Second order, chi((2)) nonlinear mixing of optical and terahertz (THz) radiation is enhanced by phase matching by the spatial distribution of THz standing waves in a slab waveguide. The interference pattern due to selectively excited waveguide modes is thus detected and modeled along a crystal slab waveguide. (c) 2012 Optical Society of America

A high-sensitivity setup for the observation of χ(2)-based, terahertz/optical, sum and difference frequency generation in liquids is presented. It relies on launching wavefront modulated terahertz radiation into a liquid traversed by an optical beam. Phase matching and polarization selection rules can be tuned to support three wave mixing via either chiral allowed electric dipole processes or higher order quadrupole/magnetic dipole processes. Under nonresonant excitation, hyperpolarizabilities from quadrupole/magnetic dipole processes are measured. Since this approach does not require terahertz transmission through macroscopic thicknesses of water, it has the potential to open a new window on the terahertz dynamics of water solvated molecules.

In this work, we investigate high-mobility two-dimensional electron gases in AlxGa1-xAs heterostructures by employing Schottky-gate-dependent measurements of the samples' electron density and mobility. Surprisingly, we found that two different sample configurations can be set in situ with mobilities differing by a factor of more than two in a wide range of densities. This observation is discussed in the context of charge redistributions between the doping layers and is relevant for the design of future gateable high-mobility electron gases.

We report on the observation of the Seebeck ratchet effect. The effect is measured in semiconductor heterostructures with a one-dimensional lateral potential excited by terahertz radiation. The photocurrent generation is based on the combined action of a spatially periodic in-plane potential and a spatially modulated light, which gives rise to a modulation of the local temperature. In addition to the polarization-independent current due to the Seebeck ratchet effect, we observe a photon helicity dependent response and propose a microscopic mechanism to interpret the experimental findings.

Direct absorption spectroscopy is used to determine the absorption coefficient of protein solutions across the spectrum from 0.1 to 3 THz. The spectra are analyzed with respect to their dependence on solution properties, protein size and protein geometry. Besides these direct absorption measurements, a non-linear two-color experiment is developed which is sensitive solely to the chirality based contribution from proteins.

We report on the observation of terahertz radiation induced photogalvanic currents in semiconductor quantum well structures with one-dimensional lateral periodic potential. The current response is well described by the phenomenological theory of the superposition of the linear and circular photogalvanic effects. Experimental data demonstrate that the inversion asymmetry of semiconductor heterostructures can be controllably varied by means of electron beam lithography. The inversion asymmetry in quantum well structures is of importance for rapidly growing field of spintronics providing an effective tool for the spin manipulation by an electric field.

Magnetotransport in a density-tunable two-dimensional electron gas that is modulated with a strong, atomically precise superlattice potential of a 15 nm period is investigated. For low densities, the system shows typical two-dimensional behavior. In the regime of large densities, however, a quasi-one-dimensional state is entered, in which the cyclotron gaps vanish and the quantum Hall effect breaks down. Both regimes, and the transition between them, are theoretically described in the framework of a quantum-mechanical tight-binding model.

We experimentally investigate the transport through a shunted surface superlattice under the influence of a magnetic field applied perpendicular to the current direction. The current–voltage characteristics of these surface superlattices exhibit a peak which is followed by a wide region of negative differential resistance. The application of a transverse magnetic field has a profound influence on the position and height of this peak. The recorded shifts are compared to the predictions of different superlattice transport theories. Since these theories predict a different dependence on the magnetic field strength, the transport mechanism in the surface superlattice structures can be uniquely determined.

We study the transport through a shunted surface superlattice system in the presence of an external magnetic field. Such a system consists of a weakly doped, instability free superlattice (shunt) that is overgrown on the edge with a two-dimensional electron system (surface superlattice). A magnetic field parallel to a static applied electric field along the superlattice axis induces current resonances in the shunt characteristics which are explained by Stark-cyclotron resonances. These resonances support the conclusion that the field alignment in the shunted surface superlattice structure is homogeneous. An increasing magnetic field perpendicular to the transport direction leads to characteristic crossings of the current-voltage characteristics of both shunt and surface superlattice. They are explained by a semiclassical model which assumes a competition between the magnetic and electric localization of the miniband electrons. We conclude that our structure indeed presents a domain free high density superlattice and might therefore be a promising candidate for the realization of an active electrically driven Bloch oscillator.

Cleaved Edge Overgrowth is employed to fabricate atomically precise single and coupled quantum wires. Single wires were investigated in the diffusive and ballistic transport regimes. In addition to a detailed understanding of the 1D sublevel structure in the complex CEO devices, clear indications of Luttinger liquid behavior of the one-dimensional electron systems were obtained. Coupled quantum wires were studied in the two limits of large and small separations. In the first case shallow ID potentials were chosen and an alternative description for commensurability oscillations in weakly modulated two-dimensional electron systems could be derived. For the coupled quantum wires with small separations and a large potential modulation the validity of the miniband transport picture leading to negative differential conductivity was demonstrated. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The fundamental condition for operating a superlattice (SL) as a Bloch oscillator is a homogeneous electric field alignment under large applied bias. This can be done by combining the nonlinear miniband transport channel with a parallel shunt channel, connected through lateral transport. We report here about the first realization of such a SL-shunt system. With the cleaved-edge-overgrowth (CEO) method we combine an undoped superlattice, which acts as the shunt, with a two-dimensional (2D) cleaved-edge surface superlattice channel. Our results confirm the long predicted current-voltage-characteristic (I-V) of a SL in a homogeneous DC electric field. Excitation experiments in the GHz regime show that our shunt approach also works in the presence of an external AC electric field.

By application of the cleaved-edge overgrowth technique, we realize a two-channel superlattice (SL) device. The structure combines the parallel transport through a low-density SL under almost homogeneous electric field conditions with that through a surface SL (SSL) with large carrier density, which is, without parallel transport, subject to pronounced field instabilities. Direct control of the SSL density allows a separation of both transport contributions. With parallel transport through the low-density SL, the current carried by the SSL is characteristic for a SL with homogeneous field distribution. In particular, it exhibits negative differential conductivity over a wide range of applied electric fields. In contrast, for current only through the SSL clear electric-field instabilities, typical for SLs at high densities are observed. Thus, by means of the parallel transport channel, field instabilities are avoided and transport in high-density SLs with a homogeneous field distribution becomes accessible.

In situ overgrowth of an undoped GaAs/Al0.3Ga0.7As superlattice with a gate electrode separated by a barrier layer produces an array of strongly coupled quantum wires. The gate allows direct control of edge channel density. The observed transport properties are very sensitive to the transport channel length. Samples with long superlattices exhibit transport characteristics dominated by an inhomogeneous density and field distribution along the channel. In shorter superlattice samples a leakage current through the bulk superlattice stabilizes the field distribution in the two-dimensional channel and allows the observation of current-voltage characteristics that exhibit strong negative differential conductance without the formation of electric field domains.

The electron transport through a short period modulated two-dimensional electron system exhibits clear negative differential conductance (NDC). The modulation parameters place the NDC peak clearly into the validity range of miniband conduction and it is therefore attributed to Bloch oscillations of electrons in the lowest miniband. In contrast to conventional superlattices the NDC in our device is stable for low carrier concentrations due to the reduced dimensionality of our system. With increasing electron concentration instabilities occur in the I–V-traces at the onset of NDC. Numerical simulations confirm this transition and a stability criterion depending on the system parameters is given. Further evidence for stable Bloch oscillations is given by coupling the device to an external high frequency field. For fixed frequency a clear suppression and shift of the peak current are observed with increasing intensity of the radiation. Both facts are predicted by the semiclassical theory when Bloch oscillations are frequency modulated by an external AC electric field.

Periodic modulation of a free two-dimensional electron system changes the bandstructure in which the electrons move. A weak modulation opens up small energy gaps in the free electron bandstructure while a strong, short-period modulation leads to the formation of well separated cosine-like minibands. We study transport in both cases using structures grown with the Cleaved-Edge-Overgrowth method. It allows to fabricate high quality two-dimensional electron systems with arbitrary modulation periods which are precise on an atomic scale. Perturbing the system only weakly with a long-period superlattice that is located some distance away from the electron layer leads to magneto-transport characteristics which exhibit each a number of different 1/B-periodic oscillations. These can be very well explained by semiclassical theory and allow the extraction of the Fermi surface of the system. The strong modulation with a short-period superlattice leads to the two-dimensional equivalent of a conventional superlattice. One very important difference lies in the stability of the expected negative differential conductance regime with respect to electric field instabilities. Geometrical considerations show that the observation of stable Bloch oscillations is possible in these systems. We will show that density dependent transport studies confirm the formation of a two-dimensional superlattice. Although negative diffential conductance, also observed in those samples, must be attributed to an inhomogeneous density distribution. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)