A device and method for controlling the polarization and direction of Cherenkov radiation

Abstract

The present invention discloses a device and method for regulating the polarization and direction of Cherenkov radiation, including a composite structure composed of a phase gradient metasurface and a waveguide. The supercell of the phase gradient metasurface satisfies: #imgabs0# where θ is the rotation angle of the supercell, d is the period of the supercell, and n eff is the refractive index of the mode in the waveguide at the wavelength of λ. The electron beam moves along the direction parallel to the waveguide to excite the mode in the waveguide to be the quasi-TE0 mode. By adjusting the movement direction of the electron beam and the rotation angle of the supercell of the phase gradient metasurface, the polarization and direction of Cherenkov radiation are regulated. The present invention realizes the simultaneous regulation of the direction and polarization state of Cherenkov radiation based on the composite structure of the phase gradient metasurface and the waveguide. The structure of the present invention is simple, the materials are common, and it is easy to manufacture.

Description

Technical Field

[0001] The invention belongs to the field of electromagnetic technology, and in particular relates to a device and method for regulating the polarization and direction of Cherenkov radiation. Background Art

[0002] When high-speed charged particles pass through a dielectric, the emitted electromagnetic waves polarize the atoms in the medium. The polarized atoms will depolarize back to their original state after the electromagnetic wave passes through, and will radiate photons. The coherence between these photons is enhanced to form Cherenkov radiation. With the development of modern physical science, Cherenkov radiation has been widely used in scientific fields such as astronomy, chemistry, optics, medicine, and nuclear physics. And its research in most fields is a hot research topic in today's scientific community. Based on the huge application potential of Cherenkov radiation in many scientific fields, it is of great significance to strengthen the basic research on Cherenkov radiation.

[0003] As a two-dimensional and easily controllable electromagnetic material, metasurface can resonantly couple with the electric or magnetic components of the incident electromagnetic wave, showing properties that have not been discovered in nature. The advantage of metasurface is that it breaks through the constraints of traditional three-dimensional metamaterials and uses existing technologies to build various structures that are easier to prepare. It shows advantages in phase, amplitude, polarization and impedance control, and has broad application prospects. Metasurfaces provide new degrees of freedom for regulating the interaction between electrons and the surrounding medium. On the one hand, it is very interesting to explore how electrons and metaatoms interact and then radiate light, which can go beyond traditional light-matter processes. On the other hand, metasurfaces can greatly affect the properties of the radiated light, opening up a new world for controlling electron-induced emission.

[0004] Phase gradient metasurface is a type of metasurface, which is composed of super units with the same structure but different rotation angles. The phase control of transmission and reflection can be achieved by simply changing the rotation angle of the unit structure, which greatly reduces the difficulty of phase gradient metasurface structure design. When an electromagnetic wave is incident on the phase gradient metasurface, each unit structure of the phase gradient metasurface will produce a different phase mutation on it. The mutation phase can be controlled by changing the rotation angle of the unit structure, so that the transmission or reflection phase reaches a certain gradient and achieves a phase coverage of 2π within one cycle, thereby realizing the regulation of the incident electromagnetic wave, providing a new design method for controlling the propagation of electromagnetic waves.

[0005] Most of the previous research focused on the direction, intensity, and frequency band of Cherenkov radiation. Regarding the polarization of Cherenkov radiation, the previous research focused on the regulation of circularly polarized Cherenkov radiation in a single direction. Summary of the invention

[0006] In view of the above-mentioned prior art, the technical problem to be solved by the present invention is to provide a device and method for controlling the polarization and direction of Cherenkov radiation, which can realize simultaneous control of the polarization and direction.

[0007] In order to solve the above technical problems, a device for regulating the polarization and direction of Cherenkov radiation of the present invention includes a composite structure consisting of a phase gradient metasurface and a waveguide, and the super unit of the phase gradient metasurface satisfies: Where θ is the rotation angle of the supercell, d is the period of the supercell, and n eff is the refractive index of the mode in the waveguide at wavelength λ. The electron beam moves in a direction parallel to the waveguide, making the excited mode in the waveguide quasi-TE 0 mode, the polarization and direction of Cherenkov radiation are controlled by adjusting the movement direction of the electron beam and the rotation angle of the superunit of the phase gradient metasurface.

[0008] Furthermore, the composite structure specifically comprises: a substrate layer, a waveguide medium layer and a phase gradient metasurface medium layer in sequence.

[0009] Furthermore, the supercells have the same structure satisfying the anisotropy condition.

[0010] Furthermore, the supercell is a rectangular block.

[0011] Furthermore, the waveguide is a rectangular waveguide.

[0012] A method for controlling the polarization and direction of Cherenkov radiation using any of the above devices: an electron beam generates Cherenkov radiation through a composite structure, satisfying:

[0013]

[0014] Among them, θ t is the radiation angle, n eff is the refractive index of the mode in the waveguide at wavelength λ, σ represents the helicity, where σ is -1, 0 and 1 respectively for left-handed circularly polarized light, linearly polarized light and right-handed circularly polarized light, θ i is the incident angle, which refers to the angle between the incident direction of the electron beam and the normal direction. When the waveguide direction is set to the x-axis direction, sinθ i =1, when the electron beam propagates in the negative direction along the x-axis, sinθ i = -1, m is the radiation order, θ is the rotation angle of the metasurface superunit, and by adjusting θ i Adjust θ and θ simultaneously t and σ.

[0015] Beneficial effects of the invention: The invention realizes the simultaneous regulation of the direction and polarization state of Cherenkov radiation based on the composite structure of phase gradient metasurface and waveguide. The invention has a simple structure, common materials, and is easy to manufacture. It combines traditional vacuum electronics with micro-nano optoelectronics, and promotes the development of integrated photonic and optoelectronic devices in the fields of polarization detectors and free electron sources. The electron beam generates Cherenkov radiation through the composite structure of waveguide and phase gradient metasurface. The high-speed electron beam is placed next to the dielectric waveguide to excite the high-purity quasi-TE in the waveguide. 0 Mode, Quasi-TE 0 The mode is decoupled through the phase gradient metasurface to emit light. By adjusting the direction of the electron beam and the rotation angle of the superunit, the polarization and direction of the Cherenkov radiation can be simultaneously controlled. The phase gradient metasurface in the present invention is universal, and the superunit can be any structure that meets the anisotropy condition. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a three-dimensional schematic diagram of the composite structure of waveguide and phase gradient metasurface;

[0017] Figure 2 is a top view of the phase gradient metasurface with adjacent cells rotated 165°;

[0018] Figure 3 is a cross-sectional view of the composite structure;

[0019] Figure 4(a) is the far-field diagram when the adjacent cells are rotated 165° and the electron beam is along the positive direction of the x-axis;

[0020] Figure 4(b) is the far-field diagram when the adjacent cells are rotated 165° and the electron beam is along the negative x-axis;

[0021] Figure 5(a) shows the variation of the polarization state of the far field of radiation with wavelength when the radiation angle is 0°;

[0022] Figure 5(b) shows the variation of the polarization state of the far-field radiation with wavelength when the radiation angle is -13°;

[0023] FIG5(c) shows the variation of the polarization state of the far-field radiation with wavelength when the radiation angle is -27°. DETAILED DESCRIPTION

[0024] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0025] Combination Figure 1 , Figure 2 and Figure 3The present invention uses a composite structure of a phase gradient metasurface and a waveguide to achieve simultaneous regulation of the polarization and direction of Cherenkov radiation. The composite structure includes a substrate layer 1, a waveguide 2, and a phase gradient metasurface 3 from bottom to top. Its radiation mechanism can be regarded as an electron beam 4 placed on the side of the waveguide 2, and the electron beam moves in a direction parallel to the waveguide 2. By changing the speed of the electron beam 4, the high-purity quasi-TE in the waveguide 2 is excited. 0 Mode. The mode in waveguide 2 is decoupled into polarized light of different polarization states through the phase gradient metasurface 3 placed on waveguide 2. The polarization and direction of Cherenkov radiation can be controlled by changing the direction of the electron beam and the rotation angle of the superunit. The direction of movement of the electron beam 4 can control the positive and negative direction of the radiation. The period d of the superunit of the phase gradient metasurface 3 and the rotation angle θ of the superunit satisfy where n eff is the refractive index of the mode in the waveguide at wavelength λ. The metasurface is composed of multiple periodic gradient superlattice units, and the phase coverage caused is 2π. The present invention is not limited to the shape of the superunit, and the superunit can be any structure that meets the anisotropy condition, such as: rectangular structure, U-shaped structure, elliptical structure, etc.

[0026] By changing the period and rotation angle of the phase gradient metasurface superunit to control the direction of the radiated light and the corresponding polarization state, polarization-adjustable Cherenkov radiation is achieved. The electron beam is composed of a series of dipoles with time phase delay (x / v). The speed of the electron beam and the mode in the waveguide meet the wave vector matching condition, and the corresponding relationship is: (where v is the speed of the electron beam, c is the speed of light in a vacuum; n eff is the refractive index of the mode in the waveguide).

[0027] The electron beam speed When moving in a direction parallel to a periodic structure (e.g., a grating), Smith-Purcell radiation is generated. The relationship between the radiation wavelength and the radiation angle is as follows:

[0028]

[0029] Where λ is the wavelength of the radiated electromagnetic wave; P is the period of the grating; m is the radiation order; v is the speed of the electron beam; c is the speed of light in a vacuum; θ t is the radiation angle, which corresponds to the angle between the outward radiation direction of the electromagnetic wave and the normal direction. The emergence of phase gradient metasurfaces provides a new degree of freedom for manipulating free electron radiation. Introducing phase gradient metasurfaces into this formula can generalize formula (1) to:

[0030]

[0031] Among them, θ iis the angle of incidence, which refers to the angle between the incident direction and the normal direction; σ is -1, 0 and 1, respectively, indicating left-handed circularly polarized light, linearly polarized light and right-handed circularly polarized light; n t is the refractive index of the medium in which the radiating light is located; n eff is the refractive index of the mode in the waveguide at wavelength λ, and n t =1, Substituting these conditions, we get:

[0032]

[0033] The above formula (3) reveals that changing the direction of the electron beam θ i The rotation angle θ of the superunit can simultaneously control the direction and polarization state of the Cherenkov radiation.

[0034] The following is an example with specific parameters:

[0035] Combination Figure 1 , Figure 2 and Figure 3 , the composite structure includes SiO 2 The base layer 1, the Si waveguide 2 and the Si medium form a phase gradient metasurface 3. The unit structure of the Si medium phase gradient metasurface 3 is a rectangular block. The length a of the rectangular block is 423 nm, the width b is 200 nm, and the height h is 1 The material is silicon medium with a refractive index of 3.46. The width w of the silicon waveguide is 600nm and the height h is 2 The phase coverage of 2π is 220nm. By rotating the angle of the super unit, a phase coverage of 2π is achieved within one period. For example, the case where the adjacent unit structure is rotated 165°, when the rotation angle of the adjacent rectangular block is 165°, the period d between the adjacent unit cells is 566.79nm, quasi-TE 0 The refractive index n of the mode at a wavelength of 1550nm eff =2.5068, the electron beam 4 is located on the side of the waveguide 2 at half the height of the Si waveguide 2, and the distance from the waveguide 2 is w 1 It is 10nm.

[0036] When the electron beam propagates in the positive direction along the x-axis, sinθ i =1(θ i is the angle between the incident direction and the normal direction) From the above formula (3), we can get:

[0037] m＝0 σ=1 <![CDATA[θ t =0°]]> m＝1 σ=0 <![CDATA[θ t =-13°]]> m＝2 σ=-1 <![CDATA[θ t =-27°]]>

[0038] When the electron beam propagates in the negative direction along the x-axis, sinθ i =-1, from the above formula (3) we can get:

[0039] m＝0 σ=-1 <![CDATA[θ t =0°]]> m＝-1 σ=0 <![CDATA[θ t =13°]]> m＝-2 σ=1 <![CDATA[θ t =27°]]>

[0040] like Figure 2 As shown. When the electron beam propagates in the positive direction along the x-axis, the electric field diagram of the far field is shown in Figure 4(a). It will radiate in three directions. When m is 0th order, the radiation angle is 0°; when m is 1st order, the radiation angle is -13°; when m is 2nd order, the radiation angle is -27°. When the electron beam propagates in the negative direction along the x-axis, it will also radiate in three directions. When m is 0th order, the radiation angle is 0°; when m is -1st order, the radiation angle is 13°; when m is -2nd order, the radiation angle is 27°, as shown in Figure 4(b). When the electron beam propagates in the positive direction along the x-axis, when θ t =0°, σ=1, and the radiated light is right-handed circularly polarized light, as shown in Figure 5(a). t =-13°, σ = 0, and the radiated light is linearly polarized, as shown in Figure 5(b). t = -27°, σ = -1, and the radiated light is left-handed circularly polarized light, as shown in Figure 5(c), where S 3 is the normalized Stokes parameter, representing the polarization state of light. For right circularly polarized light, S 3 Equal to 1, for left circularly polarized light, S 3 Equal to -1, for linearly polarized light, S 3 is equal to 0.

Claims

1. A device for regulating the polarization and direction of Cherenkov radiation, characterized in that: A composite structure consisting of a phase gradient metasurface (3) and a waveguide (2), wherein a super unit of the phase gradient metasurface (3) satisfies: Where θ is the rotation angle of the supercell, d is the period of the supercell, and n eff is the refractive index of the mode in the waveguide at a wavelength of λ, the electron beam (4) moves in a direction parallel to the waveguide (2) so that the excitation mode in the waveguide (2) is a quasi-TE0 mode, and the polarization and direction of the Cherenkov radiation are controlled by adjusting the moving direction of the electron beam (4) and the rotation angle of the super unit of the phase gradient metasurface (3).

2. The device for controlling the polarization and direction of Cherenkov radiation according to claim 1, characterized in that: The composite structure specifically comprises: a substrate layer (1), a waveguide (2) and a phase gradient metasurface (3) in sequence.

3. The device for controlling the polarization and direction of Cherenkov radiation according to claim 1, characterized in that: The super units are identical structures satisfying anisotropy conditions.

4. The device for controlling the polarization and direction of Cherenkov radiation according to claim 3, characterized in that: The super unit is a rectangular block.

5. The device for controlling the polarization and direction of Cherenkov radiation according to claim 1, characterized in that: The waveguide (2) is a rectangular waveguide.

6. A method for controlling the polarization and direction of Cherenkov radiation using any one of the devices of claims 1 to 5, characterized in that: The electron beam (4) generates Cherenkov radiation through the composite structure, satisfying: Among them, θ t is the radiation angle, n eff is the refractive index of the mode in the waveguide at wavelength λ, σ represents the helicity, σ -1, 0 and 1 represent left-handed circularly polarized light, linearly polarized light and right-handed circularly polarized light respectively, θ i is the incident angle, which refers to the angle between the incident direction of the electron beam and the normal direction. When the waveguide direction is set to the x-axis direction, sinθ i =1, when the electron beam propagates in the negative direction along the x-axis, sinθ i = -1, m is the radiation order, θ is the rotation angle of the metasurface superunit, and by adjusting θ i Adjust θ and θ simultaneously t and σ.

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