A method for realizing a chiral nanophotonic source, a method for exciting chiral light, and a method for controlling chiral light signals

Abstract

The application provides a realization method of a chiral nanometer light source, a chiral light excitation method and a chiral light signal control method. The application utilizes electron beam incidence in a square nanometer optical trap structure to generate chiral light emission, obtains a chiral radiation signal through a fixed phase difference superposition relationship. Meanwhile, different from a far field optical excitation mode of a traditional research chiral light source, the electron beam nanometer scale beam spot advantage is used to excite different superposition states of Smith-Purcell radiation in the nanometer optical trap structure, and the chiral degree of the chiral nanometer light source is effectively controlled in a hundred nanometer electron beam moving range, which has strong support power and reference value for future development.

Description

Technical Field

[0001] This invention relates to chiral nanolight sources, specifically a method for generating chiral nanolight sources by exciting Smith-Purcell radiation modulation using an electron beam incident on a periodically stacked square optical trap structure. This method can analyze the excitation and modulation of chiral Smith-Purcell radiation nanolight sources and provide guidance for optical circuit applications and binary information processing. Background Technology

[0002] Smith-Purcell radiation (SPR) is a special type of electron beam diffraction radiation. It originates from the rapid generation of polarization charges and oscillations of electrons as they graze the surface of a grating structure. Because its radiation wavelength is strongly dependent on the periodicity of the grating structure and its intensity is closely related to the incident electron beam current, it is easier to design radiation sources with richer optical performance. Through appropriate grating structure design, it is hoped to realize nanoscale light sources that carry Smith-Purcell radiation. Initially used in the design of high-power terahertz sources, free-electron lasers, and charged particle beam shaping and detection, its optical performance has been extended to linearly polarized and vortex light to meet the needs of more diverse radiation sources. However, chirality, another important property of Smith-Purcell radiation sources, often requires the coherent superposition of two orthogonally polarized linearly polarized Smith-Purcell radiation beams, which is difficult to achieve in traditional single-grating structures. This presents a greater challenge to the design of novel structures that carry Smith-Purcell radiation.

[0003] Chirality is a widely existing physical property in nature, namely, the property that an object cannot be superimposed on its mirror image. Since the two different circular polarization states of light correspond to two spin states (left-handed circular polarization and right-handed circular polarization, σ = ±1), this spin-related effect can be further extended to the field of optics, namely optical chirality (OC), which has wide applications in information processing, information storage, and optical routing. To measure the chirality of a signal, the chirality degree CD is defined as (I... LCP -I RCP ) / (I LCP +I RCP ), where I LCP and I RCPThese represent the left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) photon spin components in radiation. Since the spin angular momentum of photons can serve as a robust and high-capacity information carrier, optical chirality is predicted to enable manipulation of information carriers in optical information processing. Its orthogonality and high dimensionality have broad application prospects in information encoding and cryptography. As the most important component in optical circuits, the light source determines the storage performance and information processing capabilities of the circuits. The high integration due to the small size of chiral nanolight sources and the high dimensionality due to photon spin are of great significance for the transmission and processing of optical information in optical circuits, providing a new approach for future binary information processing and transmission.

[0004] With the rapid development of optoelectronics and integrated photonics, many optical structures have reached the nanoscale to achieve better optical performance or optical information processing capabilities, especially metallic nanostructures, microcavity structures, and nanopore 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 nanostructures. Cathodoluminescence microscopy (CFMS) is a non-invasive detection method based on scanning electron microscopy or transmission electron microscopy. It utilizes an electron beam to excite and collect the cathodoluminescence signal of the sample, offering advantages such as nanoscale spatial resolution and precise excitation, achieving fluorescence imaging with a resolution of up to ~10 nm. It is widely used in studying electron-matter interactions, nanoscale luminescence detection, and nanoscale excitation manipulation. By manipulating the incident position of the electron beam at the excitation end of the optical path, different modes of information can be excited; combined with a circular polarization detection module at the collection end of the optical path, ultra-fine circular polarization detection of the emitted optical signal at the nanoscale can be performed. In the study of Smith-Purcell radiation, cathodoluminescence microscopy has demonstrated powerful advantages, enabling flexible control of radiation wavelength and circular polarization manipulation. Summary of the Invention

[0005] The purpose of this invention is to provide a method for realizing chiral nanolight sources using electron beams, so that spin-free electron beams can emit chiral light by exciting Smith-Purcell radiation in a periodically stacked square light trap structure.

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

[0007] A method for realizing chiral nanolight sources using electron beams (see...) Figure 1This method is used for the excitation and manipulation of Smith-Purcell radiation in periodically stacked square optical trap structures. The method includes the fabrication of square nano-optical trap structures in periodically stacked gold / silicon dioxide (Au / SiO2) photonic crystals, the electron beam excitation of chiral Smith-Purcell radiation in the optical trap structures, and the chiral manipulation of the Smith-Purcell radiation in the optical trap structures.

[0008] The fabrication of the Au / SiO2 periodically stacked square optical trap structure involves two steps: 1. Fabrication of the Au / SiO2 periodically stacked photonic crystal. Au and SiO2 layers are sequentially deposited on a silicon wafer using electron beam evaporation. 2. The square optical trap hole structure is etched into the Au / SiO2 periodically stacked photonic crystal using focused ion beam etching. The Au and SiO2 layers have a period of 6–10 cycles, totaling 12–20 layers. The Au layer thickness is 200–220 nm, and the SiO2 layer thickness is 250–300 nm. The square optical trap structure is a square hole structure with a side length of 600–700 nm.

[0009] Chiral Smith-Purcell radiation in the optical trap structure is generated by an electron beam incident on the hollow portion of the optical trap. The sidewalls of the optical trap are one-dimensional Au / SiO2 grating structures. As the electron beam passes over the sidewalls, Smith-Purcell radiation is excited within them. Since the intensity of Smith-Purcell radiation decreases exponentially with increasing distance from the grating structure, we choose to incident the electron beam at a distance from the apex of the square optical trap structure and only consider the radiation generated in the nearest adjacent sidewall within the optical trap. In the square optical trap structure, the Smith-Purcell radiation generated in adjacent sidewalls exists in a set of orthogonally polarized forms, meaning that the linear polarization direction of the radiation is perpendicular to the plane of each sidewall. When an electron beam is incident at a geometrically asymmetric position in the optical trap structure, the difference in distance between the electron beam and the adjacent sidewalls leads to a phase difference between the polarization currents generated in the adjacent sidewalls. This, in turn, results in the same phase difference between the orthogonally polarized Smith-Purcell radiations generated by the polarization currents. The two orthogonal linearly polarized radiations with a fixed phase difference eventually combine to form circularly polarized light emitted from the optical trap, thus realizing the emission of specific chiral light.

[0010] The excitation of optical chirality in a square nano-optical trap radiation source is achieved by nanoscale movement of the electron beam excitation position. The greater the distance difference between the electron beam and adjacent sidewalls, the greater the phase difference in the Smith-Purcell radiation generated by the adjacent sidewalls. When the phase difference varies between -2 / π and 2 / π, the circular polarization state of the emitted light can be converted. The closer the phase difference is to -2 / π or 2 / π, the higher the chirality of the emitted light. When the phase difference is 0, the superposition principle of circularly polarized light cannot be satisfied, and the chirality is also 0. Therefore, by moving the electron beam to change the excitation region, the presence / absence and circular polarization state of the square nano-optical trap radiation source can be switched, thereby achieving manipulation.

[0011] The electron beam excitation of chiral radiation in a nano-optical trap proposed in this invention is the first chiral nano-source light source achieved by simultaneously exciting two Smith-Purcell radiations with an electron beam and superimposing them. Without spin-state injection, by changing the phase difference of a set of orthogonally polarized linearly polarized Smith-Purcell radiations, subwavelength chiral light emission is achieved. This not only significantly reduces the usable size of chiral light sources but also allows for precise alteration of the excitation position through simple nanoscale electron beam movement, thereby changing the superposition state of the linear polarizations and achieving subwavelength-scale manipulation of the chirality of the radiation. This invention focuses on the research of next-generation information carriers in nanophotonics, transforming the manipulation of spin degrees of freedom from traditional far-field laser excitation to excitation through the superposition of Smith-Purcell radiations, breaking the optical diffraction limit. This chiral nano-source light source can be applied to research on optical spin-orbit coupling and the integration of quantum information devices, featuring small scale, high sensitivity, and high robustness. Given the current widespread use of far-field optics for large-scale chiral light source realization, this invention's use of an electron beam to achieve a chiral nano-source light source has broad market prospects. Attached Figure Description

[0012] Figure 1 The diagram shows the sample structure and phenomena of a specific embodiment of the present invention.

[0013] Figure 2 The diagram illustrates the principle of a specific embodiment of the present invention.

[0014] Figure 3 The pattern distribution in the square optical trap structure of a specific embodiment of the present invention is shown.

[0015] Figure 4 The diagram shows the distribution of the phase difference between two orthogonally normalized Smith-Purcell radiations as a function of the electron beam incident position, according to a specific embodiment of the present invention. Each pixel represents the injection point of the electron beam.

[0016] Figure 5 The diagram shows the distribution of the relative intensity between two orthogonally normalized Smith-Purcell radiations as a function of the electron beam incident position, according to a specific embodiment of the present invention. Each pixel represents the injection point of the electron beam.

[0017] Figure 6 The diagram shows the left-hand and right-hand circularly polarized components in the simulated and experimental radiation signals obtained by incident a 30 keV, 20 nA electron beam at a geometrically asymmetric position in a square optical trap structure (marked in the electron microscope image) according to a specific embodiment of the present invention, as well as the calculated chirality.

[0018] Figure 7The diagram illustrates the trend of chirality of the emitted light as a function of wavelength when the electron beam is incident at various points (marked in the electron microscope image) on a straight line parallel to the diagonal, according to a specific embodiment of the present invention.

[0019] Figure 8 The diagram shows the distribution of radiation chirality as a function of the electron beam incident position, obtained by simulation according to a specific embodiment of the present invention, with a detection wavelength of 740 nm.

[0020] Figure 9 The diagram shows the intensity of the emitted light as a function of wavelength when the electron beam is incident at various points (marked in the electron microscope image) along the diagonal line according to a specific embodiment of the present invention. Detailed Implementation

[0021] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can more clearly understand the present invention.

[0022] The experimental sample structure for the excitation of chiral light in a square nano-light trap by an electron beam in this embodiment of the invention is shown in the figure below. Figure 1 As shown, the structure comprises three parts. The first part is the substrate, consisting of a Si substrate and a SiO2 spacer layer. The second part is the photonic crystal, which has a 12-layer structure with 6 periods, composed of alternating Au and SiO2 layers, with the topmost layer being the SiO2 layer. The thicknesses of the Au and SiO2 layers are 200 nm and 250 nm, respectively. The third part is the optical trap, which is a square hole structure with a side length of 700 nm and a depth of approximately 3 μm.

[0023] The following describes a method for preparing the experimental sample for electron beam excitation of chiral light in a square nano-optical trap, including the following steps:

[0024] Step 1: Deposit a SiO2 spacer layer on the Si substrate using plasma-enhanced chemical vapor deposition (PECVD) to obtain a SiO2 / Si substrate.

[0025] Step 2: Ultrasonic cleaning of the SiO2 / Si substrate using organic solvents, following the sequence 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 away the remaining deionized water on the substrate to obtain a clean SiO2 / Si substrate.

[0026] Step 3: Sequentially deposit Au and SiO2 layers on the SiO2 / Si substrate using electron beam evaporation deposition, with a deposition rate of approximately [missing information]. The final result was a periodically stacked Au / SiO2 photonic crystal.

[0027] Step 4: Using focused ion beam etching (FIB), square holes with a side length of 700 nm and a depth of approximately 3 μm are etched into the photonic crystal. Gallium (Ga) ions are selected as the etching source, with an ion source voltage of 30 kV, a beam current of approximately 9 pA, and an etching depth of 3 μm. The final result is as follows: Figure 1 The square nano-optical trap structure shown.

[0028] In this invention, the excitation of chiral light in a square nano-optical trap by an electron beam is based on the superposition of a set of orthogonally polarized Smith-Purcell radiations excited by the electron beam: the electron beam grazes into the sidewall of the optical trap, generating a polarization current in the sidewall, as shown in the schematic diagram. Figure 2 The polarization currents in the sidewalls generate Smith-Purcell radiation. Since the radiation intensity is strongly dependent on the distance between the electron beam and the grating surface, we only consider the case where the electron beam is incident near the apex of the optical trap and the influence of the polarization currents generated in the nearest adjacent sidewall. The generated Smith-Purcell radiation should satisfy the mode type in the optical trap structure. By analyzing the eigenmodes of the optical trap and the resonance condition with the electron beam dispersion curve, the intersection of the dispersion curves indicates the presence of the Smith-Purcell radiation type in the optical trap. This also determines the final wavelength of the emitted light, see [reference needed]. Figure 3 The light signal radiated from the square nano-light trap originates from the superposition of a set of orthogonally polarized Smith-Purcell radiations generated by adjacent sidewalls. When the two radiation beams have a fixed phase difference, the final superposition result will be elliptically polarized or circularly polarized light. When the electron beam is incident at a geometrically asymmetric position in the square nano-light trap, there will be a fixed distance difference between the electron beam and the adjacent sidewalls, resulting in a time delay in the effect of the electron beam on the polarization current. The phase difference between the polarization currents generated in the adjacent sidewalls when the electron beam is incident at different positions was analyzed, such as... Figure 4 As shown, it can be observed that the phase difference differs at different geometrically asymmetric positions, and the phase difference distribution is symmetrical about the diagonal, which satisfies the condition for superposition to generate a circularly polarized signal. On the other hand, the factor affecting the circular polarization characteristics of the radiated light is the intensity difference between these orthogonally polarized Smith-Purcell radiations. The greater the intensity difference, the closer the superimposed radiated light is to linearly polarized light. Therefore, we simulated the distribution of the relative intensity between the two orthogonally normalized Smith-Purcell radiations as a function of the electron beam incident position, where each pixel represents the injection point of the electron beam, as shown in the figure. Figure 5 As shown, it can be observed that the closer to the angle bisector, i.e., the geometrically symmetrical position, the smaller the intensity difference, which is more conducive to the superposition and generation of circularly polarized light. Finally, to measure the chirality of the obtained circularly polarized signal, we define the chirality CD = (I... LCP -I RCP ) / (I LCP +I RCP ), where I LCP and IRCP These are the left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) photon spin components in the radiation, respectively.

[0029] The measurement procedure for electron beam room-temperature chiral nanosource excitation is described below: In this invention, electron beam room-temperature chiral nanosource excitation is performed in a cathodoluminescence microscopy imaging system based on a scanning electron microscope. The electron beam passes through a small hole in a parabolic mirror above the sample to excite the sample. The emitted cathodoluminescence is collected by the parabolic mirror above the sample and ultimately captured by a photomultiplier tube (PMT) through the collection optical path. The circularly polarized component of the sample's cathodoluminescence is extracted by a quarter-wave plate and a linear polarizer placed in the collection optical path. The major axis of the quarter-wave plate is at ±45° to the polarization direction of the linear polarizer, allowing for the extraction of left-handed and right-handed circularly polarized components, respectively. In cathodoluminescence detection, the cathodoluminescence spectrum of the sample is collected in Pan mode.

[0030] The specific implementation method is as follows:

[0031] A square nanotrap sample was prepared following the steps described above. Figure 1 As shown, from bottom to top, there is a Si substrate and a SiO2 spacer layer, a periodic stacked layer of 200 nm thick Au and 250 nm thick SiO2, and a 700 nm × 700 nm square hole structure etched in the photonic crystal structure using a focused ion beam. Experimental measurements were performed using a cathodoluminescence microscopy system (Gatan MonoCL4 Plus) in a scanning electron microscope (FEI Quanta450FEG). A quarter-wave plate (AQWP10M-980, THORLABS) and a linear polarizer (LPVIS100, THORLABS) were used to detect the cathodoluminescence signal of the sample in the wavelength range of 690 nm to 850 nm in Pan mode.

[0032] The sample was placed into a vacuum chamber and an electron beam with a current of 20 nA at 30 keV was incident on the geometrically asymmetric position of the optical trap at room temperature (see [link]). Figure 6 (As indicated in the medium electron microscope image), left-handed and right-handed circularly polarized signals radiated from the optical trap were collected (see...). Figure 6 The spectrum (indicated by a) shows the chirality as a function of wavelength, and the calculated chirality variation. To verify the experimental results, we used the finite-difference time-domain (FDTD) simulation method to obtain results under the same excitation conditions (see [reference]). Figure 6 As shown in spectrum b), the spectral peaks are all around 740 nm, and the calculated chirality also matches well. This confirms the authenticity of the experimental results.

[0033] By changing the electron beam injection position, the movement path follows the perpendicular line to the angle bisector of the square nano-light trap (see...). Figure 7 (As indicated in the electron microscope image), the variation of chirality with wavelength at different excitation positions under the same excitation source voltage and beam current conditions (see...). Figure 7 As shown in the spectrum, it can be observed that as the electron beam gradually approaches the angle bisector from a position far from it, the chirality first increases and then decreases, reaching a maximum chirality of over 40%, before decreasing to 0 at the angle bisector. Conversely, when the electron beam moves away from the angle bisector, the chirality first increases in the opposite direction and then decreases. Considering the electron beam spot size to be approximately 10 nm × 10 nm, moving the electron beam approximately 40 nm from position III to position IV achieves the chirality phenomenon from present to absent, and moving it approximately 80 nm from position III to position VI achieves the reversal of the chirality phenomenon. This is a highly robust and spatially resolved chiral manipulation technique. To verify the authenticity of this experimental phenomenon, FDTD simulations were used to obtain the chirality distribution calculated at the radiation peak of 740 nm when the electron beam is incident at various points near the apex of a square nano-light trap. Figure 8 As shown, it can be observed that the chirality first increases and then decreases as the electron beam injection position approaches the angle bisector, and then increases and decreases in the opposite direction, confirming the experimental results.

[0034] Furthermore, the relationship between the light radiation intensity and the electron beam excitation position in the square nanostructure was investigated. Figure 9 As shown, the radiation intensity gradually decreases as the electron beam moves further away from the apex of the square nano-light trap. This provides guidance for the future application of nano-light trap light sources.

[0035] This invention is the first to utilize electron beam incident light to generate chiral light emission within a square nano-optical trap structure. By overcoming the optical diffraction limit through ultra-high spatial resolution of the electron beam, a set of orthogonally polarized Smith-Purcell radiations is precisely excited into the square optical trap structure. The chiral radiation signal is obtained through the superposition relationship of a fixed phase difference. Furthermore, unlike traditional far-field optical excitation methods for chiral light sources, this invention leverages the advantages of the nanoscale beam spot of the electron beam to excite different superposition states of Smith-Purcell radiation within the nano-optical trap structure, achieving effective manipulation of the chirality of the chiral nano-light source within a hundred-nanometer electron beam movement range. This invention represents a novel, highly sensitive, and robust method for exciting and manipulating room-temperature chiral nano-light sources at the nanoscale. Besides the square nano-optical trap structure of this size, it can be extended to other periodic optical trap structures, satisfying chiral light emission at different radiation wavelengths. With the rapid development of nanophotonic devices, the demand for new degrees of freedom in manipulating information carriers is increasingly strong. This invention provides strong support and reference value for their future development.

[0036] Finally, it should be noted that the purpose of disclosing 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 content disclosed in the embodiments, and the scope of protection of the present invention is defined by the claims.

Claims

1. A method for realizing a chiral nanolight source, characterized in that, First, a periodically stacked Au / SiO2 photonic crystal is prepared. Specifically, an Au layer and a SiO2 layer are sequentially deposited on a SiO2 / Si substrate using electron beam evaporation at a rate of 0.3–0.5 Å / s to obtain a periodically stacked Au / SiO2 photonic crystal with a thickness of 3–5 μm. Then, square holes are etched in the stacked photonic crystal to obtain a square nano-light trap structure. An electron beam is incident on the square nano-light trap structure to obtain a chiral nano-light source.

2. The method for realizing a chiral nanolight source as described in claim 1, characterized in that, Square hole structures were etched in a photonic crystal using focused ion beam etching (FIB), with gallium ions selected as the etching source. The ion source voltage was 25~30kV and the beam current was 7~9pA.

3. The method for realizing a chiral nanolight source as described in claim 1, characterized in that, The Au layer and SiO2 layer have 6 to 10 cycles, with a total of 12 to 20 layers.

4. The method for realizing a chiral nanolight source as described in claim 3, characterized in that, The Au layer has a thickness of 200~220nm, and the SiO2 layer has a thickness of 250~300nm.

5. The method for realizing a chiral nanolight source as described in claim 1, characterized in that, The square nano-optical trap structure is a square hole structure with a side length of 600~700nm.

6. A method for exciting chiral light, characterized in that, Using the chiral nanolight source described in claim 1, when an electron beam is incident at a geometrically asymmetric position in a square nanolight trap structure, the distance difference between the electron beam and the adjacent sidewalls leads to a phase difference between the polarization currents generated in the adjacent sidewalls. This results in the same phase difference between the orthogonally polarized Smith-Purcell radiations generated by the polarization currents, with the phase difference being between -2 / π and 2 / π. The two orthogonal linearly polarized radiations with a fixed phase difference are finally combined into circularly polarized light emitted from the square nanolight trap structure, thus realizing the emission of chiral light.

7. A method for controlling chiral optical signals, characterized in that, Using the chiral light excitation method described in claim 6, a chiral light signal is obtained by superimposing a fixed phase difference. When the electron beam is incident on the geometrically asymmetric position of the square nano-light trap structure, chiral light emission is generated. When the electron beam is incident on the geometrically symmetric position of the square nano-light trap structure, chiral light emission cannot be generated. The presence or absence of the chiral light signal is controlled by moving the electron beam.

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