We report on photonic technologies developed at the MPI fur Radioastronomy and the Research Center Julich to generate Terahertz Local Oscillator reference signals for use on e.g. ALMA/APEX and SOFIA. The principle is to mix two (NIR) laser colours in a biased LTG-GaAs layer, thus creating a high-frequency beat (difference) frequency signal. This output signal is coupled to free space through an antenna. In this work a systematic study of the photomixer design, in order to optimize the RF power, is presented. Part of the experiments were done with photomixers integrated to with a broadband spiral antenna designed for frequencies up to 1 THz. The LT GaAs photomixers are prepared on materials with various growth temperatures as well as using resonant cavity material structures and various finger contact geometries. An improvement in the output power up to around 3 μW of submillimeter radiation (0.5 THz) is demonstrated.
We report on fabrication and high-frequency performance of our photodetectors and photomixers based on freestanding low-temperature-grown GaAs (LT-GaAs). In our experiments, the LT-GaAs/AlAs bilayers were grown on 2-inch diameter, semi-insulating GaAs wafers by a molecular beam epitaxy. Next, the bilayer was patterned to form 10×10 μm2 to 150×150 μm2 structures using photolithography and ion beam etching. The AlAs layer was then selectively etched in diluted HF solution, and the LT-GaAs device was lifted from its substrate and transferred on top of a variety of substrates including Si, MgO/YBaCuO, Al2O3, and a plastic foil. Following the transfer, metallic coplanar transmission lines were fabricated on top of the LT-GaAs structure, forming a metal-semiconductor-metal photodetectors or photomixer structures. Our freestanding devices exhibited above 200 V breakdown voltages and dark currents at 100 V below 3×10-7 A. Device photoresponse was measured using an electro-optic sampling technique with 100-fs-wide laser pulses at wavelengths of 810 nm and 405 nm as the excitation source. For 810-nm excitation, we measured 0.55 ps-wide electrical transients with voltage amplitudes of up to 1.3 V. The signal amplitude was a linear function of the applied voltage bias, as well as a linear function of the laser excitation power, below well-defined saturation thresholds. Output power from the freestanding photomixers was measured with two-beam laser illumination experimental setup. Reported fabrication technique is suitable for the LT-GaAs integration with a range of semiconducting, superconducting, and organic materials for high-frequency hybrid optoelectronic applications.