Suppression of interactions in multimode random lasers in the Anderson localized regime

Abstract

Understanding random lasing is a formidable theoretical challenge. Unlike conventional lasers, random lasers have no resonator to trap light, they are highly multimode with potentially strong modal interactions and they are based on disordered gain media, where photons undergo random multiple scattering [1][2][3]. Interference effects notoriously modify the propagation of waves in such random media, but their fate in the presence of nonlinearity and interactions is poorly understood.Here, we present a semiclassical theory for multimode random lasing in the strongly scattering regime. We show that Anderson localization [4, 5], a wave-interference effect, is not affected by the presence of nonlinearities. To the contrary, its presence suppresses interactions between simultaneously lasing modes. Using a recently constructed theory for complex multimode lasers [6], we show analytically how Anderson localization justifies a noninteracting, single-pole approximation.Consequently, lasing modes in a strongly scattering random laser are given by long-lived, Anderson localized modes of the passive cavity, whose frequency and wave profile does not vary with pumping, even in the multi-mode regime when mode overlap spatially.

Hybridized/coupled multiple resonances in nacre

“…Indeed, such dynamical spectral evolution of mode competition/suppression as a function of the excitation energy has been studied mainly in theoretical studies, 45,46 due to the inefficient nature of highdimensional disordered resonators.…”

“…This unique spectral characteristic of multimode lasing is attributable to high competition and saturation of gain. 45,46,48,49 As nacre has low dimensionality and inherently forms hybridized localized states, which are forced to share the same gain spatially, the local depletion of excited electrons from the lasing modes suppresses other spatially overlapped modes. The importance of hole burning in this characteristic is supported by the fact that Δω from the nacre resonators is comparable to the homogenous linewidths of similar dye molecules measured at room temperature.…”

Random nanolasing in the Anderson localized regime

“…In our case, the very narrow lasing linewidths indicate the absence of chaotic operation. This linear non-interacting behaviour of lasing modes has been proposed as a tell-tale signal of Anderson localization in random lasing 16 . In diffusive random lasers the lasing wavelength is primarily determined by the spectral peak of the gain medium 7 , while only weak dispersion is incorporated into the multiple-scattering medium, enabling modest tunability of the random laser 10,26 .…”

“…1g). A recent theoretical study predicted the strong suppression of interactions in multimode random lasing in the Anderson localization regime 16 . As shown in Fig.…”

“…1h, very stable lasing wavelengths are indeed observed, even for spectrally close lasing modes. This is attributed to the fact that the lasing modes overlap very weakly due to their (on average) exponential confinement 16 . In diffusive random lasers, in contrast, the extended lasing modes imply that different modes overlap spatially, inducing strong mode competition and chaotic lasing emission and therefore spectrally broad averaged emission 14 .…”