Direct determination of the transition to localization of light in three dimensions

Diffusive wave transport in three-dimensional media should show a phase transition, with increasing disorder, to a state without transport. This transition was first discussed by Anderson1 in the context of the metal–insulator transition, but is generic for all waves, as was realized later2,3. However, the quest for the experimental demonstration of ‘Anderson’ localization in three dimensions has been a challenging task. For electrons4 and cold atoms5,6, the challenge lies in the possibility of bound states in a disordered potential. Therefore, electromagnetic and acoustic waves have been the prime candidates for the observation of Anderson localization7,8,9,10,11,12,13,14,15,16,17. The main challenge in using light lies in the distinction between the effects of absorption and localization11,12. Here, we present measurements of the time dependence of the transverse width of the transmitted-light intensity distribution, which provides a direct measure of the localization length, independent of absorption. This provides direct evidence for a localization transition in three dimensions. Experimental demonstration of Anderson localization in three dimensions is a challenging task. Here researchers present a direct and absorption-independent measure of the localization length and evidence for a localization transition in three dimensions.