Sub-wavelength polariton focusing lens and photoetching-free manufacturing method and application thereof

Abstract

The invention discloses a sub-wavelength polariton focusing lens and a photoetching-free manufacturing method and application thereof. The sub-wavelength polariton focusing lens comprises a substrate layer, a polariton layer arranged on the substrate layer, and a lens layer which is arranged on the polariton layer and has a gradually changing thickness, the polariton layer is used for exciting and bearing polaritons and compressing a light field to a nanoscale; the lens layer is used for adjusting the refractive index around the polariton layer, so that the propagation path of electromagnetic waves is adjusted, and light convergence is achieved. Dielectric environment engineering is utilized, polymer microspheres are controlled to be melted and converted into spherical caps, the dielectric thickness is gradually changed, and polaritons are regulated and controlled. The method has the advantages of high cost benefit and high efficiency, is suitable for wide in-plane isotropic polariton modes, and provides huge assistance for propelling polarization devices and systems.

Description

Technical Field

[0001] This invention belongs to the field of polariton lens technology, specifically relating to a subwavelength polariton focusing lens and its photolithography-free fabrication method and application. Background Technology

[0002] The maturation of technologies such as metasurfaces, nanomaterials, and anisotropic media has greatly promoted the development of photonic chips. Simultaneously, fields such as micro-devices and integrated photonic circuits are gradually demonstrating enormous potential, with the manipulation of light fields primarily trending towards the subwavelength range, laying a solid foundation for subwavelength electromagnetics and micro / nano photonics. Through research on confined light field manipulation, novel optical physics mechanisms at the subwavelength scale are being gradually revealed, showing promising applications in super-resolution focusing imaging, high-precision biochemical sensing, and photonic chip integration.

[0003] Polaritons are hybrid modes of photon-material excitation coupling that provide a powerful platform for manipulating light beyond the diffraction limit by confining light to a surface or interface. This strong confinement typically extends hundreds of nanometers out of the plane, making polaritons highly effective in on-chip photonics applications compatible with integrated circuits. One of the most notable applications of polaritons is the plane polarizing lens, which can focus light beams with subwavelength resolution.

[0004] Existing plane polarizing lenses are developed through polaron interference or refraction. Their working principle is to shape the polariton wavefront through the impedance interface, thereby creating a gradient refractive index lens. ([1]Duan, J. et al. Planarrefraction and lensing of highly confined polaritons in anisotropic media.Nat. Commun. 12, 4325 (2021). [2]Hu, H. et al. Doping-driven topological polaritons in graphene / a-MoO3 heterostructures. Nat. Nanotechnol. 17, 940-946 (2022). [3]Hu, H. et al. Gate-tunable negative refraction of mid-infrared polaritons. Science 379, 558-561 (2023).

[0005] However, creating gradient refractive index polariton lenses remains a significant challenge due to the lack of practical and cost-effective manufacturing methods. Summary of the Invention

[0006] To address the aforementioned shortcomings in existing technologies, this invention provides a subwavelength polariton focusing lens, its photolithography-free fabrication method, and its applications. This method leverages the combined effects of polymer melt flow under gravity at high temperatures and material evaporation, utilizing a spherical cap model to fit the polariton focusing lens configuration and achieve a subwavelength-scale focusing effect.

[0007] The present invention first provides a subwavelength polariton focusing lens, comprising a substrate layer, a polariton layer disposed on the substrate layer, and a lens layer disposed on the polariton layer having a gradually varying thickness. The polariton layer is used to excite and support polaritons, compressing the optical field to the nanoscale. The lens layer is used to adjust the refractive index around the polariton layer, thereby adjusting the propagation path of electromagnetic waves and achieving light convergence.

[0008] Preferably, the substrate layer is made of an inorganic dielectric material; The polariton layer is made of a van der Waals semiconductor material or a metal material capable of exciting polaritons. The lens layer is made of a plastic polymer.

[0009] More preferably, the substrate layer is made of at least one of silicon dioxide, calcium fluoride, barium fluoride, and high-resistivity silicon. The polariton layer is made of at least one of hexagonal boron nitride, graphene, gold, and silver. The lens layer is made of at least one of polymethyl methacrylate, polystyrene, and polyvinylidene fluoride.

[0010] Preferably, the thickness of the polariton layer is 0.34 nm-1 μm; the maximum thickness of the lens layer is 1 nm-2 μm.

[0011] More preferably, the thickness of the polariton layer is 60 nm-89 nm; and the maximum thickness of the lens layer is 18 nm-122 nm.

[0012] Preferably, the substrate layer and the polariton layer are polar crystals, and the dielectric constant is fitted using the Lorentz model: ; in, ε ( ω ) is the dielectric constant of the material. ω It's frequency. ε ∞is the high-frequency dielectric constant, j is the Reststrahlen band number within the considered frequency range, j takes values ​​from 1 to n, and n is the index of the last term in the multiplication sign where j is taken. ω LO,j and ω TO,j These refer to the longitudinal and transverse optical phonon frequencies of the j-th term, γ, respectively. j Let i represent the damping coefficient of the j-th term, where i is the imaginary unit; The lens layer is a non-polar crystal with a fitted dielectric constant: ; in, F It is the intensity of the oscillation. ω ν γ represents the vibration frequency, and γ represents the damping coefficient.

[0013] This invention further provides a photolithography-free fabrication method for the subwavelength polariton focusing lens, comprising the following steps: (1) Preparation of the substrate layer; (2) Prepare a polariton layer on the substrate; (3) Drop the microsphere solution for preparing the lens layer onto the polariton layer, heat it to melt it, and then cool it to obtain a lens layer with a gradually varying thickness.

[0014] Preferably, in step (2), the material for preparing the polariton layer is transferred onto the substrate layer to obtain the polariton layer.

[0015] The present invention also provides the application of the subwavelength polariton focusing lens in the fabrication of Luneburg lenses, Maxwell fisheye lenses or Mikaelian lenses.

[0016] Beneficial effects of this invention: This invention relates to a subwavelength polariton focusing lens comprising, from bottom to top, a substrate layer, a polariton layer, and a lens layer with a gradually varying thickness. The invention utilizes dielectric environment engineering, in which polymer microspheres are controlled to melt and transform into spherical caps, resulting in a gradual change in dielectric thickness, thereby achieving control over the polaritons. This process can fabricate polarization Luneburg lenses, Maxwell fisheye lenses, Mikaelian lenses, and other gradient refractive index lenses with tunable focusing characteristics. This method is cost-effective, efficient, and applicable to a wide range of in-plane isotropic polariton modes, providing significant assistance in advancing polarization devices and systems. Attached Figure Description

[0017] Figure 1 These are a longitudinal cross-sectional view (left) and a top view (right) of the subwavelength polariton focusing lens described in this invention.

[0018] Figure 2 This is a flowchart illustrating the fabrication process of the subwavelength polariton focusing lens described in this invention.

[0019] Figure 3 This is a three-dimensional schematic diagram of the morphological changes of microspheres obtained from multiple tests during the implementation of this invention, and an example of actual combustion test of PMMA microspheres. Figure 3 (a) Two-dimensional cross-sectional view of microsphere morphology changes ( Figure 3 (b) in the middle.

[0020] Figure 4 The graph shows the relationship between the change in the radius ratio of PMMA microspheres and the combustion time, and the graph shows the morphological changes of PMMA at different time points observed under an optical microscope.

[0021] Figure 5 This is a longitudinal cross-sectional view (left) and an experimental test diagram (right) of the device described in Embodiment 1 of the present invention.

[0022] Figure 6 This is a longitudinal cross-sectional view (left) and an experimental test diagram (right) of the device described in Embodiment 2 of the present invention.

[0023] Figure 7 The diagram shows a longitudinal cross-sectional view (left) and a simulated top view of the electric field distribution of the device described in Embodiment 3 of the present invention (right).

[0024] Figure 8 The diagram shows a longitudinal cross-sectional view (left) and a simulated top view of the electric field distribution of the device described in Embodiment 4 of the present invention (right).

[0025] Figure 9 The images show a three-dimensional schematic diagram (top), a longitudinal cross-sectional schematic diagram (bottom left), and a top view of the electric field distribution simulation (bottom right) of the device described in Embodiment 5 of the present invention. Detailed Implementation

[0026] The purpose of this invention is to provide a photolithography-free fabrication method for subwavelength polariton focusing lenses, which enables the fabrication of subwavelength on-chip focusing lenses in a simplified manner.

[0027] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0028] like Figure 1As shown, the subwavelength polariton focusing lens provided by this invention includes a substrate layer 1, a polariton layer 2, and a lens layer 3 with a gradually varying thickness, arranged sequentially from bottom to top. The upper surface of the subwavelength polariton focusing lens is in contact with air. Taking a Luneburg lens as an example, to achieve a perfect Luneburg lens effect, the thickness variation of the lens layer 3 has a theoretical relationship with the thickness of the polariton layer 2, the dielectric constant of the polariton layer 2, the dielectric constant of the substrate layer 1, the dielectric constant of the polymer material, and the incident light frequency. The microsphere melting method can well satisfy this variation trend. The lens layer with varying thickness can change the dielectric environment around the polariton layer, thereby changing the behavior mechanism of the polaritons and achieving a subwavelength-scale focusing effect.

[0029] Specifically, this invention provides a photolithography-free fabrication method for a subwavelength polariton focusing lens. By constructing a lens layer with gradually varying thickness and controlling the dielectric environment around the polariton layer, a subwavelength-scale focusing effect is achieved, and the focal length can be adjusted by changing the incident light frequency. The thickness variation obtained by this method matches the thickness variation of a standard lens.

[0030] The methods for exciting polaritons are: directly exciting polaritons using the scattered light from the tip of a mid-infrared scattering scanning near-field optical microscope, or exciting polaritons by irradiating the antenna with incident infrared light and then resonating the antenna.

[0031] The substrate material in this invention is an inorganic dielectric material. The polariton layer material is hexagonal boron nitride (hBN), graphene, gold, silver, or other metallic materials or van der Waals semiconductor materials. The polariton layer material itself is required to have in-plane isotropy and thermal stability; hexagonal boron nitride is preferred. The lens layer is a plastic polymer. The thickness of the polariton layer is 0.34 nm–1 μm. The maximum thickness of the lens layer is 1 nm–2 μm.

[0032] Polar crystals refer to the polariton layer and the substrate layer, while non-polar crystals refer to the lens layer. The material parameters used in other embodiments are approximate parameters.

[0033] For polar crystals, such as hexagonal boron nitride and silicon dioxide, we use the following Lorentz model to fit their dielectric constant:

[0034] in, ε ( ω ) is the dielectric constant of the material. ω It's frequency. ε ∞It is the high-frequency dielectric constant. j is the Reststrahlen band number within the considered frequency range, with j ranging from 1 to n, where n is the index of the last term of j in the multiplication sign. ω LO,j and ω TO,j These refer to the longitudinal (LO) and transverse (TO) optical phonon frequencies of the j-th term, respectively, γ j This represents the damping coefficient of the j-th term. i is the imaginary unit.

[0035] For nonpolar crystals, such as polymethyl methacrylate, we fit their dielectric constant using the following formula:

[0036] in, F It is the intensity of the oscillation. ω ν γ represents the vibration frequency, and γ represents the damping coefficient. i is the imaginary unit.

[0037] The specific embodiments of this application will now be described in detail with reference to the accompanying drawings.

[0038] Example 1

[0039] like Figure 5 The image shows a longitudinal cross-sectional schematic diagram and experimental test diagram of a subwavelength polariton focusing lens.

[0040] The substrate layer 1 used is silicon dioxide, the polariton layer 2 is hexagonal boron nitride, and the lens layer 3 with a gradient thickness is polymethyl methacrylate (PMMA).

[0041] Reference Figure 2 The flowchart shown is a photolithography-free fabrication method for a subwavelength polariton focusing lens according to the present invention. Figure 5 The specific steps of the fabrication method of the subwavelength polariton focusing lens shown are as follows: Step 1: Select silicon dioxide as the substrate material and prepare substrate 1.

[0042] Step 2: Fabrication of Polariton Layer 2. Using tweezers, place the hexagonal boron nitride crystal onto Scotch tape, folding it repeatedly until the three-dimensional granular crystal spreads into a two-dimensional planar shape on the tape. Secure the spread tape to a glass slide for subsequent operations. Fix the cleaned silicon wafer (substrate 1) onto the glass slide, then press the silicon wafer and tape together for several tens of seconds before separating. At this point, the two-dimensional hexagonal boron nitride has been transferred onto the silicon wafer, completing the fabrication of Polariton Layer 2. When selecting the location of the hexagonal boron nitride, it is necessary to ensure that its size can support several wavelengths and that it does not have material corners or other shapes that would cause reflection. Therefore, there are certain requirements for the minimum planar geometry; generally, a planar material with a side length of at least 10 µm is selected. In this embodiment, the thickness of the polariton layer is 89 nm.

[0043] Step 3: Prepare the polymer microsphere solution. Using a dropper, add one drop of a 50 mg / ml polymethyl methacrylate microsphere solution (water as solvent) to 40 ml of isopropanol. Then, stir using an ultrasonic oscillator for 10 min to obtain a microsphere solution of suitable concentration. Use a dropper to add one drop of the solution to a silicon wafer that has been transferred with hexagonal boron nitride material. After allowing it to air dry, observe it. If microspheres are located at the edge of the hexagonal boron nitride material, it can be used as a backup. If not, repeat the process until microspheres land in suitable positions. Note that if too much solution is added to the same silicon wafer, a large area of ​​microsphere aggregation will occur. In this case, a new silicon wafer with hexagonal boron nitride crystals should be used.

[0044] Step 4, fabricate lens layer 3 with a gradient thickness. According to... Figure 3 , Figure 4 A schematic diagram of the microsphere morphology change is shown. Under certain material parameters, the thickness change consistent with theoretical calculations can generally be obtained in about two hours. Taking the Luneburg lens as an example, since its effect is to focus light on the other side of the lens, achieving this effect only requires the lens thickness change to conform to the theoretical calculations, without strict requirements on the specific planar geometry of the lens. To reduce the impact of losses in the polariton layer material itself, and to ensure that the microspheres are easy to shape, we selected PMMA microspheres with an initial diameter of 1 μm for processing. After obtaining a suitable initial structure, the entire structure was placed in a tube furnace and treated at 250 °C in an air atmosphere for 2 hours, and then removed and allowed to cool naturally. The device fabrication is thus complete. The maximum thickness of the lens layer obtained in this embodiment is 95 nm.

[0045] The material parameters used in this embodiment are as follows, where xyz are the crystal axis directions:

[0046] Near-field optical imaging was performed using an s-SNOM system. A PtIr-coated atomic force microscope tip was used as a scattering near-field probe, allowing simultaneous acquisition of sample morphology during imaging. During the s-SNOM scan, the tip oscillated vertically with an amplitude of approximately 60 nm and a frequency of approximately 273 kHz. Illumination was provided by a wavelength-tunable continuous-wave quantum cascade laser. The backscattered signal was recorded using a pseudo-heterodyne Michelson interferometer and demodulated at the second harmonic of the tip oscillation frequency. The complex-valued near-field signal (…) σ Using equations σ = se iφ Based on the recorded amplitude ( s ) and phase signal ( φ The calculations were performed. To observe obvious experimental phenomena, the distance was 1500 cm. −1 Testing at a certain frequency reveals a noticeable stripe bending phenomenon, with a focal point formed at a certain distance outside the sphere, demonstrating that the device has a good polariton modulation effect and can achieve subwavelength on-chip focusing effect.

[0047] Example 2

[0048] like Figure 6 The image shows a longitudinal cross-sectional schematic diagram and experimental test diagram of a subwavelength polariton focusing lens.

[0049] The substrate layer 1 used is silicon dioxide, the polariton layer 2 is hexagonal boron nitride, and the lens layer 3 with a gradually varying thickness is polystyrene (PS). ε PS = 2.4025).

[0050] Reference Figure 2 The flowchart shown is a photolithography-free fabrication method for a subwavelength polariton focusing lens according to the present invention. Figure 6 The specific steps of the fabrication method of the subwavelength polariton focusing lens shown are as follows: Step 1: Select silicon dioxide as the substrate material and prepare substrate 1.

[0051] Step 2: Fabrication of Polariton Layer 2. Using tweezers, place the hexagonal boron nitride crystal onto Scotch tape, folding it repeatedly until the three-dimensional granular crystal spreads into a two-dimensional planar shape on the tape. Secure the spread tape to a glass slide for subsequent operations. Fix the cleaned silicon wafer (substrate 1) onto the glass slide, then press the silicon wafer and tape together for several tens of seconds before separating. At this point, the two-dimensional hexagonal boron nitride has been transferred onto the silicon wafer, completing the fabrication of Polariton Layer 2. When selecting the location of the hexagonal boron nitride, it is necessary to ensure that its size can support several wavelengths and that it does not have material corners or other shapes that would cause reflection. Therefore, there are certain requirements for the minimum planar geometry; generally, a planar material with a side length of at least 10 µm is selected. In this embodiment, the thickness of the polariton layer is 60 nm.

[0052] Step 3: Prepare the polymer microsphere solution. Using a dropper, add one drop of a 50 mg / ml polystyrene microsphere solution (water as solvent) to 40 ml of isopropanol. Then, stir using an ultrasonic oscillator for 10 min to obtain a microsphere solution of suitable concentration. Use a dropper to add one drop of the solution to a silicon wafer that has been transferred with hexagonal boron nitride material. After allowing it to air dry, observe it. If microspheres are located at the edge of the hexagonal boron nitride material, it can be used as a backup. If not, repeat the process until microspheres are found in suitable positions. Note that if too much solution is added to the same silicon wafer, a large area of ​​microsphere aggregation will occur. In this case, a new silicon wafer with hexagonal boron nitride crystals should be used.

[0053] Step 4, fabricate lens layer 3 with a gradient thickness. According to... Figure 3 , Figure 4 A schematic diagram of the microsphere morphology changes is shown. Since PS has a higher dielectric constant than PMMA, achieving the same effect requires a longer high-temperature treatment time. To reduce the impact of losses in the polariton layer material itself, and to ensure the microspheres are easy to shape, we selected PS microspheres with an initial diameter of 1 μm for processing. After obtaining a suitable structure, the entire structure was placed in a tube furnace and treated at 250 °C in an air atmosphere for 2 h 45 min, followed by natural cooling. The device fabrication was thus completed. The maximum thickness of the lens layer obtained in this embodiment is 122 nm.

[0054] The material parameters used in this embodiment are as follows:

[0055] Near-field optical imaging was performed using an s-SNOM system. A PtIr-coated atomic force microscope tip was used as a scattering near-field probe, allowing simultaneous acquisition of sample morphology during imaging. During the s-SNOM scan, the tip oscillated vertically with an amplitude of approximately 60 nm and a frequency of approximately 273 kHz. Illumination was provided by a wavelength-tunable continuous-wave quantum cascade laser. The backscattered signal was recorded using a pseudo-heterodyne Michelson interferometer and demodulated at the second harmonic of the tip oscillation frequency. The complex-valued near-field signal (…) σ Using equations σ = se iφ Based on the recorded amplitude ( s ) and phase signal ( φ This was calculated at 1445 cm. −1 Testing at a certain frequency reveals a clear stripe bending phenomenon, demonstrating that the device has a good polariton modulation effect and can achieve subwavelength on-chip focusing effect.

[0056] Example 3

[0057] like Figure 7 The figure shows a longitudinal cross-sectional schematic diagram and a top view of the simulated electric field distribution of a subwavelength polariton focusing lens.

[0058] The substrate layer 1 used is silicon dioxide, the polariton layer 2 is graphene, and the lens layer 3 with a gradually varying thickness is polymethyl methacrylate (PMMA).

[0059] Reference Figure 2 The flowchart shown is a photolithography-free fabrication method for a subwavelength polariton focusing lens according to the present invention. Figure 7 The specific steps of the fabrication method of the subwavelength polariton focusing lens shown are as follows: Step 1: Select silicon dioxide as the substrate material and prepare substrate 1.

[0060] Step 2: Fabrication of polariton layer 2. Using tweezers, place the graphene onto Scotch tape, folding it repeatedly until the graphene spreads out on the tape. Secure the spread tape to a glass slide for subsequent operations. Fix the cleaned silicon wafer (substrate 1) onto the glass slide, then press the silicon wafer and tape together for several tens of seconds before separating. At this point, the two-dimensional graphene has been transferred onto the silicon wafer, thus fabricating polariton layer 2. This invention uses two-dimensional graphene. Graphene is generally a single-layer structure, so the thickness of polariton layer 2 was not characterized after fabrication.

[0061] Step 3: Prepare the polymer microsphere solution. Using a dropper, add one drop of a 50 mg / ml polymethyl methacrylate microsphere solution (water as solvent) to 40 ml of isopropanol. Then, stir using an ultrasonic oscillator for 10 min to obtain a microsphere solution of suitable concentration. Use a dropper to add one drop of the solution to a silicon wafer that has been transferred with graphene. After allowing it to air dry, observe it. If microspheres are located at the edge of the graphene, it can be used as a backup. If not, repeat the process until microspheres are found in suitable positions. Note that if too much solution is added to the same silicon wafer, large areas of microsphere aggregation will occur. In this case, a new silicon wafer with graphene should be used for the next operation.

[0062] Step 4: Fabricate a lens layer with a gradually varying thickness. After obtaining a suitable structure, place the entire structure in a tube furnace and treat it at 250 °C in an air atmosphere for a certain period of time, then remove it and allow it to cool naturally. At this point, the device fabrication is complete. The maximum thickness of the lens layer used in this embodiment is 18 nm.

[0063] The material parameters used in this embodiment are as follows:

[0064] In COMSOL simulation at a frequency of 1300 cm⁻¹ −1 Simulation tests were conducted under the condition of Fermi level of 0.37 eV, and obvious stripe bending phenomenon was observed, indicating that the device has good polariton modulation effect and can realize subwavelength on-chip focusing effect.

[0065] Example 4

[0066] like Figure 8 The image shows a longitudinal cross-sectional schematic diagram and experimental test diagram of a subwavelength polariton focusing lens.

[0067] The substrate layer 1 used is silicon dioxide, the polariton layer 2 is hexagonal boron nitride, and the lens layer 3 with a gradually varying thickness is polyvinylidene fluoride (PVDF). ε PVDF = 7.5).

[0068] Reference Figure 2 The flowchart shown is a photolithography-free fabrication method for a subwavelength polariton focusing lens according to the present invention. Figure 8 The specific steps of the fabrication method of the subwavelength polariton focusing lens shown are as follows: Step 1: Select silicon dioxide as the substrate material and prepare substrate 1.

[0069] Step 2: Fabrication of Polariton Layer 2. Using tweezers, place the hexagonal boron nitride crystal onto Scotch tape, folding it repeatedly until the three-dimensional granular crystal spreads into a two-dimensional planar shape on the tape. Secure the spread tape to a glass slide for subsequent operations. Fix the cleaned silicon wafer (substrate) onto the glass slide, then press the silicon wafer and tape together for several seconds before separating. At this point, the two-dimensional hexagonal boron nitride has been transferred onto the silicon wafer, completing the fabrication of Polariton Layer 2. When selecting the location of the hexagonal boron nitride, it is necessary to ensure that its size can support several wavelengths and that it does not have sharp corners or other shapes that would cause reflection. Therefore, there are certain requirements for the minimum planar geometry; generally, a planar material with a side length of at least 10 µm is selected. In this embodiment, the thickness of the polariton layer is 60 nm.

[0070] Step 3: Prepare the polymer microsphere solution. Using a dropper, add one drop of a 50 mg / ml polyvinylidene fluoride microsphere solution (water as solvent) to 40 ml of isopropanol. Then, stir using an ultrasonic oscillator for 10 min to obtain a microsphere solution of suitable concentration. Place one drop of the solution onto a silicon wafer that has been transferred with hexagonal boron nitride material. After allowing it to air dry, observe it. If microspheres are located in the center of the hexagonal boron nitride material, it can be used as a backup. If not, repeat the process until microspheres are found in suitable positions. Note that if too much solution is applied to the same silicon wafer, large-area microsphere aggregation will occur. In this case, a new silicon wafer with hexagonal boron nitride crystals should be used.

[0071] Step 4: Fabricate lens layer 3 with a gradient thickness. After obtaining a suitable initial structure, place the entire structure in a tube furnace and treat it at a high temperature of 250 °C in air atmosphere for a certain period of time, then remove it and allow it to cool naturally. At this point, the device fabrication is complete. The maximum thickness of the lens layer used in this embodiment is 91 nm.

[0072] The material parameters used in this embodiment are as follows:

[0073] In COMSOL simulation at 1400 cm −1 Simulation tests of the structure at a certain frequency revealed obvious stripe bending, indicating that the device has good polaron modulation effect and can achieve subwavelength on-chip focusing effect.

[0074] Example 5: like Figure 9 The image shows a three-dimensional schematic diagram, a longitudinal cross-sectional schematic diagram, and an experimental test diagram of a subwavelength polariton focusing lens.

[0075] The substrate layer 1 used is silicon dioxide, the polariton layer 2 is hexagonal boron nitride, and the lens layer 3 with a gradient thickness is polymethyl methacrylate (PMMA).

[0076] Reference Figure 2 The flowchart shown is a photolithography-free fabrication method for a subwavelength polariton focusing lens according to the present invention. Figure 9 The specific steps of the fabrication method of the subwavelength polariton focusing lens shown are as follows: Step 1: Select silicon dioxide as the substrate material and prepare substrate 1.

[0077] Step 2: Fabrication of Polariton Layer 2. Using tweezers, place the hexagonal boron nitride crystal onto Scotch tape, folding it repeatedly until the three-dimensional granular crystal spreads into a two-dimensional planar shape on the tape. Secure the spread tape to a glass slide for subsequent operations. Fix the cleaned silicon wafer (substrate) onto the glass slide, then press the silicon wafer and tape together for several seconds before separating. At this point, the two-dimensional hexagonal boron nitride has been transferred to the silicon wafer, completing the fabrication of Polariton Layer 2. When selecting the location of the hexagonal boron nitride, it is necessary to ensure that its size can support several wavelengths and that it does not have material corners or other shapes that would cause reflection. Therefore, there are certain requirements for the minimum planar geometry; generally, a planar material with a side length of at least 10 µm is selected. In this embodiment, the thickness of the polariton layer is 20 nm.

[0078] Step 3: Prepare the polymer microrod solution. Since the Mikaelian lens phenomenon propagates periodically, its characteristic is reflected in the refractive index distribution across the cross-section. Therefore, using microrods can approximate a structure with the required cross-sectional shape within the propagation range. Using a dropper, add one drop of a 50 mg / ml polymethyl methacrylate microrod solution (water as solvent) to 40 ml of isopropanol, then stir with an ultrasonic oscillator for 10 min to obtain a microrod solution of suitable concentration. Use a dropper to add one drop of the solution to a silicon wafer that has been transferred with hexagonal boron nitride material. After allowing it to air dry, observe. If microrods are located in the center of the hexagonal boron nitride material, they can be used as a backup. If not, repeat the solution application until microrods land in suitable positions. Note that if too much solution is applied to the same silicon wafer, a large area of ​​microrod aggregation will occur. In this case, a new silicon wafer with hexagonal boron nitride crystals needs to be used.

[0079] Step 4: Fabricate lens layer 3 with a gradually varying thickness. After obtaining a suitable initial structure, place the entire structure in a tube furnace and treat it at 250 °C in an air atmosphere for a certain period of time, then remove it and allow it to cool naturally. At this point, the device fabrication is complete. The maximum thickness of the lens layer used in this embodiment is 57 nm.

[0080] The material parameters used in this embodiment are as follows:

[0081] In COMSOL simulation at 1430 cm −1 Simulation tests of the structure at a certain frequency revealed a significant periodic stripe bending phenomenon, indicating that the device has a good polariton modulation effect and can achieve subwavelength on-chip focusing effect.

Claims

1. A subwavelength polariton focusing lens, characterized in that, It includes a substrate layer, a polariton layer disposed on the substrate layer, and a lens layer disposed on the polariton layer with a gradually varying thickness; The polariton layer is used to excite and support polaritons, compressing the optical field to the nanoscale. The lens layer is used to adjust the refractive index around the polariton layer, thereby adjusting the propagation path of electromagnetic waves and achieving light convergence.

2. The subwavelength polariton focusing lens according to claim 1, characterized in that, The substrate layer is made of an inorganic dielectric material; The polariton layer is made of a van der Waals semiconductor material or a metal material capable of exciting polaritons. The lens layer is made of a plastic polymer.

3. The subwavelength polariton focusing lens according to claim 2, characterized in that, The substrate layer is made of at least one of silicon dioxide, calcium fluoride, barium fluoride, and high-resistivity silicon. The polariton layer is made of at least one of hexagonal boron nitride, graphene, gold, and silver. The lens layer is made of at least one of polymethyl methacrylate, polystyrene, and polyvinylidene fluoride.

4. The subwavelength polariton focusing lens according to claim 1, characterized in that, The thickness of the polariton layer is 0.34 nm-1 μm; the maximum thickness of the lens layer is 1 nm-2 μm.

5. The subwavelength polariton focusing lens according to claim 4, characterized in that, The thickness of the polariton layer is 60 nm-89 nm; the maximum thickness of the lens layer is 18 nm-122 nm.

6. The subwavelength polariton focusing lens according to claim 1, characterized in that, The substrate layer and polariton layer are polar crystals, and their dielectric constants are fitted using the Lorentz model. ； in, ε ( ω ) is the dielectric constant of the material. ω It's frequency. ε ∞ is the high-frequency dielectric constant, j is the Reststrahlen band number within the considered frequency range, j takes values ​​from 1 to n, and n is the index of the last term in the multiplication sign where j is taken. ω LO,j and ω TO,j These refer to the longitudinal and transverse optical phonon frequencies of the j-th term, γ, respectively. j Let i represent the damping coefficient of the j-th term, where i is the imaginary unit; The lens layer is a non-polar crystal with a fitted dielectric constant: ； in, F It is the intensity of the oscillation. ω ν γ represents the vibration frequency, and γ represents the damping coefficient.

7. The method for fabricating a subwavelength polariton focusing lens without photolithography according to any one of claims 1-6, characterized in that, Includes the following steps: (1) Preparation of the substrate layer; (2) Fabrication of a polariton layer on a substrate; (3) Drop the microsphere solution for preparing the lens layer onto the polariton layer, heat it to melt it, and then cool it to obtain a lens layer with a gradually varying thickness.

8. The method for fabricating a subwavelength polariton focusing lens without photolithography according to claim 7, characterized in that, In step (3), a microsphere solution of a certain concentration is dropped onto a suitable position of the polariton layer, and after high-temperature treatment for a certain period of time, a lens layer with a gradual thickness is obtained.

9. The use of the subwavelength polariton focusing lens according to any one of claims 1-6 in the fabrication of a Luneburg lens, a Maxwell fisheye lens, or a Mikaelian lens.

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