Abstract
Mode locking is rarely observed in random lasing devices, as they are based on the strong optical scattering by random nanostructures and the requirement for strong nonlinear interactions in conventional mode-locking mechanisms cannot be satisfied in them. Our discovery of the cascaded absorption and stimulated emission (CASE) effect solved this challenge, with the strong overlap between the absorption and emission spectra in hybrid organic-inorganic perovskites being the underlying physics. Previously, phase-locked random lasing was observed in randomly distributed crystalline nanoparticles, which provide strong optical scattering and facilitate gain mechanisms for random lasing. In this work, we demonstrate phase-locked random lasing in continuous crystalline microstripes embedded with nanoholes. These nanoholes enable the formation of porous stripes of the active material, implying a different mechanism for random lasing. Microchannels with different widths were produced by photolithography using mask gratings with different periods and the same duty cycle of 50%. Microstripes were then created by spin-coating the green-emitting MAPbBr(3) precursor solution onto the mask gratings. Thus, nanohole-structured MAPbBr(3) microstripes formed in the grating grooves. Using femtosecond transient absorption spectroscopy, we were able to resolve the CASE features through the oscillations in the TA dynamics, which have a period of less than 400 fs. Phase-locked random lasing modes were identified by equally spaced spectral lines with a mode separation of 1.26-1.58 nm. Thus, phase-locking mechanisms for random lasers were verified in a nanoporous scheme.