Molecular terahertz spectrum measurement method and system based on plasmon nanometer optical tweezers

Abstract

The invention discloses a molecular terahertz spectrum measurement method and system based on plasmon nano optical tweezers, and belongs to the technical field of molecular spectrum detection and nano manipulation. A local enhanced light field generated by a plasmon nano structure is utilized to capture and fix nanoparticles of a single target molecule or a loaded molecule in an aqueous solution; generating frequency-tunable terahertz waves by adopting a double-laser beat frequency technology, and coupling the terahertz waves to the local light field to excite a captured target; monitoring the optical signal change caused thereby in real time; acquiring a characteristic vibration spectrum of the target molecule by scanning the terahertz frequency; the plasmon nano optical tweezers and the optical beat frequency terahertz technology are combined, so that the terahertz vibration spectrum of the single molecule or nano particle level in the aqueous solution environment is directly measured, and the core problem that the traditional terahertz technology cannot be used for liquid-phase biological detection due to strong absorption of water is solved.

Description

Technical Field

[0001] This invention relates to the field of molecular spectroscopy detection and nanomanipulation technology, specifically to a method and system for measuring molecular terahertz spectra based on plasmonic nanotweezers. Background Technology

[0002] Plasmon optical tweezers are a novel nanomanipulation technique based on surface plasmon resonance. By exciting localized plasmon fields on the surface of metal nanostructures, they achieve spatial focusing and enhancement of optical forces at the subwavelength scale. This technique utilizes the strong localized electromagnetic fields generated by noble metal nanostructures under specific wavelength illumination, overcoming the optical diffraction limit to stably capture particles with diameters down to the nanometer scale, including biological nanoparticles such as viral particles, exosomes, and protein complexes. Compared to traditional optical tweezers, which rely on high numerical aperture objectives with millimeter-level working distance limitations, plasmon optical tweezers, through the integration of nanoantenna arrays, can precisely confine the manipulation area within 100 nm of the substrate surface, simultaneously enabling parallel manipulation of multiple particles and real-time dynamic monitoring. In single-molecule biophysics research, this technique has been successfully applied in areas such as DNA conformation regulation and the analysis of molecular motor mechanical properties.

[0003] Terahertz waves are electromagnetic waves with frequencies ranging from 0.1 THz to 10 THz, corresponding to wavelengths from 30 m to 3 mm, falling within the spectrum between microwaves and infrared light. This band has attracted significant attention due to its unique physical properties. Its photon energy is only in the millielectron volt range (4.1 meV at 1 THz), far below the ionization threshold of X-rays, giving it a non-destructive advantage in biological sample detection. Simultaneously, terahertz waves can excite weak intermolecular interactions, forming unique fingerprint spectra that provide high specificity for substance identification. Furthermore, terahertz waves have the ability to penetrate nonpolar materials and are sensitive to polar substances; this characteristic is widely used in security imaging and materials analysis.

[0004] However, the strong absorption of terahertz waves by water severely limits its application in aquatic environments. Traditional terahertz spectroscopy typically requires samples to be prepared in a dry state, which often leads to changes in the structure of biomolecules in biomedical applications, affecting the accuracy of detection results. Plasmon optical tweezers utilize the localized surface plasmon resonance effect induced by the surface of metal nanostructures to overcome the diffraction limit of traditional optical trapping techniques, achieving nanoscale precision particle manipulation. However, traditional plasmon optical tweezers suffer from thermal effects; the heat generated when lasers irradiate metal nanostructures may affect the structure and function of biomolecules, leading to distorted measurement results.

[0005] While there have been independent studies on plasmonic optical tweezers and terahertz spectroscopy in the existing technology, there is still no systematic solution that effectively combines the two to simultaneously solve the problems of thermal effects and application limitations in aquatic environments.

[0006] Therefore, there is an urgent need for a measurement method that can combine the precise manipulation capabilities of plasmonic optical tweezers with the non-destructive detection capabilities of terahertz spectroscopy, while avoiding the thermal effects of plasmonic optical tweezers. Summary of the Invention

[0007] The purpose of this invention is to provide a method and system for measuring molecular terahertz spectroscopy based on plasmonic nanotweezers, so as to achieve high-sensitivity and high-spatial-resolution terahertz vibrational spectrum detection of single molecules or nanoparticles in an aqueous environment. Furthermore, this invention is supported by the Shanxi Provincial Postgraduate Innovation Project (Project Approval No.: 2024SJ270).

[0008] Therefore, this invention provides a method for measuring molecular terahertz spectroscopy based on plasmonic nanotweezers, comprising the following steps: S1. Prepare plasmonic nanostructures with local electric field enhancement effect for optical tweezers trapping units; S2. Place the sample solution to be tested on the plasmonic nanostructure, and use the incident laser to excite the plasmonic resonance of the structure to form an optical potential well in a specific region of the structure, which is used to locate and stably bind the molecule to be tested to the detection region. S3. A frequency-tunable terahertz wave signal is generated using dual-laser beat frequency technology, and the signal is coupled with the trapping optical field in step S2 to act on the trapped target molecules or nanoparticles to excite their characteristic vibrations in the terahertz frequency band. S4. Monitor and record in real time the changes in transmitted light, scattered light or reflected light signals caused by the trapped particles under the excitation of the terahertz signal; S5. Scan the frequency of the terahertz signal and obtain the terahertz vibration spectrum of the target molecule or nanoparticle based on the changes in the optical signal recorded in step S4.

[0009] Preferably, in step S1, the plasmonic nanostructure is a dual nanopore structure prepared on a metal thin film. The dual nanopore structure includes two cylindrical holes etched on a gold film with a thickness of 100 nm. The radius of the cylindrical holes is 110 nm, and the distance between the tips of the two holes is 30 nm.

[0010] Preferably, the plasmonic nanostructures are prepared by electron beam lithography and focused ion beam etching processes.

[0011] Preferably, in step S2, the specific steps for locating and stably binding the analyte molecule within the detection region are as follows: (201) Initial parameter settings: Set its output wavelength to 852nm, which matches the resonant wavelength of the designed plasmonic structure, and set the laser operating current to about 136mA through the laser driver so that the actual incident light power reaching the chip plane after optical path attenuation is about 6mW. (202) Sample loading and chip positioning: The prepared plasmonic chip is fixed on the sample stage, the sample solution to be tested is dropped onto the chip surface, and a coverslip is placed on top to form a liquid immersion environment. The marking lines on the chip surface are observed through the CCD camera of the microscope. (203) Optical field alignment to establish potential well: manipulate the three-dimensional displacement stage to precisely move the selected target DNH structure to the focal position of the capture laser, slightly adjust the displacement stage, and monitor the transmitted light intensity signal detected by the APD in real time. The process ends when the laser focal point and the hot spot area of ​​the DNH structure are optimally coincident. (204) Target particle capture confirmation: Maintain the alignment between the laser and the structure, and confirm whether the target particle has been captured by monitoring the voltage output signal of the APD. When a single particle is successfully captured, the APD signal will undergo a significant step-like upward jump. (205) State maintenance: After confirming that a single particle has been stably captured, the system enters the test state, keeping all optical and electrical parameters unchanged, and waiting for the introduction of a terahertz excitation signal.

[0012] Preferably, in step S2, the wavelength of the incident laser matches the resonant wavelength of the plasmonic nanostructure, and the polarization direction of the incident laser is parallel to the line connecting the two tips of the dual nanopores.

[0013] Preferably, in step S3, the dual-laser beat frequency technology uses two lasers with similar wavelengths and coherent phases to interfere, one of which is a fixed laser with a fixed wavelength and the other is a tunable laser with a tunable wavelength. The frequency difference between the two lasers is the frequency of the generated terahertz signal.

[0014] Preferably, in step S3, the specific steps for coupling the frequency-tunable terahertz wave signal with the captured optical field are as follows: (301) Laser parameter settings: The parameters of the fixed laser are set as follows: turn on the numerically controlled tunable semiconductor laser, set the output wavelength to a fixed value according to the terahertz frequency band of the target detection through its control software, set its operating current to a stable value, and preheat for a sufficient time to ensure wavelength and power stability; The parameters of the tunable laser are set as follows: turn on the distributed Bragg reflector laser and its matching laser diode driver, set its operating current to a fixed value, then set its initial operating temperature through the driver, and wait for the temperature control system to stabilize. (302) Beat frequency system calibration: First, calibrate the tunable laser, gradually change its set temperature, and at the same time use a high-precision wavelength meter to measure the actual output wavelength corresponding to each temperature point, and build a lookup table of set temperature and output wavelength. (303) Calculate and verify the beat frequency: based on In the formula, is the speed of light. To fix the wavelength of the laser, To calculate the wavelength of the tunable laser, different... Corresponding beat frequency Using a free-space ultrafast photodetector, temporarily connected to the beat frequency optical path, verification measurements were performed at several frequency points until it was confirmed that the frequency of the actual generated beat frequency signal matched the calculated value. (304) Beat frequency signal generation: Connect the output fibers of the fixed laser and the tunable laser to the two input ports of a 2x2 fiber coupler respectively. The two lasers with similar frequencies and coherent phases interfere in the coupler and generate a beat frequency optical signal with intensity periodically modulated by the frequency difference at one of its output ports, which is the carrier of terahertz excitation. (305) Optical path coupling: The output of the fiber optic cable that generates the beat frequency signal is collimated into spatial light through a collimator, guided by a series of mirrors and lenses, and finally combined with the 852nm optical path from the capture laser through a dichroic mirror. The combined beam is collected and focused onto the chip by the same 100x oil immersion objective lens. (306) Start terahertz excitation and system status confirmation: After confirming that the target molecule has been stably captured, turn on the fixed laser and the tunable laser at the same time, introduce the beat frequency signal into the system, and closely monitor the voltage signal of the APD.

[0015] Preferably, in step S4, the change in transmitted light intensity is monitored by a photodetector, and the fluctuation signal of the light intensity is used to characterize the vibrational response of the captured particles under terahertz excitation.

[0016] Preferably, in step S5, the characteristic vibrational peaks of the target molecule are identified by analyzing the standard deviation, probability density function, or calculated free energy curve of the light intensity fluctuation signal with terahertz frequency.

[0017] A molecular terahertz spectrum measurement system based on plasmonic nano-optical tweezers includes an optical tweezers system, a beat frequency excitation subsystem, and a signal detection subsystem; The optical tweezers system includes a trapping laser, a beam shaping assembly, a dichroic mirror, a high numerical aperture objective lens, and a three-dimensional displacement stage, used to form and manipulate an optical potential well on a plasmonic nanostructure.

[0018] The beat frequency excitation subsystem includes a fixed wavelength laser, a tunable wavelength laser, an optical fiber coupler, and a beam combiner, used to generate and couple a tunable terahertz wave to the capture region of the optical tweezers system. The signal detection subsystem includes an avalanche photodetector, a data acquisition card, and a computer, which are used to collect and process changes in optical signals caused by the captured particles in real time.

[0019] The present invention proposes a method and system for measuring molecular terahertz spectroscopy based on plasmonic nanotweezers, the advantages of which are as follows: This invention combines plasmonic nanotweezers with optical beat frequency terahertz technology. By utilizing the nanohot spots of the optical tweezers, the target molecules are precisely bound and enriched in a very small detection volume, making the interaction strength between the terahertz waves and the molecules far exceed the absorption of bulk water. This enables the direct measurement of the terahertz vibration spectrum at the level of single molecules or nanoparticles in an aqueous environment, solving the core problem that traditional terahertz technology cannot be used for liquid phase biological detection due to strong water absorption. The local field enhancement generated by the plasmon structure by more than 100 times greatly amplifies the weak optical response signal generated by molecules under terahertz excitation, significantly improving the detection sensitivity. Moreover, the nanoscale manipulation precision of the optical tweezers provides a super spatial resolution far superior to the diffraction limit, enabling the localization and detection of individual biomolecules. Plasmon enhancement allows the system to operate stably at low incident laser power in the milliwatt range, greatly reducing the damage of photothermal effects to sensitive biological samples, ensuring the in-situ nature and biological activity of the detection process, and laying the foundation for activity and dynamic studies. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application or related technologies, the accompanying drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 Figure for this invention; Figure 2 This is a schematic diagram of the DNH structure of the present invention; Figure 3 This is a schematic diagram of the single-particle trapping optical tweezers device system of the present invention; Figure 4 This is a schematic diagram of the dual-laser beat frequency signal of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described in this specification are merely for explaining the invention and are not intended to limit the invention.

[0023] Example: Please see Figure 1-4 This invention provides a method for measuring molecular terahertz spectroscopy based on plasmonic nanotweezers, taking the detection of the neurotransmitter molecule aminobutyric acid (GABA) as an example: GABA, as the most important inhibitory neurotransmitter in the central nervous system, ensures the balanced transmission of nerve signals by regulating neuronal excitability. Abnormal GABA levels or function may lead to neurological disorders such as anxiety, epilepsy, and insomnia. The GABA molecule contains an amino group (… ) and a carboxyl group ( It consists of a chain of four carbon atoms. It exhibits a unique absorption peak in the range of 0.6 THz to 2.5 THz. When molecules or functional groups absorb terahertz waves, absorption at different wavelengths leads to different vibrational modes. Traditional terahertz spectroscopy detection methods (such as THz-TDS and FDS) provide important tools for biomolecular conformation, interactions, and pathological diagnosis through time-domain or frequency-domain analysis. However, their application is limited by water absorption, and the analyte usually needs to be in a dry state.

[0024] However, water is not only a solvent for biomolecules, but also a regulator of their structural stability, dynamic behavior, and functional realization. Detecting the terahertz vibrational spectra of biomolecules in an aqueous environment can better characterize important information about molecular function. A dual-nanopore structure was fabricated on a glass substrate covered with a 100 nm gold film (with a 2 nm titanium adhesion layer) using focused ion beam (FIB) etching. The structural parameters are: hole radius R = 110 nm, distance between the two hole tips W = 30 nm. This structure is effective at 852 nm wavelength and polarization angle... Under laser excitation, a local electric field enhancement of approximately 150 times can be generated; GABA molecules were covalently coupled to 20 nm gold nanoparticles with polyethylene glycol surface modification using the carbodiimide method to form... The complex was dispersed in deionized water.

[0025] The trapping laser, with a wavelength of 852nm, is expanded and collimated before being focused onto a dual-nanopore structure on the chip via a dichroic mirror and a 100x oil-immersion objective lens, exciting plasmon resonance and forming a trapping potential well. A fixed-wavelength laser (852.121nm) and a tunable laser (with a wavelength range of 852.15-853.11nm) are combined via an optical fiber coupler to generate a beat frequency signal tunable in the range of 1.0-1.6THz. This beat frequency signal is introduced through another optical path and superimposed on the trapping light field. The sample solution was dropped onto the chip, and the stage was adjusted to focus the laser onto a dual-nanopore structure. The capture laser was then activated at approximately 6mW, allowing the system to stably capture a single nanopore. Composite particles. The dual-laser beat frequency system is activated, the temperature of the second laser is set, and a fixed frequency is generated corresponding to a specific wavelength. excitation; Avalanche photodetectors were used to monitor the light intensity signal passing through the dual nanopore structure in real time, and the signal was recorded via a data acquisition card. While maintaining the capture state, the temperature of the second laser was controlled by a program, and frequency scanning was performed in two bands: 1.05-1.24 THz and 1.41-1.63 THz, in increments of approximately 23 GHz. Light intensity signals were acquired for 20 seconds at each frequency point. The standard deviation of the light intensity signal collected at each frequency point was calculated. A vibrational spectrum was plotted with frequency in THz on the x-axis and standard deviation (STD) on the y-axis. The results showed distinct vibrational peaks near 1.18 THz and 1.58 THz, consistent with the theoretical vibrational modes of the GABA molecule. To verify that the signal originates from GABA, a control experiment was conducted using uncoupled GABA. When particles are scanned under the same conditions, their STD signal remains at a low background level with no obvious peak. The original signals at three frequency points—1.18 THz (peak), 1.08 THz, and 1.23 THz (valley)—were analyzed using probability density function (PDF) and free energy calculations. The results show that the signal fluctuation amplitude is largest at 1.18 THz, the PDF distribution is more dispersed, and the free energy potential well is wider and shallower, confirming that the molecules are effectively excited to produce significant vibrations at this frequency.

[0026] See Figure 4An 852nm laser, after collimation and beam expansion, is projected onto a 100x oil immersion microscope objective via a dichroic mirror. Markings are used to locate the DNH (Dichroic Noise Detector) structure. The stage is adjusted to focus the incident laser onto the DNH structure. The optical signal is transmitted to an avalanche photodetector via a mirror, converting the optical information into an electrical signal, which is then transmitted to a computer via a data acquisition card. The laser is turned on or off by controlling the LOn / Off switch of the TL4000 Series device. APD: Avalanche photodetector; DM: Dichroic mirror; BE: Beam expander; MR: Mirror; 100OIMO: 100x oil immersion objective; 10MO: 10x objective; CCD: Camera. The experiment uses a Toptica digitally controlled tunable semiconductor laser, DLCDLpro850, with a wavelength range of 840nm to 875nm, a linewidth of 100kHz, and an operating current of 136mA. The photodetector used is Thorlabs (APD440A2), with a detection surface diameter of 1.0 mm and a wavelength range of 200 nm to 1000 nm.

[0027] This invention combines plasmonic nanotweezers with optical beat frequency terahertz technology. By utilizing the nanohotspots of the optical tweezers to precisely bind and enrich target molecules within a very small detection volume, the interaction strength between terahertz waves and molecules far exceeds the absorption of bulk water. This enables direct measurement of terahertz vibrational spectra at the level of single molecules or nanoparticles in an aqueous environment, solving the core problem that traditional terahertz technology cannot be used for liquid-phase biological detection due to strong water absorption.

[0028] A molecular terahertz spectroscopy measurement system based on plasmonic nanotweezers It includes an optical tweezers system, a beat frequency excitation subsystem, and a signal detection subsystem; the optical tweezers system includes a trapping laser, a beam shaping assembly, a dichroic mirror, a high numerical aperture objective lens, and a three-dimensional displacement stage, used to form and manipulate an optical potential well on a plasmonic nanostructure; The laser acquisition unit employs a Toptica DLCDLpro850 digitally controlled tunable semiconductor laser with a working wavelength of 852 nm and a linewidth of 100 kHz. The output power is adjusted via current, and after optical attenuation, approximately 6 mW reaches the chip plane. The beam shaping assembly includes a beam expander and collimator to expand and collimate the laser beam into uniform parallel light. A dichroic mirror reflects the acquired laser to a high numerical aperture objective lens, which focuses it to form a stable optical potential well on the surface of the plasmon nanostructure for capturing target single molecules or nanoparticles. A three-dimensional displacement stage enables precise control of the sample chip, ensuring the acquisition position coincides with the terahertz near-field hotspot. The beat frequency excitation subsystem generates a difference frequency signal from two tunable terahertz sources to excite characteristic vibrations of the target material. After being fixed by optical tweezers, the signal enhances the local electromagnetic field, significantly improving the signal-to-noise ratio. The system also includes an LED light source, a 10x objective lens, and a CCD camera to assist in locating marker lines on the chip and observing the sample area.

[0029] The beat frequency excitation subsystem includes a fixed wavelength laser, a tunable wavelength laser, an optical fiber coupler, and a beam combiner, used to generate and couple a tunable terahertz wave to the capture region of the optical tweezers system. The fixed-wavelength laser, also known as the aforementioned trapping laser, serves as a fixed-frequency light source in this subsystem. The tunable-wavelength laser is a Thorlabs DBR852PN distributed Bragg reflector laser, controlled by temperature and current via its matching laser diode driver (CLD1015), achieving precise tuning within the wavelength range of 852.15nm to 853.11nm, with tuning steps corresponding to frequency variations of approximately 23GHz. The fiber coupling and combining unit connects the output fibers of the two laser beams to the two input ports of a 22-mode single-mode fiber coupler. Interference occurs within the coupler, generating a beat-frequency optical signal at one of its output ports, with the intensity periodically modulated by the frequency difference between the two laser beams. This refers to the generated terahertz excitation frequency (range 1.0-1.6THz); Spatial light coupling optical path: The beat frequency signal fiber output end is converted into spatial light by a collimator, guided by a mirror, and finally coaxially coupled with the captured laser optical path through the same dichroic mirror (used to capture laser beam combining), and then collected and focused by a 100x oil immersion objective lens.

[0030] The signal detection subsystem includes an avalanche photodetector, a data acquisition card, and a computer, which are used to collect and process changes in optical signals caused by trapped particles in real time. The photodetector is a Thorlabs APD440A2 avalanche photodetector with a detection surface diameter of 1.0 mm and a spectral response range of 200-1000 nm, used for high-sensitivity detection of transmitted light intensity through plasmon structures. The voltage signal output by the APD is acquired in real time by a high-speed data acquisition card (sampling rate 150 kHz) and transmitted to a computer. Signal processing is performed using dedicated software, including calculation of the standard deviation (STD), probability density function (PDF), and free energy curve.

[0031] This system supports two high-performance plasmon trapping structures, both fabricated on a chip. The first is a dual-nanopore structure, with a cleaned glass substrate. Metal thin film deposition is performed using magnetron sputtering, sequentially depositing a 2nm titanium adhesion layer and a 100nm gold layer. Focused ion beam etching is then used to etch the dual-nanopore structure onto the 100nm gold film. The key geometric parameters are: radius R = 110nm for a single cylindrical hole and distance W = 30nm between the tips of the two holes. The first structure, when excited by laser at a wavelength of 852 nm with a polarization direction parallel to the line connecting the two holes, can generate a local electric field enhancement of approximately 150 times. The second type is a silicon-gold ring hybrid nanostructure. Its substrate is silicon dioxide. A silicon disk with a thickness of 100 nm and a radius of 450 nm is fabricated using electron beam lithography and reactive ion etching. Identical double nanopores are etched in the central region of the silicon disk using FIB. Then, using electron beam lithography and metal lift-off processes, a gold ring with an inner radius of 500 nm, a ring width of 600 nm, and a thickness of 100 nm is fabricated at a horizontal distance of 100 nm from the edge of the silicon disk. This hybrid structure excites the Anapole mode of the silicon disk near 888 nm and couples it with the plasmon resonance of the gold ring, achieving an electric field enhancement of over 80 times in the DNH region while significantly reducing the Ohmic heating effect.

[0032] The prepared plasmonic laser chip, taking the first type as an example, is fixed on the sample stage. The CCD camera and illumination are turned on, and the displacement stage is moved. Using the marking lines on the chip, the target dual-nanopore structure is roughly positioned to the center of the field of view. The trapping laser is turned on, and the three-dimensional displacement stage is finely adjusted while the APD signal is monitored in real time. When the laser focus optimally coincides with the hot spot region of the DNH structure, the transmitted light intensity signal tends to stabilize, indicating that the optical potential well has been established. The sample solution is dropped onto the chip, and a coverslip is placed on top. Under Brownian motion, the probe particles diffuse to the vicinity of the potential well. When a single particle is trapped, the voltage signal output by the APD will show a significant step-like upward jump, indicating successful trapping and the system entering a stable trapping state.

[0033] Maintain the capture state and activate the beat frequency excitation subsystem. Set the fixed laser wavelength to 852.121 nm, and control the tunable laser according to a pre-calibrated temperature-wavelength relationship table. Set the scanning frequency range, gradually changing the temperature of the tunable laser in approximately 23 GHz increments, thereby altering the beat frequency. At each frequency point, the APD continuously acquires the transmitted light intensity signal for 20 seconds, which is then recorded by the data acquisition card.

[0034] The standard deviation of the time-domain optical intensity signal acquired at each frequency point is calculated, and a vibration spectrum is plotted with terahertz frequency as the x-axis and STD value as the y-axis. The system features modular design for each subsystem, clear operation procedures, good repeatability and stability, and high integration.

[0035] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for measuring molecular terahertz spectrum based on plasmonic nanophotonic tweezers, characterized in that: The method comprises the following steps: S1, preparing a plasmonic nanostructure with a local electric field enhancement effect for a light tweezer capture unit; S2, placing a sample solution to be tested on the plasmonic nanostructure, exciting plasmonic resonance of the structure by using incident laser light, and forming an optical potential well in a specific region of the structure for positioning and stably binding the molecules to be tested in a detection region; S3, generating a frequency-tunable terahertz wave signal by using a double-laser beat frequency technique, coupling the signal with the capture optical field in step S2, and acting on the captured target molecules or nanoparticles to excite characteristic vibrations in the terahertz band of the target molecules or nanoparticles; S4, monitoring and recording in real time changes in the transmission light, scattering light or reflection light signals caused by the captured particles under the excitation of the terahertz signal; S5, scanning the frequency of the terahertz signal, and obtaining a terahertz vibration spectrum of the target molecules or nanoparticles according to the recorded changes in the light signals in step S4.

2. The method according to claim 1, wherein the method is characterized by: In step S1, the plasmonic nanostructure is a double-nanopore structure prepared on a metal thin film, and the double-nanopore structure comprises two cylindrical pores etched on a gold film with a thickness of 100 nm, the radius of the cylindrical pores is 110 nm, and the distance between the tips of the two pores is 30 nm.

3. The method according to claim 1, wherein the method is characterized by: The plasmonic nanostructure is prepared by electron beam lithography and focused ion beam etching processes.

4. The method according to claim 1, wherein the method is characterized by: In step S2, the specific operation steps for positioning and stably binding the molecules to be tested in the detection region are as follows: (201) initial parameter setting: the output wavelength is set to 852 nm which matches the resonance wavelength of the designed plasmonic structure, and the laser current is set to about 136 mA through a laser driver, so that the actual incident light power reaching the chip plane after light attenuation is about 6 mW; (202) sample loading and chip positioning: fixing the prepared plasmonic chip on a sample stage, dropping the sample solution to be tested on the chip surface, covering it with a cover glass to form a liquid immersion environment, and observing the mark line on the chip surface through the CCD camera of the microscope; (203) light field alignment and potential well establishment: controlling the three-dimensional displacement table to accurately move the selected target DNH structure to the focal point position of the capture laser, slightly adjusting the displacement table, and monitoring the transmission light intensity signal detected by the APD in real time, and ending when the laser focal point and the hot spot region of the DNH structure are optimally coincided; (204) target particle capture confirmation: keeping the alignment state of the laser and the structure, confirming whether the target particle is captured by monitoring the voltage output signal of the APD, and when a single particle is successfully captured, the APD signal will have a significant stepwise upward jump; (205) state maintenance: confirming that a single particle is stably captured, the system enters a detection state, all optical and electrical parameters remain unchanged, and the terahertz excitation signal is introduced.

5. The method according to claim 1, wherein the method is characterized by: In step S2, the wavelength of the incident laser matches the resonance wavelength of the plasmonic nanostructure, and the polarization direction of the incident laser is parallel to the direction of the line connecting the tips of the two nanopores.

6. The method according to claim 5, wherein the method is characterized by: In step S3, the dual laser beat frequency technology uses two phase-coherent lasers with similar wavelengths to interfere, one of which is a fixed laser with a fixed wavelength, and the other is a tunable laser with a tunable wavelength, and the frequency difference between the two lasers is the frequency of the generated terahertz signal.

7. The method according to claim 1, wherein the method is characterized by: In step S3, the specific steps of coupling the frequency-tunable terahertz wave signal with the captured light field are: (301) Laser parameter setting: the parameter setting of the fixed laser is to turn on the numerical control adjustable semiconductor laser, set the output wavelength to a fixed value according to the target detection terahertz frequency band through its control software, set the working current to a stable value, and preheat for a sufficient time to ensure the stability of the wavelength and power; the parameter setting of the tunable laser is to turn on the distributed Bragg reflector laser and its matching laser diode driver, set the working current to a fixed value, then set the initial working temperature through the driver, and wait for the temperature control system to stabilize; (302) Beat frequency system calibration: first, calibrate the tunable laser, gradually change the set temperature, and measure the actual output wavelength at each temperature point using a high-precision wavemeter to build a lookup table of temperature and output wavelength; (303) Calculate the verification beat frequency: according to , where c is the speed of light, is the wavelength of the fixed laser, is the wavelength of the adjustable laser, calculate the different corresponding beat frequency , using a free-space ultrafast photodetector, temporarily access the beat frequency light path, and perform verification measurements on several frequency points until the actual generated beat signal frequency is confirmed to match the calculated value; (304) Beat frequency signal generation: connect the output fibers of the fixed laser and the tunable laser to the two input ports of a 2x2 fiber coupler, respectively, and the two phase-coherent lasers with similar frequencies interfere in the coupler to generate a beat frequency light signal with periodic intensity modulation of the frequency difference at one output port, which is the terahertz-excited carrier; (305) Optical coupling: the fiber output end of the beat frequency signal is converted into spatial light by a collimator, guided by a series of mirrors and lenses, and finally combined with the 852nm light path from the capture laser through a dichroic mirror. The combined beam is collected and focused onto the chip by the same 100x oil immersion objective. In step S4, the change of the transmitted light intensity is monitored by the photodetector, and the fluctuation signal of the light intensity is used to represent the vibration response of the captured particles under the terahertz excitation.

8. The method according to claim 1, wherein the method is characterized by: In step S5, the characteristic vibration peak of the target molecule is identified by analyzing the standard deviation, probability density function of the light intensity fluctuation signal, or the free energy curve calculated as a function of the terahertz frequency.

9. The method according to claim 1, wherein the method is characterized by: It comprises an optical tweezer system, a beat frequency excitation system, and a signal detection system. 10.The plasmonic nanophotonic tweezers based molecular terahertz spectrum measurement system of claim 1, wherein: The optical tweezer system comprises a capture laser, a beam shaping assembly, a dichroic mirror, a high numerical aperture objective, and a three-dimensional displacement stage, which is used to form and manipulate optical potential wells on plasmonic nanostructures. The beat frequency excitation system comprises a fixed wavelength laser, a tunable wavelength laser, a fiber coupler, and a beam combining path, which is used to generate and couple a tunable terahertz wave to the capture region of the optical tweezer system. ​ The signal detection subsystem includes an avalanche photodiode, a data acquisition card and a computer, which are used to collect and process the optical signal changes caused by the captured particles in real time.

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