CN114594097A - A method to characterize real-space characteristics of two-dimensional polaritons - Google Patents
A method to characterize real-space characteristics of two-dimensional polaritons Download PDFInfo
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Abstract
本发明提供了一种表征二维极化激元实空间特征的方法，包括如下步骤：S1：将被测样品置于SiO2/Si基底上，使被测样品的一条边缘与散射型近场光学显微镜的悬梁臂边缘呈45°；S2：使散射型近场光学显微镜的针尖沿着垂直于被测样品的边缘扫描，依次改变激发激光光源的波长，散射型近场光学显微镜通过扫描得到近场实空间图像；S3：对步骤二中得到的近场实空间图像一一进行傅里叶变换处理，分别得到杂化极化激元模式的波矢量实部k′p和传播长度Lp；S4：根据公式求得各个波长下的杂化极化激元的波矢k′p和Lp。本发明通过散射型近场光学显微镜突破衍射极限，探究近场区域内光与物质的相互作用的物理现象，呈现二维极化激元的实空间特征。
The present invention provides a method for characterizing real space characteristics of two-dimensional polaritons, 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 connected to the scattering type near-field optical The cantilever edge of the microscope is at 45°; S2: Scan the tip of the scattering type near-field optical microscope along the edge perpendicular to the sample to be tested, and change the wavelength of the excitation laser light source in turn. The scattering type near-field optical microscope obtains the near field by scanning. Real-space image; S3: Perform Fourier transform processing on the near-field real-space images obtained in step 2 one by one, and obtain the wave vector real part k′ p and propagation length L p of the hybrid polariton mode respectively; S4 : According to the formula, the wave vector k′ p and L p of the hybrid polariton at each wavelength are obtained. The invention breaks through the diffraction limit through a scattering-type 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 a two-dimensional polariton.
Description
Technical Field
The invention relates to the technical field of nano optical imaging of near-field optical detection, in particular to a method for representing real space characteristics of two-dimensional polaritons formed by a heterojunction formed by van der Waals semiconductor layered materials or van der Waals semiconductor layered materials and metals with nano-scale thickness by a scattering type near-field optical microscope.
Background
Optical imaging plays an important role in modern science, however, the spatial resolution of the traditional optical imaging technology is bound by diffraction limit, the research work of optical phenomena under the nanometer scale cannot be solved, and the problem can be just solved by the near-field optical imaging technology which can break through the traditional optical diffraction limit.
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 incident light wavelength is much larger than the distance between the sample and the optical probe, then the point spread function of the sample in near-field optics is completely determined by the size of the optical probe. The scanning type near-field optical microscope has two types, namely an aperture type scanning type near-field optical microscope and a scattering type scanning type near-field optical microscope, but the application of the aperture type scanning type near-field optical microscope is also limited, because the equipment leads incident laser into an optical fiber as an optical probe and approaches to a near-field area on the surface of a sample, the preparation of a needle point is very complicated, and the prepared optical probe inevitably has the problem of light leakage.
In the prior art, means for studying such physical phenomena in the near field region include electron imaging techniques, electron energy loss spectroscopy, and the like, which detect the optical local density of states (LDOS) of a sample by using energy change after interaction between electrons and substances or cathode ray fluorescence. The related prior art is as follows: (1) chinese patent CN 107655891 a, a method for characterizing van der waals crystal optical anisotropy at nanoscale thickness, published No. 2018.02.02; (2) david T, Schoen, Aa Ron L, et al, binding the electrical switching of a memrisic optical antenna by STEM EELS, Nature communications 2016; (3) raza S, Esfandyarpoor M, Koh A L, et al, Electron energy-loss spectroscopy of branched gap plasma reactors, Nature Communications, 2016. However, the prior art has some defects and shortcomings: firstly, the technologies are optical local state density of a detection mode, and the relationship between the optical state density and a polariton eigenmode is very complex and difficult to explain clearly; secondly, these techniques require the experimental conditions to be in 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 condition of optical local state density in space, and the morphological characteristics of the hybrid polariton mode in real space are not directly presented.
Therefore, a method for simply and conveniently characterizing the two-dimensional polariton real space features is needed.
Disclosure of Invention
The invention mainly solves the technical problem that the scattering type near-field optical microscope breaks through the diffraction limit, explores the physical phenomenon of the interaction of light and substances in a near-field area and presents the real space characteristic of two-dimensional polariton.
In order to solve the above problems, the present invention provides a method for characterizing a two-dimensional polariton real space feature, comprising the following steps:
the method comprises the following steps: placing the sample to be detected on a SiO2/Si substrate, and enabling one edge of the sample to be detected to form an angle of 45 degrees with the edge of a cantilever arm of the scattering type near-field optical microscope;
step two: scanning a needle point of the scattering type near-field optical microscope along the edge vertical to a measured sample, sequentially changing the wavelength of an excitation laser light source, and obtaining a near-field real space image through scanning by the scattering type near-field optical microscope;
step three: fourier transform processing is carried out on the near-field real space images obtained in the step two one by one to respectively obtain wave vector real parts k 'of the hybrid polarization mode'pAnd propagation length Lp；
Step four: according to formula (1)
And formula (2)
Obtaining a wavevector k 'of hybrid polarization excimer at each wavelength'pAnd LpWherein, k'pFor hybridizing the real part of the polariton wave vector, lambda0Is the wavelength of the incident light source, d is the period of the interference fringes, and α is the incident light wave vector k0Angle to the sample plane, beta is incident light wave vector k0Projection k in the sample planexyAngle k with respect to the normal direction of the edge of the sample to be measured0Denotes the wave vector, k, of the incident light0＝2π/λ0，LpTo hybridize the propagation length of the polariton, W is the full width at half maximum of the fourier peak resulting from the fourier transform.
In the invention, the inventor adopts a scattering type near-field optical microscope to complete real-space imaging on the hybrid polariton and ensure the integrity and repeatability of the sample.
Furthermore, the tested sample is a van der Waals semiconductor layered material or a heterojunction of the van der Waals semiconductor layered material and metal, and the transverse size of the tested sample is less than or equal to 140 micrometers.
Furthermore, the tested sample is Van der Waals semiconductor layered material or a heterojunction of the Van der Waals semiconductor layered material and metal, and the longitudinal thickness of the tested sample is less than or equal to 4 micrometers.
Further, the van der waals semiconductor layered material is a thin film that may be WS2, WSe2, MoS2, or MoSe 2.
In a preferred embodiment of the invention, the sample tested was a heterojunction of WS2 with a single crystal silver disk with a lateral dimension of 70 microns and a longitudinal thickness of 2 microns.
The invention has the following technical effects:
the invention uses a scattering type scanning near-field optical microscope (s-SNOM) to excite the interaction between a waveguide mode and excitons in a van der Waals semiconductor layered material or the interaction between the excitons of the van der Waals semiconductor layered material and surface plasmons of metal, and carries out near-field imaging on the interactions, and further obtains a dispersion relation diagram of a measured sample by analyzing a near-field image. The method firstly overcomes the limitation of a far-field measurement means on the size of a sample, can present optical information in a near-field range of the sample, overcomes the defect of complex measurement of an electronic imaging technology, and visually represents the real space characteristics of two-dimensional polaritons.
Drawings
Embodiments of the presently disclosed subject matter will be described with reference to the accompanying drawings, in which:
FIG. 1 is a schematic diagram of the experimental measurements performed in the present invention.
FIG. 2 is a near field image obtained by scanning a light source of different excitation wavelengths using the s-SNOM in the present invention.
Fig. 3 is a dispersion relation graph of heterojunction samples under excitation light sources of different wavelengths, obtained by fourier transform of the near-field image shown in fig. 2.
Fig. 4 is a graph showing the relationship between the propagation lengths of the hybrid modes obtained by fourier transforming the near-field image shown in fig. 2.
Detailed Description
The objects and functions of the present invention and methods for accomplishing the same are explained with reference to exemplary embodiments.
The invention is further illustrated by the following examples in conjunction with the drawings.
The method of the present invention is used to characterize the spatial characteristics of a heterojunction formed by a thin film of van der waals layered semiconductor material WS2 and a single crystal silver disc. The heterojunction has a lateral dimension of 70 microns and a longitudinal thickness of 2 microns.
Referring to fig. 1 to 4, the present invention provides a method for characterizing a two-dimensional polariton real space feature, which includes the following steps:
FIG. 1 is a schematic diagram of the structure of experimental measurements in the present invention. As shown in FIG. 1, the cantilever arm of a scattering type scanning near-field optical microscope (s-SNOM) is taken as a reference, wherein alpha is an incident light wave vector k0Angle between the sample surface and beta is incident light wave vector k0Projection k in the sample planexyThe included angle between the angle alpha and the normal direction of the straight edge of the tested sample is 30 degrees, and k isxyThe included angle between the edge of the sample and the straight edge of the tested sample is beta, and the beta exists due to the different arrangement of the sample in the actual experimentOn the other hand, when λ ═ 617nm, a laser light source is irradiated on a tip of a scattering-type near-field optical microscope (s-SNOM) to excite a surface plasmon mode of a metal surface, which mode propagates in the form of a cylindrical wave on the metal surface, and is scattered at the edge of a sample and collected by an optical probe and returned to a detector. Therefore, a near-field image obtained by a scattering scanning near-field optical microscope (s-SNOM) is mainly a fringe formed by interference between a surface plasmon excited by the tip of the tip and scattered light at the edge of the sample.
Referring to fig. 1, in step one: placing a heterojunction sample to be detected on a SiO2/Si substrate, and enabling one edge of the edge of a silver disc and a cantilever arm of a scattering type scanning near-field optical microscope (s-SNOM) to form an angle of 45 degrees, namely the positive direction of x in figure 1;
referring to fig. 1 and 2, in step two: scanning the heterojunction sample by the tip of the s-SNOM along the direction vertical to the edge of the sample to be detected (namely along the positive v direction in the figure 1), and sequentially changing the wavelength of an excitation light source to obtain a near-field image;
FIG. 2 is a near field image obtained by measuring WS 2/single crystal silver disk heterojunction under different laser light source excitations by using the scattering type scanning near field optical microscope (s-SNOM), wherein the wavelengths of the exciting light source are respectively 610nm, 617nm, 620nm, 622nm, 630nm and 633 nm;
step three: respectively carrying out Fourier transform on the near-field images obtained in the step two to respectively obtain wave vector real parts k 'of hybrid polarization modes'pAnd propagation length Lp；
FIG. 3 is a graph of dispersion relationship obtained by Fourier transform of the near field image shown in step two corresponding to WS 2/single crystal silver disk heterojunction measured at different excitation wavelengths. The hybrid polarization excimer wave vector real part k 'obtained by directly reading the dispersion relation graph'p。
FIG. 4 shows Fourier transform of the near field image shown in step two, and the propagation length of the hybrid polariton in WS 2/single crystal silver disk heterojunction can be determined. The propagation length L of the hybrid polariton obtained by directly reading the propagation length mapp。
Step four: according to formula (1)
And formula (2)
Obtaining polarization excimer wave vector k 'at each wavelength'pAnd LpOf which k'pFor hybridization of real part of polariton wave vector, LpIs the propagation length of the hybrid polariton;
as can be seen from the figure: k 'can be obtained by substituting 610nm λ 1 and 3.016 μm d into equations (1) and (2)'p＝1.0613，Lp＝4.645μm；
As can be seen from the figure: k 'can be obtained by substituting λ 2 ═ 617nm, d ═ 3.034 μm, and equation (1) and equation (2)'p＝1.0409，Lp＝3.394μm；
As can be seen from the figure: k 'can be obtained by substituting equation (1) and equation (2) with λ 3 ═ 620nm and d ═ 3.063 μm'p＝1.0404，Lp＝3.394μm；
As can be seen from the figure: k 'can be obtained by substituting λ 4 ═ 622nm, d ═ 2.981 μm into equations (1) and (2)'p＝1.044，Lp＝3.845μm；
As can be seen from the figure: k 'can be obtained by substituting λ 5 ═ 630nm, d ═ 3.0568 μm into equations (1) and (2)'p＝1.0598，Lp＝9.186μm；
As can be seen from the figure: k 'can be obtained by substituting λ 6 ═ 633nm, d ═ 3.0546 μm, and equation (1) and equation (2)'p＝1.0409，Lp＝6.464μm；
In conclusion: the real-space characteristics of the hybrid polaritons in the WS 2/single crystal silver disk heterojunction in the 605nm-660nm wave band can be characterized by a scattering type scanning near-field optical microscope (s-SNOM).
The invention uses a scattering type scanning near-field optical microscope (s-SNOM) to form a hybrid polariton mode in a van der Waals semiconductor layered material, the mode interacts with an exciton of WS2 to form a hybrid polariton mode, near-field real-space imaging is carried out on the hybrid mode, and then a dispersion relation map of the measured van der Waals semiconductor layered material exciton and the surface plasmon mode is obtained by analyzing images. The method firstly overcomes the limitation of a far-field measurement means on the size of a sample, can present optical information in a near-field range of the sample, also overcomes the defect of complex measurement of an electronic imaging technology, and visually represents the real space characteristics of the two-dimensional polaritons.
Other embodiments of the present invention will readily suggest themselves to such skilled persons, and will not necessarily be described herein in connection with the above-described illustrated embodiments. The true scope and spirit of the invention are defined by the following claims.
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| CN115420705A (en) * | 2022-08-31 | 2022-12-02 | 中国科学院国家空间科学中心 | Two-three-dimensional joint simulation method for scattering type near-field scanning imaging transceiving design |
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