A method and control of the optical spin Hall effect using electron beam excitation

Abstract

The present invention discloses a method and control method of using electron beam to excite the optical spin Hall effect. Plasmon resonance is generated by bombarding metal nanostructures with electron beams, so that the radiation directions of left-handed circularly polarized components are separated to excite the optical spin Hall effect. The presence or absence of the optical spin Hall effect and the switching of the radiation directions of left-handed circularly polarized light can be realized by moving the electron beam to change the excitation area, thereby realizing the control of the optical spin angular momentum. The present invention uses electron beam incidence to generate the optical spin Hall effect for the first time, breaks through the optical diffraction limit through the ultra-high spatial resolution of the electron beam, realizes the detection of the optical spin Hall effect at the sub-wavelength scale, greatly reduces the control scale of the photon spin angular momentum degree of freedom, has the characteristics of small scale, high sensitivity, strong robustness, etc., and can be applied to the fields of optical spin and orbit coupling research, nanophotonics information carrier research, and quantum information device integration.

Description

Technical Field

[0001] The present invention relates to the excitation of the photo-spin Hall effect, specifically a method of using electron beams to bombard metals to generate plasmons and then stimulate the photo-spin Hall effect to modulate the photon spin angular momentum. The method can analyze the excitation of the photo-spin Hall effect and the modulation of the photo-spin angular momentum, and provide guidance for the application of quantum information devices. Background Art

[0002] The spin Hall effect (SHE) was originally used to describe the phenomenon of spin-electron separation in materials. Its discovery provides a unique method for transmitting and storing information through the electron spin degree of freedom, and opens up the research field of spin electronics. Since the two different circular polarization states of light correspond to two spins (σ = ± 1), this spin-related effect can be further extended to the field of optics and is called the optical spin Hall effect (OSHE). Using the optical spin Hall effect, light with different spins can be separated in space, which is of great significance for the transmission and processing of optical information using the spin degree of freedom of photons.

[0003] The optical spin Hall effect provides a method to manipulate the spin angular momentum of photons. Theoretical research on spin-orbit coupling related to photon spin separation, as well as application research in metasurface materials, planar microcavities, precision measurement, etc., have attracted the attention of researchers. Due to the great value of the optical spin Hall effect in theoretical research and practical applications, in recent years, more and more researchers have been committed to realizing and studying this optical effect in various optical structures, such as dielectric cylinders, metal nanoparticles, plasmonic waveguides, and metasurface materials. Since the spin angular momentum of photons can be used as a robust and large-capacity information carrier, the optical spin Hall effect is predicted to be able to manipulate information carriers in quantum information. Its orthogonality and high-dimensionality have broad application prospects in information coding and information cryptography. The research related to the manipulation of the optical spin Hall effect mainly focuses on the research related to the Berry Phase, and the spatial separation of different spin components and the separation of radiation directions are achieved by designing the structure of the metasurface. In the future application of the optical spin Hall effect in quantum information technology, it is necessary to realize the modulation of photon spin angular momentum at the nanoscale, which will help the integration of quantum devices and the construction of quantum networks.

[0004] On the other hand, with the development of integrated photonics, many optical structures have reached subwavelength dimensions, especially plasmon structures. Therefore, achieving high spatial resolution excitation and detection at subwavelength or even deep subwavelength scales is becoming increasingly important for studying the optical properties of micro-nano structures. Cathodoluminescence (CL) microscopy uses a highly focused electron beam to excite micro-nano structures and collect the cathode fluorescence signals of the samples. It can not only achieve extremely high spatial resolution (~10nm) cathode fluorescence imaging, but also study the rich optical properties of nanostructures through dynamic local excitation, and realize the detection and characterization of local electromagnetic field modes. Therefore, it plays an important role in the study of micro-nano photonics. Further, under electron beam excitation, the subwavelength-scale electron beam excitation site movement directly modulates the local photon state density, realizing the manipulation of far-field cathode fluorescence signals. The angular-resolved cathodoluminescence microscopy technique is used to characterize the radiation direction of photons. Combined with the optical polarization detection module, the polarization information in the cathodoluminescence signal and the photon spin angular momentum can be characterized, and polarization-related optical phenomena can be detected with ultra-fine resolution at the nanoscale, and related application scenarios can be explored. In the study of plasmon nanostructures, cathodoluminescence microscopy has revealed many optical properties and phenomena that cannot be studied by conventional optical detection methods, such as hidden optical chirality in non-chiral structures and directional optical radiation in disc-shaped nanoparticles. Summary of the invention

[0005] The object of the present invention is to provide an excitation method for realizing the optical spin Hall effect by using an electron beam, so that the electron beam excitation without spin injection can modulate the photon spin angular momentum through the plasmon resonance supported by the metal nanostructure.

[0006] The technical solution of the present invention is as follows:

[0007] A method for exciting the optical spin Hall effect (see Figure 1 ) is used for modulation of photon spin angular momentum without spin injection. The method includes three aspects: metal nanostructure processing and preparation, electron beam bombardment of metal nanostructure to stimulate photo-spin Hall effect, and nanoscale manipulation of photon spin angular momentum.

[0008] The metal nanostructure is rectangular and is usually prepared on an insulating substrate (such as a SiO2 / Si substrate). The steps of processing and preparing the metal nanostructure are: 1. Preparing a nano-pattern on the substrate by electron beam etching; 2. Processing a rectangular metal nanostructure by electron beam evaporation.

[0009] The metal nanostructure is preferably a rectangle with a length of 150 to 250 nm and a width of 50 to 100 nm, and a thickness of 30 to 60 nm. The material of the metal nanostructure is preferably a metal that generates plasmon resonance, such as gold, silver and aluminum.

[0010] The optical spin Hall effect is generated by the electron beam bombarding the specific position of the metal nanostructure. Electron beam bombardment of the metal nanostructure produces plasmon resonance. Due to the small excitation area, precise excitation at the center of the long edge of the rectangle can form a circularly polarized electromagnetic mode formed by the superposition of the dipole electromagnetic mode and the quadrupole electromagnetic mode. The radiation electric field directions of the dipole electromagnetic mode and the quadrupole electromagnetic mode to the left and right are perpendicular to each other, and there is a 90° phase difference between the two electric field components. After the electric field is superimposed, the light field propagating to both sides is circularly polarized light with opposite spins, which stimulates the optical spin Hall effect (see Figure 2 ).

[0011] The manipulation of the optical spin angular momentum of metal nanostructures is produced by the nanoscale movement of the electron beam excitation position. Electron beam bombardment of the center of the long edge of the rectangle can produce mutually perpendicular circularly polarized dipole and circularly polarized quadrupole electromagnetic modes. Changing the electron beam excitation position can achieve phase switching of the circularly polarized dipole electromagnetic mode, thereby causing the radiation direction of left-handed and right-handed light to be reversed, thereby achieving photon spin angular momentum manipulation. When the electron beam bombards the center of the rectangle, no separation of the radiation directions of left-handed and right-handed light is observed. Therefore, moving the electron beam to change the excitation area can realize the presence or absence of the optical spin Hall effect and the switching of the radiation direction of left-handed and right-handed light, thereby achieving the manipulation of the optical spin angular momentum (see Figure 2 ).

[0012] The electron beam excited optical spin Hall effect proposed in the present invention is the first to use electron beam excited plasmons to achieve the regulation of photon spin angular momentum. In the absence of spin injection, the spatial separation of the left and right components of the light beam is achieved through the circular polarization electromagnetic mode of the metal nanostructure, and the detection of the optical spin Hall effect at the subwavelength scale is realized, which greatly reduces the control scale of the photon spin angular momentum degree of freedom. The excitation position is accurately changed by the nanoscale movement of the electron beam, and then the electromagnetic mode of the metal nanostructure is changed to achieve the subwavelength scale control of the photon spin angular momentum. The present invention focuses on the research of a new generation of information carriers in nanophotonics, and the control of the spin freedom degree is transformed from traditional far-field laser excitation to near-field interaction excitation using metal plasmons, breaking through the optical diffraction limit. This method can be applied to the study of optical spin and orbital coupling and the integration of quantum information devices, and has the characteristics of small scale, high sensitivity, and high robustness. Under the premise of the widespread application of far-field optical methods to excite the optical spin Hall effect at a large scale, the electron beam excited plasmon room temperature induced optical spin Hall effect and photon spin angular momentum regulation of the present invention will have broad market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 A schematic diagram of sample structure and phenomenon of a specific embodiment of the present invention is shown.

[0014] Figure 2 It is a schematic diagram of the principle of the present invention of exciting the optical spin Hall effect by bombarding a metal nanostructure with an electron beam.

[0015] Figure 3 The hotspot modes of plasmon resonances of different spins of the metal nanostructure under 30 keV electron beam excitation in a specific embodiment of the present invention are shown.

[0016] Figure 4 The left-handed circularly polarized and right-handed circularly polarized cathode fluorescence spectral components and cathode fluorescence chirality intensities of the metal nanostructure in a specific embodiment of the present invention under 30 keV electron beam excitation are shown.

[0017] Figure 5 The numerical simulation of the metal nanostructure in a specific embodiment of the present invention shows the left-right hand signal radiation direction pattern under different electron beam excitation sites at the central wavelength.

[0018] Figure 6 The left-handed circularly polarized cathode fluorescence scattering intensity of the metal nanostructure under 30keV electron beam excitation in a specific embodiment of the present invention is shown, wherein "upper", "middle" and "lower" represent that the electron beam bombardment site is located at the midpoint of the upper edge, the center of the rectangle and the midpoint of the lower edge of the rectangular metal nanostructure, respectively.

[0019] Figure 7 It shows the ratio of the left-handed and right-handed circularly polarized cathode fluorescence scattering intensities of the metal nanostructure under 30 keV electron beam excitation in a specific embodiment of the present invention, wherein "upper", "middle" and "lower" represent that the electron beam bombardment site is located at the midpoint of the upper edge, the center of the rectangle and the midpoint of the lower edge of the rectangular metal nanostructure, respectively.

[0020] Figure 8 The radiation direction pattern of left-right circularly polarized cathode fluorescence at the central wavelength under light excitation of numerical simulation of the metal nanostructure in a specific embodiment of the present invention is shown.

[0021] Fig. 9 The directional patterns of left- and right-handed circularly polarized cathode fluorescence radiation under electron beam excitation of numerically simulated metal nanostructures of different sizes in a specific embodiment of the present invention are shown.

[0022] Fig.10 The numerical simulation of the metal nanostructure in a specific embodiment of the present invention shows the left-right circularly polarized cathode fluorescence radiation direction pattern at different electron beam excitation sites at a wavelength deviating from the center.

[0023] Fig.11The invention shows the application of the electron beam excited optical spin Hall effect in spin coding. DETAILED DESCRIPTION

[0024] 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.

[0025] The electron beam stimulated optical spin Hall effect is achieved by bombarding the metal nanostructure with electron beam to generate plasmon resonance. The experimental sample structure in this embodiment is as follows Figure 1 As shown: it includes a Si substrate 1, a SiO2 spacer layer 2 and a metal nanostructure 3, wherein the SiO2 spacer layer 2 is located on the Si substrate 1, and the metal nanostructure 3 is located on the SiO2 spacer layer 2. In the present invention, the excitation of metal plasmon resonance by electron beam is based on the reciprocity principle of electron beam excitation and circularly polarized light irradiation: under the excitation of x and y polarized light, the rectangular metal nanostructure can support the basic electromagnetic mode oscillating along the long side. When switched to circularly polarized light incidence, this electromagnetic mode can also be effectively excited. The left (right) circularly polarized light incidence introduces a 90° (-90°) phase difference, and the final phase difference between the electromagnetic modes is 180° (0°). The dipole moment is along the diagonal direction of the rectangle, and plasmon hotspots are generated at the upper left (upper right) and lower right (lower left) corners, forming a circularly polarized dipole electromagnetic mode. As shown Figure 3 As shown in the figure, when the electron beam bombards the metal nanostructure position corresponding to the plasmon hotspot, the metal nanostructure can generate a circularly polarized dipole electromagnetic mode similar to that when circularly polarized light is incident, and radiate circularly polarized outgoing light corresponding to the incident circular polarization state. When the excitation site is located at the center of the long edge of the metal nanostructure or the center of the metal nanostructure, the electron beam excitation of the circularly polarized plasmon hotspot is avoided, and the intensities of the left-handed component and the right-handed component of the outgoing light are equal.

[0026] In order to find the appropriate wavelength to realize the optical spin Hall effect, the optical circular dichroism signal ( Figure 4 ), the calculation formula of optical circular dichroism far-field signal is as follows:

[0027] CD=(CL LCP -CL RCP ) / (CL LCP +CL RCP )

[0028] In the above formula, CD represents the circular dichroism spectrum, CL LCP Represents the left-handed cathodoluminescence emitted by metal nanostructures, CL RCP Represents the right-handed cathodoluminescence emitted by a metal nanostructure.

[0029] The following is a method for preparing an experimental sample for electron beam excitation optical spin Hall effect, which includes the following steps:

[0030] Step 1: Use plasma enhanced chemical vapor deposition (PECVD) to deposit a SiO2 spacer layer 2 on a Si substrate 1 to obtain a SiO2 / Si substrate.

[0031] Step 2: Use an organic solvent to ultrasonically clean the SiO2 / Si substrate 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.

[0032] Step 3: Spin-coat PMMA A2 glue (3000rad / s, 51s) on the upper surface of the SiO2 / Si substrate obtained in the previous step, and dry it at 180°C for 5min. Then use the electron beam exposure (EBL) system to etch out the designed structural shape, and put it into the developer (MIBK) for development (about 50s). After development, immediately put it into the isopropanol solution for fixing (about 5min), then take out the sample and blow dry the residual isopropanol solution with a nitrogen gun. Then, use electron beam evaporation to evaporate the selected gold target. Finally, put the entire sample into an acetone solution for about 5 hours, and finally peel it off to obtain the metal nanostructure 3 by rinsing with the acetone solution.

[0033] The measurement process of the electron beam optical spin Hall effect excitation is given below: In the present invention, the electron beam optical spin Hall effect excitation is carried out in a cathode fluorescence microscopy imaging system based on a scanning electron microscope. The electron beam passes through the small hole of the parabolic mirror above the sample to excite the sample. The radiated cathode fluorescence is collected by the parabolic mirror above the sample and finally captured by a complementary metal oxide semiconductor element (Complementary Metal Oxide Semiconductor, referred to as CMOS) through the collection light path. The circular polarization component of the cathode fluorescence of the sample is extracted by a quarter wave plate and a linear polarizer placed in the collection light path. The long axis of the quarter wave plate is ±45° with the polarization direction of the linear polarizer, and the left-handed and right-handed circular polarization components can be extracted respectively. In cathode fluorescence detection, the cathode fluorescence spectrum of the sample is collected in Pan mode, and in Mono mode, the information of the optical angle-resolved radiation direction is detected by placing a filter in the collection light path, and the cathode fluorescence image of the selected wavelength range of the sample is detected.

[0034] Example 1

[0035] According to the above steps, a three-layer metal nanostructure sample was prepared. Figure 1As shown, from bottom to top, Si substrate 1, 100nm thick SiO2 spacer 2, 200nm long, 80nm wide, 30nm high rectangular metal nanostructure 3. In the numerical simulation, the plasmon resonance peak generated by the electron beam excitation of the rectangular metal nanostructure is 630nm, which is consistent with the cathode fluorescence spectrum obtained by experimental detection. The experimental measurement was carried out in a cathode fluorescence microscopy imaging system (Sparc) in a scanning electron microscope (FEI Quattro C). A quarter wave plate (AQWP10M-600, THORLABS) and a linear polarizer (LPVIS100, THORLABS) were used to detect the sample cathode fluorescence signal in the wavelength range of 500nm to 800nm ​​through Pan mode. In Mono mode, a notch filter with a central wavelength of 650nm was placed in the optical path to collect the angular resolved radiation direction information and cathode fluorescence image of the sample at a wavelength of 650nm.

[0036] The sample was placed in a vacuum chamber, and the metal nanostructure was bombarded with a 30 keV electron beam at room temperature, and the spectral characteristics of plasmon resonance at different chirality were collected (see Figure 4 ), the fluorescence signal peaks in the three polarization states of left-handed circular polarization, right-handed circular polarization, and full polarization are all located at 630nm, and the plasmon resonance peak of the metal nanostructure matches the optical circular dichroism peak.

[0037] A 30 keV electron beam was used to bombard the upper edge midpoint, lower edge midpoint, and center of the rectangular metal nanostructure, respectively, and the radiation direction information of the left-handed circularly polarized and right-handed circularly polarized cathode fluorescence signals generated by the three positions were collected (see Figure 5 ), the collection wavelength range is located near the peak of the fluorescence signal at around 630nm. It can be seen that when the electron beam excites the midpoint of the upper edge and the midpoint of the lower edge of the long side of the metal structure, the radiation directions of the left-handed circular polarization component and the right-handed circular polarization component are inconsistent, respectively. The radiation direction of the left-handed circular polarization component is located in the left hemisphere (see Figure 5 ), the radiation direction of the right circular polarization component is located in the right hemisphere, and the radiation direction of the left circular polarization component is located in the right hemisphere, and the radiation direction of the right circular polarization component is located in the left hemisphere, that is, the radiation directions of the left and right circular polarization components are separated by stimulating these two positions, and neither the left nor right circular polarization hotspots are excited, and the left-right intensity ratio is close to 1 (see Figure 7), which means that the intensities of the left and right circular polarization components are basically equal, and it can be considered that an effective optical spin Hall effect phenomenon has occurred. When the electron beam bombards the center of the rectangle, there is no obvious separation in the radiation directions of the left and right circular polarization components, and it can be considered that no effective optical spin Hall effect phenomenon has occurred. Considering that the electron beam spot is about 10nm×10nm in size, the optical spin Hall effect phenomenon can be turned from presence to absence by moving the electron beam from the midpoint of the upper edge of the metal structure to the center of the rectangle by about 40nm, and the radiation direction of the left and right circular polarization components can be reversed by moving the electron beam from the midpoint of the upper edge of the metal structure to the midpoint of the lower edge of the metal structure by about 80nm.

[0038] To further verify the specificity of the electron beam in realizing the optical spin Hall effect, we used white light to excite the same metal nanostructure and collected the radiation direction information of the left-handed circular polarization component and the full polarization. Figure 8 The results of angle-resolved detection are shown. The radiation intensity in all directions is basically the same, and no optical spin Hall effect phenomenon is shown. It can be judged that for a single rectangular metal nanostructure, optical excitation cannot control the photon spin angular momentum. In order to further study the influence of the size of metal nanostructures on the realization of the optical spin Hall effect by electron beams, we collected the radiation direction information of the left and right circularly polarized components excited by electron beams of rectangular metal nanostructures of different sizes (see Fig. 9 ), the optical spin Hall effect is significant at larger nanostructure sizes. To further study the influence of the detection band on the optical spin Hall effect of electron beams, we collected the radiation direction information of the left and right circularly polarized components excited by the metal nanostructure electron beam at different central wavelengths (see Fig.10 ), the photo-spin Hall effect phenomenon weakens away from the peak of the fluorescence signal.

[0039] The electron beam stimulated photo-spin Hall effect provides a new way to control the spin angular momentum of photons at the nanoscale, and provides photon spin as a new information carrier, which has broad application prospects in the fields of optical information and quantum information. Fig.11The application mode of the present invention in spin coding is given. The far-field angular resolution mode in which the radiation direction of the left-handed circular polarization component is located in the left hemisphere and the radiation direction of the right-handed circular polarization component is located in the right hemisphere is defined as "0"; the far-field angular resolution mode in which the radiation direction of the left-handed circular polarization component is located in the right hemisphere and the radiation direction of the right-handed circular polarization component is located in the left hemisphere is defined as "1"; the far-field angular resolution mode in which the radiation directions of the left and right circular polarization components are not separated is defined as an erasure code, so as to improve the accuracy of the encoded information. The three modes of "0", "1" and erasure code correspond to the electron beam excitation sites located at the upper edge midpoint, the lower edge midpoint and the center of the rectangle of the long side of the metal nanostructure respectively. The entire encoding process is integrated in a sub-wavelength scale metal structure nanounit. By designing the electron beam scanning path, the output of different encoded information is realized. Photon spin coding provides a coding method different from level intensity, and the new information carrier increases the information carrying capacity of the optical signal. The electron beam excitation optical spin Hall effect provides a way to manipulate the photon spin angular momentum at a sub-wavelength scale, thereby improving the integration in the application of information devices.

[0040] The present invention uses electron beam incident for the first time to produce the optical spin Hall effect, breaks through the optical diffraction limit through the ultra-high spatial resolution of the electron beam, accurately excites the metal nanostructure to produce a circularly polarized electromagnetic mode, and realizes the separation of the left-handed and right-handed components of the cathode fluorescence signal in the radiation direction. Through the plasmon resonance of the metal nanostructure, the electron beam that does not carry a single spin state can produce a consistent optical spin Hall effect phenomenon with the circularly polarized light incident with a single spin state. At the same time, it is different from the traditional far-field optical excitation method for studying the manipulation of photon angular momentum. With the advantage of the electron beam nanoscale spot, the circularly polarized electromagnetic mode of the metal nanostructure is excited to produce the optical spin Hall effect phenomenon at the sub-wavelength scale, and the radiation direction of the left-handed and right-handed components can be effectively controlled within the range of a hundred nanometer electron beam movement. The present invention is a novel method for excitation of optical spin Hall effect and manipulation of photon spin angular momentum at nanoscale, with high sensitivity and strong robustness. In addition to metal rectangular nanostructures, it is also applicable to metal metasurfaces of other structures. With the rapid development of nanophotonic devices and quantum information devices today, the demand for new degrees of freedom for manipulating information carriers is becoming increasingly urgent. The present invention has extremely strong supporting power and reference value for their future development.

[0041] 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 manipulating the spin angular momentum of light, using an electron beam to bombard a rectangular metal nanostructure: when the electron beam bombards the midpoint of the long side edge of the rectangular metal nanostructure, the radiation directions of the left and right circular polarization components are separated, and circularly polarized light with opposite spin directions is propagated to both sides, and when the bombardment position is switched between the midpoints of the two long sides, the radiation directions of the generated left and right circular polarization light are reversed; when the electron beam bombards the center of the rectangular metal nanostructure, the radiation directions of the left and right circular polarization components are not separated; switching the bombardment position of the electron beam between the midpoint of the long side edge of the rectangular metal nanostructure and the center of the rectangular metal nanostructure realizes the on-off regulation of the light spin Hall effect, and switching the bombardment position of the electron beam between the midpoints of the two long sides of the rectangular metal nanostructure realizes the switching of the radiation directions of the left and right circular polarization light, thereby realizing the manipulation of the spin angular momentum of light.

2. The method for controlling the spin angular momentum of light according to claim 1, characterized in that: The metal nanostructure is a rectangle with a length of 150-250 nm, a width of 50-100 nm, and a thickness of 30-60 nm.

3. The method for controlling the spin angular momentum of light according to claim 1, characterized in that: The material of the metal nanostructure is gold, silver or aluminum.

4. The method for controlling the spin angular momentum of light according to claim 1, wherein: The metal nanostructure is prepared on an insulating substrate.

5. A method for using photon spin as an information carrier, performing photon spin encoding based on the method for manipulating light spin angular momentum as described in claim 1, defining the far-field angular resolution mode in which the radiation direction of the left-handed circular polarization component is in the left hemisphere and the radiation direction of the right-handed circular polarization component is in the right hemisphere as "0"; defining the far-field angular resolution mode in which the radiation direction of the left-handed circular polarization component is in the right hemisphere and the radiation direction of the right-handed circular polarization component is in the left hemisphere as "1"; defining the far-field angular resolution mode in which the radiation directions of the left and right circular polarization components are not separated as an erasure code; the three modes of "0", "1" and erasure code correspond to the electron beam bombardment site being located at the upper edge midpoint of the long side of the rectangular metal nanostructure, the lower edge midpoint and the center of the rectangle respectively; and by designing the electron beam scanning path, the output of different coded information is achieved.

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