Disorder is generally considered an undesired element in lasing action. However, in random lasers (RLs) whose feedback mechanism is based on random scattering events, disorder plays a very important role. 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 certain 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.

In this seminar, 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. Furthermore, It is shown that by aligning the nanowires the scattering efficiency and quality factor of the modes can be increased, thereby favouring the resonant (coherent) feedback lasing. Finally, practical methods for managing the type of feedback mechanism are shown and discussed.

To conclude, in this seminar, 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.

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