Thomas Feil

Ratchet Effects Induced by Terahertz Radiation in Heterostructures with a Lateral Periodic Potential

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.

Electric-field stabilization in a high-density surface superlattice

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.

Terahertz/optical sum and difference frequency generation in liquids

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.

Inelastic light scattering from terahertz standing waves in a slab waveguide

Second order, χ(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.

Terahertz photocurrents in heterostructures with one-dimensional lateral periodic potential

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.

Studying protein dynamics in aqueous solutions through linear and non-linear THz spectroscopy

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.

Negative differential conductance in cleaved edge overgrown surface superlattices

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.