Localized fluorescence excitation in opaque media by time-reversed ultrasonically encoded (TRUE) optical focusing

To focus light beyond one transport mean free path, time-reversed ultrasonically encoded (TRUE) optical focusing has previously been implemented by both analog and digital devices. By allowing wavefront recording with finer resolution and larger aperture, the analog scheme, which uses photorefractive materials as the phase-conjugate mirror, generates a more complete set of time-reversed optical modes than the digital scheme. Here, we report the direct visualization of localized fluorescence excitation inside a turbid medium by photorefractive time reversal. Further, we imaged fluorescent targets embedded in a turbid phantom whose thickness was four transport mean free paths.