Subpicosecond Observation of Photoexcited Carrier Thermalization and Relaxation in InP-Based Films1

Advancements in microfabrication techniques and thin film growth have led to complex integrated photonic devices. The performance of these devices relies upon precise control of the band gap and absorption mechanisms in the thin film structures, as well as a fundamental understanding of the photoexcited carrier thermalization and relaxation processes. Using a pumpprobe technique, it is possible to monitor the transient thermalization and relaxation of hot electrons and holes on a sub-picosecond time scale. This method relies upon the generation of hot carriers by the absorption of an intense ultrashort laser pulse (∼135 fs). Transient changes in reflectance due to the pump pulse excitation are monitored using a weaker probe pulse. Control of the relative time delay between the pump and probe pulses allows for temporal measurements with resolution limited only by the pulse width. The transient change in reflectance is the result of the transient change in the electron and hole distributions. Observation of the reflectance response of InP films on a subpicosecond timescale allows for detailed examination of thermalization and relaxation processes of the excited carriers. Longer timescales (>100 ps) are useful for correlating the transient reflectance response to slower processes such as thermal conduction and recombination. A description of this technique and results for several InP-based films are presented.