In biomedical imaging applications, optical scattering disrupts the predictability of the light path, challenging the achievement of high-resolution optical imaging in deep tissue. Even state-of-the-art microscopy is limited to operating at roughly one millimeter in depth using visible light. Overcoming the scattering effect for deep tissue imaging remains a significant challenge. Wavefront shaping methods present a promising solution, allowing researchers to achieve high-resolution imaging through scattering media. By modulating the phase of incident light and compensating for wavefront distortion due to scattering, these methods effectively refocus scattered light, enabling high-resolution imaging in deep tissue.

Wavefront shaping methods can be categorized into three types: feedback-based, transmission matrix-based, and optical phase conjugation-based. These methods differ in system complexity and time effectiveness for obtaining the phase map. Feedback-based wavefront shaping, the first successful method for focusing light through scattering media, has a simple setup and low algorithm complexity. Research in this area has focused on improving optimization algorithms to find the optimal phase distribution, enhancing robustness and convergence speed. Transmission matrix-based wavefront shaping models light propagation in a scattering medium by using a linear transmission matrix, enabling wide-field imaging of hidden objects after obtaining the optical transmission matrix. Neural networks excel in manipulating nonlinear scattering and assist in wavefront shaping, particularly in scenarios involving multimode gain fibers and strongly absorbing tissue. Optical phase conjugation-based wavefront shaping is the most efficient method, requiring only a one-time measurement to determine a row vector of the transmission matrix. It efficiently acquires and controls information about the scattered light field, demonstrating advantages in dynamic scattering processes. In applications such as imaging living tissue, where optical scattering is dynamic on a millisecond to microsecond timescale due to physiological processes, optical phase conjugation-based wavefront shaping stands out as a promising method for high-resolution imaging.

To achieve internal focusing and imaging within scattering media, wavefront shaping methods need to be combined with guiding stars. Guiding stars within the scattering medium result from local interactions with scattered light, causing observable changes in intensity, phase, and frequency. Wavefront shaping locates guiding stars by perceiving these changes, guiding scattered light to achieve focus at the guiding star's location. Focused ultrasound serves as a “virtual guiding star”, freely adjustable within biological tissue. This allows researchers, under the influence of optical scattering, to achieve a bright optical focus guided by focused ultrasound. Scanning this focus and measuring the intensity of frequency-shifted light enable the reconstruction of absorption distribution images of objects within the scattering medium.

In summary, wavefront shaping methods offer new possibilities for achieving high-resolution imaging through scattering media. By modulating the incident light phase and compensating for wavefront distortion, these methods efficiently refocus scattered light for high-resolution imaging in deep tissue. Combining wavefront shaping with guiding stars holds promise for internal focusing and imaging within scattering media, particularly in biomedical imaging applications.

For the first time, a research group at the University of Twente introduced the wavefront shaping method (Fig. 2). They utilized feedback-based wavefront shaping to refocus scattered light by adjusting the target function, successfully achieving simultaneous focusing of scattered light at multiple target positions (Fig. 4). Another research group then employed a coaxial interference method to obtain the optical transmission matrix of scattering media. With the acquired transmission matrix, they could focus light to arbitrary positions on the output plane (Fig. 7). To mitigate coherent noise caused by the external reference beam, direct intensity detection approaches for retrieving the transmission matrix were proposed and validated. The optical phase conjugation-based wavefront shaping method stood out as the most efficient approach for focusing light through scattering media, relying on the time-reversal symmetry of the optical wave propagation equation.

In 2008, researchers used a phase conjugate mirror based on photorefractive crystals to focus scattered light through biological tissue. Although phase conjugate mirrors based on photorefractive crystals have advantages in processing speed and controlling mode count, their limited efficiency in generating conjugate wavefronts restricts their applicability. Consequently, the combination of high-performance cameras and spatial light modulators (SLMs) has gradually become the mainstream solution (Fig. 10). Digital optical phase conjugate mirrors have an advantage in modulation efficiency and wavelength insensitivity, establishing their dominance in optical imaging, control, and therapeutic applications.

In 2011, ultrasonic guided stars were first proposed and applied in optical phase conjugation-based wavefront shaping, successfully achieving optical focusing and imaging within scattering media. Two independent research groups in the United States demonstrated focusing light deep inside scattering media with ultrasonic guided stars (Fig. 12). In summary, the development of small-sized, high-contrast, non-invasive, and easily controllable guiding stars has become a current focus in wavefront shaping research.

Wavefront shaping offers an effective approach for comprehensive exploration and precise control of scattered light. This capability allows for the alleviation of information disorder caused by scattering, facilitating high-resolution optical imaging within scattering media. This review delves into the historical development of wavefront shaping, discusses various wavefront shaping methods, highlights their applications in overcoming optical scattering for deep tissue imaging, and provides insights into future trends in the advancement of wavefront shaping techniques.