58 research outputs found
Dynamic bound states in the continuum
All-dielectric metasurfaces are a versatile platform to investigate a host of unconventional physical scattering responses. Effects, including high absorption and Huygens’ surfaces, have been demonstrated; however, a more exotic materialization—termed bound states in the continuum (BSCs)—exists and consists of nonradiating localized waves that lie within the energy spectrum of the continuum. Here we experimentally demonstrate a dynamic BSC at terahertz frequencies that realizes a material-limited high-quality factor () resonance =8.7×103, which may be modified by over 2 orders of magnitude through photodoping with band gap light. We elucidate the nature of the BSC resonance, and our experimental results are well supported by eigenvalue and -parameter simulations. The demonstrated system and underlying theory establish a path to realize extremely high- dynamic resonances, which may be useful for detection of hazardous materials and frequency-diverse imaging.U.S. Department of Energy (DOE) (DESC0014372); Australian Research Council (ARC); National Science Foundation (NSF) (ECCS-1542015); National Nanotechnology Coordinated Infrastructure (NNCI)
Decoupling Crossover in Asymmetric Broadside Coupled Split Ring Resonators at Terahertz Frequencies
We investigate the electromagnetic response of asymmetric broadside coupled
split ring resonators (ABC-SRRs) as a function of the relative in-plane
displacement between the two component SRRs. The asymmetry is defined as the
difference in the capacitive gap widths (\Delta g) between the two resonators
comprising a coupled unit. We characterize the response of ABC-SRRs both
numerically and experimentally via terahertz time-domain spectroscopy. As with
symmetric BC-SRRs (\Delta g=0 \mu m), a large redshift in the LC resonance is
observed with increasing displacement, resulting from changes in the capacitive
and inductive coupling. However, for ABC-SRRs, in-plane shifting between the
two resonators by more than 0.375Lo (Lo=SRR sidelength) results in a transition
to a response with two resonant modes, associated with decoupling in the
ABC-SRRs. For increasing \Delta g, the decoupling transition begins at the same
relative shift (0.375Lo), though with an increase in the oscillator strength of
the new mode. This strongly contrasts with symmetric BC-SRRs which present only
one resonance for shifts up to 0.75Lo. Since all BC-SRRs are effectively
asymmetric when placed on a substrate, an understanding of ABC-SRR behavior is
essential for a complete understanding of BC-SRR based metamaterials
Three-dimensional broadband tunable terahertz metamaterials
We present optically tunable magnetic 3D metamaterials at terahertz (THz)
frequencies which exhibit a tuning range of ~30% of the resonance frequency.
This is accomplished by fabricating 3D array structures consisting of
double-split-ring resonators (DSRRs) on silicon-on-sapphire, fabricated using
multilayer electroplating. Photoexcitation of free carriers in the silicon
within the capacitive region of the DSRR results in a red-shift of the resonant
frequency from 1.74 THz to 1.16 THz. The observed frequency shift leads to a
transition from a magnetic-to-bianisotropic response as verified through
electromagnetic simulations and parameter retrieval. Our approach extends
dynamic metamaterial tuning to magnetic control, and may find applications in
switching and modulation, polarization control, or tunable perfect absorbers.Comment: 5page
Nonlinear terahertz metamaterials via field-enhanced carrier dynamics in GaAs
We demonstrate nonlinear metamaterial split ring resonators (SRRs) on GaAs at
terahertz frequencies. For SRRs on doped GaAs films, incident terahertz
radiation with peak fields of ~20 - 160 kV/cm drives intervalley scattering.
This reduces the carrier mobility and enhances the SRR LC response due to a
conductivity decrease in the doped thin film. Above ~160 kV/cm, electric field
enhancement within the SRR gaps leads to efficient impact ionization,
increasing the carrier density and the conductivity which, in turn, suppresses
the SRR resonance. We demonstrate an increase of up to 10 orders of magnitude
in the carrier density in the SRR gaps on semi-insulating GaAs substrate.
Furthermore, we show that the effective permittivity can be swept from negative
to positive values with increasing terahertz field strength in the impact
ionization regime, enabling new possibilities for nonlinear metamaterials.Comment: 5 pages, 4 figure
Infrared all-dielectric Kerker metasurfaces
The unidirectional scattering of electromagnetic waves in the backward and forward direction, termed Kerkers’ first and second conditions, respectively, is a prominent feature of
sub-wavelength particles, which also has been found recently in all-dielectric metasurfaces. Here we formulate the exact polarizability requirements necessary to achieve both Kerker
conditions simultaneously with dipole terms only and demonstrate its equivalence to so-called “invisible metasurfaces”. We further describe the perfect absorption mechanism in all-dielectric
metasurfaces through development of an extended Kerker formalism. The phenomena of both invisibility and perfect absorption is shown in a 2D hexagonal array of cylindrical resonators, where only the resonator height is modified to switch between the two states. The developed framework provides critical insight into the range of scattering response possible with all-dielectric metasurfaces, providing a methodology for studying exotic electromagnetic phenomena.National Nanotechnology Coordinating Office; National Science Foundation (ECCS-1542015); Australian Research Council (UNSW Scientia Fellowship); U.S. Department of Energy (DESC0014372)
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