There were three key aspects to EPNRL: design, fabrication and characterisation.
Design involved optimising both the individual components (waveguides) as well as the network structure to reduce the threshold of network lasers. For the waveguide design, we used finite-difference time-domain (FDTD) simulations to optimise the waveguide dimensions to reduce scattering losses at network nodes, which are the main source of loss in the network. For the network design, we used a home-built code based on solving Maxwell’s Equations on a graph, to model the passive modes in networks and estimate their Q factors. This code, however, did not include gain and mode competition, which is necessary for modelling lasing. So later on in the project, through a collaboration, we extended the graph model to include gain (netSALT), and used it for modelling lasing in network lasers with non-uniform pumping and for comparing lasing in different network structures.
The fabrication of semiconductor networks involved developing the process and optimising the steps to achieve high quality structures. We focused on making networks with Indium Phosphide (InP) because of its low surface recombination and thus high optical quality. During the project, we developed the process for fabricating high-quality InP networks, optimising the lithography and reactive ion etching steps to minimise the waveguide roughness and reduce ion damage. This part of the project took longer than expected, as it needed access to facilities (affected by closures and equipment downtimes) and materials (purchasing of wafers, resist and resins). During the project, we also collaborated with many external groups, with established III-V fabrication labs, to get advice and help with the fabrication. These collaborations were fruitful in obtaining first working samples, which exhibited optically pumped room temperature lasing (manuscript under preparation).
We characterised lasing in InP networks by optical pumping using a fs-pulsed laser equipped to a microscope. During the fabrication process development, we used micro-photoluminescence to assess the optical quality of etched structures and scanning electron beam to measure the sidewall roughness and dimensions of fabricates structures. We also performed experiments to illuminate the network lasers with a spatially non-uniform pump profile, using a digital micromirror device (DMD), and demonstrated spectral control in the polymer nanofiber network lasers (manuscript under preparation).
A summary of the lasing results from InP networks is shown in the attached figure. Panel a shows a top view image of a fabricated InP network with the intensity profile of a network mode overlaid on top. Schematic of the InP waveguide cross-section is given in the inset. Panel b shows the optical image of the InP network laser above threshold and panels c-d show the lasing spectrum and total intensity from the network as a function of pump fluence, respectively.
