Chiral planar band continuum bound state metasurface and preparation method

Abstract

The application discloses a chiral quasi-continuous band gap bound state surface and a preparation method thereof, and belongs to the fields of micro-nano photonics and nonlinear optics. The structure is composed of a substrate and a high refractive index medium unit arranged in a rectangular lattice, adopts a chiral cross configuration with C2 rotational symmetry and no in-plane mirror symmetry, and excites chiral quasi-BICs through an asymmetric parameter. Four groups of reverse mode coupling are formed by regulating the lattice period, a near-dispersion-free band is formed, and the quality factor and robustness are improved. The device can realize third harmonic output, and the conversion efficiency reaches 2.3*10-3 under 1 MW / cm2 pumping, and is suitable for micro nonlinear chiral light sources and chiral devices.

Description

Technical Field

[0001] This invention relates to the fields of materials science and nano-optics, specifically to a chiral flat band continuum bound metasurface and its preparation method. Background Technology

[0002] Chirality, stemming from the breaking of mirror or rotational symmetry, is a fundamental property of nature and has significant implications in numerous scientific and technological fields. In optics, chirality arises from the selective interaction between chiral objects and the spin angular momentum of photons, and is quantitatively characterized by circular dichroism (CD) spectroscopy. Typically, natural materials exhibit weak CD responses due to the scale mismatch between chiral molecules and light wavelengths. Artificially designed chiral metasurfaces have been developed to enhance these weakly chiral optical signals. However, their practical application in chiral optics remains limited by significant ohmic and scattering losses within the supported optical modes, which typically result in low quality factors (Q factors).

[0003] Continuum-bound states (BICs) are nonradiative eigenmodes embedded in a radiative continuum, exhibiting Bloch resonances in periodic lattice structures. With their extremely high quality factor and strong optical field localization capabilities, BICs can significantly amplify chiral responses induced by structural asymmetries, proving to be a revolutionary solution to key bottlenecks in chiral photonics. While chiral quasi-bound states realized based on BICs possess near-infinite radiative lifetimes, unavoidable radiative losses still exist in their proximity resonances, and strong angular dispersion causes rapid decay of the quality factor, severely limiting their further application in practical devices such as spin modulation and valley photonics.

[0004] Both flat-band and merged bipolar interphase (BICs) can reduce the Q-value decay rate of BICs. However, existing flat-band BIC implementation schemes mostly employ moiré superlattices and disorder-induced band folding, which are extremely difficult to fabricate experimentally. Furthermore, the chiral flat bands and related nonlinear properties involved in this type of BIC research have not been fully explored, failing to meet the application requirements of miniaturized, wide-angle, and high-performance chiral photonic devices. Summary of the Invention

[0005] Purpose of the invention: In view of the above-mentioned prior art, a chiral flat-band continuum bound state metasurface and its preparation method are proposed to realize flat-band continuum bound state and efficient third harmonic generation within a wide incident angle range, so that the device has a high quality factor and near-unit circle dichroism.

[0006] Technical solution: A chiral flat-band continuum bound state metasurface includes a substrate and structural units, wherein the structural units are periodically arranged in a rectangular lattice on the substrate surface; the reference structure of the structural units is a chiral cross structure with C2 rotational symmetry and no in-plane mirror symmetry, and the C2 rotational symmetry of the reference structure is broken by introducing asymmetric parameters to excite chiral quasi-continuum bound states; the coupling strength of the reverse propagation folded waveguide is controlled by adjusting the periodic parameters of the rectangular lattice to form a flat-band dispersive band structure, thereby constituting a chiral flat-band continuum bound state.

[0007] Furthermore, the reference structure includes four nanoarms intersecting at the center point: the nanoarm extending along the +x direction has a width of W1 and a length of (I3+W2); the nanoarm extending along the -x direction has a width of W1 and a length of (I3+W3); the nanoarm extending along the +y direction has a width of W2 and a length of I1; and the nanoarm extending along the -y direction has a width of W3 and a length of I1.

[0008] Furthermore, the asymmetric parameter α = W3 - W2 is formed by adjusting the width of the nanoarms extending in the +y and -y directions.

[0009] Furthermore, the period P of the rectangular lattice along the x-direction x =485nm, period P along the y direction y =600nm.

[0010] Furthermore, the height of the structural unit is H=260nm, W1=160nm, W2=W3=45nm, I1=185nm, and I3=215nm.

[0011] Furthermore, the asymmetry parameter α = 10 nm.

[0012] Furthermore, the structural unit is made of silicon, and the substrate is made of silicon dioxide.

[0013] Working principle: By simulating the band structure and Q-factor distribution of the metasurface in k-space, three eigenmodes are determined to exist within the frequency band. Modes 1 and 2 both exhibit resonance peaks at the Γ point where the Q-factor tends to infinity, indicating complete nonradiativity at this momentum, thus confirming that they are symmetry-protected continuous-domain bound states (SP-BICs). In contrast, the Q-factor of Mode 3 increases with the in-plane wave vector (k... xThe increase in ) remains almost constant, indicating that it is a waveguide. Mode 2 is verified to be an inherently chiral BICs mode by the non-zero helicity vortex polarization singularity (V point) surrounded by elliptical eigenstates. An ultra-narrowband chiral resonance was observed at a wavelength of 1022 nm in the simulated transmission spectrum of Mode 2. By introducing the asymmetry parameter αW3-W2=10 nm to break the C2 rotational symmetry of the baseline design, the excitation of the chiral quasi-continuum bound state was induced.

[0014] When a plane wave illuminates a metasurface, it excites a resonant mode in a single chiral cross-cell unit. The unit couples energy to its four neighboring directions via its four arms. This flat-band effect can be achieved by modulating the interaction of the four waveguides in different directions within the chiral metasurface, i.e., by adjusting the lattice period P. y Achieve. With P y As the chromaticity increases, the dispersion curvature of the mode gradually decreases, while the intersection point of the waveguide in the y-direction and the waveguide in the x-direction redshifts. This is achieved by adjusting the lattice period P in the y-direction. y Changing the coupling strength in the +y and -y directions alters the frequency supported by the waveguide along the y-direction. Under suitable parameters, the waveguide in the y-direction will couple with the waveguide in the x-direction, thus forming a chiral flat-band continuum bound state. When P y When adjusted to 600nm, the waveguide mode in the y-direction undergoes a frequency redshift due to the change in coupling phase, making it degenerate with the waveguide mode in the x-direction. The two hybridize through strong coupling, causing the coupling phase relationship in the four directions to enter a critical state, and the dispersion curvature approaches zero, forming a photonic flat band. Combined with the chiral structure design (without mirror symmetry) and the breaking of the C2 symmetry by the small asymmetric parameter α, this flat band mode merges with the continuum bound state, ultimately realizing a chiral flat band quasi-continuum bound state.

[0015] Metasurfaces generate chirality by modulating the destructive interference between two intrinsic linearly polarized modes, exhibiting opposite electric field distributions under the incidence of orthogonally linearly polarized light with a phase difference of π / 2. Due to the significant difference between left- and right-hand circularly polarized excitations on the metasurface, the third harmonic generated on this basis also exhibits a significant chiral optical response. Unlike conventional bipolar integrated circuits (BICs), the nondispersive BICs of this invention can maintain effective optical field confinement, and their third harmonic generation (THG) signal intensity is two orders of magnitude higher than that of conventional dispersive BICs. Furthermore, the flat-band nondispersive BICs, due to their near-zero group velocity and infinite effective mass, exhibit strong localization of the optical field within the structure, resulting in lower radiation and scattering losses at the structure edges, thereby significantly enhancing the structure's CD and Q values.

[0016] Beneficial effects: Compared with existing conventional dispersive chiral BICs: This invention obtains chiral flat-band BICs through four waveguide couplings, making the high-quality factor chiral resonance less sensitive to the incident angle, and can stably maintain Q>20000 and a circular dichroism peak of 0.91 over a wide angle range; under a pump light intensity of 1MW / cm², the third harmonic conversion efficiency reaches 2.3×10⁻⁶. -3 The THG signal is two orders of magnitude higher than that of traditional dispersive BICs. Furthermore, compared to existing flat-band BICs, this invention employs a simple single-layer chiral cross structure, eliminating complex fabrication processes and significantly reducing processing difficulty. It can be used for the design and fabrication of miniaturized nonlinear chiral light sources, chiral lasers, advanced nonlinear polarization imaging, and various novel chiral optical functional devices. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of the chiral flat band BICs metasurface of the present invention;

[0018] Figure 2 This is a schematic diagram of the planar structure of the structural unit of the present invention;

[0019] Figure 3 This is a schematic diagram of the energy band structure supported by the metasurface of the present invention;

[0020] Figure 4 This is a quality factor diagram corresponding to the metasurface energy bands of the present invention;

[0021] Figure 5 The calculated transmission and circular dichroism spectra of the chiral metasurface under different circularly polarized light incident conditions are presented in this invention.

[0022] Figure 6 The angle-resolved transmission spectrum and circular dichroism spectrum of the flat band mode of this invention under LCP and RCP incident conditions are shown in the figure. (f) represents the Mode 2 transmission spectrum, (g) represents the Mode 2 RCP transmission spectrum, and (h) represents the Mode 2 CD (circular dichroism) spectrum.

[0023] Figure 7 This is a schematic diagram of the electric field distribution of structural unit mode 2 of the present invention, where (i) is the normalized optical chiral field distribution, (j) is the normalized in-plane electric field distribution under x-polarized excitation with a resonant wavelength of 1015nm and a phase difference of π / 2, and (k) is the normalized in-plane electric field distribution under y-polarized excitation with a resonant wavelength of 1015nm and a phase difference of π / 2.

[0024] Figure 8 The five waveguide modes calculated for the example are shown, where (l) corresponds to P. y =485 nm, (m) corresponds to P y =525nm, (n) corresponds to P y=550 nm, (o) corresponds to P y =575 nm, (p) corresponds to P y =600 nm;

[0025] Figure 9 Simulated third harmonic generation (THG) spectra for dispersive and non-dispersive BICs modes. Detailed Implementation

[0026] The invention will now be further explained with reference to the accompanying drawings.

[0027] like Figure 1 , Figure 2 As shown, this embodiment provides a chiral flat-band continuum bound-state metasurface structure, including: a silicon dioxide substrate 1 and silicon structural units 2. The silicon structural units 2 are arranged periodically on the surface of the silicon dioxide substrate 1 in a rectangular lattice, and the period P of the rectangular lattice along the x-direction is... x =485nm, period P along the y direction y =600nm.

[0028] The baseline structure of silicon structural unit 2 is a chiral cross structure with C2 rotational symmetry and no in-plane mirror symmetry. This baseline structure consists of four nanoarms intersecting at the center point: the nanoarm extending along the +x direction has a width of W1 and a length of (I3+W2); the nanoarm extending along the -x direction has a width of W1 and a length of (I3+W3); the nanoarm extending along the +y direction has a width of W2 and a length of I1; and the nanoarm extending along the -y direction has a width of W3 and a length of I1. The asymmetry parameter α = W3 - W2, i.e., adjusting the widths of the nanoarms extending along the +y and -y directions, forms the asymmetry parameter, breaking the C2 rotational symmetry of the baseline structure. The specific dimensions of the baseline structure are: height H = 260 nm, W1 = 160 nm, W2 = W3 = 45 nm, I1 = 185 nm, I2 = 25 nm, I3 = 215 nm, and I4 = 115 nm.

[0029] Figure 3 and Figure 4 The band structure and quality factor calculated based on the aforementioned baseline structure are presented. Three eigenmodes with different characteristics were identified within the studied energy range. Modes 1 and 2 both exhibit divergent high quality factors at the Γ point, confirming their nature as two symmetry-protected continuum bound states (SP-BICs). In contrast, mode 3 has a lower quality factor and its quality factor increases with the in-plane wave vector (k... x The increase in ) remains almost constant, indicating that it is a waveguide. Furthermore, compared to the two emerging BICs modes, mode 2 has a higher quality factor and a slower attenuation rate, suggesting they have different radiative attenuation characteristics. This difference reveals a significant difference in the radiative attenuation channels of the two modes. Figure 7 The distribution of the intrinsic polarization vector field of Mode 2 in momentum space is shown. By using the non-zero helicity vortex polarization singularity (V point) surrounded by elliptical eigenstates, it is verified that Mode 2 is an intrinsically chiral BICs mode.

[0030] Figure 5 The different polarization components T of Mode 2 under normal illumination of LCP and RCP are given. ij The simulated transmission spectrum, T ij In this diagram, the subscripts "i" and "j" represent the polarization states of the transmitted and incident light, respectively. An ultra-narrow-band chiral resonance was observed at a wavelength of 1022 nm, with a transmittance peak Ti. rl =0.95, transmittance valley T rr =T lr =T ll ≈0, where r represents RCP and l represents LCP. An asymmetric parameter α = W3 - W2 = 10 nm is introduced, where W2 = 45 nm and W3 = 55 nm, to break the C2 rotational symmetry, thereby inducing the excitation of the chiral quasi-continuum bound state. Based on λ0 / ∆λ, the Q factor is estimated to be approximately 20000, where λ0 is the resonance wavelength and ∆λ is the full width at half maximum (FWHM). The calculated peak value of the circular dichroism is 0.91.

[0031] Figure 6 Angle-resolved transmission and circular dichroism spectra of Mode 2 under LCP and RCP incidence are presented, further demonstrating the performance of the flat band dispersion lacking the normal point (Γ point) supported by SP-BICs in the transmission and circular dichroism spectra. Mode 2 is a typical magnetic dipole mode. Due to the group velocity approaching zero and the effective mass being infinitely large, the light field exhibits strong locality within the structure, resulting in low radiation and scattering losses at the structure edges. Angle-resolved LCP and RCP transmission spectra and circular dichroism simulations reveal the mode's dispersion-free characteristics and its differential response to different circularly polarized light.

[0032] Figure 7 The normalized optical chirality field distribution of mode 2 and the normalized in-plane electric field distribution under x-polarized and y-polarized excitation at a resonant wavelength of 1015 nm and a phase difference of π / 2 are presented. The significant and non-uniform sign distribution in the calculated near-field optical chirality indicates a strong chirality enhancement potential and optical nonlinearity in the far field. The opposite electric field distribution with a phase difference of π / 2 under orthogonally linearly polarized incidence confirms that the observed chirality originates from the destructive interference between two intrinsic linearly polarized modes.

[0033] Figure 8 Four waveguide modes supported by metasurfaces are presented. As the period P... yAs P increases, the dispersion curvature of Mode 2 gradually decreases, while the intersection point of the waveguide in the y-direction and the waveguide in the x-direction redshifts. y At 600 nm, the waveguide redshifts along the y-direction and hybridizes with the waveguide in the x-direction, thus forming a flat-band continuum bound state. In this design, the physical origin of the chiral flat-band continuum bound state is the intrinsic flat band formed by the y-direction waveguide within the band structure in the x-direction.

[0034] Figure 9 Simulated third harmonic generation (THG) spectra of dispersive and nondispersive bipolar inductively coupled plasma (BIC) modes are presented. A significant comparison of THG intensities under LCP and RCP excitation verifies the chiral nonlinear enhancement effect. Notably, the THG signal intensity of the nondispersive BICs is two orders of magnitude higher than that of the conventional dispersive BICs, highlighting the inherent difference in enhancement capabilities between the two modes. This result is consistent with the magnetic field distribution curves, indicating that stronger field localization directly enhances the THG output. Based on this, and using the formula... , where P THG and P FW Representing the radiated power of the third harmonic and the fundamental frequency, respectively, the nonlinear conversion efficiency of the nondispersive BICs is estimated to be 2.3 × 10⁻⁶. - ³, a value significantly higher than that reported in most previous studies.

[0035] A method for preparing a chiral flat-band continuum bound metasurface includes the following steps:

[0036] Step 1: Substrate preparation and cleaning: Clean and treat the quartz sheet to remove organic contaminants, particles and metal ions from the substrate surface;

[0037] Step 2: Silicon Thin Film Deposition: A 260 nm thick amorphous silicon thin film is deposited on the cleaned SiO2 substrate surface using plasma-enhanced chemical vapor deposition. Compared to crystalline silicon, amorphous silicon exhibits better isotropy during etching, making it easier to control the morphology.

[0038] Step 3: Electron beam lithography: Photoresist is coated on the surface of the silicon thin film. Using a focused electron beam, the photoresist is scanned and exposed according to the designed chiral flat band structure unit pattern. The photoresist in the exposed area is dissolved and removed, forming a photoresist mask on the surface of the silicon thin film.

[0039] Step 4: Etching: Using photoresist as a mask, perform inductively coupled plasma etching on the silicon thin film to accurately transfer the structural pattern to the silicon layer. After removing the photoresist, the metasurface fabrication is completed.

[0040] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A chiral flat-band continuum bound metasurface, characterized in that, The system includes a substrate and structural units, wherein the structural units are periodically arranged in a rectangular lattice on the substrate surface; the reference structure of the structural units is a chiral cross structure with C2 rotational symmetry and no in-plane mirror symmetry, and the C2 rotational symmetry of the reference structure is broken by introducing asymmetric parameters to excite chiral quasi-continuum bound states; the coupling strength of the reverse propagation folded waveguide is controlled by adjusting the periodic parameters of the rectangular lattice to form a flat-band dispersive band structure, which constitutes a chiral flat-band continuum bound state.

2. The chiral planar continuum bound metasurface according to claim 1, characterized in that, The reference structure includes four nanoarms that intersect at the center point: the nanoarm extending along the +x direction has a width of W1 and a length of (I3+W2); the nanoarm extending along the -x direction has a width of W1 and a length of (I3+W3); the nanoarm extending along the +y direction has a width of W2 and a length of I1; and the nanoarm extending along the -y direction has a width of W3 and a length of I1.

3. The chiral planar continuum bound metasurface according to claim 2, characterized in that, The asymmetric parameter α = W3 - W2 is formed by adjusting the width of the nanoarms extending in the +y and -y directions.

4. The chiral planar continuum bound metasurface according to claim 1, characterized in that, The period P of the rectangular lattice along the x-direction x =485nm, period P along the y direction y =600nm.

5. The chiral planar continuum bound metasurface according to claim 2, characterized in that, The structural unit has a height H=260nm, W1=160nm, W2=W3=45nm, I1=185nm, and I3=215nm.

6. The chiral planar continuum bound metasurface according to claim 3, characterized in that, The asymmetry parameter α = 10 nm.

7. The chiral planar continuum bound metasurface according to claim 1, characterized in that, The structural unit is made of silicon, and the substrate is made of silicon dioxide.

8. The method for preparing a chiral flat-band continuum bound metasurface according to any one of claims 1-7, characterized in that, include: Step 1: Clean and treat the SiO2 substrate to remove organic contaminants, particles and metal ions from the surface; Step 2: Deposit an amorphous silicon thin film on the cleaned SiO2 substrate; Step 3: Using electron beam lithography, prepare a photoresist mask with the corresponding structural unit pattern on the surface of a silicon thin film; Step 4: The pattern of the photoresist mask is transferred to the silicon thin film through an etching process to complete the metasurface fabrication.

9. The preparation method according to claim 7, characterized in that, In step 2, plasma-enhanced chemical vapor deposition is used to deposit amorphous silicon thin films.

10. The preparation method according to claim 7, characterized in that, In step 4, inductively coupled plasma etching is used to complete the pattern transfer.

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