Ultrafast Nanowire Lasers as On-Chip Devices

Microchips that operate with light instead of electronics have become a vision that has been a key driver of semiconductor and nano research during the past years. The consequent drive to replace electronic components by photonic devices has reached a level where today’s communication and computation speed limit, as well as their energy consumption appears antiqued. The data capacity of fiber technology has evolved even faster than the downscaling of microchips and provides huge potential especially for short haul and chip-level interconnects in the near future. In contrast, the increasingly problematic of heat dissipation in electronic microchips and rising costs per bit make it very difficult to enhance speed and computing power simply by following constant field scaling. However, the inefficiency of silicon light sources and the incompatibility of the majority of optically active gain materials for the far advanced silicon technology has hindered the integration of efficient light sources on microchips. Offering a solution to this challenge, semiconductor nanowires can be monolithically integrated directly on silicon whilst maintaining the exceptional optical properties of crystalline direct bandgap semiconductors. Furthermore, the optical cavity formed by the unique nanowire geometry, in combination with the nanowire gain material constitutes all the required components for a nano-scale laser. Integrated on silicon, such coherent on-chip light sources could represent a major step towards the realization of chip-level optical interconnects or even photonic computers that, in the next decade, might not only remain a vision. However, the deleterious impact of non-radiative surface recombinations on the optical properties of III-V semiconductors has inhibited the demonstration of nanowire lasers that operate in the technologically important near infrared spectral range. Furthermore, in previous studies, nanowires typically had to be removed from their growth substrate to enable lasing, negating the huge technological advantage of monolithic integration and rendering their integration impractical. Besides their strong potential for future applications, nanowire lasers constitute a unique system that provide entirely new insights into fundamental gain dynamics at the ultimate downscaling limit of photonic lasers. Although, lasing operation of nanowires has been demonstrated from a broad range of material systems, coherent light-matter interactions within nanowire lasers have not received significant attraction. This thesis demonstrates how GaAs based nanowires are carefully designed to achieve lasing operation in the near infrared spectral range. The presented results demonstrate the first observation of room temperature lasing from GaAs nanowire lasers subject to pulsed optical excitation. Experiments using continuous optical pumping demonstrate continuous wave operation and show spectral linewidths below 200meV. Investigations of the gain and photon dynamics of the nanowire lasers reveal a novel mechanism that allows long-term mutual phase locking of picosecond pulse pairs and potential operation at modulation frequencies >200GHz. Finally, a novel nanowire laser geometry that allows the monolithic integration of nanowire lasers onto silicon microchips is introduced, successfully demonstrated in experiments and has been submitted as patent application. The exceptional properties of GaAs nanowire lasers combined with their unique ability for monolithic integration onto silicon provide nanowire lasers with the potential to impact a broad range of integrated optoelectronic technologies and could be a step along the road toward the vision of photonic microchips.