Karl Friedrich Renk

Interference Filters and Fabry-Perot Interferometers for the Far Infrared

This paper describes measurements on transmission interference filters for the far infrared (100–800 μ region). The reflection layers of these filters are metallic mesh. The measured coefficients of finesse, ten to sixty, are comparable with the values which can be obtained for interference filters in other spectral regions. Peak-transmission values up to 0.9 were reached. From these measurements the absorptivity of the single grids has been determined. Measurements on a variable spacing interference filter showed that the finesse is proportional to the square of the wavelength in accordance with theoretical predictions for interference filters constructed with reflection layers of one-dimensional wire gratings.

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.

Broadband ultrafast superconducting NbN detector for electromagnetic radiation

An ultrafast detector that is sensitive to radiation in a broad spectral range from submillimeter waves to visible light is reported. It consists of a structured NbN thin film cooled to a temperature below Tc (∼11 K). Using 20 ps pulses of a GaAs laser, we observed signal pulses with both rise and decay time of about 50 ps. From the analysis of a mixing experiment with submillimeter radiation we estimate an intrinsic response time of the detector of ∼12 ps. The sensitivity was found to be similar for the near‐infrared and submillimeter radiation. Broadband sensitivity and short response time are attributed to a quasiparticle heating effect.

High-frequency self-sustained current oscillation in an Esaki-Tsu superlattice monitored via microwave emission

Abstract We report the observation of a high-frequency self-sustained current oscillation in an n doped GaAs AlAs superlattice (at room temperature) that shows a negative differential conductance (Esaki-Tsu superlattice). The superlattice biased in the region of the negative differential conductance emitted microwave radiation at a fundamental frequency of about 6 GHz and at harmonics. We attribute the oscillation to space charge instabilities inside the superlattice caused by Bragg reflection of electrons in the lowest, wide miniband.

Far-Infrared Reflectivity and Dynamical Conductivity of an Epitaxial YBa2Cu3O7-δ Thin Film

We report on measurements of the far-infrared reflectivity of an epitaxial, oriented YBa2Cu3O7-δ thin film for radiation with the electric-field vector parallel to the crystal (a, b)-plane and present an analysis that delivers evidence for two different superconducting energy gaps, with 2Δ(0)/kB Tc ≈ 1.9 and ≈ 5.8.

THz-field induced nonlinear transport and dc voltage generation in a semiconductor superlattice due to Bloch oscillations

We report on a theoretical analysis of terahertz (THz-) field induced nonlinear dynamics of electrons in a semiconductor superlattice that are capable to perform Bloch oscillations. Our results suggest that for a strong THz-field a dc voltage should be generated. We have analyzed the real-time dynamics using a balance equation approach to describe the electron transport in a superlattice miniband. Taking account of both Bloch oscillations of electrons in a superlattice miniband and dissipation, we studied the influence of a strong THz-field on currently available superlattices at room temperature. We found that a THz-field can lead to a negative conductance resulting in turn in a THz-field induced dc voltage, and that the voltage per superlattice period should show, for varying amplitue of the THz-field, a form of wisted plateaus with the middle points being with high precision equal to the photon energy divided by the electron charge. We show voltage to the finite voltage state, and that in the finite voltage state dynamic localization of the electrons in a miniband occurs.

Pulsed‐laser deposition of smooth high‐Tc superconducting films using a synchronous velocity filter

Pulsed‐laser deposition of smooth high‐Tc superconducting films almost free from droplets and precipitates with the use of velocity filtration of plasma particles is reported. We have removed droplets from laser‐induced plasma by using a shutter technique; a reduction of the droplet density by a factor of 105 has been achieved. We have applied the technique to the preparation of high quality YBa2Cu3O7−δ films on (100)‐oriented SrTiO3, MgO, Y2O3‐stabilized ZrO2 (YSZ) substrates and, furthermore, on (1102)‐oriented sapphire covered with (100) sublayers of Si and (100) YSZ buffer layers.

Subnanosecond photoresponse of a YBaCuO thin film to infrared and visible radiation by quasiparticle induced suppression of superconductivity

We observed subnanosecond photoresponse of a structured superconducting YBa2Cu3O7−δ thin film to infrared and visible radiation. We measured the voltage response of a current biased film (thickness 700 A) in a resistive state to radiation pulses. From our results we conclude a response time of about 90 ps and a responsivity of about 4×1010 Ω/J. We attribute the response to Cooper pair breaking and suppression of the superconducting energy gap induced by nonequilibrium quasiparticles.

Self-sustained current oscillation above 100 GHz in a GaAs/AlAs superlattice

A GaAs/AlAs superlattice with a large miniband (120 meV) showed self-sustained current oscillation at a frequency of 103 GHz giving rise to microwave emission (power 0.5 mW). The emission line had a linewidth of about 1 MHz and was tuneable by about 800 MHz. An analysis suggests that the transport in the superlattice was mainly due to electrons in the lowest miniband and that the oscillation was caused by traveling dipole domains. We also observed frequency locking of the current oscillation attributed to a synchronization of domain propagation by the external high-frequency field.

Broadband semiconductor superlattice detector for THz radiation

We report on a broadband GaAs/AlAs superlattice detector for THz radiation; a THz field reduces the current through a superlattice, which is carried by miniband electrons, due to modulation of the Bloch oscillations of the miniband electrons. We studied the detector response, by use of a free electron laser, in a large frequency range (5–12 THz). The responsivity showed strong minima at frequencies of infrared active phonons of the superlattice. A theoretical analysis of the detector delivers an understanding of the role of phonons and gives a characterization of the responsivity.