The dynamics of room-temperature ionic liquids (RTILs) were studied by investigating their dielectric relaxation (DR) and timeresolved optical Kerr-effect (OKE) spectra in the frequency range of ∼10 MHz to ∼20 THz. For the studied RTILs the OKE and DR spectra are dominated by a relaxation in the GHz region and extend to a relatively sharp band at around 10 THz. Whilst the first feature is mainly associated with the structural relaxation of the fluid through ion rotation (α relaxation), the second indicates the short-time limit of intermolecular dynamics. The rather featureless intermediate region is mainly associated with intermolecular vibrations that are strongly coupled to hindered rotations. In contrast to other RTILs, imidazolium salts show an additional sub-α relaxation which dominates the OKE signal and is indicative of the breathing motion of rather long-lived cages.Mixed with polar solvents RTILs were found to retain their ionic liquid-like character up to relatively high levels of dilution, but with the overall dynamics considerably speeded up. Below RTIL mole fractions of ∼0.2-0.4 these systems behave like conventional electrolyte solutions with more or less pronounced ion pairing.
Using terahertz near-field imaging we experimentally investigate the interaction between split-ring resonators (SRRs) in metamaterial arrays. Depending on the inter-SRR spacing two regimes can be distinguished for which strong coupling between SRRs occurs. For dense arrays SRRs couple via their electric and magnetic near-fields. In this case distinct deformations of the SRRs' characteristic near-field patterns are observed as a signature of their strong interaction. For larger separations with a periodicity matching the resonance wavelength, the SRRs become diffractively coupled via their radiated fields. In this regime hybridization between plasmonic and lattice modes can be clearly identified in the experimentally obtained near-field maps.
Optimal focusing of surface plasmon polaritons in the center of a metal disc illuminated by radially polarized terahertz pulses is demonstrated. By matching the cylindrical symmetry of the metal structure with the radially polarized terahertz field, surface plasmons are excited along its entire circumference. Constructive interference in the disc center produces a sharp frequency-dependent focal spot well described by a zero-order Bessel function. We map the field distributions on the disc by terahertz (THz) nearfield microscopy and compare our results with numerical simulations. For comparison, the behavior of the plasmonic lens under linearly polarized THz illumination is also characterized. The remarkable focusing capabilities of such a plasmonic lens together with its simple structure offer considerable potential for THz sensing and imaging applications.
A metamaterial consisting of three-dimensional submillimeter solenoidal coils is presented. The structures have been fabricated by an automated wire-bonding technique and are characterized by terahertz time-domain spectroscopy and numerical simulations. We demonstrate the excitation of electric and magnetic resonances in the microcoils at frequencies between 0.05 and 0.6 THz. The implementation of these microstructures provides a promising route for the design of nonplanar and intrinsically chiral metamaterials for gigahertz to terahertz frequencies.
By terahertz microscopy based on a photoconductive antenna as scanning near-field probe we demonstrate mapping of electromagnetic fields close to microstructures. Our approach visualizes electric and magnetic near-fields with sub-ps temporal and sub-wavelength spatial resolution.
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