Figure 1.

Schematic of the experimental setup for TRUE optical focusing. EOM, electro-optic modulator; PBS_1-3, polarizing beam splitter; S_1-3, shutter; HWP_1-3, half-wave plate; M_1-3, mirror; AOM_1-2, acousto-optic modulator; L_1-3, lens; PD_1,2, photodiode; BE_1,2, beam expander; R, reference beam; R*, conjugate reference beam; WT, water tank filled with deionized water; UT, ultrasonic transducer; S, collected diffused signal light through the sample; S*, time-reversed signal light; PRC, photorefractive crystal [a 10 × 10 × 5 mm³ Bi₁₂SiO₂₀ (BSO) crystal was used in the study]; HV ac, high voltage ac electrical field; xyz, system coordinates (x = sample scanning axis, y = acoustic axis, and z = axis of incident signal beam propagation).