Terahertz spin-selective perfect absorption enabled by quasi-bound states in the continuum

Local control of polarization and geometric phase in thermal metasurfaces

Thermal emission from a hot body is inherently challenging to control due to its incoherent nature. Recent advances have shown that patterned surfaces can transform thermal emission into partially coherent beams with tailored directionality and frequency selectivity. Here we experimentally demonstrate polarization-selective, unidirectional and narrowband thermal emission using single-layer metasurfaces. By implementing polarization gradients across the surface, we unveil a generalization of the photonic Rashba effect from circular polarizations to any pair of orthogonal polarizations and apply it to thermal emission. Leveraging pointwise specification of arbitrary elliptical polarization, we implement a thermal geometric phase and leverage it to prove previous theoretical predictions that asymmetric chiral emission is possible without violating reciprocity. This general platform can be extended to other frequency regimes in efforts to compactify metasurface optics technologies without the need for external coherent sources.

From Local to Nonlocal High-Q Plasmonic Metasurfaces

The physics of bound states in the continuum (BICs) allows the design and demonstration of optical resonant structures with large values of the quality factor (Q factor) by employing dielectric structures with low losses. However, BIC is a general wave phenomenon that should be observed in many systems, including the metal-dielectric structures supporting surface plasmon polaritons where optical resonances are hindered by losses. Here we suggest and develop a comprehensive strategy to achieve high-Q resonances in plasmonic metasurfaces by effectively tailoring the resonant modes from local to nonlocal regimes, thus transitioning from quasi-isolated localized resonances to extended resonant modes involving strong interaction among neighboring structure metaunits.

Switchable Terahertz Metasurfaces for Spin-Selective Absorption and Anomalous Reflection Based on Vanadium Dioxide

Conventional chiral metasurfaces are constrained by predetermined functionalities and have limited versatility. To address these constraints, we propose a novel chirality-switchable terahertz (THz) metasurface with integrated heating control circuits tailored for spin-selective anomalous reflection, leveraging the phase-change material vanadium dioxide (VO2). The reversible and abrupt insulator-to-metal phase transition feature of VO2 is exploited to facilitate a chiral meta-atom with spin-selectivity capabilities. By employing the Pancharatnam-Berry phase principle, complete 2π reflection phase coverage is achieved by adjusting the orientation of the chiral structure. At the resonant frequency of 0.137 THz, the designed metasurface achieves selective absorption of a circularly polarized wave corresponding to the state of the VO2 patches. Concurrently, it reflects the circularly polarized wave of the opposite chirality anomalously at an angle of 28.4° while maintaining its handedness. This chirality-switchable THz metasurface exhibits promising potential across various applications, including wireless communication data capacity enlargement, polarization modulation, and chirality detection.

Using quasi-bound states in the continuum in an all-dielectric metasurface array to enhance terahertz fingerprint sensing

The terahertz absorption fingerprint spectrum is crucial for qualitative spectral analysis, revealing the rotational or vibrational energy levels of numerous biological macromolecules and chemicals within the THz frequency range. However, conventional sensing in this band is hindered by weak interactions with trace analytes, leading to subtle signals. In this Letter, an all-dielectric metasurface array is proposed to enhance the absorption fingerprint spectrum using quasi-bound states in the continuum (BIC) resonance. The observable quasi-BIC resonance is achieved by breaking the symmetry of the C2v structure. The periodic dimensions of the structure are adjusted to excite quasi-BIC resonances at different frequencies, thereby enhancing the fingerprint spectra of four different substances. By exploiting the correlation between the Q-factor and absorption across different frequencies, calibration of the molecular absorption fingerprint spectrum obtained through metasurface sensing yields precise enhanced absorption fingerprint spectra for various substances within the 0.55-1.6 THz range. Our Letter introduces a novel, to the best of our knowledge, strategy for trace sensing in the THz frequency range, demonstrating the promising potential for enhanced absorption fingerprint spectrum sensing.

Passive trapping of biomolecules in hotspots with all-dielectric terahertz metamaterials

Electromagnetic metamaterials feature the capability of squeezing photons into hotspot regions of high intensity near-field enhancement for strong light-matter interaction, underpinning the next generation of emerging biosensors. However, randomly dispersed biomolecules around the hotspots lead to weak interactions. Here, we demonstrate an all-silicon dielectric terahertz metamaterial sensor design capable of passively trapping biomoleculars into the resonant cavities confined with powerful electric field. Specifically, multiple controllable high-quality factor resonances driven by bound states in the continuum (BIC) are realized by employing longitudinal symmetry breaking. The dielectric metamaterial sensor with nearly 15.2 experimental figure-of-merit enabling qualitative and quantitative identification of different amino acids by delivering biomolecules to the hotspots for strong light-matter interactions. It is envisioned that the presented strategy will enlighten high-performance meta-sensors design from microwaves to visible frequencies, and serve as a potential platform for microfluidic sensing, biomolecular capture, and sorting devices.

Flexible design of chiroptical response of planar chiral metamaterials using deep learning

Optical chirality is highly demanded for biochemical sensing, spectral detection, and advanced imaging, however, conventional design schemes for chiral metamaterials require highly computational cost due to the trial-and-error strategy, and it is crucial to accelerate the design process particularly in comparably simple planar chiral metamaterials. Herein, we construct a bidirectional deep learning (BDL) network consists of spectra predicting network (SPN) and design predicting network (DPN) to accelerate the prediction of spectra and inverse design of chiroptical response of planar chiral metamaterials. It is shown that the proposed BDL network can accelerate the design process and exhibit high prediction accuracy. The average process of prediction only takes ∼15 ms, which is 1 in 40000 compared to finite-difference time-domain (FDTD). The mean-square error (MSE) loss of forward and inverse prediction reaches 0.0085 after 100 epochs. Over 95.2% of training samples have MSE ≤ 0.0042 and MSE ≤ 0.0044 for SPN and DPN, respectively; indicating that the BDL network is robust in the inverse deign without underfitting or overfitting for both SPN and DPN. Our founding shows great potentials in accelerating the on-demand design of planar chiral metamaterials.

Coupling-Based Multiple Bound States in the Continuum and Grating-Assisted Permittivity Retrieval in the Terahertz Metasurface

The terahertz (THz) metasurfaces that support bound states in the continuum (BICs) provide a promising platform for various applications due to their high Q-factor resonance. In this study, we experimentally demonstrate multiple BICs with different resonance symmetries in the THz metasurface based on mode coupling. The proposed metasurface is composed of 2 × 2 split ring resonators (SRRs) metamolecules. The SRRs of two different gap angles in the metamolecule lattice provide intrinsic resonance with different frequencies, and the coupling between them exhibits high transmission quasi-BIC resonance, which can be tuned by varying the gap angle. The arrangement of SRRs in the 2 × 2 metamolecule lattice determines the types of coupling that govern the resonance symmetry of quasi-BIC. More interestingly, the multiple quasi-BICs enabled by different couplings can be simultaneously achieved in a metasurface. Apart from tuning the gap angles, the permittivity in the vicinity of SRRs also changes the resonance frequency. Consequently, quasi-BIC can be artificially formed by deliberately constructing the permittivity difference of the dielectric environment on the SRRs. In view of this, we introduce the scheme of permittivity retrieval for the dispersive analyte, assisted by the fixed-permittivity gratings. In addition, we experimentally demonstrate the metasurface in combination with the microfluidic chip for the sensing of the glucose solution concentration. Our findings offer a possible strategy for the existing manufactured metasurface to achieve quasi-BIC resonance and provide a promising candidate for approaching the spectral analysis of the biochemical.

Ultrahigh-Q and angle-robust chiroptical resonances beyond BIC splitting

Chiroptical resonances inspired by bound states in the continuum (BICs) open a new, to the best of our knowledge, avenue to enhance chiral light-matter interaction. Symmetry breaking is the widely employed way, wherein the circularly polarized states (CPSs) arise from BIC splitting. Here, we utilize a far-field interference mechanism to create ultrahigh-Q (typically, 2.36 × 106) chiroptical resonance beyond BIC splitting, in which CPSs coexist with BICs in the momentum space. Accordingly, the spin-selective absorption with ultranarrow linewidth is achieved at the CPS points, which can be regulated by monolayer transition metal dichalcogenides (TMDCs). In addition, the chiral response of our scheme exhibits the incident-direction robustness and flexible tunability. Our findings may facilitate potential applications in light manipulation, spin-valley interaction, and chiral sensing.

Active control of circular dichroism in a graphene-metal hybridized metamaterial driven by symmetry-protected bound states in the continuum

Active control of chirality in plasmonic metamaterials is of great importance due to their potential for diverse applications in imaging, communication and spectroscopy. Recently, inspired by the concept of bound states in the continuum (BIC), strong chiroptical responses are constructed in metamaterials by introducing structural asymmetries. However, most of these chiral metamaterials are static and cannot be modulated. Herein, we theoretically demonstrate a novel approach for manipulating chiroptical responses with enhanced circular dichroism (CD) and large modulation depths in a graphene-metal hybridized metamaterial. By introducing a structured graphene and adjusting the Fermi energy (EF), the conversion between BIC and quasi-BIC states is achieved successfully. The proposed device demonstrates a tuneable CD in the range of 0.693-0.008 when EF is adjusted from 0.01 eV to 1.0 eV, which can be further improved by optimizing its geometry. The proposed graphene-metal hybridized metamaterial paves a new way for manipulating polarization states at terahertz frequencies and is of great potential for practical applications such as dynamic display and optoelectronic modulation.

Spin-dependent and tunable perfect absorption in a Fabry-Perot cavity containing a multi-Weyl semimetal

Spin-dependent absorption has been widely studied in metamaterials and metasurfaces with chirality since it develops significant applications in multiplexed holograms, photodection, and filtering. Here, the one-dimensional photonic crystal Fabry-Perot (FP) cavity containing a multi-Weyl semimetal (mWSM) defect is proposed to investigate the spin-dependent perfect absorption. Results denote that the distinct refractive indices of right hand circularly polarized (RCP) and left hand circularly polarized (LCP) waves are present due to the nonzero off-diagonal term of mWSM, thus supporting the perfect absorption of RCP and LCP waves at distinct resonant wavelengths. The different perfect absorption wavelengths of RCP and LCP waves reveal the spin-dependent perfect absorption. By altering the Fermi energy, tilt degree of Weyl cones, Weyl nodes separation, topological charge, and thickness of the mWSM layer, the perfect absorption wavelength of RCP and LCP waves can be regulated conveniently. Particularly, the linear tunable perfect absorption wavelength with thickness of the mWSM layer supports the accurate determination of perfect absorption wavelength at distinct mWSM thicknesses. Our studies develop simple and effective approaches to acquire the spin-dependent and adjustable perfect absorption without the external magnetic field, and can find practical applications in spin-dependent photonic devices.

Photonic spin-selective perfect absorptance on planar metasurfaces driven by chiral quasi-bound states in the continuum

Optical metasurfaces with high-quality-factor resonances and selective chirality simultaneously are desired for nanophotonics. Here, an all-dielectric planar chiral metasurface is theoretically proposed and numerically proved to support the astonishing symmetry-protected bound state in the continuum (BIC), due to the preserved π rotational symmetry around the z axis and up-down mirror symmetry simultaneously. Importantly, such BIC is a vortex polarization singularity enclosed by elliptical eigenstate polarizations with non-vanishing helicity, owing to the broken in-plane mirror symmetry. Under the oblique incidence, companied by the BIC transforming into a quasi-BIC (Q-BIC), the strong extrinsic chirality manifests. Assisted by the single-port critical coupling, the planar metasurface can selectively and near-perfectly absorb one circularly polarized light but non-resonantly reflect its counterparts. The circular dichroism (CD) approaching 0.812 is achieved. Intriguingly, the sign of CD (namely, the handedness of the chiral metasurface) can be flexibly manipulated only via varying the azimuthal angle of incident light, due to the periodic helicity sign flip in eigen polarizations around the BIC. Numerical results are consistent with the coupled-mode theory and multipole decomposition method. The spin-selective metasurface absorber empowered by the physics of chiral Q-BICs undoubtedly may promise various applications such as optical filters, polarization detectors, and chiral imaging.

Tunable dual quasi-bound states in continuum and electromagnetically induced transparency enabled by the broken material symmetry in all-dielectric compound gratings

Dual quasi-bound states in continuum (quasi-BICs) enabled by the broken geometric symmetry offer an effective way to design high-quality photonic devices, yet challenged by tunable functionalities. Here we employ the material asymmetry originating from the tunable material property of phase-change materials to design quasi-BICs in all-dielectric compound gratings. We find the even and odd quasi-BICs are modulated by the geometric and material asymmetries, respectively, and this effect is ensured by two different types of structural symmetries in the compound structure. Particularly, tunable electromagnetically induced transparency (EIT) can be achieved by modulating the material asymmetry. Furthermore, we systematically design the compound gratings consisting of the phase-change material of Sb₂Se₃ to demonstrate tunable dual quasi-BICs and EITs. Analytical calculations and numerical simulations are performed to verify these findings. Our work provides a promising way to enhance the flexibility of realizing quasi-BICs, which may boost tunable applications in nanodevices assisted by quasi-BICs.

Observation of nonvanishing optical helicity in thermal radiation from symmetry-broken metasurfaces

Spinning thermal radiation is a unique phenomenon observed in condensed astronomical objects, including the Wolf-Rayet star EZ-CMa and the red degenerate star G99-47, due to the existence of strong magnetic fields. Here, by designing symmetry-broken metasurfaces, we demonstrate that spinning thermal radiation with a nonvanishing optical helicity can be realized even without applying a magnetic field. We design nonvanishing optical helicity by engineering a dispersionless band that emits omnidirectional spinning thermal radiation, where our design reaches 39% of the fundamental limit. Our results firmly suggest that metasurfaces can impart spin coherence in the incoherent radiation excited by thermal fluctuations. The symmetry-based design strategy also provides a general pathway for controlling thermal radiation in its temporal and spin coherence.

Highly sensitive terahertz sensing with 3D-printed metasurfaces empowered by a toroidal dipole

Highly sensitive terahertz (THz) sensing with metasurfaces has attracted considerable attention recently. However, ultrahigh sensing sensitivity remains a huge challenge for practical applications. To improve the sensitivity of these devices, herein we have proposed an out-of-plane metasurface-assisted THz sensor consisting of periodically arranged bar-like meta-atoms. Benefiting from elaborate out-of-plane structures, the proposed THz sensor with high sensing sensitivity of 325 GHz/RIU can be easily fabricated via a simple three-step fabrication process, and the maximum sensing sensitivity can be ascribed to toroidal dipole resonance-enhanced THz-matter interactions. The sensing ability of the fabricated sensor is experimentally characterized by the detection of three types of analytes. It is believed that the proposed THz sensor with ultrahigh sensing sensitivity and its fabrication method might provide great potential in emerging THz sensing applications.

Angle-selective chiral absorption induced by diffractive coupling in metasurfaces

Here we report that a simple chiral metasurface with twisted metallic cut-wire arrays enables highly efficient and continuously tunable chiral absorption over a broad spectral range by scanning the incidence angle over a few degrees. The angle-selective chiral absorption results from the surface plasmon resonance (SPR) excited by diffractive effects of the metasurface. The diffraction-assisted chiral metasurface provides a straightforward strategy for achieving dynamically tunable chiral devices and offers intriguing possibilities for various applications in on-chip chiral detectors/emitters, chiral spectrometers, chiral lasers, and so on.

THz absorbers with an ultrahigh Q-factor empowered by the quasi-bound states in the continuum for sensing application

The exceptional resonances excited by symmetry-protected quasi-bound states in the continuum (QBICs) have provided significant potential in high-sensitive sensing applications. Herein, we have proposed a type of metal-insulator-metal (MIM) absorbers supported by QBIC-induced resonances, and the ideal Q-factors of QBIC-induced resonances can be enhanced up to 10⁵ in the THz regime. The coupled mode theory and the multipole scattering theory are employed to thoroughly interpret the QBIC-induced absorption mechanism. Furthermore, the refractive index sensing capacities of the as-presented absorbers have been investigated, where the maximum values of the sensing sensitivity and figure of merit (FOM) can reach up to 187 GHz per refractive index unit and 286, respectively. Therefore, it is believed that the proposed absorbers enabled by QBIC-induced resonances hold promising potential in a broad range of highly demanding sensing applications.

Quasi-Bound States in the Continuum Enabled Strong Terahertz Chiroptical Response in Bilayer Metallic Metasurfaces

Bound state in the continuum (BIC) as a novel non-radiating state of light in the continuum of propagating modes has received great attention in photonics. Recently, chiral BICs have been introduced in the terahertz regime. However, strong chiroptical effects of transmitted waves remain challenging to achieve in metallic terahertz metasurfaces, especially for intrinsic chirality at normal incidences. Here, we propose a chiral quasi-BIC by simultaneously breaking the out-of-plane mirror and in-plane C2 rotation symmetries in a bilayer metallic metasurface, in which spin-selective terahertz transmittance is successfully realized. Benefiting from the symmetry-protected nature of our proposed BIC, precise tuning of structural parameters can lead to anticipated chiroptical performance. As a degree of freedom, the rotation angle of the split ring gaps can fully determine the handedness, linewidth, and working frequency with strong circular dichroism. Besides, the sensing performance shows a surrounding refractive index sensitivity of 200 GHz/RIU, which is similar to those of previous works based on terahertz metasurfaces. Taking advantage exclusively of symmetry-protected BICs to realize transmitted terahertz chiroptical response provides fresh insights into the creation of novel BICs, which enables profound advancements in the surging field of novel terahertz devices.

Ultra-narrow-band circular dichroism by surface lattice resonances in an asymmetric dimer-on-mirror metasurface

Narrow-linewidth circular dichroism (CD) spectroscopy is a promising candidate to push the limits of molecular handedness detection toward a monolayer or even to a single molecule level. Here, we designed a hybrid metasurface consisting of a periodic array of symmetry-breaking dielectric dimers on a gold substrate, which can generate strong CD of 0.44 with an extremely-narrow linewidth of 0.40 nm in the near-infrared. We found that two surface lattice resonance modes can be excited in the designed metasurface, which can be superimposed in the crossing spectral region, enabling a remarkable differential absorption with a high Q-factor for circular polarizations. The multipole decomposition of the resonance modes shows that the magnetic dipole component contributes most to the CD. Our simulation results also show that the CD response of the chiral structure can be engineered by modulating the structural parameters to reach the optimal CD performance. Ultra-narrow-linewidth CD response offered by the proposed metasurface with dissymmetry provides new possibilities towards design of the high-sensitive polarization detecting, chiral sensing and efficient chiral light emitting devices.