Probing localized states with spectrally resolved speckle techniques

We have studied the spatial extent of localized states in random media using speckle correlation techniques. Optical gain is introduced to a local region of a random medium to induce lasing of the localized states. The far-field speckle pattern of laser emission from a localized state gives its spatial field correlation function at the surface of the random medium. The envelope of the spatial field correlation function decays exponentially with the transverse coordinate on the surface. The decay length is independent of the pumping rate and the excitation area. We demonstrate that localized states exist in a disordered system that does not reach the localization threshold.