Fast wavefront shaping for two-photon brain imaging with multipatch correction.

Nonlinear fluorescence microscopy promotes in-vivo optical imaging of cellular structure at diffraction-limited resolution deep inside scattering biological tissues. Active compensation of tissue-induced aberrations and light scattering through adaptive wavefront correction further extends the accessible depth by restoring high resolution at large depth. However, those corrections are only valid over a very limited field of view within the angular memory effect. To overcome this limitation, we introduce an acousto-optic light modulation technique for fluorescence imaging with simultaneous wavefront correction at pixel scan speed. Biaxial wavefront corrections are first learned by adaptive optimization at multiple locations in the image field. During image acquisition, the learned corrections are then switched on the fly according to the position of the excitation focus during the raster scan. The proposed microscope is applied to in vivo transcranial neuron imaging and demonstrates multi-patch correction of thinned skull-induced aberrations and scattering at 40-kHz data acquisition speed.

Coherent enhancement of optical remission in diffusive media.

Remitted waves are used for sensing and imaging in diverse diffusive media from the Earth's crust to the human brain. Separating the source and detector increases the penetration depth of light, but the signal strength decreases rapidly, leading to a poor signal-to-noise ratio. Here, we show, experimentally and numerically, that wavefront shaping a laser beam incident on a diffusive sample enables an enhancement of remission by an order of magnitude at depths of up to 10 transport mean free paths. We develop a theoretical model which predicts the maximal remission enhancement. Our analysis reveals a significant improvement in the sensitivity of remitted waves to local changes of absorption deep inside diffusive media. This work illustrates the potential of coherent wavefront control for noninvasive diffuse wave imaging applications, such as diffuse optical tomography and functional near-infrared spectroscopy.

Polariton Microfluidics for Nonreciprocal Dragging and Reconfigurable Shaping of Polaritons.

Dielectric environment engineering is an efficient and general approach to manipulating polaritons. Liquids serving as the surrounding media of polaritons have been used to shift polariton dispersions and tailor polariton wavefronts. However, those liquid-based methods have so far been limited to their static states, not fully unleashing the promise offered by the mobility of liquids. Here, we propose a microfluidic strategy for polariton manipulation by merging polaritonics with microfluidics. The diffusion of fluids causes gradient refractive indices over microchannels, which breaks the symmetry of polariton dispersions and realizes the microfluidic analogue to nonreciprocal polariton dragging. Based on polariton microfluidics, we also designed a set of on-chip polaritonic elements to actively shape polaritons, including planar lenses, off-axis lenses, Janus lenses, bends, and splitters. Our strategy expands the toolkit for the manipulation of polaritons at the subwavelength scale and possesses potential in the fields of polariton biochemistry and molecular sensing.

Efficient Redirection of Trapped Broad-Band Fluorescence from Substrates into Free Space Using c-Si Metasurfaces.

Fluorescent dye films on transparent substrates are essential for OLEDs, flexible displays, X-ray detection, and wireless optical communications. However, their efficiency is often hampered by fluorescence trapping due to total internal reflection (TIR) and waveguiding. This study tackles this longstanding challenge by reconceptualizing the integration of dye films with nanoantenna metasurfaces. Traditional methods involve directly spin-coating films onto c-Si metasurfaces on quartz substrates, resulting in edge luminescence and weak inner signals. We present a straightforward, adjustable approach by integrating dye films on the opposite side of quartz substrates, reaching a 2.5-fold photoluminescence enhancement and improving the uniformity of the emission compared to the conventional methods. These gains stem from redirecting a significant portion of leaked fluorescence light trapped inside the substrate into free space, surpassing TIR conditions through in-plane diffraction orders of the metasurfaces across the full RGB spectrum. Our findings facilitate the design of more efficient luminescent devices.

Excitonic van der Waals Metasurfaces for Resonant Wavefront Shaping at Deep Subwavelength Thickness Scale.

Dielectric phase gradient metasurfaces have emerged as promising candidates to shrink bulky optical elements to subwavelength thickness scale based on dielectric meta-atoms. These meta-atoms strongly interact with light, thus offering excellent phase manipulation of incident light. However, to fulfill 2π phase control using meta-atoms, the metasurface thickness, to date, is limited to the order of 102 nm. Here, we present the thickness scaling down of phase gradient metasurfaces to <λ/20 by using excitonic van der Waals metasurfaces. High-refractive-index enabled by exciton resonances and symmetry-breaking nanostructures in the patterned layered tungsten disulfide (WS2) corporately enable quasibound states in the continuum in WS2 metasurfaces, which consequently yield complete phase regulation of 2π with the thickness down to 35 nm. To illustrate the concept, we have experimentally demonstrated beam steering, focusing, and holographic display using WS2 metasurfaces. We envision our results unveiling new venues for ultimate thin phase gradient metasurfaces.

In-depth polarisation resolved SHG microscopy in biological tissues using iterative wavefront optimisation.

Polarised nonlinear microscopy has been extensively developed to study molecular organisation in biological tissues, quantifying the response of nonlinear signals to a varying incident linear polarisation. Polarisation Second harmonic Generation (PSHG) in particular is a powerful tool to decipher sub-microscopic modifications of fibrillar collagen organisation in type I and III collagen-rich tissues. The quality of SHG imaging is however limited to about one scattering mean free path in depth (typically 100 micrometres in biological tissues), due to the loss of focus quality, induced by wavefront aberrations and scattering at even larger depths. In this work, we study how optical depth penetration in biological tissues affects the quality of polarisation control, a crucial parameter for quantitative assessment of PSHG measurements. We apply wavefront shaping to correct for SHG signal quality in two regimes, adaptive optics for smooth aberration modes corrections at shallow depth, and wavefront shaping of higher spatial frequencies for optical focus correction at larger depths. Using nonlinear SHG active nanocrystals as guide stars, we quantify the capabilities of such optimisation methods to recover a high-quality linear polarisation and investigate how this approach can be applied to in-depth PSHG imaging in tissues, namely tendon and mouse cranial bone.

Focusing Coherent Light through Volume Scattering Phantoms via Wavefront Shaping.

Manipulating the wavefront of coherent light incident on scattering media to enhance the imaging depth, sensitivity, and resolution is a common technique in biomedical applications. Local phase variations cause changes in the interference and can be used to create a focus inside or behind a scattering medium. We use wavefront shaping (WFS) to force constructive interference at an arbitrary location. The amount of light transmitted into a given region strongly depends on the scattering and absorption characteristics. These are described by their respective coefficients µs and µa and the scattering phase function. Controlling the scattering and absorption coefficients, we study the behavior of wavefront shaping and the achievable intensity enhancement behind volume scattering media with well-defined optical properties. The phantoms designed in this publication are made of epoxy resin. Into these epoxy matrices, specific amounts of scattering and absorbing particles, such as titanium dioxide pigments and molecular dyes, are mixed. The mixture obtained is filled into 3D-printed frames of various thicknesses. After a precise fabrication procedure, an integrating sphere-based setup characterizes the phantoms experimentally. It detects the total hemispherical transmission and reflection. Further theoretical characterization is performed with a newly developed hybrid PN method. This method senses the flux of light into a particular angular range at the lower boundary of a slab. The calculations are performed without suffering from ringing and fulfill the exact boundary conditions there. A decoupled two-path detection system allows for fast optimization as well as sensitive detection. The measurements yield results that agree well with the theoretically expected behavior.

Multimode Optical Interconnects on Silicon Interposer Enable Confidential Hardware-to-Hardware Communication.

Following Moore's law, the density of integrated circuits is increasing in all dimensions, for instance, in 3D stacked chip networks. Amongst other electro-optic solutions, multimode optical interconnects on a silicon interposer promise to enable high throughput for modern hardware platforms in a restricted space. Such integrated architectures require confidential communication between multiple chips as a key factor for high-performance infrastructures in the 5G era and beyond. Physical layer security is an approach providing information theoretic security among network participants, exploiting the uniqueness of the data channel. We experimentally project orthogonal and non-orthogonal symbols through 380 µm long multimode on-chip interconnects by wavefront shaping. These interconnects are investigated for their uniqueness by repeating these experiments across multiple channels and samples. We show that the detected speckle patterns resulting from modal crosstalk can be recognized by training a deep neural network, which is used to transform these patterns into a corresponding readable output. The results showcase the feasibility of applying physical layer security to multimode interconnects on silicon interposers for confidential optical 3D chip networks.

Cylindrical Metalens for Generation and Focusing of Free-Electron Radiation.

Metasurfaces constitute a powerful approach to generate and control light by engineering optical material properties at the subwavelength scale. Recently, this concept was applied to manipulate free-electron radiation phenomena, rendering versatile light sources with unique functionalities. In this Letter, we experimentally demonstrate spectral and angular control over coherent light emission by metasurfaces that interact with free-electrons under grazing incidence. Specifically, we study metalenses based on chirped metagratings that simultaneously emit and shape Smith-Purcell radiation in the visible and near-infrared spectral regime. In good agreement with theory, we observe the far-field signatures of strongly convergent and divergent cylindrical radiation wavefronts using in situ hyperspectral angle-resolved light detection in a scanning electron microscope. Furthermore, we theoretically explore simultaneous control over the polarization and wavefront of Smith-Purcell radiation via a split-ring-resonator metasurface, enabling tunable operation by spatially selective mode excitation at nanometer resolution. Our work highlights the potential of merging metasurfaces with free-electron excitations for versatile and highly tunable radiation sources in wide-ranging spectral regimes.

Shaping the branched flow of light through disordered media.

Electronic matter waves traveling through the weak and smoothly varying disorder potential of a semiconductor show a characteristic branching behavior instead of a smooth spreading of flow. By transferring this phenomenon to optics, we demonstrate numerically how the branched flow of light can be controlled to propagate along a single branch rather than along many of them at the same time. Our method is based on shaping the incoming wavefront and only requires partial knowledge of the system's transmission matrix. We show that the light flowing along a single branch has a broadband frequency stability such that one can even steer pulses along selected branches-a prospect with many interesting possibilities for wave control in disordered environments.

Tunable mode control through myriad-mode fibers.

Multimode fibers are attractive for imaging, communication, computation, and energy delivery. Unfortunately, intermodal and polarization coupling precludes direct control of the delivered mode composition. We present a technique to tailor the mode composition at the output of a multimode fiber with thousands of modes, which we refer to as myriad-mode fiber, using its experimentally measured transmission matrix. While precise mode control has been demonstrated in typical multimode fibers with up to 210 modes, the method proposed here is particularly useful for high mode number fibers, such as when the number of modes is comparable to the number of modes of the wavefront shaping spatial light modulator. To illustrate the technique, we select different subsets of modes to create focal spots at the output of a fiber with 7140 modes. Importantly, we define efficiency and fidelity metrics to evaluate the mode control and demonstrate the relationship between efficiency, fidelity, and the spatial location of the spots across the distal fiber cross-section.

Holographic Imaging and Stimulation of Neural Circuits.

A critical neuroscience challenge is the need to optically image and manipulate neural activity with high spatiotemporal resolution over large brain volumes. The last three decades have seen the development of calcium imaging to record activity from neuronal populations, as well as optochemistry and optogenetics to optically manipulate neural activity. These methods are typically implemented with wide-field or laser-scanning microscopes. While the former approach has a good temporal resolution, it generally lacks spatial resolution or specificity, particularly in scattering tissues such as the nervous system; meanwhile, the latter approach, particularly when combined with two-photon excitation, has high spatial resolution and specificity but poor temporal resolution. As a new technique, holographic microscopy combines the advantages of both approaches. By projecting a holographic pattern on the brain through a spatial light modulator, the activity of specific groups of neurons in 3D brain volumes can be imaged or stimulated with high spatiotemporal resolution. In a combination of other techniques such as fast scanning or temporal focusing, this high spatiotemporal resolution can be further improved. Holographic microscopy enables all-optical interrogating of neural activity in 3D, a critical tool to dissect the function of neural circuits.

Shaping reverberating sound fields with an actively tunable metasurface.

A reverberating environment is a common complex medium for airborne sound, with familiar examples such as music halls and lecture theaters. The complexity of reverberating sound fields has hindered their meaningful control. Here, by combining acoustic metasurface and adaptive wavefield shaping, we demonstrate the versatile control of reverberating sound fields in a room. This is achieved through the design and the realization of a binary phase-modulating spatial sound modulator that is based on an actively reconfigurable acoustic metasurface. We demonstrate useful functionalities including the creation of quiet zones and hotspots in a typical reverberating environment.

Far-Field Wavefront Control of Nonlinear Luminescence in Disordered Gold Metasurfaces.

We demonstrate the local optimization of nonlinear luminescence from disordered gold metasurfaces by shaping the phase of femtosecond excitation. This process is enabled by the far-field wavefront control of plasmonic modes delocalized over the sample surface, leading to a coherent enhancement of subwavelength electric fields. In practice, the increase in nonlinear luminescence is strongly sensitive to both the nanometer-scale morphology and the level of structural complexity of the gold metasurface. We typically observe a 2 orders of magnitude enhancement of the luminescence signal for an optimized excitation wavefront compared to a random one. These results demonstrate how disordered metasurfaces made of randomly coupled plasmonic resonators, together with wavefront shaping, provide numerous degrees of freedom to program locally optimized nonlinear responses and optical hotspots.

Wavefront Shaping Concepts for Application in Optical Coherence Tomography-A Review.

Optical coherence tomography (OCT) enables three-dimensional imaging with resolution on the micrometer scale. The technique relies on the time-of-flight gated detection of light scattered from a sample and has received enormous interest in applications as versatile as non-destructive testing, metrology and non-invasive medical diagnostics. However, in strongly scattering media such as biological tissue, the penetration depth and imaging resolution are limited. Combining OCT imaging with wavefront shaping approaches significantly leverages the capabilities of the technique by controlling the scattered light field through manipulation of the field incident on the sample. This article reviews the main concepts developed so far in the field and discusses the latest results achieved with a focus on signal enhancement and imaging.

High-resolution in vivo imaging of mouse brain through the intact skull.

Multiphoton microscopy is the current method of choice for in vivo deep-tissue imaging. The long laser wavelength suffers less scattering, and the 3D-confined excitation permits the use of scattered signal light. However, the imaging depth is still limited because of the complex refractive index distribution of biological tissue, which scrambles the incident light and destroys the optical focus needed for high resolution imaging. Here, we demonstrate a wavefront-shaping scheme that allows clear imaging through extremely turbid biological tissue, such as the skull, over an extended corrected field of view (FOV). The complex wavefront correction is obtained and directly conjugated to the turbid layer in a noninvasive manner. Using this technique, we demonstrate in vivo submicron-resolution imaging of neural dendrites and microglia dynamics through the intact skulls of adult mice. This is the first observation, to our knowledge, of dynamic morphological changes of microglia through the intact skull, allowing truly noninvasive studies of microglial immune activities free from external perturbations.

All-Optical Interrogation of Neural Circuits.

There have been two recent revolutionary advances in neuroscience: First, genetically encoded activity sensors have brought the goal of optical detection of single action potentials in vivo within reach. Second, optogenetic actuators now allow the activity of neurons to be controlled with millisecond precision. These revolutions have now been combined, together with advanced microscopies, to allow "all-optical" readout and manipulation of activity in neural circuits with single-spike and single-neuron precision. This is a transformational advance that will open new frontiers in neuroscience research. Harnessing the power of light in the all-optical approach requires coexpression of genetically encoded activity sensors and optogenetic probes in the same neurons, as well as the ability to simultaneously target and record the light from the selected neurons. It has recently become possible to combine sensors and optical strategies that are sufficiently sensitive and cross talk free to enable single-action-potential sensitivity and precision for both readout and manipulation in the intact brain. The combination of simultaneous readout and manipulation from the same genetically defined cells will enable a wide range of new experiments as well as inspire new technologies for interacting with the brain. The advances described in this review herald a future where the traditional tools used for generations by physiologists to study and interact with the brain-stimulation and recording electrodes-can largely be replaced by light. We outline potential future developments in this field and discuss how the all-optical strategy can be applied to solve fundamental problems in neuroscience. SIGNIFICANCE STATEMENT: This review describes the nexus of dramatic recent developments in optogenetic probes, genetically encoded activity sensors, and novel microscopies, which together allow the activity of neural circuits to be recorded and manipulated entirely using light. The optical and protein engineering strategies that form the basis of this "all-optical" approach are now sufficiently advanced to enable single-neuron and single-action potential precision for simultaneous readout and manipulation from the same functionally defined neurons in the intact brain. These advances promise to illuminate many fundamental challenges in neuroscience, including transforming our search for the neural code and the links between neural circuit activity and behavior.

Graded metascreens to enable a new degree of nanoscale light management.

Optical metasurfaces, typically referred to as two-dimensional metamaterials, are arrays of engineered subwavelength inclusions suitably designed to tailor the light properties, including amplitude, phase and polarization state, over deeply subwavelength scales. By exploiting anomalous localized interactions of surface elements with optical waves, metasurfaces can go beyond the functionalities offered by conventional diffractive optical gratings. The innate simplicity of implementation and the distinct underlying physics of their wave-matter interaction distinguish metasurfaces from three-dimensional metamaterials and provide a valuable means of moulding optical waves in the desired manner. Here, we introduce a general approach based on the electromagnetic equivalence principle to develop and synthesize graded, non-periodic metasurfaces to generate arbitrarily prescribed distributions of electromagnetic waves. Graded metasurfaces are realized with a single layer of spatially modulated, electrically polarizable nanoparticles, tailoring the scattering response of the surface with nanoscale resolutions. We discuss promising applications based on the proposed local wave management technique, including the design of ultrathin optical carpet cloaks, alignment-free polarization beam splitters and a novel approach to enable broadband light absorption enhancement in thin-film solar cells. This concept opens up a practical route towards efficient planarized optical structures with potential impact on the integrated nanophotonic technology.

Subsurface Spectroscopy of Thermal Degradation Inside an Inert Plastic Bonded Explosive (PBX) Simulant Using Feedback-Assisted Wavefront Shaping.

We characterize the subsurface thermal degradation of an inert analog of high-explosive molecular crystals (Eu:Y(acac)3(DPEPO)) (EYAD) embedded inside of a plastic bonded explosive simulant using feedback-assisted wavefront shaping-based fluorescence and Raman spectroscopies. This technique utilizes wavefront shaping to focus pump light inside a heterogeneous material onto a target particle, which significantly improves its spectroscopic signature. We find that embedding the EYAD crystals in the heterogeneous polymer results in improved thermal stability, relative to bare crystal measurements, with the crystal remaining fluorescent to >612 K inside of the heterogeneous material, while the bare crystal's fluorescence is fully quenched by 500 K. We hypothesize that this improvement is due to the polymer restricting the effects of EYAD melting, which occurs at 400 K and is the primary mechanism for spectroscopic changes in the temperature range explored.

An Ultrathin, Fast-Response, Large-Scale Liquid-Crystal-Facilitated Multi-Functional Reconfigurable Metasurface for Comprehensive Wavefront Modulation.

The rapid advancement of prevailing communication/sensing technologies necessitates cost-effective millimeter-wave arrays equipped with a massive number of phase-shifting cells to perform complicated beamforming tasks. Conventional approaches employing semiconductor switch/varactor components or tunable materials encounter obstacles such as quantization loss, high cost, high complexity, and limited adaptability for realizing large-scale arrays. Here, a low-cost, ultrathin, fast-response, and large-scale solution relying on metasurface concepts combined together with liquid crystal (LC) materials requiring a layer thickness of only 5 µm is reported. Rather than immersing resonant structures in LCs, a joint material-circuit-based strategy is devised, via integrating deep-subwavelength-thick LCs into slow-wave structures, to achieve constitutive metacells with continuous phase shifting and stable reflectivity. An LC-facilitated reconfigurable metasurface sub-system containing more than 2300 metacells is realized with its unprecedented comprehensive wavefront manipulation capacity validated through various beamforming functions, including beam focusing/steering, reconfigurable vortex beams, and tunable holograms, demonstrating a milli-second-level function-switching speed. The proposed methodology offers a paradigm shift for modulating electromagnetic waves in a non-resonating broadband fashion with fast-response and low-cost properties by exploiting functionalized LC-enabled metasurfaces. Moreover, this extremely agile metasurface-enabled antenna technology will facilitate a transformative impact on communication/sensing systems and empower new possibilities for wavefront engineering and diffractive wave calculation/inference.