Optical rectification in semiconductor waveguides
In this thesis, we study optical to microwave conversion and generation of ultrashort electrical pulses by the use of optical rectification at telecommunication wavelengths, λ = 1550 nm. By using optical rectification, an electromagnetic pulse is generated in a completely passive semiconductor waveguide. This pulse is coupled in a microwave transmission line with periodically loaded ground electrodes to create a velocity-matched structure. The optical waveguide and the microwave transmission line form the optical rectification device. Although in theory, the width of the electrical pulse in a travelling wave structure is limited only by the duration of the optical excitation pulse, imperfections in the velocity matching will attenuate and disperse most of the electrical pulse. The calculated effective optical refractive index of the rectification devices, nopt - 3.30, matches the measured effective microwave index in one of our structures namely DevO68 (nmw = 3.30). If the structure is slightly velocity-mismatched, losses as high as 14 dB/mm at frequencies of 1 THz will affect the propagation of the electrical pulse. The optical rectification device was fabricated using conventional photolithography techniques and e-beam lithography techniques. The advantages of e-beam lithography are: better pattern definition, perfect alignment and easier lift-off process. The only disadvantage is the cost associated with running the e-beam writer and maybe the time it takes to complete a pattern. The semiconductor material system of choice for the rectification devices is GaAs / AlGaAs due to its well-known large nonlinear coefficient. The use of GaAs/AlGaAs with light at λ = 1550 nm, presents serious absorption effects. The absorption effects mask the pure optical rectification signal and therefore must be minimised. The most significant absorption effect at λ = 1550 nm is two-photon absorption (TPA), which in more than one experiment gave us pulses of a few nanosecons duration. Our rectification device is engineered to minimise TPA, and this is the perhaps the hardest challenge in the design of the device. This also maybe the reason why there is not rectification devices such as ours reported in the literature working at λ = 1550 nm. The reason why we wanted to work with GaAs/AlGaAs is the potential integration of the rectification device in optoelectronic systems. In the final rectification device, we could observe a clear polarization dependence of the generated signal indicating optical rectification. The signal detected was small in magnitude, ~75 dBm and on top of an offset signal which is believed to be TPA. Nevertheless, we proved that an optical rectification signal could be generated and detected by experimental means. Finally, Q-switched diode lasers in Al-quaternary material were fabricated and evaluated as possible sources for the rectification devices. The lasers produced a pulse train ranging from 1 GHz to 2 GHz depending on the bias current. We reckon that our measurement set-up is not ideal to characterize the rectification signal but is the simplest set-up capable of giving us an indicative result. The time domain observation of the optical rectification signal has still to be done and the integration of a photoconductive switch to the optical rectification device seems to be the most obvious solution to achieve this.