A method for exciting polariton backward Cherenkov radiation

Abstract

The present invention discloses a method for exciting polariton inverse Cherenkov radiation (CR), which relates to the technical fields of nano light sources and electron excitation technologies, and includes: S1. Preparing a hyperbolic polariton material or a hyperbolic polariton heterojunction material supporting negative group velocity dispersion on a chip substrate; S2. Based on the prepared hyperbolic polariton material, using moving charged particles to excite polariton inverse CR; S3. Placing the moving charged particles on the surface of the hyperbolic polariton material or the hyperbolic polariton heterojunction material. When the wave vector of the moving charged particles and the polariton satisfy the wave vector matching condition, inverse CR occurs. The present invention 1) can effectively excite polariton inverse CR by using the plasmon of nanowires to imitate moving charged particles; 2) can be further adjusted in other heterostructures based on hyperbolic polariton materials.

Description

Technical Field

[0001] The present invention relates to the technical fields of nano light sources and electronic excitation technologies, and in particular to a method for exciting the reverse Cherenkov radiation of polaritons. Background Art

[0002] In on-chip photonic circuits, it is crucial to achieve a nano light source that breaks through the diffraction limit. Cherenkov Radiation (CR) is electromagnetic radiation emitted by charged particles moving faster than the speed of light in a medium, with an ultra-wide frequency coverage range (from microwaves to ultraviolet light). Forward CR is emitted in a conical shape, and its radiation direction (energy flow direction) forms an acute angle with the moving direction of the fast-moving charged particles, which will cause interference between the radiation signal and the charged particle signal; in order to separate the light radiation generated by the charged particles from the exciting particles, reverse CR provides a solution. By designing materials with negative refractive index or negative group velocity dispersion, the radiation direction of the charged particles in the medium forms an obtuse angle with the moving direction of the fast-moving charged particles. In 1968, Veselago theoretically predicted that reverse CR could be achieved in a left-handed medium with a negative refractive index (i.e., simultaneously having a negative dielectric constant and a negative magnetic permeability). In the microwave frequency band, reverse CR has been experimentally demonstrated in artificial left-handed media. However, in the infrared frequency band, due to the complex design of the structural units of left-handed metamaterials and large scattering losses, it is difficult to achieve efficient and tunable Cherenkov radiation, which limits its wide application.

[0003] In recent years, birefringent crystals with low-loss hyperbolic phonon polariton response and negative group velocity dispersion can significantly slow down the speed of light in the medium, which is beneficial to reducing the velocity threshold of the charged particles required for reverse CR excitation and provides a competitive platform for reverse CR at optical frequencies. Currently, the reverse CR of hyperbolic polariton materials based on negative group velocity dispersion has been theoretically predicted. However, due to the large momentum mismatch between the fast-moving particles and the polaritons, the reverse CR phenomenon of hyperbolic polaritons has not been observed in experiments yet.

[0004] Therefore, proposing a method for exciting the reverse Cherenkov radiation of polaritons to solve the difficulties existing in the prior art is an urgent problem that needs to be solved by those skilled in the art. Summary of the Invention

[0005] In view of this, the present invention provides a method for exciting the reverse Cherenkov radiation of polaritons, which can effectively excite the reverse CR of type-I phonon polaritons by means of moving charged particles or conductive surface plasmons imitating moving charged particles.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for exciting the reverse Cherenkov radiation of polaritons, comprising the following steps:

[0008] S1. Prepare a hyperbolic polariton material or a hyperbolic polariton heterojunction material with negative group velocity dispersion on a chip substrate;

[0009] S2. Based on the prepared hyperbolic polariton material or hyperbolic polariton heterojunction material, use moving charged particles to excite reverse CR;

[0010] S3. Place the moving charged particles above the hyperbolic polariton material or hyperbolic polariton heterojunction material. When the wave vector of the moving charged particles and the polariton satisfy the wave vector matching condition, reverse CR occurs.

[0011] In the above method, optionally, the chip substrate material in S1 includes but is not limited to SiO 2 , Si, Au, Ag, Cu, Ni, lithium niobate, glass.

[0012] In the above method, optionally, the hyperbolic polariton material in S1 has negative group velocity dispersion, including but not limited to MoO 3 , hBN, graphene, Bi 2 Se 3 , Bi 2 Te 3 , WS 2 , TaS 2 , TiS 2 , WSe 2 , TaSe 2 , TiSe 2 , SnS 2 , HfS 2 , PdS 2 , with a thickness of 0 - 5000 nm.

[0013] In the above method, optionally, the hyperbolic polariton heterojunction in S1 is constructed by a hyperbolic polariton material and a dielectric;

[0014] The dielectric includes but is not limited to graphene, h - BN, WS 2 , MoS 2 , HfO 2 , SiO 2 , Al 2 O 3 , CaF 2 , MgF 2 ;

[0015] The thickness of the dielectric is 0 - 100 nm.

[0016] The above method, optionally, analogous moving charged particles include but are not limited to tunneling electrons, electron beams, and antennas with polariton responses.

[0017] The above method, optionally, antennas with polariton responses include but are not limited to Au nanowires, Ag nanowires, BN nanoribbons, BN nanotubes, carbon nanotubes, graphene nanoribbons, MoO 3 nanowires and MoO 3 nanoribbons.

[0018] The above method, optionally, changes the symmetry of the reverse CR by changing the angle between the moving charged particles or the antenna with polariton response and the hyperbolic polariton material; changes the radiation angle and radiation efficiency of the reverse CR by changing the type or thickness of the medium between the moving charged particles and the hyperbolic polariton material, thereby optimizing the performance of the reverse CR.

[0019] From the above technical solutions, compared with the prior art, the present invention provides a method for exciting polariton reverse Cherenkov radiation. Using the polaritons of moving charged particles or nanowires to imitate moving charged particles can effectively excite polariton reverse CR, and the frequency range depends on the frequency band of the hyperbolic polariton material with negative group velocity dispersion response; further research on the real-space image of the reverse CR shows that the radiation distribution and reverse CR angle are closely related to the planar isofrequency contours (IFCs) of the hyperbolic polariton material and can be further adjusted in other heterostructures based on hyperbolic polariton materials; this device design and experimental research method can be generally applied to other materials; in addition, by constructing a heterojunction, it is possible to integrate the reverse CR operating frequency bands of different materials; this invention of a phonon polariton reverse CR nanolight source provides an effective method for applications such as nanoimaging, optical sensing, and nanoscale energy transfer. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained according to the provided drawings without creative efforts.

[0021] Figure 1 It is a flowchart of a method for exciting polariton reverse Cherenkov radiation provided by the present invention;

[0022] Figure 2 It is a schematic diagram of s-SNOM test provided by an embodiment of the present invention;

[0023] Figure 3The typical s-SNOM near-field distribution diagram of reverse CR provided by the embodiments of the present invention;

[0024] Figure 4 The typical s-SNOM near-field distribution diagram of non-CR provided by the embodiments of the present invention;

[0025] Figure 5 The contrast diagram of the polariton interference fringe intensities of reverse CR and non-CR provided by the embodiments of the present invention. Detailed implementation manners

[0026] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] Refer to Figure 1 As shown, the present invention discloses a method for exciting reverse Cherenkov radiation of polaritons, including the following steps:

[0028] S1. Prepare a hyperbolic polariton material or a hyperbolic polariton heterojunction material that supports negative group velocity dispersion on a chip substrate;

[0029] S2. Based on the prepared hyperbolic polariton material or hyperbolic polariton heterojunction material, use moving charged particles to excite reverse CR;

[0030] S3. Place the moving charged particles above the hyperbolic polariton material or hyperbolic polariton heterojunction material. When the wave vector of the moving charged particles and the polariton satisfy the wave vector matching condition, reverse CR occurs.

[0031] Further, the chip substrate material in S1 includes but is not limited to SiO 2 , Si, Au, Ag, Cu, Ni, lithium niobate, glass.

[0032] Further, the hyperbolic polariton material in S1 has negative group velocity dispersion, including but not limited to MoO 3 , hBN, graphene, Bi 2 Se 3 , Bi 2 Te 3 , WS 2 , TaS 2 , TiS 2 , WSe 2 , TaSe 2 , TiSe 2 , SnS 2, HfS 2 , PdS 2 , with a thickness of 0 - 5000 nm.

[0033] Furthermore, the hyperbolic polariton heterojunction in S1 is constructed by a hyperbolic polariton material and a medium together;

[0034] The medium includes but is not limited to graphene, h - BN, WS 2 , MoS 2 , HfO 2 , SiO 2 , Al 2 O 3 , CaF 2 , MgF 2 ;

[0035] The thickness of the medium is 0 - 100 nm.

[0036] Furthermore, the analogous moving charged particles include but are not limited to tunneling electrons, electron beams, and antennas with polariton responses.

[0037] Furthermore, the antennas with polariton responses include but are not limited to Au nanowires, Ag nanowires, BN nanoribbons, BN nanotubes, carbon nanotubes, graphene nanoribbons, MoO 3 nanowires and MoO 3 nanoribbons.

[0038] Furthermore, by changing the angle between the moving charged particles or the antennas with polariton responses and the hyperbolic polariton material, the symmetry of the reverse CR is changed; by changing the type or thickness of the medium between the moving charged particles and the hyperbolic polariton material, the radiation angle and radiation efficiency of the reverse CR are changed, thereby optimizing the performance of the reverse CR.

[0039] In a specific embodiment, the specific content is as follows:

[0040] Selection of materials: MoO 3 Phonon polaritons have excellent optical properties. First, low loss; compared with the most studied phonon polariton material h - BN, due to the significant difference in the masses of Mo and O atoms, the frequency difference between its optical and acoustic phonons is large, thus significantly reducing the loss of phonon polaritons. On a silicon oxide substrate, the measured lifetime of MoO 3 phonon polaritons can reach 8 - 22 ps at room temperature; by isotope purification of Mo atoms, its lifetime can be further increased by nearly two times. Second, in - plane and out - of - plane hyperbolic characteristics; due to MoO 3In-plane and out-of-plane anisotropic optical parameters result in in-plane hyperbolic characteristics within Band 1 and Band 2, and out-of-plane hyperbolic characteristics within Band 3. Within the remaining ray bands, the MoO 3 surface is expected to support phonon polaritons. The first type of remaining ray band (Band 1) originates from the phonon mode along the

[001] crystal orientation and is called the Type II hyperbolic band (ε x >0, ε y <0, ε z >0), with a range of 545 - 851 cm -1 ; the second type of remaining ray band (Band 2) is also called the Type II hyperbolic band (ε x <0, ε y >0, ε z >0), which originates from the phonon mode along the

[100] crystal orientation and has a range of 820 - 972 cm -1 ; the third type of remaining ray band (Band 3) is also called the Type I hyperbolic band (ε x >0, ε y >0, ε z >0), which originates from the phonon mode along the

[010] crystal orientation and has a range of 958 - 1010 cm -1 . Third, it is easy to tune; due to the relatively large unit cell of MoO 3 and the relatively large distance between adjacent molecular layers, external atoms / ions can be easily inserted into the interlayer or intralayer, thereby effectively regulating its electronic band structure and phonon mode. Therefore, MoO 3 with mid-infrared response, ultra-low loss, and anisotropy provides an ideal platform for realizing backward CR in the infrared band. To study the backward CR of MoO 3 phonon polaritons in the atmospheric environment, optical antennas are often used to mimic moving charged particles. Plasmons propagating along one dimension on silver / gold nanowires (i.e., dynamic charge density waves) are used to mimic superluminal moving charged particles. In the mid-infrared region, the phase velocity of plasmons on silver / gold nanowires is close to the speed of light in free space, much larger than the phase velocity of MoO 3 phonon polaritons. Therefore, backward CR of MoO 3 phonon polaritons can be excited. In addition, this experimental design can conveniently characterize the radiation characteristics of backward CR of phonon polaritons in real space by means of the s-SNOM characterization method, including the radiation angle and radiation efficiency.

[0041] Device fabrication: First, with the help of Nitto blue tape, the purchased high-quality MoO 3 crystals and h-BN crystals are mechanically exfoliated into large-area nanosheets, and then the tape with these nanosheets is pasted on a clean thermally oxidized Si wafer (285 nm SiO2 , 500 μm Si, purchased from svm), on the surface, heat it with a hot stage (90 degrees Celsius) for about 2 minutes. Since the adhesion force between the material and the Si wafer is stronger than the intermolecular van der Waals force, a part of the nanosheets on the tape is transferred to the surface of the Si wafer. Prepare h-BN nanosheets using the same method. When preparing the h-BN / MoO 3 heterojunction, use PDMS to position and transfer h-BN onto MoO 3 above. Then, transfer silver / gold nanowires onto the surface of the clean and large-area MoO 3 nanosheets. Another method for preparing the antenna: utilize micro-nano processing technology to fabricate high-quality gold nanowires on the surface of the MoO 3 and h-BN / MoO 3 heterojunction samples. Specifically, coat approximately 350 nm of 950K polymethyl-methacrylate (PMMA) photoresist on the surface of the MoO 3 nanosheets, use electron beam lithography to write nanowire patterns with different lengths and different angles of rotation in specific areas; then in a vacuum chamber with a pressure less than 1×10 - 6 torr, use electron beam evaporation to deposit 10 nm of chromium and 100 nm of gold, immerse the sample in hot acetone in a water bath (60 °C, 40 minutes) to remove the excess PMMA and Au, soak it in isopropyl alcohol for 3 minutes, and finally take it out and dry it with flowing nitrogen.

[0042] In terms of characterization tests, use s-SNOM (Neaspec) with a wavelength-tunable quantum cascade laser (890 - 1700 cm -1 ) to perform near-field imaging on the device, as Figure 2 shown. The radius of curvature of the probe tip is approximately 10 - 20 nm (purchased from Nanoworld), and the tip tapping frequency and amplitude are set to approximately 270 kHz and 50 - 100 nm respectively. The obliquely incident infrared laser beam is focused on the tip and the sample, and the spot size is approximately 25 microns, which is sufficient to cover the nanowires and the large-area sample. To simulate the movement directions of different charged particles, we rotate the sample to control the angle of the infrared polarized light relative to the long axis of the nanowires. Among them, the thicknesses of MoO 3 and h-BN are 380 nm and 7 nm respectively.

[0043] Figure 3 and Figure 4 show the reverse CR and non-CR under two different incident polarized light directions for the same MoO 3Typical s-SNOM images of the sample. When the polarization direction of the incident light is along the x direction, it can effectively drive the surface plasmons propagating along the long axis of the nanowire, similar to electrons moving along the long axis. Therefore, it can excite the polaritonic backward CR. When the polarization direction of the incident light is along the y direction, the polarization direction of the surface plasmon electrons is along the y direction, and the nanowire only acts as a boundary, exciting ordinary phonon polaritons. Therefore, the radiation angle of the backward CR is 0.

[0044] As Figure 3 and Figure 4 shown, there are obvious differences in the propagation directions of the interference fringes measured for the two different incident polarization light directions of backward CR and no CR (red dashed arrows). Figure 3 The parallel bright fringes are due to the interference between the polaritonic backward CR excited by the surface plasmons in the silver nanowire and the tip-reflected light, which Figure 4 verifies the successful observation of the backward CR phenomenon in the experiment by comparison. By extracting the phonon polariton interference fringes of backward CR and no CR at the same position, it can be found that the intensity of the interference fringes with backward CR is about 6 times higher than that without CR, as Figure 5 shown. Further comparing the backward CR at different excitation frequencies, it is found that as the excitation frequency increases, the wave vector of the MoO 3 phonon polaritons shrinks, so θ k increases.

[0045] In the experiment, the radiation angle and radiation efficiency of the backward CR can be effectively modulated by changing the negative group velocity dispersion and the motion direction of charged particles. In addition, by constructing the h-BN / MoO 3 heterojunction, not only can the radiation angle be adjusted, but also the quality factor of the backward CR can be improved. MoO 3 and the backward CR based on the vdW heterojunction provide a possible path for electron-excited infrared nanolight sources and demonstrate their application potential for on-chip photonic chips.

[0046] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for exciting the reverse Cherenkov radiation of polaritons, characterized in that, it includes the following steps: S1. Prepare a hyperbolic polariton material or a hyperbolic polariton heterojunction material that supports negative group velocity dispersion on a chip substrate; S2. Based on the prepared hyperbolic polariton material or hyperbolic polariton heterojunction material, use moving charged particles to excite reverse Cherenkov radiation; S3. Place the moving charged particles above the hyperbolic polariton material or hyperbolic polariton heterojunction material. When the wave vector of the moving charged particles and the polaritons satisfy the wave vector matching condition, reverse Cherenkov radiation occurs.

2. The method for exciting the reverse Cherenkov radiation of polaritons according to claim 1, characterized in that, The chip substrate material in S1 is one of SiO 2 , Si, Au, Ag, Cu, Ni, lithium niobate or glass.

3. The method for exciting the reverse Cherenkov radiation of polaritons according to claim 1, characterized in that, The hyperbolic polariton material in S1 has negative group velocity dispersion, such as MoO 3 , hBN, graphene, Bi 2 Se 3 , Bi 2 Te 3 , WS 2 , TaS 2 , TiS 2 , WSe 2 , TaSe 2 , TiSe 2 , SnS 2 , HfS 2 , or PdS 2 , with a thickness of 0 - 5000 nm.

4. The method for exciting the reverse Cherenkov radiation of polaritons according to claim 1, characterized in that, The hyperbolic polariton heterojunction in S1 is constructed by a hyperbolic polariton material and a medium; The medium is one of graphene, h-BN, WS 2 , MoS 2 , HfO 2 , SiO 2 , Al 2 O 3 , CaF 2 or MgF 2 ; The thickness of the medium is 0 - 100 nm.

5. The method for exciting the reverse Cherenkov radiation of polaritons according to claim 1, characterized in that, The analogous moving charged particles are one of tunneling electrons, electron beams or antennas with polariton responses.

6. The method for exciting the reverse Cherenkov radiation of polaritons according to claim 5, characterized in that, The antennas with polariton responses are one of Au nanowires, Ag nanowires, BN nanoribbons, BN nanotubes, carbon nanotubes, graphene nanoribbons, MoO 3 nanowires or MoO 3 nanoribbons.

7. The method for exciting the reverse Cherenkov radiation of polaritons according to any one of claims 1 - 6, characterized in that, By changing the angle between the moving charged particles or the antenna with polariton response and the hyperbolic polariton material, the symmetry of the reverse Cherenkov radiation is changed; by changing the type or thickness of the medium between the moving charged particles and the hyperbolic polariton material, the radiation angle and radiation efficiency of the reverse Cherenkov radiation are changed, thereby optimizing the performance of the reverse Cherenkov radiation.

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