Exciton state energy level detection method based on plasmon enhanced nonlinear spectrum

Abstract

The invention relates to the field of nano photonics and optical materials, in particular to an exciton state energy level detection method based on a plasmon enhanced nonlinear spectrum, which comprises the following steps: constructing a mixed nano cavity structure in which a single-layer two-dimensional material is coupled with a plasmon cavity; collecting a two-photon photoluminescence spectrum, a second harmonic spectrum and a third harmonic spectrum of the single-layer two-dimensional material in the mixed nanocavity structure; the intensity of a nonlinear optical signal is improved through a local enhancement effect of plasmons; according to the two-photon photoluminescence spectrum, the second harmonic spectrum and the third harmonic spectrum of the single-layer two-dimensional material, the dark exciton state energy level, the bright state energy level, the dark exciton state energy level and the high-energy exciton state energy level of the single-layer two-dimensional material are determined. According to the method, the energy level of the Rydberg exciton state of the semiconductor material with the nanoscale thickness can be accurately detected.

Description

Technical Field

[0001] The invention relates to the fields of nanophotonics and optical materials, and in particular to an exciton state energy level detection method based on plasmon enhanced nonlinear spectroscopy. Background Art

[0002] Since the successful exfoliation of single-layer graphene in 2004, two-dimensional layered materials have attracted much attention due to their atomically thick properties. Graphene's excellent mechanical properties, ultra-high carrier rate and broadband optical response give it a huge advantage in the field of optoelectronic devices. However, graphene's zero bandgap characteristics limit its application in the optical field. Therefore, the search for materials with excellent optoelectronic properties similar to graphene has become a research hotspot. At this time, two-dimensional transition metal sulfides (TNDCs) have entered people's field of vision as representative materials. Unlike semi-metallic graphene, the band gap range of layered transition metal sulfides covers the visible light to near-infrared band. In addition, it is interesting that transition metal sulfides also show a layer number dependence phenomenon. When the thickness is reduced to a single layer, transition metal sulfides change from indirect bandgap semiconductors to direct bandgap semiconductors, showing interesting optical phenomena. For example: valley-selected circular dichroism, strong photocurrent response. For a single layer of two-dimensional transition metal sulfide, its optoelectronic properties are mainly dominated by excitons, which is due to the reduction of dielectric screening and quantum confinement effects when its thickness is reduced from three-dimensional to two-dimensional, showing a significantly enhanced Coulomb interaction between electron and hole pairs. This leads to the formation of Rydberg-like exciton states below the free quasiparticle band gap. The exciton states of two-dimensional materials, especially the Rydberg exciton states, have excellent characteristics such as long lifetime, large binding energy and environmental sensitivity. Therefore, the precise understanding of high-order Rydberg exciton states is of great significance in the understanding of basic science such as many-body effects and device applications such as optical switches.

[0003] The existing method for determining the energy level of exciton states is to use traditional spectroscopy methods for detection. However, this method faces many challenges, such as low signal-to-noise ratio, wide spectral linewidth, high requirements on sample quality, and extremely low experimental temperature. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a method for detecting the energy level of an exciton state based on plasmon enhanced nonlinear spectroscopy, so as to achieve accurate detection of the energy level of the Rydberg exciton state, thereby playing its important role in optoelectronic devices.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is:

[0006] A method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy comprises the following steps:

[0007] Construct a hybrid nanocavity structure that couples a single-layer two-dimensional material with a plasmon cavity;

[0008] Collect two-photon photoluminescence spectra, second harmonic spectra, and third harmonic excitation spectra of single-layer two-dimensional materials in hybrid nanocavity structures;

[0009] According to the two-photon photoluminescence spectrum, second harmonic spectrum and third harmonic excitation spectrum of the single-layer two-dimensional material, the relationship between the two-photon photoluminescence, second harmonic and third harmonic signal intensity and the corresponding emission wavelength are drawn respectively;

[0010] Extract the energy value corresponding to the peak value of the light signal in the two-photon photoluminescence spectrum from the relationship diagram between the two-photon photoluminescence signal intensity and the emission wavelength, and determine the dark exciton state energy level of the single-layer two-dimensional material;

[0011] Extract the energy value corresponding to the peak value of the optical signal in the second harmonic spectrum from the relationship between the second harmonic signal intensity and the emission wavelength, and determine the energy levels of the bright and dark exciton states of the single-layer two-dimensional material;

[0012] The energy value corresponding to the peak value of the optical signal in the third harmonic spectrum is extracted from the relationship diagram between the third harmonic signal intensity and the emission wavelength, and the energy level of the high-energy exciton state of the single-layer two-dimensional material is determined.

[0013] Preferably, when constructing a hybrid nanocavity structure in which a single-layer two-dimensional material is coupled with a plasmon cavity, a plasmon cavity having a nanoparticle reflective structure is formed by successively dripping a polyallylamine hydrochloride solution and gold nanohexagons coated with hexadecyltrimethylammonium bromide onto the surface of an Au / SiO2 substrate; wherein the single-layer two-dimensional material is transferred to the Au / SiO2 substrate by a polymer-assisted wet transfer method, so that the single-layer two-dimensional material is placed in the plasmon cavity to achieve strong coupling.

[0014] Preferably, when constructing a hybrid nanocavity structure in which a single-layer two-dimensional material is coupled to a plasmon cavity, dark-field scattering spectra of plasmon cavities with gold nanohexagons of different aspect ratios and photoluminescence spectra of single-layer two-dimensional materials are collected, and the aspect ratio size of the gold nanohexagon when the central wavelength of the dark-field scattering spectrum of the plasmon cavity overlaps with the central wavelength of the photoluminescence spectrum of the single-layer two-dimensional material and the excitation wavelength of the nonlinear optical response is selected as the optimal plasmon cavity.

[0015] Preferably, when constructing a hybrid nanocavity structure in which a single-layer two-dimensional material is coupled to a plasmon cavity, the hybrid nanocavity structure is placed in the sample area of ​​a microscope, and the dark-field scattering spectrum of the plasmon cavity is collected using a white-light halogen lamp light source, a microscope, and a collection optical path; based on the collected dark-field scattering spectrum, a relationship diagram between the intensity of the dark-field scattered light and the wavelength is plotted, the position corresponding to the maximum dark-field scattered light signal is extracted, and the central wavelength of the dark-field scattering spectrum of the plasmon cavity is determined.

[0016] Preferably, when constructing a hybrid nanocavity structure in which a single-layer two-dimensional material is coupled with a plasmon cavity, the hybrid nanocavity structure is placed in the sample area of ​​a microscope, and the photoluminescence spectrum of the single-layer two-dimensional material is collected through a continuous excitation light source, a microscope and a collection optical path; based on the collected photoluminescence spectrum, a relationship diagram between the photoluminescence intensity and the wavelength is drawn, the position corresponding to the peak of the photoluminescence signal is extracted, and the central wavelength of the photoluminescence spectrum of the single-layer two-dimensional material is determined.

[0017] Preferably, when collecting the two-photon photoluminescence spectrum, second harmonic spectrum and third harmonic spectrum of a single-layer two-dimensional material in a hybrid nanocavity structure, the hybrid nanocavity structure is placed in the sample area of ​​a microscope, and then the excitation light emitted by a femtosecond excitation light source is collimated and focused on the sample area through the microscope objective lens through the excitation light path to excite the two-photon photoluminescence signal, second harmonic signal and third harmonic signal in the sample, and then the two-photon photoluminescence spectrum, second harmonic spectrum and third harmonic spectrum of the sample area are collected through the collection light path.

[0018] Preferably, when collecting the two-photon photoluminescence spectrum, second harmonic spectrum and third harmonic spectrum of a single-layer two-dimensional material in a hybrid nanocavity structure, the excitation light power is adjusted to ensure a high signal-to-noise ratio and no saturation effect, and then the wavelength of the excitation light is adjusted to excite and enhance the nonlinear optical effects of the bright exciton state and the dark exciton state of the single-layer two-dimensional material.

[0019] Preferably, when collecting the two-photon photoluminescence spectrum, second harmonic spectrum and third harmonic spectrum of a single-layer two-dimensional material in a hybrid nanocavity structure, the power of the excitation light is increased to ensure a high signal-to-noise ratio and collect the third harmonic spectrum at multiple set wavelengths.

[0020] Preferably, filters are placed before both the excitation light path and the collection light path.

[0021] The present invention has the following beneficial effects:

[0022] The present invention utilizes a 430nm continuous light or femtosecond wavelength adjustable pulse light excitation optical path, a microscope and a CCD imaging system to determine the area of ​​a single-layer material to be measured from a substrate, and realizes the measurement of a linear spectrum and a nonlinear spectrum; wherein a wavelength adjustable continuous laser excitation optical path, a microscope and a spectrum collection optical path can be used to determine the resonance wavelength of a plasmon cavity, and realize the measurement of a dark field scattering spectrum; a 430nm continuous light excitation optical path, a microscope and a spectrum collection optical path can be used to determine the resonance position of an exciton state of a single-layer material, and realize the measurement of a steady-state photoluminescence spectrum; a wavelength adjustable femtosecond pulse light excitation optical path, a microscope and a spectrum collection optical path can be used to determine the energy level of a high-order Rydberg exciton state of a single-layer material, and realize the measurement of a two-photon photoluminescence spectrum, a second harmonic spectrum and a third harmonic spectrum; and thus the energy level of a Rydberg exciton state can be accurately determined. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of a method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy;

[0024] Figure 2 The schematic diagram of the optical path structure of the present invention using plasmon-enhanced nonlinear spectroscopy to detect discrete exciton state energy levels;

[0025] Figure 3 The optical photograph and structural schematic diagram of the representative measurement sample WS2-plasmon cavity integrated under a microscope in the present invention;

[0026] Figure 4 Optical photographs of gold nanohexagons of different sizes and scattering spectra of plasmon cavities with gold nanohexagons of different sizes in the present invention;

[0027] Figure 5 The photoluminescence spectrum of the sample WS2 and the scattering spectrum of the optimized plasmon cavity are measured in the present invention;

[0028] Figure 6 This is the emission spectra of a single AuNH and a monolayer WS2 on an Au / SiO2 substrate and a WS2-AuNPoM hybrid structure with AuNH of AR=3.8 under 1200nm femtosecond laser excitation in the present invention.

[0029] Figure 7 The two-photon photoluminescence spectrum, second harmonic spectrum and third harmonic spectrum of the single-layer WS2 in the present invention and the WS2 sample in AuNPoM were tested at an interval of 5nm, and the comparison diagram of the relationship between the signal peak value and the fundamental frequency wavelength of the two-photon photoluminescence spectrum, the second harmonic spectrum and the third harmonic spectrum was selected.

[0030] Figure 8 Schematic diagram of the plasmon-enhanced WS2 nonlinear optical (two-photon fluorescence 2PPL, second harmonic SHG, and third harmonic THG) response (energy of 1s, 2p, 2s, 3s, 3s, 4s and 3p states of A, B, C, D excitons in monolayer WS2) at room temperature in the present invention. DETAILED DESCRIPTION

[0031] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations utilizing the concept of the present invention are protected.

[0032] like Figure 1 As shown, the embodiment of the present invention provides a method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy, comprising the following steps S1 to S6:

[0033] S1. Construct a hybrid nanocavity structure that couples a single-layer two-dimensional material with a plasmon cavity;

[0034] In an optional embodiment of the present invention, the preparation of a single-layer two-dimensional material can be prepared by chemical vapor deposition and mechanical stripping. Usually, the size of the sample stripped out mechanically is about a few microns, while the chemical vapor deposition method can synthesize samples with a size of hundreds of microns. The WS2 sample selected in the example is currently prepared at the commercial or industrial level at the wafer level. The plasmon cavity (AuNPoM cavity) is composed of Au nanohexagons (AuNH) and Au / SiO2 film. The AuNPoM cavity is prepared by sequentially dripping AuNHs coated with polyallylamine hydrochloride (PAH) solution and hexadecyltrimethylammonium bromide (CTAB) onto the surface of the Au (50nm) / SiO2 (10nm) substrate, which play the role of adhesion and isolation respectively. The single-layer two-dimensional material is transferred to the Au / SiO2 substrate by a polymer (bisphenol A carbonate) (PBAC)-assisted wet transfer method.

[0035] The AuNPoM cavity is further optimized by finely adjusting the size and aspect ratio (AR) of the gold nanohexagons in the cavity to maximize the interaction between the cavity and the monolayer two-dimensional material and achieve a strong coupling effect between the two. The specific implementation process is as follows: First, select gold nanohexagons of various sizes, and use advanced dark field microscopy, scanning electron microscopy and atomic force microscopy techniques to perform detailed size characterization, and accurately record the aspect ratio (AR) of each gold nanohexagon, that is, the ratio of the distance (L) between its two opposite edges (L) to the thickness (T). Construct plasmonic cavities containing gold nanohexagons with different aspect ratios, and measure the dark field scattering spectra of these cavities respectively, and at the same time, measure the photoluminescence spectrum of the monolayer WS2. Through comparative analysis, screen out the gold nanohexagon sizes whose dark field scattering spectrum center wavelength of the plasmon cavity overlaps well with the photoluminescence spectrum center wavelength of the monolayer WS2 and the excitation wavelength of the nonlinear optical response in the spectrum. When this condition is met, it is expected that the plasmon cavity can achieve efficient coupling with the monolayer WS2, thereby significantly enhancing the nonlinear optical response of WS2. Based on the above selection, the optimal parameters of the gold nanohexagon are determined to complete the optimization design of the cavity.

[0036] In this embodiment, the dark field scattering spectrum of the plasmon cavity is collected through the optical path of dark field microscopic observation; the steady-state photoluminescence spectrum is collected through the 430nm continuous light excitation optical path; based on the collected dark field scattering spectrum, a relationship diagram between the dark field scattered light intensity and the wavelength is plotted, and the position corresponding to the maximum dark field scattered light signal, that is, the resonant wavelength of the plasmon cavity is extracted; based on the photoluminescence spectrum of the single-layer layered material in the plasmon cavity, a relationship diagram between the photoluminescence intensity and the wavelength is plotted, and the position corresponding to the peak of the photoluminescence signal is extracted, that is, the resonant position of the exciton state of the single-layer material; when the resonant wavelength of the plasmon cavity overlaps with the central wavelength of the photoluminescence spectrum of the single-layer two-dimensional material and the wavelength of the excitation light, and the second harmonic emission wavelength, the nonlinear optical response signal is significantly enhanced, and this is the optimized plasmon cavity.

[0037] In this embodiment, a monolayer material is precisely placed in an optimized plasmon cavity, and the plasmon cavity finely adjusts its geometric dimensions and material properties to maximize the coupling efficiency with the monolayer material, thereby achieving effective localization and enhancement of the light field.

[0038] S2, collecting two-photon photoluminescence spectra, second harmonic spectra and third harmonic spectra of single-layer two-dimensional materials in hybrid nanocavity structures;

[0039] In an optional embodiment of the present invention, a nanocavity structure in which a carefully designed single-layer two-dimensional material is coupled to a plasmon cavity is first precisely placed in the center of the field of view of a microscope to ensure that the sample can be accurately irradiated by the laser. Subsequently, the microscope is used to determine the specific position of the sample to be tested, and a femtosecond laser of a specific wavelength and power is introduced through a designed optical path system to ensure that the laser can be accurately focused on the sample area. Finally, the two-photon photoluminescence spectrum, second harmonic spectrum, and third harmonic spectrum of the sample area are collected and recorded respectively through a spectrum acquisition device.

[0040] This embodiment significantly enhances the nonlinear optical response of the single-layer material by adjusting and changing the excitation wavelength, and then collects the two-photon photoluminescence, second harmonic, and third harmonic spectra of the single-layer layered material in the plasmon cavity under a microscope. Specifically, the excitation light power is adjusted to an appropriate level to effectively avoid the occurrence of sample saturation effect while ensuring a high signal-to-noise ratio. In addition, under the premise of keeping other test conditions consistent, the wavelength of the excitation light is adjusted at a fixed interval of 5nm to systematically test and record the two-photon photoluminescence spectrum, second harmonic spectrum, and third harmonic spectrum of WS2 at different wavelengths. In view of the weak response of the third harmonic signal under low-power conditions, the present invention proposes an optimized measurement method. At several specific wavelengths, the power of the excitation light is appropriately increased to enhance the intensity of the third harmonic signal, so as to accurately measure and record the third harmonic spectrum at these wavelengths. According to the implementation of the above-mentioned measurement method, the integrity of nonlinear spectrum acquisition is facilitated.

[0041] In this embodiment, filters need to be added before the excitation light and the collected spectral signal, because it is necessary to filter out other stray light except the excitation light before the incident light is focused on the sample and place the filter before the optical fiber collection port of the spectrometer to ensure that no excitation light enters the spectrometer and is affected by the excitation light itself.

[0042] In this embodiment, the grating scale of the spectrometer is selected so that the spectrum collection range of the spectrometer corresponds to the wavelength of the nonlinear optical signal of the excitation light.

[0043] This embodiment achieves significant nonlinear optical enhancement effects at room temperature or near room temperature by optimizing the integrated design of the plasmon cavity and the single-layer two-dimensional material, thus getting rid of the dependence of traditional high-order exciton state detection technology on low-temperature environment and reducing equipment cost and operation complexity.

[0044] S3, plotting the relationship between the intensity of two-photon photoluminescence, second harmonic, and third harmonic and wavelength respectively according to the two-photon photoluminescence spectrum, second harmonic spectrum, and third harmonic spectrum of the single-layer two-dimensional material;

[0045] S4, extracting the position corresponding to the peak of the light signal in the two-photon photoluminescence spectrum from the relationship diagram between the two-photon photoluminescence and the wavelength, and determining the dark exciton state energy level of the single-layer two-dimensional material;

[0046] S5. Extracting the position corresponding to the peak of the optical signal in the second harmonic spectrum from the relationship diagram between the second harmonic and the wavelength, and determining the energy levels of the bright state and the dark exciton state of the single-layer two-dimensional material;

[0047] S6. Extract the position corresponding to the peak of the optical signal in the third harmonic spectrum from the relationship diagram between the third harmonic intensity and the wavelength, and determine the energy level of the Rydberg exciton state of the single-layer two-dimensional material.

[0048] This embodiment extracts the peak value of the nonlinear optical signal under different excitation wavelengths based on the collected nonlinear spectrum, and draws a graph of the relationship between the peak value of the nonlinear spectrum signal and the wavelength. Specifically, according to the collected two-photon photoluminescence spectrum, second harmonic spectrum and third harmonic spectrum of the single-layer material in the plasmon cavity, the graphs of the relationship between the peak value of two-photon photoluminescence, second harmonic and third harmonic under different excitation wavelengths are drawn respectively, and the position corresponding to the peak value of the light signal in the two-photon photoluminescence spectrum is extracted to determine the dark exciton state "np" of the single-layer material; the position corresponding to the peak value of the light signal in the second harmonic spectrum is extracted to determine the bright state and dark exciton state "ns" and "np" of the single-layer material; the position corresponding to the peak value of the light signal in the third harmonic spectrum is extracted to determine the higher-order Rydberg exciton state energy level "ns" of the single-layer material, and the energy level of the exciton state of the single-layer material is accurately determined by combining the linear spectrum with the three complementary nonlinear spectra.

[0049] like Figure 2 As shown, this is a diagram of the optical path structure of the present embodiment using plasmon-enhanced nonlinear spectroscopy to detect discrete exciton state energy levels. This embodiment is applicable to the measurement of nonlinear spectroscopy and linear spectroscopy of two-dimensional materials. The optical path structure includes:

[0050] Femtosecond pulse polarized excitation light source, used to excite two-photon photoluminescence signals, second harmonic signals and third harmonic signals in the sample;

[0051] 430nm continuous excitation light source, used to measure the steady-state luminescence spectrum of the sample;

[0052] White light halogen lamp excitation light source, used for dark field scattering measurement of samples;

[0053] The excitation light path is used to collimate and focus different excitation light sources on the sample area through the microscope objective;

[0054] A microscope is used to move the laser to the sample area to be tested by adjusting the objective lens magnification and the focusing knob.

[0055] Nanopositioning stages are used to achieve nanometer-level motion resolution and motion control accuracy.

[0056] The spectrum collection optical path is used to collect the nonlinear optical spectrum, steady-state luminescence spectrum and dark-field scattering spectrum of the single-layer sample in the plasmon cavity.

[0057] Nonlinear optical spectroscopy, steady-state luminescence spectroscopy, and dark-field scattering spectroscopy data processing modules are used to determine the exciton state energy levels corresponding to the peak values ​​of signals in different spectra.

[0058] For the collection of nonlinear spectra, the incident excitation light is ultrafast pulse light at the femtosecond level. The femtosecond laser source is an optical parametric oscillator with a central wavelength adjustable in the range of 1050 to 1500 nm and a pulse width of ~200 fs, which is focused on the sample area through a microscope objective.

[0059] like Figure 3 As shown, this embodiment selects a hybrid nanocavity structure coupled with a WS2-AuNPoM cavity. Bright field and dark field optical images, scanning electron microscope images and structural schematic diagrams of the WS2-AuNPoM cavity structure designed under a microscope.

[0060] Figure 4 and Figure 5 This is the WS2-AuNPoM cavity optimization process provided in this embodiment. Figure 4 It is a characterization of the parameters of the selected gold nanohexagons of different sizes and a description of the scattering spectra of the plasmon cavities with gold nanohexagons of different sizes. Figure 5The scattering spectrum of the optimized plasmon cavity and the photoluminescence spectrum of the representative measured sample WS2 are screened out. Figure 5 The central wavelength of the photoluminescence spectrum of the WS2 sample shown overlaps well with the central wavelength of the dark-field scattering spectrum of the screened AR=3.8 plasmon cavity. Based on this, the optimized design of the plasmon cavity is completed, ensuring that the best signal enhancement effect and resolution can be obtained in subsequent spectral detection experiments.

[0061] Figure 6 1 is the emission spectrum of a single AuNH and a single layer WS2 on an Au / SiO2 substrate and a WS2-AuNPoM hybrid structure with an AuNH having an AR of 3.8 under 1200nm femtosecond laser excitation provided in this embodiment. Figure 6 , the horizontal axis represents the wavelength range, and the vertical axis represents the nonlinear optical signal intensity.

[0062] Based on the above embodiments, this embodiment sets the femtosecond excitation light to be tested in a wide tunable range of 1050nm to 1500nm, and uses 5nm as a fine test interval to ensure the comprehensiveness and continuity of the spectral data. By changing the wavelength of the excitation light, the two-photon photoluminescence spectrum, second harmonic spectrum, and third harmonic spectrum of WS2 and the integrated hybrid structure (WS2-AuNPoM) are systematically collected. Data processing is performed based on the collected nonlinear spectra, and the peak values ​​of the nonlinear optical signals at different excitation wavelengths are extracted, and a graph showing the relationship between the peak value of the nonlinear spectral signal and the wavelength is drawn. From Figure 7 It can be directly observed that the plasmon cavity significantly enhances the local electromagnetic field, effectively improving the signal-to-noise ratio and resolution of the spectral signal. Figure 7 , respectively, the two-photon photoluminescence spectra, second harmonic and third harmonic spectrum excitation diagrams of WS2 and WS2-AuNPoM. The horizontal axis represents the excitation wavelength, and the vertical axis represents the peak intensity of the nonlinear optical signal at different excitation wavelengths.

[0063] Figure 8 It is a schematic diagram of the plasmon-enhanced WS2 nonlinear optical (2PPL, SHG, and THG) response (energy of 1s, 2p, 2s, 3s, 3s, 4s, and 3p states of A, B, C, and D excitons in a monolayer WS2) at room temperature provided by an embodiment of the present invention.

[0064] Based on the above embodiments, Figure 8It is a process of combining the nonlinear spectrum collected in the above-mentioned embodiment 6 with the linear spectrum of embodiment 4 to identify the energy level of the exciton state of the monolayer WS2. According to the electron transition selection rule, the linear spectroscopy technology can directly detect the bright exciton state ("ns" state), and the second harmonic excitation spectrum can detect the bright and dark excitons "ns" and "np" states, thereby determining the exciton energy of the bright "ns" state. The dark exciton state "np" state can be detected according to the two-photon photoluminescence spectrum, and the exciton state energy of the "np" state can be determined. The third harmonic is a high-order nonlinear optical process. By measuring and analyzing the third harmonic, the energy level structure of the high-order exciton state "ns" state can be obtained. In summary, according to the characteristics of plasmon nanostructures that can effectively localize and enhance the electromagnetic field, thereby significantly improving the efficiency of nonlinear optical processes, the three complementary nonlinear spectra of two-photon photoluminescence, second harmonic, and third harmonic collected are combined with the linear spectrum to accurately determine the energy level of the exciton state. Finally, the relationship between photon energy and wavelength is expressed by conversion, see Figure 8 , the horizontal axis is the photon energy E, and the vertical axis is the state density.

[0065] The present invention uses specific embodiments to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present invention.

[0066] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. A method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy, characterized in that: The following steps are involved: Construct a hybrid nanocavity structure that couples a single-layer two-dimensional material with a plasmon cavity; Collect two-photon photoluminescence spectra, second harmonic spectra, and third harmonic excitation spectra of single-layer two-dimensional materials in hybrid nanocavity structures; According to the two-photon photoluminescence spectrum, second harmonic spectrum and third harmonic excitation spectrum of the single-layer two-dimensional material, the relationship between the two-photon photoluminescence, second harmonic and third harmonic signal intensity and the corresponding emission wavelength are drawn respectively; Extract the energy value corresponding to the peak value of the light signal in the two-photon photoluminescence spectrum from the relationship diagram between the two-photon photoluminescence signal intensity and the emission wavelength, and determine the dark exciton state energy level of the single-layer two-dimensional material; Extract the energy value corresponding to the peak value of the optical signal in the second harmonic spectrum from the relationship between the second harmonic signal intensity and the emission wavelength, and determine the energy levels of the bright and dark exciton states of the single-layer two-dimensional material; The energy value corresponding to the peak value of the optical signal in the third harmonic spectrum is extracted from the relationship diagram between the third harmonic signal intensity and the emission wavelength, and the energy level of the high-energy exciton state of the single-layer two-dimensional material is determined.

2. The method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy according to claim 1, characterized in that: When constructing a hybrid nanocavity structure in which a single-layer two-dimensional material is coupled with a plasmon cavity, a plasmon cavity with a nanoparticle reflection structure is formed by sequentially dripping a polyallylamine hydrochloride solution and gold nanohexagons coated with hexadecyltrimethylammonium bromide onto the surface of an Au / SiO2 substrate; The single-layer two-dimensional material is transferred to the Au / SiO2 substrate by a polymer-assisted wet transfer method, so that the single-layer two-dimensional material is placed in the plasmon cavity to achieve strong coupling.

3. The method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy according to claim 1, characterized in that: When constructing a hybrid nanocavity structure coupled with a monolayer two-dimensional material and a plasmon cavity, the dark-field scattering spectra of the plasmon cavity with gold nanohexagons of different aspect ratios and the photoluminescence spectra of the monolayer two-dimensional material are collected, and the aspect ratio size of the gold nanohexagon when the central wavelength of the dark-field scattering spectrum of the plasmon cavity overlaps with the central wavelength of the photoluminescence spectrum of the monolayer two-dimensional material and the emission wavelength of the nonlinear optical response is selected as the optimal plasmon cavity.

4. The method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy according to claim 3, characterized in that: When constructing a hybrid nanocavity structure in which a single-layer two-dimensional material is coupled with a plasmon cavity, the hybrid nanocavity structure is placed in the sample area of ​​a microscope, and then the dark-field scattering spectrum of the plasmon cavity is collected through a halogen white light source, a microscope, and a collection optical path; According to the collected dark field scattering spectrum, a relationship diagram between the dark field scattered light intensity and the wavelength is drawn, the position corresponding to the maximum dark field scattered light signal is extracted, and the central wavelength of the dark field scattering spectrum of the plasmon cavity is determined.

5. The method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy according to claim 3, characterized in that: When constructing a hybrid nanocavity structure in which a single-layer two-dimensional material is coupled with a plasmon cavity, the hybrid nanocavity structure is placed in the sample area of ​​a microscope, and the photoluminescence spectrum of the single-layer two-dimensional material is collected through a continuous excitation light source, a microscope, and a collection optical path; based on the collected photoluminescence spectrum, a relationship diagram between the photoluminescence intensity and the wavelength is drawn, the position corresponding to the peak of the photoluminescence signal is extracted, and the central wavelength of the photoluminescence spectrum of the single-layer two-dimensional material is determined.

6. The method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy according to claim 1, characterized in that: When collecting the two-photon photoluminescence spectrum, second harmonic spectrum and third harmonic spectrum of a single-layer two-dimensional material in a hybrid nanocavity structure, the hybrid nanocavity structure is placed in the sample area of ​​a microscope, and then the excitation light emitted by a femtosecond excitation light source is collimated and focused on the sample area through the microscope objective lens through the excitation light path to excite the two-photon photoluminescence signal, second harmonic signal and third harmonic signal in the sample, and then the two-photon photoluminescence spectrum, second harmonic spectrum and third harmonic spectrum of the sample area are collected through the collection light path.

7. The method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy according to claim 6, characterized in that: When collecting the two-photon photoluminescence spectrum, second harmonic spectrum and third harmonic spectrum of a single-layer two-dimensional material in a hybrid nanocavity structure, the excitation light power is adjusted to ensure a high signal-to-noise ratio and no saturation effect, and then the wavelength of the excitation light is adjusted to excite and enhance the nonlinear optical effects of the bright exciton state and the dark exciton state of the single-layer two-dimensional material.

8. The method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy according to claim 6, characterized in that: When collecting the third harmonic spectrum of a single layer of two-dimensional material in a hybrid nanocavity structure, the power of the excitation light is increased to ensure a high signal-to-noise ratio and collect the third harmonic spectrum at multiple set wavelengths.

9. The method for detecting exciton state energy levels based on plasmon enhanced nonlinear spectroscopy according to claim 6, characterized in that: Filters are placed before the excitation light path and the collection light path to eliminate stray light interference and improve the signal-to-noise ratio and measurement accuracy of nonlinear spectral signals.

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