Florian Klappenberger

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.

Semiconductor–superlattice frequency multiplier for generation of submillimeter waves

We report on a semiconductor–superlattice frequency multiplier for the generation of submillimeter waves. A monochromatic microwave-pump field (frequency near 100 GHz) periodically excited a superlattice into states of negative differential conductivity giving rise to space-charge domain creation and annihilation joint with a nonsinusoidal high-frequency current and radiation at harmonics of the pump field. A highly doped GaAs/AlAs superlattice allowed the conversion of pump to third-harmonic radiation at 300 GHz with an efficiency of about 1% (power∼30 μW).

Electric-field-induced reversible avalanche breakdown in a GaAs microcrystal due to cross band gap impact ionization

We report on a reversible avalanche breakdown due to free-carrier multiplication caused by cross band gap impact ionization in a GaAs microcrystal. The n GaAs microcrystal (length 1 μm, diameter 1 μm) was embedded between n+ GaAs layers serving as electric contacts. We guided an electric pulse to the sample and determined, from the reflected and transmitted pulse, the I(V) characteristic. The breakdown was indicated by a sudden current rise and voltage drop and a hysteresis effect and, furthermore, by electron-hole recombination radiation. We reached the threshold field for ionization by making use of a high-field domain whose formation was based on the Gunn effect. The microcrystal could reproducibly be switched into the nonequilibrium avalanche state. Our analysis indicates that the effect provides a basis for the development of an ultrafast electric switch.

Transient-pulse nonlinear spectroscopy with the radiation of a multimode THz gas laser

We report on transient-pulse nonlinear spectroscopy with the radiation of a multimode THz gas laser. The method is demonstrated for studying the nonlinear response of a current-carrying superlattice to THz radiation; the current through a superlattice can be suppressed by a strong THz field. The method makes use of the pulses of a high power multimode THz gas laser. By splitting the laser beam for selected laser modes into a main beam and a reference beam we monitored with a reference detector the transient power in the main beam. The simultaneous measurement of both the instantaneous response and the instantaneous power allows to obtain the power dependence of the response within a single laser pulse. The method is suitable to study the nonlinear response of matter to THz radiation fields in a large dynamic range.

Superlattice detector as a fast direct detector and autocorrelator for terahertz radiation

We report on a GaAs/AlAs superlattice detector as a novel direct detector and autocorrelator for THz radiation. It is based on a doped wide-miniband GaAs/AlAs superlattice, with submonolayer AlAs barrier layers; the superlattice is operated at room temperature. THz radiation, generated by a free-electron laser and a mode locked p-Ge laser, was coupled into the superlattice via a corner cube antenna system. THz-irradiation of the biased superlattice resulted in a current reduction, which was monitored. The direct detector showed a fast response (20 ps, limited by the electronic circuit) and was robust against intense radiation pulses (peak power 10 kW). The responsivity was 100 times higher than the responsivity of detectors of comparable risetime and comparable robustness. Intense THz radiation caused a complete suppression of the current through the superlattice. This is the basis of the superlattice autocorrelator. The superlattice autocorrelator could resolve picosecond radiation pulses.

Fast GaAs detector for THz radiation

We present a fast detector for THz radiation. The detector consists of a current carrying n doped GaAs crystal built in a corner cube antenna system. Irradiation with THz radiation leads to a reduction of the direct current. We demonstrate the feasibility for detection by analysing single radiation pulses of a TEA CO/sub 2/ laser pumped FIR gas laser.

Ultra-fast domain formation in a semiconductor superlattice

Presents experimental evidence for ultra-fast domain formation in a semiconductor superlattice. An InGaAs/InAlAs superlattice device was mounted in a corner cube. The superlattice device showed a region of negative differential conductance in the current-voltage characteristic for voltages larger than an onset voltage. When the superlattice device was irradiated with radiation smaller than 1.2 THz, the onset voltage shifted, with increasing THz radiation power, to lower voltages. Taking an analysis based on a drift-diffusion model into account, we suggest that dipole domains form within about 400 fs.