Terahertz diffraction enhanced transparency probed in the near field

Abstract

Electromagnetically induced transparency in metamaterials allows to engineer structures which transmit narrow spectral ranges of radiation while exhibiting a large group index. Implementation of this phenomenon frequently calls for strong near-field coupling of bright (dipolar) resonances to dark (multipolar) resonances in the metamolecules comprising the metamaterials. The sharpness and contrast of the resulting transparency windows thus depends strongly on how closely these metamolecules can be placed to one another, placing constraints on fabrication capabilities. In this manuscript, we demonstrate that the reliance on near-field interaction strength can be relaxed, and the magnitude of the electromagnetic-induced transparency enhanced, by exploiting the long-range coupling between metamolecules in periodic lattices. By placing dolmen structures resonant at THz frequencies in a periodic lattice, we show a significant increase of the transparency window when the in-plane diffraction is tuned to the resonant frequency of the metamolecules, as confirmed by direct mapping of the THz near-field amplitude across a lattice of dolmens. Through the direct interrogation of the dark resonance in the near field, we show the interplay of near-and far-field couplings in optimizing the response of planar dolmen arrays via diffraction-enhanced transparency.

Lattice‐Enhanced Fano Resonances from Bound States in the Continuum Metasurfaces

“…In this article, we experimentally study the coupling of the Fano resonance of an asymmetric split ring resonator (ASRR) to the FOLM of the resonator array. Previous studies have shown enhanced Q factors by coupling a lattice mode to inductive–capacitive (LC) resonances and electromagnetically induced transparency‐type (EIT‐type) resonances. Here, we demonstrate FOLM coupling to a Fano resonance obtained through magnetic coupling and observe both Q factor and FoM enhancement.…”

Spectral and temporal evidence of robust photonic bound states in the continuum on terahertz metasurfaces

“…Nonetheless, there are no fundamental works describing how to achieve robust BICs on a general basis, except for well-known symmetry-protected and accidental degeneracies appearing in photonic crystals [3] and asymmetric metasurfaces [7]. A variety of resonant phenomena, such as surface plasmon lattice resonances in the optical domain [13][14][15][16][17][18][19], Fano resonances [20][21][22][23][24][25][26], and electromagnetically induced transparency at optical and THz frequencies [27][28][29][30][31][32][33][34][35][36], have been reported in metasurfaces; however, robust symmetry-protected BICs remain unexplored. Metasurfaces are especially interesting for this purpose due to their enhanced collective response [21,26,[37][38][39][40].…”

High‐Q Transparency Band in All‐Dielectric Metasurfaces Induced by a Quasi Bound State in the Continuum

“…On the other hand, recall the classical analogue of quantum electromagnetically‐induced transparency (EIT) has been widely explored in metasurfaces, where the three level system needed is typically replaced by mode coupling of two resonances of various kinds. [ 34–45 ] Nonetheless, even though, as mentioned above for BICs in general, metasurface quasi‐BICs with high Q‐factors have been widely investigated, [ 5,13,18,46–48 ] the use of BICs to achieve ultra‐narrow EIT has not been explored thus far.…”