Showing papers on "Light scattering published in 2016"

Dynamic light scattering: a practical guide and applications in biomedical sciences.

Abstract: Dynamic light scattering (DLS), also known as photon correlation spectroscopy (PCS), is a very powerful tool for studying the diffusion behaviour of macromolecules in solution. The diffusion coefficient, and hence the hydrodynamic radii calculated from it, depends on the size and shape of macromolecules. In this review, we provide evidence of the usefulness of DLS to study the homogeneity of proteins, nucleic acids, and complexes of protein–protein or protein–nucleic acid preparations, as well as to study protein–small molecule interactions. Further, we provide examples of DLS’s application both as a complementary method to analytical ultracentrifugation studies and as a screening tool to validate solution scattering models using determined hydrodynamic radii.

Underwater Depth Estimation and Image Restoration Based on Single Images

Abstract: In underwater environments, the scattering and absorption phenomena affect the propagation of light, degrading the quality of captured images. In this work, the authors present a method based on a physical model of light propagation that takes into account the most significant effects to image degradation: absorption, scattering, and backscattering. The proposed method uses statistical priors to restore the visual quality of the images acquired in typical underwater scenarios.

Polarized light interaction with tissues

Abstract: This tutorial-review introduces the fundamentals of polarized light interaction with biological tissues and presents some of the recent key polarization optical methods that have made possible the quantitative studies essential for biomedical diagnostics. Tissue structures and the corresponding models showing linear and circular birefringence, dichroism, and chirality are analyzed. As the basis for a quantitative description of the interaction of polarized light with tissues, the theory of polarization transfer in a random medium is used. This theory employs the modified transfer equation for Stokes parameters to predict the polarization properties of single- and multiple-scattered optical fields. The near-order of scatterers in tissues is accounted for to provide an adequate description of tissue polarization properties. Biomedical diagnostic techniques based on polarized light detection, including polarization imaging and spectroscopy, amplitude and intensity light scattering matrix measurements, and polarization-sensitive optical coherence tomography are described. Examples of biomedical applications of these techniques for early diagnostics of cataracts, detection of precancer, and prediction of skin disease are presented. The substantial reduction of light scattering multiplicity at tissue optical clearing that leads to a lesser influence of scattering on the measured intrinsic polarization properties of the tissue and allows for more precise quantification of these properties is demonstrated.

Triple-Resonant Brillouin Light Scattering in Magneto-Optical Cavities.

Abstract: An enhancement in Brillouin light scattering of optical photons with magnons is demonstrated in magneto-optical whispering gallery mode resonators tuned to a triple-resonance point. This occurs when both the input and output optical modes are resonant with those of the whispering gallery resonator, with a separation given by the ferromagnetic resonance frequency. The identification and excitation of specific optical modes allows us to gain a clear understanding of the mode-matching conditions. A selection rule due to wave vector matching leads to an intrinsic single-sideband excitation. Strong suppression of one sideband is essential for one-to-one frequency mapping in coherent optical-to-microwave conversion.

Optical theorem and multipole scattering of light by arbitrarily shaped nanoparticles

Abstract: The application of Cartesian multipoles in irreducible representations provides the possibility to get explicit contributions of the toroidal multipole terms in the extinction and scattering power without the introduction of special form factors. In the framework of the Cartesian multipoles, we obtained multipole decomposition (up to the third order) of the induced polarization (current) inside an arbitrarily shaped scatterer (nanoparticle). The third-order decomposition includes the toroidal dipole, magnetic quadrupole, electric octupole terms, and also nonradiating terms. The corresponding multipole decomposition of the scattering cross section, taking into account the electric octupole term, is derived and compared with the multipole decomposition of the extinction cross section obtained using the optical theorem. We show that the role of multipoles in the optical theorem (light extinction) and scattering by arbitrarily shaped nanoparticles can be different. This can result in seemingly paradoxical conclusions with respect to the appearance of multipole contributions in the scattering and extinction cross sections. This fact is especially important for absorptionless nanoparticles, for which the scattering cross section can be calculated using the optical theorem, because in this case extinction is solely determined by scattering. Demonstrative results concerning the role of third-order multipoles in the resonant optical response of high-refractive-index dielectric nanodisks, with and without a through hole at the center, are presented. It is shown that the optical theorem results in a negligible role of the third-order multipoles in the extinction cross sections, whereas these multipoles provide the main contribution in the scattering cross sections.

Plasmonic nanoparticle-semiconductor composites for efficient solar water splitting

Abstract: Photoelectrochemical (PEC) water splitting is a promising technology that uses light absorbing semiconductors to convert solar energy directly into a chemical fuel (i.e., hydrogen). PEC water splitting has the potential to become a key technology in achieving a sustainable society, if high solar to fuel energy conversion efficiencies are obtained with earth abundant materials. This review article discusses recent developments and discoveries in the mechanisms by which the localized surface plasmon resonance (LSPR) in metallic nanoparticles can increase or complement a neighbouring semiconductor in light absorption for catalytic water splitting applications. These mechanisms can mitigate the intrinsic optical limitations of semiconductors (e.g., metal oxides) for efficient solar water splitting. We identify four types of enhancement mechanisms in the recent literature: (i) light scattering, (ii) light concentration, (iii) hot electron injection (HEI), and (iv) plasmon-induced resonance energy transfer (PIRET). (i) Light scattering and (ii) light concentration are light trapping mechanisms that can increase the absorption of light with energies above the semiconductor optical band-edge. These two mechanisms are ideal to enhance the absorption of promising semiconductors with narrow bandgap energies that suffer from limited absorption coefficients and bulk charge recombination. On the other hand, (iii) HEI and the recently discovered (iv) PIRET are mechanisms that can enhance the absorption also below the semiconductor optical band-edge. Therefore, HEI and PIRET have the potential to extend the light utilization to visible and near-infrared wavelengths of semiconductors with excellent electrochemical properties, but with large bandgap energies. New techniques and theories that have been developed to elucidate the above mentioned plasmonic mechanisms are presented and discussed for their application in metal oxide photoelectrodes. Finally, other plasmonic and non-plasmonic effects that do not increase the device absorption, but affect the electrochemical properties of the semiconductor (e.g., charge carrier transport) are also discussed, since a complete understanding of these phenomena is fundamental for the design of an efficient plasmonic NP-semiconductor water splitting device.

Preparing monodisperse macromolecular samples for successful biological small-angle X-ray and neutron-scattering experiments.

Abstract: Small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) are techniques used to extract structural parameters and determine the overall structures and shapes of biological macromolecules, complexes and assemblies in solution. The scattering intensities measured from a sample contain contributions from all atoms within the illuminated sample volume, including the solvent and buffer components, as well as the macromolecules of interest. To obtain structural information, it is essential to prepare an exactly matched solvent blank so that background scattering contributions can be accurately subtracted from the sample scattering to obtain the net scattering from the macromolecules in the sample. In addition, sample heterogeneity caused by contaminants, aggregates, mismatched solvents, radiation damage or other factors can severely influence and complicate data analysis, so it is essential that the samples be pure and monodisperse for the duration of the experiment. This protocol outlines the basic physics of SAXS and SANS, and it reveals how the underlying conceptual principles of the techniques ultimately 'translate' into practical laboratory guidance for the production of samples of sufficiently high quality for scattering experiments. The procedure describes how to prepare and characterize protein and nucleic acid samples for both SAXS and SANS using gel electrophoresis, size-exclusion chromatography (SEC) and light scattering. Also included are procedures that are specific to X-rays (in-line SEC-SAXS) and neutrons, specifically preparing samples for contrast matching or variation experiments and deuterium labeling of proteins.

Application of polarization effects in light scattering: A new

Abstract: We demonstrate that a newly developed in- strument which measures all polarization and intensity infor- mation contained in differentially and elastically scattered light has valuable applications in biology. The polarization states of light scattered differentially from suspensions of bi- ological scatterers are shown to contain structural informa- tion about those systems. The scatterers are discussed in the context of a 16 component matrix which completely charac- terizes the scattering process. The instrument and method are described in terms of the corresponding matrix algebra. We also discuss the use of the instrument as a device for dis- tinguishing between closely related structural systems and as a tool for following time-dependent structural changes. A technique has recently been developed (1) which can measure all possible information contained in the light dif- ferentially and elastically scattered from a suspension of small particles. We used the technique to examine the polar- ization states of light scattered from suspensions of bacterial spores and showed that it will have valuable and powerful applications in biology. Initial results show that of 10 possi- ble scattering parameters, one in particular is extremely valuable as an indicator of structural changes in biological systems. This parameter can distinguish between two mu- tant varieties of bacterial spores more easily than the differ- ential scattered light intensity does. Light scattering is widely used in biological research to determine particle numbers, particle sizes, axial ratios, size distributions, particle mobilities, and indices of refraction. Most of these studies measure only the small-angle differen- tial scattered light intensity even though much more addi- tional information is contained in the polarization states of the differentially scattered light. Previous studies of polar- ization effects have been restricted mostly to optical rotatory dispersion (ORD) and circular dichroism (CD) measure- ments in the forward direction, which do not contain the

Seeing cells in a new light: a renaissance of Brillouin spectroscopy

Abstract: Over the years, light scattering from acoustic waves has grown to be increasingly important in the fields of biology and medicine. This type of scattering, known as Brillouin scattering, has already seen a plethora of applications in fields such as physics. However, the potential for Brillouin scattering for medical imaging and diagnostics has only recently been considered. In this work, we summarize most of the applications of Brillouin scattering in biology to date, and some current work in our lab showing how Brillouin scattering is a worthy prospect for many emerging problems in biology and medical diagnostics.

Giant in-particle field concentration and Fano resonances at light scattering by high-refractive-index particles

Abstract: We present the results of a detailed analytical study of light scattering by a particle with high refractive index $m+i\ensuremath{\kappa}$ and low losses ($m\ensuremath{\gg}1,\phantom{\rule{0.28em}{0ex}}0l\ensuremath{\kappa}\ensuremath{\ll}1$) based on the exact Mie solution. We show that there is a dramatic difference in the behavior of the electromagnetic field within the particle (inner problem) and outside it (outer problem). With an increase in $m$ at fixed values of the other parameters, the field within the particle asymptotically converges to a periodic function of $m$. The electric and magnetic type Mie resonances of different orders overlap substantially. It may lead to a giant concentration of the electromagnetic energy within the particle. At the same time, we demonstrate that the solution for the outer problem makes it possible to present each partial scattered wave as a sum of two partitions. One of them corresponds to the $m$-independent wave, scattered by a perfectly reflecting particle and plays the role of a background, while the other is associated with the excitation of a sharply $m$-dependent resonant Mie mode. The interference of the partitions brings about a typical asymmetric Fano profile. The profile is obtained from the exact Mie solution by means of identical transformations without any additional assumptions and/or fitting. It makes it possible to generalize rigorously the Fano theory to the case of finite dissipation. At an increase in $m$ the Fano resonances in the outer problem die out and the scattered field converges to the universal, $m$-independent profile. The behavior of the resonances at a fixed $m$ and varying particle size parameter $(x)$ is also discussed in detail. The similarities and differences of the two cases (fixed $x$, varying $m$ and fixed $m$, varying $x$) are disclosed. We also show that under certain very general conditions the scattering cross section of a large lossy sphere cannot be smaller than half its geometric cross section, while its absorption cross section cannot exceed three halves of the geometric one. Numerical estimates of most discussed effects for a gallium phosphide particle irradiated by the second harmonic of a Nd:YAG laser are presented as an example. In addition to purely academic interest, the obtained results may be employed to design new highly nonlinear heterogenic nanostructures and other metamaterials.

Realistic Numerical and Analytical Modeling of Light Scattering in Brain Tissue for Optogenetic Applications

Abstract: In recent years, optogenetics has become a central tool in neuroscience research. Estimating the transmission of visible light through brain tissue is of crucial importance for controlling the activation levels of neurons in different depths, designing optical systems, and avoiding lesions from excessive power density. The Kubelka–Munk model and Monte Carlo simulations have previously been used to model light propagation through rodents' brain tissue, however, these prior attempts suffer from fundamental shortcomings. Here, we introduce and study two modified approaches for modeling the distributions of light emanating from a multimode fiber and scattering through tissue, using both realistic numerical Monte Carlo simulations and an analytical approach based on the beam-spread function approach. We demonstrate a good agreement of the new methods' predictions both with recently published data, and with new measurements in mouse brain cortical slices, where our results yield a new cortical scattering length estimate of ∼47 µ m at λ = 473 nm, significantly shorter than ordinarily assumed in optogenetic applications.

Unidirectional light scattering with high efficiency at optical frequencies based on low-loss dielectric nanoantennas

Abstract: Dielectric nanoparticles offer low optical losses and access to both electric and magnetic Mie resonances. This enables unidirectional scattering along the incident axis of light, owing to the interference between these two resonances. Here we theoretically and experimentally demonstrate that an asymmetric dimer of dielectric nanoparticles can provide unidirectional forward scattering with high efficiency. Theoretical analyses reveal that the dimer configuration can satisfy the first Kerker condition at the resonant peaks of electric and magnetic dipolar modes, therefore showing highly efficient directional forward scattering. The unidirectional forward scattering with high efficiency is confirmed in our experiments using a silicon nanodisk dimer on a transparent substrate. This study will boost the realization of practical applications using low-loss and efficient subwavelength all-dielectric nanoantennas.

Multi-band light perfect absorption by a metal layer-coupled dielectric metamaterial.

Abstract: Multispectral light perfect absorption is desired for many applications. Herein, we propose and demonstrate a novel multi-band light perfect absorber (MLPA) scheme based on a triple-layer dielectric meta-material structure coupled with a metal substrate. Four absorption bands with the maximal absorbance up to 98.9% and the narrow bandwidth down to 2 nm are achieved in the visible range. Optical cavity resonances and the plasmon-like dipolar resonance of the high-index dielectric resonators and their hybridization effects contribute to the observed absorption behaviors. Moreover, the obtained MLPA is with high scalability in the frequency range by tuning the structural parameters. These features pave a new and feasible way for multispectral light absorption and hold applications in the optoelectronic detection, filtering and imaging.

Precision of a data-driven estimate of hadronic light-by-light scattering in the muon $g-2$: Pseudoscalar-pole contribution

Abstract: Within a dispersive approach to hadronic light-by-light scattering in the muon g-2, the evaluation of the numerically dominant pseudoscalar-pole contribution involves the pseudoscalar-photon transition form factor FPγ*γ*(-Q12,-Q22) with P=π0,η,η′ and, in general, two off-shell photons with spacelike momenta Q1,22. We show that for π0(η,η′), the region of photon momenta below about 1(1.5) GeV gives the main contribution to hadronic light-by-light scattering. We then discuss how the precision of current and future measurements of the single- and double-virtual transition form factor in different momentum regions impacts the precision of a data-driven estimate of this contribution to hadronic light-by-light scattering. Based on Monte Carlo simulations for a planned first measurement of the double-virtual form factor at BESIII, we find that for the π0,η,η′-pole contributions a precision of 14%, 23%, 15% seems feasible. Further improvements can be expected from other experimental data and also from the use of dispersion relations for the different form factors themselves.

Plasmonic fiber-optic vector magnetometer

Abstract: A compact fiber-optic vector magnetometer based on directional scattering between polarized plasmon waves and ferro-magnetic nanoparticles is demonstrated. The sensor configuration reported in this work uses a short section of tilted fiber Bragg grating (TFBG) coated with a nanometer scale gold film and packaged with a magnetic fluid (Fe3O4) inside a capillary. The transmission spectrum of the sensor provides a fine comb of narrowband resonances that overlap with a broader absorption of the surface plasmonresonance (SPR). The wavelength of the SPR attenuation in transmission shows high sensitivity to slight perturbations by magnetic fields, due to the strong directional scattering between the SPR attenuated cladding modes and the magnetic fluid near the fiber surface. Both the orientation (2 nm/deg) and the intensity (1.8 nm/mT) of magnetic fields can be determined unambiguously from the TFBG spectrum. Temperature cross sensitivity can be referenced out by monitoring the wavelength of the core mode resonance simultaneously.

Deep transmission of Laguerre-Gaussian vortex beams through turbid scattering media.

Abstract: Light scattering and transmission of Gaussian (G) and Laguerre–Gaussian (LG) vortex beams with different orbital angular momentum (L) in various turbid media were investigated. Transmittance was measured with varied ratios of sample thickness (z) to scattering mean free path (ls) of turbid media, z/ls. In the ballistic region, the LG and G beams were found to have no significant difference on transmittance, while in the diffusive region, the LG beams showed a higher received signal than the G beams, and the LG beams with higher L values showed a higher received signal than those with lower L values. The transition points from ballistic to diffusive regions for different scattering media were determined. This newly observed transmittance difference of LG and G beams may be used for deep target detection in turbid media through LG beam imaging.

Light scattering from dense cold atomic media

Abstract: Light shifts in a cold atomic medium are studied using a microscopic coherent dipole model, a random walk model, and a semiclassical model that directly accounts for motional effects. However, even with the motion of the atoms considered, the current models still fall short of explaining the large density-dependent light shifts measured with ${}^{88}$Sr atoms.

Multimodal Dispersion of Nanoparticles: A Comprehensive Evaluation of Size Distribution with 9 Size Measurement Methods

Abstract: Evaluation of particle size distribution (PSD) of multimodal dispersion of nanoparticles is a difficult task due to inherent limitations of size measurement methods. The present work reports the evaluation of PSD of a dispersion of poly(isobutylcyanoacrylate) nanoparticles decorated with dextran known as multimodal and developed as nanomedecine. The nine methods used were classified as batch particle i.e. Static Light Scattering (SLS) and Dynamic Light Scattering (DLS), single particle i.e. Electron Microscopy (EM), Atomic Force Microscopy (AFM), Tunable Resistive Pulse Sensing (TRPS) and Nanoparticle Tracking Analysis (NTA) and separative particle i.e. Asymmetrical Flow Field-Flow Fractionation coupled with DLS (AsFlFFF) size measurement methods. The multimodal dispersion was identified using AFM, TRPS and NTA and results were consistent with those provided with the method based on a separation step prior to on-line size measurements. None of the light scattering batch methods could reveal the complexity of the PSD of the dispersion. Difference between PSD obtained from all size measurement methods tested suggested that study of the PSD of multimodal dispersion required to analyze samples by at least one of the single size particle measurement method or a method that uses a separation step prior PSD measurement.

Solving inverse scattering problems in biological samples by quantitative phase imaging

Abstract: Quantitative phase imaging (QPI), a method that precisely recovers the wavefront of an electromagnetic field scattered by a transparent, weakly scattering object, is a rapidly growing field of study. By solving the inverse scattering problem, the structure of the scattering object can be reconstructed from QPI data. In the past decade, 3D optical tomographic reconstruction methods based on QPI techniques to solve inverse scattering problems have made significant progress. In this review, we highlight a number of these advances and developments. In particular, we cover in depth Fourier transform light scattering (FTLS), optical diffraction tomography (ODT), and white-light diffraction tomography (WDT).

Light scattering by fractal dust aggregates. i. angular dependence of scattering

Abstract: In protoplanetary disks, micron-sized dust grains coagulate to form highly porous dust aggregates. Because the optical properties of these aggregates are not completely understood, it is important to investigate how porous dust aggregates scatter light. In this study, the light scattering properties of porous dust aggregates were calculated using a rigorous method, the T-matrix method, and the results were then compared with those obtained using the Rayleigh–Gans–Debye (RGD) theory and Mie theory with the effective medium approximation (EMT). The RGD theory is applicable to moderately large aggregates made of nearly transparent monomers. This study considered two types of porous dust aggregates—ballistic cluster–cluster agglomerates (BCCAs) and ballistic particle–cluster agglomerates. First, the angular dependence of the scattered intensity was shown to reflect the hierarchical structure of dust aggregates; the large-scale structure of the aggregates is responsible for the intensity at small scattering angles, and their small-scale structure determines the intensity at large scattering angles. Second, it was determined that the EMT underestimates the backward scattering intensity by multiple orders of magnitude, especially in BCCAs, because the EMT averages the structure within the size of the aggregates. It was concluded that the RGD theory is a very useful method for calculating the optical properties of BCCAs.

Focusing light through biological tissue and tissue-mimicking phantoms up to 9.6 cm in thickness with digital optical phase conjugation.

Abstract: Optical phase conjugation (OPC)-based wavefront shaping techniques focus light through or within scattering media, which is critically important for deep-tissue optical imaging, manipulation, and therapy. However, to date, the sample thickness in OPC experiments has been limited to only a few millimeters. Here, by using a laser with a long coherence length and an optimized digital OPC system that can safely deliver more light power, we focused 532-nm light through tissue-mimicking phantoms up to 9.6 cm thick, as well as through ex vivo chicken breast tissue up to 2.5 cm thick. Our results demonstrate that OPC can be achieved even when photons have experienced on average 1000 scattering events. The demonstrated penetration of nearly 10 cm (∼100 transport mean free paths) has never been achieved before by any optical focusing technique, and it shows the promise of OPC for deep-tissue noninvasive optical imaging, manipulation, and therapy.

Organic electroluminescent element

Abstract: This organic electroluminescent element, which is provided with a light scattering film obtained by curing a composition for light scattering film formation containing, for example, a triazine ring-containing polymer represented by formula (AA), a crosslinking agent and a light-diffusing agent, has excellent current efficiency

Cloud chamber experiments on the origin of ice crystal complexity in cirrus clouds

Abstract: . This study reports on the origin of small-scale ice crystal complexity and its influence on the angular light scattering properties of cirrus clouds. Cloud simulation experiments were conducted at the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) cloud chamber of the Karlsruhe Institute of Technology (KIT). A new experimental procedure was applied to grow and sublimate ice particles at defined super- and subsaturated ice conditions and for temperatures in the −40 to −60 °C range. The experiments were performed for ice clouds generated via homogeneous and heterogeneous initial nucleation. Small-scale ice crystal complexity was deduced from measurements of spatially resolved single particle light scattering patterns by the latest version of the Small Ice Detector (SID-3). It was found that a high crystal complexity dominates the microphysics of the simulated clouds and the degree of this complexity is dependent on the available water vapor during the crystal growth. Indications were found that the small-scale crystal complexity is influenced by unfrozen H2SO4 / H2O residuals in the case of homogeneous initial ice nucleation. Angular light scattering functions of the simulated ice clouds were measured by the two currently available airborne polar nephelometers: the polar nephelometer (PN) probe of Laboratoire de Meterologie et Physique (LaMP) and the Particle Habit Imaging and Polar Scattering (PHIPS-HALO) probe of KIT. The measured scattering functions are featureless and flat in the side and backward scattering directions. It was found that these functions have a rather low sensitivity to the small-scale crystal complexity for ice clouds that were grown under typical atmospheric conditions. These results have implications for the microphysical properties of cirrus clouds and for the radiative transfer through these clouds.

Focusing light through scattering media by full-polarization digital optical phase conjugation.

Abstract: Digital optical phase conjugation (DOPC) is an emerging technique for focusing light through or within scattering media such as biological tissue. Since DOPC systems are based on time reversal, they benefit from collecting as much information about the scattered light as possible. However, existing DOPC techniques record and subsequently phase-conjugate the scattered light in only a single-polarization state, limited by the operating principle of spatial light modulators. Here, we develop the first, to the best of our knowledge, full-polarization DOPC system that records and phase-conjugates scattered light along two orthogonal polarizations. When focusing light through thick scattering media, such as 2 mm and 4 mm-thick chicken breast tissue, our full-polarization DOPC system on average doubles the focal peak-to-background ratio achieved by single-polarization DOPC systems and improves the phase-conjugation fidelity.

Strong light scattering and broadband (UV to IR) photoabsorption in stretchable 3D hybrid architectures based on Aerographite decorated by ZnO nanocrystallites

Abstract: In present work, the nano- and microscale tetrapods from zinc oxide were integrated on the surface of Aerographite material (as backbone) in carbon-metal oxide hybrid hierarchical network via a simple and single step magnetron sputtering process. The fabricated hybrid networks are characterized for morphology, microstructural and optical properties. The cathodoluminescence investigations revealed interesting luminescence features related to carbon impurities and inherent host defects in zinc oxide. Because of the wide bandgap of zinc oxide and its intrinsic defects, the hybrid network absorbs light in the UV and visible regions, however, this broadband photoabsorption behavior extends to the infrared (IR) region due to the dependence of the optical properties of ZnO architectures upon size and shape of constituent nanostructures and their doping by carbon impurities. Such a phenomenon of broadband photoabsorption ranging from UV to IR for zinc oxide based hybrid materials is novel. Additionally, the fabricated network exhibits strong visible light scattering behavior. The developed Aerographite/nanocrystalline ZnO hybrid network materials, equipped with broadband photoabsorption and strong light scattering, are very promising candidates for optoelectronic technologies.

Stochastic methods for light propagation and recurrent scattering in saturated and nonsaturated atomic ensembles

Abstract: We derive equations for the strongly coupled system of light and dense atomic ensembles. The formalism includes an arbitrary internal-level structure for the atoms and is not restricted to weak excitation of atoms by light. In the low-light-intensity limit for atoms with a single electronic ground state, the full quantum field-theoretical representation of the model can be solved exactly by means of classical stochastic electrodynamics simulations for stationary atoms that represent cold atomic ensembles. Simulations for the optical response of atoms in a quantum degenerate regime require one to synthesize a stochastic ensemble of atomic positions that generates the corresponding quantum statistical position correlations between the atoms. In the case of multiple ground levels or at light intensities where saturation becomes important, the classical simulations require approximations that neglect quantum fluctuations between the levels. We show how the model is extended to incorporate corrections due to quantum fluctuations that result from virtual scattering processes. In the low-light-intensity limit, we illustrate the simulations in a system of atoms in a Mott-insulator state in a two-dimensional optical lattice, where recurrent scattering of light induces strong interatomic correlations. These correlations result in collective many-atom subradiant and superradiant states and a strong dependence of the response on the spatial confinement within the lattice sites

Exploiting Optical Asymmetry for Controlled Guiding of Particles with Light

Abstract: Conventional methods of manipulating particles with light, such as optical tweezers and optical tractor beams, rely on beam-shaping to realize complex electromagnetic field profiles and are thus sensitive to scattering. Here, we show that, by introducing tailored optical asymmetry in the particle, we can realize a novel guiding method that is controllable by the frequency of light, without regard to the direction or the shape of the light beam. With detailed stochastic simulations, we demonstrate guiding of a two-faced nanoparticle where the optically induced thermophoretic drift serves as the propulsion mechanism. Exploiting the difference in resonant absorption spectra of the two materials, we create a bidirectional local thermal gradient that is externally switchable. This is advantageous because the frequency of a light beam, unlike its shape or coherence, is preserved even in strongly scattering environments. Since this approach is insensitive to scattering and applicable to many particles at once, a...