All-dielectric sandwich type terahertz broadband achromatic superlens and preparation method thereof

Abstract

The application discloses a full dielectric sandwich type terahertz broadband achromatic superlens and a preparation method thereof, and the superlens comprises a full dielectric sandwich type unit structure, wherein the unit structure comprises a middle fused quartz substrate and high-resistance silicon column microstructures located on both sides of the fused quartz substrate; based on a focusing formula of the superlens and a corresponding relationship between a library phase of the unit structure and geometric parameters, the size of the unit structure selected at each position is determined, the selected unit structure is placed into a corresponding spatial position, and the above steps are repeated to construct a complete superlens. The application realizes a propagation phase range which is twice that of a conventional single-layer full dielectric unit structure, has more abundant dispersion control capacity, and thus realizes a terahertz achromatic superlens which has a larger bandwidth and a numerical aperture compared with known inventions and maintains high focusing efficiency in the whole frequency band.

Description

Technical Field

[0001] This invention relates to the field of terahertz metasurfaces, and more particularly to an all-dielectric sandwich-type terahertz broadband achromatic superlens and its preparation method. Background Technology

[0002] Terahertz (THz) generally refers to electromagnetic waves in the band between millimeter waves (0.1 THz) and far-infrared waves (10 THz). With the rapid development of terahertz science and technology, the research fields of terahertz, such as sensing, imaging, and communication, have placed higher demands on the corresponding functional devices.

[0003] Metasurfaces, as two-dimensional arrays composed of subwavelength artificial microstructures, are increasingly demonstrating their potential to replace or even surpass traditional functional devices due to their compact structural characteristics and flexible electromagnetic control capabilities. Ordinary terahertz lenses require varying the thickness of the raw material at different light ray positions to achieve phase modulation through optical path accumulation, which is typically quite bulky. However, metalenses primarily create a phase gradient at the interface by generating different phase abrupt changes through different unit cell structures, thereby altering the shape of the outgoing wavefront. However, the strong modal dispersion of the unit cell structure results in significant chromatic aberration in metalenses, preventing waves of different frequencies from converging at the same focal point, which severely hinders the practical application of metalenses.

[0004] Currently, there are only sporadic studies on achromatic metasurfaces in the terahertz band. These metasurfaces either rely on geometric phase to achieve phase and dispersion control, which results in low overall device efficiency; or they only use phase transmission, but due to the limitations of micro-nano fabrication on the etching aspect ratio of high-resistivity silicon, the range of phase and dispersion control is extremely limited, which has led to a bottleneck in further improving the bandwidth and numerical aperture of the devices.

[0005] In summary, the fundamental problem hindering the development of terahertz achromatic metasurfaces is the lack of unit structures that can achieve large propagation phase and flexible dispersion control with low loss. Summary of the Invention

[0006] This invention provides an all-dielectric sandwich-type terahertz broadband achromatic superlens and its fabrication method. The superlens is constructed from a single-unit structure composed of a silicon-quartz-silicon material system. This single-unit structure effectively expands the propagation phase and dispersion modulation range. Based on this single-unit structure, a terahertz broadband achromatic superlens with superior performance is designed, and a corresponding fabrication method is proposed. The performance of the superlens is characterized in an experimental system, as detailed below:

[0007] A fully dielectric sandwich-type unit structure, the unit structure comprising: a fused silica substrate in the middle and high-resistivity silicon pillar microstructures located on both sides thereof.

[0008] A fully dielectric sandwich-type terahertz broadband achromatic superlens, the superlens comprising: a fully dielectric sandwich-type unit structure,

[0009] Based on the focusing formula of the superlens and the correspondence between the phase and geometric parameters of the unit structure library, the size of the selected unit structure at each location is determined, and the selected unit structure is placed in the corresponding spatial position. By doing so, a complete superlens is constructed.

[0010] The superlens can achieve achromatic focusing with a numerical aperture of 0.47 in the frequency band of 0.5 to 1.1 THz, with an average focusing efficiency of 43.1%.

[0011] A method for fabricating an all-dielectric sandwich-type terahertz broadband achromatic superlens, the method comprising:

[0012] Two high-resistivity silicon wafers and a piece of fused silica of the same size are bonded together with BCB adhesive. The double-polished silicon wafers are then baked in a nitrogen oven to remove surface moisture and stains.

[0013] A preset thickness of BCB adhesive is continuously coated onto the silicon wafer at a preset rotation speed to serve as a bonding layer; the BCB-coated silicon wafer is then preheated on a hot plate.

[0014] A fused silica substrate is sandwiched between two pre-treated silicon wafers. The three wafers are aligned and placed into a wafer bonding machine. A graphite sheet is placed on top of the uppermost silicon wafer. Pressure is applied inside the bonding machine, and the wafers are bonded sequentially under a temperature gradient while maintaining the pressure. After the operating chamber cools down, the bonded wafers are removed.

[0015] A silicon dioxide layer of a predetermined thickness is deposited on the surface of a silicon wafer using chemical vapor deposition as a hard mask. An antioxidant layer of another predetermined thickness is then spin-coated onto the silicon wafer. The wafer is then baked and exposed to ultraviolet light in a photolithography machine, and finally developed in a developing solution.

[0016] The pattern is transferred to a SiO2 hard mask layer by reactive ion etching, and silicon is etched by inductively coupled plasma reactive ion etching using Bosch technology; the patterned SiO2 hard mask is removed by a highly selective buffered oxide etching solution to obtain the first side of the target sample.

[0017] The sample is flipped over so that the alignment mark on the mask is aligned with the alignment mark that has been processed on the first side. The above photolithography, etching and mask removal process is repeated to finally obtain the sample with double-sided microstructure.

[0018] The beneficial effects of the technical solution provided by this invention are:

[0019] (1) This invention provides a novel metasurface unit structure system based on silicon-quartz-silicon, which achieves a propagation phase range twice that of the traditional single-layer all-dielectric unit structure and has richer dispersion control capabilities.

[0020] (2) The present invention utilizes the above unit structure to design a superlens that can achieve achromatic focusing with a numerical aperture of 0.47 in the range of 0.5 to 1.1 THz, and its average focusing efficiency can reach 43.1%.

[0021] (3) The present invention provides a benzocyclobutene (BCB)-assisted wafer bonding and a preparation scheme combining traditional photolithography and dry deep silicon etching techniques, making the practical application of the above-mentioned superlens possible. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the unit structure;

[0023] Figure 2A This is a schematic diagram of the phase spectrum and amplitude spectrum of the unit structure;

[0024] Figure 2B A cross-sectional view of the normalized magnetic field energy distribution of a unit structure;

[0025] Figure 3 A schematic diagram of a continuous broadband achromatic superlens;

[0026] Figure 4 A flowchart for sample preparation;

[0027] Figure 5A This is an optical microscope image of the sample;

[0028] Figure 5B This is a partial scanning electron microscope image of the sample.

[0029] Figure 6 The experimental results for the achromatic superlens are shown in the figure.

[0030] Figure 7 This is a schematic diagram of the focusing efficiency curve of an achromatic superlens. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below.

[0032] The terminology used in this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0033] The term "phase" as used in this invention refers to the transmission phase generated by the unit cell structure at the minimum frequency within the target frequency band. The term "dispersion" as used in this invention refers to the difference between the transmission phases generated by the unit cell structure at the maximum and minimum frequencies within the target frequency band.

[0034] It should also be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0035] The all-dielectric sandwich-type unit structure provided in the embodiments of the present invention is as follows: Figure 1 As shown, the unit structure of this system includes a central fused silica substrate (JGS3) and high-resistivity silicon pillar microstructures (Si) on both sides. In the terahertz band, the measured refractive index of the fused silica JGS3 is approximately between 1.92 and 1.96, fluctuating slightly with frequency, while the refractive index of silicon (Si) is 3.45. This embodiment of the invention does not limit the above values; it is merely used as an example for illustration.

[0036] For the fused silica substrate, its function is to provide mechanical support for the microstructure on it. Its thickness is designed to be T = 200 μm. This distance makes the wavefront deformation of the first metasurface negligible and is also sufficient to effectively avoid coupling between the two metasurfaces.

[0037] For the silicon pillars on both sides, considering that the etching depth of high-resistivity silicon is typically required to be less than 300 μm and the aspect ratio is usually less than 10:1 in the terahertz band, the height of the silicon pillars on both sides is the etching depth required during fabrication. Therefore, when establishing the unit structure model, the heights H1 and H2 of the silicon pillars on both sides are both set to 250 μm. Since there are microstructures on both sides of the substrate, it is equivalent to doubling the equivalent height of the unit structure. According to waveguide principles, the range of phase and dispersion is also doubled as a result.

[0038] Furthermore, the square cross-sectional shape of the unit structure exhibits C4 symmetry, ensuring the metasurface's polarization-independent properties. Since the dimensions of the silicon pillars on both sides can be adjusted independently, the design freedom of the unit structure increases from n to [value missing]. By performing parameter scanning on the side lengths W1 and W2 of the silicon pillars respectively, a unit structure library corresponding to different amplitudes and phase responses was constructed, that is, a number of unit structures constituted the unit structure library.

[0039] Considering the characteristics of the sandwich structure, the period of the unit structure needs to be subwavelength for the minimum equivalent wavelength of the target frequency band in fused silica, while also ensuring that the side length has a sufficient range of variation. Therefore, the period P is set to 80μm.

[0040] The transmission spectrum of the unit cell structure was calculated using CST Microwave Studio simulation software (a technique well-known in the art). The incident wave was x-polarized, with periodic boundary conditions in the x and y directions and a perfect absorbing wall (open) boundary condition in the z direction. Its amplitude and phase spectra are as follows: Figure 2A As shown, this indicates that the unit structure has high and relatively smooth transmittance within the target frequency band, and the phase and frequency are basically linearly related. Therefore, the phase dispersion is constant, and its value is the slope of the phase-frequency graph. Within a certain target frequency band, the dispersion can be equivalently described by the phase difference corresponding to the maximum and minimum frequencies. The normalized magnetic field energy distribution of the unit structure in the frequency range of 0.5–1.1 THz is shown below. Figure 2B As shown, it can be intuitively seen that the electromagnetic characteristics of this unit structure can well satisfy the waveguide model.

[0041] Furthermore, based on this unit structure, the design of the achromatic superlens in this embodiment of the invention mainly adopts the following steps S1 to S3.

[0042] Step S1: Determine the scope of the unit structure library;

[0043] In this embodiment of the invention, the transmission phase range of the all-dielectric sandwich-type unit structure library composed of silicon-quartz-silicon material system is 478.05°, and the dispersion range is 472.26°.

[0044] Step S2: Determine the structural parameters of the superlens;

[0045] Based on this structure library, and considering bandwidth and numerical aperture, the parameters of the achromatic superlens provided in this embodiment of the invention are set as follows: diameter D = 6.96 mm, focal length f = 10 mm, and designed operating frequency band of 0.6–1.0 THz. A schematic diagram is shown below. Figure 3 As shown.

[0046] Step S3: Determine the unit structure selected at each location of the superlens;

[0047] The focusing formula for the superlens is:

[0048]

[0049] Where c represents the speed of light in a vacuum, and ω is the angular frequency. Let f be the distance from the unit structure located at coordinates (x, y) on the lens plane to the center of the lens, and let f be the focal length of the lens. The reference phase refers to the phase delay generated by the unit structure located at the center of the lens.

[0050] Since a spatial location is a pair of (x,y) coordinates, the most suitable unit structure is selected according to the phase formula, and then the unit structure of this size is placed at (x,y). By doing so, a complete superlens can be constructed.

[0051] Two silicon wafers and a quartz wafer are bonded together using a benzocyclobutene (BCB)-assisted wafer bonding process to form a "sandwich" structure. Then, a metasurface composed of silicon pillar microstructures is fabricated on the silicon wafers on both sides of the quartz substrate using a photolithography process combined with dry deep silicon etching technology. Finally, the photoresist and hard mask on the surface are removed to obtain the final sample.

[0052] Furthermore, the sample was characterized using a terahertz near / far-field time-domain spectroscopy system, and the electric field intensity distribution on the propagation surface and focal plane of the superlens was obtained by scanning, thereby experimentally verifying the design method and fabrication process.

[0053] Furthermore, this embodiment of the invention also provides a method for fabricating a metasurface based on a silicon-quartz-silicon unit structure for processing the aforementioned broadband terahertz achromatic superlens, the method comprising at least the following steps S1 to S4.

[0054] Step S1: Wafer bonding;

[0055] Two 4-inch diameter, 250μm thick high-resistivity silicon wafers and a 200μm thick fused silica wafer of the same size are bonded together using BCB adhesive with a relative permittivity of 2.67 and a loss tangent of 0.012. Before spin-coating the BCB, the double-polished silicon wafers are baked in a nitrogen oven at 200°C for 12 minutes to remove surface moisture and contaminants. Then, a 5μm thick layer of BCB adhesive is spin-coated onto the silicon wafer at 4000rpm for one minute to serve as the bonding layer. Next, the BCB-coated silicon wafer is preheated on a hot plate at 100°C for 100 seconds. The above steps are repeated for the other silicon wafer. A fused silica substrate is then sandwiched between the two pre-treated silicon wafers, and all three are aligned and placed in the CB6L wafer bonding machine. To prevent adhesive splattering during bonding, a graphite sheet is placed on top of the uppermost silicon wafer. A pressure of 0.4 MPa was applied inside the bonding machine, and bonding was performed sequentially at temperature gradients of 100℃ for 10 min, 150℃ for 10 min, 200℃ for 10 min, and 250℃ for 60 min. The pressurized conditions were maintained, and the bonded wafers were removed after the operating chamber cooled down.

[0056] Step S2: Photolithography;

[0057] First, a 3μm thick silicon dioxide layer is deposited on the silicon wafer surface as a hard mask using chemical vapor deposition (CVD). Then, a 3μm thick AZ4620 antioxidant layer is spin-coated on it and baked at 120°C for 120 seconds. Next, it is exposed to ultraviolet light in a SUSS MA4 lithography machine and developed in AZ400K developer for 18 seconds. This process transfers the sample pattern from the mask onto the photoresist.

[0058] This embodiment of the invention uses the SUSS MA4 lithography machine as an example for illustration. In specific implementation, this embodiment of the invention does not impose any limitations on this.

[0059] Step S3: Etching;

[0060] First, the pattern is transferred to a SiO2 hard mask layer using reactive ion etching (RIE). Then, the silicon is etched using a Bosch inductively coupled plasma reactive ion etching (ICP-RIE) system. In each reaction cycle, SF6 gas is used for 4 seconds of surface etching, followed by 2.2 seconds of passivation of the silicon sidewalls using C4F8 gas. After removing the patterned SiO2 hard mask with a highly selective buffered oxide etching (BOE) solution, the first surface of the target sample is obtained.

[0061] Step S4: Double-sided processing;

[0062] First, the sample is flipped over, with the unetched side facing up and the etched side facing down. However, the structure is not damaged due to the protection of the surrounding silicon wafer. Next, the alignment mark on the mask corresponding to this side is aligned with the alignment mark already processed on the first side (i.e., on the sample pattern in the photolithography step S2). Then, steps S2 and S3 are repeated to obtain the final sample with a double-sided microstructure.

[0063] Figure 5A and Figure 5B The images shown are actual images of the superlens prepared according to the processing method provided in the embodiments of the present invention under an optical microscope and a scanning electron microscope.

[0064] Furthermore, the performance of the superlens in the embodiments of the present invention is characterized using a fiber-optic terahertz near / far-field time-domain spectroscopy system. Figure 6 The figures shown are experimental results of the terahertz achromatic superlens in this embodiment of the invention, including the normalized intensity distribution map on the propagation plane (top), where the dashed line represents the position of the focal plane, and the normalized intensity map on the focal plane (middle) and the corresponding intensity distribution curve along the x-axis (bottom). Figure 6As can be seen, terahertz waves from 0.5 to 1.1 THz converge approximately to the same focal point, indicating that the superlens achieves good achromatic focusing performance over a wide bandwidth. The average focal length is 6.5 mm, thus yielding an actual numerical aperture of 0.47. Furthermore, the maximum focal length shift is 0.67 mm within the target frequency band of 0.6–1.0 THz, and only 0.69 mm within the observation frequency band of 0.5–1.1 THz, far less than the depth of focus (DOF = λ / NA). 2 Meanwhile, the focal spots of different frequencies all have good spot shapes and Gaussian intensity distributions, and the half-width at half maximum (FWHM) of the focal spot decreases with increasing frequency, which is consistent with the general performance of a lens. Figure 7 This is the focusing efficiency curve of the superlens. In this embodiment of the invention, the focusing efficiency is defined as the sum of the powers on the focal plane of the superlens divided by the power of a lens with the same area (3.5 × 3.5 mm). 2 The ratio of the sum of incident light power within a certain frequency band is used. Measurements show that the highest efficiency is achieved at 1.05 THz, reaching 54.2%, with an average of 43.1% across the entire observation band. This indicates that the superlens has very high focusing efficiency.

[0065] Unless otherwise specified, the model numbers of the various devices in this embodiment of the invention are not limited, and any device that can perform the above functions is acceptable.

[0066] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of a preferred embodiment, and the sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0067] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for fabricating an all-dielectric sandwich-type terahertz broadband achromatic superlens, wherein the all-dielectric sandwich-type terahertz broadband achromatic superlens comprises: A unit structure, comprising: a central fused silica substrate and high-resistivity silicon pillar microstructures on either side thereof; based on the focusing formula of a superlens and the correspondence between the phase and geometric parameters of the unit structure library, the size of the selected unit structure at each location is determined, and the selected unit structure is placed in the corresponding spatial position, and so on, to construct a complete superlens, characterized in that the fabrication method of the superlens includes: Two high-resistivity silicon wafers and a piece of fused silica of the same size are bonded together with BCB adhesive. The double-polished silicon wafers are then baked in a nitrogen oven to remove surface moisture and stains. A preset thickness of BCB adhesive is continuously coated onto the silicon wafer at a preset rotation speed to serve as a bonding layer; the BCB-coated silicon wafer is then preheated on a hot plate. A fused silica substrate is sandwiched between two pre-treated silicon wafers. The three wafers are aligned and placed into a wafer bonding machine. A graphite sheet is placed on top of the uppermost silicon wafer. Pressure is applied inside the bonding machine, and the wafers are bonded sequentially under a temperature gradient while maintaining the pressure. After the operating chamber cools down, the bonded wafers are removed. A silicon dioxide layer of a predetermined thickness is deposited on the surface of a silicon wafer using chemical vapor deposition as a hard mask. An antioxidant layer of another predetermined thickness is then spin-coated onto the silicon wafer. The wafer is then baked and exposed to ultraviolet light in a photolithography machine, and finally developed in a developing solution. The pattern is transferred to a SiO2 hard mask layer by reactive ion etching, and silicon is etched by inductively coupled plasma reactive ion etching using Bosch technology; the patterned SiO2 hard mask is removed by a highly selective buffered oxide etching solution to obtain the first side of the target sample. The sample is flipped over so that the alignment mark on the mask is aligned with the alignment mark that has been processed on the first side. The above photolithography, etching and mask removal process is repeated to finally obtain the sample with double-sided microstructure.

2. The method for fabricating an all-dielectric sandwich-type terahertz broadband achromatic superlens according to claim 1, characterized in that, The superlens can achieve achromatic focusing with a numerical aperture of 0.47 in the frequency band of 0.5~1.1 THz, with an average focusing efficiency of 43.1%.

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