A method for separating the emission directions of two-dimensional material valleys using electron beams

Abstract

The present invention discloses a method for realizing separation of emission directions of energy valleys of two-dimensional materials by using electron beams, and controlling coupling between plasmon resonance modes of nanometal structures and valley excitons inside two-dimensional materials by using focused electron beams, realizing selective guidance of photons with different spins emitted by different valley excitons, so that light signals associated with different degrees of freedom of energy valleys propagate in different directions. The present invention builds a bridge connecting far-field optical signals with energy valley information inside materials, and has the characteristics of simple realization, flexible design, high precision, and high-density integration. It can be used for cutting-edge research on coupling of two-dimensional materials with light spin orbits and new information technology projects such as energy valley optoelectronic devices, nano optoelectronic chips, and quantum communication devices, and has broad development prospects.

Description

Technical Field

[0001] The present invention belongs to the field of materials and relates to a method for manipulating the valley degree of freedom of a two-dimensional material using a focused electron beam and achieving far-field separation. Specifically, based on the selective excitation characteristics of the electron beam on the surface plasmon resonance mode, the coupling between the valley excitons of the two-dimensional material and the resonance mode of the nanometal structure is manipulated, and then the far-field interference between different radiation modes is utilized to guide and separate the emission directions of photons with different spin angular momenta radiated by different valley excitons, thereby building a bridge connecting the far-field optical signal and the valley information inside the material. Background Art

[0002] The energy valley is a property of the electronic band structure of common condensed materials such as silicon, diamond, and silicon dioxide, existing at the lowest point of the conduction band and the highest point of the valence band. The energy valley characteristic of electrons is similar to spin, which is an intrinsic degree of freedom. Therefore, it is also called pseudospin and has great potential in the application direction of traditional electronic devices that go beyond charge manipulation. However, due to the lack of physical quantities directly related to the energy valley degree of freedom, the characterization and manipulation of the energy valley degree of freedom is very challenging, which greatly limits the development of valley electronics devices based on the energy valley degree of freedom.

[0003] In recent years, the emergence of two-dimensional transition metal dichalcogenides (TMDCs) has provided a feasible solution to this long-standing problem and has attracted great attention from researchers and industry to valley electronics. Compared with traditional condensed matter materials, TMDCs have the characteristics of broken spatial inversion symmetry but preserved time inversion symmetry. In addition, the spin-orbit coupling effect generated between the d orbitals of metal atoms and the carriers at the edge of the valence band causes the valence band to split and the spin to degenerate, which ultimately leads to the spins of the excited transition electrons in different valence bands being locked to the valleys, which is commonly known as the spin-valley-locking effect. Through the spin-valley-locking effect, we can use circularly polarized light or external electric and magnetic fields to control the valleys, so that the population numbers at different valleys are different, thereby generating valley polarization and other valley-dependent binary physical phenomena, such as valley-polarized emission, valley-dependent strong coupling, valley Hall effect, topological valley electronics, etc. As a valley electronics and photonics interface that is very likely to be realized, valley-related directional propagation is of great significance to future valley-related information networks and optoelectronic chips. However, due to the limitation of the diffraction limit, it is difficult for traditional optical methods to control the emission directions of different valleys to achieve valley information processing and transmission while meeting the requirements of high-density chip integration. Therefore, the development of a technical means to control and separate valley emission directions on a sub-wavelength scale is of great significance for the preparation of nano optoelectronic chips.

[0004] With the development of nanophotonics, nanometal structures have become an important unit for designing and constructing functional devices at the subwavelength scale. Free electrons inside metals can generate a kind of elementary excitation, namely surface plasmons, under external stimulation, which can compress the optical mode volume to the nanoscale and greatly enhance the interaction between light and matter. By changing the geometric morphology of the structure, the electromagnetic field properties near the structure can be effectively controlled to achieve the modulation of the luminous intensity and radiation direction of the material. At the same time, by controlling the coupling interference between different surface plasmon modes, similar spin-orbit coupling can be achieved to achieve the locking between the photon spin angular momentum and the propagation direction, which is specifically manifested as the separation of photons with different spins in the far field.

[0005] In recent years, a large number of studies have shown that focused electron beams with spatial resolution at the nanoscale can effectively excite surface plasmons. As an excitation source, free electrons can cover an extremely wide band from x-rays to near-infrared, achieving simultaneous excitation of multiple optical modes. In addition, careful control of the excitation position of the electron beam can achieve excellent local excitation, thereby selectively exciting specific plasmon resonance modes of nanometal structures, and regulating the band, chirality, and direction of the luminescence of the metal structure. On the other hand, electron beams are also widely used to study and regulate the optical properties of materials such as semiconductors and hybrid perovskites. Thanks to the spatial resolution of electron beams far exceeding that of traditional optical excitation methods and their unique selective excitation characteristics, using electron beams to control the luminescence characteristics of composite systems of nanometal structures and two-dimensional materials is a very important means. Summary of the invention

[0006] The purpose of the present invention is to provide a method for transmitting valley information from the near field to the far field at the nanoscale, using a focused electron beam to control the coupling between the nanometal structure and the valley excitons inside the two-dimensional material, thereby achieving selective guidance of photons with different spins emitted by different valley excitons, so that light signals associated with different valley degrees of freedom propagate in different directions.

[0007] The technical solution of the present invention is as follows:

[0008] A method for separating the emission directions of valleys of two-dimensional materials using electron beams. First, a composite heterostructure device is prepared in which a nanometal structure is located on a two-dimensional material. The nanometal structure has two localized surface plasmon resonance modes with energies close to the wavelength of valley exciton emission of the two-dimensional material but with different symmetries. The electron beam is focused at a specific position of the nanometal structure to excite the two-dimensional material below the nanometal structure to emit light, so that valley excitons of two different valleys (K valley and K' valley) inside the two-dimensional material are coupled to the two resonance modes of the nanometal structure respectively, and the radiation directions are guided respectively. Under the constraint of spin-orbit coupling, photons with different spins carrying valley information propagate in different directions.

[0009] In the above method, the two-dimensional material can be graphene, black phosphorus, a single-layer two-dimensional transition metal chalcogenide, preferably a single-layer two-dimensional transition metal chalcogenide, such as a single-layer tungsten disulfide, molybdenum disulfide, tungsten diselenide, molybdenum diselenide, molybdenum ditelluride, tungsten ditelluride, etc.

[0010] In order to reduce the damage of the electron beam to the two-dimensional material, the two-dimensional material can be encapsulated between two hexagonal boron nitride sheets, and the nanometal structure is located on the upper hexagonal boron nitride sheet. Preferably, the thickness of the hexagonal boron nitride sheet located at the bottom layer is 100 to 200 nanometers, and the thickness of the hexagonal boron nitride sheet located at the upper layer is 10 to 30 nanometers.

[0011] The material of the nano-metal structure is, for example, gold, aluminum, silver, copper, etc., and a suitable nano-metal structure can be prepared by selecting the material according to the wavelength band of the luminescence of the two-dimensional material to be regulated.

[0012] The true frequencies of the two localized surface plasmon resonance modes of the nanometal structure are close to the center frequency of the valley exciton luminescence of the two-dimensional material, and effective coupling can be achieved. The plasmon resonance frequency of the nanometal structure can be adjusted to the vicinity of the required frequency by changing its shape and size. In the design of the geometric morphology and specific parameters of the nanometal structure, the morphological characteristics of the nanometal structure can be continuously adjusted by combining numerical simulation with scanning parameters, so that it has two localized surface plasmon resonance modes with energy close to the wavelength of the valley exciton luminescence of the two-dimensional material but different symmetry. At the same time, only when the local excitation is in some specific positions, the two resonance modes are in phase after being locally excited and the amplitude is not much different, can the effect of far-field separation be achieved.

[0013] In some embodiments of the present invention, the two-dimensional material is a monolayer of tungsten disulfide, and the nanometal structure is a "two"-shaped structure composed of a short gold rod and a long gold rod, in which there are two localized surface plasmon resonance modes with different symmetries, namely, a dipole mode and a quadrupole mode. Under the designed geometric parameters (for example, the length, width and height of the long gold rod are 200-220 nanometers, 65-75 nanometers, and 35-45 nanometers, respectively, and the length, width and height of the short gold rod are 60-70 nanometers, 20-30 nanometers, and 35-45 nanometers, respectively), the eigenfrequencies of the two resonance modes are close to the center frequency of the luminescence of the valley exciton of tungsten disulfide, and effective coupling with the monolayer of tungsten disulfide can be achieved. When the local excitation is at some specific positions, such as the center of the short rod, the two resonance modes are in phase after being locally excited and the amplitudes are not much different, thereby achieving the effect of far-field separation.

[0014] In other embodiments of the present invention, the two-dimensional material is a monolayer of tungsten disulfide, and the nanometal structure is a gold disk with a diameter of 450 to 550 nanometers and a thickness of 35 to 45 nanometers. There are two localized surface plasmon resonance modes with different symmetries, namely, a hexapole mode and an octupole mode. When the focused electron beam excitation position is the edge of the disk, the emission directions of the K valley and K' valley excitons of the monolayer of tungsten disulfide are guided separately, and the generated photons with different spins propagate in different directions, achieving the effect of far-field separation.

[0015] The present invention uses an electron beam to realize a method for transmitting the energy valley information of a two-dimensional material from the near field to the far field (see Figure 1 ), which is specifically manifested as the far-field radiation of the exciton luminescence of different energy valleys inside the separated two-dimensional material. The method includes three parts: preparation of a composite heterostructure device of nanometal structure and two-dimensional material, luminescence of the two-dimensional material in the composite heterostructure device, and binary separation of the luminescence direction of the valley exciton of the two-dimensional material.

[0016] In some embodiments of the present invention, the preparation of a composite heterostructure device of a nanometal structure and a two-dimensional material is divided into the following three steps: 1. Preparation of hexagonal boron nitride and monolayer tungsten disulfide samples. Use tape to peel off the hexagonal boron nitride flakes and the monolayer tungsten disulfide from the bulk material. Select a peeled hexagonal boron nitride and place it on a clean silicon dioxide / silicon substrate, then place a monolayer of tungsten disulfide on the hexagonal boron nitride, and characterize the sample using an optical microscope and an atomic force microscope to determine the relative position and specific thickness of the hexagonal boron nitride and tungsten disulfide samples. 2. Select a thinner hexagonal boron nitride flake and place it on another clean silicon dioxide / silicon substrate, then spin-coat a layer of electron beam photoresist on the substrate, and use electron beam exposure to transfer the designed pattern on the hexagonal boron nitride. Subsequently, the gold film and the Ti film as the adhesion layer were evaporated on the silicon wafer by the electron beam evaporation coating method, and the residual electron beam photoresist was dissolved by acetone, and finally a specific nanometal structure located on the hexagonal boron nitride was obtained. 3. The processed hexagonal boron nitride / metal sample was transferred to the hexagonal boron nitride / tungsten disulfide sample on another silicon wafer as the top layer. Finally, a composite system of tungsten disulfide encapsulated by hexagonal boron nitride and a specific nanometal structure was obtained. The reason why hexagonal boron nitride is required to encapsulate tungsten disulfide is, first, to reduce the damage of the electron beam to the monolayer tungsten disulfide sample and extend the life of the sample; second, hexagonal boron nitride can provide more holes to the monolayer tungsten disulfide, thereby enhancing the probability of electron-hole pair recombination, enhancing the intensity of valley exciton luminescence, and making the phenomenon of separation of valley exciton luminescence direction more obvious.

[0017] The luminescence of the two-dimensional material in the composite heterostructure device is generated by exciting the two-dimensional material located below the nanometal structure with a focused electron beam. The local electromagnetic field carried by the electron beam transfers energy from free electrons to the valence band electrons of the two-dimensional material through near-field interaction, exciting them to the conduction band and leaving a hole in the valence band. Due to the splitting characteristics of the valence band of the two-dimensional material, only electrons with spin up are excited in the K valley, while only electrons with spin down are excited in the K' valley. These two electron-hole pairs with different spins form a bound state, namely valley excitons, under the Coulomb interaction. Subsequently, different valley excitons, under the constraints of the optical transition selection rule, undergo radiative decay and generate photons with different spin angular momentum. Due to the symmetry of electron beam excitation, there is no difference in the number of valley excitons in the K valley and the K' valley. Therefore, in the light emitted by the composite heterostructure device, there is no obvious difference in the intensity of left-handed circularly polarized light and right-handed circularly polarized light.

[0018] By controlling the local excitation position of the electron beam, the spatial position of the excited valley exciton can be effectively changed. Then the coupling between the valley exciton and the specific surface plasmon resonance mode of the nanometal structure can be regulated. Since the radiation transition dipole of the valley is a circular polarization dipole, the circular polarization transition dipole of the valley excitons in the K valley and K' valley has opposite signs. Therefore, when coupling with the surface plasmon resonance mode close to the energy, the different signs of the circular polarization dipole will cause different results of the far-field interference of multiple radiation modes, and the radiation direction of the valley exciton will be modulated differently. By carefully designing the geometric morphology of the nanometal structure, adjusting the amplitude and phase of the multiple plasmon resonance modes with similar energies of the structure, and then regulating the results of the far-field interference, the control of the emission direction of different valley excitons can be achieved through spin-orbit coupling. At the same time, the switch of the separation of the emission direction of the valley exciton can also be controlled by adjusting the excitation site of the electron beam, that is, separation can be generated when the electron beam excites certain specific sites, and when the electron beam position is slightly moved, the separation disappears. This also proves the flexibility of this method.

[0019] The method proposed by the present invention for controlling and separating the emission of valleys of two-dimensional materials by using electron beams is the first means that can use electron beam excitation and nanometal structures to control and separate the emission directions of valley excitons in different valleys of two-dimensional materials in a sub-wavelength scale, and can realize the transmission of valley information inside the material to optical information in the far field. By preparing a composite heterostructure device of two-dimensional materials and specific nanometal structures, at room temperature and without an external electric field or magnetic field, the radiation directions of valley excitons in two different valleys (K and K') inside the two-dimensional material can be guided separately by relying only on the excitation of focused electron beams with ultra-high spatial resolution at certain specific points. Under the constraint of spin-orbit coupling, photons carrying valley information with different spins will propagate in different directions, realizing the locking of valley-photon spin-propagation direction. Thus, through regulation within the nanoscale, the information carried by valley excitons in a single-layer two-dimensional material is finally converted into photon information with different spins transmitted in different directions in the far field.

[0020] The present invention focuses on the research of a new generation of information carriers in nanophotonics. The control of valley freedom is transformed from traditional methods such as far-field light excitation and external electric and magnetic fields to the use of electron beams to control the coupling between valley excitons and nanometal structure plasmon resonance modes, breaking through the optical diffraction limit. By guiding the radiation direction of different valley excitons, the purpose of transferring valley information from the inside of the material and the near-field space to the far field is achieved. The invention has the characteristics of simple implementation, flexible design, high precision, and high-density integration. It can be used in cutting-edge research on two-dimensional materials and optical spin-orbit coupling and new information technology projects such as valley optoelectronic devices, nano optoelectronic chips, and quantum communication devices, and has broad market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic diagram of the sample structure and phenomenon of a specific embodiment of the present invention (left figure) and an electron microscope photograph of the sample (right figure) are shown.

[0022] Figure 2 A schematic diagram of the research optical path of a specific implementation method of the present invention is shown.

[0023] Figure 3 The resonance mode distribution diagram of the "two"-shaped nanometal structure in a specific embodiment of the present invention is shown.

[0024] Figure 4 The intensity and relative phase of each resonance mode of the "two"-shaped nanometal structure in the specific embodiment of the present invention are shown.

[0025] Figure 5 Electron beam excitation spectra of monolayer tungsten disulfide and "2"-shaped gold nanostructures are shown.

[0026] Figure 6 The results show that when the electron beam is focused on a specific position of the composite heterostructure device of tungsten disulfide and nanometal structure in Example 1 of the present invention, the effect of separating the emission directions of the two valley excitons can be produced.

[0027] Figure 7 The numerical simulation diagram showing the separation of the emission directions of two valley excitons when the electron beam is focused on a specific position of a composite heterostructure device of tungsten disulfide and nanometal structure in Example 1 of the present invention is shown.

[0028] Figure 8 The two resonance mode distribution diagrams of the circular nanometal structure used in Example 2 of the present invention are shown.

[0029] Fig. 9 It shows that the electron beam focused on a specific position of the composite heterostructure device of tungsten disulfide and nanometal structure in Example 2 of the present invention can produce an effect of separating the emission directions of the two valley excitons. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below through specific embodiments in conjunction with the accompanying drawings so that those skilled in the art can understand the present invention more clearly.

[0031] See also Figure 1The composite heterostructure device of the two-dimensional material and nanometal structure of the present invention is processed by dry transfer and electron beam etching. The designed nanometal structure is processed on the surface of a single layer of tungsten disulfide encapsulated by hexagonal boron nitride. From bottom to top, it is: bottom hexagonal boron nitride, single layer tungsten disulfide, upper hexagonal boron nitride, nanometal structure array, and the spacing between the nanometal structures in the array is greater than 1 micron. The entire composite heterostructure device is placed on a clean silicon wafer. The nanometal structure designed in this example can be a "two" shaped structure composed of long gold rods with a length, width and height of 210 nanometers, 70 nanometers, and 40 nanometers respectively, and short gold rods with a length, width and height of 65 nanometers, 25 nanometers, and 40 nanometers respectively; it can also be a disk structure with a diameter of 250nm, or other nanometal structure morphology that can meet the requirements. The following is a method for preparing an experimental sample for realizing the separation of the emission direction of the energy valley of two-dimensional materials by electron beam, comprising the following steps:

[0032] 1. Using plasma enhanced chemical vapor deposition (PECVD) to deposit SiO2 spacer layers on two Si substrates to obtain two SiO2 / Si substrates;

[0033] 2. Use organic solvents to ultrasonically clean the two SiO2 / Si substrates in the order of acetone (cleaning time 10-15 min) → ethanol (cleaning time 10-20 min) → deionized water (cleaning time 20-30 min). Finally, use a nitrogen gun to blow dry the deionized water remaining on the substrate to obtain a clean SiO2 / Si substrate.

[0034] 3. Use PDMS (polymethyl methacrylate) tape to tear off multiple pieces of hexagonal boron nitride flakes and single-layer tungsten disulfide from the block hexagonal boron nitride sample and tungsten disulfide sample, respectively, and place them on the same clean SiO2 / Si substrate;

[0035] 4. Use optical microscopy and atomic force microscopy to characterize, select a hexagonal boron nitride sheet with a thickness of about 100 nanometers as the bottom layer, and transfer the monolayer of tungsten disulfide to a suitable hexagonal boron nitride sheet;

[0036] 5. Transfer the thinner hexagonal boron nitride sheet to another clean SiO2 / Si substrate;

[0037] 6. Use an optical microscope to determine the relative position of the hexagonal boron nitride sheet transferred in step 5 on the second SiO2 / Si substrate, and spin-coat a layer of electron beam photoresist (MicroChem PMMA A2) with a thickness of about 100 nanometers on it at a coating speed of 3000 rpm. After coating, bake at 180°C for 5 minutes;

[0038] 7. Etch the designed metal nanostructure shape using an electron beam exposure (EBL) system, with the etched area located on the hexagonal boron nitride sheet described in step 6, and place it in a developer (MIBK) for development (about 50 seconds), and immediately place it in an isopropanol solution for fixing (about 5 minutes) after development, then take out the sample and use a nitrogen gun to blow dry the remaining isopropanol solution;

[0039] 8. Using electron beam evaporation to deposit titanium with a thickness of 2 nm and gold with a thickness of 40 nm on the sample treated in step 7, wherein titanium mainly plays a bonding role to ensure that the gold structure is firmly fixed on the surface of the hexagonal boron nitride; placing the entire sample in an acetone solution for about 5 hours, and finally peeling off the nano-metal structure located on the hexagonal boron nitride sheet by rinsing with the acetone solution;

[0040] 9. The hexagonal boron nitride + nanometal structure obtained in step 8 is simultaneously transferred to the hexagonal boron nitride + monolayer tungsten disulfide on the first SiO2 / Si substrate to complete the packaging. The final sample morphology is shown in Figure 1 .

[0041] In addition, the specific method of stripping and transferring may vary according to the actual situation. Here, the specific steps are described according to the actual situation:

[0042] 1. At room temperature, use USI adhesive plastic film P / N: 1005R to stick on tungsten disulfide and boron nitride blocks, then tear off the tape film and stick it on PDMS, press the tape film gently with a cotton swab, then tear off the tape film, and find the single-layer tungsten disulfide and hexagonal boron nitride flakes on PDMS according to the color of the flakes under an optical microscope. Generally speaking, the peeling of a single layer of tungsten diselenide requires multiple repetitions to achieve.

[0043] 2. Under an optical microscope, align the single-layer tungsten disulfide or hexagonal boron nitride sheet on the PDMS with the structural position that needs to be transferred, adjust the position of the PDMS so that the PDMS is in the critical condition of contact with the substrate, heat it to 45°C, use the thermal expansion of the PDMS to fit the substrate, and then adjust the PDMS away from the substrate.

[0044] The present invention uses a focused electron beam to excite a composite heterostructure device of a two-dimensional material and a nanometal structure to achieve separation of the radiated luminescence directions of different energy valley excitons. To achieve separation of luminescence directions, the geometric morphology and specific parameters of the nanometal structure need to be carefully designed. By combining numerical simulation with scanning parameters, the morphological characteristics of the nanometal structure are continuously adjusted so that it has two localized surface plasmon resonance modes with energies close to the luminescence wavelength of the valley excitons of the two-dimensional material but with different symmetries. The simplest combination is Figure 3The dipole mode and quadrupole mode are shown. The charge distribution of the dipole mode is antisymmetric with respect to the symmetry axis of the structure, while the charge distribution of the quadrupole mode is symmetric with respect to the symmetry axis of the structure. And under the designed geometric parameters, the eigenfrequencies of these two modes are close to the center frequency of the luminescence of the tungsten disulfide valley exciton, which can achieve effective coupling with the monolayer tungsten disulfide. In order to achieve the effect of far-field separation, it is also required that the two resonant modes are in phase and have similar amplitudes after being locally excited. It is worth noting that the effect of local excitation has obvious position dependence. Therefore, only when the local excitation is in some specific positions, such as the center of the short rod, can the requirements of being in phase and having similar amplitudes be met. As Figure 4 As shown, only when the local excitation is located at the center of the short rod, the x-direction dipole mode (Px) and the quadrupole mode (Qxx) are close in intensity in the 600-650nm band, and the phase difference is close to 0, indicating that the phases are in phase and can meet the requirements. When the local excitation position is located at the end of the long rod, the x-direction dipole mode (Px) and the quadrupole mode (Qxx) have a large difference in intensity in the 600-650nm band, which cannot meet the requirements. When the requirements are met, it is possible to make these two modes interact with the circularly polarized dipoles of different valley excitons at the same time. Due to the interference between the modes, spin-orbit coupling occurs, resulting in the separate guidance of the emission directions of different valley excitons, and ultimately causing photons of different spins to propagate in different directions, that is, a one-to-one correspondence between valley-photon spin-propagation direction. After determining the nanometal structure morphology that can meet the requirements through careful numerical simulation, samples that may produce the expected effect can be prepared, and it can also be determined how to control the switch of the separation effect by controlling the position of the electron beam.

[0045] Based on the above principles and designs, after the sample is prepared, it is necessary to use an angle-resolved fluorescence detection system to characterize the luminescence quality and luminescence angle of the sample. The following is a process for measuring the radiation characteristics of the composite heterostructure device at room temperature using an electron beam. The optical path diagram can be found in Figure 2:The characterization of radiation characteristics of the present invention is carried out in a cathode fluorescence microscopic imaging system based on a scanning electron microscope. The electron beam excites the sample after passing through the small hole of the parabolic mirror above the sample. The cathode fluorescence radiated is collected by the parabolic mirror above and converted into parallel light, which is collected and analyzed through the optical path. The collection and analysis optical path can be switched between two modes: Mode 1 is a polarization angle resolution optical path. The collected cathode fluorescence is collected and imaged by a CMOS camera after passing through a polarization measurement component to obtain a two-dimensional intensity distribution diagram of left-handed or right-handed light at a specific wavelength. Then, by analyzing the correspondence between each pixel point in the intensity diagram and the stereo angle of the sample's luminous direction, the transmission angle distribution diagram of photons radiated by the sample with a spin of +1 or -1 can be obtained, that is, the luminous intensity as a function of the stereo angles θ and φ; Mode 2 is a spectral measurement optical path. The collected cathode fluorescence is collected by a photomultiplier tube after passing through a spectrometer to obtain luminous intensity information at different wavelengths. A polarization analysis component composed of a quarter wave plate and a linear polarizer can also be added between the photomultiplier tube and the spectrometer to obtain spectral information of a certain polarization component.

[0046] Example 1

[0047] The morphology of a composite heterostructure device of a single-layer tungsten disulfide and nano-metal structure prepared according to the above steps is as follows Figure 1 As shown, the nanometal structure is a "two" shaped structure composed of long gold rods with length, width and height of 210 nanometers, 70 nanometers and 40 nanometers respectively, and short gold rods with length, width and height of 65 nanometers, 25 nanometers and 40 nanometers respectively. In order to ensure that there is no near-field interaction between adjacent samples, the period of the nanometal structure array is 1 micron, which is much larger than the spatial scale of near-field interaction. The radiation characteristics of the composite heterostructure device samples were characterized in a cathode fluorescence detection imaging system (Delmic SPARC) based on a scanning electron microscope (ThermoFisherQuattro C). After the sample was sent into the vacuum chamber, a focused electron beam with an acceleration voltage of 5 kilovolts, a beam current of 2.0 nanoamperes and a beam spot size of 3 nanometers was used at room temperature to locally excite the sample at a selected position. Mode 1 was used to measure the angle and intensity information of the left-handed circularly polarized light and the right-handed circularly polarized light emitted by the sample under electron beam excitation. When the electron beam excitation position is the center of the short rod of the "two" shaped structure, the emission directions of the K valley and K' valley excitons are guided separately, and the generated photons with different spins propagate in opposite directions. When the electron beam excitation position is the end point of the long rod of the "two" shaped structure, the emission directions of the K valley and K' valley excitons are basically the same, and there is no obvious difference in the propagation directions of the photons with different spins generated.

[0048] In order to clearly show the propagation direction of the two types of photons, we plotted the difference in intensity between right-handed circularly polarized light and left-handed circularly polarized light as a function of the solid angle of emission based on the experimental measurement results, as shown in Figure 6 As shown. The excitation position of the electron beam is represented by a white dot. When the white dot is located at the center of the short rod, the right-handed circularly polarized light is mainly located in the left hemisphere, and the left-handed circularly polarized light is located in the right hemisphere, which means that the "on" state of emission direction separation is achieved. When the white dot is located at the end of the long rod, the right-handed and left-handed circularly polarized light are evenly distributed over the entire spherical surface without obvious directionality, which means that the "off" state of emission direction separation is achieved. In addition, the results of numerical simulation ( Figure 7 ) is in good agreement with the experimental results. The above results prove that the separation and active control of the emission direction of valley excitons in two-dimensional materials can be achieved by moving the electron beam in the nanometer scale.

[0049] Example 2

[0050] A composite heterostructure device of a single-layer tungsten disulfide and nano-metal structure is prepared according to the above steps, and its morphology is as follows Figure 1 As shown in Figure 1, the nanometal structure is a gold disk with a radius of 250 nanometers and a height of 40 nanometers. The energy of the gold disk is close to the wavelength of tungsten disulfide emission. The distribution diagram of the two modes with different symmetries is shown in Figure 1. Figure 8 As shown. In order to ensure that there is no interaction between adjacent samples, the minimum distance between nanometal structures is greater than one micron, which is much larger than the scale of near-field interaction. The localized surface plasmon resonance modes of the gold disk structure are hexapole mode and octupole mode. The radiation characteristics of the composite heterostructure device samples were characterized in a cathode fluorescence detection imaging system (Delmic SPARC) based on a scanning electron microscope (ThermoFisher Quattro C). After the sample was sent into the vacuum chamber, the sample was locally excited at a selected position at room temperature using a focused electron beam with an acceleration voltage of 5 kilovolts, a beam current of 2.0 nanoamperes, and a beam spot size of 3 nanometers in diameter. Mode 1 was used to measure the angle and intensity information of left-handed circularly polarized light and right-handed circularly polarized light emitted by the sample under electron beam excitation, and the difference in the intensity of right-handed circularly polarized light and left-handed circularly polarized light as a function of the luminous solid angle was plotted, as shown in Fig. 9 shown. Fig. 9In the electron microscope image, the white dots represent the electron beam excitation positions. When the electron beam excitation position is at the lower edge of the disk, the emission directions of the K valley and K' valley excitons are guided separately, and the generated photons with different spins propagate in different directions: the left side of the dotted line in the figure is mainly left-handed circularly polarized light, and the right side of the dotted line is right-handed circularly polarized light, realizing the "on" state with separated emission directions. When the electron beam excitation position is the center of the gold disk, the emission directions of the K valley and K' valley excitons are basically the same, and there is no obvious difference in the emission angles of the two circularly polarized lights, that is, the "off" state with separated emission directions is realized. In addition, due to the rotational symmetry of the gold disk structure, when the electron beam position is located to the left or right of the lower edge, it can be seen that the "on" state with separated emission directions still appears, and the separated symmetry axis (as shown by the dotted line in the figure) also undergoes corresponding deflection. The above results prove that the separation and active control of the emission direction of valley excitons in two-dimensional materials can be achieved by using the movement of the electron beam in the nanoscale.

[0051] It should be noted that the material used to make the nanometal structure in both examples is gold, because the radiation center wavelength of the valley exciton of a monolayer of tungsten disulfide is about 610nm. The plasmon resonance frequency of the metal nanostructure made of gold can be easily adjusted to near this frequency by changing the shape and size. However, in specific implementations, other metals, such as aluminum, silver, copper, etc., can also be selected to prepare suitable nanometal structures according to the different wavelengths of the luminescence of the two-dimensional material to be regulated, so that their localized surface plasmon resonance mode energy matches the luminescence frequency of the selected two-dimensional material.

[0052] The present invention uses electron beam excitation and nano-metal structures to separate the luminescence direction of valley excitons of two-dimensional materials for the first time at the nanoscale, completes the transmission of valley information inside the material to far-field light information, and can actively control the progress and interruption of this transmission, solving the problem that valley information is difficult to directly utilize, reducing the utilization requirements of valley information of electrons in a single-layer two-dimensional material, and at the same time greatly compressing the geometric size of the device and improving the integration. In addition, the photons generated but with different spin angular momentum may also be further used in quantum communication networks based on photon spin encoding. The present invention builds a bridge connecting far-field optical signals and valley information inside the material at the nanoscale, realizes an ultra-small valley-light interface, and can be used in platforms such as valley optoelectronic devices, nano optoelectronic chips, and quantum communication networks. Due to the flexibility of metal nanostructure design, this method can be extended to the regulation of a variety of radiation radiation properties of a wider range of fluorescent materials, which has extremely high guiding significance and reference significance for future research in nanophotonics.

[0053] Finally, it should be noted that the purpose of publishing the embodiments is to help further understand the present invention. Those skilled in the art should understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the contents disclosed in the embodiments, and the scope of protection claimed by the present invention shall be subject to the scope defined in the claims.

Claims

1. A method for separating the emission directions of valleys of two-dimensional materials using electron beams. First, a composite heterostructure device is prepared in which a nanometal structure is located on a two-dimensional material. The nanometal structure has two localized surface plasmon resonance modes with energies close to the emission wavelengths of valley excitons of the two-dimensional material but with different symmetries. After being locally excited, the two resonance modes are in phase and have similar amplitudes. A focused electron beam is used for local excitation at a specific position of the nanometal structure, so that valley excitons of two different energy valleys, K valley and K' valley, inside the two-dimensional material below the nanometal structure are coupled to the two resonance modes of the nanometal structure respectively, and the radiation directions are guided respectively. Under the constraint of spin-orbit coupling, photons carrying valley information and having different spins propagate in different directions.

2. The method according to claim 1, characterized in that The two-dimensional material is a single-layer two-dimensional transition metal chalcogenide.

3. The method according to claim 2, characterized in that The two-dimensional transition metal chalcogenide compound is selected from one of tungsten disulfide, molybdenum disulfide, tungsten diselenide, molybdenum diselenide, molybdenum ditelluride and tungsten ditelluride.

4. The method according to claim 1, characterized in that The two-dimensional material is encapsulated between two hexagonal boron nitride sheets, and the nano-metal structure is located on the upper hexagonal boron nitride sheet.

5. The method according to claim 4, characterized in that The thickness of the bottom hexagonal boron nitride flake is 100~200 nanometers, and the thickness of the upper hexagonal boron nitride flake is 10~30 nanometers.

6. The method according to claim 1, characterized in that The material of the nano-metal structure is selected from one or more of gold, aluminum, silver and copper.

7. The method according to claim 1, characterized in that The morphological characteristics of the nanometal structure are continuously adjusted through numerical simulation combined with scanning parameters, so that there are two localized surface plasmon resonance modes with energy close to the emission wavelength of the valley exciton of the two-dimensional material but different symmetry.

8. The method according to claim 1, characterized in that The two-dimensional material is a single layer of tungsten disulfide, and the nanometal structure is a "two" shaped structure consisting of a short gold rod and a long gold rod. The focused electron beam performs local excitation at the center of the short gold rod.

9. The method according to claim 1, characterized in that The two-dimensional material is a single layer of tungsten disulfide, the nano-metal structure is a gold disk with a diameter of 450-550 nanometers and a thickness of 35-45 nanometers, and the focused electron beam is locally excited at the edge of the disk.

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