Optical Wavefront Shaping Research Articles

On-chip optical wavefront shaping by transverse-spin-induced Pancharatanam-Berry phase.

Pancharatnam-Berry (PB) metasurfaces can be applied to manipulate the phase and polarization of light within subwavelength thickness. The underlying mechanism is attributed to the geometric phase originating from the longitudinal spin of light. Here, we demonstrate, to the best of our knowledge, a new type of PB geometric phase derived from the intrinsic transverse spin of guided light. Using full-wave numerical simulations, we show that the rotation of a metallic nano-bar sitting on a metal substrate can induce a geometric phase covering 2π full range for the surface plasmons carrying an intrinsic transverse spin. Especially, the geometric phase is different for the surface plasmons propagating in opposite directions due to spin-momentum locking. We apply the geometric phase to design metasurfaces to manipulate the wavefront of surface plasmons to achieve steering and focusing. Our work provides a new mechanism for on-chip light manipulations with potential applications in designing ultra-compact optical devices for imaging and sensing.

Focusing scattering light field with different states based on iterative algorithm

Transmitting optical information through scattering medium has broad application prospects in biomedical, aerospace and other fields. However, the light passing through the scattering medium will cause wavefront distortion and optical information blurring. Wavefront shaping technology uses a mathematical matrix to characterize the characteristics of scattering medium, which can achieve refocusing and imaging after light propagation through the scattering medium. It mainly includes optical phase conjugation, optical transmission matrix and wavefront shaping based on iterative optimization. However, the iterative wavefront shaping is considered to be a cost-effective method. Based on the wavefront amplitude modulation technology, the wavefront amplitude of the incident light is continuously adjusted by using the optimization algorithm to find the corresponding wavefront amplitude distribution that can maximize the light intensity in the target area. The system generates binary patterns implemented with digital-micromirror device (DMD) based on on-off state of micromirror, where “on” represents 1 and “off” refers to 0. The DMD has a high refresh rate and can achieve high speed wavefront amplitude modulation by using the iteration algorithm. In the experiment, the scattering medium is prepared with TiO<sub>2</sub>, water and gelatin, whose persistence times are controlled with the water-gelatin ratio (WGR). In addition, the Pearson correlation coefficient (Cor) curve obtained through 300-s-measurement under different WGR conditions, which shows that the greater WGR, the shorter the persistence time is. The experiment mainly studies the focusing of the spatial light through scattering media by wavefront amplitude modulation, and discusses the ability of point guard algorithm (PGA) and genetic algorithm (GA) to control the scattered light field with different persistence times in 64 × 64 segments. The experimental results show that the PGA can achieve higher enhancement factor and more uniform multi-point focusing than the GA after 1000 iterations in the scattering medium with the same persistence time. The relative standard deviation value is inversely proportional to the WGR value when multi-point focusing can be completed. We also demonstrate that GA can only achieve single-point focusing when WGR = 40, and it cannot accomplish multi-point focusing in self-made scattering medium. This study not only verifies a method to achieve focusing scattering light field, but also provides a new scheme for testing the performance of the iterative wavefront shaping.

Performance enhancement in wavefront shaping of multiply scattered light: a review.

In nonballistic regime, optical scattering impedes high-resolution imaging through/inside complex media, such as milky liquid, fog, multimode fiber, and biological tissues, where confocal and multiphoton modalities fail. The significant tissue inhomogeneity-induced distortions need to be overcome and a technique referred as optical wavefront shaping (WFS), first proposed in 2007, has been becoming a promising solution, allowing for flexible and powerful light control. Understanding the principle and development of WFS may inspire exciting innovations for effective optical manipulation, imaging, stimulation, and therapy at depths in tissue or tissue-like complex media. We aim to provide insights about what limits the WFS towards biomedical applications, and how recent efforts advance the performance of WFS among different trade-offs. By differentiating the two implementation directions in the field, i.e., precompensation WFS and optical phase conjugation (OPC), improvement strategies are summarized and discussed. For biomedical applications, improving the speed of WFS is most essential in both directions, and a system-compatible wavefront modulator driven by fast apparatus is desired. In addition to that, algorithm efficiency and adaptability to perturbations/noise is of concern in precompensation WFS, while for OPC significant improvements rely heavily on integrating physical mechanisms and delicate system design for faster response and higher energy gain. Substantial improvements in WFS implementations, from the aspects of physics, engineering, and computing, have inspired many novel and exciting optical applications that used to be optically inaccessible. It is envisioned that continuous efforts in the field can further advance WFS towards biomedical applications and guide our vision into deep biological tissues.

On the exploration of structured light transmission through a multimode fiber in a reference-less system

Recent advancements in optical wavefront shaping have brought multimode fibers (MMFs) into the spotlight as potential contenders for long-haul communication, positioning them as promising substitutes to single-mode fibers. MMFs offer greater data rates, countering the impending congestion of fiber-based networks. Additionally, their suitability for single fiber endoscope procedures presents them as compelling alternatives for minimally invasive endoscopy, providing information comparable to, if not surpassing, current cutting-edge technology. However, the complex modal behavior of light in MMFs hinders the implementation of these promising applications. Hence, precise modal excitation and control are crucial for improving the transmission of structured light in MMFs. This study introduces a groundbreaking approach that achieves the retrieval of the transmission matrix in a single step, thereby facilitating coherent light propagation through highly dispersive MMFs. By combining iterative phase retrieval algorithms with the measurement of phase shifts between experimentally established focal points, potential arbitrary interference control is enabled, leading to effective phase correction. The efficacy of our method is validated through the successful transmission of diverse structured light beams, including Laguerre–Gauss and Hermite–Gaussian types, as well as handwritten characters via MMF. The examination of structured light is simplified using an off-axis holographic technique that accurately captures both intensity and phase information. These results hold significant potential, paving the way for major advancements in long-distance communication and minimally invasive medical procedures, thereby transforming the telecommunications and healthcare sectors.

Spatially resolved readout of a Fabry-Perot ultrasound sensor interrogated through a multimode optical fiber using wavefront shaping.

The spatially resolved interrogation of a Fabry-Perot ultrasound sensor using a laser beam focused through a multimode fiber is demonstrated. To scan the beam across the sensor as required to read it out, optical wavefront shaping was employed to compensate for the scrambling of light in the fiber. By providing a means to map ultrasound through inexpensive, lightweight fibers, this could lead to new ultrasonic and photoacoustic imaging systems, such as endoscopes and flexible handheld probes.

Arbitrary Optical Wavefront Shaping with Holographic Plasmonic Gap Waveguides

AbstractManipulations of free‐space light are usually achieved through various metasurfaces and spatial light modulators. However, an external light source is required to excite these devices, making complete on‐chip integrations difficult. Integrated photonics enables the miniaturization and multi‐functionalization of optical systems by densely packing numerous optical components on a single chip. Particularly, plasmons have recently attracted extensive attention due to their unique abilities to enable the routing and manipulation of light at the nanoscale. Here optical wavefront shaping achieved by holographic metal‐insulator‐metal plasmonic gap waveguides is reported. An arbitrary free‐space wave's amplitude and phase information can be recorded in the waveguide by the amplitude variation of the guided wave. By elaborately designing the plasmonic gap waveguide, the guided waves can be molded into any desired free‐space light field, thus enabling complex free‐space functions including highly directional beams, dual beams with arbitrarily tailorable power ratio and radiation angles, focusing beams, and Airy beam generation. This study opens a new route for optical wavefront shaping via modulating guided waves. It paves the way for optical interconnects across multifunctional photonic integrated devices and free space, holding potential in optical communications, light detection and ranging, imaging, and displays.

On‐Chip Electrically Driven Tunable Meta‐Lens for Dynamic Focusing and Beam Steering

AbstractThe on‐chip meta‐lenses consisting of subwavelength slot arrays with lateral gradients have shown great potential for parallel signal processing and optical computing. The dynamic tuning mechanisms which are necessary steps toward exploiting the applications of the on‐chip meta‐system have been rarely mentioned yet. Here, a free‐form functional microheater via topology design is proposed to manipulate the refractive index distribution, allowing for flexible optical wavefront shaping. For a proof‐of‐concept, wide range of dynamic focusing and beam steering are demonstrated via a tunable meta‐lens on silicon photonic platform. The continuous scanning of the focused beam is experimentally achieved with a maximum travel range of 100 µm in the longitudinal direction (≈95 nm mW−1) and 25 µm in the lateral direction (≈20 nm mW−1). The thermal‐optic tuning elements can response the electrical control within ≈41.2 µs. A waveguide array at the focal plane is used to characterize the device. The on‐chip loss of the tunable meta‐lens is measured to be <1 dB including the coupling loss from slab to single‐mode waveguide. The proposed tunable meta‐lenses are fabricated via a standard silicon photonic process and allow for arbitrarily two dimensional (2D) beam steering. It offers great versatility for on‐chip meta‐systems with a wider scope of functionalities and applications.

Towards ideal focusing of diffused light via optical wavefront shaping

Controllable optical propagation, such as forming diffraction-limited optical focusing, beyond the diffusion limit in biological tissue or tissue-like scattering media, has been desired for long yet considered challenging. In the past two decades, optical wavefront shaping (WFS) has been proposed and has progressed, demonstrating its remarkable potential. That said, inherent tradeoffs still exist among optimization speed, control degree of freedom, and energy gain, which has hindered wide applications of the technology. Most recently, an analogue optical phase conjugation system was developed, equipped with stimulated emission light amplification that effectively achieves the least tradeoff ever, yielding high-gain and high-speed performance of optical focusing through dynamic thick media.

Nanocavity-Integrated van der Waals Heterobilayers for Nano-excitonic Transistor.

Optical computing with optical transistors has emerged as a possible solution to the exponentially growing computational workloads, yet an on-chip nano-optical modulation remains a challenge due to the intrinsically noninteracting nature of photons in addition to the diffraction limit. Here, we present an all-optical approach toward nano-excitonic transistors using an atomically thin WSe2/Mo0.5W0.5Se2 heterobilayer inside a plasmonic tip-based nanocavity. Through optical wavefront shaping, we selectively modulate tip-enhanced photoluminescence (TEPL) responses of intra- and interlayer excitons in a ∼25 nm2 area, demonstrating the enabling concept of an ultrathin 2-bit nano-excitonic transistor. We suggest a simple theoretical model describing the underlying adaptive TEPL modulation mechanism, which relies on the additional spatial degree of freedom provided by the presence of the plasmonic tip. Furthermore, we experimentally demonstrate a concept of a 2-trit nano-excitonic transistor, which can provide a technical basis for processing the massive amounts of data generated by emerging artificial intelligence technologies.

Exploring the hidden dimensions of an optical extreme learning machine

Extreme Learning Machines (ELMs) are a versatile Machine Learning (ML) algorithm that features as the main advantage the possibility of a seamless implementation with physical systems. Yet, despite the success of the physical implementations of ELMs, there is still a lack of fundamental understanding in regard to their optical implementations. In this context, this work makes use of an optical complex media and wavefront shaping techniques to implement a versatile optical ELM playground to gain a deeper insight into these machines. In particular, we present experimental evidences on the correlation between the effective dimensionality of the hidden space and its generalization capability, thus bringing the inner workings of optical ELMs under a new light and opening paths toward future technological implementations of similar principles.

Tunable Polarization-Multiplexed Achromatic Dielectric Metalens.

Tunable metasurfaces provide a compact and efficient strategy for optical active wavefront shaping. Varifocal metalens is one of the most important applications. However, the existing tunable metalens rarely serves broadband wavelengths restricting their applications in broadband imaging and color display due to chromatic aberration. Herein, an electrically tunable polarization-multiplexed achromatic metalens integrated with twisted nematic liquid crystals (TNLCs) in the visible region is demonstrated. The phase profiles at different wavelengths under two orthogonal polarization channels are customized by the particle swarm optimization algorithm and matched with the dielectric metaunits database to achieve polarization-multiplexed achromatic performance. By combining the broadband linear polarization conversion ability of TNLC, the tunability of varifocal achromatic metalens is realized by applying different voltages. Further, the electrically tunable customized dispersion-manipulated metalens and switchable color metaholograms are demonstrated. The proposed devices will accelerate the application of metasurfaces in broadband zoom imaging, AR/VR displays and spectral detection.

Shaping the propagation of light in complex media

The main obstacle for optical imaging or for sending information through turbid media such as paint, clouds and biological tissue is the random scattering of light. Owing to its immense complexity, the process of multiple scattering has long been described by the diffusion equation, which ignores the interference of scattered light. Recent developments in optical wavefront shaping and phase recording techniques have enabled the breaking of the diffusion limit and the control of coherent light transport in complex media, including strongly scattering tissues and multimode optical fibres with random mode mixing. Great advances have been made in focusing and controlling the transmission of light through such complex systems and in performing various tasks behind them, such as optical micro-manipulation. Here, we summarize the amazing power and the fundamental limits of controlling multiple light scattering, which lay the physical foundation to harness multiply-scattered light for imaging and communication purposes. Connections to practical applications are illustrated, in particular in those areas covered in the companion articles in this issue. Multiple scattering fundamentally complicates the task of sending light through turbid media, as many applications require. This Review summarizes the theoretical framework and experimental techniques to understand and control these processes.

Multiple feedback based wavefront shaping method to retrieve hidden signal

We present an optical wavefront shaping approach that allows tracking and localization of a signal hidden inside or behind a scattering medium. The method combines traditional feedback based wavefront shaping together with a switch function, controlled by two different signals. A simple, in transmission imaging system is used with two detectors: one monitors the speckle signature and the other tracks the fully hidden signal (e.g., fluorescent beads). The algorithm initially finds the optimal incident wavefront to maximize light transmission to generate a focus in the scattering medium. This modulation process redirects the scattered input signal, inducing instantaneous changes in both monitored signals, which, in turn, locates the hidden objects. Once the response from the hidden target becomes distinct, the algorithm switches to use this signal as the feedback. We provide experimental demonstrations as a proof of concept of our approach. Potential applications of our method include extracting information from biological samples and developing noninvasive diagnosis methods.

Optically Selective Neuron Stimulation with a Wavefront Shaping‐Empowered Multimode Fiber

Optogenetics is proved to be a powerful tool for exploring the connection between behavior and neural circuits. The progress greatly benefits from the advances of optical techniques that enable high spatiotemporal resolution for selective single‐neuron stimulation through modulating light. Efficient propagation of modulated light, however, is handicapped by strong optical scattering in biological tissues, which results in inherent tradeoff between penetration depth and resolution. Schemes like graded index (GRIN) lens‐based microendoscopes are developed to yield more confined delivery of light, but tissue damage caused by the insertion of the bulky components cannot be ignored. Herein, an optically selective precise neuron stimulation using an ultrathin multimode fiber (MMF) is demonstrated, which is empowered by optical wavefront shaping to achieve light focusing and rapid raster scanning without mechanical movement at the distal end of the MMF and even through a mouse skull. With this method, primary neurons expressing Chr2 can be regulated spatiotemporally in experiment. Although a lot shall be further improved, the work may open up new venues for noninvasive or minimally invasive all‐optical investigation of neural circuits in used‐to‐be optically inaccessible brain regions.

Nonlinear wavefront engineering with metasurface decorated quartz crystal

Abstract In linear optical processes, compact and effective wavefront shaping techniques have been developed with the artificially engineered materials and devices in the past decades. Recently, wavefront shaping of light at newly generated frequencies was also demonstrated using nonlinear photonic crystals and metasurfaces. However, the nonlinear wave-shaping devices with both high nonlinear optical efficiency and high wave shaping efficiency are difficult to realize. To circumvent this constraint, we propose the idea of metasurface decorated optical crystal to take the best aspects of both traditional nonlinear crystals and photonic metasurfaces. In the proof-of-concept experiment, we show that a silicon nitride metasurface decorated quartz crystal can be used for the wavefront shaping of the second harmonic waves generated in quartz. With this crystal-metasurface hybrid platform, the nonlinear vortex beam generation and nonlinear holography were successfully demonstrated. The proposed methodology may have important applications in nonlinear structured light generation, super-resolution imaging, and optical information processing, etc.

Observation of mutual extinction and transparency in light scattering

Interference of scattered waves is fundamental for modern light-scattering techniques, such as optical wavefront shaping. Recently, a new type of wavefront shaping was introduced where the extinction is manipulated instead of the scattered intensity. The underlying idea is that upon changing the phases or the amplitudes of incident beams, the total extinction will change due to interference described by the cross terms between different incident beams. Here, we experimentally demonstrate the mutual extinction and transparency effects in scattering media, in particular, a human hair and a silicon bar. To this end, we send two light beams with a variable mutual angle on the sample. Depending on the relative phase of the incident beams, we observe either nearly zero extinction, mutual transparency, or almost twice the single-beam extinction, mutual extinction, in agreement with theory. We use an analytical approximation for the scattering amplitude, starting from a completely opaque object, and we discuss the limitations of our approximation. We discuss the applications of the mutual extinction and transparency effects in various fields such as non-line-of-sight communications, microscopy, and biomedical imaging.

Dynamic mutation enhanced particle swarm optimization for optical wavefront shaping.

Particle swarm optimization (PSO) is a well-known iterative algorithm commonly adopted in wavefront shaping for focusing light through or inside scattering media. The performance is, however, limited by premature convergence in an unstable environment. Therefore, we aim to solve this problem and enhance the focusing performance by adding a dynamic mutation operation into the plain PSO. With dynamic mutation, the "particles," or the optimized masks, are mutated with quantifiable discrepancy between the current and theoretical optimal solution, i.e., the "error rate." Gauged by that, the diversity of the "particles" is effectively expanded, and the adaptability of the algorithm to noise and instability is significantly promoted, yielding optimization approaching the theoretical optimum. The simulation and experimental results show that PSO with dynamic mutation demonstrates considerably better performance than PSO without mutation or with a constant mutation, especially under a noisy environment.

Spatial coherence control and analysis via micromirror-based mixed-state ptychography

Flexible and fast control of the phase and amplitude of coherent light, enabled by digital micromirror devices (DMDs) and spatial light modulators, has been a driving force for recent advances in optical tweezers, nonlinear microscopy, and wavefront shaping. In contrast, engineering spatially partially coherent light remains widely elusive due to the lack of tools enabling a joint analysis and control sequence. Here, we report an approach to coherence engineering that combines a quasi-monochromatic, thermal source and a DMD together with a ptychographic scanning microscope. The reported method opens up new routes to low-cost coherence control, with applications in micromanipulation, nanophotonics, and quantitative phase contrast imaging.

Optical information transmission through complex scattering media with optical-channel-based intensity streaming

For the past decade, optical wavefront shaping has been the standard technique to control light through scattering media. Implicit in this dominance is the assumption that manipulating optical interference is a necessity for optical control through scattering media. In this paper, we challenge this assumption by reporting on an alternate approach for light control through a disordered scattering medium – optical-channel-based intensity streaming (OCIS). Instead of actively tuning the interference between the optical paths via wavefront shaping, OCIS controls light and transmits information through scattering media through linear intensity operations. We demonstrate a set of OCIS experiments that connect to some wavefront shaping implementations, i.e. iterative wavefront optimization, digital optical phase conjugation, image transmission through transmission matrix, and direct imaging through scattering media. We experimentally created focus patterns through scattering media on a sub-millisecond timescale. We also demonstrate that OCIS enables a scattering medium mediated secure optical communication application.

Adaptive optical focusing through perturbed scattering media with a dynamic mutation algorithm

Optical imaging through or inside scattering media, such as multimode fiber and biological tissues, has a significant impact in biomedicine yet is considered challenging due to the strong scattering nature of light. In the past decade, promising progress has been made in the field, largely benefiting from the invention of iterative optical wavefront shaping, with which deep-tissue high-resolution optical focusing and hence imaging becomes possible. Most of the reported iterative algorithms can overcome small perturbations on the noise level but fail to effectively adapt beyond the noise level, e.g., sudden strong perturbations. Reoptimizations are usually needed for significant decorrelation to the medium since these algorithms heavily rely on the optimization performance in the previous iterations. Such ineffectiveness is probably due to the absence of a metric that can gauge the deviation of the instant wavefront from the optimum compensation based on the concurrently measured optical focusing. In this study, a square rule of binary-amplitude modulation, directly relating the measured focusing performance with the error in the optimized wavefront, is theoretically proved and experimentally validated. With this simple rule, it is feasible to quantify how many pixels on the spatial light modulator incorrectly modulate the wavefront for the instant status of the medium or the whole system. As an example of application, we propose a novel algorithm, the dynamic mutation algorithm, which has high adaptability against perturbations by probing how far the optimization has gone toward the theoretically optimal performance. The diminished focus of scattered light can be effectively recovered when perturbations to the medium cause a significant drop in the focusing performance, which no existing algorithms can achieve due to their inherent strong dependence on previous optimizations. With further improvement, the square rule and the new algorithm may boost or inspire many applications, such as high-resolution optical imaging and stimulation, in instable or dynamic scattering environments.