Limiting the incident NA for efficient wavefront shaping through thin anisotropic scattering media

Optical scattering is a fundamental problem in biomedical optics and limits most optical techniques to shallow operating depths less than 1 millimeter. However, although the scattering behavior of tissue scrambles the information it contains, it does not destroy it. Therefore, if you can unscramble the scattered light, it increases the accessible imaging depths up the absorption limit of light (several centimeters deep). One such way to beat optical scattering is using wavefront shaping. Borrowing ideas from adaptive optics in astronomy and phased arrays in radar and ultrasonic imaging, the basic concept of wavefront shaping is to control the phase and amplitude of the light field in order to harness scattered light. Using wavefront shaping techniques, scattered light can be used to form focal spots or transmit information through or inside optically scattering media. Furthermore, even without correcting for scattering directly by shaping the input light field, the properties of the scattered light can be analyzed to recover information about the structure and dynamic properties of a sample using methods from diffuse optics. The main contributions of this thesis are along these two lines of research: moving wavefront shaping toward more practical applications and developing new techniques to recover useful physiological information from scattered light. This is developed through three main projects: (1) an investigation of how dynamic samples impact the scattering process and the practical implications of these dynamics on wavefront shaping systems, (2) the development of a wavefront shaping system combining light and ultrasound to focus light inside acute brain slices to improve light delivery for optogenetics, (3) a novel method to sensitively detect the dynamics of scattered light and use it to tease out information about the flow of blood within the tissue sample of interest.

Optical imaging through highly scattering media such as biological tissue is limited by light scat tering. Recently, it has been shown that wavefront shaping is a powerful tool to overcome this problem. In this work, wavefront shaping using spatial light modulators is used to compensate static scattering media (piece of translucent tape) to allow focu sing of different intensity distributions. Light propagation is engineered into a specific region of interest. For this purpose, a sequential phase shape algorithm was implemented experimentally. This algorithm is used to encode a phase distribution on an incident beam to pre -compensate phase distortions acquired by the beam after propagating through the tape. The sequential algorithm combined with a spatial light modulator is used to synthesize a phase distribution required for re -directing light using wav efront shaping. The scattered light was re -directed at the detector plane, in order to be: i) focused at a single pixel, ii) at squared regions of 3x3 and 5x5 pixeles and iii ) a line pattern of 41 pixels of the camera. Furthermore, the region of interest was placed outside the central area of the camera opening the possibility of image formation. Keywords: Focusing light, controlling light propagation, scattering media, wavefront shaping, SLM

Photoacoustic guided wavefront shaping using digital micromirror devices

Wavefront shaping is becoming increasingly attractive as it promises to enable various biomedical applications by breaking through the optical diffusion limit that prevents light focusing at depths larger than in biological tissue. However, despite recent advancements in wavefront shaping technology, such as those exploiting non-invasive photoacoustic-guidance, in vivo demonstrations remain challenging mainly due to rapid tissue speckle decorrelation. In this work, we report a high-speed photoacoustic-guided wavefront shaping method with a relatively simple experimental setup, based on the characterization of a scattering medium with a real-valued intensity transmission matrix. We demonstrated light focusing through an optical diffuser by optimizing 4096 binary amplitude modulation modes of a digital micromirror device within , leading to a system runtime of 75 µs per input mode, which is 3 orders of magnitude smaller than the smallest runtime reported in literature so far using photoacoustic-guided wavefront shaping. Thus, our method is a solid step forward toward in vivo applications of wavefront shaping.

Controllable light delivery to the region of interest is essential to biomedical optical imaging methods like photoacoustic microscopy. It is, however, challenging beyond superficial depths in biological tissue (~1 mm beneath human skin) due to the strong scattering of light that scrambles the photon propagation paths. Recently, optical wavefront shaping has been proposed to modulate the incident light wavefront to compensate for the scattering-induced phase distortions, and consequentially, convey light optimally to a desired location behind or inside turbid media. To reach an optimum wavefront, a searching algorithm is usually required to optimize a feedback signal. In this work, we present our latest explorations, which use photoacoustic signals as the feedback to remotely and non-invasively guide the wavefront shaping process. Our method does not require direct optical access to the target region or the invasive embedding of fluorescence probes inside turbid media. Experimentally, we have demonstrated that diffuse light can be converged to the ultrasound focus by maximizing the amplitude of photoacoustic emissions from the intended absorbing site. Moreover, we show that wavefront-shaped light focusing can enhance existing optical imaging modalities like photoacoustic microscopy, in regard to signal-to-noise ratio, imaging depth, and potentially, resolution.

In biological applications, optical focusing is limited by the diffusion of light, which prevents focusing at depths greater than ∼1 mm in soft tissue. Wavefront shaping extends the depth by compensating for phase distortions induced by scattering and thus allows for focusing light through biological tissue beyond the optical diffusion limit by using constructive interference. However, due to physiological motion, light scattering in tissue is deterministic only within a brief speckle correlation time. In in vivo tissue, this speckle correlation time is on the order of milliseconds, and so the wavefront must be optimized within this brief period. The speed of digital wavefront shaping has typically been limited by the relatively long time required to measure and display the optimal phase pattern. This limitation stems from the low speeds of cameras, data transfer and processing, and spatial light modulators. While binary-phase modulation requiring only two images for the phase measurement has recently been reported, most techniques require at least three frames for the full-phase measurement. Here, we present a full-phase digital optical phase conjugation method based on off-axis holography for single-shot optical focusing through scattering media. By using off-axis holography in conjunction with graphics processing unit based processing, we take advantage of the single-shot full-phase measurement while using parallel computation to quickly reconstruct the phase map. With this system, we can focus light through scattering media with a system latency of approximately 9 ms, on the order of the in vivo speckle correlation time.

A new class of planar optical components and devices has emerged using subwavelength metastructures with a strong contrast in refractive indices. High-contrast metastructures have shown promises to manipulate optical fields in an extraordinary way and to replace conventional bulky optical elements by their low-profile analogs, typically with subwavelength-scale features. We elucidate the underlying principle, how these seemingly low-profile geometries render unique optical responses, using the coupled-mode analysis in a multimode waveguide. Moreover, strong field localization in high-index structures allows us to interpret each single element in the metastructures as a low-quality-factor resonator (or a localized scatterer), permitting us to realize designer surface that shapes phase, amplitude, and polarization of light in free space, also known as an optical metasurface. The remainder of the thesis is devoted to explore novel applications in optics using high-contrast metastructures. One of the particularly interesting applications is to use them in an optical resonator. Specifically, we demonstrate to incorporate high-contrast subwavelength grating reflectors and dielectric metasufaces in a vertical Fabry–Perot cavity, and show that we can flexibly tune the resonance frequency by the subwavelength patterning. With this technique, we envision the realization of compact, on-chip spectrometers when integrating them on a photodetector array. Secondly, we investigate the use of high-contrast subwavelength gratings in visible wavelengths. We perform the optimization of their geometries and demonstrate a set of RGB color filters, down to near a micrometer in the pixel size. This platform exhibits unique performances such as high efficiency, angular insensitivity, and color tunability by the design. A novel device concept is also explored, where a high-contrast subwavelength grating reflector is integrated on a silicon platform to constitute an active resonant antenna, enabling high-speed, phase-dominant modulation by means of thermo-optic effect of silicon. We demonstrate an array of such active antennas, yielding a beam deflection capability. This justifies the robustness of our device design, enabling a large-scale integration of high-speed, phase-dominant spatial light modulators. Finally, we introduce a disorder-engineered metasurface in the context of wavefront shaping. Recently, wavefront shaping with disordered media has demonstrated optical manipulation capabilities beyond those of conventional optics, but translating this class of technology into a practical use has remained challenging due to enormous amounts of information needed to be characterized as the input-output responses. As a paradigm shift, we propose the use of disorder-engineered metasurface in wavefront shaping, where the disorder is programmatically designed and makes the system characterization-free prior to use. With this approach, we demonstrate high numerical aperture focusing in an extended volume as well as wide-field fluorescence imaging with unprecedented performances.

Controlling light propagation intentionally through turbid media such as ground glass or biological tissue has been demonstrated for many useful applications. Due to random scattering effect, one of the important goals is to draw a desired shape behind turbid media with a swift and precise method. Feedback wavefront shaping method which is known as a very effective approach to focus the light, is restricted by slow optimization process for obtaining multiple spots. Here we propose a technique to implement feedback wavefront shaping with optical memory effect and optical 4f system to speedy move focus spot and form shapes in 3D space behind scattering media. Starting with only one optimization process to achieve a focusing spot, the advantages of the optical configuration and full digital control allow us to move the focus spot with high quality at the speed of SLM frame rate. Multiple focusing spots can be achieved simultaneously by combining multiple phase patterns on a single SLM. By inheriting the phase patterns in the initial focusing process, we can enhance the intensity of the focusing spot at the edge of memory effect in with 50% reduction in optimization time. With a new focusing spot, we have two partially overlapped memory effect regions, expanding our 3D scanning range. With fast wavefront shaping devices, our proposed technique could potentially find appealing applications with biological tissues.

Diffraction-limited optical imaging through scattering media has the potential to transform many applications such as airborne and space-based imaging (through the atmosphere), bioimaging (through skin and human tissue), and fiber-based imaging (through fiber bundles). Existing wavefront shaping methods can image through scattering media and other obscurants by optically correcting wavefront aberrations using high-resolution spatial light modulators-but these methods generally require (i) guidestars, (ii) controlled illumination, (iii) point scanning, and/or (iv) statics scenes and aberrations. We propose neural wavefront shaping (NeuWS), a scanning-free wavefront shaping technique that integrates maximum likelihood estimation, measurement modulation, and neural signal representations to reconstruct diffraction-limited images through strong static and dynamic scattering media without guidestars, sparse targets, controlled illumination, nor specialized image sensors. We experimentally demonstrate guidestar-free, wide field-of-view, high-resolution, diffraction-limited imaging of extended, nonsparse, and static/dynamic scenes captured through static/dynamic aberrations.

When light propagates through complex medium, such as biological tissue and multimode fiber, refractive index inhomogeneity causes multiple scattering and distortion. This phenomenon is usually seen as obstacles for biomedical imaging, telecommunications, photodynamic therapy and so on. Thus, manipulation of the incident wavefront to compensate for the wavefront distortion due to multiple scattering has been an interdisciplinary subject of interest. Fortunately, wavefront shaping technologies have emerged to provide versatile solutions to minimize the influence of light scattering. By modulating the incident light into a special wavefront with a spatial light modulator, focusing through scattering medium is obtained. To date, several wavefront shaping techniques have been proposed, mainly including transmission matrix inversion, feedback based iterative optimization, and digital optical phase conjugation. Unlike a planar wavefront, the modulated light with special wavefront is transformed into a bright optical focus spot or a desired focus pattern after the scattering medium. Among the proposed approaches, the transmission matrix is considered as a significant tool to characterize a multiple scattering medium with the purpose of manipulating light propagation through it, which contains all the information related to the input field and the scattered output field. In this work, we experimentally measure the transmission matrix of scattering media based on self-reference interference method with a digital micromirror device. Unlike the conventional setup, which divides the incident wavefront into a signal part and reference part, in the self-reference interference method, the reference light is superimposed directly on the signal light to form a new set of input light fields. This self-reference interference method effectively improves the degree of freedom of optical field modulation. Moreover, the intensity ratio between the signal light and the reference light can be adjusted conveniently. In our experiment, this superimposed field is generated by a digital micromirror device with superpixel method. We measure the Hadamard basis and the OAM-basis transmission matrices of scattering medium, respectively. With the measured transmission matrices, single-spot, multi-spot and vortex focusing are achieved after scattering medium, verifying the accuracy of the measured transmission matrices. The strong diagonal presented in the norm of focusing operator also proves the accuracy of the measured transmission matrices. The proposed method may have potential applications in optical imaging and optical communication under scattering environment.

The problem of optical scattering was long thought to fundamentally limit the depth at which light could be focused through turbid media such as fog or biological tissue. However, recent work in the field of wavefront shaping has demonstrated that by properly shaping the input light field, light can be noninvasively focused to desired locations deep inside scattering media. This has led to the development of several new techniques which have the potential to enhance the capabilities of existing optical tools in biomedicine. Unfortunately, extending these methods to living tissue has a number of challenges related to the requirements for noninvasive guidestar operation, speed, and focusing fidelity. Of existing wavefront shaping methods, time-reversed ultrasonically encoded (TRUE) focusing is well suited for applications in living tissue since it uses ultrasound as a guidestar which enables noninvasive operation and provides compatibility with optical phase conjugation for high-speed operation. In this paper, we will discuss the results of our recent work to apply TRUE focusing for optogenetic modulation, which enables enhanced optogenetic stimulation deep in tissue with a 4-fold spatial resolution improvement in 800-micron thick acute brain slices compared to conventional focusing, and summarize future directions to further extend the impact of wavefront shaping technologies in biomedicine.

Wavefront shaping techniques allow one to focus light inside and through turbid media. In order to gain a better understanding how light evolves inside a scattering random medium while shaping the incident wavefront, it is beneficial to know the electromagnetic field within the entire medium, which is difficult to achieve in experiments. Here we investigate wavefront shaping techniques with numerical simulations based on Maxwell’s equations to examine various effects of wave propagation in randomly scattering media. First, we calculate numerically monochromatic plane wave solutions for a given scattering medium for different incident k-vectors. With these plane wave solutions we are able to simulate complex-shaped incident wavefronts impinging on the scattering medium. This enables us to scan and tilt the incident complex wavefront after solving Maxwell’s equations and to examine the electromagnetic field inside the medium. Due to the fact that we have access to the electromagnetic field of the plane wave illumination from different angles, it is easy to phase-optimize an incident wavefront by conjugating the known phase at a certain location. Because of the separate calculation of the plane wave solutions and the possibility to simulate different incident wavefronts by superimposing these plane wave solutions, our method is very dynamic and versatile. In summary, our simulation approach contributes to the understanding of light propagation in strongly scattering media such as biological tissue and to the improvement of medical imaging techniques.

Light propagation in complex media, such as paint, clouds, or biological tissues, is a very challenging phenomenon, encompassing fundamental aspects in mesoscopic and statistical physics. It is also of utmost applied interest, in particular for imaging. Wavefront shaping has revolutionized the ability to image through or in complex media. I will discuss how computational tools and machine learning allows to develop further wavefront shaping for imaging applications, and conversely discuss how the same complexity can be leveraged for optical computing tasks.

Multimode fibres (MMFs) have gained attention for their potential in high-resolution, minimally invasive imaging applications due to their small diameter and high-density signal transmission. However, challenges such as mode interference, environmental sensitivity, and the need for frequent recalibration have limited their practical imaging applications. Optical imaging using MMFs usually relies on wavefront shaping and raster-scanning techniques, which require extended calibration and imaging time along with high computational resources. Here we present a dual-modal MMF imaging system based on compressive sensing, enabling both optical imaging and photocurrent mapping. By projecting random speckle patterns onto photovoltaic surfaces, photocurrent mapping is achieved without the need for raster scanning. Experimental results demonstrate the system’s capability to achieve micron-level spatial characterization, with spatial resolution determined by the fibre’s numerical aperture and operating wavelength. Calibration and imaging processes are completed within 2 seconds, with a compression ratio 28 times below the Nyquist limit. This dual-modal imaging approach paves the way for fibre-based endoscopes capable of simultaneous optical and electrical characterizations, offering new opportunities in biomedical imaging and material science.

Imaging in thick biological tissues is often degraded by sample-induced aberrations, which reduce resolution and contrast, particularly in super-resolution techniques. While hardware-based adaptive optics (AO) using wavefront shaping can correct these aberrations, their complexity and cost hinder widespread adoption. Here, we present a computational AO framework for multiphoton structured illumination microscopy, enabling deep-tissue super-resolution imaging with minimal hardware modifications. By replacing the photodetector with a camera from the conventional laser-scanning multiphoton microscope, we capture a sequence of scanned images. Using virtual structured illumination, we develop a dual deconvolution algorithm that independently corrects excitation and emission aberrations, recovering an aberration-free object spectrum with an extended spatial frequency bandwidth. We experimentally validate this framework through two-photon super-resolution imaging, achieving a lateral resolution of 130 nm-one-fourth of the emission wavelength-at a depth of 180 μm in thick mouse brain tissue, where conventional deconvolution fails to maintain super-resolution capability. This approach provides a cost-effective and accessible alternative to hardware-based AO, expanding the potential for high-resolution deep-tissue imaging in biological research.