Single mode emission and non-stochastic laser system based on disordered point-sized structures: toward a tuneable random laser

Continuous-wave quasi-single-mode random lasing in CH3NH3PbBr3 perovskite films on patterned sapphire substrates

We report intriguing continuous-wave quasi-single-mode random lasing in methylammonium lead bromide (CH3NH3PbBr3) perovskite films synthesized on a patterned sapphire substrate (PSS) under excitation of a 532-nm laser diode. The random laser emission evolves from a typical multi-mode to a quasi-single-mode with increasing pump fluences. The full width at half-maximum of the lasing peak is as narrow as 0.06 nm at ∼547.8 nm, corresponding to a high Q-factor of ∼9000. Such excellent random lasing performance is plausibly ascribed to the exciton resonance in optical absorption at 532 nm and the enhanced optical resonance due to the increased likelihood for randomly scattered light to re-enter the optical loops formed among the perovskite grains by multi-reflection at the perovskite/PSS interfaces. This work demonstrates the promise of single-mode perovskite random lasers by introducing the exciton resonance effect and ingeniously designed periodic nano/micro optical structure.

Random lasing over gap states from a quasi-one-dimensional amplifying periodic-on-average random superlattice

We report the experimental implementation and theoretical analysis of a random laser system that constitutes an amplifying periodic-on-average random superlattice (PARS). The stringent conditions on monodispersity required for a periodic-on-average random superlattice system are fulfilled using a linear array of spherical microresonators whose separation and size distribution can be controlled. Statistical studies of the lasing frequency reveal a frequency-controlled behavior. We perform transfer matrix calculations with gain to analyze the origin of the lasing modes, their thresholds, and frequency statistics. The results confirm that the experimentally observed lasing modes arise from states introduced into the stop gap of the underlying periodic system. On virtue of the fact that these high-quality gap states are restricted to a band of frequencies, the consequent random lasing exhibits significant reduction in frequency fluctuations.

Pump-controlled directional light emission from random lasers

The angular emission pattern of a random laser is typically very irregular and difficult to tune. Here we show by detailed numerical calculations that one can overcome the lack of control over this emission pattern by actively shaping the spatial pump distribution. We demonstrate, in particular, how to obtain customized pump profiles to achieve highly directional emission. Going beyond the regime of strongly scattering media where localized modes with a given directionality can simply be selected by the pump, we present an optimization-based approach which shapes extended lasing modes in the weakly scattering regime according to any predetermined emission pattern.

Taming random lasers through active spatial control of the pump

Active control of the spatial pump profile is proposed to exercise control over random laser emission. We demonstrate numerically the selection of any desired lasing mode from the emission spectrum. An iterative optimization method is employed, first in the regime of strong scattering where modes are spatially localized and can be easily selected using local pumping. Remarkably, this method works efficiently even in the weakly scattering regime, where strong spatial overlap of the modes precludes spatial selectivity. A complex optimized pump profile is found, which selects the desired lasing mode at the expense of others, thus demonstrating the potential of pump shaping for robust and controllable single mode operation of a random laser.