High speed signal generation, guidance and detection in the millimeter-wave regime by ultrafast optical techniques.

Abstract

This dissertation has investigated ultrafast optoelectronic techniques and their application to the generation, guiding and measurement of picosecond electrical pulses. The ability to produce short-duration test signals, transmit them over significant distances, and measure them has also been used to demonstrate unique device characterization schemes. For picosecond pulse generation, low-temperature-grown GaAs (LT-GaAs), with its low leakage current and high responsivity, is shown to be the most important material. The origin of the short carrier lifetime in this material is found to be more related to point defects than to structural defects. For metal-semiconductor-metal (MSM) photodetectors fabricated on LT-GaAs, the high optical power saturation mechanism is attributed to the Schottky barrier lowering instead of the trap saturation. For picosecond pulse generation on semi-insulating GaAs (which has a long carrier lifetime) by edge illumination, the ultrafast response mechanism is found to be due to field localization toward the positive bias electrode. The metal-semiconductor contact property has also been studied, and it is discovered that with a thin Ge layer, the leakage current can be greatly reduced. A new process called van der Waals bonding is used to integrate the LT-GaAs thin film onto various host substrates in order to produce an in situ electric pulser. In order to deliver the bandwidth of picosecond pulses to the desired destination, planar waveguides fabricated on low-permittivity substrates have been studied. In particular, a coplanar stripline on a micromachined membrane substrate has been demonstrated to have a bandwidth in excess of one terahertz. This was possible because of the complete elimination of radiation loss and dispersion on the coplanar line, and as a result, the distortionless propagation of picosecond pulses over a long distance was attained. Electro-optic sampling is the primary tool currently available for high-speed measurement. However, the high permittivity crystal typically can lead to the acquisition of distorted waveforms. A new electro-optic probe based on an organic electro-optic polymer is suggested. By using a novel probe structure to squeeze the electric field into the active layer, this probe should exhibit no degradation in sensitivity while having extremely low invasiveness. In regards to measurement applications, a HEMT transistor has been characterized in the time domain to demonstrate the capability of optically-based instrumentation. A 7-ps, 0.8-V switching signal, which can not be obtained using conventional, all-electronic instrumentation, is observed. Two passive electrical components have also been characterized in the frequency domain up to a bandwidth of 300 Ghz.