G. Loata

THz-photomixer based on quasi-ballistic transport

We report on a novel concept for THz photomixers with high conversion efficiency up to several THz. In contrast to the conventional pin photomixer we can overcome the trade-off between either optimizing transit-time or RC-roll-off. Using quasi-ballistic transport in nano-pin-diodes the transport path can be optimized regarding both path length and transit time. Independently, the capacitance can be kept small by using a sufficiently large number of optimized nano-pin-diodes in series. The concept is presented in detail and first experimental results are reported which corroborate our theoretical expectations.

Radiation field screening in photoconductive antennae studied via pulsed terahertz emission spectroscopy

We report terahertz emission experiments on low-temperature-grown GaAs photoconductive antennae. Two field-screening effects determine the device response: space-charge screening on a long time scale and radiation field screening of the local electric field. This latter effect is the principal cause for saturation of terahertz emission observed when the emitters are driven hard with high-repetition-rate femtosecond laser pulses. We present an equivalent-circuit model consisting of three elements: a resistor with time-dependent conductance (photoswitch), a time-dependent voltage source (space-charge screening), and the antenna impedance (terahertz emission and radiation field screening). The simulations with this voltage divider reproduce the measured data well.

All-optoelectronic continuous-wave terahertz systems.

We discuss the optoelectronic generation and detection of continuous–wave terahertz (THz) radiation by the mixing of visible/near–infrared laser radiation in photoconductive antennas. We review attempts to reach higher THz output–power levels by reverting from mobility–lifetime–limited photomixers to transit–time–limited p–i–n photodiodes. We then describe our implementation of a THz spectroscopy and imaging–measurement system and demonstrate its imaging performance with several examples. Possible application areas of THz imaging in the biomedical field and in surface characterization for industrial purposes are explored.

Evidence for long-living charge carriers in electrically biased low-temperature-grown GaAs photoconductive switches

Low-temperature-grown GaAs continues to be one of the most important materials of ultrafast optoelectronics. Little is known, however, about the recombination dynamics of photogenerated charge carriers under the influence of an applied electric field, and it has remained unclear to what extent biased photoswitches exhibit field screening effects. Here, the authors investigate the screening in biased few-micrometer-sized photoconductive gaps quantitatively and find that it can amount to tens of percent of the applied field. They find that a subgroup of the photogenerated carriers recombines on an unexpectedly long excitation-density-dependent time scale of nanoseconds to tens of nanoseconds.

Optical Far-IR wave generation - state-of-the-art and advanced device structures

A recent study initiated by the European Space Agency aimed at identifying the most promising technologies to significantly improve on the generation of coherent electromagnetic radiation in the THz regime. The desired improvements include, amongst others, higher output powers and efficiencies at increasingly higher frequencies, wider tunability and miniaturization. The baseline technologies considered revolve around Photomixing and novel laser based technologies compared to all electronic techniques. Some of the most significant findings will be presented together with technological developments and experimental results selected for medium to short term development. These technologies include advanced p-i-n photomixer with superlattice structures and, THz quantum cascade lasers. Recent results achieved in these fields will be put into the potential perspective for the respective technology in the future.

Terahertz surface and interface characterization

We describe our recent work on various all-optoelectronic imaging systems for the terahertz frequency range and outline their application for surface and interface characterization. Our systems are based either on pulsed or continuous-wave techniques, and they operate both in reflection and transmission geometry. Dark-field techniques are implemented in order to increase the sensitivity to diffraction and scattering effects. Possible application areas of THz imaging in the biomedical field as well as in surface and interface characterization for industrial purposes are explored.

A new THz-photomixer based on a n-i-p-n doping superlattice

We investigate a THz-photomixer based on quasi-ballistic transport in an asymmetric n-i-p-n superlattice. The measured frequency roll-off is due to the device RC time constant alone whilst the transit-time limitation is overcome in this device. Measured power levels compare well with those from standard LT-GaAs photomixers.

All-optically generated ultrashort voltage pulses on planar transmission lines

We show that shift currents generated by all-optical excitation of (110)-oriented bulk GaAs can be employed to launch ultrashort voltage pulses with frequency components exceeding 1 THz on planar transmission lines.

The nipnip‐THz‐emitter: photomixing based on ballistic transport

We report on a novel concept for THz‐photomixers based on quasi‐ballistic transport in an asymmetric nipnip‐doping‐superlattice. Due to tansport‐optimized i‐layers the emitted powers are not transit‐time‐limited up to 1 THz. Furthermore the capacitance and hence the RC‐roll‐off is minimized by increasing the number of pin‐periods. The frequency‐dependence of the nipnip‐emitter proofs to be superior to corresponding pin‐photomixers.