Wilhelm Prettl

Experimental Separation of Rashba and Dresselhaus Spin Splittings in Semiconductor Quantum Wells

The relative strengths of Rashba and Dresselhaus terms describing the spin-orbit coupling in semiconductor quantum well (QW) structures are extracted from photocurrent measurements on n-type InAs QWs containing a two-dimensional electron gas (2DEG). This novel technique makes use of the angular distribution of the spin-galvanic effect at certain directions of spin orientation in the plane of a QW. The ratio of the relevant Rashba and Dresselhaus coefficients can be deduced directly from experiment and does not relay on theoretically obtained quantities. Thus our experiments open a new way to determine the different contributions to spin-orbit coupling.

Spin photocurrents in quantum wells

Spin photocurrents generated by homogeneous optical excitation with circularly polarized radiation in quantum wells (QWs) are reviewed. The absorption of circularly polarized light results in optical spin orientation due to the transfer of the angular momentum of photons to electrons of a two-dimensional electron gas. It is shown that in QWs belonging to one of the gyrotropic crystal classes a non-equilibrium spin polarization of uniformly distributed electrons causes a directed motion of electrons in the plane of the QW. A characteristic feature of this electric current, which occurs in unbiased samples, is that it reverses its direction upon changing the radiation helicity from left-handed to right-handed and vice versa. Two microscopic mechanisms are responsible for the occurrence of an electric current linked to a uniform spin polarization in a QW: the spin polarization-induced circular photogalvanic effect and the spin-galvanic effect. In both effects the current flow is driven by an asymmetric distribution of spin-polarized carriers in k-space of systems with lifted spin degeneracy due to k-linear terms in the Hamiltonian. Spin photocurrents provide methods to investigate spin relaxation and to reach a conclusion as regards the in-plane symmetry of QWs. The effect can also be utilized to develop fast detectors for determining the degree of circular polarization of a radiation beam. Furthermore, spin photocurrents under infrared excitation were used to demonstrate and investigate monopolar spin orientation of free carriers.

Spin-galvanic effect

There is much recent interest in exploiting the spin of conduction electrons in semiconductor heterostructures together with their charge to realize new device concepts. Electrical currents are usually generated by electric or magnetic fields, or by gradients of, for example, carrier concentration or temperature. The electron spin in a spin-polarized electron gas can, in principle, also drive an electrical current, even at room temperature, if some general symmetry requirements are met. Here we demonstrate such a ‘spin-galvanic’ effect in semiconductor heterostructures, induced by a non-equilibrium, but uniform population of electron spins. The microscopic origin for this effect is that the two electronic sub-bands for spin-up and spin-down electrons are shifted in momentum space and, although the electron distribution in each sub-band is symmetric, there is an inherent asymmetry in the spin-flip scattering events between the two sub-bands. The resulting current flow has been detected by applying a magnetic field to rotate an optically oriented non-equilibrium spin polarization in the direction of the sample plane. In contrast to previous experiments, where spin-polarized currents were driven by electric fields in semiconductor, we have here the complementary situation where electron spins drive a current without the need of an external electric field.

Giant voltages upon surface heating in normal YBa2Cu3O7−δ films suggesting an atomic layer thermopile

Experiments are reported which show that temperature gradients perpendicular to the surface of epitaxial normal conducting YBa2Cu3O7−δ films give rise to large transversal voltages between contacts on the film surface. The temperature gradients have been produced by pulsed laser irradiation and by continuous heating of the films by heater wires. To explain the large lateral voltages, an atomic layer thermopile is proposed, which may be formed by the layered structure of the material.

Spin-dependent tunnelling through a symmetric barrier

The problem of electron tunneling through a symmetric semiconductor barrier based on zinc-blende-structure material is studied. The k3 Dresselhaus terms in the effective Hamiltonian of bulk semiconductor of the barrier are shown to result in a dependence of the tunneling transmission on the spin orientation. The difference of the transmission probabilities for opposite spin orientations can achieve several percents for the reasonable width of the barriers.

Rashba and Dresselhaus spin splittings in semiconductor quantum wells measured by spin photocurrents

The spin-galvanic effect and the circular photogalvanic effect induced by terahertz radiation are applied to determine the relative strengths of Rashba and Dresselhaus band spin splitting in 001-grown GaAs and InAs based two dimensional electron systems. We observed that shifting the -doping plane from one side of the quantum well to the other results in a change of sign of the photocurrent caused by Rashba spin splitting while the sign of the Dresselhaus term induced photocurrent remains. The measurements give the necessary feedback for technologists looking for structures with equal Rashba and Dresselhaus spin splittings or perfectly symmetric structures with zero Rashba constant.

Planar leaky light-guides and couplers

A quantitative theory of light propagation in a dielectric slab guide with general cladding media is presented. It is based on a plane wave which bounces in a zigzag fashion along the guide as a result of total or partial reflections at the two surfaces of the film. Two mechanisms are considered which contribute to the attenuation of the guide: losses due to absorption in the slab and cladding materials, and radiation losses if the guide is a leaky one. We point out the significance of the Goos-Hänchen effect for all questions relating to the power flow in the slab guide. The theory is illustrated by discussing dispersion and attenuation of guides with various low-index and high-index claddings, operating above and below cutoff. The low-index leaky guide is considered particularly in detail. Its high attenuation by leakage can be reduced to practically acceptable values (<1d B/cm) by increasing the film thickness to ≳ 40λ. One application of this guide is in the leaky wave coupler. This coupler may be viewed as a prism-film coupler simplified by omission of the gap. It offers a new approach to the problem of broad-band coupling to thin-film light guides.

Demonstration of Rashba spin splitting in GaN-based heterostructures

The circular photogalvanic effect, induced by infrared radiation, has been observed in (0001)-oriented n‐GaN low dimensional structures. The photocurrent changes sign upon reversing the radiation helicity demonstrating the existence of spin splitting of the conduction band in k space in this type of materials. The observation suggests the presence of a sizeable Rashba type of spin splitting, caused by the built-in asymmetry at the AlGaN∕GaN interface.

Fast thermoelectric response of normal state YBa2Cu3O7−δ films

Normal state YBa2Cu3O7−δ films, epitaxially grown ‘‘off‐c axis’’ with tilt angles up to 20° are shown to be fast thermoelectric detectors for radiation from UV to far infrared wavelengths. Upon radiation heating of the tilted films a thermoelectric voltage arises due to the anisotropy of the thermopower in YBa2Cu3O7−δ. The response time is limited by the decaying temperature gradient and thus by heat diffusion. For thin films we have measured a response time from ≲1 ns in the UV to ∼5 ns in the far infrared. Because of the wavelength dependent reflectivity, the responsivity of the films varies between 0.5 V/MW and 20 V/MW. Thus, thin tilted YBa2Cu3O7−δ films can be used as fast room temperature detectors over a wide spectral range.

Optical bistability in InSb at room temperature with two‐photon excitation

We report the observation of optical bistability in an InSb resonator at room temperature. This effect and fringe shifts were caused by nonlinear refraction induced by two‐photon absorption of radiation from a single longitudinal mode injection‐locked pulsed CO2 laser operating at 9.6–10.6 μm. Intensities as low as 100 kW/cm2 were found to be sufficient to tune the 250‐μm‐thick cavity through a fringe maximum. From our results we deduce a value of χ(3) of the order of 10−4 esu over the range of intensities investigated.