Investigating Optical-Field-Induced Currents in GaN using Ultrafast Lasers

Abstract: This study saw the development of an experimental setup capable of generating and measuring optical-field-induced currents in a variety of nanodevices fabricated specifically for this project. Each device design features two metallic contacts, closely separated by about 5 micrometers, deposited onto a semiconductor or insulator substrate. The region between these two contacts is a junction, being where the laser is focused to generate, and consequently measure, the current. In total, 15 separate substrate samples had nanodevices fabricated onto them. Most devices were made using gold etching, placing Au on mostly GaN substrates, but some SiC and SiO2 substrates were also used. Devices were also fabricated using a lift-off procedure, allowing for a Ti/Au-GaN device to be made. Two separate laser systems were used to investigate the generation of optical-field-induced currents. One, a titanium-sapphire laser generating a field asymmetry by ultrashort pulses, the other a ytterbium laser generating it by a fundamental and second harmonic superposition. However, while currents were measured with both laser systems, several characteristics of the signal raised further questions. The phase modulation of the Ti:Sa laser did not result in a current oscillation, nor did the device only produce a current when the pulse was polarised in the direction of the junction, instead generating a current even with a perpendicularly polarised pulse. On the other hand, the Yb laser measurements seemed to confirm that the current direction has a polarisation dependence, since no current was generated with perpendicularly polarised pulses, and when reversing the polarisation there was a sign change in the current. However, modulating the relative delay of the two pulses from the Yb laser generated no current oscillation, unlike what was expected from the model. Furthermore, the high intensities required to induce a current would ablate both the gold and substrate materials, destroying the devices. Likewise, the phase modulation of the Ti:Sa also saw no change in current. To substantiate the experimental efforts, a model derived from Bloch equations developed by Khurgin[1] to estimate the photoinduced charge produced by a single, or two cross-polarised, laser pulses on a device was recreated. It was also expanded upon to cover two-colour experiments, like the wave superposition used in the Yb laser experiments. Finally, a fully featured GUI was written to more easily control the parameters of the model, having the potential to quickly create and compare experimental results to the expected modelled outcome.