Measuring nanoparticles shape by structured illumination.

Exploiting the size and shape of nanoparticles is critical for engineering the optical and mechanical properties of nanoparticle systems that are ubiquitous in everyday life. However, accurate determination of nanoparticle morphology usually requires elaborated methods such as XRD or TEM, which are not suitable for non-invasive and rapid control. Dynamic light scattering on the other hand, relies on the motion of nanoparticles and mixes different rotational and translational diffusion coefficients to infer synthetic information about the shape in terms of effective hydrodynamic characteristics. Here, we introduce a new scattering approach for measuring shape. We demonstrate analytically, numerically, and experimentally that the contrast of low-intensity fluctuations arising from the scattering of classically entangled optical fields allows determining the polarimetric anisotropy of nanoparticles. By leveraging the active variation of illumination structuring, we control the non-Gaussian statistics of the measured fluctuations, which, in turn, provides means to improve the measurement sensitivity. This technique offers practical opportunities for applications ranging from molecular chemistry to drug delivery to nanostructures synthesis where the real-time, quantitative assessment of nanoparticles shapes is indispensable.

Propagation of asymmetric optical vortex beams through turbulence and evolution of their OAM spectra.

In the realm of wave propagation through turbulent media, the spectrum of the orbital angular momentum of optical vortex beams is known to undergo symmetric broadening. However, the evolution of beams that are initially azimuthally asymmetric represents a distinct phenomenon. In this work, we have developed an analytical model describing the propagation of asymmetric OAM beams through the so-called Kolmogorov turbulence. Our results describe how the perturbation strength and the initial beam properties lead to a nonsymmetric spectrum of OAM modes. These findings lay the groundwork for further use of asymmetric fields that propagate in inhomogeneous media and their applications such as communications and sensing.

Self-referenced single-shot low-power Stokes polarimetry.

We demonstrate a Stokes polarimeter that not only preserves the power of the light to be analyzed but also requires only a single measurement. The novel design relies on the distinctive characteristics of a corner-cube retroreflector. It is simple and robust, and it circumvents the need for a local oscillator or a controllable reference beam.

First-order statistics of intensity and phase in Laguerre-Gauss speckles.

Laguerre-Gaussian (LG) beams are characterized by an azimuthal index or topological charge (m), associated with the orbital angular momentum, and by a radial index (p), which represents the number of the rings in the intensity distribution. We present a detailed, systematic study of the first-order phase statistics of the speckle fields created when LG beams of different order interact with random phase screens with different optical roughness. The phase properties of the LG speckle fields are studied in both the Fresnel and the Fraunhofer regimes using the equiprobability density ellipse formalism such that analytical expressions can be derived for the phase statistics.

Simultaneous Spatiotemporal Measurement of Structural Evolution in Dynamic Complex Media.

When the properties of soft materials evolve in time, the simultaneous measurement of different characteristics is critical. Here, we demonstrate an experimental system that permits monitoring both the spatial and temporal evolution of the optical and mechanical properties. An integrated fiber-optic-based system allows determining the mechanical vibrations of structural elements over 5 orders of magnitude and over a broad frequency range. At the same time, the optical properties can be obtained within seconds from high-resolution measurements of the path-length distribution of reflected light. With proper cyclical scanning, the temporal evolution of the mesoscopic light scattering properties can be obtained in a depth-resolved manner. The performance of this integrated measurement is validated in the particular case of drying paint films. For these typical nonstationary media, we show how our approach provides unique access to the spatiotemporal material properties and how this information permits identifying the specific stages of structural evolution.

Phase memory of optical vortex beams.

Optical vortex beams are under considerable scrutiny due to their demonstrated potential for applications ranging from quantum optics to optical communications and from material processing to particle trapping. However, upon interaction with inhomogeneous material systems, their deterministic properties are altered. The way these structured beams are affected by different levels of disturbances is critical for their uses. Here, for the first time, we quantify the degradation of perfect optical vortex beams after their interaction with localized random media. We developed an analytical model that (1) describes how the spatial correlation and the phase variance of disturbance affect the phase distribution across the vortex beams and (2) establishes the regimes of randomness for which the beams maintain the memory of their initial vorticity. Systematic numerical simulations and controlled experiments demonstrate the extent of this memory effect for beams with different vorticity indices.

Computational Investigation of the Ordered Water System Around Microtubules: Implications for Protein Interactions.

The existence of an exclusion zone in which particles of a colloidal suspension in water are repelled from hydrophilic surfaces has been experimentally demonstrated in numerous studies, especially in the case of Nafion surfaces. Various explanations have been proposed for the origin of this phenomenon, which is not completely understood yet. In particular, the existence of a fourth phase of water has been proposed by G. Pollack and if this theory is proven correct, its implications on our understanding of the properties of water, especially in biological systems, would be profound and could give rise to new medical therapies. Here, a simple approach based on the linearized Poisson-Boltzmann equation is developed in order to study the repulsive forces mediated by ordered water and involving the following interacting biomolecules: 1) microtubule and a tubulin dimer, 2) two tubulin dimers and 3) a tubulin sheet and a tubulin dimer. The choice of microtubules in this study is motivated because they could be a good candidate for the generation of an exclusion zone in the cell and these models could be a starting point for detailed experimental investigations of this phenomenon.

Passive high-frequency microrheology of blood.

Indicative of various pathologies, blood properties are under intense scrutiny. The hemorheological characteristics are traditionally gauged by bulk, low-frequency indicators that average out critical information about the complex, multi-scale, and multi-component structure. In particular, one cannot discriminate between the erythrocytes contribution to global rheology and the impact of plasma. Nevertheless, in their fast stochastic movement, before they encounter each other, the erythrocytes probe the subtle viscoelasticity of their protein-rich environment. Thus, if these short time scales can be resolved experimentally, the plasma properties could be determined without having to separate the blood components; the blood is practically testing itself. This microrheological description of blood plasma provides a direct link between the composition of whole blood and its coagulability status. We present a parametric model for the viscoelasticity of plasma, which is probed by the erythrocytes over frequency ranges of kilohertz in a picoliter-sized volume. The model is validated both in vitro, using artificial hemo-systems where the composition is controlled, as well as on whole blood where continuous measurements provide real-time information. We also discuss the possibility of using this passive microrheology as an in vivo assay for clinically relevant situations where the blood clotting condition must be observed and managed continuously for diagnosis or during therapeutic procedures at different stages of hemostatic and thrombotic processes.

Continuous Optical Measurement of Internal Dynamics in Drying Colloidal Droplets.

Accessing the colloidal dynamics non-invasively and continuously during the phase transition of a colloidal system is challenging but critical. Here, we demonstrate the use of spatiotemporal coherence-gated light scattering for studying the internal dynamics of drying colloidal droplets. The continuously acquired signal originates from a picoliter-sized volume located at the droplet-substrate interface. The measurement is non-contact, non-invasive, and label free and permits real-time observations of both optical and mechanical changes in the measurement volume. Additionally, some macroscopic descriptors of the drying process can be constructed from the microscopic measurement, providing ample information of the process. Implemented with an endoscopic-like probe, this system can be easily incorporated into the existing drop profiling instruments, which is potential for the full characterization of dynamic colloidal droplets.

3D intensity correlations in random fields created by vortex structured beams.

We develop an analytical model for the 3D spatial coherence function of speckle fields generated by scattering of vortex and perfect optical vortex beams. The model is general and describes the spatial coherence along both the transversal and the longitudinal directions. We found that, on propagation, the 3D spatial coherence evolves differently for the different types of initially structured beams, which may affect their use in a variety of sensing applications.

Colloidal density control with Bessel-Gauss beams.

Optical manipulation of colloidal systems is of high interest for both fundamental studies and practical applications. It has been shown that optically induced thermophoresis and nonlinear interactions can significantly affect the properties of dense colloidal media. However, macroscopic scale phenomena can also be generated at thermal equilibrium. Here, we demonstrate that steady-state variations of particle density can be created over large, three-dimensional regions by appropriately structured external optical fields. We prove analytically and experimentally that an optical vortex beam can dynamically control the spatial density of microscopic particles along the direction of its propagation. We show that these artificial steady-states can be generated at will and can be maintained indefinitely, which can be beneficial for applications such as path clearing and mass transportation.

Single-shot omnidirectional Stokes polarimetry.

Many active sensing applications benefit from measuring, as fast as possible, the polarization state of target reflections. Traditional polarimetry, however, relies on (1) the assumption of field transversality and (2) a given direction of wave propagation. When this is not known, one must regard the field as being three-dimensional, which inherently complicates the polarimetry due to experimental constraints imposed by the planar geometry of detector arrays. We demonstrate a single-shot, Stokes polarimetry approach that alleviates these limitations. The approach is based on the spatial Fourier analysis of the interference between the unknown wave and controlled reference fields.

Nonstationary Intensity Statistics in Diffusive Waves.

It is a long-standing belief that, in the diffusion regime, the intensity statistics is always stationary and its probability distribution follows a negative exponential decay. Here, we demonstrate that, in fact, in reflection from strong disordered media, the intensity statistics changes through different stages of the diffusion. We present a statistical model that describes this nonstationary property and takes into account the evolving balance between recurrent scattering and near field coupling. The predictions are further verified by systematic experiments in the optical regime. This statistical nonstationary is akin to the nonequilibrium but steady-state diffusion of particulate systems.

Vector wave simulation of active imaging through random media.

When a target is embedded in random media, the quality of optical imaging can be improved by actively controlling the illumination and exploiting vector wave properties. A rigorous description, however, requires expensive computational resources to fully account for the electromagnetic boundary conditions. Here, we introduce a statistically equivalent scaling model that allows for reducing the complexity of the problem. The new scheme describes the entanglement between the local wave vector and the polarization state in random media and also accounts for cumulative properties such as geometric phase. The approach is validated for different scenarios where the coherent background noise alters substantially the performance of active imaging.

First-order statistics of the phase in optical vortex speckles.

We present the theoretical analysis of first-order statistics of the phase in a far-field speckle field, which originates from an optical vortex passing through a random phase screen. By using the concept of the equiprobability density ellipse, we show that the standard deviation of the phase in a speckle field varies non-monotonically in the radial direction and, more interestingly, it exhibits a minimum at a certain radial position determined by the topological charge. In the limit of zero topological charge, the phase statistics naturally converges to the expectation corresponding to the incident Gaussian beam.

Discriminating randomly polarized fields.

We introduce the scalar average similarity of an ensemble of randomly polarized states. This global measure is based on the complex degree of mutual polarization between any pair of vector fields in the ensemble. We show that, in the case of fully correlated and globally unpolarized fields, the variation of this parameter is bounded, and its value can effectively discriminate between different configurations of pure states.

Tubulin Polarizability in Aqueous Suspensions.

We report accurate optical measurements of tubulin polarizability in aqueous suspensions. We determined the dependence of polarizability on tubulin concentration and on the suspension's pH, providing benchmark numbers for quantifying the optical response of this protein in various artificial and cellular environments. We compare our measurement data with a few estimates found in the previous literature and also with our simplified model estimations.

Polarization-encoded field measurement in subwavelength scattering.

We demonstrate that polarization encoding provides a convenient way to realize a robust common-path interferometer for measuring both the phase and the amplitude of scattered optical fields. Moreover, for a given detector array, the design allows maximizing the interferometric visibility and, therefore, permits reaching the sensitivity limit for the field measurement. The approach is of particular interest for inefficient scattering scenarios such as subwavelength scattering.

Spatial intensity correlations of a vortex beam and a perfect optical vortex beam.

We present a model based on the Fresnel diffraction scheme for the spatial coherence function of random fields created by scattering optical vortex and perfect vortex beams. By using the spatial coherence function we showed analytically, numerically, and experimentally the dependence and independence of the speckle size of an optical vortex and perfect optical vortex (POV) with a topological charge, respectively. We also showed in both cases the linear dependence of speckle size on the distance of propagation. Furthermore, we describe a regime in which the spatial coherence function is nonevolving for the optical vortex beam and the POV beam with the propagation distance.

Behavior of α, ß tubulin in DMSO-containing electrolytes.

α, ß-tubulin is a cytoskeletal protein that forms cylindrical structures termed microtubules, which are crucial to the cell for a variety of roles. Microtubules are frequently modelled as one-dimensional bionanowires that act as ion transporters in the cell. In this work, we used dynamic light scattering (DLS) to measure the hydrodynamic diameter of tubulin in the presence of a polar aprotic co-solvent. We found that the hydrodynamic diameter increased with increasing DMSO volume fraction, almost doubling at 20% DMSO. To evaluate if this was due to an enlarged solvation shell, we performed reference interaction site model (RISM) simulations and found that the extent of solvation was unchanged. Using fluorescence microscopy, we then showed that tubulin was polymerization competent in the presence of colchicine, and thus inferred the presence of oligomers in the presence of DMSO, which points to its mechanism of action as a microtubule polymerization enhancing agent. Tubulin oligomers are known to form when microtubules depolymerize and are controversially implicated in microtubule polymerization. We show that DLS may be used to monitor early-state microtubule polymerization and is a viable alternative to fluorescence and electron microscopy-based methods. Our findings showing that DMSO causes tubulin oligomerization are thus of critical importance, both for creating bio-inspired nanotechnology and determining its biophysical roles in the cell.