Gain,Laser Aléatoire et Désordre – GLAD

In conventional lasers with mirrors cavities, emission is stimulated into well-defined cavity modes and emerges as a coherent beam. Care is taken to suppress scattering within the cavity since this would shorten the photon residence time in the lasing mode and thereby raise the excitation power. In mirrorless random lasers, feedback derives from disorder-induced scattering. Multiple scattering impedes the flow of light out of the scattering amplifying medium and facilitates lasing action in colloidal suspensions in dye solutions or in semiconductor and rare earth powders excited by an external source. However, isotropic laser emission and high threshold hinder their applications. Efficient optical confinement is therefore necessary to reduce the number of modes in a random laser and thereby improve its spectral and spatial characteristics. This can be achieved in the presence of extreme scattering, where light can be localized within a small region of space, due to the complex interplay of partial waves interferences. Indeed, in this regime of Anderson localization, the electromagnetic modes are exponentially localized in space. If the medium is active, these localized modes are naturally selected by the gain. They act as the regular cavity modes of a conventional laser. but significant lowering of the lasing threshold is expected, due to the spatial confinement of the modes. The regime of strong localization is therefore favorable to efficient lasing action in mirrorless multiple scattering media. The originality and specificity of this project is to bring 'down to the clean room' a theoretical concept of fundamental physics. The aims are (1) to propose and to design a micro-laser which will demonstrate random lasing action with field feedback in the regime of strong localization, using a technological approach giving unprecedented control and reproducibility of the random geometry (2) to characterize this device and define the frontier of operation in the localized regime, (3) to explore the sensitivity of this device to local perturbation and to propose possible applications of this random laser based on these characteristics. The success of this project will rest on the synergy between fundamental competence and laser expertise. The two partners in this project, the Photonic Group at LAAS-CNRS in Toulouse and the Wave Propagation in Complex Media Group at LPMC in Nice have been preparing this project for a year in a PEPS (ST2I) program to learn the fundamental concepts, appreciate the technological challenges and constitute a task force. They will bring their respective expertise in microcavity semiconductor lasers and in modeling of light scattering and localization to design and develop such a micro- random laser. The LAAS-CNRS group has developed particularly innovative concepts of AlGaAs/ GaAs quantum well lasers. In this project, this group will design and characterize an optically pumped planar random laser, based on a randomly perforated hanging GaAs membrane, incorporating a GaInAs quantum well for an emission wavelength around 1µm. Optical pumping will be performed from the top surface to excite a selective part of the perforated surface, for a selective excitation of localized modes. The experience built up by the LPMC-CNRS group in multiple scattering and localization of waves, will serve to provide with the optimal parameters for a rapid demonstration of random lasing action and with physical intuitions to take advantage of the localized nature of the modes and their spectral sensitivity to local perturbation. As compared to other approaches, the close mapping between the 2D numerical model and the planar geometry will provide with a unique one-to-one comparison of the simulated and experimental systems. A close feedback between experiment and theory will help in improving the model. If successful, we will be able to numerically design random lasers with any desired performance and characteristics. Of the key points addressed by LAAS Photonic group activities, one of the most important is the understanding of semiconductor laser cavities concepts and developments. In that project, the Photonic group will gain in depth understanding of the working principles of mirrorless cavities defined by a disordered media. This will complete its already ongoing work on photonic crystal lasers, were cavities are defined by a well ordered media. The comparison of both approaches will help in finding potential applications and breakthrough of random lasers. For the LPMC group, this is a unique opportunity to test fundamental concepts in the real world and propose serious applications. It will learn from the expertise of the LAAS group where to contribute in the domain of laser physics, by identifying the specific potential of random lasers. In turn, this micro-laser will provide for a test device to explore further fundamental questions.