Exploiting volumetric wave correlation for enhanced depth imaging in scattering medium.

Imaging an object embedded within a scattering medium requires the correction of complex sample-induced wave distortions. Existing approaches have been designed to resolve them by optimizing signal waves recorded in each 2D image. Here, we present a volumetric image reconstruction framework that merges two fundamental degrees of freedom, the wavelength and propagation angles of light waves, based on the object momentum conservation principle. On this basis, we propose methods for exploiting the correlation of signal waves from volumetric images to better cope with multiple scattering. By constructing experimental systems scanning both wavelength and illumination angle of the light source, we demonstrated a 32-fold increase in the use of signal waves compared with that of existing 2D-based approaches and achieved ultrahigh volumetric resolution (lateral resolution: 0.41 [Formula: see text], axial resolution: 0.60 [Formula: see text]) even within complex scattering medium owing to the optimal coherent use of the broad spectral bandwidth (225 nm).

Through-skull brain imaging in vivo at visible wavelengths via dimensionality reduction adaptive-optical microscopy.

Compensation of sample-induced optical aberrations is crucial for visualizing microscopic structures deep within biological tissues. However, strong multiple scattering poses a fundamental limitation for identifying and correcting the tissue-induced aberrations. Here, we introduce a label-free deep-tissue imaging technique termed dimensionality reduction adaptive-optical microscopy (DReAM) to selectively attenuate multiple scattering. We established a theoretical framework in which dimensionality reduction of a time-gated reflection matrix can attenuate uncorrelated multiple scattering while retaining a single-scattering signal with a strong wave correlation, irrespective of sample-induced aberrations. We performed mouse brain imaging in vivo through the intact skull with the probe beam at visible wavelengths. Despite the strong scattering and aberrations, DReAM offered a 17-fold enhancement of single scattering-to-multiple scattering ratio and provided high-contrast images of neural fibers in the brain cortex with the diffraction-limited spatial resolution of 412 nanometers and a 33-fold enhanced Strehl ratio.

Optical transfer function of time-gated coherent imaging in the presence of a scattering medium.

Optical imaging of objects embedded within scattering media such as biological tissues suffers from the loss of resolving power. In our previous work, we proposed an approach called collective accumulation of single scattering (CASS) microscopy that attenuates this detrimental effect of multiple light scattering by combining the time-gated detection and spatial input-output correlation. In the present work, we perform a rigorous theoretical analysis on the effect of multiple light scattering to the optical transfer function of CASS microscopy. In particular, the spatial frequency-dependent signal to noise ratio (SNR) is derived depending on the intensity ratio of the single- and multiple-scattered waves. This allows us to determine the depth-dependent resolving power. We conducted experiments using a Siemens star-like target having various spatial frequency components and supported the theoretical derived SNR spectra. Our study provides a theoretical framework for understanding the effect of multiple light scattering in high-resolution and deep-tissue optical imaging.

Time-gated iterative phase conjugation for efficient light energy delivery in scattering media.

Light waves propagating through complex biological tissues are spatially spread by multiple light scattering, and this spread limits the working depth in optical bioimaging, phototherapy, and optogenetics. Here, we propose the iterative phase conjugation of time-gated backscattered waves for enhancing the light energy delivered to a target object embedded in a scattering medium. We demonstrate the enhancement of light energy delivered to a target object hidden behind a 200-µm-thick mouse skull by more than ten times in comparison with the initial random input. The maximum enhancement was reached in only 10 iterations, more than a hundred times smaller than existing methods based on either a time-gated reflection matrix or iterative feedback optimization of the time-gated reflection intensity. Consequently, the proposed method is less sensitive to sample perturbations. Furthermore, the number of images required for optimization remained almost unchanged with an increase in the illumination area, unlike existing methods, where the convergence time scales with the illumination area. The proposed method provides high operation speed over a wide illumination area, which can facilitate the use of wavefront shaping in practical applications.

Iterative optimization of time-gated reflectance for the efficient light energy delivery within scattering media.

In complex media, light waves are diffused both in space and time due to multiple light scattering, and its intensity is attenuated with the increase of propagation depth. In this paper, we propose an iterative wavefront shaping method for enhancing time-gated reflection intensity, which leads to efficient light energy delivery to a target object embedded in a highly scattering medium. We achieved an over 10 times enhancement of reflectance at the specific flight time and demonstrated the focusing of light energy to the target object. Since the proposed method does not require reflection matrix measurement, it will be highly suited to samples in mechanically dynamic conditions.

Analytic framework for understanding the competing multiple light scattering processes.

In many complex physical phenomena such as wave propagation in scattering media, the process of interest often cannot be easily distinguished from other processes because only the total combined process is accessible. This makes it difficult to extract the precise knowledge of each subprocess. Here, we derive an analytic expression describing the way the eigenchannel coupling of the total process distributes its energy to the individual subprocesses, with only partial information on each subprocess such as the average eigenvalue 〈τ〉 and enhancement factor η. We found that the ratio of (η - 1)〈τ〉 between two subprocesses is a critical parameter determining the preferable subprocess in the energy coupling. This work provides a new analytic framework for understanding the effect of wavefront shaping in the control of wave propagation in disordered media.

Itinerant ferromagnetism in a Fermi gas of ultracold atoms.

Can a gas of spin-up and spin-down fermions become ferromagnetic because of repulsive interactions? We addressed this question, for which there is not yet a definitive theoretical answer, in an experiment with an ultracold two-component Fermi gas. The observation of nonmonotonic behavior of lifetime, kinetic energy, and size for increasing repulsive interactions provides strong evidence for a phase transition to a ferromagnetic state. Our observations imply that itinerant ferromagnetism of delocalized fermions is possible without lattice and band structure, and our data validate the most basic model for ferromagnetism introduced by Stoner.