Design, fabrication and characterisation of III-V nanowire random lasers

Abstract

Disorder is generally considered an undesired element in lasing action. However, disorder plays a significant and critical role in random lasers (RLs), whose feedback mechanism is based on random scattering events. Even though some unique properties in RLs such as large-angle emission, lasing from different surfaces, large area manufacturability, and wavelength tunability can be advantageous in specific applications, the applicability of RLs has been limited due to the chaotic fluctuations and instability of the lasing modes. The instability issue of RLs has resulted in fluctuating sources whose properties could not be controlled. Therefore, even though RLs, unlike other lasers based on traditional Fabry-Perot cavities, are much simpler and cost-effective sources to fabricate, because of the chaotic fluctuations and instability of the lasing modes, controlling their lasing properties is challenging. In this dissertation, using random arrays of III-V nanowires as the disordered medium, it is shown that mode localisation could reduce the spatial overlap between the various lasing modes, thus preventing mode competition and improving stability, leading to laser sources with high-quality factors and very low thresholds. We present the experimental evidence of strong light localisation in multi-mode random nanowire lasers, which are temporally stable, leading to controllable emission. We further demonstrate that by changing the design parameters of the nanowire array, such as filling factor, dimensions of the nanowires, degree of randomness, and array size, the properties of the lasing modes, including the number of modes, lasing wavelengths, and lasing threshold can be controlled. To localise and decouple the lasing modes, the disorder in the medium should support modes that have a resonant type of feedback where there is a chance for the scattered light to create cavities or closed loops. If the scattering efficiency is not strong enough, the modes are coupled which are not conducive to supporting closed-loop paths. In these types of RLs, the feedback mechanism becomes non-resonant (non-coherent) without creating any cavities. In other words, light just gets amplified, and the emission becomes amplified spontaneous emission without any coherency. We show that by aligning the nanowires, the modes' scattering efficiency and quality factor can be increased, thereby favouring the resonant feedback lasing. Besides, practical methods for managing the type of feedback mechanism are also shown and discussed. Switching between resonant and non-resonant lasing modes opens up new opportunities to use these lasers in many applications. In this thesis, different issues of RLs such as lasing mode instability, challenges of controlling the RLs emission properties and feedback mechanisms were investigated and addressed. The practical methods introduced to solve these challenges could open up the realisation of broadband nanoscale lasers for the next-generation photonic devices, with all the advantages offered by RLs. Show more