Material chirality detection method based on ultrafast terahertz emission spectrum

Abstract

The invention provides a chiral detection method which is widely suitable for testing opaque materials and is convenient to use. The material chirality detection method disclosed by the invention adopts an ultrafast terahertz emission spectrum technology, and the technology is very mature and has been widely applied to the field of scientific research. A circular polarization modulation technology required for exciting the circular polarization photoelectric effect is one of the most basic optical technologies. Therefore, based on a mature technology system, reliable and stable operation of chiral detection testing is facilitated. Meanwhile, the method disclosed by the invention is an all-optical non-contact detection means, does not need a transparent sample, has low requirements on sample treatment and test environment, is high in test speed, is visual and readable in result, and is suitable for being used as a widely applicable and efficient material chiral detection method.

Description

Technical Field

[0001] This invention belongs to the field of ultrafast lasers, and specifically relates to a method for detecting the chirality of a material under test. Background Technology

[0002] Chirality, as a novel quantum degree of freedom, holds immense promise for controlling material properties and developing multifunctional devices. One of the primary tasks in chirality research is determining the chirality of chiral materials before proceeding with studies on their properties and applications. The most mature and convenient method for chirality detection is circular dichroism-based spectroscopy; however, the signal intensity of this method is proportional to the interaction distance between the material and light, making it suitable only for chemical molecules and transparent materials. For opaque materials, such as metals, the light penetration depth is typically in the hundreds of nanometers, resulting in an insufficient interaction distance to generate observable signals, rendering this method unsuitable. Current methods for chirality detection in such materials include single-crystal X-ray diffraction, angle-resolved photoelectron spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction. However, these methods have stringent requirements for sample preparation, testing environment, and data analysis capabilities, resulting in low testing efficiency and limited widespread application. Therefore, there is an urgent need to develop a widely applicable and convenient chirality detection method, including for opaque materials. Summary of the Invention

[0003] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a chiral detection method that is widely applicable to testing, including opaque materials, and is convenient to use.

[0004] Before describing the content of this invention, the following terms are defined as follows:

[0005] The term "ultrafast pulsed laser" refers to a pulsed laser with a pulse duration of ≤1000 picoseconds.

[0006] The term "THz" refers to terahertz, also known as Terahertz or Terahertz waves. Terahertz (THz) waves generally refer to electromagnetic waves with frequencies ranging from 0.1 to 10 THz (wavelengths from 3000 to 30 μm). In the long-wavelength band, they overlap with millimeter waves, and in the short-wavelength band, they overlap with infrared light. It is a transitional zone from macroscopic classical theory to microscopic quantum theory, and also a transitional zone from electronics to photonics. The wavelength range of terahertz (THz) waves can cover the characteristic spectra of substances such as semiconductors, plasmas, organisms, and biological macromolecules.

[0007] The term "CPGE" refers to the circularly polarized photocurrent effect, also known as the circularly polarized photoelectric effect. It is a photocurrent phenomenon in which circularly polarized light incident on a solid material generates a directional current.

[0008] The term "current J" refers to the circularly polarized photovoltaic effect current, also known as CPGE current, denoted by the symbol J. It is a photocurrent generated by a second-order photoresponse, the magnitude of which is proportional to the light intensity (the square of the electric field strength of the light), and it is in the form of an injected current.

[0009] In the description of this invention, it should be understood that the terms "center", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0010] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0011] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0012] It should be noted that the structure described in this invention applies only to ultrafast pulsed laser amplifiers and does not include the laser oscillation generation and seed source sections. As long as the energy required for the amplifier input is met, the laser amplifier of this invention is not limited to the form or structure of the pre-stage laser oscillation stage or seed source.

[0013] To achieve the above objectives, the present invention provides a method for detecting the chirality of a test material, the method comprising:

[0014] 1) Irradiate a sample of the test material using a circularly polarized or ultrafast pulsed laser containing a circularly polarized component; and

[0015] 2) Analyze the terahertz waves emitted by the transient circularly polarized photocurrent generated in step 1).

[0016] Preferably, according to the method described above, the pulse range of the ultrafast pulsed laser is ≤100 picoseconds, more preferably less than 10 picoseconds, more preferably less than 1 picosecond, and most preferably less than 0.2 picoseconds.

[0017] Preferably, according to the method described above, step 2) further includes analyzing the direction of the transient circularly polarized photocurrent generated in step 1).

[0018] More preferably, according to the method described above, in step 2), the direction of the transient circularly polarized photocurrent generated in step 1) is analyzed by detecting the phase of the emitted terahertz wave.

[0019] Preferably, according to the method described above, the terahertz wave is collected by one or more methods selected from: parabolic mirror, terahertz lens, concave mirror, reflector, waveguide.

[0020] Preferably, according to the method described above, when analyzing the terahertz wave in step 2), if there is no calibration information or reference signal to assist, the electro-optic sampling signal of the excitation light with left or right circular polarization is detected to determine the phase direction information.

[0021] Preferably, according to the method described above, the analysis of the terahertz wave in step 2) is performed according to the following formula:

[0022] E THz (t)∝dJ / dt,

[0023] Where THz represents terahertz waves, and E... THz Let be the electric field intensity of the THz wave, J be the circularly polarized photovoltaic effect current, t be time, and dJ / dt be the time derivative of J.

[0024] Preferably, according to the method described above, the material being tested comprises an opaque material.

[0025] Preferably, according to the method described above, when analyzing the terahertz wave in step 2), the analysis includes detecting and analyzing the relative phase and absolute phase of the terahertz electric field.

[0026] Preferably, according to the method described above, the method is an all-optical and non-contact detection method.

[0027] Specifically, according to a preferred embodiment of the present invention, a method for detecting the chirality of materials based on ultrafast terahertz emission spectroscopy is provided. The detection method irradiates the chiral material with ultrafast pulsed light to generate a transient circularly polarized photoelectric effect that is dependent on the chirality of the material. The chirality of the material is detected by detecting and analyzing the terahertz waves emitted by this effect.

[0028] According to the preferred embodiment of the chiral detection method, the detection method selects an appropriate excitation wavelength based on the chiral electronic structure characteristics of the chiral material to ensure that the excitation of one type of chiral electron or a chiral electron within a specific momentum range is dominant.

[0029] According to the preferred embodiment of the chiral detection method, the detection method uses a circularly polarized state or an ultrafast light pulse containing a circularly polarized component to excite the chiral material to generate a transient circularly polarized photoelectric effect. The pulse width of the light pulse is no greater than the picosecond level, thereby enabling the radiation of terahertz waves through the transient circularly polarized photocurrent.

[0030] According to the preferred embodiment of the chirality detection method, the direction of the transient circularly polarized photocurrent depends on the chirality of the excited chiral electron, and the chirality of the electron is determined by the chirality of the material. Therefore, the chirality of the material can be detected by detecting the phase of the emitted terahertz wave (which reflects the direction of the transient circularly polarized photocurrent).

[0031] According to the preferred embodiment of the chirality detection method, the terahertz wave carrying material chirality information can be collected by means of a parabolic mirror, a terahertz lens, a concave mirror, a reflecting mirror, or a waveguide.

[0032] According to the chirality detection method described in this preferred embodiment, if there is no calibration information or reference signal to assist in terahertz signal analysis, the electro-optic sampling signal of the excitation light with left or right circular polarization can be detected. Taking advantage of the fact that this signal has a definite phase direction and has a clear configurational correlation with the terahertz signal of the sample, it is compared with the phase of the terahertz signal of the sample under test to determine the absolute chirality of the material.

[0033] In summary, this invention addresses the bottleneck problem in material chirality detection in existing technologies by proposing a novel ultrafast THz emission spectroscopy chirality detection method based on nonlinear optical effects. This method is based on the circularly polarized photoelectric effect (CPGE), a nonlinear optical effect closely related to the chiral electronic structure, which is determined by the chirality of the material. Since circularly polarized photons carry a spin angular momentum of |s| = 1, they can selectively excite electrons of a specific chirality in the material, causing their spin directions to flip, thus producing chiral anomalies (electronic chirality imbalance). Because chiral electrons have spin-momentum locked characteristics and chiral materials exhibit spatial symmetry breaking, chiral anomalies generate photocurrents with specific directions that depend on the material's chirality and the light's circular polarization state. Therefore, by detecting the direction of the CPGE current J excited by a certain circularly polarized light, the chirality of the material can be obtained: the J directions generated by left-handed and right-handed chirality are opposite (due to the opposite spin configurations of the corresponding chiral electrons). However, directly probing the photocurrent using conventional methods requires fabricating the material into micro / nano optoelectronic devices and carefully eliminating interference effects introduced by the device structure, which is still not conducive to efficient chiral detection. Therefore, this invention proposes irradiating chiral samples with circularly polarized or ultrafast pulsed lasers (picosecond or narrower pulse widths) containing circularly polarized components. This will generate a transient CPGE current, and according to the dipole radiation model, its picosecond-scale varying component can radiate terahertz (THz) waves: E THz (t)∝dJ / dt, where THz is the terahertz wave, E THz Let be the electric field intensity of the THz wave, J be the circularly polarized photovoltaic effect current, t be time, and dJ / dt be the time derivative of J.

[0034] Thus, the directional information of the aforementioned chiral current will be reflected in the phase of the THz wave: the phases of the THz waves generated by different chiralities of the material are opposite, such as... Figure 1 As shown, this method is an all-optical, non-contact detection technique that requires no transparent sample, has low requirements for sample processing and testing environment, offers fast testing speed, and provides intuitive and easy-to-read results. It is suitable as a widely applicable and efficient method for detecting the chirality of materials.

[0035] The material chirality detection method based on ultrafast THz emission spectroscopy proposed in this invention includes the following aspects:

[0036] 1) The chirality of the material under test is detected using a THz emission spectroscopy system and related techniques based on ultrafast pulsed laser (picosecond or narrower pulse width) excitation.

[0037] 2) Select an appropriate excitation wavelength based on the electronic structure of the material being tested, such as the bandgap, chiral electron distribution, photoresponse window, and Fermi level position.

[0038] 3) At the selected excitation wavelength, the ultrafast pulse light is modulated into a circularly polarized state or a state with a circularly polarized component (such as elliptically polarized) by a quarter-wave plate or other means, and then irradiated into the chiral material to excite the CPGE effect and thereby emit THz waves.

[0039] 4) The THz waves emitted by the material sample under test are collected by components such as parabolic mirrors, THz lenses, concave mirrors, reflectors, and waveguides, and the phase information of the emitted THz signals is detected by time-domain waveform detection methods such as electro-optic sampling.

[0040] 5) Since the specific direction of the CPGE current depends on the specific symmetry of the material and its relative orientation and azimuth angle in the testing system, and the direction of the CPGE current determines the emission direction of the THz wave, in order to ensure that the system can collect the THz signal emitted by the sample, the preferred configuration of the system is "45°-45°" "light incident-THz signal collection". At the same time, the sample holder must have the ability to rotate and adjust the sample in three axes to easily adjust the relative orientation of the sample, i.e. the direction of the CPGE current.

[0041] 6) If the signal of the material sample being tested is weak, the signal can be optimized by adjusting the light intensity, spot size, focusing, selecting a stronger light response wavelength, adjusting the polarization state, adjusting the relative orientation, and increasing the signal integration time. However, it should be noted that the light flux should be less than the damage threshold of the material being tested.

[0042] 7) Compare the phase information of the THz signal of the test material sample with the calibration information or reference signal to determine the chirality of the material; if there is no calibration information or reference signal, the electro-optic sampling signal of the left or right circularly polarized excitation light can be directly detected. Since this signal has a definite phase direction, it can replace the calibration information and be compared with the phase of the THz signal of the test sample to determine the chirality of the material.

[0043] This invention provides a method for detecting the chirality of materials based on ultrafast terahertz emission spectroscopy. Based on the principle that the electronic chiral structure of a material corresponds to its atomic chiral arrangement, and that there is an essential interaction between the electronic chiral structure and circularly polarized light, this invention utilizes ultrafast laser pulses containing circularly polarized components to irradiate the chiral material, exciting a transient circularly polarized photocurrent with a chirality-dependent direction. This current radiates terahertz waves, and the material's chirality information is reflected in the phase direction of the terahertz radiation. By detecting the phase of this terahertz radiation, the chirality of the material can be detected. This chiral detection method is an all-optical, non-contact detection technique with mature basic research, simple processing methods, wide applicability to various sample types, no need for transparent samples, low requirements for sample handling and testing environment, fast testing speed, and intuitive and easy-to-read results. It is suitable as a widely applicable and high-throughput method for detecting the chirality of materials.

[0044] Compared with existing technologies, the material chirality detection method based on ultrafast terahertz emission spectroscopy of the present invention can have, but is not limited to, the following beneficial effects:

[0045] 1. This invention utilizes ultrafast THz emission spectroscopy, a mature technology widely applied in scientific research. The circularly polarized light modulation technique required to excite the CPGE effect is one of the most fundamental optical techniques. Therefore, this invention is based on a mature technology system, facilitating reliable and stable operation of chiral detection testing.

[0046] 2. The method of the present invention is an all-optical non-contact detection method that does not require transparent samples, has low requirements for sample processing and testing environment, is fast in testing, and produces intuitive and easy-to-read results. It is suitable as a widely applicable and efficient method for detecting the chirality of materials. Attached Figure Description

[0047] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein:

[0048] Figure 1 This is a schematic diagram illustrating the principle of the material chirality detection method using ultrafast THz emission spectroscopy of the present invention.

[0049] Figure 2 This is a schematic diagram of the topological chiral rhodium tin oxide (RhSn) half-metal in Embodiment 1 of the present invention. The left side of the figure shows the first Brillouin zone and its high symmetry point, the right side shows the corresponding band structure, and the chiral band region is schematically marked in the dashed circle.

[0050] Figure 3 This is a schematic diagram of the chiral detection system based on ultrafast THz emission spectrum in Embodiment 1 of the present invention;

[0051] Figure 4 This is a schematic diagram of the detection method process in Embodiment 1 of the present invention;

[0052] Figure 5 The THz time-domain waveforms emitted by the rhodium tinide samples (sample-1 and sample-2) with different chiralities obtained in Example 1 of the present invention under 2μm light excitation with left circular polarization and right circular polarization;

[0053] Figure 6 The atomic arrangement along the

[111] direction is reconstructed from the single-crystal X-ray diffraction test data of rhodium tin oxide samples (sample-1 and sample-2) with different chirities in Example 1 of the present invention, wherein the smaller the size of the same atom, the deeper its depth, and the arrow indicates the direction of the helical arrangement of the atoms;

[0054] Figure 7This is a schematic diagram of the electro-optic sampling signals of the circularly polarized excitation pulse and the THz wave emitted by the sample under test, which are related to the crystal chirality, and the phase-locked relationship between the two, specifically a comparison of the electro-optic sampling signals of the left-circularly polarized and right-circularly polarized 2μm excitation light, and the electro-optic sampling signal of the right-circularly polarized 2μm excitation light and the THz wave it emits. Detailed Implementation

[0055] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, it should be understood that these embodiments are merely for more detailed and specific explanation and should not be construed as limiting the present invention in any way.

[0056] This section provides a general description of the materials and testing methods used in the experiments of this invention. While many of the materials and methods of operation used to achieve the objectives of this invention are well known in the art, the invention is still described in as much detail as possible herein. It will be apparent to those skilled in the art that, unless otherwise stated in the context, the materials and methods of operation used in this invention are well known in the art.

[0057] To further illustrate the invention, the chirality detection of topologically chiral rhodium tin oxide single crystal particles using this technique is demonstrated herein.

[0058] Example 1

[0059] This embodiment is an exemplary illustration of the material chirality detection method based on ultrafast terahertz emission spectroscopy of the present invention.

[0060] The detection method exemplified in this embodiment includes:

[0061] 1) Irradiate the sample of the topologically chiral half-metal rhodium tin (RhSn) test material using a pulsed laser generated by a 250kHz Ti:sapphire amplifier; and

[0062] 2) Analyze the terahertz waves emitted by the transient circularly polarized photocurrent generated in step 1).

[0063] like Figure 1 As shown, the chiral material selected for testing in Example 1 is a topologically chiral half-metal rhodium tin, which has a silvery-white metallic luster, is metallic, and is opaque. The single crystal particle size is about 2 mm, and the largest naturally exposed crystal face is about 1 mm, which is the (111) face, generally triangular. In this example, this face is uniformly selected for experimental testing.

[0064] like Figure 2As shown, based on the band structure of the tested material rhodium tinide, electrons with opposite chirality are distributed near the conical band structures at points Γ and R, respectively, and the energy difference between the topological chirality points Γ and R is approximately 0.3 eV, with point Γ being closer to the Fermi surface. Based on this characteristic and considering the laser parameters, an excitation wavelength of 2 μm was selected here.

[0065] Figure 3 A schematic diagram of the ultrafast THz emission spectroscopy system is shown. The excitation source is a pulsed laser generated by a 250 kHz Ti:sapphire amplifier, with a center wavelength of approximately 800 nm and a pulse width of approximately 100 fs. The 800 nm laser is amplified by an optical parametric amplification system to generate a 2 μm laser as the excitation light. The remaining 800 nm laser is attenuated and used as the probe light for electro-optic sampling and detection of the THz time-domain waveform. The system adopts a "45°-45°" "light incidence-THz signal collection" configuration: the linearly polarized 2 μm laser is modulated into a circularly polarized state by a quarter-wave plate and then incident on the rhodium tinplate (111) surface at a 45° incident angle, with a typical intensity of approximately 0.75 mJ / cm². 2 The spot diameter is 0.1 mm. The emitted THz wave is collected and collimated into parallel light by a gold parabolic mirror (aperture / focal length: 2-inch) with a 45° reflection direction, and then focused onto a 0.5 mm-ZnTe(110) crystal by another gold parabolic mirror. The 800 nm probe light is coaxially focused onto the ZnTe(110) crystal with the THz wave by a plano-convex lens with a focal length of 100 mm. The time-domain waveform of the THz wave is detected using standard electro-optic sampling technology and a balanced photodetector. The detection signal is output to a lock-in amplifier based on the optical chopper frequency for amplification.

[0066] Figure 4 A schematic diagram is shown of the chirality detection of topological chiral half-metal rhodium tinide single crystal particles using this ultrafast THz emission spectroscopy system.

[0067] Figure 5 The time-domain waveforms of THz waves emitted by rhodium tinide single crystal particles with opposite chirality, excited by the CPGE effect with left- and right-circularly polarized light, are shown. It can be seen that the THz waves excited by left- and right-circularly polarized light have opposite phases. Furthermore, for the same type of circularly polarized light excitation, the THz wave phases of different single crystal samples are also opposite, indicating that they have opposite chirality.

[0068] Figure 6 The crystal structures of the two rhodium tinide single crystal particles, reconstructed by single-crystal X-ray diffraction, are shown. It is evident that the helical arrangement of tin and rhodium atoms in the two samples is reversed, indicating that they indeed possess atomic structures with opposite chiral arrangements.

[0069] Figure 7The electro-optic sampling signals of circularly polarized excitation pulses and THz waves emitted by the sample under test, which are related to the crystal chirality, are shown, along with their phase-locking relationship. Specifically, the electro-optic sampling signals of left- and right-circularly polarized 2μm excitation pulses are compared, as well as the electro-optic sampling signals of right-circularly polarized 2μm excitation pulses and their emitted THz waves.

[0070] Because THz signals generated by different chiralities have opposite phases, the relative chirality of a sample can be detected; that is, samples with opposite chirality have opposite THz signals. However, in the absence of a reference signal, since the chirality of a sample cannot be determined by its sign, the positive or negative phase itself is meaningless.

[0071] Through extensive experiments, the inventors discovered that the reflected signal of a circularly polarized excitation light has a fixed phase relationship with the THz signal generated by a sample with a specific chirality, which can be used as a reference signal to detect the absolute chirality of the sample. In this embodiment, the electro-optic sampling waveforms of left- and right-circularly polarized 2μm light pulses also have opposite phases, exhibiting a square wave shape. By controlling their polarization, the tester obtains a 2μm light pulse electro-optic sampling waveform with a defined phase. By comparing the waveforms of the 2μm light and the THz light it excites, it can be found that the right edge of the 2μm waveform coincides with the left peak of the THz wave, thus accurately locating their relationship. Then, by utilizing the characteristic of the fixed phase of the electro-optic sampling of a given circularly polarized 2μm light pulse, the absolute chirality of the sample can be easily determined by analyzing whether the THz peak phase is in phase or out of phase with respect to the 2μm light signal.

[0072] While the effects of some embodiments have been shown above, those skilled in the art should understand that, based on the concept of the invention, other embodiments not specifically shown or other technical solutions of the invention not shown in the embodiments can also achieve the same technical effects as those claimed in the summary section:

[0073] 1. This invention utilizes ultrafast THz emission spectroscopy, a mature technology widely applied in scientific research. The circularly polarized light modulation technique required to excite the CPGE effect is one of the most fundamental optical techniques. Therefore, this invention is based on a mature technology system, facilitating reliable and stable operation of chiral detection testing.

[0074] 2. The method of the present invention is an all-optical non-contact detection method that does not require transparent samples, has low requirements for sample processing and testing environment, is fast in testing, and produces intuitive and easy-to-read results. It is suitable as a widely applicable and efficient method for detecting the chirality of materials.

[0075] Although the invention has been described to a certain extent, it is apparent that appropriate variations can be made to the various conditions without departing from the spirit and scope of the invention. It is understood that the invention is not limited to the described embodiments, but falls within the scope of the claims, which include equivalent substitutions for each of the elements.

Claims

1. A method for detecting the chirality of a test material, characterized in that, The method includes: 1) Irradiate a sample of the test material using a circularly polarized or ultrafast pulsed laser containing a circularly polarized component; and 2) Analyze the terahertz waves emitted by the transient circularly polarized photocurrent generated in step 1).

2. The method according to claim 1, characterized in that, The pulse range of the ultrafast pulsed laser is less than 100 picoseconds, preferably less than 10 picoseconds, more preferably less than 1 picosecond, and most preferably less than 0.2 picoseconds.

3. The method according to claim 1 or 2, characterized in that, Step 2) also includes analyzing the direction of the transient circularly polarized photocurrent generated in step 1).

4. The method according to claim 3, characterized in that, In step 2), the direction of the transient circularly polarized photocurrent generated in step 1) is analyzed by detecting the phase of the emitted terahertz wave.

5. The method according to any one of claims 1-4, characterized in that, The terahertz waves are collected by one or more of the following methods: parabolic mirror, terahertz lens, concave mirror, reflector, waveguide.

6. The method according to any one of claims 1-5, characterized in that, In step 2), when analyzing the terahertz wave, if there is no calibration information or reference signal to assist, the electro-optic sampling signal of the excitation light with left or right circular polarization is detected to determine the phase direction information.

7. The method according to any one of claims 1-6, characterized in that, The analysis of the terahertz wave in step 2) is performed according to the following formula: E THz (t)∝dJ / dt, Where THz represents terahertz waves, and E... THz Let be the electric field intensity of the THz wave, J be the circularly polarized photovoltaic effect current, t be time, and dJ / dt be the time derivative of J.

8. The method according to any one of claims 1-7, characterized in that, The material being tested includes opaque materials.

9. The method according to any one of claims 1-8, characterized in that, When analyzing the terahertz wave in step 2), the relative phase and absolute phase of the terahertz electric field are detected and analyzed.

10. The method according to any one of claims 1-9, characterized in that, The method described is an all-optical and non-contact detection method.

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