32 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
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
Overview of JET results for optimising ITER operation
The JET 2019–2020 scientific and technological programme exploited the results of years of concerted scientific and engineering work, including the ITER-like wall (ILW: Be wall and W divertor) installed in 2010, improved diagnostic capabilities now fully available, a major neutral beam injection upgrade providing record power in 2019–2020, and tested the technical and procedural preparation for safe operation with tritium. Research along three complementary axes yielded a wealth of new results. Firstly, the JET plasma programme delivered scenarios suitable for high fusion power and alpha particle (α) physics in the coming D–T campaign (DTE2), with record sustained neutron rates, as well as plasmas for clarifying the impact of isotope mass on plasma core, edge and plasma-wall interactions, and for ITER pre-fusion power operation. The efficacy of the newly installed shattered pellet injector for mitigating disruption forces and runaway electrons was demonstrated. Secondly, research on the consequences of long-term exposure to JET-ILW plasma was completed, with emphasis on wall damage and fuel retention, and with analyses of wall materials and dust particles that will help validate assumptions and codes for design and operation of ITER and DEMO. Thirdly, the nuclear technology programme aiming to deliver maximum technological return from operations in D, T and D–T benefited from the highest D–D neutron yield in years, securing results for validating radiation transport and activation codes, and nuclear data for ITER
Overview of JET results for optimising ITER operation
The JET 2019–2020 scientific and technological programme exploited the results of years of concerted scientific and engineering work, including the ITER-like wall (ILW: Be wall and W divertor) installed in 2010, improved diagnostic capabilities now fully available, a major neutral beam injection upgrade providing record power in 2019–2020, and tested the technical and procedural preparation for safe operation with tritium. Research along three complementary axes yielded a wealth of new results. Firstly, the JET plasma programme delivered scenarios suitable for high fusion power and alpha particle (α) physics in the coming D–T campaign (DTE2), with record sustained neutron rates, as well as plasmas for clarifying the impact of isotope mass on plasma core, edge and plasma-wall interactions, and for ITER pre-fusion power operation. The efficacy of the newly installed shattered pellet injector for mitigating disruption forces and runaway electrons was demonstrated. Secondly, research on the consequences of long-term exposure to JET-ILW plasma was completed, with emphasis on wall damage and fuel retention, and with analyses of wall materials and dust particles that will help validate assumptions and codes for design and operation of ITER and DEMO. Thirdly, the nuclear technology programme aiming to deliver maximum technological return from operations in D, T and D–T benefited from the highest D–D neutron yield in years, securing results for validating radiation transport and activation codes, and nuclear data for ITER