Designed semiconductor network random lasers

Conventional lasers typically support a well-defined comb of modes. Coupling many resonators together to form larger complex cavities enables the design of the spatial and spectral distribution of modes, for sensitive and controllable on-chip light sources. Network lasers, formed from a mesh of dye-doped polymer interconnecting waveguides, have shown great potential for random lasing with a highly sensitive and customisable lasing spectrum albeit suffering from gain bleaching. Here we introduce on-chip semiconductor network lasers, fabricated by etching an InP epilayer bonded onto a $SiO_2$/Si wafer, as a reproducible, stable and designable random laser with a rich multimodal spectrum and low room temperature lasing threshold. We observe thresholds as low as 60 {\mu}J$cm^{-2}$ pulse{-1}$ for InP networks with optimum link width of 450 nm and thickness 120 nm. We further show, both experimentally and numerically, that the network density directly affects the mode spatial distribution, and lasing modes are spatially localised over only 10-20 connected links in large dense networks. The InP network lasers are also stable to pump illumination and sensitive to small variations in the pump pattern. These studies lay the ground for future design of random lasers tailored to application in robust semiconductor platforms with impact for sensing, signal processing, cryptography, and machine learning.