A terahertz near-field fluorescence signal analysis system based on scanning tunneling microscope

Abstract

The present invention discloses a terahertz (THz) near-field fluorescence signal analysis system based on a scanning tunneling microscope (STM). The system comprises a THz radiation system, a THz transmission-characterization-modulation system, a THz coupling and fluorescence collection system, and a THz fluorescence data analysis system, all connected in sequence. The present invention enhances the ability to control THz characteristics such as polarity, carrier phase, polarization direction, intensity, and operating mode, and performs autocorrelation splitting and combining of THz beams, as well as measurement of far-field THz time-domain waveforms. By adopting a structure of a symmetrical plane mirror and an off-axis parabolic mirror that moves with the needle tip, THz can be conveniently coupled to the vicinity of the STM needle tip, and the THz incident direction can be planned in a two-dimensional plane, avoiding complex analysis in three-dimensional space. The fluorescence collection efficiency is significantly increased by designing an independent fluorescence collection structure. LABVIEW software is used to achieve synchronous analysis of THz near-field fluorescence signal measurement and STM operating system, expanding the research capabilities of THz near-field fluorescence phenomena.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz near-field fluorescence signal analysis, and in particular relates to a terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope. Background Art

[0002] In 1976, Lambe et al. at the University of Michigan discovered a new fluorescence mechanism: a spectral pattern generated by inelastic collisions between tunneling electrons. In 1989, this inelastic tunneling radiation spectroscopy effect was incorporated into scanning tunneling microscopy (Gimzewski 1989), enabling the acquisition of nanoscale fluorescence spectra. This fluorescence spectrum can be applied to micro- and nanoscale spectral imaging (Hoffmann 2002, Umeno 1996). The resulting STM tunneling spectroscopy technique is widely used to identify nanoscale molecular fluorescence phenomena (Hoffmann 2002, Imada 2017). In these fluorescence systems, no external excitation light is added; instead, fluorescence emission is generated by localized plasmon modes provided by a steady-state bias voltage. Further adding external excitation light to study tunnel junction fluorescence under excitation light has led to the development of tip-enhanced Raman fluorescence spectroscopy at the micro- and nanoscale, opening up new possibilities for the application of fluorescence systems in the micro- and nanoscale realm. However, in this type of tip-enhanced Raman spectroscopy, the introduction of external laser light can generate very strong thermal effects at the tip, leading to system instability. It wasn't until 2013, when Frank's group at the University of Alberta (Cocker 2013) first proposed the experimental architecture for THz-STM, that this thermal effect problem was completely resolved. Furthermore, in 2021, Kimura's group observed fluorescence radiation from THz-excited localized plasmon modes for the first time, sparking a wave of research into THz near-field fluorescence.

[0003] This type of terahertz near-field fluorescence research provides strong technical support for studying terahertz fluorescence effects in the micro- and nanoscale fields, laying the technical foundation for the development of new terahertz fluorescent materials and, consequently, low-cost, high-performance terahertz imaging devices. The present invention provides a complete and efficient system solution for studying terahertz-induced near-field fluorescence effects.

[0004] The existing terahertz-induced near-field fluorescence observation system mainly comes from the research work of Kimura's group. In their experimental architecture, Figure 1As shown, THz radiation is generated from a lithium niobate crystal source, collected by an off-axis parabolic mirror, and transmitted to the scanning tunneling microscope (STM) window. A THz beamsplitter can align the guided light and THz radiation, thereby directing the THz transmission path. Two gold wire-grid THz polarizers can change the THz intensity by rotating the polarizers. After being guided to the STM window, the THz radiation is incident on the ultra-high vacuum cavity inside the STM, passing through the Z-cut quartz window and the Tsurupica THz window, and finally focused onto the STM tip by the Tsurupica THz lens. The THz-induced fluorescence is then collected by a glass lens and transmitted through the glass window out of the STM cavity, where it enters the CCD for observation.

[0005] The shortcomings of existing fluorescence systems can be summarized as follows:

[0006] (1) Existing fluorescence systems have insufficient spatial resolution in the terahertz excitation band. Most existing systems analyze the far-field region, focusing through optical means to excite the fluorescence phenomenon in the material, thereby obtaining information such as the material's Raman spectrum and fluorescence spectrum. In the terahertz frequency band, due to the long wavelength of terahertz, the method of focusing through the objective lens cannot break through the diffraction limit, thus limiting the spatial resolution of terahertz-induced fluorescence to the order of 100 μm, which is far from sufficient to analyze terahertz-induced near-field fluorescence signals in the nm region.

[0007] (2) Existing near-field fluorescence analysis systems lack excitation light in the terahertz band. There are some near-field methods that can break through the diffraction limit to achieve nanometer-level spatial resolution, such as Raman fluorescence spectroscopy analysis systems based on scanning tunneling microscopes, but the excitation light bands of these systems lack the terahertz band.

[0008] (3) The existing terahertz near-field fluorescence analysis system structure design makes it difficult to control the characteristics of THz light. In these systems, the terahertz optical path design is very simple, making it difficult to solve the problems of THz autocorrelation beam splitting and combining, carrier phase modulation, polarity modulation, etc., and can only modulate the THz intensity.

[0009] (4) The existing terahertz near-field fluorescence analysis system structure design has a relatively high difficulty in THz light coupling. The light introduction structure of these systems is complex and requires oblique incidence conditions in three-dimensional space.

[0010] (5) The existing terahertz near-field fluorescence analysis system lacks a separate design for the fluorescence collection device outside the STM cavity, making it difficult to adjust the fluorescence signal and having poor optimization capabilities.

[0011] (6) Existing terahertz near-field fluorescence analysis systems lack software control. These systems do not provide a structure or code design for the fluorescence signal optimization process, making it difficult to optimize and enhance the fluorescence signal. Moreover, these systems cannot synchronize sampling with STM scans and cannot synchronize observation of fluorescence signals with changes in STM parameters, making it difficult to perform experiments such as THz fluorescence imaging, THz fluorescence current curve measurement, and THz fluorescence voltage curve measurement. Summary of the Invention

[0012] The main purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art and to provide a terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope.

[0013] In order to achieve the above object, the present invention adopts the following technical solutions:

[0014] In one aspect of the present invention, a terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope is provided, comprising a THz radiation system, a THz transmission-characterization-modulation system, a THz coupling and fluorescence collection system, and a THz fluorescence data analysis system connected in sequence;

[0015] The THz radiation system is used to excite and output THz radiation signals;

[0016] The THz transmission-characterization-modulation system includes a beam divergence suppression module, a THz intensity modulation module, a working mode modulation module, a THz carrier phase modulation module, a secondary divergence suppression module, a THz polarity modulation module and a THz waveform characterization module, which are sequentially arranged along the optical path;

[0017] The THz coupling and fluorescence collection system includes a THz coupling system and a THz fluorescence collection system arranged in sequence along the optical path; the THz coupling system includes two reflectors and an off-axis parabolic mirror symmetrically distributed and mounted on a fixture that moves with the needle tip; the THz fluorescence collection system includes an achromatic lens and an optical fiber connected to a spectrometer, and the achromatic lens and optical fiber are capable of three-dimensional movement and rotation;

[0018] The THz fluorescence data analysis system includes a host computer, a spectrometer and an STM operating system, and is used to image and analyze the collected fluorescence signals.

[0019] As a preferred technical solution, the THz radiation system includes a spot size adjustment device arranged in sequence along the optical path, a grating for tilting the pulse wavefront to match the THz radiation plane, a 4F imaging system for imaging the grating onto the THz radiation plane, and a lithium niobate crystal;

[0020] The light spot size adjustment device includes a convex lens L1 and a convex lens L2 arranged in sequence along the optical path, and the distance between the convex lens L1 and the convex lens L2 can be adjusted to adjust the incident pump light power density of the Gaussian beam on the surface of the lithium niobate crystal;

[0021] The 4F imaging system includes a convex lens L3 and a convex lens L4 arranged in sequence along the optical path, wherein the focal length F3 of the convex lens L3 and the focal length F4 of the convex lens L4 satisfy .

[0022] As a preferred technical solution, the beam divergence suppression module includes a plurality of off-axis parabolic mirrors sequentially arranged along the optical path; the secondary divergence suppression module includes a THz lens and an off-axis parabolic mirror sequentially arranged along the optical path.

[0023] As a preferred technical solution, the intensity modulation module includes a first gold wire grid THz polarizer, a reflector and a second gold wire grid THz polarizer arranged in sequence along the optical path. The angle of the first gold wire grid THz polarizer can be rotated to adjust the field strength of the THz radiation signal output by the second gold wire grid THz polarizer; the angle of the second gold wire grid THz polarizer can be rotated to adjust the polarization direction of the THz radiation signal output from the second gold wire grid THz polarizer.

[0024] The carrier phase modulation module includes a reflector and a regular triangular prism arranged in sequence along the optical path; the reflector is set on a one-dimensional displacement platform, used to change the distance between the reflector and the regular triangular prism to modulate the carrier phase of the emitted THz radiation signal; the refractive index of the regular triangular prism for the THz radiation signal is 1.4-1.5.

[0025] As a preferred technical solution, the working mode modulation module includes a 1:1 THz beam splitter, reflectors M2, M3, M4, M5 and M6; wherein the THz beam splitter is mounted on a detachable magnetic device, reflector M2 is mounted on a motorized translation stage, reflectors M3 and M4 share the same magnetic base, and reflector M6 is mounted on a detachable magnetic device, and the following connection method is set:

[0026] Autocorrelation mode: Install the THz beam splitter and remove the reflector M6. The THz radiation signal is incident on the THz beam splitter at a 45° angle. One beam is incident on the reflector M2 at a 90° angle and reflected back to the THz beam splitter. The other beam is incident on the reflector M3 at a 90° angle and reflected back to the THz beam splitter. The two beams are combined into one for transmission.

[0027] Total reflection mode: Install reflector M6 and remove the THz beam splitter, and allow the THz radiation signal to pass through reflectors M4, M5, and M6 in sequence, outputting a single THz radiation signal with a power twice that of the autocorrelation mode.

[0028] As a preferred technical solution, the THz polar modulation module includes reflectors M8, M9 and M10, wherein the reflectors M8 and M10 are both mounted on a detachable magnetic device and are connected in the following manner:

[0029] Connection method 1: Install reflector M8 and remove reflector M10, so that the THz radiation signal passes through reflector M8 and reflector M9 in sequence and is output;

[0030] Connection method 2: Install reflector M10 and remove reflector M8, so that the THz radiation signal passes through reflector M10 and is output;

[0031] By increasing or decreasing the number of reflectors that the THz radiation signal passes through through the above connection method, the carrier phase of the THz radiation signal is changed by utilizing the half-wave loss of the reflectors, and THz radiation signals of two polarities are output.

[0032] As a preferred technical solution, the THz waveform characterization module includes an off-axis parabolic mirror with a hole, an electro-optical crystal, a 1 / 4 wavelength glass, a Wollaston prism, a balanced detector and a lock-in amplifier connected in sequence, and also includes a chopper;

[0033] The off-axis parabolic mirror with a hole is used to focus the THz radiation signal on the electro-optical crystal, and an additional lens is configured to focus the detection light through the small hole on the same position on the electro-optical crystal;

[0034] The 1 / 4 wavelength glass plate is used to convert the detection light modulated by the THz radiation signal from linear polarization to circular polarization;

[0035] The Wollaston prism is used to separate the two orthogonal components corresponding to the circular polarization and transmit them to the balanced detector;

[0036] The balanced detector is used to convert the optical signals of the two orthogonal components into differential electrical signals and transmit them to the lock-in amplifier;

[0037] The chopper is used to perform chopping modulation on the THz radiation signal and transmit the modulated signal to the lock-in amplifier;

[0038] The optical path difference between the THz radiation signal and the detection light is controlled by a motorized translation stage;

[0039] The lock-in amplifier is used to extract the time domain waveform of the THz radiation signal.

[0040] As a preferred technical solution, the THz coupling system includes reflectors M''1 and M''2, off-axis parabolic mirrors P''1 and P''2, and is configured as follows:

[0041] The THz radiation is focused to the needle tip through the reflector M''1 and the off-axis parabolic mirror P''1, and then emitted through the aluminum-coated off-axis parabolic mirror P''2 and the aluminum-coated reflector M''1;

[0042] The reflecting mirrors M''1 and M''2 and the off-axis parabolic mirrors P''1 and P''2 are symmetrically distributed and are all mounted on a fixing device that moves with the needle tip.

[0043] As a preferred technical solution, the THz fluorescence collection system further includes an optical fiber support seat and a light shield, wherein the light shield is used to eliminate stray light interference; the optical fiber support seat is used to fix the optical fiber;

[0044] The achromatic lens and the optical fiber support seat are installed through a slider rail structure or a cage structure, and are used to adjust the optical fiber entrance to the focal plane of the lens;

[0045] The achromatic lens and the optical fiber support seat are moved by a three-dimensional displacement system to align the fluorescence to be collected with the optical fiber inlet;

[0046] The achromatic lens and the optical fiber support seat are rotated by a rotating table to match the emission direction of the fluorescence to be collected.

[0047] As a preferred technical solution, the THz fluorescence data analysis system detects the synchronization signal of the action instruction issued by the STM operating system through LABVIEW, and establishes a connection with the acquisition action of the spectrometer to implement the synchronization of STM parameter changes and spectrometer acquisition actions.

[0048] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0049] (1) The present invention generates strong THz radiation through a lithium niobate crystal source. By designing a reasonable optical layout, the ability to control the characteristics of THz in terms of polarity, carrier phase, polarization direction, intensity and working mode is improved, and the autocorrelation splitting and combining of THz beams and the measurement of far-field THz time domain waveforms can be achieved.

[0050] (2) The present invention adopts a design structure of a symmetrical plane mirror that moves with the needle tip and an off-axis parabolic mirror, which can conveniently couple THz to the vicinity of the scanning tunneling microscope tip, and can plan the incident direction of THz in a two-dimensional plane, avoiding the complex analysis of the THz incident direction in three-dimensional space; in addition, the off-axis parabolic mirror of this coupling structure can achieve a large numerical aperture, which can significantly increase the fluorescence collection efficiency.

[0051] (3) The present invention couples THz to the vicinity of the scanning tunneling microscope tip, which can induce local near-field fluorescence, and can import the fluorescence signal into the spectrometer through the fluorescence collection device, and then conduct data linkage with the host computer. This terahertz fluorescence analysis system combined with STM can achieve terahertz-induced fluorescence resolution in the nanoscale spatial range, and make up for the shortcoming of the existing STM fluorescence system that lacks terahertz band excitation light.

[0052] (4) The present invention realizes the synchronous analysis of THz near-field fluorescence signal measurement and STM operating system through LABVIEW software design. It can analyze the fluorescence signal while adjusting the STM parameters, and can expand experimental scenarios such as fluorescence spectroscopy nano-imaging, greatly expanding the research capabilities of THz near-field fluorescence phenomena.

[0053] (5) The present invention provides sufficient degrees of freedom for the alignment and collection of fluorescence by designing an independent fluorescence collection structure. At the same time, the fluorescence intensity can be quantified with the help of LABVIEW programming, which greatly facilitates the optimization of fluorescence collection efficiency and ensures the most efficient fluorescence collection. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 1 is a schematic diagram of the overall architecture of a terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope according to an embodiment of the present invention;

[0055] Figure 2 is a schematic diagram of the optimal grating constant according to an embodiment of the present invention;

[0056] Figure 3 is a schematic structural diagram of a THz radiation system according to an embodiment of the present invention;

[0057] Figure 4 Schematic diagram of the structure of a THz transmission-characterization-modulation system according to an embodiment of the present invention;

[0058] Figure 5 2 is a schematic structural diagram of a THz waveform characterization module according to an embodiment of the present invention;

[0059] Figure 6 Schematic diagram of the structure of the THz coupling and fluorescence collection system according to an embodiment of the present invention;

[0060] Figure 7 Schematic diagram of the structure of the THz fluorescence data analysis system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0061] In order to enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0062] Example:

[0063] like Figure 1 As shown, this embodiment provides a terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope, including a THz radiation system, a THz transmission-characterization-modulation system, a THz coupling and fluorescence collection system, and a THz fluorescence data analysis system.

[0064] (1) The THz radiation system is used to excite and output THz radiation signals.

[0065] In a preferred embodiment, the THz radiation system includes a spot size adjustment device arranged in sequence along the optical path, several reflectors, a grating for tilting the pulse wave front to match the THz radiation plane, a 4F imaging system for imaging the grating onto the THz radiation plane, and a lithium niobate crystal.

[0066] This example uses a commercial femtosecond pulse laser with a central wavelength of 1030 nm, a repetition rate of 520.8 kHz, and a pulse width of 200 fs to construct a lithium niobate crystal terahertz radiation source. According to existing lithium niobate crystal radiation theory, maximizing efficiency and achieving THz radiation requires meeting three conditions: grating image coincidence matching, tilted wavefront matching, and grating diffraction matching:

[0067] (1);

[0068] (2);

[0069] (3);

[0070] (4);

[0071] in, γ is the phase matching angle for achieving lithium niobate crystal radiation, defined by formula (1). By querying the phase velocity and group velocity of THz and pump light (1030 nm) in lithium niobate crystal, it can be calculated that its magnitude is about 63 o , this angle is also the apex angle of lithium niobate crystal; θ ris the angle of the diffracted light direction relative to the grating normal direction; n is the refractive index of 1030 nm light in lithium niobate crystal, which is 2.2342; M is the lateral magnification of the optical system from the grating to the lithium niobate crystal; λ is 1030 nm wavelength of light; p is the grating constant, the number of grating lines per unit length; n g is the group refractive index of 1030nm light in lithium niobate crystal, which is 2.3019; θ i is the angle of the incident light direction relative to the grating normal direction; the second formula in formula (4) is the Litjou condition. For the blazed grating, only the diffraction with the incident angle within that range can achieve a higher diffraction efficiency.

[0072] Combining formula (2)-formula (4), when the grating constant changes p When , the corresponding incident angle dependence can be calculated, such as Figure 2 As shown, combined with the Littru angle range for optimal diffraction efficiency, it can be determined that the optimal grating constant is 1600 lines / mm. At this time, the formula can be used to calculate θ i 56.6 o , θ r 54 o ,and M It is 0.6511.

[0073] In a preferred embodiment, Figure 3 As shown, the spot size adjustment device includes convex lenses L1 and L2, where the focal length F1 of L1 is preferably 100mm and the focal length F2 of L2 is preferably 50mm, forming a Galilean telescope design. By varying the spacing between L1 and L2, the pump light spot size of the Gaussian beam on the crystal surface of the terahertz radiation source can be adjusted, thereby adjusting the power density of the incident pump light. This design provides additional freedom in optical power density, significantly improving terahertz radiation efficiency. The pump light output by the spot size adjustment device passes through mirrors M'3 and M'4 in sequence and is incident on grating G1 at a specific angle.

[0074] In a preferred embodiment, Figure 3 As shown, the grating G1 is a transmission grating, which is used to tilt the pulse wavefront (black ellipse) to match the THz radiation plane. The grating constant p It is 1600 lines / mm.

[0075] In a preferred embodiment, Figure 3As shown, the 4F system with small aberration is selected as the lateral magnification M The implementation mechanism is that the convex lens L3 and the convex lens L4 are sequentially arranged along the optical path, and the focal length F3 of the convex lens L3 and the focal length F4 of the convex lens L4 satisfy , so as to image the grating onto the THz radiation plane, satisfy the grating image coincidence matching condition, and ensure the effective THz radiation in the lithium niobate crystal LN. Among them, the focal length F3 of L3 is preferably 150mm, and the focal length F4 of L4 is preferably 100mm.

[0076] In a preferred embodiment, Figure 3 As shown, the top angle of lithium niobate crystal LN γ =63°. θ i 、 θ r are the angles between the incident light of grating G1 and the normal of grating G1, and the angle between the first-order diffracted light and the normal, δθ It describes the central wavelength λ Angular dispersion of the 0=1030 nm wave packet after passing through grating G1.

[0077] In the lithium niobate architecture of the above-mentioned embodiment, multiple factors such as the incident angle of the grating, the diffraction angle of the grating, the focusing position of the circular lens, the focusing position on the crystal, and the spacing between the optical elements will seriously affect the terahertz radiation efficiency. Therefore, it is necessary to set a sufficiently high degree of freedom for these optimization conditions to perform adjustment and optimization.

[0078] In particular, in the THz radiation system, the lithium niobate source is only one of many options. This system is of course compatible with other types of THz sources, such as photoconductive antenna THz radiation sources and gallium phosphide crystal THz radiation sources.

[0079] (2) The THz transmission-characterization-modulation system includes a beam divergence suppression module, a THz intensity modulation module, a working mode modulation module, a THz carrier phase modulation module, a secondary divergence suppression module, a THz polarity modulation module and a THz waveform characterization module, which are sequentially arranged along the optical path.

[0080] The present invention has made special designs in the control capability of THz optical properties, which can modulate the intensity, carrier phase, polarity and operating mode of the THz radiation signal, enriching the adjustment freedom of the incident THz, as follows:

[0081] The beam divergence suppression module includes a plurality of off-axis parabolic mirrors arranged in sequence along the optical path.

[0082] In a preferred embodiment, Figure 4As shown in FIG5 , off-axis parabolic mirrors P1, P2, and P3 are set, and their focal lengths are 152 mm, 50.8 mm, and 76.2 mm, respectively, which can ensure uniform and collimated transmission of THz radiation signals within a range of 1 m.

[0083] The intensity modulation module includes a first gold wire grid THz polarizer, a gold-plated reflector M1, and a second gold wire grid THz polarizer (the gold wire grid THz polarizer is arranged in sequence along the optical path) Figure 4 The middle mark is WG), the angle of the first gold wire grid THz polarizer can be rotated to adjust the field strength of the THz radiation signal output by the second gold wire grid THz polarizer; the angle of the second gold wire grid THz polarizer can be rotated to adjust the polarization direction of the THz radiation signal output from the second gold wire grid THz polarizer.

[0084] The working mode modulation module includes a 1:1 THz beam splitter and gold-plated mirrors M2-M6; wherein the THz beam splitter is mounted on a detachable magnetic device, the gold-plated mirror M2 is mounted on a motorized translation stage, the gold-plated mirrors M3 and M4 share the same magnetic base, and the gold-plated mirror M6 is mounted on a detachable magnetic device, and preferably the following connection method is set:

[0085] Autocorrelation mode: Install the THz beam splitter and remove the gold-plated reflector M6. The THz radiation signal is incident on the THz beam splitter at a 45° angle. One beam is incident on the gold-plated reflector M2 at a 90° angle and reflected back to the THz beam splitter. The other beam is incident on the gold-plated reflector M3 at a 90° angle and reflected back to the THz beam splitter. The two beams are combined into one for transmission.

[0086] Total reflection mode: Install the gold-plated reflector M6 and remove the THz beam splitter, and make the THz radiation signal pass through the gold-plated reflectors M4, M5, and M6 at 45° in sequence, outputting a single THz radiation signal with a power twice that of the autocorrelation mode.

[0087] The carrier phase modulation module includes a gold-plated reflector M7 and a regular triangular prism arranged in sequence along the optical path; the reflector is set on a one-dimensional displacement platform, which is used to change the distance between the reflector and the regular triangular prism to modulate the carrier phase of the emitted THz; the refractive index of the regular triangular prism for THz is 1.4-1.5.

[0088] In a preferred embodiment, Figure 4 As shown, a regular triangular prism made of polytetrafluoroethylene (PTFE) material is used. It should be understood that as long as the refractive index of the THz radiation signal is around 1.4 and a sufficiently strong THz radiation signal transmittance is retained, such as ZEONEX cycloolefin polymer material, the solution of this application can be implemented.

[0089] The secondary divergence suppression module includes a THz lens and an off-axis parabolic mirror arranged in sequence along the optical path.

[0090] In a preferred embodiment, Figure 4 As shown in the figure, the focal length of the THz lens is 50.8 mm, and the focal length of the off-axis parabolic mirror P4 is 101 mm. The two work together to suppress the secondary divergence of the THz beam and ensure that the THz beam spot incident on the STM is uniform and stable.

[0091] The THz polar modulation module includes gold-plated reflectors M8-M10, wherein the gold-plated reflectors M8 and M10 are both mounted on a detachable magnetic device and are connected in the following manner:

[0092] Connection method 1: Install the gold-plated reflector M8 and remove the gold-plated reflector M10, so that the THz radiation signal passes through the gold-plated reflector M8 and gold-plated reflector M9 in sequence and is output;

[0093] Connection method 2: Install the gold-plated reflector M10 and remove the gold-plated reflector M8, so that the THz radiation signal passes through the gold-plated reflector M10 at a 45° angle and is output;

[0094] By increasing or decreasing the number of reflectors that the THz radiation signal passes through through the above connection method, the carrier phase of the THz radiation signal (the phase is π) is changed by utilizing the half-wave loss of the reflectors, and THz radiation signals of two polarities are output.

[0095] The THz waveform characterization module includes an off-axis parabolic mirror P'1 with a hole, an electro-optical crystal T1, a 1 / 4 wavelength glass plate T2, a Wollaston prism T3, a balanced detector and a lock-in amplifier, which are connected in sequence, and also includes a chopper.

[0096] The off-axis parabolic mirror P'1 with a hole is used to focus the THz radiation signal on the electro-optical crystal T1, and an additional lens L1 is configured to focus the detection light through the small hole on the same position on the electro-optical crystal T1;

[0097] The 1 / 4 wavelength glass plate T2 is used to convert the detection light modulated by the THz radiation signal from linear polarization to circular polarization, and transmit it to the Wollaston prism T3 through the gold-plated reflector M'1;

[0098] The Wollaston prism T3 is used to separate the two orthogonal components corresponding to the circular polarization and transmit them to the balanced detector through the gold-plated mirrors M'2 and M'3 respectively;

[0099] The balanced detector is used to convert the optical signals of the two orthogonal components into differential electrical signals and transmit them to the lock-in amplifier;

[0100] The chopper is used to perform chopping modulation on the THz radiation signal and transmit the modulated signal to the lock-in amplifier;

[0101] The optical path difference between the THz radiation signal and the detection light is controlled by an electric translation stage. The THz radiation signal is chopped and modulated, and the modulated signal is transmitted to the phase-locked amplifier. When the optical path difference between the THz radiation signal and the detection light is scanned, the time domain waveform of the THz radiation signal can be extracted from the phase-locked amplifier. The synchronization of the two is controlled by LABVIEW software.

[0102] In a preferred embodiment, Figure 5 As shown, the focal length of the off-axis parabolic mirror P'1 with a hole is 50.8 mm, the electro-optical crystal T1 is a 1 mm thick ZnTe crystal, the focal length of the plano-convex lens L'1 is 150 mm, the focal length of the plano-convex lens L'2 is 100 mm, the wavelength of the 1 / 4 wavelength glass T2 is 1030 nm, and M'1-M'3 are gold-plated mirrors.

[0103] In particular, in this application, for fluorescence analysis, the reflector is required to have the largest possible light wavelength coverage. Gold and aluminum can achieve a reflectivity of more than 90% for almost all wavelengths of light. Therefore, these materials are selected in this embodiment. In principle, as long as the material can exist stably in an ultra-high vacuum and ultra-low temperature environment and can fully reflect most wavelengths of light, the solution of this application can be implemented.

[0104] (3) The THz coupling and fluorescence collection system includes a THz coupling system and a THz fluorescence collection system arranged in sequence along the optical path; the THz coupling system includes two aluminum-coated reflectors and an aluminum-coated off-axis parabolic mirror that are symmetrically distributed and mounted on a fixing device that moves with the needle tip; the THz fluorescence collection system includes an achromatic lens and an optical fiber, and the optical fiber is connected to a spectrometer, and the achromatic lens and the optical fiber can be moved and rotated in three dimensions.

[0105] In a preferred embodiment, Figure 6As shown, the THz coupling system includes aluminum-coated mirrors M''1 and M''2, and aluminum-coated off-axis parabolic mirrors P''1 and P''2, all with a focal length of 33.85 mm. The aluminum-coated reflector M''1 / aluminum-coated off-axis parabolic mirror P''1 and the aluminum-coated reflector M''2 / aluminum-coated off-axis parabolic mirror P''2 are symmetrically distributed and are both mounted on a fixture that moves with the needle tip. Therefore, when the needle tip moves, the THz focused light spot always remains near the needle tip, thereby ensuring the stability of THz coupling. In addition, the off-axis parabolic mirrors P''1 and P''2 of this design can achieve a large numerical aperture, thereby improving the fluorescence collection efficiency. Finally, due to the presence of the built-in aluminum-coated reflector M''1, the incident direction of the vertically polarized THz radiation signal can be conveniently designed in the YZ two-dimensional space without considering the complex three-dimensional space THz radiation signal transmission.

[0106] In a preferred embodiment, Figure 6 As shown, the THz fluorescence collection system consists of an achromatic lens and a fiber spectrometer. The achromatic lens has a transmission range of 400-1100 nm and a focal length of 180 mm. To effectively collect fluorescence, the fluorescence must be perfectly aligned with the fiber inlet, which relies on a combination of various mechanical structures. First, the achromatic lens and fiber support are mounted on a slider that matches the guide rail. Both can be easily moved in the Z-axis, allowing for coarse adjustment of the fiber inlet to the lens' focal plane, maximizing fluorescence collection. Furthermore, a two-dimensional translation stage is added to each slider to further precisely control the focal plane alignment in the Z direction and the port alignment in the Y direction. The port alignment in the X direction can be easily adjusted by combining the XY-axis adjustable features of the fiber support. Second, the slider, along with the entire guide rail, is mounted on a rotation stage that rotates about the Z-axis. This can be used to align the guide rail direction with the STM fluorescence beam output direction, ensuring they are aligned. Furthermore, by adding a long-range one-dimensional translation stage along the Y direction to the entire system, combined with an appropriate height in the X direction (this is usually determined by the height of the breadboard; a modulation module of the overall height can also be added below the long-range one-dimensional translation stage), the strict collinearity of the fluorescence emission direction and the guide rail direction can be ensured.

[0107] In particular, in the THz coupling and fluorescence collection system, the solution for aligning the direction of the emitted fluorescence with a slider rail is used. Of course, a cage structure can also be used for alignment. The connection relationship is still lens-fiber-spectrometer, which can realize the function of orienting the slide rail and changing the relative distance, and the alignment effect is better, but the adjustment range is small.

[0108] Finally, after completing the alignment between the fluorescence and the fiber port in the Z-axis focal plane and the XY-axis space, the fluorescence signal can be presented on the spectrometer interface through optical fiber transmission.

[0109] (4) The THz fluorescence data analysis system includes a host computer, a spectrometer, and an STM operating system, which are used to image and analyze the collected fluorescence signals, and detect the synchronization signals of various action instructions issued by the STM operating system through LABVIEW, and establish a connection with the acquisition action of the spectrometer to implement the synchronization of STM parameter changes and spectrometer acquisition actions.

[0110] In a preferred embodiment, after completing the collection of fluorescence data, the present application further designs LABVIEW software to control the linkage between the STM operating system and the spectrometer acquisition action, thereby enriching the research methods for terahertz near-field fluorescence signal analysis. Specifically, Figure 7 As shown, the STM operating system and spectrometer are linked within the host computer control software through LABVIEW programming. The core of this LABVIEW design is to utilize the trigger detection function of the Zurich Instruments HF2LI lock-in amplifier to detect synchronization signals from the various action commands issued by the STM operating system. This signal is then linked to the spectrometer's acquisition operations through LABVIEW software, achieving synchronization between STM parameter changes and spectrometer acquisition. Obviously, any level trigger can be used for this synchronization; the HF2LI's trigger detection function is just one application of this lock-in amplifier.

[0111] On this basis, this embodiment also develops a terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope and host computer software in the above embodiment to achieve the function of optimizing the fluorescence collection efficiency; at the same time, it realizes the synchronization of STM scanning and fluorescence signal acquisition, thereby realizing the fluorescence imaging function; further expands the customized experimental environment, and can diversify the analysis of the influencing factors of the terahertz near-field fluorescence signal, especially its response to changes in STM voltage and current parameters.

[0112] It should be noted here that the system provided in the above embodiment is only illustrated by the division of the above functional modules. In actual applications, the above functions can be assigned to different functional modules as needed, that is, the internal structure can be divided into different functional modules to complete all or part of the functions described above.

[0113] It should be understood that various parts of the present application can be implemented using hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented using software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented using hardware, as in another embodiment, any one of the following technologies known in the art or a combination thereof can be used to implement: a discrete logic circuit having a logic gate circuit for implementing a logic function on a data signal, an application-specific integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0114] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope, characterized in that: It includes a THz radiation system, a THz transmission-characterization-modulation system, a THz coupling and fluorescence collection system, and a THz fluorescence data analysis system connected in sequence; The THz radiation system is used to excite and output THz radiation signals, including a spot size adjustment device arranged in sequence along the optical path, a grating for tilting the pulse wavefront to match the THz radiation plane, a 4F imaging system for imaging the grating onto the THz radiation plane, and a lithium niobate crystal; wherein the central wavelength of the incident pump light is λ is 1030nm, phase matching angle γ The top angle of lithium niobate crystal is 63 o , grating constant p The angle of the incident light direction relative to the grating normal direction is 1600 lines / mm. θ i 56.6 o , the angle of the diffracted light direction relative to the grating normal direction θ r 54 o , the lateral magnification of the optical system from the grating to the lithium niobate crystal M is 0.6511; The light spot size adjustment device includes a convex lens L1 and a convex lens L2 arranged in sequence along the optical path, and the distance between the convex lens L1 and the convex lens L2 can be adjusted to adjust the incident pump light power density of the Gaussian beam on the surface of the lithium niobate crystal; The 4F imaging system includes a convex lens L3 and a convex lens L4 arranged in sequence along the optical path, wherein the focal length F3 of the convex lens L3 and the focal length F4 of the convex lens L4 satisfy ; The THz transmission-characterization-modulation system includes a beam divergence suppression module, a THz intensity modulation module, a working mode modulation module, a THz carrier phase modulation module, a secondary divergence suppression module, a THz polarity modulation module and a THz waveform characterization module, which are sequentially arranged along the optical path; The THz coupling and fluorescence collection system includes a THz coupling system and a THz fluorescence collection system arranged in sequence along the optical path; the THz coupling system includes two reflectors and an off-axis parabolic mirror symmetrically distributed and mounted on a fixture that moves with the needle tip; the THz fluorescence collection system includes an achromatic lens and an optical fiber connected to a spectrometer, and the achromatic lens and optical fiber are capable of three-dimensional movement and rotation; The THz coupling system includes reflectors M''1 and M''2, off-axis parabolic mirrors P''1 and P''2, and is configured as follows: The THz radiation is focused to the needle tip through the reflector M''1 and the off-axis parabolic mirror P''1, and then emitted through the aluminum-coated off-axis parabolic mirror P''2 and the aluminum-coated reflector M''1; The reflectors M''1 and M''2, and the off-axis parabolic mirrors P''1 and P''2 are symmetrically distributed and are all mounted on a fixture that moves with the needle tip; The THz fluorescence data analysis system includes a host computer, a spectrometer, and an STM operating system, which are used to image and analyze the collected fluorescence signals; The THz fluorescence data analysis system detects the synchronization signal of the action instruction issued by the STM operating system through LABVIEW, and establishes a connection with the acquisition action of the spectrometer to implement the synchronization of the STM parameter change and the spectrometer acquisition action.

2. The terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope according to claim 1, characterized in that: The beam divergence suppression module includes a plurality of off-axis parabolic mirrors sequentially arranged along the optical path; the secondary divergence suppression module includes a THz lens and an off-axis parabolic mirror sequentially arranged along the optical path.

3. The terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope according to claim 1, characterized in that: The intensity modulation module includes a first gold wire grid THz polarizer, a reflector, and a second gold wire grid THz polarizer arranged in sequence along the optical path, wherein the angle of the first gold wire grid THz polarizer can be rotated to adjust the field intensity of the THz radiation signal output by the second gold wire grid THz polarizer; the angle of the second gold wire grid THz polarizer can be rotated to adjust the polarization direction of the THz radiation signal output from the second gold wire grid THz polarizer; The carrier phase modulation module includes a reflector and a regular triangular prism arranged in sequence along the optical path; the reflector is set on a one-dimensional displacement platform, used to change the distance between the reflector and the regular triangular prism to modulate the carrier phase of the emitted THz radiation signal; the refractive index of the regular triangular prism for the THz radiation signal is 1.4-1.

5.

4. The terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope according to claim 1, characterized in that: The working mode modulation module includes a 1:1 THz beam splitter, reflectors M2, M3, M4, M5, and M6; wherein the THz beam splitter is mounted on a detachable magnetic device, reflector M2 is mounted on a motorized translation stage, reflectors M3 and M4 share the same magnetic base, and reflector M6 is mounted on a detachable magnetic device, and the following connection method is set: Autocorrelation mode: Install the THz beam splitter and remove the reflector M6. The THz radiation signal is incident on the THz beam splitter at a 45° angle. One beam is incident on the reflector M2 at a 90° angle and reflected back to the THz beam splitter. The other beam is incident on the reflector M3 at a 90° angle and reflected back to the THz beam splitter. The two beams are combined into one for transmission. Total reflection mode: Install reflector M6 and remove the THz beam splitter, and allow the THz radiation signal to pass through reflectors M4, M5, and M6 in sequence, outputting a single THz radiation signal with a power twice that of the autocorrelation mode.

5. The terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope according to claim 1, characterized in that: The THz polar modulation module includes reflectors M8, M9, and M10, wherein the reflectors M8 and M10 are both mounted on a detachable magnetic device and are connected in the following manner: Connection method 1: Install reflector M8 and remove reflector M10, so that the THz radiation signal passes through reflector M8 and reflector M9 in sequence and is output; Connection method 2: Install reflector M10 and remove reflector M8, so that the THz radiation signal passes through reflector M10 and is output; By increasing or decreasing the number of reflectors that the THz radiation signal passes through through the above connection method, the carrier phase of the THz radiation signal is changed by utilizing the half-wave loss of the reflectors, and THz radiation signals of two polarities are output.

6. The terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope according to claim 1, characterized in that: The THz waveform characterization module includes an off-axis parabolic mirror with a hole, an electro-optical crystal, a 1 / 4 wavelength glass plate, a Wollaston prism, a balanced detector and a lock-in amplifier connected in sequence, and also includes a chopper; The off-axis parabolic mirror with a hole is used to focus the THz radiation signal on the electro-optical crystal, and an additional lens is configured to focus the detection light through the small hole on the same position on the electro-optical crystal; The 1 / 4 wavelength glass plate is used to convert the detection light modulated by the THz radiation signal from linear polarization to circular polarization; The Wollaston prism is used to separate the two orthogonal components corresponding to the circular polarization and transmit them to the balanced detector; The balanced detector is used to convert the optical signals of the two orthogonal components into differential electrical signals and transmit them to the lock-in amplifier; The chopper is used to perform chopping modulation on the THz radiation signal and transmit the modulated signal to the lock-in amplifier; The optical path difference between the THz radiation signal and the detection light is controlled by a motorized translation stage; The lock-in amplifier is used to extract the time domain waveform of the THz radiation signal.

7. The terahertz near-field fluorescence signal analysis system based on a scanning tunneling microscope according to claim 1, characterized in that: The THz fluorescence collection system further includes an optical fiber support seat and a light shield, wherein the light shield is used to eliminate stray light interference; the optical fiber support seat is used to fix the optical fiber; The achromatic lens and the optical fiber support seat are installed through a slider rail structure or a cage structure, and are used to adjust the optical fiber entrance to the focal plane of the lens; The achromatic lens and the optical fiber support seat are moved by a three-dimensional displacement system to align the fluorescence to be collected with the optical fiber inlet; The achromatic lens and the optical fiber support seat are rotated by a rotating table to match the emission direction of the fluorescence to be collected.

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