Development of detectors and modulators for multi-hundred gigahertz operation.

Abstract

High performance optoelectronic devices are required for modern optical and optoelectronic systems. The thesis covers the development of novel ultrafast detectors and modulators as well as the work on quantum well modulators. A detector with a bandwidth of 375 GHz, the fastest reported to date, has been realized based on low-temperature-grown GaAs. The detector uses 0.2 $\mu$m interdigitated metal-semiconductor-metal (MSM) structure. The responsivity of the device is 0.1 A/W. Such high responsivity is comparable to a conventional MSM-photodiode, which is also experimentally investigated in the thesis. In addition, we have found that the same device can also function as a high efficiency picosecond photoconductive gate and picosecond electrical signal generator. We have combined two such components to form a sampling optical temporal analyzer to measure weak optical signals. The system response time is 1.9 ps, with noise equivalent power of 500 pW. Attempts were made to produce multi-hundred gigahertz traveling-wave modulators. We have successfully demonstrated single picosecond pulse propagation over 10 mm coplanar strip transmission lines. The propagation speed is found to be matched with that of the optical signal to within 1%. Based on the need to use superconductive transmission lines for the modulator, Nb thin film growth and fabrication techniques were developed in-house. Furthermore, a YBaCuO high-Tc thin film was successfully grown on GaAs for the first time. A buffer Al$\sb2$O$\sb3$ layer is used to prevent interdiffusion between the high-Tc material and the GaAs substrate. This technique can be used for traveling-wave modulators, as well as for other novel superconductive microwave devices. Research has also been done in the area of quantum well electronics. We have carried out a systematic study on temperature dependence of linewidths and lifetimes of excitonic transitions in quantum wells using both photoluminescence and optical absorption. These results are used for designing quantum well modulators. We fabricated the first InGaAs/GaAs strained quantum well modulator, and measured strong intensity and phase modulation when it was operated near the excitonic transition edge.