E. Mohler

Indium-tin-oxide-coated glass as dichroic mirror for far-infrared electromagnetic radiation

We propose to utilize glass plates coated with indium tin oxide (ITO) as far-infrared dichroic mirrors, e.g., in optoelectronic terahertz spectroscopy for purposes such as electro-optic detection of terahertz radiation or in time-resolved experiments with pump pulses in the visible or near-infrared spectral range and probe pulses at terahertz frequencies. Measurements of the complex reflection and transmission coefficients in the spectral range from 0.1 to 2.8 THz indicate that commercially available ITO glass is suitable for this purpose.

Optical determination of the oxygen content of YBa2Cu3O6+x thin films by IR reflectance and transmittance measurements

Abstract Recent studies of the optical properties of YBa2Cu3O6+x (YBCO) single crystals by several authors have shown that the optical conductivity in the infrared spectral region is a sensitive function of the oxygen content of the samples. Infrared spectroscopy thus offers a possibility for oxygen concentration analysis and is an alternative to other methods such as (i) the X-ray determination of the length of the c-axis and (ii) spectroscopic ellipsometry at an electronic transition band centered around 4.1 eV whose strength decreases with increasing oxygen content. We explore the applicability of the IR optical method for the case of YBCO thin films of about 300 nm thickness which are epitaxially grown on SrTiO3 substrates. We perform normal-incidence transmittance and reflectance measurements between 2000 and 3000 nm to determine the refractive index and the absorption coefficient of the films. Due to the large penetration depth of the IR light it is essential to pay regard to the interferometric aspects of light propagation in the film and to take care of ambiguities in the mathematical inversion. The latter problem is solved by using the asymmetry of the reflectance measured on the two different sides of the sample. It is shown that the accuracy of the results depends on the quality of the films and on the accuracy with which their thickness can be determined by an independent measurement. Despite these difficulties the method provides a simple means for monitoring the oxygen content of YBCO films in the whole doping regime (0⩽x⩽1).

Examining the terahertz signal from a photoexcited biased semiconductor superlattice for evidence of gain

We present a careful analysis of the coherent terahertz emission from an undoped biased semiconductor superlattice excited via an ultrashort optical pulse. We use both a semiclassical model and a fully quantum-mechanical model that includes the excitation process and excitonic effects to analyze emission. We conclude that, in contrast to what has recently been claimed, it is not possible to deduce whether there is terahertz gain from the analysis of the emitted coherent terahertz pulse.

The coherent Hall effect of charge carriers in a superlattice: semiclassical description of the wavepacket dynamics

The coherent Hall effect of charge carriers is the quantum mechanical analogue of the classical Hall effect for the case where the charge carriers are a coherent ensemble of wavepackets. Kosevich considered this effect for the first time for semiconductor superlattices subjected both to an electric field along the growth direction and to a perpendicular magnetic field (Kosevich Y A 1999 Ann.?Phys.?8 SI-145; 2001 Phys.?Rev.?B?63 205313). The dynamics of the charge carriers can be described successfully by means of a semiclassical model mapping the equation of motion to a simple pendulum equation with the deflection angle in the case of the pendulum being replaced by the k-vector of the wavepackets along the superlattices growth direction. While Kosevich has presented an analytical treatment of the equation of motion, we numerically solve it and compute the real-space and k-space trajectories of the charge carriers. We thus arrive at a more illustrative and detailed analysis of the wavepacket dynamics and make predictions for optical pump?probe experiments which detect either the terahertz radiation emitted by the oscillating charge carriers or the internal electric field as a function of time after impulsive excitation of charge carriers.

CARRIER-DENSITY DEPENDENCE OF THE EXCHANGE COUPLING BETWEEN MAGNETIC IONS AND CONDUCTION BAND ELECTRONS IN HEAVILY n-TYPE Zn(1-x)MnxSe AND OPTICALLY PUMPED Cd(1-x)MnxTe

We report on the analysis of the influence of carrier concentration on the exchange coupling between conduction band electrons and Mn d-electrons in diluted magnetic semiconductors. For this analysis we employed (i) electronic spin-flip Raman spectroscopy on heavily n-doped (Zn,Mn)Se, (ii) time-resolved Faraday rotation on optically pumped (Cd,Mn)Te. With increasing carrier density, a reduction of the exchange energy N0α up to 30 % is observed. It is explained by a weakening of the ferromagnetic character of the coupling of conducton-band electrons to the Mn d-electrons by an admixture of an antiferromagnetic (p-d) coupling contribution to the ferromagnetic (s-d) one for q ≠ 0.

Terahertz optical properties of thin doped contact layers in GaAs device structures

We investigate the transmittance of thin doped GaAs layers in the terahertz (THz) frequency range taking into account multiple reflections. Our experimental and theoretical study aims at providing a guideline for designing the top-side contact layers for THz emitters and receivers with direct, antenna-free coupling of the radiation. It is shown that the surface conductivity of the contact layer is the determining factor for the THz transmittance.

Carrier-density dependence of Faraday rotation and spin splitting in Cd1-xMnxTe

We employ spin-quantum-beat spectroscopy to investigate the carrier-density dependence of the spin-precession frequency and the magnitude of the Faraday rotation of Cd1−xMnxTe samples at fixed magnetic field. We find an onset of saturation of the Faraday rotation at carrier densities as low as 4× 1016 cm−3 and attribute it to electrons (not holes which dominate in other types of experiments). The spin splitting at fixed magnetic field remains density dependent down to 3 × 1015 cm−3 (the lowest density accessible in our measurements) which suggests a direct influence of the carrier-density on the sp–d exchange not mediated by screening effects.

Mirrors and output couplers for THz and optical radiation

Summary form only given. We use time-resolved THz spectroscopy in order to investigate the properties of highly conductive thin films. In this contribution we present experimental data on two of such film systems, which can be used in free-space THz radiation applications. We perform experiments on a thin indium tin oxide (ITO) film evaporated on a highly insulating glass substrate, in order to investigate its properties as a dichroic beamsplitter for THz and optical frequencies. In addition we study three different samples consisting of epitaxially grown p/sup +/-doped GaAs layers on GaAs substrates with different doping concentrations and layer thicknesses which are commonly used as contact layers in semiconductor devices (e.g. THz emitter structures).