Figure 1

(a–d) Schematic illustration of the iterative focus improvement.(a) The initial incident light field (purple) propagates to the ultrasound focus (yellow circle). A simulated speckle pattern at the sound focus (location marked with the white arrows) is shown in the right inset. A small portion of the input light is frequency shifted (green). (b) In the first DOPC iteration, the green light field is time-reversed and is re-focused into the ultrasound focus, resulting in a more confined sound light interaction (right inset). A portion of the light is frequency shifted (purple). (c) The purple light field is time reversed and is re-focused into the ultrasound zone, further shrinking the sound-light interaction zone. (d) After nine iterations, the time-reversed purple light field results in a much improved focus. (e) Experimental setup; PO, Pockels cell; I, Isolator; BS, beam splitter; AOM, acousto-optical modulator; ND, neutral density filter, BB, beam block; DL, delay line; BE, beam expander; P, polarizer; BP, bandpass filter; L1, f = 35 mm lens; L2, f = 50 mm lens; D, fluorescence detector. Scalebar: 10 microns.