Roman Dietz

Ordered n-type ZnO nanorod arrays

Using indium as catalyst for growth and simultaneously as doping source, ordered arrays of n-type ZnO single crystal nanorods have been perpendicularly grown on p-GaN∕Al2O3 substrates with a vapor phase transport growth method. The low temperature photoluminescence measurements of the n-ZnO nanorods show dominant In-related neutral donor bound exciton emission in the ultraviolet region. Electrical transport measurements performed on single n-ZnO nanorods yield resistances of about 50–200kΩ and a typical specific resistivity of 2.0×10−2Ωcm. The resistivity is one order of magnitude reduced by introducing In compared to the nominally undoped ZnO nanorods.

64 μW pulsed terahertz emission from growth optimized InGaAs/InAlAs heterostructures with separated photoconductive and trapping regions

We present results on optimized growth temperatures and layer structure design of high mobility photoconductive Terahertz (THz) emitters based on molecular beam epitaxy grown In0.53Ga0.47As/In0.52Al0.48As multilayer heterostructures (MLHS). The photoconductive antennas made of these MLHS are evaluated as THz emitters in a THz time domain spectrometer and with a Golay cell. We measured a THz bandwidth in excess of 4 THz and average THz powers of up to 64 μW corresponding to an optical power-to-THz power conversion efficiency of up to 2 × 10−3.

Carrier dynamics in Beryllium doped low-temperature-grown InGaAs/InAlAs

The electron and hole dynamics in low-temperature-grown InGaAs/InAlAs multiple quantum well structures are studied by optical pump-probe transmission measurements for Beryllium (Be) doping levels between 3 × 1017 cm−3 and 4 × 1018 cm−3. We investigate electron dynamics in the limit cases of unsaturated and completely saturated electron trapping. By expanding a rate equation model in these limits, the details of carrier dynamics are revealed. Electrons are trapped by ionized arsenic antisites, whereas recombination occurs between trapped electrons and holes trapped by negatively charged Be dopants.

Terahertz generation and detection with InGaAs-based large-area photoconductive devices excited at 1.55 μm

We report on scalable large-area terahertz emitters and detectors based on In0.53Ga0.47As/In0.52Al0.48As heterostructures for excitation with 1.55 μm radiation. Different geometries involving three different electrode gap sizes are compared with respect to terahertz (THz) emission, bias field distribution, and Joule heating. The field distribution becomes more favorable for THz emission as gap size increases, while Joule heating exhibits the opposite dependence. Devices with three different gap sizes, namely 3 μm, 5 μm, and 7.5 μm, have been investigated experimentally, the emitter with a gap size of 7.5 μm showed the best performance. The scalable devices are furthermore employed as detectors. The scalable electrode geometry enables spatially integrated detection, which is attractive for specific applications, e.g., where an unfocused THz beam has to be used.

Handheld terahertz systems based on telecom technologies

The potential of Terahertz technology for various applications in security or material inspection has now been demonstrated widely. However, THz systems have to shrink in size and price to become acceptable for real-world applications. Our way to approach this target is to exploit technologies developed and matured originally for 1.55 μm fiber-optic telecom applications. Key devices are photoconductive antennas for 1.55 μm operation. They were developed using a novel InGaAs/InAlAs multi-layer structure. The antennas are packaged into fiber coupled modules. These handheld THz heads are driven by a 1.55 μm pulsed fiber laser. A tunable delay with both an oscillating mirror and a precise linear drive allow for real-time measurements as well as for high resolution spectroscopy. The system performance up to 4 THz is demonstrated.

Gain competition and mode shifts in ZnO nanorod lasers

We study the lasing of individual ZnO nanorod resonators. We focus on the dynamics of the laser modes in terms of gain competition and shifts. Furthermore the influence of temperature on lasing thresholds is discussed.