of the unit structure is 5-15 mu m, the structure is asymmetric to the polarization direction of incident light, is not overlapped with a mirror image of the incident light, and has chirality.
The basic working principle of the invention is as follows: the linear polarization terahertz plane wave with the electric field vibrating along the vertical direction is transmitted along the horizontal direction, the polarization degree is improved through the front polarization piece (1), the linear polarization terahertz plane wave is focused through the front terahertz lens (2), and the terahertz plane wave is reflected to the chiral microstructure sensor (4) at a certain angle through the metal reflecting prism (3); terahertz is reflected on a contact surface of the chiral spiral metal microstructure (9) and a sample, recovered into parallel-transmitted plane waves through a rear terahertz lens (6), and emitted out through a rear polarizer (7); the reflection system can be arranged in a terahertz parallel light path of a traditional time-domain spectroscopy system, and a sample cell (5) is prepared for containing a sample solution.
In order to be able to perform a sensing detection of the liquid sample, a reflective sensing system is used. The reflective sensing system does not need THz waves to pass through a sample, so that the absorption loss of the liquid sample to the THz is avoided, the intensity of a detection signal can be obviously improved, and the signal-to-noise ratio of the system is improved. Terahertz polaroids are respectively arranged in front of and behind the reflection system, a group of orthogonal signals of emergent waves can be obtained through detection by rotating the directions of the polaroids, complete polarization state information is restored, a reflection type time domain polarization spectrum (RTDPS) system is formed, and detection of the polarization state and chiral information of a sample in a broadband range is realized while the detection sensitivity is improved. In addition, the single-layer spiral microstructure is used as a sensor, and has strong chirality under the oblique incidence condition of electromagnetic waves. When the sample is placed on the surface of the microstructure, the optical response of a natural chiral sample can be greatly enhanced through local resonance and artificial chiral response, the detection sensitivity is further improved, and the chiral response between two different enantiomers can be effectively distinguished.
The sample was characterized by a functional me polarization rotation angle (PEA) spectrum, a polarization elliptical corner (PRA) spectrum, a Circular Dichroism (CD) spectrum, and an Optical Activity (OA) spectrum. The PEA spectrum and the PRA spectrum respectively represent the change of the polarization state of emergent light when incident light is in a linear polarization state, and the polarization conversion capability of a sample and a microstructure is embodied; CD and OA respectively represent the reflection difference of LCP and RCP when the incident light is in a circular polarization state, and the chiral and optical characteristics of the sample and the microstructure are reflected. The characterization means can not only carry out sensing detection on different concentrations of the same sample solution, but also distinguish chiral enantiomers of the same compound molecule under the condition of the same concentration.
The invention has the advantages that:
1. by adopting the reflective THz sensing system, terahertz waves do not need to pass through the whole sample solution, so that huge absorption loss caused by the solution is avoided, the intensity of a detection signal can be obviously increased, and the signal-to-noise ratio is improved.
2. The terahertz polaroids are respectively arranged in front of and behind the sample position, a group of orthogonal signals of emergent waves can be obtained through detection by rotating the directions of the polaroids, complete polarization state information is restored, an RTDPS system is formed, and detection of the polarization state and chiral information of the sample in a broadband range is realized while the detection sensitivity is improved.
3. The single-layer chiral spiral metal microstructure is used as a sensor, the local resonance and artificial chiral response of the chiral microstructure can obviously enhance the natural chiral optical response of the sample, and the detection sensitivity is further improved.
4. The PEA spectrum, the PRA spectrum, the CD spectrum and the OA spectrum are utilized to characterize the sample, not only can the sensing detection be carried out on different concentrations of the same sample solution, but also the chiral enantiomers of the same compound molecules can be distinguished under the condition of the same concentration.
Description of the drawings:
FIG. 1 is a schematic diagram of a terahertz microstructure polarization sensing system facing a liquid chiral sample;
FIG. 2 illustrates the polarization degree and minimum polarization angle of an RTDPS system; the inset is the time domain signal when the rear polarizer rotates to 0 ° and 90 °;
FIG. 3 shows the geometric and dimensional parameters of the chiral microstructure sensor;
FIG. 4 shows (a) linear polarization reflection (R) of the chiral microstructure sensor LP ) Spectrum, (b) circularly polarized reflection (R) CP ) Spectra, (c) PEA and PRA spectra, (d) CD and OA spectra;
FIG. 5 shows (a) PEA spectrum and (b) PRA spectrum of aqueous solutions of D-tyrosine and L-tyrosine at 0g/mL, 0.02g/mL and 0.04g/mL, respectively, under linearly polarized light incidence;
fig. 6 shows the case of circularly polarized light incidence. (a) CD and (b) OA profiles at 0g/mL, 0.02g/mL and 0.04g/mL for aqueous solutions of D-tyrosine and L-tyrosine, respectively;
FIG. 7 (a) shows the difference between the PEA and PRA spectra at 0.04g/mL for aqueous solutions of D-and L-tyrosine; (b) Is the difference between the CD spectrum and the OA spectrum of the D-tyrosine aqueous solution and the L-tyrosine aqueous solution at 0.04 g/mL; (c) Is the emergent wave polarization at 0.04g/mL of D-and L-tyrosine at the 0.55THz position;
the numerals in parentheses in the above figures respectively represent the devices and structures: the device comprises a (1) a front terahertz polaroid, (2) a front terahertz lens, (3) a metal reflecting prism, (4) a chiral microstructure sensor, (5) a sample pool, (6) a rear terahertz lens, (7) a rear terahertz polaroid, (8) a quartz glass substrate, and (9) a chiral spiral metal microstructure.
The specific implementation mode is as follows:
the working principle and method of the present invention are explained below:
the invention relates to a terahertz microstructure polarization sensing system facing a liquid chiral sample and a detection method thereof. In order to realize the sensing detection of the liquid sample, a reflective sensing system is adopted, the absorption loss of the liquid sample to THz is avoided, the intensity of a detection signal can be obviously improved, and the signal to noise ratio of the system is improved. Terahertz polaroids are respectively arranged in front of and behind the reflection system, a group of orthogonal signals of emergent waves can be obtained through detection by rotating the directions of the polaroids, complete polarization state information is restored, an RTDPS system is formed, and detection of the polarization state and chiral information of a sample in a broadband range is realized while detection sensitivity is improved. In addition, when a sample is placed on the surface of the single-layer spiral microstructure sensor, the optical response of a natural chiral sample can be greatly enhanced by the local resonance and artificial chiral response of the structure. As shown in fig. 1.
In order to obtain the complete polarization state of the emergent wave in an experiment, a rear polarizer is rotated, linear polarization components of +45 degrees and-45 degrees are detected, and the amplitude and the phase of two orthogonal components of the emergent terahertz wave are obtained: a. The ±45° (ω) and δ ±45° (omega) and then processing the amplitude and phase of the two orthogonal signals to obtainTo two polarization sensing parameters PEA 'epsilon' and PRA 'psi', the calculation formula is as follows:
tan 2ε(ω)＝sin 2β(ω)sinΔδ(ω) (1)
sin 2ψ(ω)＝tan 2β(ω)cosΔδ(ω) (2)
wherein tan beta (omega) = A +45° (ω)/A -45° (ω)，Δδ(ω)＝δ +45。 (ω)-δ -45° (ω). PEA reflects the polarization state of emergent light, positive values indicate right-handed rotation, and negative values indicate left-handed rotation. PEA ranges from-45 to +45, where 0 corresponds to linear polarization, -45 corresponds to LCP and +45 corresponds to RCP. PRA denotes the rotation angle of the polarization direction of the output wave with respect to the incident wave, ranging from-90 ° to +90 °, positive clockwise and negative counter-clockwise.
To obtain the reflection coefficient of circularly polarized waves, i.e. R ++ ，R -+ ，R +- And R -- The reflection coefficient R of linear polarization co-polarization and cross polarization is measured by experiments xx ，R yx ，R xy And R yy The reflection coefficient of the circularly polarized wave is obtained using the following formula:
where the first and second subscripts refer to the polarization states of the incident and reflected waves, "+" and "-" refer to RCP and LCP, respectively, and "x" and "y" refer to linearly polarized light with electric fields in two orthogonal directions, respectively.
After obtaining the reflection coefficient of circularly polarized light, CD and OA can be obtained by the following formulas:
CD indicates the reflection difference of co-polarized circularly polarized light in the range of-45 ° to +45 °, -45 ° indicates that only LCP light can be reflected, +45 ° indicates that only RCP light can be reflected, and 0 ° indicates that the reflectivities of LCP and RCP are the same. OA represents the optical rotation characteristics of the structure and the sample, and is related to the phase difference of the co-polarized circularly polarized light, ranging from-90 to + 90.
The detection accuracy of the polarization parameter in the sensing system mainly depends on the polarization angle resolution of the RTDPS system. The angular resolution of polarization is mainly determined by the signal-to-noise ratio and the degree of polarization (DOP) of the RTDPS system. FIG. 2 shows DOP and minimum polarization angle (M) for an RTDPS system PA ) The formula is as follows:
wherein A is || And A ⊥ Is the amplitude of the two terahertz polarizers in parallel and orthogonal. As can be seen from FIG. 2, the DOP of the system reaches 99.8%, M, in the detection frequency range PA And was 0.05 deg.
The invention is further illustrated by the following specific embodiment; it is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present invention without any creative work belong to the protection scope of the present invention. The specific characteristics of the chiral microstructure sensor (4) include: the chiral helical metal microstructures (9) are periodically attached to the front surface of the quartz glass substrate (8), as shown in fig. 3; the thickness of the quartz glass substrate (8) is 500 mu m; the chiral spiral metal microstructure (9) is made of gold material with the thickness of 200 nm; the period along the x-axis and y-axis directions is 200 μm, and the structure line width is 10 μm. An aqueous tyrosine solution with chiral enantiomers (D-form and L-form) was used as the sensing probe sample.
FIG. 4 shows some performance parameters of a chiral helical microstructure sensor. The results show that: r of the sensor LP And R CP The difference is significant at a specific frequency, PEA and PRA reach 44.5 degrees and 89.5 degrees at 0.67THz respectively, and the peak values of CD and OA spectra reach 44.8 degrees and 90 degrees at 0.58THz respectively, which shows that the sensor has stronger polarization conversion and artificial optical chirality at the specific frequency.
The RTDPS system is used for sensing and detecting D-tyrosine and L-tyrosine aqueous solutions with different concentrations. FIG. 5 shows the results of the PEA and PRA spectra sensing at linearly polarized incidence, where it can be seen that the peak frequency of the PEA spectrum is blue-shifted and the peak angle is significantly reduced around 0.57THz as the sample concentration increases; for the PRA spectrum, the peak values of different concentrations can reach the extreme value of 90 °, but the frequency of the abrupt change in the PRA spectrum shifts to a high frequency with the increase in concentration. Around the mutation frequency, the size of PRA varies dramatically with concentration. D-and L-tyrosine share a similar phenomenon for PEA and PRA. For polarization sensing, sensitivity can be defined as the change in polarization angle per change in sample concentration, i.e., S θ Where Δ θ is the change in the polarization angle at a certain frequency and Δ c is the corresponding change in the sample concentration, = Δ θ/Δ c, and S can be calculated from the experimental results of fig. 5 θ(D-PEA) ＝457.5°·mL/g，S θ(L-PEA) ＝727.7°·mL/g，S θ(D-PRA) ＝680.7°·mL/g，S θ(L-PRA) ＝1393.3°·mL/g。
From the above, the minimum polarization angle M of the RTDPS system PA Up to 0.05 deg., and therefore the minimum detection accuracy result based on the polarization angle parameter will be very high, as shown in the following equation:
where FoM (figure of merit) is used as a parameter to describe the sensitivity of sensing in a sample and the minimum concentration MC is the overall detection accuracy of the system. And calculating to obtain: the minimum detection concentration of PEA sensing and PEA sensing reaches 10 respectively -4 g/mL and 10 -5 On the order of g/mL. Therefore, the sample concentration can be detected by detecting the polarization state of the emergent waveHigh sensitivity sensing is achieved.
FIG. 6 shows the CD and OA spectral sensing results for D-and L-tyrosine at circularly polarized incidence. From the results, the peak value of the CD spectrum increases with the increase of the concentration of the sample at 0.56THz, and the peak values of the OA spectrum at different concentrations can reach an extreme value of 90 degrees, but the mutation frequency moves towards a high frequency direction with the increase of the concentration, which shows that the CD and OA spectra can well reflect the change of the concentration of the sample. S can be calculated through experimental results θ(D-CD) ＝586.5°·mL/g，S θ(L-CD) ＝648.5°·mL/g，S θ(D-OA) ＝886.9°·mL/g，S θ(L-OA) =912.4 °. ML/g. The CD sensing and the OA sensing reach 10 by the calculation of the formula (8) -4 On the order of g/mL.
The spectral differences between the chiral enantiomers were obtained by subtracting the polarization sensing spectra of D-and L-tyrosine, and the results are shown in FIG. 7. FIG. 7a shows the difference between the PEA and PRA spectra of D-and L-tyrosine at a concentration of 0.04g/mL, which can reach 18 and 40 degrees around 0.57THz, respectively; FIG. 7b shows the difference in CD and OA spectra for D-and L-tyrosine at a concentration of 0.04g/mL, which can reach 13 and 19 around 0.56THz, respectively. Fig. 7c shows the polarization ellipses of the output THz waves of D-and L-tyrosine at 0.57THz, and it can be seen that the output polarization states of D-and L-tyrosine, especially at the polarization rotation angle, have significant differences at the same concentration and frequency.
In addition, concentration sensing and chiral identification are carried out on other types of amino acid aqueous solutions, and similar experimental results are obtained. The method has the advantages that the high-sensitivity quantitative detection of the amino acid aqueous solution is realized by using the RTDPS system and the chiral spiral microstructure sensor, and the qualitative identification of the D-type enantiomer and the L-type enantiomer is further realized. Sensitivity of polarization sensing exceeds 10 3 The concentration sensing precision reaches 10 DEG.mL/g -5 The g/mL order far exceeds the sensitivity and precision of the traditional THz sensing system. The novel terahertz sensing system and the sensing method are expected to become an effective non-contact and non-labeled sensing method, and can be used for quantitative concentration detection and chiral enantiomer qualitative identification of chiral biological and chemical samples.
Claims (4)
1. A terahertz microstructure polarization sensing system facing a liquid chiral sample is characterized by comprising a front terahertz polarizer (1), a front terahertz lens (2), a metal reflecting prism (3), a chiral microstructure sensor (4), a sample pool (5), a rear terahertz lens (6) and a rear terahertz polarizer (7), wherein a chiral spiral metal microstructure (9) is periodically attached to the front surface of a quartz glass substrate (8) to form the chiral microstructure sensor (4), the chiral microstructure sensor (4) serves as the bottom of the sample pool (5), and the sample pool (5) is positioned above the metal reflecting prism (3); the linear polarization terahertz plane wave with the incident electric field vibrating along the vertical direction is transmitted along the horizontal direction, the polarization degree is improved through the front polarization piece (1), the linear polarization terahertz plane wave is focused through the front terahertz lens (2), and the terahertz plane wave is reflected to the chiral microstructure sensor (4) at a certain angle through the metal reflecting prism (3); the terahertz waves are reflected on the contact surface of the chiral spiral metal microstructure (9) and the sample, are recovered into plane waves which are transmitted in parallel through the rear terahertz lens (6), and are emitted through the rear polarizer (7); the reflection system can be arranged in a terahertz parallel light path of a traditional time domain spectroscopy system, and a sample preparation pool (5) is used for containing a sample solution; in addition, the amplitude and the phase of the emergent terahertz wave in two orthogonal directions can be respectively detected by rotating the direction of the rear terahertz polarizer (7), the complete polarization state of the emergent wave is reconstructed, and a polarization rotation angle spectrum, a polarization ellipse angle spectrum, a circular dichroism spectrum and an optical activity spectrum are obtained to characterize the polarization parameters of the detection sample; the polarization degree of the system reaches 99.8 percent in the detection frequency range, and the detectable minimum polarization angle is 0.05 degrees.
2. The terahertz microstructure polarization sensing system for the liquid chiral sample as claimed in claim 1, wherein: the quartz glass substrate (8) has a thickness of 300 to 500 [ mu ] m.
3. The terahertz microstructure polarization sensing system for the liquid chiral sample as recited in claim 1, wherein: the chiral spiral metal microstructure (9) is formed by etching on a gold film with the thickness of 200 nm; the period along the x-axis and the y-axis is 200 mu m, the line width of the unit structure is 5-15 mu m, the structure is asymmetric to the polarization direction of incident light, is not overlapped with a mirror image of the incident light, and has chirality.
4. The terahertz microstructure polarization sensing system for the liquid chiral sample as recited in claim 1, wherein: the concentration detection precision of the amino acid aqueous solution sample is 10 by utilizing the chiral microstructure sensor -5 g/mL magnitude; by the obvious difference of the polarization spectrum at the characteristic frequency position, the D-type chiral enantiomer and the L-type chiral enantiomer of the amino acid can be identified under the condition of the same concentration.
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