Amplitude-masked photoacoustic wavefront shaping and application in flowmetry

Optical-resolution photoacoustic flowmetry (PAF) allows noninvasive single-cell flow measurements. However, its operational depth is limited by optical diffusion, which prevents focusing beyond shallow depths in scattering media, as well as reducing the measurement signal-to-noise ratio (SNR). To overcome this limitation, we used binary-amplitude wavefront shaping to enhance light focusing in the presence of scattering. Here, the transmission modes that contributed constructively to the intensity at the optical focus were identified and selectively illuminated, resulting in a 14-fold intensity increase and a corresponding increase in SNR. This technique can potentially extend the operational depth of optical-resolution PAF beyond 1 mm in tissue

Hybridized wavefront shaping for high-speed, high-efficiency focusing through dynamic diffusive media

One of the prime limiting factors of optical imaging in biological applications is the diffusion of light by tissue, which prevents focusing at depths greater than the optical diffusion limit (typically ∼1  mm). To overcome this challenge, wavefront shaping techniques that use a spatial light modulator (SLM) to correct the phase of the incident wavefront have recently been developed. These techniques are able to focus light through scattering media beyond the optical diffusion limit. However, the low speeds of typically used liquid crystal SLMs limit the focusing speed. Here, we present a method using a digital micromirror device (DMD) and an electro-optic modulator (EOM) to measure the scattering-induced aberrations, and using a liquid crystal SLM to apply the correction to the illuminating wavefront. By combining phase modulation from an EOM with the DMD’s ability to provide selective illumination, we exploit the DMD’s higher refresh rate for phase measurement. We achieved focusing through scattering media in less than 8 ms, which is sufficiently short for certain in vivo applications, as it is comparable to the speckle correlation time of living tissue

Digital reflection-mode time-reversed ultrasonically encoded (TRUE) optical focusing

To achieve localized light delivery beyond turbid layers, TRUE optical focusing has been previously implemented by both analog and digital devices. The digital scheme offers a higher energy gain than the analog version. In many biological applications, the reflection-mode configuration, which uses backscattered light from the sample, is more suitable than the transmission-mode configuration. Although reflection-mode analog TRUE focusing has been demonstrated, its digital implementation has not been explored. Here, we report a reflection-mode digital TRUE focusing to concentrate light through a turbid layer. Further, by simply moving the ultrasound focus, we show the system's dynamic focusing capability

Continuous scanning of a time-reversed ultrasonically encoded optical focus by reflection-mode digital phase conjugation

Time-reversed ultrasonically encoded (TRUE) optical focusing in turbid media was previously implemented using both analog and digital phase conjugation. The digital approach, in addition to its large energy gain, can improve the focal intensity and resolution by iterative focusing. However, performing iterative focusing at each focal position can be time-consuming. Here, we show that by gradually moving the focal position, the TRUE focal intensity is improved, as in iterative focusing at a fixed position, and can be continuously scanned to image fluorescent targets in a shorter time. In addition, our setup is, to the best of our knowledge, the first demonstration of TRUE focusing using a digital phase conjugate mirror in a reflection mode, which is more suitable for practical applications

Photoacoustically guided wavefront shaping for enhanced optical focusing in scattering media

Non-invasively focusing light into strongly scattering media, such as biological tissue, is highly desirable but challenging. Recently, ultrasonically guided wavefront-shaping technologies have been developed to address this limitation. So far, the focusing resolution of most implementations has been limited by acoustic diffraction. Here, we introduce nonlinear photoacoustically guided wavefront shaping (PAWS), which achieves optical diffraction-limited focusing in scattering media. We develop an efficient dual-pulse excitation approach to generate strong nonlinear photoacoustic signals based on the Grueneisen relaxation effect. These nonlinear photoacoustic signals are used as feedback to guide iterative wavefront optimization. As a result, light is effectively focused to a single optical speckle grain on the scale of 5–7 μm, which is ∼10 times smaller than the acoustic focus, with an enhancement factor of ∼6,000 in peak fluence. This technology has the potential to benefit many applications that require a highly confined strong optical focus in tissue

Ultrasonically encoded wavefront shaping for focusing into random media

Phase distortions due to scattering in random media restrict optical focusing beyond one transport mean free path. However, scattering can be compensated for by applying a correction to the illumination wavefront using spatial light modulators. One method of obtaining the wavefront correction is by iterative determination using an optimization algorithm. In the past, obtaining a feedback signal required either direct optical access to the target region, or invasive embedding of molecular probes within the random media. Here, we propose using ultrasonically encoded light as feedback to guide the optimization dynamically and non-invasively. In our proof-of-principle demonstration, diffuse light was refocused to the ultrasound focal zone, with a focus-to-background ratio of more than one order of magnitude after 600 iterations. With further improvements, especially in optimization speed, the proposed method should find broad applications in deep tissue optical imaging and therapy