Measurement of the anomalous phase velocity of ballistic light in a randommedium by use of a novel interferometer

Ballistic light, i.e., radiation that propagates undeflected through a turb id medium, undergoes a small change in phase velocity and exhibits unusual dispersion because of its wave nature. We use a novel highly sensitive diff erential phase optical interferometer to study these previously unmeasurabl e phenomena. We find that ballistic propagation can be classified into thre e regimes based on the wavelength-to-size ratio. In the regime in which the scatterer size is comparable with the wavelength, there is an anomalous ph ase-velocity increase as a result of adding scatterers of higher refractive index. We also observe an anomaly in the relative phase velocity, where re d light is slowed more than blue light even though the added scatterers are made of material with normal dispersion. (C) 2001 Optical Society of Ameri ca OCIS codes: 000.2690, 030.1640, 120.3180, 120.5050.