Focusing light into scattering media with ultrasound-induced field perturbation

Abstract

Focusing light into scattering media, although challenging, is highly desirable in many realms. With the invention of time-reversed ultrasonically encoded (TRUE) optical focusing, acousto-optic modulation was demonstrated as a promising guidestar mechanism for achieving noninvasive and addressable optical focusing into scattering media. Here, we report a new ultrasound-assisted technique, ultrasound-induced field perturbation optical focusing, abbreviated as UFP. Unlike in conventional TRUE optical focusing, where only the weak frequency-shifted first-order diffracted photons due to acousto-optic modulation are useful, here UFP leverages the brighter zeroth-order photons diffracted by an ultrasonic guidestar as information carriers to guide optical focusing. We find that the zeroth-order diffracted photons, although not frequency-shifted, do have a field perturbation caused by the existence of the ultrasonic guidestar. By detecting and time-reversing the differential field of the frequency-unshifted photons when the ultrasound is alternately ON and OFF, we can focus light to the position where the field perturbation occurs inside the scattering medium. We demonstrate here that UFP optical focusing has superior performance to conventional TRUE optical focusing, which benefits from the more intense zeroth-order photons. We further show that UFP optical focusing can be easily and flexibly developed into double-shot realization or even single-shot realization, which is desirable for high-speed wavefront shaping. This new method upsets conventional thinking on the utility of an ultrasonic guidestar and broadens the horizon of light control in scattering media. We hope that it provides a more efficient and flexible mechanism for implementing ultrasound-guided wavefront shaping.

Fig. 1. Principle of UFP optical focusing.

In UFP optical focusing, we detect only the frequency-unshifted scattered photons because their higher energy dominates over that of the extremely weak frequency-shifted photons by an ultrasound field. a The scattered field through the scattering medium is directly measured via interference with a reference beam while the ultrasound is OFF. b Repeat the same measurement as in a while the ultrasound is ON. c The signals measured in a and b are different because of the ultrasound-induced field perturbation. Playing back the phase-conjugated differential field via a DOPC system generates a time-reversed beam that converges to the location where the perturbation occurs

Fig. 2. Conceptual simulations of UFP optical focusing into scattering media.

a Schematic of the simulation model. b1 Amplitude time sequence of the optical field at a given pixel of the observation plane when the ultrasound field is ON. b2, Fourier transform amplitude of the time sequence in b1. c1–c2 Amplitude and phase maps, respectively, of the frequency-unshifted optical field in the observation plane when the ultrasound is ON. d1–d2 Amplitude and phase maps, respectively, of the complex differential field between the frequency-unshifted photons when the ultrasound is on the ON and OFF states. The amplitude map is normalized here. e1 A 3D representation of the time-reversed optical intensity in the scattering medium, using UFP optical focusing. The intensity is normalized. e2 Time-reversed optical focus inside the scattering medium. Inset, the zoomed-in view of the focus. f1 A 3D representation of the optical intensity in the scattering medium when playing back a random optical wavefront. The intensity is normalized to the maximum intensity of the time-reversed optical focus in e1. f2 Only speckles can be seen at the focal position of the ultrasound when playing back a random optical wavefront

Fig. 3. Schematic of the experimental setup.

AOM acousto-optic modulator, BS beam splitter (nonpolarizing), HWP half-wave plate, L lens, M mirror, PBS polarizing beam splitter, SLM spatial light modulator, SM scattering medium, UT ultrasound transducer

Fig. 4. UFP optical focusing into scattering media via a full-phase modulation scheme.

a Phase map displayed on the SLM, based on the differential signal between the measured complex fields when the ultrasound was OFF and ON. b Time-reversed focus from UFP optical focusing. c No optical focus was observed in a control experiment conducted without launching the ultrasound. d Time-reversed focus from conventional TRUE optical focusing under the same condition. e Line profiles of the central rows in b–d. Scale bars: 1 mm

Fig. 5. Double-shot UFP optical focusing into scattering media.

a Time sequence for double-shot UFP optical focusing. b Phase map generated from the two recorded hologram frames. c Time-reversed focus from double-shot UFP optical focusing. d In a control experiment, no focus was observed on the observation camera in a double-shot measurement without ultrasound. e Line profiles of the central rows in c and d. Scale bar: 1 mm

Fig. 6. Single-shot UFP optical focusing into scattering media.

a Time sequence for the single-shot realization of UFP optical focusing. b In general interferometers, if a light beam with a spherical wavefront interferes with a plane reference beam, a typical interferogram with many circular fringes will be seen. c In our system, because the phases of the reference beam were reversed during the first and the second halves of the camera exposure period, the interference terms canceled out completely, resulting in no interference fringes. d Phase map generated from the recorded single hologram. e Time-reversed focus from single-shot UFP optical focusing. f A control experiment without launching the ultrasound, yielding no focus. g Line profiles of the central rows in e and f. Scale bar: 1 mm