H. Nemec

Ultrafast opto-terahertz photonic crystal modulator

We present an agile optically controlled switch or modulator of terahertz (THz) radiation. The element is based on a one-dimensional photonic crystal with a GaAs wafer inserted in the middle as a defect layer. The THz electric field is enhanced in the photonic structure at the surfaces of the GaAs wafer. Excitation of the front GaAs surface by ultrashort 810 nm laser pulses then leads to an efficient modulation of the THz beam even at low photocarrier concentrations (approximately 10(16) cm(-3)). The response time of the element to pulsed photoexcitation is about 130 ps.

Broadband dielectric terahertz metamaterials with negative permeability

We present a design of dielectric metamaterials exhibiting a broad range of negative effective permeability in the terahertz spectral region. The investigated structures consist of an array of high-permittivity rods that exhibit a series of Mie resonances giving rise to the effective magnetic response. The spectral positions of resonances depend on the geometrical parameters of the rods and on their permittivity, which define the resonant confinement of the electromagnetic field within the rods. The electromagnetic coupling between the adjacent rods is negligible. With a suitable aspect ratio of the rods, a broadband magnetic response can be obtained.

Charge Transport in

The depolarization fields play an important role in terahertz experiments on nanostructured samples with complex nanoparticle morphologies and percolation pathways. Namely, their effects can hide or distort peculiarities of nanoscopic charge transport in the spectra measured on these structures. We calculate the local fields for a large number of percolated and non-percolated two-dimensional model structures by numerical solving of Maxwell equations in the quasi-static limit. The results strongly suggest that in a broad family of structures a simple effective medium approximation model can be applied to characterize the effective response. The model consists in an equivalent circuit composed of a resistance accounting for the percolated chains with an additional parallel RC-branch describing the non-percolated part. The physical meaning of this model is discussed in the frame of the Bergman spectral representation of effective medium. We show a recipe for the retrieval of a response connected to the depolarization fields and to the nanoscale transport mechanisms from transient terahertz spectra. Finally, we use the model to interpret our THz photoconductivity spectra in various

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Films With Complex Percolation Pathways Investigated by Time-Resolved Terahertz Spectroscopy

films with nanofabricated percolation pathways.

Optical two-photon absorption in GaAs measured by optical pump-terahertz probe spectroscopy

A novel scheme of optical pump-THz probe experiments was used to study the nonlinear absorption in GaAs. The crystal was excited by ultrashort laser pulses with wavelength 810 nm and fluences up to 4.2 mJ/cm/sup 2/, for which the single-photon absorption (SPA) saturates and the two-photon absorption (TPA) dominates. The TPA coefficient /spl beta/ and its anisotropy were determined.

Fishnet metamaterials on thin polymer film for terahertz applications

An original double side photolithography method on ultrathin polymer and a laser micromachining technique were used to fabricate different fishnet structures. We performed simulations and experiments by terahertz time domain spectroscopy (THz-TDS from 100GHz to 3THz) respectively. The effective parameters were extracted with a single shot approach.

Terahertz conductivity of degenerate electron gas confined in 2D model nanostructures

Terahertz conductivity spectra of degenerate electron gas confined in various model 2D nanostructures were calculated using semi-classical Monte-Carlo calculations based on Kubo formula. We show theoretically that the conductivity of high-quality nanostructures containing a degenerate electron gas may exhibit pronounced spectral structure. This is in contrast with general featureless character of experimental terahertz conductivity spectra measured so far in most semiconductor nanostructures..

Ultrafast carrier transport in silicon nanocrystal superlattices

Time-resolved terahertz spectroscopy is employed to study silicon nanocrystals prepared by thermal decomposition of silicon-rich SiOx layers. We observe that higher silicon content favors formation of larger silicon clusters with short-range percolation.

Effects of Depolarization Fields on Transient Terahertz Spectra of Nanostructured Materials

Analysis of the relationship between the microscopic conductivity, morphology represented by percolation degree and terahertz transmission spectra allowed us to retrieve microscopic properties of several nanostructured materials from measurements by terahertz spectroscopy.