59 research outputs found
Structural Control of Metamaterial Oscillator Strength and Electric Field Enhancement at Terahertz Frequencies
The design of artificial nonlinear materials requires control over the
internal resonant charge densities and local electric field distributions. We
present a MM design with a structurally controllable oscillator strength and
local electric field enhancement at terahertz frequencies. The MM consists of a
split ring resonator (SRR) array stacked above an array of nonresonant closed
conducting rings. An in-plane, lateral shift of a half unit cell between the
SRR and closed ring arrays results in a decrease of the MM oscillator strength
by a factor of 4 and a 40% change in the amplitude of the resonant electric
field enhancement in the SRR capacitive gap. We use terahertz time-domain
spectroscopy and numerical simulations to confirm our results and we propose a
qualitative inductive coupling model to explain the observed electromagnetic
reponse.Comment: 11 pages, 5 figure
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
- …