Measurement of the anomalous phase velocity of ballistic light in a random medium by use of a novel interferometer.

Ballistic light, i.e., radiation that propagates undeflected through a turbid medium, undergoes a small change in phase velocity and exhibits unusual dispersion because of its wave nature. We use a novel highly sensitive differential phase optical interferometer to study these previously unmeasurable phenomena. We find that ballistic propagation can be classified into three 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 phase-velocity increase as a result of adding scatterers of higher refractive index. We also observe an anomaly in the relative phase velocity, where red light is slowed more than blue light even though the added scatterers are made of material with normal dispersion.