Random lasersA laser is usually constructed from two basic elements: a material that provides optical gain via stimulated emission and an optical cavity that traps the light. When the total gain in the cavity is larger then the losses, the system reaches a threshold and lases. It is the cavity that determines the modes of a laser. Random lasers work on the same principles, but the modes are determined by multiple scattering and not by a laser cavity.
Random amplifying media can be realized in several ways. For example, one can dissolve laser dye in a particle suspension, powder a laser crystal, or powder a semiconductor like zinc oxide. The emission properties of such media show similarities with a regular laser. The system exhibits a threshold behaviour and when the gain overcomes the losses the emitted spectrum shows band narrowing and laser spiking. In addition, random laser materials can exhibit ultra-narrow emission modes either due to interference effects like Anderson localization that provide coherent feedback, or due to exponential gain in extended modes in systems with diffusive feedback. Even in the absence of interference effects, the photon statistics of a random laser can be Poissonian, indicating a high degree of coherence. Random lasers use multiple scattering of light as feedback mechanism to achieve gain that exceeds losses and thereby obtain lasing.
PDLC Random lasersPolymer dispersed liquid crystals (PDLCs) consists of submicron to micron-sized droplets of liquid crystal dispersed in a polymer matrix and is strongly scattering because of a refractive index mismatch between the polymer matrix and the liquid crystal. When an electric field is applied, the nematic director of the liquid crystal aligns along the field direction and the material appears transparent.
Examples recent papers on this subject
Resonance-driven random lasing - S. Gottardo, R. Sapienza, P.D. García, A. Blanco, D.S. Wiersma, C. López Nature Photonics, 2 429 (2008).
Cahotic behaviour of a random laser with static disorder - S. Mujumdar, V. Turk, R. Torre, D.S. Wiersma, Physical Review A, 76 033807 (2007).
Quasi-two-dimensional diffusive random laser action - S. Gottardo, S. Cavalieri, O. Yaroshchuk, D.S. Wiersma, Physical Review Letters, 93 263901 (2004).