Time-reversed ultrasonically encoded optical focusing into scattering media

Abstract

Light focusing plays a central role in biomedical imaging, manipulation, and therapy. In scattering media, direct light focusing becomes infeasible beyond one transport mean free path. All previous methods1–3 to overcome this diffusion limit lack a practical internal “guide star.”4 Here we proposed and experimentally validated a novel concept, called Time-Reversed Ultrasonically Encoded (TRUE) optical focusing, to deliver light into any dynamically defined location inside a scattering medium. First, diffused coherent light is encoded by a focused ultrasonic wave to provide a virtual internal “guide star”; then, only the encoded light is time-reversed and transmitted back to the ultrasonic focus. The TRUE optical focus–defined by the ultrasonic wave–is unaffected by multiple scattering of light. Such focusing is especially desirable in biological tissue where ultrasonic scattering is ~1000 times weaker than optical scattering. Various fields including biomedical and colloidal optics can benefit from TRUE optical focusing.

Controlling waves in space and time for imaging and focusing in complex media

“…A method for focusing light at a chosen position in a turbid medium would therefore be of great value. An important step in this direction was made by Xu and co-workers 80 , who employed an acousto-optic technique. They illuminated a sample with light and ultrasound simultaneously.…”

One-Wave Optical Phase Conjugation Mirror by Actively Coupling Arbitrary Light Fields into a Single-Mode Reflector

“…The PCM conjugates the phase on the mirror surface, and causes the reflected light to rewind through the path that the input light has travelled. This "time-reversal" property of PCMs has been tried in various optical applications such as aberration canceling [2,3], pulse compression [4,5], laser resonators [6,7], holography [8], and the suppression of multiple light scattering in biological tissues [9][10][11][12][13][14]. More direct demonstrations have also been performed using acoustic waves [15] and microwaves [16], where conventional electronics can directly measure and generate the phase profile in real time.…”

Effects of digital phase-conjugate light intensity on time-reversal imaging through animal tissue

“…Therefore, some technique to suppress scattering effects is necessary for practical applications of light in these fields. In recent years, a principle of scattering suppression using the time-reverse capability of phaseconjugate light has been proposed [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Various attempts based on this principle have been undertaken, such as the time-reversed ultrasonically encoded (TRUE) method [1,[4][5][6][7][8].…”