Abstract

Traditional lasers are composed by three basic elements: an amplifying medium, an external pumping setup, and an optical cavity that confines and shapes the emitted light in well-determined modes and directions. However, several modern approaches are extending this traditional laser paradigm into new avenues. Cavity-free stimulated emission of radiation has been widely studied in random lasers (RLs) [1], where the optical cavity modes of traditional lasers are replaced with multiple scattering in disordered media, while the interplay between gain and scattering determines the lasing properties. In spite of their striking potential applications, RLs lack external tunability, reproducibility, and control over the spatial pattern of the output beam. Overcoming these limitations is central for the development and application of cost-effective cavity-free lasers. Inspired by the aforementioned challenges, here we investigate the optical properties of randomly-oriented undoped graphene flakes embedded in externally pumped amplifying media. We demonstrate a novel mechanism leading to stable and tunable single-mode cavity-free lasing characterized by a well-determined and highly coherent spatial pattern [2]. We find that the transverse size of the localized output beam, ranging from a few to several hundreds microns, can be accurately manipulated through the external pumping and through the volume density of graphene flakes. This cavity-free lasing mechanism profoundly relies on the extraordinary optical properties of graphene, and particularly on its highly-saturated absorption at rather modest light intensities, a remarkable property which enables self-organization of light into a well determined spatial mode profile.

© 2017 IEEE