Development of epitaxial strategies towards the growth of novel semiconductor saturable absorber mirrors (SESAMs) for passively mode-locked optically pumped Vertical-external-cavity surface-emitting lasers (VECSELs)

Mode-locked VECSEL systems using SESAMs are a relatively less complex and cost-effective alternative to state-of-the-art ultrafast lasers based on solid-state or fiber lasers. This system has seen considerable progress in device performance in terms of pulse width and peak power in the recent years. However, these device characteristics still have to be improved greatly and this system has to become more reliable to become commercially relevant. The realization of femtosecond pulses over long periods of time is non-trivial due to the high sensitivity of SESAM characteristics to minor drifts in growth or cavity conditions. This work focuses on developing novel epitaxial strategies for the growth and optimization of SESAMs towards obtaining sub-100fs pulse durations and increasing the damage threshold for these devices. As part of this work, we have described in detail a comprehensive growth strategy for InGaAs quantum well (QW) -based SESAMs that are capable of supporting femtosecond pulses. These SESAMs are characterized for reflectivity, temperature-dependence, dispersion control and lifetimes. Through this process, we were able to achieve pulse durations as short as 128fs from InGaAs QW-based SESAMs. This is one of the shortest pulse durations reported to date from mode-locked VECSEL systems. However, it is found that the QW-based SESAMs exhibit poor temporal performance. As an alternative to this system, the latter part of the thesis explores the possibility of using InAs/GaAs submonolayer (SML) quantum dots (QDots) as the saturable absorber for SESAMs around 1µm. Along with higher output power, we were able to realize mode-locking of VECSELs using SML QDot-based SESAMs with pulses as short as 185 fs. To the best of our knowledge, this is the first time SML QDots are used as saturable absorbers for SESAMs in the femtosecond regime. We also found that QDot-based SESAMs have substantially longer lifetimes compared to QW-based SESAMs