Modes of random lasers

Abstract

In conventional lasers, the optical cavity that confines the photons also determines essential characteristics of the lasing modes such as wavelength, emission pattern, directivity, and polarization. In random lasers, which do not have mirrors or a well-defined cavity, light is confined within the gain medium by means of multiple scattering. The sharp peaks in the emission spectra of semiconductor powders, first observed in 1999, has therefore lead to an intense debate about the nature of the lasing modes in these so-called lasers with resonant feedback. In this paper, we review numerical and theoretical studies aimed at clarifying the nature of the lasing modes in disordered scattering systems with gain. The last decade has witnessed the emergence of the idea that even the low-Q resonances of such open systems could play a role similar to the cavity modes of a conventional laser and produce sharp lasing peaks. We will focus here on the near-threshold single-mode lasing regime where nonlinear effects associated with gain saturation and mode competition can be neglected. We will discuss in particular the link between random laser modes near threshold and the resonances or quasi-bound (QB) states of the passive system without gain. For random lasers in the localized (strong scattering) regime, QB states and threshold lasing modes (TLM) were found to be nearly identical within the scattering medium. These studies were later extended to the case of more lossy systems such as random systems in the diffusive regime where it was observed that increasing the openness of such systems eventually resulted in measurable and increasing differences between quasi-bound states and lasing modes. Very recently, a theory able to treat lasers with arbitrarily complex and open cavities such as random lasers established that the threshold lasing modes are in fact distinct from QB states of the passive system and are better described in terms of a new class of states, the so-called constant-flux states. The correspondence between QB states and lasing modes is found to improve in the strong scattering limit, confirming the validity of initial work in the strong scattering limit.

Random lasing in Nd:LuVO_4 crystal powder

“…In this type of cavityless lasers, multiple light scattering replaces the standard optical cavity of a conventional laser, and the interplay between gain and scattering determines the lasing properties. A detailed discussion about the latest results and theories concerning the mechanisms responsible for random lasing and the precise nature of the random laser modes can be found in Refs [1,[4][5][6].…”

“…Recently Nd 3+ :LuVO 4 has attracted much attention since it was first reported by C. Maunier et al in 2002 [12]. It has the highest absorption and emission cross-sections among vanadates, 6 :LuVO 4 crystal has been recently confirmed by the demonstration of passively Qswitched laser-diode pumped nanosecond self-Raman laser operating at cascade downconverted frequency [13].…”

Microwave conductance in random waveguides in the cross-over to Anderson localization and single-parameter scaling

“…Such fluctuations make it impossible to predict transport in an individual disordered sample. However, fluctuations of the field within disordered samples may be exploited to sharpen focusing in random systems (18,19) and lower the threshold for lasing (20,21). A full account of transport in a random system would begin with the measurement of the probability distributions of conductance in ensembles of random samples with different physical dimensions in the cross-over to Anderson localization.…”

Non-locality and collective emission in disordered lasing resonators

“…11 RLs are among the most complex systems in photonics, encompassing structural disorder, nonlinearity, 12 strong nonlinear interaction 13 and different photon statistics 14 in systems ranging from micron-sized optical cavities 15 to kilometer-long fibers. 16 First-principles timedomain simulations show that the modes of a RL arise from localized electromagnetic states, [17][18][19] which may appear in a localized or an extended fashion. Several experiments have been reported on the nature of these modes, 15,[20][21][22] and these studies attempted to address the correlation between the structure and the degree of localization.…”