Random Lasers

One essential component of a laser is the cavity, which provides optical confinement and feedback for lasing oscillation. In a highly disordered medium, light experiences multiple scattering and undergoes a random walk. Surprisingly, lasing can occur in a random system without well-defined cavities. Such lasers are called random lasers, whose development was dated back to the early years of laser development. Over the past two decades there have been extensive experimental and theoretical studies on random lasers. I will review the random laser development and describe the lasing mechanism. In particular, I will discuss coherent lasing with resonant feedback in strongly scattering semiconductor power and polycrystalline films. The interference of multiply scattered light also provides a novel mechanism of three-dimensional optical confinement in disordered microlasers. Finally, I will describe practical applications that will benefit from the unique characteristics of random lasers. For example, a random laser can provide intense radiation with low spatial coherence, thus combining the high brightness of a laser with the low coherence of a LED. Full-field images taken under random laser illumination are free of speckle noise and coherent crosstalk.