A method for characterizing the real-space characteristics of two-dimensional polaritons

Abstract

The present invention provides a method for characterizing the real-space characteristics of a two-dimensional polariton, comprising the following steps: S1: placing a sample to be tested on a SiO2 / Si substrate, so that an edge of the sample to be tested is at 45° with the edge of a cantilever arm of a scattering type near-field optical microscope; S2: scanning the needle tip of the scattering type near-field optical microscope along an edge perpendicular to the sample to be tested, sequentially changing the wavelength of an excitation laser light source, and obtaining a near-field real-space image by scanning the scattering type near-field optical microscope; S3: performing Fourier transform processing on the near-field real-space images obtained in step 2 one by one, and obtaining the real part k′ of the wave vector of the hybrid polariton mode respectively. p and propagation length L p ; S4: According to the formula, the wave vector k′ of the hybrid polaritons at each wavelength is obtained p and L p The present invention breaks through the diffraction limit through a scattering near-field optical microscope, explores the physical phenomenon of the interaction between light and matter in the near-field region, and presents the real-space characteristics of two-dimensional polaritons.

Description

Technical Field

[0001] The present invention relates to the field of nano-optical imaging technology for near-field optical detection, and in particular to a method for characterizing the real-space characteristics of two-dimensional polaritons formed by nanometer-level-thick van der Waals semiconductor layered materials or heterojunctions composed of van der Waals semiconductor layered materials and metals using a scattering near-field optical microscope. Background Art

[0002] Optical imaging plays an important role in modern science. However, the spatial resolution of traditional optical imaging technology is constrained by the diffraction limit and cannot solve the research work of optical phenomena at the nanoscale. Near-field optical imaging technology, which can break through the diffraction limit of traditional optics, can solve this problem.

[0003] The spatial resolution of near-field optical imaging is determined by the distance between the sample and the optical probe and the size of the optical probe. When the wavelength of the incident light is much larger than the distance between the sample and the optical probe, the point spread function of the sample in near-field optics is completely determined by the size of the optical probe. There are two types of scanning near-field optical microscopes: aperture scanning near-field optical microscopes and scattering scanning near-field optical microscopes. The application of aperture scanning near-field optical microscopes is also very limited because the device guides the incident laser into the optical fiber as an optical probe and then approaches the near-field area of ​​the sample surface. Therefore, the preparation of the needle tip is very complicated, and the optical probe prepared in this way inevitably has the problem of light leakage.

[0004] In the prior art, the means of studying the physical phenomena in the near-field region include electron imaging technology, electron energy loss spectroscopy, etc. These technologies all detect the optical localized density of states (LDOS) of the sample by utilizing the energy change after the interaction between electrons and matter or cathode ray fluorescence. Related prior art include: (1) Chinese patent CN 107655891 A, a method for characterizing the optical anisotropy of nanometer-thick van der Waals crystals, published on February 2, 2018; (2) David T, Schoen, Aa Ron L, et al. Probing the electrical switching of a memristiveoptical antenna by STEM EELS. Nature communications. 2016; (3) Raza S, Esfandyarpour M, Koh AL, et al. Electron energy-loss spectroscopy of branchedgap plasmon resonators. Nature Communications. 2016. However, there are still some defects and deficiencies in the existing technologies: first, these technologies are all about detecting the optical local state density of the mode, and the relationship between the optical state density and the polariton eigenmode is very complicated and difficult to explain clearly; second, these technologies require the experimental conditions to be in a vacuum, which increases the complexity of the equipment and the difficulty of the experiment. For example, the spectrum obtained by the electron energy loss spectrum only reflects the distribution of the optical local state density in space, and does not directly present the morphological characteristics of the hybrid polariton mode in real space.

[0005] Therefore, a method is needed to simply and conveniently characterize the real-space characteristics of two-dimensional polaritons. Summary of the invention

[0006] The main technical problem solved by the present invention is to break through the diffraction limit through a scattering near-field optical microscope, explore the physical phenomenon of the interaction between light and matter in the near-field region, and present the real-space characteristics of two-dimensional polaritons.

[0007] In order to solve the above problems, the present invention provides a method for characterizing the real-space characteristics of two-dimensional polaritons, the steps of which are as follows:

[0008] Step 1: Place the sample to be tested on a SiO2 / Si substrate so that one edge of the sample to be tested is at a 45° angle to the edge of the cantilever arm of the scattering near-field optical microscope;

[0009] Step 2: Scan the tip of the scattering near-field optical microscope along the edge perpendicular to the sample to be measured, and change the wavelength of the excitation laser light source in sequence. The scattering near-field optical microscope obtains a near-field real space image by scanning;

[0010] Step 3: Perform Fourier transform on the near-field real-space images obtained in step 2 one by one to obtain the real part of the wave vector k′ of the hybrid polariton mode p and propagation length L p ;

[0011] Step 4: According to formula (1) and formula (2) Obtain the wave vector k′ of the hybrid polaritons at each wavelength p and L p , where k′ p is the real part of the hybrid polariton wave vector, λ0 is the wavelength of the incident light source, d is the period of the interference fringes, α is the angle between the incident light wave vector k0 and the sample plane, and β is the projection k of the incident light wave vector k0 in the sample plane. xy The angle with the normal direction of the edge of the sample being measured, k0 represents the incident light wave vector, k0 = 2π / λ0, L p is the propagation length of the hybrid polariton, and W is the half-height width of the Fourier peak obtained by Fourier transform.

[0012] In the present invention, the inventors use a scattering near-field optical microscope to complete real-space imaging of hybrid polaritons and ensure the integrity and repeatability of the sample.

[0013] Furthermore, the sample to be tested is a van der Waals semiconductor layered material or a heterojunction of a van der Waals semiconductor layered material and a metal, and its lateral size is ≤140 microns.

[0014] Furthermore, the sample to be tested is a van der Waals semiconductor layered material or a heterojunction of a van der Waals semiconductor layered material and a metal, and its longitudinal thickness is ≤4 microns.

[0015] Furthermore, the van der Waals semiconductor layered material may be a thin film of WS2, WSe2, MoS2 or MoSe2.

[0016] In a preferred embodiment of the present invention, the sample to be tested is a heterojunction consisting of WS2 and a single crystal silver disk, with a lateral dimension of 70 microns and a longitudinal thickness of 2 microns.

[0017] The present invention has the following technical effects:

[0018] The present invention uses a scattering scanning near-field optical microscope (s-SNOM) to excite the interaction between the waveguide mode and the exciton in the van der Waals semiconductor layered material or the interaction between the exciton of the van der Waals semiconductor layered material and the surface plasmon of the metal, and performs near-field imaging on it, and then obtains the dispersion relation diagram of the measured sample by analyzing the near-field image. This method first overcomes the limitation of the far-field measurement method on the sample size, can present the optical information within the near-field range of the sample, and also overcomes the drawback of the complex measurement of electronic imaging technology, and intuitively characterizes the real space characteristics of the two-dimensional polaritons. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Embodiments of the subject matter disclosed herein will be described with reference to the accompanying drawings, in which:

[0020] Figure 1 It is a schematic diagram of the structure of the experimental measurement of the present invention.

[0021] Figure 2 This is the near-field image obtained by scanning light sources with different excitation wavelengths using the s-SNOM in the present invention.

[0022] Figure 3 Yes Figure 2 The dispersion relation diagram of the heterojunction sample under different wavelength excitation light sources is obtained by Fourier transforming the near-field image shown.

[0023] Figure 4 Yes Figure 2 The relationship between the propagation length of the hybrid mode and the near-field image shown in the figure is obtained by Fourier transform. DETAILED DESCRIPTION

[0024] The objects and functions of the present invention and methods for achieving the objects and functions are clarified by referring to exemplary embodiments.

[0025] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0026] Taking the heterojunction formed by the van der Waals semiconductor layered material WS2 film and the single crystal silver disk as an example, the method described in the present invention is used to characterize its spatial characteristics. The lateral size of this heterojunction is 70 microns and the longitudinal thickness is 2 microns.

[0027] refer to Figure 1-Figure 4 The present invention provides a method for characterizing the real-space characteristics of two-dimensional polaritons, the steps of which are as follows:

[0028] Figure 1 Schematic diagram of the structure of the experimental measurement in the present invention. Figure 1As shown in the figure, the cantilever arm of the scattering scanning near-field optical microscope (s-SNOM) is used as a reference, where α is the angle between the incident light wave vector k0 and the sample surface, and β is the projection k of the incident light wave vector k0 in the sample plane. xy The angle between the straight edge and the normal direction of the sample under test, and α = 30°, k xy The angle with the straight edge of the sample under test is β. Due to the different placement of the sample in the actual experiment, β is different. When λ = 617nm, the laser light source irradiates the tip of the scattering near-field optical microscope (s-SNOM) to excite the surface plasmon mode of the metal surface. This mode propagates on the metal surface in the form of cylindrical waves, and is scattered at the edge of the sample and collected by the optical probe and returned to the detector. Therefore, the near-field image obtained by the scattering scanning near-field optical microscope (s-SNOM) is mainly formed by the interference of the surface plasmon excited by the tip of the needle and the scattered light at the edge of the sample.

[0029] See also Figure 1 In step 1, the heterojunction sample to be tested is placed on the Sio2 / Si substrate so that one edge of the silver disk is at a 45° angle to the cantilever arm of the s-SNOM, that is, Figure 1 The positive x direction in ;

[0030] See also Figure 1 , Figure 2 In step 2, the s-SNOM tip is moved along a direction perpendicular to the edge of the sample being measured (i.e. Figure 1 The heterojunction sample is scanned along the positive direction of v in the figure, and the wavelength of the excitation light source is changed in sequence to obtain a near-field image;

[0031] Figure 2 The near-field images obtained by measuring the WS2 / single crystal silver disk heterojunction under different laser light source excitation using the scattering scanning near-field optical microscope (s-SNOM), the excitation light source wavelengths shown are 610nm, 617nm, 620nm, 622nm, 630nm, 633nm respectively;

[0032] Step 3: Perform Fourier transform on the near-field images obtained in step 2 to obtain the real part of the wave vector k′ of the hybrid polariton mode p and propagation length L p ;

[0033] Figure 3 The dispersion relation diagram corresponding to the WS2 / single crystal silver disk heterojunction measured at different excitation wavelengths is obtained by Fourier transforming the near-field image shown in step 2. The real part k′ of the hybrid polariton wave vector can be directly read out from the dispersion relation diagram p .

[0034] Figure 4 To perform Fourier transform on the near-field image shown in step 2, the propagation length of the hybrid polaritons in the WS2 / single crystal silver disk heterojunction can be obtained. The propagation length L of the hybrid polaritons can be directly read from the propagation length diagram. p .

[0035] Step 4: According to formula (1) and formula (2) Obtain the polariton wave vector k′ at each wavelength p and L p , where k′ p is the real part of the hybrid polariton wave vector, L p is the propagation length of the hybrid polaritons;

[0036] From the figure, we can see that: λ1=610nm, d=3.016μm, and substituting into formula (1) and formula (2), we can get k′ p =1.0613,L p =4.645μm;

[0037] From the figure, we can see that: λ2=617nm, d=3.034μm, and substituting into formula (1) and formula (2), we can get k′ p =1.0409,L p =3.394μm;

[0038] From the figure, we can see that: λ3=620nm, d=3.063μm, and substituting into formula (1) and formula (2), we can get k′ p =1.0404,L p =3.394μm;

[0039] From the figure, we can see that: λ4=622nm, d=2.981μm, and substituting into formula (1) and formula (2), we can get k′ p =1.044,L p =3.845μm;

[0040] From the figure, we can see that: λ5=630nm, d=3.0568μm, and substituting into formula (1) and formula (2), we can get k′ p =1.0598,L p =9.186μm;

[0041] From the figure, we can see that: λ6=633nm, d=3.0546μm, and substituting into formula (1) and formula (2), we can get k′ p =1.0409,L p =6.464μm;

[0042] In summary, in the 605nm-660nm band, the real-space characteristics of the hybrid polaritons in the WS2 / single-crystal silver disk heterojunction can be characterized by scattering scanning near-field optical microscopy (s-SNOM).

[0043] The present invention uses a scattering scanning near-field optical microscope (s-SNOM) to detect surface plasmon modes in van der Waals semiconductor layered materials, which interact with the excitons of WS2 to form hybrid polaritons, and performs near-field real-space imaging on the hybrid modes, and then obtains the dispersion relationship spectrum of the excitons and surface plasmon modes of the measured van der Waals semiconductor layered materials through image analysis. This method first overcomes the limitation of far-field measurement methods on sample size, and can present optical information within the near-field range of the sample, and also overcomes the drawbacks of complex measurement of electronic imaging technology, and intuitively characterizes the real-space characteristics of two-dimensional polaritons.

[0044] In combination with the above-described examples, other examples of the present invention are easy to think of and grasp for those skilled in the art, and it is not necessary to list all the embodiments here. The true scope and spirit of the present invention are defined by the claims.

Claims

1. A method for characterizing the real-space characteristics of two-dimensional polaritons, characterized in that: The steps include: S1: Place the sample to be tested on a SiO2 / Si substrate so that one edge of the sample to be tested is at a 45° angle to the edge of the cantilever arm of the scattering type near-field optical microscope; S2: Scanning the tip of the scattering near-field optical microscope along the edge perpendicular to the sample to be measured, changing the wavelength of the excitation laser light source in sequence, and obtaining a near-field real space image by scanning the scattering near-field optical microscope; S3: Perform Fourier transform processing on the near-field real-space images obtained in step S2 one by one to obtain the real part k' of the wave vector of the hybrid polariton mode p and propagation length L p ; S4: According to formula (1) and formula (2) Obtain the wave vector k' of the hybrid polaritons at each wavelength p and L p , where k' p is the real part of the hybrid polariton wave vector, λ0 is the wavelength of the incident light source, d is the period of the interference fringes, α is the angle between the incident light wave vector k0 and the sample plane, and β is the projection k of the incident light wave vector k0 in the sample plane. xy The angle with the normal direction of the edge of the sample being measured, k0 represents the incident light wave vector, k0 = 2π / λ0, L p is the propagation length of the hybrid polariton, and W is the half-height width of the Fourier peak obtained by Fourier transform.

2. The method for characterizing the real-space characteristics of two-dimensional polaritons according to claim 1, characterized in that: The sample to be tested is a van der Waals semiconductor layer material or a heterojunction of a van der Waals semiconductor layer material and a metal, and its lateral size is ≤140 microns.

3. The method for characterizing the real-space characteristics of two-dimensional polaritons according to claim 1 or 2, characterized in that: The sample to be tested is a van der Waals semiconductor layer material or a heterojunction formed by a van der Waals semiconductor layer material and a metal, and its longitudinal thickness is ≤4 microns.

4. The method for characterizing the real-space characteristics of two-dimensional polaritons according to claim 2, characterized in that: The van der Waals semiconductor layered material is a thin film of WS2, WSe2, MoS2 or MoSe2.

5. The method for characterizing the real-space characteristics of two-dimensional polaritons according to claim 3, characterized in that: The van der Waals semiconductor layered material is a thin film of WS2, WSe2, MoS2 or MoSe2.

6. The method for characterizing the real-space characteristics of two-dimensional polaritons according to claim 1, characterized in that: The sample under test is a heterojunction formed by WS2 and a single crystal silver disk, with a lateral dimension of 70 microns and a longitudinal thickness of 2 microns.

Images