We examine planar double split-ring resonators (SRRs) consisting of two concentric rings with either opposite,
similar, or asymmetric gap orientation. Depending on the geometry we observe resonance hybridization, metamaterial
induced transparency, or the excitation of dark resonances. These properties can be used for SRR
based sensing applications, to realize strongly dispersive behavior, or for determining the optical properties of
metals. We further find that THz SRRs featuring very narrow gaps on the micro- or nanoscale can provide
in-gap enhancement factors of several 10,000, a property particularly useful for the realization of nonlinear THz
experiments.
To exploit the great potential of room-temperature ionic liquids (RTILs) as solvents that offer both low environmental
impact and product selectivity, an understanding of the liquid structure, the microscopic dynamics, and the way in which
the pertinent macroscopic properties, such as viscosity, thermal conductivity, ionic diffusion, and solvation dynamics
depend on these properties, is essential. We have measured the intermolecular dynamics of the 1,3-dialkylimidazoliumbased
RTILs [emim][BF4], [emim][DCA], and [bmim][DCA], at 25 °C from below 1 GHz to 10 THz by ultrafast optical
Kerr effect (OKE) spectroscopy and dielectric relaxation spectroscopy (DRS) augmented by time-domain terahertz and
far-infrared FTIR spectroscopy. This concerted approach allows a more detailed analysis to be made of the relatively
featureless terahertz region, where the higher frequency diffusional modes are strongly overlapped with librations and
intermolecular vibrations. In the terahertz region, the signal-to-noise ratio of the OKE spectra is particularly high and the
data show that there is a greater number of librational and intermolecular vibrational modes than previously detected. Of
greatest interest though, is an intense low frequency (sub-alpha) relaxation that we show is in strong accordance with
recent simulations that observe mesoscopic structure arising from aggregates or clusters; structure that explains the
anomalous and inconveniently-high viscosities of these liquids.
KEYWORDS: Near field, Terahertz radiation, Magnetism, Metamaterials, Sensors, Near field optics, Phase measurement, Spatial resolution, Split ring resonators, Metals
Experimental investigations of the microscopic electric and in particular the magnetic near-fields in metamaterials
remain highly challenging and current studies rely mostly on numerical simulations to characterize their resonant
microscopic behavior. Here, we present a terahertz imaging technique, which allows us to measure the amplitude,
phase and polarization of the electric near-fields in the vicinity of the resonant structures in planar metamaterials.
By our approach we are able to trace the electric field vectors close to the structures after their excitation on
sub-ps time scales with sub-wavelength spatial resolution. From the measured in-plane electric vector fields we
are able to reconstruct the out-of-plane magnetic field vectors. As a result we obtain a comprehensive microscopic
picture of the electromagnetic response in metamaterials.
We have applied a newly developed transient terahertz time- domain spectrometer to study the temporal development of the dynamics of photogenerated carriers in semiconductor materials. The study presented here include semi-insulating (SI) and low-temperature-grown (LT) GaAs. By measuring the detailed shape of a subpicosecond electrical field pulse (THz pulse) transmitted through the sample at a time T after excitation with a femtosecond laser pulse, the absorption coefficient and refractive index in the region between 0.1 THz and 3 THz can be measured with high accuracy. By varying the time T, the transient absorption and index spectra can be measured with subpicosecond time resolution. Temporal and spectral behavior of the carrier dynamics in SI and LT GaAs, in dependence of intensity and wavelength of the excitation pulse, is measured. We directly observe carrier scattering to the sidevalleys and the subsequent return of the carriers to the central valley. The experimental data strongly suggest that the transmission of the THz pulse through the photoconducting surface layer of the semiconductor can be described as instantaneous tunneling of the electric field through a metal-like barrier.
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