Development of high-resolution deep tissue optical focusing that combines $2^nd$-order acousto-optic signal measurement and optical phase conjugation

Abstract

Noninvasive light focusing inside the scattering media has been developed such as time-reversed ultrasonically encoded (TRUE) optical focusing, which uses ultrasound as an guide-star. Acousto-optic imaging (AO) is a promising technique that is able to tag the photon in a localized manner, however, the spatial resolution of the technique is limited to the diffraction limit of the ultrasound ($~30 \mu m$ for $50 MHz$). In Raman-Nath theory, it was demonstrated that the nonlinear response of the AO modulation efficiency to the ultrasound pressure occurs, resulting in a reduction of the tagged zone size. We demonstrated that the combined approach of TRUE focusing and nonlinear response can reduce the focal spot size by about 30% compared to that of the conventional TRUE light focus.