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1. Circular dichroism in achiral metasurfaces induced by spatially selective coupling with molecules

   https://doi.org/10.1364/OL.537725  

   Han, Baojuan; Yang, Xiaodong; Gao, Jie (January 2025, Optics Letters)

   Achiral metasurfaces with near-field optical chirality have attracted great attention in molecular sensing and chiral emission control. Here, the circular dichroism (CD) response of an achiral metasurface induced by spatially selective coupling with polymethyl methacrylate (PMMA) molecules is demonstrated. A designed achiral metasurface with a V-shaped resonator exhibits large optical chirality with a strongly dissymmetric distribution under circular polarization. By introducing a PMMA molecule layer on top of the metasurface, which covers the area with large optical chirality, CD in absorption of 0.38 and a dissymmetric factor of optical chiralityg_cof 0.16 are obtained. Furthermore, an analysis of the coupled harmonic oscillator model reveals stronger coupling strength between the PMMA layer and the metasurface under RCP incidence, compared to the LCP case. Moreover, it is shown that the far-field CD response of the metasurface is linearly correlated with the dissymmetric near-field optical chirality distribution. The demonstrated results present the potential for advancing applications in chiral molecule vibrational sensing, thermal emission control, and infrared chiral imaging. 

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2. Electroelastic metasurface with resonant piezoelectric shunts for tunable wavefront control

   https://doi.org/10.1088/1361-6463/acbd5f  

   Lin, Z.; Tol, S. (March 2023, Journal of Physics D: Applied Physics)

   Abstract In this paper, we design a tunable phase-modulated metasurface composed of periodically distributed piezoelectric patches with resonant-type shunt circuits. The electroelastic metasurface can control the wavefront of the lowest antisymmetric mode Lamb wave (A₀mode) in a small footprint due to its subwavelength features. The fully coupled electromechanical model is established to study the transmission characteristics of the metasurface unit and validated through numerical and experimental studies. Based on the analysis of the metasurface unit, we first explore the performance of electroelastic metasurface with single-resonant shunts and then extend its capability with multi-resonant shunts. By only tuning the electric loads in the shunt circuits, we utilize the proposed metasurface to accomplish wave deflection and wave focusing ofA₀mode Lamb waves at different angles and focal points, respectively. Numerical simulations show that the metasurface with single-resonant shunts can deflect the wavefront of 5 kHz and 6 kHz flexural waves by desired angles with less than $2\mathrm{\%}$deviation. In addition, it can be tuned to achieve nearly three times displacement amplification at the designed focal point for a wide range of angles from $-{75}^{\circ }$to 75^∘. Furthermore, with multi-resonant shunts, the piezoelectric-based metasurface can accomplish anomalous wave control over flexural waves at multiple frequencies (i.e. simultaneously at 5 kHz and 10 kHz), developing new potentials toward a broad range of engineering applications such as demultiplexing various frequency components or guiding and focusing them at different positions. 

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3. Enhanced biochemical sensing with high- Q transmission resonances in free-standing membrane metasurfaces

   https://doi.org/10.1364/OPTICA.549393  

   Rosas, Samir; Adi, Wihan; Beisenova, Aidana; Biswas, Shovasis_Kumar; Kuruoglu, Furkan; Mei, Hongyan; Kats, Mikhail_A; Czaplewski, David_A; Kivshar, Yuri_S; Yesilkoy, Filiz (February 2025, Optica)

   Optical metasurfaces provide solutions to label-free biochemical sensing by localizing light resonantly beyond the diffraction limit, thereby selectively enhancing light–matter interactions for improved analytical performance. However, high-Qresonances in metasurfaces are usually achieved in the reflection mode, which impedes metasurface integration into compact imaging systems. Here, we demonstrate a metasurface platform for advanced biochemical sensing based on the physics of the bound states in the continuum (BIC) and electromagnetically induced transparency (EIT) modes, which arise when two interfering resonances from a periodic pattern of tilted elliptic holes overlap both spectrally and spatially, creating a narrow transparency window in the mid-infrared spectrum. We experimentally measure these resonant peaks observed in transmission mode (Q∼734 atλ∼8.8µm) in free-standing silicon membranes and confirm their tunability through geometric scaling. We also demonstrate the strong coupling of the BIC-EIT modes with a thinly coated PMMA film on the metasurface, characterized by a large Rabi splitting (32cm⁻¹) and biosensing of protein monolayers in transmission mode. Our new photonic platform can facilitate the integration of metasurface biochemical sensors into compact and monolithic optical systems while being compatible with scalable manufacturing, thereby clearing the way for on-site biochemical sensing in everyday applications. 

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4. Strong Metasurface–Josephson Plasma Resonance Coupling in Superconducting La _{2− x} Sr _x CuO ₄

   https://doi.org/10.1002/adom.201900712  

   Schalch, Jacob_S; Post, Kirk; Duan, Guangwu; Zhao, Xiaoguang; Kim, Young_Duck; Hone, James; Fogler, Michael_M; Zhang, Xin; Basov, Dmitri_N; Averitt, Richard_D (August 2019, Advanced Optical Materials)

   Abstract Terahertz spectroscopy of thec‐axis Josephson plasma resonance (JPR) in high‐temperature cuprates is a powerful probe of superconductivity, providing a route to couple to and interact with the condensate. Electromagnetic coupling between metasurface arrays of split ring resonators (SRRs) and the JPR of a La_2−xSr_xCuO₄single crystal (T_c= 32 K) is investigated. The metasurface resonance frequency (ω_MM), determined by the SRR geometry, is swept through the JPR frequency (ω_JPR= 1.53 THz) using a series of interchangeable tapes applied to the same single crystal. Terahertz reflectivity measurements on the resulting hybrid superconducting metamaterials (HSMMs) reveal anticrossing behavior characteristic of strong coupling. The experimental results, validated with numerical simulations, indicate a normalized Rabi frequency of Ω_R= 0.29. Further, it is shown that HSMMs with ω_MM> ω_JPRprovide a route to couple to hyperbolic waveguide modes inc‐axis cuprate samples. This work informs future possibilities for optimizing the coupling strength of HSMMs and investigating nonlinear superconductivity under high field terahertz excitation. 

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5. Effective medium metasurfaces using nanoimprinting of the refractive index: design, performance, and predictive tolerance analysis

   https://doi.org/10.1364/OME.515617  

   Panipinto, Matthew; Ryckman, Judson_D (March 2024, Optical Materials Express)

   Production of flat optics incorporating subwavelength features, particularly at visible frequencies, remains a significant challenge. Here, we establish a framework for the design of effective medium metasurfaces (EMM), relying on nanoimprinting of mesoporous silicon to realize a patterned refractive indexn(x,y) corresponding to an arbitrary transmitted phase profileϕ(x,y). The method is used to design the stamp profile required to produce a Fresnel lens and the theoretical performance of the metalens is examined using the finite-difference time-domain method. Additionally, we demonstrate neural network aided Monte Carlo analysis as a method to model the effects of metasurface fabrications errors on EMM performance and process yield. 

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