Wavefront shaping for forward scattering.

Aberrations and scattering limit the ability of optical microscopy to penetrate deep tissues. Adaptive optics (AO) is a very effective technique to correct for smooth aberrations. Wavefront shaping (WFS) techniques, on the other hand, compensate both scattering and aberrations and have guaranteed convergence even for finding high-dimensional corrections. However, if it is known in advance that the required corrections should be smooth, WFS is suboptimal because it does not use this a priori information. Here, we combine the best of AO and WFS by introducing a WFS method that takes into account the smoothness of the required correction. Our method is numerically stable and robust against noise, and it can find the corrections for multiple targets simultaneously. We experimentally confirmed that this method outperforms existing WFS techniques, especially in forward scattering samples.

Noise-tolerant wavefront shaping in a Hadamard basis.

Light scattering is the main limitation for optical imaging. However, light can be focused through or inside turbid media by spatially shaping the incident wavefront. Wavefront shaping is ultimately limited by the available photon budget. We developed a new 'dual reference' wavefront shaping algorithm that optimally uses the available light. Our method allows for multi-target wavefront shaping, making it suitable for transmission matrix measurements or transmitting images. We experimentally confirmed the improvement of the focus intensity compared to existing methods.

Ultra-thin boundary layer for high-accuracy simulations of light propagation.

The modified Born series method is currently one of the most efficient methods available for simulating light scattering in large inhomogeneous media. However, to achieve high accuracy, the method requires thick gradually absorbing layers around the simulation domain. Here, we introduce new boundary conditions, combining a padding-free acyclic convolution with an ultra-thin boundary layer. Our new boundary conditions minimize the wrap-around and reflection artefacts originating from the edges of the simulation domain, while also greatly reducing the computational costs and the memory requirements of the method. Our GPU-accelerated Matlab implementation is available on GitHub.

Model-based wavefront shaping microscopy.

Wavefront shaping is increasingly being used in modern microscopy to obtain high-resolution images deep inside inhomogeneous media. Wavefront shaping methods typically rely on the presence of a "guide star" to find the optimal wavefront to mitigate the scattering of light. However, the use of guide stars poses severe limitations. Notably, only objects in the close vicinity of the guide star can be imaged. Here, we introduce a guide-star-free wavefront shaping method in which the optimal wavefront is computed using a digital model of the sample. The refractive index model of the sample, that serves as the input for the computation, is constructed in situ by the microscope itself. In a proof of principle imaging experiment, we demonstrate a large improvement in the two-photon fluorescence signal through a diffuse medium, outperforming state-of-the-art wavefront shaping by a factor of two in imaging depth.

Quantum key establishment via a multimode fiber.

Quantum communication aims to provide absolutely secure transmission of secret information. State-of-the-art methods encode symbols into single photons or coherent light with much less than one photon on average. For long-distance communication, typically a single-mode fiber is used and significant effort has been devoted already to increase the data carrying capacity of a single optical line. Here we propose and demonstrate a fundamentally new concept for remote key establishment. Our method allows high-dimensional alphabets using spatial degrees of freedom by transmitting information through a light-scrambling multimode fiber and exploiting the no-cloning theorem. Eavesdropper attacks can be detected without using randomly switched mutually unbiased bases. We prove the security against a common class of intercept-resend and beam-splitting attacks with single-photon Fock states and with weak coherent light. Since it is optical fiber based, our method allows to naturally extend secure communication to larger distances. We experimentally demonstrate this new type of key exchange method by encoding information into a few-photon light pulse decomposed over guided modes of an easily available multimode fiber.

Scanning a focus through scattering media without using the optical memory effect.

Wavefront shaping makes it possible to form a focus through opaque scattering materials. In some cases, this focus may be scanned over a small distance using the optical memory effect. However, in many cases of interest, the optical memory effect has a limited range or is even too small to be measured. In such cases, one often resorts to measuring the full transmission matrix (TM) of the sample to completely control the light transmission. However, this process is time-consuming and may not always be possible. We introduce a new method, to the best of our knowledge, for focusing and scanning the focus at any arbitrary position behind the medium by measuring only a subset of the TM, called sparse field focusing (SFF). With SFF, the scan range is not limited to the memory effect, and there is no need to measure the full TM. Our experimental results agree well with our theoretical model. We expect that this method will find applications in imaging through scattering media, especially when the optical memory effect range is small.

Reconfigurable beam system for non-line-of-sight free-space optical communication.

In this paper, we propose a reconfigurable beam-shaping system to permit energy-efficient non-line-of-sight (NLOS) free-space optical communication. Light is steered around obstacles blocking the direct communication pathway and reaches a receiver after reflecting off of a diffuse surface. A coherent array optical transmitter (CAO-Tx) is used to spatially shape the wavefront of the light incident on a diffuse surface. Wavefront shaping is used to enhance the amount of diffusely reflected light reaching the optical receiver. Synthetic NLOS experiments for a signal reflected over an angular range of 20° are presented. A record-breaking 30-Gbit/s orthogonal frequency-division multiplexing signal is transmitted over a diffused optical wireless link with a >17-dB gain.

Blind focusing through strongly scattering media using wavefront shaping with nonlinear feedback.

Scattering prevents light from being focused in turbid media. The effect of scattering can be negated through wavefront shaping techniques when a localized form of feedback is available. Even in the absence of such a localized reporter, wavefront shaping can blindly form a single diffraction-limited focus when the feedback response is nonlinear. We developed and experimentally validated a model that accurately describes the statistics of this blind focusing process. We show that maximizing the nonlinear feedback signal only results in the formation of a focus when a limited number of reporters are contributing to the signal. Using our model, we can calculate the minimal requirements for the number of controlled spatial light modulator segments and the order of nonlinearity to blindly focus light through strongly scattering media.

Spatially confined quantification of bilirubin concentrations by spectroscopic visible-light optical coherence tomography.

Spatially confined measurements of bilirubin in tissue can be of great value for noninvasive bilirubin estimations during neonatal jaundice, as well as our understanding of the physiology behind bilirubin extravasation. This work shows the potential of spectroscopic visible-light optical coherence tomography (sOCT) for this purpose. At the bilirubin absorption peak around 460 nm, sOCT suffers from a strong signal decay with depth, which we overcome by optimizing our system sensitivity through a combination of zero-delay acquisition and focus tracking. In a phantom study, we demonstrate the quantification of bilirubin concentrations between 0 and 650 µM with only a 10% difference to the expected value, thereby covering the entire clinical pathophysiological range.

Imaging blood flow inside highly scattering media using ultrasound modulated optical tomography.

We report the use of ultrasound modulated optical tomography (UOT) with heterodyne parallel detection to locally sense and image blood flow deep inside a highly scattering medium. We demonstrate that the UOT signal is sensitive to the speed of the blood flow in the ultrasound focus and present an analytical model that relates UOT signals to the optical properties (i. e. scattering coefficient, anisotropy, absorption, and flow speed) of the blood and the background medium. We found an excellent agreement between the experimental data and the analytical model. By varying the integration time of the camera in our setup, we were able to spatially resolve blood flow in a scattering medium with a lateral resolution of 1.5 mm.

Mapping the energy density of shaped waves in scattering media onto a complete set of diffusion modes.

We study the energy density of shaped waves inside a quasi-1D disordered waveguide. We find that the spatial energy density of optimally shaped waves, when expanded in the complete set of eigenfunctions of the diffusion equation, is well described by considering only a few of the lowest eigenfunctions. Taking into account only the fundamental eigenfunction, the total internal energy inside the sample is underestimated by only 2%. The spatial distribution of the shaped energy density is very similar to the fundamental eigenfunction, up to a cosine distance of about 0.01. We obtain the energy density of transmission eigenchannels inside the sample by numerical simulation of the scattering matrix. Computing the transmission-averaged energy density over all transmission channels yields the ensemble averaged energy density of shaped waves. From the averaged energy density, we reconstruct its spatial distribution using the eigenfunctions of the diffusion equation. The results of our study have exciting applications in controlled biomedical imaging, efficient light harvesting in solar cells, enhanced energy conversion in solid-state lighting, and low threshold random lasers.

Roadmap on neurophotonics.

Mechanistic understanding of how the brain gives rise to complex behavioral and cognitive functions is one of science's grand challenges. The technical challenges that we face as we attempt to gain a systems-level understanding of the brain are manifold. The brain's structural complexity requires us to push the limit of imaging resolution and depth, while being able to cover large areas, resulting in enormous data acquisition and processing needs. Furthermore, it is necessary to detect functional activities and 'map' them onto the structural features. The functional activity occurs at multiple levels, using electrical and chemical signals. Certain electrical signals are only decipherable with sub-millisecond timescale resolution, while other modes of signals occur in minutes to hours. For these reasons, there is a wide consensus that new tools are necessary to undertake this daunting task. Optical techniques, due to their versatile and scalable nature, have great potentials to answer these challenges. Optical microscopy can now image beyond the diffraction limit, record multiple types of brain activity, and trace structural features across large areas of tissue. Genetically encoded molecular tools opened doors to controlling and detecting neural activity using light in specific cell types within the intact brain. Novel sample preparation methods that reduce light scattering have been developed, allowing whole brain imaging in rodent models. Adaptive optical methods have the potential to resolve images from deep brain regions. In this roadmap article, we showcase a few major advances in this area, survey the current challenges, and identify potential future needs that may be used as a guideline for the next steps to be taken.

Feedback-based wavefront shaping.

Light scattering was thought to be the fundamental limitation for the depth at which optical imaging methods can retain their resolution and sensitivity. However, it was shown that light can be focused inside even the most strongly scattering objects by spatially shaping the wavefront of the incident light. This review summarizes recently developed feedback-based approaches for focusing light inside and through scattering objects.

Relation between speckle decorrelation and optical phase conjugation (OPC)-based turbidity suppression through dynamic scattering media: a study on in vivo mouse skin.

Light scattering in biological tissue significantly limits the accessible depth for localized optical interrogation and deep-tissue optical imaging. This challenge can be overcome by exploiting the time-reversal property of optical phase conjugation (OPC) to reverse multiple scattering events or suppress turbidity. However, in living tissue, scatterers are highly movable and the movement can disrupt time-reversal symmetry when there is a latency in the OPC playback. In this paper, we show that the motion-induced degradation of the OPC turbidity-suppression effect through a dynamic scattering medium shares the same decorrelation time constant as that determined from speckle intensity autocorrelation - a popular conventional measure of scatterer movement. We investigated this decorrelation characteristic time through a 1.5-mm-thick dorsal skin flap of a living mouse and found that it ranges from 50 ms to 2.5 s depending on the level of immobilization. This study provides information on relevant time scales for applying OPC to living tissues.

Physical key-protected one-time pad.

We describe an encrypted communication principle that forms a secure link between two parties without electronically saving either of their keys. Instead, random cryptographic bits are kept safe within the unique mesoscopic randomness of two volumetric scattering materials. We demonstrate how a shared set of patterned optical probes can generate 10 gigabits of statistically verified randomness between a pair of unique 2 mm(3) scattering objects. This shared randomness is used to facilitate information-theoretically secure communication following a modified one-time pad protocol. Benefits of volumetric physical storage over electronic memory include the inability to probe, duplicate or selectively reset any bits without fundamentally altering the entire key space. Our ability to securely couple the randomness contained within two unique physical objects can extend to strengthen hardware required by a variety of cryptographic protocols, which is currently a critically weak link in the security pipeline of our increasingly mobile communication culture.

Digital optical phase conjugation of fluorescence in turbid tissue.

We demonstrate a method for phase conjugating fluorescence. Our method, called reference free digital optical phase conjugation, can conjugate extremely weak, incoherent optical signals. It was used to phase conjugate fluorescent light originating from a bead covered with 0.5 mm of light-scattering tissue. The phase conjugated beam refocuses onto the bead and causes a local increase of over two orders of magnitude in the light intensity. Potential applications are in imaging, optical trapping, and targeted photochemical activation inside turbid tissue.

Turbidity suppression from the ballistic to the diffusive regime in biological tissues using optical phase conjugation.

We describe the amplitude and resolution trends of the signals acquired by turbidity suppression through optical phase conjugation (TSOPC) with samples that span the ballistic and diffusive scattering regimes. In these experiments, the light field scattered through a turbid material is written into a hologram, and a time-reversed copy of the light field is played back through the sample. In this manner, the wavefront originally incident on the sample is reconstructed. We examine a range of scattering samples including chicken breast tissue sections of increasing thickness and polyacrylamide tissue-mimicking phantoms with increasing scattering coefficients. Our results indicate that only a small portion of the scattered wavefront (<0.02%) must be collected to reconstruct a TSOPC signal. Provided the sample is highly scattering, all essential angular information is contained within such small portions of the scattered wavefront due to randomization by scattering. A model is fitted to our results, describing the dependence of the TSOPC signal on other measurable values within the system and shedding light on the efficiency of the phase conjugation process. Our results describe the highest level of scattering that has been phase conjugated in biological tissues to date.

Scattered light fluorescence microscopy: imaging through turbid layers.

A major limitation of any type of microscope is the penetration depth in turbid tissue. Here, we demonstrate a fundamentally novel kind of fluorescence microscope that images through optically thick turbid layers. The microscope uses scattered light, rather than light propagating along a straight path, for imaging with subwavelength resolution. Our method uses constructive interference to focus scattered laser light through the turbid layer. Microscopic fluorescent structures behind the layer were imaged by raster scanning the focus.