Governance of Friedrich-Wintgen bound states in the continuum by tuning the internal coupling of meta-atoms.

Bound state in the continuum (BIC) is a phenomenon that describes the perfect confinement of electromagnetic waves despite their resonant frequencies lying in the continuous radiative spectrum. BICs can be realized by introducing a destructive interference between distinct modes, referred to as Friedrich-Wintgen BICs (FW-BICs). Herein, we demonstrate that FW-BICs can be derived from coupled modes of individual split-ring resonators (SRR) in the terahertz band. The eigenmode results manifest that FW-BICs are in the center of the far-field polarization vortices. Quasi-BIC-I keeps an ultrahigh quality factor (Q factor) in a broad momentum range along the Γ-X direction, while the Q factor of the quasi-BIC-II drops rapidly. Our results can facilitate the design of devices with high-Q factors with extreme robustness against the incident angle.

Terahertz inner and outer edge modes in a tetramer of strongly coupled spoof localized surface plasmons.

Photonic edge mode confining light in cavities of surface plasmons is beneficial in image and biosensor applications. In the terahertz band, however, the edge mode in a cavity of spoof localized surface plasmons has not matured sufficiently. Herein, a cost-effective strategy to achieve a terahertz photonic edge mode using a metasurface of strongly coupled fourfold spoof localized surface plasmons in a tetramer layout is demonstrated. The quality factors of edge modes decrease when the tetramer shrinks, as revealed by the terahertz dielectric functions. The edge modes that emerge can be categorized as inner and outer edge modes, as deduced from the simulated electric field distribution. Our results show that the edge modes are due to the interaction of spoof localized surface plasmons in the terahertz band.

Tuning symmetry-protected quasi bound state in the continuum using terahertz meta-atoms of rotational and reflectional symmetry.

Conventionally, a symmetry-protected quasi bound state of the continuum (BIC) becomes achievable by breaking the C2 symmetry of meta-atoms. Our work exhibits a novel approach to achieving dual band quasi-BIC by breaking the C2v symmetry into Cs symmetry. Also, we show that a single band quasi-BIC can be achieved by breaking the C2v symmetry into C2 symmetry. Our metasurface of C2v symmetry is composed of double gaps split ring resonator (DSRR), and it degrades to C2 symmetry when the double gaps are displaced in opposite directions. One band quasi-BIC can be observed occurring at around 0.36 and 0.61 THz respectively with the metasurface excited by x- and y-polarized terahertz radiation, respectively. A couple of dark dipole oscillator dominates the quasi-BIC at 0.36 THz, while a quadruple-like oscillator dominates the quasi-BIC at 0.61 THz. The damping ratio and coupling coefficients of the above single quasi-BIC are close to the orthogonal polarization of the incident terahertz wave. However, the metasurface of the DSRR array degrades down to Cs symmetry when the double gaps are displaced in the same directions. A dual band quasi-BIC (0.23 THz and 0.62 THz) is found to be sensitive to the y-polarized terahertz radiation. It is found that the inductive-capacitive (LC) resonance results in quasi-BIC at 0.23 THz, while a quadrupole-like oscillation results in quasi-BIC at 0.62 THz. The quasi-BIC at 0.62 THz has a higher coupling coefficient and lower damping ratio than quasi-BIC at 0.23 THz in a metasurface of Cs symmetry. The realization of the above locally symmetric breaking on the quasi-BIC of terahertz metasurfaces is helpful for the innovation of multi-band terahertz biosensors.

Coherently controllable terahertz plasmon-induced transparency using a coupled Fano-Lorentzian metasurface.

Metamaterials have been engineered to achieve electromagnetically induced transparency (EIT)-like behavior, analogous to those in quantum optical systems. These meta-devices are opening new paradigms in terahertz communication, ultra-sensitive sensing and EIT-like anti-reflection. The controlled coupling between a sub-radiant and a super-radiant particle in the unit cells of these metamaterial can enable multiple narrow plasmon induced transparency (PIT) windows over a broad band, with considerable group delay of electromagnetic field (slow light effect). Phase coherence between these PIT windows is highly desired for next-generation multichannel communication network. Herein, we numerically and experimentally validate a controllable frequency hopping mechanism between "slow light" windows in the terahertz (THz) regime. The effective media are composed of plasmonic "molecules" in which an asymmetric split-ring resonator (ASRR) or Fano resonator is displaced on the side of a cut-wire (Lorentz oscillator). Two metasurfaces where ASRR is on opposite side of the cut-wire are investigated. In these two cases, the proximity of the cut-wire to the gap on the ASRR having asymmetry is different. On one side, when the gap is nearer to the cut wire, displacing the ASRR along the cut-wire, produces only one narrow transparency window at 0.8 THz, corresponding to 20 ps group delay. When the ASRR is positioned on the opposite side, such that the gap is further, two transparency windows are observed when the ASRR is displaced along the cut-wire. That is, the transparency window hops from 0.8 THz to 1.2 THz. This corresponds to an increase from 20 to 30 ps in slow light effect. Numerical simulations suggest these single or multiple PIT windows occur if the couplings between the plasmonic modes in the different arrangements are either in-phase or out-of-phase, respectively.

Demonstration of group delay above 40 ps at terahertz plasmon-induced transparency windows.

Herein, we demonstrate one of the highest terahertz group delay of 42.4 ps achieved experimentally at 0.23 THz, on a flexible planar metamaterial. The unit cell of metasurface is made up of a textured closed cavity and another experimentally concentric metallic arc. By tuning the central angle of the metallic arc, its intrinsic dipolar mode is in destructive interference with the spoof localized surface plasmon (SLSP) on textured closed cavity, which results in a plasmon-induced transparency phenomenon. The measured transmittances of as-fabricated samples using terahertz-time domain spectroscopy validate numerical results using extended coupled Lorentz oscillator model. It is found that the coupling coefficient and damping ratio of SLSP relies on the radius of the ring structure of textured closed cavity. As a consequence, the slow light maximum values become manoeuverable in strength at certain frequencies of induced transparency windows. To the best of our knowledge, our experimental result is currently the highest value demonstrated so far within metasurface at terahertz band.

Active Control of Nanodielectric-Induced THz Quasi-BIC in Flexible Metasurfaces: A Platform for Modulation and Sensing.

A bound state in the continuum (BIC) is a nonradiating state of light embedded in the continuum of propagating modes providing drastic enhancement of the electromagnetic field and its localization at micro-nanoscale. However, access to such modes in the far-field requires symmetry breaking. Here, it is demonstrated that a nanometric dielectric or semiconductor layer, 1000 times thinner than the resonant wavelength (λ/1000), induces a dynamically controllable quasi-bound state in the continuum (QBIC) with ultrahigh quality factor in a symmetric metallic metasurface at terahertz frequencies. Photoexcitation of nanostrips of germanium activates ultrafast switching of a QBIC resonance with 200% transmission intensity modulation and complete recovery within 7 ps on a low-loss flexible substrate. The nanostrips also form microchannels that provide an opportunity for BIC-based refractive index sensing. An optimization model is presented for (switchable) QBIC resonances of metamaterial arrays of planar symmetric resonators modified with any (active) dielectric for inverse metamaterial design that can serve as an enabling platform for active micro-nanophotonic devices.