Photoacoustic-guided convergence of light through optically diffusive media

Abstract

We demonstrate that laser beams can be converged toward a light-absorbing target through optically diffusive media by using photoacoustic-guided interferometric focusing. The convergence of light is achieved by shaping the wavefront of the incident light with a deformable mirror to maximize the photoacoustic signal, which is proportional to the scattered light intensity at the light absorber.

© 2011 Optical Society of America

Figures

Fig. 1

(Color online) Schematic of experimental system. DM, deformable mirror; UT, ultrasound transducer. The diffuser, when introduced, is placed at 2 mm in front of the graphite absorber.

Fig. 2

Profiles of laser beam as viewed through the carbon tape (a) before and (b) after being guided to the focus of the ultrasound transducer (marked by the arrows) by wavefront shaping. (c) Photo of graphite particle (marked by wide arrow) after being shifted to 10μm from the laser beam. (d) Refocused laser beam centered at the graphite particle after phase shaping. The narrow arrows mark the locations of the center of ultrasonic focus. The diameter of the ultrasound focus is 90 μm. (e) Time evolution of the amplitudes of the photoacoustic signal as the deformable mirrors undergo three iterations of phase shaping, each iteration scanning through 140 pixels. Also shown is the preshift signal level.

Fig. 3

(Color online) (a) Scattered laser beam after passing through paraffin films, (b) reconverged laser beam centered at the 10 μm graphite particle after wavefront shaping. The wide arrows mark the location of the graphite particle and the narrow arrows mark the location of the center of the ultrasound focus. (c) Scattered laser beam after passing through paraffin films with regions of overexposure in gray dot below dotted circle. (d) Reconverged laser beam centered at the 50 μm-diameter graphite particle after wavefront shaping. The dashed curves mark the contours of the graphite particle.