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Terahertz confocal imaging allows 3D see‐through of a non‐metallic object with high resolution. Conventional methods acquiring 3D images of thick objects suffer from limited depth‐of‐field, constrained depth resolution, and/or inconsistent spatial resolution at different depths. To address these limitations, the intrinsic chromatic aberration of a typical focusing metasurface is exploited to achieve frequency‐dependent focal lengths. An object located within this extended focal range can be readily 3D inspected by performing 2D raster scans. A rigorous analysis reveals that the focal spot maintains a constant waist diameter of 2.4 mm (equivalent to 2.2λ0$\lambda _{0}$ at 275 GHz) and migrates 68.1 mm (equivalent to 62.4λ0$\lambda _{0}$, or 16.4 times of Rayleigh length, or 1.4‐fold of the designed focal length at 275 GHz) from 175 to 525 GHz, and thus achieving a consistent spatial resolution and a large depth‐of‐field for 3D imaging. Importantly, this large depth‐of‐field is achieved with a relatively high numerical aperture of around 0.42. Measurements conducted between 220 and 330 GHz exhibit close agreement with the calculation. To demonstrate its imaging functionality, two stacked papers with different texts, a mobile phone, and earphones concealed in a charging case are imaged, where a short‐time Fourier transform is implemented in the time‐domain terahertz images to enhance image contrast. The presented metasurface is technologically significant for imaging systems to rapidly inspect objects in 3D with exceptional resolutions. Its potential applications include in‐situ defect detection and object identification in security screening. [ABSTRACT FROM AUTHOR]

Harnessing confined light on a subwavelength scale within Fano metasurfaces enhances light‐matter interaction on a flexible, low‐loss substrate. This approach enables the integration of ultra‐thin semiconducting films for designing low‐power, ultrafast switchable terahertz, and optical meta devices. Here, an ultra‐thin, ∼λ/6000 germanium overlayers of 25nm or 50nm is thermally evaporated onto a flexible Fano metallic metasurface to achieve ultrafast optical modulation of terahertz radiation. A remarkable 85% modulation depth with an ultrafast speed of 400 GHz is obtained in the resonant transmission. These ultrathin, flexible, and ultrafast functional metasurfaces pave the way for developing flexible terahertz active meta devices based on nanometric scale germanium films, offering the additional advantage of compatibility with complementary metal‐oxide‐semiconductor (CMOS) technology in microelectronics. [ABSTRACT FROM AUTHOR]

The control of the speed of terahertz waves is always a challenge since the bandgap of most optical materials is much larger beyond meV with subtle nonlinear susceptibility. Moiré metasurfaces are shown to exhibit wide tunable optical properties and extraordinary physical phenomena at specific commensurate angles. These can be achieved by a careful design of the metasurface to manipulate terahertz slow light. Herein, we demonstrate a triple layer Moiré metasurface with a distinct electromagnetically induced transparency (EIT) phenomenon at commensurate angles. The proposed metasurface is composed of an intrinsic square lattice embedded into another Moiré photonic superlattice made of twisted square lattice at commensurate angles of 10.39° and 7.63°. The coupling between adjacent meta-atoms on the combined metasurface leads to destructive interference of dual trapped lattice modes, which results in a transparency window at the terahertz band. A maximum group delay of 9.76 ps is found at the transparent window of 0.84 THz when the commensurate angle is 10.39°. When the commensurate angle reduces to 7.63°, the transparency window shifts to 0.57 THz with a 5.96 ps group delay. The coupled Lorentz oscillator model indicates that the nonlinear optical susceptibility at transparency windows is above zero. Our results create an approach to tune the EIT as well as slow light in the terahertz band. Our device can have potential applications in terahertz signal processing and storage. [ABSTRACT FROM AUTHOR]

In a system of C2 symmetry, symmetry-protected bound states in the continuum (SP-BICs) exist with a continuous spectrum of radiating waves that can carry energy away and enable an infinite radiative quality (Q) factor and zero linewidth. However, the SP-BICs transform into quasi-BICs by breaking the C2 symmetry of the system, where the resonance lifetime and linewidth become finite and measurable. As such, the quasi-BICs are very sensitive to the polarization of incident radiation. Owing to the application of a biosensor or a lasing device, it is unavoidable to work with an unpolarized radiative beam. Herein, we propose a metasurface in a C4 symmetric layout, which exhibits polarization-insensitive terahertz symmetry-protected quasi-BICs. The orientations of adjacent two meta-molecules (MMs) are designed to be orthogonal to each other. By tuning the degree of asymmetry along the orientation of MMs, the quasi-BICs exhibit insensitivity to the polarization of the incident terahertz wave. A large degree of asymmetry results in a deformation of an electric quadrupole, which forms an energy leaky channel to the free space. Due to the translational symmetry, the wave-vector of the lattice in C4 symmetry is conserved so that the electric components of transmitted radiation along the x axis is identical to that along the y axis, Txx = Tyy. As such, the leaky channel of electromagnetic scattering becomes insensitive to the incident polarization. Our results present an approach to achieve polarization-insensitive quasi-BICs in a topologically symmetric metasurface, which is helpful for the innovation of terahertz biosensor. [ABSTRACT FROM AUTHOR]

The excitation and manipulation of symmetry-protected bound states in the continuum (SP-BIC) is significantly valuable for metasurface-based biosensors. The interactions between adjacent meta-atoms determine the fundamental properties of SP-BIC; however, this topic has not been profoundly explored. In this work, we experimentally and numerically investigate the effects of contactless and contact interactions between adjacent dual-gap split-ring resonators (DSRRs) on the SP-BIC. We demonstrate that there is only one SP-BIC at 0.9 THz when the incident radiation polarization is parallel to the gap in both contactless and contact coupling conditions. When the polarization is vertical to the gap, the individual SP-BIC shifts the frequency to 0.8 THz under contactless coupling. Under contact coupling, the SP-BIC degrades to be an electromagnetically induced transparency (EIT) windows at 0.8 THz. We calculated a 3.6 ps group delay of slow light for EIT. Numerical simulations indicate that the combination of one magnetic dipole (MD) in the inner arm and another electric dipole in the outer arm of DSRR results in quasi-BIC at 0.9 THz and 0.8 THz under contactless coupling. Under contact coupling conditions, the formation of quasi-BIC at 0.9 THz is similar to contactless coupling. However, two MDs of opposite polarity results in the EIT windows at 0.8 THz. Our results reveal excitation and manipulation of terahertz SP-BIC via contactless and contact coupling, which is significant for the innovation of terahertz biosensors. [ABSTRACT FROM AUTHOR]

The study of optical resonators is of significant importance in terms of their ability to confine light in optical devices. A major drawback of optical resonators is the phenomenon of light emission due to their limited capacity for light confinement. Bound states in the continuum are gaining significant attention in the realization of optical devices due to their unique ability for reducing light scattering via interference mechanisms. This process can potentially suppress scattering, leading to improved optical performance. Using this concept, a metasurface having two elliptical silicon (Si) resonators nonidentically angled to create an out‐of‐plane asymmetry is studied. Various parameters are optimized by employing a genetic algorithm (GA) to subsequently achieve a high‐Q factor at terahertz frequencies. Herein, the device is fabricated using a novel method, and a thick high‐index resonator is achieved. Terahertz measurements are carried out to validate the results. It is indicated in the experimental results that plasmons appear at the top surface of the metasurface and create strong sharp resonances that are sensitive to the external environment. Owing to strong field confinement ability, and high‐Q factor, the metasurface is sensitive to its surrounding environment and can be essentially employed in terahertz sensing applications. [ABSTRACT FROM AUTHOR]

Manipulation of terahertz radiation opens new opportunities that underpin application areas in communication, security, material sensing, and characterization. Metasurfaces employed for terahertz manipulation of phase, amplitude, or polarization of terahertz waves have limitations in radiation efficiency which is attributed to losses in the materials constituting the devices. Metallic resonators‐based terahertz devices suffer from high ohmic losses, while dielectric substrates and spacers with high relative permittivity and loss tangent also reduce bandwidth and efficiency. To overcome these issues, a proper choice of low loss and low relative permittivity dielectric layers and substrates can improve field confinement and reduce dissipation. Alternatively, replacing metallic resonators with a moderate relative permittivity dielectric material that supports cavity mode resonances also reduces dissipation due to the absence of conduction current. Herein, an overview of dielectric materials employed as spacers and dielectric resonators is provided, and the fabrication methods employed to realize these devices at the terahertz frequency range are also presented. Material selection guidelines, material‐specific and application‐specific fabrication quality metrics are outlined, and new techniques are proposed. [ABSTRACT FROM AUTHOR]

Moiré metasurfaces exhibit high optical tuneabilities and versatile light manipulation capabilities. Both infinite quality factor (Q factor) and topological vortex configurations in momentum space ( k -space) of the bound state in the continuum (BIC) have introduced new dimensions for light modulation. Herein, we propose a moiré metasurface comprising two identical square photonic lattices superimposed with a commensurate angle of 12.68°. By tuning the incidence angle, the symmetric-protected BICs, Friedrich-Wintgen BIC, and accidental BIC can be achieved simultaneously in our moiré metasurfaces. It is found that the quasi-BICs maintain an ultrahigh Q factor beyond 10 7 . The photonic band structures manifest that the three types of BICs are at the center of far-field polarization vortices in k -space, which have their own topological charges. We experimentally show that these BICs exhibit high sensitivity to subtle changes in analyte refractive index for thin-film sensor application. Our discovery predicts an approach to a highly sensitive multi-channel terahertz biosensor.