An ITO electrically tunable harmonic waveguide-based metasurface circular dichroism dynamic regulation structure and a preparation method thereof

Abstract

This invention discloses a dynamically modulated metasurface circular dichroism (CD) structure based on an ITO electrically tunable waveguide and its fabrication method. The structure is a metal-ITO-metal waveguide structure (bottom metal reflective layer, middle indium tin oxide (ITO) waveguide layer, and top metal diatomic umbrella-shaped structure), supporting guided mode resonance (GMR). The two top umbrella-shaped resonators have different rotation angles, breaking mirror symmetry and generating an intrinsic chiral circular dichroism (CD) response. Applying a DC bias between the bottom reflective layer and the top metal layer allows for adjustment of the ITO carrier concentration, changing its effective refractive index and modulating the interference phase difference between the two GMR modes, achieving continuous and reversible electrical control of the reflected CD amplitude and sign at a fixed wavelength. COMSOL simulations verify that the CD sign continuously inverts with changes in the ITO dielectric constant. Compared to traditional metasurfaces, this invention offers advantages such as fully electrically continuous tuning, simplified structure, CMOS compatibility, and pixel-level independent addressing, and can be applied to chiral optical switches and spin-selective spatial light modulators.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano photonics and dynamically tunable metasurface technology. Specifically, it relates to a circular dichroism (CD) dynamically tunable metasurface structure based on guided mode resonance (GMR) interference phase modulation in a metal-indium tin oxide (ITO)-metal (MIM) waveguide structure, its electrical modulation method and its fabrication method. This invention operates in the 1550nm band and can be applied to fields such as dynamic polarization optical modulation, chiral optical switches, spin-selective optical communication and adaptive optics systems. Background Technology

[0002] Circular dichroism (CD) refers to the difference in reflectance or transmittance of a material to left-handed circularly polarized light (LCP) and right-handed circularly polarized light (RCP), and is a core parameter characterizing chiral optical properties. Metasurfaces, by arranging subwavelength artificial microstructures on a two-dimensional plane, can arbitrarily control the amplitude, phase, and polarization state of electromagnetic waves. Among them, chiral metasurfaces, by deliberately introducing in-plane mirror asymmetry, can achieve CD values ​​close to the theoretical limit (>0.9), and have important application prospects in fields such as circularly polarized light detection, chiral molecule sensing, spin-selective optical switches, and quantum optics.

[0003] However, existing chiral metasurface technologies suffer from the following key limitations: (a) Static structure limitation: The vast majority of reported chiral metasurfaces are static structures—their CD response is fixed after fabrication and cannot be adjusted in real time. Typical examples include three-dimensional L-shaped, helical, and Ω-shaped metallic nanostructures (see: Z. Wang et al., "Three-dimensional plasmonic chiral metamaterials," *Nature Communications*, vol. 7, p. 12409, 2016; M. Hentschel et al., "Chiral plasmonics," *Science Advances*, vol. 3, e1602735, 2017). Although these structures achieve strong CD, their CD amplitude and sign remain fixed once fabricated. (b) Static CD inversion based on geometric parameter control: Cheng et al. (J. Cheng et al., "Counterintuitive Reversal of Circular Dichroism via Controllable Plasmonic Guided Mode Resonance in Diatomic Metasurfaces," *Laser & Photonics Reviews*, vol. 19, no. 8, 2025, DOI: 10.1002 / lpor.202401184) reported an umbrella-shaped diatomic metasurface based on an Al / SiO2 / AlMIM structure. By adjusting the interatomic spacing d at a fixed wavelength to change the GMR interference phase difference, the CD sign was inverted from +0.55 to -0.36. However, this method still relies on passive changes to the structural geometric parameters—once the d value is fixed, the CD cannot be further adjusted. This "one-time" adjustment method cannot meet the requirements of real-time continuous control of the CD in practical applications. (c) Switching control based on phase change materials: The refractive index difference between the amorphous and crystalline states of phase change materials such as GST (Ge2Sb2Te5) can be used to switch the CD (see: X. Yin et al., "Activechiral plasmonics," *Nano Letters*, vol. 15, pp. 4255–4260, 2015; Y. Chen et al., "Phase-change chiral metasurface," *Advanced Optical Materials*, vol. 8, p. 2000375, 2020).However, the GST phase transition requires thermal actuation, and the switching speed is limited by the thermal diffusion time (in the order of μs–ms). Furthermore, it can only provide discrete switching between two states (amorphous / crystalline), making continuous tuning impossible. In addition, GST exhibits significant absorption loss in the visible to near-infrared band, affecting the device's optical efficiency. (d) Mechanical manipulation based on liquid crystals and MEMS: Liquid crystal manipulation schemes (see: SJ Kim et al., "Liquid crystal integrated metasurface for active control of chirality," *ACS Photonics*, vol. 8, pp. 2100–2107, 2021) are limited by the response speed of liquid crystal molecules (in the order of ms); MEMS schemes (see: M. Liu et al., "MEMS-based reconfigurable chiral metamaterial," *Advanced Materials*, vol. 32, p.1907972, 2020) face problems such as high manufacturing complexity, mechanical fatigue, and reliability issues. Neither approach is suitable for applications requiring high speed (>MHz), solid-state, and addressable manipulation. (e) The scheme of combining BIC (bound states in a continuum) with tunable materials: combining polyaniline (PANI) electrochromic materials or graphene with BIC metasurfaces (see: S. Han et al., "Electrically tunable BIC metasurface," *Nature Nanotechnology*, vol. 16, pp.1090–1095, 2021; TCW Tan et al., "Graphene-based tunable BIC for chiralsensing," *ACS Nano*, vol. 16, pp. 6989–6997, 2022), although it can achieve high Q-value CD response, BIC is extremely sensitive to geometric processing errors (sub-nanometer tolerance requirements), experimental preparation tolerance is very small, and the monolayer atomic characteristics of graphene limit its tunability.

[0004] In summary, current technologies have not yet achieved a solid-state metasurface CD manipulation scheme that is electrically addressable, continuously tunable, has a loose fabrication tolerance, and possesses CD sign inversion capability. This technological gap severely restricts the development of adaptive polarization optics, high-speed chiral optical switches, and addressable spin optics.

[0005] The above application scenarios place clear demands on dynamic CD control: (1) Dynamic polarization optical modulation requires continuous adjustment of the circular polarization component ratio of the output light; (2) Chiral optical switches need to switch at high speed (>MHz) between LCP and RCP reflection states; (3) Adaptive optics systems need to adjust the polarization response in real time according to environmental changes; (4) Spatial light modulators need to independently address the circular polarization response of each pixel electrically. The prerequisite for realizing the above functions is to develop a solid-state metasurface that can continuously and reversibly control the CD amplitude and sign through electrical means.

[0006] This invention operates in the 1550nm band, the core band in optical communication, primarily due to its combination of lowest transmission loss and usable optical amplification. Optical fibers exhibit varying losses at different wavelengths, with three low-loss windows existing around 850nm, 1310nm, and 1550nm. Among these, 1550nm offers the lowest loss: attenuation is only about 0.2 dB / km, approaching the theoretical limit of silica fiber, making it a core window for long-distance and coherent communication. Compared to 850nm, the transmission loss at 1550nm is approximately 1 / 10, making it more suitable for long-distance transmission. The key to the 1550nm window lies in its precise matching with the operating wavelength of the erbium-doped fiber amplifier (EDFA). The EDFA can amplify only a narrow segment of light around 1550nm, achieving a gain of approximately 30dB and a bandwidth exceeding 20nm, allowing simultaneous amplification of multiple wavelength division multiplexing (WDM) channels. It is this combination of "1550nm low loss + EDFA optical amplification" that allows transoceanic optical cables and ultra-long-distance trunk lines to replace electronic repeaters with optical amplifiers, achieving repeaterless transmission. The 1550nm wavelength is crucial in this technology because it represents the lowest loss transmission window for silica optical fiber and is perfectly matched with EDFA optical amplification technology, thus supporting modern ultra-long-distance, high-capacity optical communication systems. Summary of the Invention

[0007] The present invention aims to solve the following technical problems: to achieve real-time, continuous, and reversible control of the amplitude and sign of circular dichroism (CD) through purely electrical means without changing the geometry of the metasurface. Specifically, it includes: (1) overcoming the limitation that the CD response of existing chiral metasurfaces is fixed once it is prepared; (2) overcoming the deficiency that CD can only be changed by changing the geometric parameter d and cannot be dynamically controlled in situ; (3) overcoming the limitation that the GST phase change material scheme can only achieve binary switching and cannot be continuously tuned; (4) overcoming the problems of slow response speed and poor reliability of liquid crystal and MEMS schemes; (5) overcoming the difficulties of BIC schemes in fabrication with extremely small tolerance and difficulty in engineering.

[0008] The present invention provides the following technical solution: a metasurface circular dichroism dynamic control structure based on ITO electrically tunable waveguide (claim 1), comprising: a bottom metal reflective layer (100): made of metal material Al or Au, with a thickness of 150–250 nm, serving as both an optical reflector (R>85% in the target band) and a bottom electrode; a middle indium tin oxide (ITO) waveguide layer (200): with a thickness of 150–200 nm, serving as a dielectric spacer layer for the MIM waveguide and as a carrier transport channel—its effective refractive index can be controlled by an applied electric field; and a top metal nanostructure layer (300): made of Al, with a thickness of 60–80 nm, patterned as a periodically arranged array of biatomic umbrella resonators, serving as both an optical resonator and a top electrode.

[0009] The three layers described above constitute a metal-ITO-metal waveguide structure, supporting guided-mode resonance (GMR). In this invention, the term "MIM" should be broadly understood as a "metal-dielectric / semiconductor-metal" quasi-parallel-plate waveguide structure—ITO, as a transparent conductive oxide, exhibits dielectric properties (real part of dielectric constant ε_r > 0) in the depletion / low-doping state, effectively supporting GMR mode propagation; in the electron accumulation state, its dielectric constant transitions towards ENZ and even negative values, significantly altering the waveguide's dispersion characteristics. This continuous transition from "dielectric-like" to "metal-like" is the physical basis for achieving electrical tuning in this invention.

[0010] Each diatomic umbrella resonator unit comprises a first umbrella resonator with a first rotation angle θ_A and a second umbrella resonator with a second rotation angle θ_B, where θ_A ≠ θ_B. This asymmetric rotation angle configuration is a key design feature for breaking the unit's mirror symmetry and generating an intrinsic chiral response.

[0011] A method for electrically controlling the circular dichroism of a metasurface (claim 2) includes: (a) applying a DC bias voltage V_g between a bottom metal reflective layer (100) and a top metal nanostructure layer (300); (b) changing the electron carrier concentration N in the ITO waveguide layer by adjusting the amplitude and polarity of V_g, thereby controlling its effective refractive index n_eff; (c) changing the interference phase condition of GMR in the waveguide by changing n_eff; and (d) achieving continuous electrical control of the CD amplitude and sign at a fixed incident wavelength.

[0012] A method for fabricating the structure (claim 3) is characterized by requiring only three thin-film deposition processes and one etching process: S1 depositing a bottom Al reflective layer → S2 depositing an ITO waveguide layer and annealing → S3 depositing a top Al layer → S4 electron beam lithography + ICP-RIE etching to form a dual-atom umbrella-shaped array. Compared to traditional electrically tunable MIM devices based on insulating barrier layers (requiring 4–5 deposition processes + precise control of the barrier layer), this invention eliminates the additional insulating barrier layer deposition step, resulting in a simpler process.

[0013] The core principle of this invention is to achieve dynamic tuning of CD by controlling the GMR interference phase through electrical means.

[0014] When circularly polarized light is incident on a diatomic umbrella-shaped metasurface, the first and second umbrella-shaped resonators excite GMR modes, respectively. Because the rotation angles of the two resonators are different (θ_A ≠ θ_B), their coupling phases with respect to the incident light are also different. The interference of the two GMR modes in the far field determines the CD of the reflected light: CD ∝ sin(Δφ). Where Δφ = (2π / λ)·n_eff·d + Δφ0 is the total interference phase difference between the two GMR modes, including the phase accumulated by waveguide propagation (2π / λ)·n_eff·d and the initial coupling phase difference Δφ0.

[0015] When a DC bias voltage V_g is applied between the top and bottom metal layers: V_g>0 (positive bias, electron accumulation)**: The electron concentration N at the ITO / metal interface increases → According to the Drude model, the plasma frequency ω_p increases → The real part of the ITO dielectric constant ε_r decreases → The refractive index n decreases → n_eff decreases → Δφ decreases → The CD amplitude and sign change. V_g<0 (negative bias, electron depletion): N decreases → ε_r increases → n increases → n_eff increases → Δφ increases → The CD amplitude and sign change in opposite directions. By continuously adjusting the amplitude and polarity of V_g, continuous, reversible, and bidirectional control of the CD, including sign reversal, can be achieved. The key advantage of this scheme is that CD tuning is achieved entirely through electrical means, without involving any change to the geometric parameter d or the resonator rotation angle, fundamentally overcoming the limitations of the scheme of Cheng et al. (2025).

[0016] This invention is based on the ITO electrical tuning physical mechanism, which has been verified by multiple independent experimental groups, as follows: The Drude model describes the dielectric function of ITO in the near-infrared band as: ε(ω) = ε_∞ − ω_p² / [ω(ω+ iγ)], where the plasma frequency ω_p = √(N·e² / ε0·m*), directly dependent on the carrier concentration N. By changing N through electrical bias, continuous control of ε(ω) and the refractive index n(ω) can be achieved.

[0017] The ENZ effect enhances tuning sensitivity: When N is tuned to bring the real part of ε close to zero (ENZ region), the refractive index of ITO becomes extremely sensitive to changes in N (dε / dN → large value). This invention designs the zero-bias carrier concentration of ITO near the ENZ region (approximately N0 = 3 × 10²). 0 (cm⁻³), which means that a small voltage change can produce a significant change in refractive index.

[0018] The tuning range of this invention is: near a wavelength of 1550 nm, the ITO carrier concentration ranges from N = 1 × 10². 0 cm⁻³ (depletion state, n ≈ 1.8) changes to N = 5 × 10² 0 cm⁻³ (accumulation state, n → ENZ transition region) can achieve an effective refractive index change Δn_eff ≈ 0.3–0.5. COMSOL simulations show that this Δn range is sufficient to drive a significant change in the CD amplitude and achieve sign inversion.

[0019] The electrical bias structure of this invention can be equivalent to a metal-semiconductor oxide-metal (MSM) capacitor: bottom electrode: Al reflective layer (200 nm), connected to the external circuit via an ohmic contact; semiconductor oxide layer: ITO (150–200 nm); carrier concentration N0 ≈ 3 × 10² 0 cm⁻³, top electrode: Al nanostructure layer (70 nm), connected to the external circuit via Schottky contact.

[0020] Operating parameters: DC bias voltage range: V_g = -10 V to +10 V. Polarity convention: V_g>0 indicates that the top electrode is at a positive potential relative to the bottom electrode (electrons accumulate at the ITO / top electrode interface). ENZ crossing point: approximately V_g = +2 V to +3 V (depending on the initial ITO doping concentration and thickness). Power consumption: Quasi-static bias, current is leakage current (nA–μA level), actual power consumption is extremely low (<μW / pixel). Innovative operating features that distinguish it from existing electrically tuned schemes: Direct bias, no insulating barrier layer: Unlike the HfO2 / Al2O3 insulating barrier layer used by Park et al. (2015) and Liu et al. (2017), the Al top electrode of this invention directly contacts ITO, eliminating the precise deposition step of the barrier layer and simplifying the device structure to only three layers. 2. Bidirectional Polarity Utilization: Existing ITO electrical tuning research mainly utilizes forward bias (electron accumulation). This invention utilizes both forward (accumulation) and reverse (depletion) biases simultaneously to achieve bidirectional refractive index control, maximizing the tuning range of CD. 3. Preset ENZ Operating Point: By designing the initial doping concentration of ITO near the ENZ region, the refractive index tuning efficiency per unit voltage is maximized.

[0021] Compared with the prior art, the present invention has the following beneficial effects: (1) Continuous Electrical CD Tuning and Sign Inversion: Within a DC bias range of -10V to +10V, continuous adjustment of CD amplitude and sign inversion (e.g., tuning from -0.90 to +0.32 or a larger range) can be achieved without altering the metasurface geometry during tuning. This is the first time that an electrical CD dynamic tuning scheme based on GMR interference phase has been realized. (2) Simplified Structure and Reduced Fabrication Difficulty: Compared to traditional electrically tunable MIM devices that require an HfO2 / Al2O3 insulating barrier layer, this invention uses a direct stacked Al / ITO / Al three-layer structure, reducing the deposition steps from 4–5 times to 3 times, and eliminating the need for sub-nanometer precision control of the barrier layer thickness. (3) CMOS-Compatible Fabrication Process: Al and ITO are both standard CMOS process materials, suitable for wafer-level large-scale manufacturing. The fabrication process only involves magnetron sputtering, electron beam lithography, and ICP-RIE etching, and is fully compatible with existing semiconductor production lines. (4) Pixel-level independent addressing capability: The top and bottom metal layers of each diatomic unit can be connected to the row and column addressing lines respectively, supporting independent electrical bias control of large-scale pixel arrays, suitable for spatial light modulators and programmable spin optical devices. (5) High-speed response potential: The carrier accumulation / depletion of ITO is a field-effect process, and the intrinsic response time is mainly limited by the RC time constant (determined by the device capacitance and contact resistance), which can theoretically reach the MHz–GHz range, much faster than GST thermal phase transition (μs–ms), liquid crystal (ms) and MEMS (μs–ms). (6) Verified physical basis: The ENZ electrical tuning effect of ITO has been verified in multiple independent experiments (Park 2015, Liu 2017, Rajput 2023), and the feasibility of this invention has a solid experimental basis. Attached Figure Description

[0022] Figure 1 The schematic diagram (xz plane) of the layered cross-section of the metasurface structure of the present invention shows the three-layer stacking relationship of the bottom Al reflective layer (100), the middle ITO waveguide layer (200) and the top Al umbrella-shaped nanostructure layer (300), as well as the method of applying the DC bias voltage V_g.

[0023] Figure 2 Top view (xy plane) of the diatomic umbrella-shaped resonator unit structure, with key geometric parameters labeled: x-direction period Px, y-direction period Py, outer radius r_e, inner radius r_i, connecting rod width w, first resonator rotation angle θ_A, second resonator rotation angle θ_B, and diatomic spacing d.

[0024] Figure 3The simulation results of CD changing with wavelength and the real part of ITO dielectric constant ε'_wg are shown in the family of CD(λ) curves. Each curve corresponds to an ε'_wg value (2.0–5.0, step size 0.2), showing the evolution of CD sign reversal with ε'_wg. The CD=0 reference line is marked.

[0025] Figure 4 : A three-dimensional surface plot of CD as a function of frequency and ε'_wg. The X-axis represents frequency (THz, range 166–250 THz), the Y-axis represents ε'_wg (range 2.0–5.0), and the Z-axis represents CD amplitude. Color mapping distinguishes between CD>0 (warm) and CD<0 (cool) regions, and the CD=0 isosurface is labeled.

[0026] Figure 5 The curves showing the relationship between the refractive index n and extinction coefficient k of ITO and the gate voltage V_g are calculated based on the Drude model and the MOS capacitor accumulation / depletion model, and the ENZ operating point and the zero bias initial operating point are marked.

[0027] Figure 6 The schematic diagram of the fabrication process shows the flow of the four main fabrication steps: S1 → magnetron sputtering deposition of the bottom Al reflective layer; S2 → magnetron sputtering deposition of the ITO waveguide layer + annealing; S3 → magnetron sputtering deposition of the top Al layer; S4 → electron beam lithography + ICP-RIE etching to form a two-atom umbrella array.

[0028] Figure 7 A schematic diagram of the metasurface array system's control process, illustrating the overall workflow and principles of the device, from the refractive index change caused by electrical tuning to the dynamic adjustment of the resulting CD value. Detailed Implementation

[0029] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] Reference Figure 1 and Figure 2 The metasurface structure in this embodiment includes: Bottom Al reflective layer (100): 200 nm thick, deposited on a quartz substrate by magnetron sputtering, serving as both an optical mirror and a bottom electrode; Central ITO waveguide layer (200): 180 nm thick, zero bias carrier concentration approximately 3 × 10² 0 cm⁻³, annealed at 300°C in an oxygen-containing atmosphere for 60 min to control crystallinity and carrier concentration; Top Al nanostructure layer (300): 70 nm thick, patterned as a diatomic umbrella-shaped resonator array.

[0031] Geometric parameters of the diatomic umbrella resonator unit (refer to) Figure 2 ): Among them, θ_A ≠ θ_B (80° ≠ 65°) is the key to breaking the mirror symmetry and generating the intrinsic chiral CD response. The arc-shaped umbrella surface of the umbrella resonator is connected collinearly with the adjacent resonator through the connecting rod, forming a conjoined diatomic configuration of umbrella-rod-umbrella.

[0032] The fabrication process in this embodiment includes the following steps: S1: A 200 nm thick Al film is deposited on a clean quartz substrate using DC magnetron sputtering at a sputtering power of 200 W, an Ar atmosphere, and a working pressure of 3 mTorr. This layer serves as the bottom reflector and bottom electrode. S2: A 180 nm thick ITO film is deposited on the Al layer using RF magnetron sputtering at a sputtering power of 150 W, an Ar / O2 mixed atmosphere (Ar:O2 = 98:2), and a working pressure of 5 mTorr. After deposition, annealing is performed in a tube furnace at 300°C, in a flowing O2 atmosphere (O2 flow rate 50 sccm), for 60 min. The purpose of annealing is to control the crystallinity and carrier concentration of ITO, so that its zero-bias carrier concentration reaches approximately 3 × 10². 0 cm⁻³. S3: A 70 nm thick Al film was deposited on the ITO layer using DC magnetron sputtering, under the same conditions as S1. S4: Electron beam resist (PMMA 950K, approximately 200 nm thick) was spin-coated onto the Al top layer, and a biatomic umbrella-shaped resonator pattern was defined using electron beam lithography (accelerating voltage 100 keV, dose approximately 1200 μC / cm²). After development, Al etching was performed using ICP-RIE: a Cl₂ / BCl₃ mixed gas (Cl₂:BCl₃ = 4:1), ICP power 500 W, RF bias power 100 W, working pressure 10 mTorr, until the ITO layer was exposed. The final device structure was obtained after resist removal.

[0033] The key features of this preparation method are: (1) it involves only three thin film deposition steps (compared to the 4–5 deposition steps of traditional HfO2 / Al2O3-based electrically tunable devices); (2) it has no precise growth steps of insulating barrier layer, which reduces the difficulty of the process; (3) all materials (Al, ITO, quartz) and processes (magnetron sputtering, electron beam lithography, ICP-RIE) are standard materials and processes in the semiconductor industry.

[0034] The electrical tuning operation steps in this embodiment are as follows: Circuit connection: Connect the bottom Al reflective layer (100) to the negative terminal (or ground) of the DC voltage source via wire bonding; connect the top Al nanostructure layer (300) to the positive terminal of the DC voltage source via probes or bonding wires. Initial state: When V_g = 0, the ITO waveguide layer is in a zero bias state, and the carrier concentration N0 ≈ 3 × 10² 0 cm⁻³, the real part of the ITO dielectric constant ε_r ≈ 3.9, and the effective refractive index n_eff ≈ 1.8–2.0. At this point, CD has a specific initial amplitude and sign at the target wavelength (e.g., 1550 nm). Forward bias (accumulation mode): V_g is slowly increased from 0 to +10 V. As V_g increases, electrons accumulate at the ITO interface near the top electrode, and the local carrier concentration N increases. When N increases from 3 × 10² 0 Increased to 5×10² 0 At cm⁻³, ε_r decreases from approximately 3.9 to near zero (ENZ region), and n decreases from approximately 1.8 to approximately 0.3+ik (metallic transition). The amplitude of CD is observed to change continuously, with a sign reversal occurring near the ENZ crossing point. Negative bias (depletion mode): V_g is slowly changed from 0 to -10 V. Electrons are depleted at the ITO interface, and N decreases to approximately 1 × 10². 0 cm⁻³, ε_r increases to approximately 5.0 or higher, and n increases to approximately 2.2 or higher. The amplitude of CD changes in the opposite direction relative to the initial state.

[0035] Specific tuning example (based on COMSOL simulation): V_g = -10 V (n_eff ≈ 2.24, ε_r ≈ 5.0): CD ≈ +0.32 (LCP reflection is dominant) V_g = 0 V (n_eff ≈ 1.9, ε_r ≈ 3.9): CD ≈ -0.15 (RCP reflection is slightly better) V_g = +5 V(n_eff → ENZ, ε_r → 0): CD ≈ -0.90(RCP reflection is significantly dominant) The results show that continuous tuning of CD from +0.32 to -0.90, including sign inversion, was achieved in the range of -10 V to +5 V. The geometry of the metasurface remained unchanged during tuning.

[0036] Element boundary conditions: Floquet periodic boundary (x and y directions) Excitation Port: Periodic Port, with incident light of x-polarized and y-polarized light respectively. Frequency range: 166–250 THz (corresponding wavelength 1200–1800 nm) The real part of the dielectric constant ε'_wg of the ITO waveguide layer is 2.0–5.0 (step size 0.2), corresponding to n = 1.41–2.24. Using the Jones matrix method: extract the complex reflection fields r_xx, r_yx, r_xy, and r_yy under x- and y-polarized incident incidence, calculate the LCP and RCP reflectivities, and obtain CD = |r_LCP|² - |r_RCP|² Simulation results ( Figure 3 and Figure 4 )show: (1) When ε'_wg = 2.0 (n ≈ 1.41, corresponding to a strong accumulation state), CD reaches a negative maximum of about -0.90 at about 1530 nm (RCP reflection is significantly dominant). (2) As ε'_wg gradually increases (n increases, corresponding to a change in voltage from positive to negative), the amplitude of CD changes continuously; (3) When ε'_wg = 5.0 (n ≈ 2.24, corresponding to the dissipated state), the CD sign flips to a positive value (LCP reflection is dominant). (4) The CD sign inversion occurs in the interval ε'_wg ≈ 2.5–3.5 (corresponding to the ENZ region of ITO).

Claims

1. A metasurface circular dichroism dynamically modulated structure based on an ITO electrically tunable waveguide, characterized in that, include: The bottom metal reflective layer (100), made of metallic material Al or Au, has a thickness of 150–250 nm and also serves as the bottom electrode of the structure; The central indium tin oxide (ITO) waveguide layer (200), with a thickness of 150–200 nm, serves as both an electrically tunable dielectric and a carrier transport channel. The top metal nanostructure layer (300), made of metallic material Al, with a thickness of 60–80 nm, is patterned into a periodically arranged array of biatomic umbrella-shaped resonators and also serves as the top electrode of the structure. The bottom metal reflective layer (100), the middle ITO waveguide layer (200) and the top metal nanostructure layer (300) constitute a metal-ITO-metal (MIM) waveguide structure, which supports guided mode resonance (GMR). Each of the diatomic umbrella resonator units includes a first umbrella resonator (310) with a first rotation angle θ_A and a second umbrella resonator (320) with a second rotation angle θ_B, and θ_A ≠ θ_B, in order to break the mirror symmetry of the unit structure; The effective refractive index n_eff of the middle ITO waveguide layer (200) is configured to be tuned by an external DC bias voltage V_g applied between the bottom metal reflective layer (100) and the top metal nanostructure layer (300), thereby modulating the interference phase difference between the guided modes excited by the first umbrella resonator and the second umbrella resonator respectively, and realizing continuous electrical control of the amplitude and sign of the circular dichroism (CD) of the reflection.

2. A method for electrically controlling the circular dichroism of a metasurface, characterized in that, Using the structure of claim 1 includes the following steps: (a) A DC bias voltage V_g is applied between the bottom metal reflective layer (100) and the top metal nanostructure layer (300); (b) By adjusting the amplitude and polarity of the DC bias voltage V_g, the electron carrier concentration N in the middle ITO waveguide layer (200) is changed, thereby controlling its effective refractive index n_eff; (c) By changing the effective refractive index n_eff, the interference phase condition of guided mode resonance in the metal-ITO-metal waveguide structure is altered; (d) Achieve continuous amplitude adjustment and sign reversal of the circular dichroism (CD) of reflection at a fixed incident wavelength.

3. A method for preparing the dynamically tunable metasurface circular dichroism structure of claim 1, characterized in that, Includes the following steps: S1: Deposit a bottom metal reflective layer (100) on a transparent substrate using magnetron sputtering, with a thickness of 150–250 nm; S2: Deposit a central ITO waveguide layer (200) on the bottom metal reflective layer (100) using magnetron sputtering with a thickness of 150–200 nm, and anneal it at 250–350°C in an oxygen-containing atmosphere; S3: Deposit a top metal layer on the central ITO waveguide layer (200) using a magnetron sputtering process, with a thickness of 60–80 nm; S4: Perform electron beam lithography and inductively coupled plasma reactive ion etching (ICP-RIE) on the top metal layer to form the dual-atom umbrella-shaped resonator array; The preparation method involves only three thin film deposition processes and one etching process, without the need for an additional insulating barrier layer deposition step.

4. The structure according to claim 1, characterized in that, The geometric parameters of the diatomic umbrella-shaped resonator unit are as follows: The x-direction period Px = 1000–1400 nm; The period in the y-direction is Py = 500–700 nm; The outer radius of the umbrella-shaped resonator is r_e = 210–250 nm; The inner radius of the umbrella-shaped resonator is r_i = 140–180 nm; The width of the connecting rod, w_stick, is 60–80 nm. The rotation angle θ_A of the first umbrella-shaped resonator is 75°–85°; The rotation angle θ_B of the second umbrella-shaped resonator is 60°–70°; The interatomic distance d = 400–700 nm.

5. The method according to claim 2, characterized in that, The DC bias voltage V_g is adjustable from -10 V to +10 V, where: When V_g > 0, the carrier concentration increases and the effective refractive index decreases; When V_g < 0, electrons are depleted at the interface, carrier concentration decreases, and effective refractive index increases; Within the specified range, the sign of circular dichroism (CD) is inverted at least once.

6. The method according to claim 5, characterized in that, The polarity reversal of the DC bias voltage V_g causes the circular dichroism sign to switch from a positive value (dominant left-handed circular polarization reflection) to a negative value (dominant right-handed circular polarization reflection), or vice versa, and the switching process does not involve any mechanical change to the geometric parameters of the structure.

7. The structure according to claim 1, characterized in that, The diatomic umbrella-shaped resonator is a non-closed ring structure with an arc-shaped umbrella surface. The arc-shaped umbrella surface is collinearly connected to the connecting rod of the adjacent resonator unit through the connecting rod, forming an umbrella-rod-umbrella-shaped conjoined diatomic configuration.

8. A spatial light modulator, characterized in that, The array comprises a pixel unit array consisting of the structures described in any one of claims 1 to 7, wherein each pixel unit has an independent DC bias voltage control, and can spatially selectively control the circular polarization component of the incident light.

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