Doppler encoded excitation pattern tomographic optical microscopy

Most far-field optical imaging systems rely on lenses and spatially resolved detection to probe distinct locations on the object. We describe and demonstrate a high-speed wide-field approach to imaging that instead measures the complex spatial Fourier transform of the object by detecting its spatially integrated response to dynamic acousto-optically synthesized structured illumination. Tomographic filtered backprojection is applied to reconstruct the object in two or three dimensions. This technique decouples depth of field and working distance from resolution, in contrast to conventional imaging, and can be used to image biological and synthetic structures in fluoresced or scattered light employing coherent or broadband illumination. We discuss the electronically programmable transfer function of the optical system and its implications for imaging dynamic processes. We also explore wide-field fluorescence imaging in scattering media by coherence gating. Finally, we present two-dimensional high-resolution tomographic image reconstructions in both scattered and fluoresced light demonstrating a thousandfold improvement in the depth of field compared to conventional lens-based microscopy.     

Variable-coherence tomography for inverse scattering problems

We propose a technique for determining the pair-correlation function of a quasi-homogeneous medium. The method uses the variation of the spatial-coherence properties of the incident beam to generate two separate volumes of coherence where the field is correlated. Using this specially prepared beam, we reconstruct experimentally the correlation function of a scattering potential by recording the scattered intensity in only one direction.     

Influence of multiple scattering on three-dimensional imaging with optical diffraction tomography

Optical diffraction tomography is an imaging technique that permits retrieval of the map of permittivity of an object from its scattered far field. Most reconstruction procedures assume that single scattering is dominant so that the scattered far field is linearly linked to the permittivity. In this work, we present a nonlinear inversion method and apply it to complex three-dimensional samples. We show that multiple scattering permits one to obtain a power of resolution beyond the classical limit imposed by the use of propagative incident and diffracted waves. Moreover, we stress that our imaging method is robust with respect to correlated and uncorrelated noise.     

Near-normal incidence dark-field microscopy: applications to nanoplasmonic spectroscopy

The spectroscopic characterization of individual nanostructures is of fundamental importance to understanding a broad range of physical and chemical processes. One general and powerful technique that addresses this aim is dark-field microscopy, with which the scattered light from an individual structure can be analyzed with minimal background noise. We present the spectroscopic analysis of individual plasmonic nanostructures using dark-field illumination with incidence nearly normal to the substrate. We show that, compared to large incidence angle approaches, the near-normal incidence approach provides significantly higher signal-to-background ratios and reduced retardation field effects. To demonstrate the utility of this technique, we characterize an individual chemically synthesized gold nanoshell and a lithographically defined heptamer exhibiting a pronounced Fano-like resonance. We show that the line shape of the latter strongly depends on the incidence angle. Near-normal incidence dark-field microscopy can be used to characterize a broad range of molecules and nanostructures and can be adapted to most microscopy setups.     

Light scattering and crossover critical phenomena in polymer solutions

An accurate photon-correlation spectrometer capable of measuring scattered light at two fixed scattering angles simultaneously in the temperature range from 10 to 120 degrees C is described. For the detection of the correlation function of the scattered light both an original correlator (PhotoCor-SP) with a linear-time-scale channel spacing and an ALV GmbH correlator (Model ALV-5000) with a logarithmic-time-scale channel spacing are used. High-resolution static and dynamic light-scattering measurements near the critical point of a polystyrene solution were performed with this instrument. The static-intensity measurements reveal that the crossover from mean-field to Ising behavior occurs at a temperature at which the correlation length of the critical fluctuations becomes equal to the radius of gyration of the polymer molecule. We found that the wave-number dependence and the temperature dependence of the diffusion coefficient of the concentration fluctuations are not consistent with existing theory for critical dynamics in low-molecular-weight liquid mixtures.     

3D Strain Field Measurement by Correlation of Volume Images Using Scattered Light: Recording of Images and Choice of Marks

Abstract:  We have extended the Digital Image Correlation technique to the case in three dimensions. This new technique, allowing the full three-dimensional (3D) strain measurement in the bulk of a solid, needs volume images containing a 3D variation of the grey levels. Generally, volume images are obtained by X-ray computed tomography. In this paper, we present a procedure that is easier to implement and enables to generate volume image in transparent materials. The principle consists in the optical slicing of the specimen. To obtain a random distribution of grey levels within the volume image, we use the scattered light phenomenon induced by particles included in the specimen. The recording of 3D images by optical slicing is presented and the influence of different kinds of particles on the scattered light and on the accuracy of measurement is described. Through several tests involving rigid body displacements and a tensile test we show the performance of this technique and we evaluate the measurement error of displacement and strain components.     

Second-harmonic specular and scattered generated light: application to the experimental study of zinc-sulfide thin films

We present measurements of second-harmonic generation with zinc-sulfide thin films. Both scattered light and specular light are investigated in linear optics as well as in second-harmonic generation. We show that second-harmonic generation is a powerful tool for understanding the nonlinear properties of thin films; it allows the study of the anisotropy found in the scattered and specular second harmonic. Using the symmetry of susceptibility tensors, we show that the films cannot be considered homogeneous when the crystallites are large. Finally, we outline nonlinear scattering measurements, which bring out the usefulness of second-harmonic light in probing the structure of thin films.     

胶体中布朗粒子简谐束缚下准直高斯光束的动态光散射

在将散射粒子间的相互作用看作简谐力、入射光为准直高斯光束、散射光为理想平面光波的模型下研究了胶体溶液中布朗粒子的统计特征并求解了动态光散射的散射光电场强度的自相关函数；     

Influence of correlated errors on the estimation of the relaxation time spectrum in dynamic light scattering

An important step in analyzing data from dynamic light scattering is estimating the relaxation time spectrum from the correlation time function. This estimation is frequently done by regularization methods. To obtain good results with this step, the statistical errors of the correlation time function must be taken into account [J. Phys. A 6, 1897 (1973)]. So far error models assuming independent statistical errors have been used in the estimation. We show that results for the relaxation time spectrum are better if correlation between statistical errors is taken into account. There are two possible ways to obtain the error sizes and their correlations. On the one hand, they can be calculated from the correlation time function by use of a model derived by Sch?tzel. On the other hand, they can be computed directly from the time series of the scattered light. Simulations demonstrate that the best results are obtained with the latter method. This method requires, however, storing the time series of the scattered light during the experiment. Therefore a modified experimental setup is needed. Nevertheless the simulations also show improvement in the resulting relaxation time spectra if the error model of Sch?tzel is used. This improvement is confirmed when a lattice with a bimodal sphere size distribution is applied to experimental data.     

Comparison of field correlations in multiply scattered quasi-monochromatic light

An experiment is described that directly compares the degradations, with the number of scattering mean free paths, of two field correlations that may be used to form gates for imaging techniques in scattered light: the correlation of the scattered wave with an unscattered reference wave and the correlation of two wave-vector components of the scattered wave itself. Results for 20-mum polymer spheres show that the latter correlation is consistently larger well into the multiple-scattering regime (up to 10 mean free paths) for wave-vector separations less than at least 50 mm(-1) and that the two correlations tend to merge in this scattering regime for larger wave-vector separations.     

Theory of the excitonic fluctuation in a strong magnetic field. I. Development of the physical picture and a self-consistent Hartree theory

The physical picture of the appearance of the excitonic correlation in a strong magnetic field is developed in an electron-hole system which consists of one conduction band and one valence band. The field is assumed to be sufficiently strong that all the electrons and holes occupy their lowest Landau levels. Kinematical and dynamical aspects of such a truncated system are discussed in detail. The discussion does not deny the possibility of the excitonic phase transition in a strong magnetic field. The essence of the present physics is in the high degeneracy of the single-particle state and the strong shrinking of the electron wave function due to the magnetic field. In order to study the physics explicitly we construct a self-consistent Hartree theory. Further introducing static and local approximations for the excitonic fluctuation. We can determine the quasiparticle state and the fluctuation exactly near the transition point. We present the transition temperature in two limiting cases of the band configuration. We indicate the self-consistent Hartree theory combined with the static and local approximations is an unsatisfactory theory in the light of our physical picture. We also discuss possible improvements of the theory briefly.     

Improved axial position detection in optical tweezers measurements

We investigate the axial position detection of a trapped microsphere in an optical trap by using a quadrant photodiode. By replacing the photodiode with a CCD camera, we obtain detailed information on the light scattered by the microsphere. The correlation of the interference pattern with the axial position displays complex behavior with regions of positive and negative interference. By analyzing the scattered light intensity as a function of the axial position of the trapped sphere, we propose a simple method to increase the sensitivity and control the linear range of axial position detection.     

Near-field intensity distribution of waves scattered from slightly rough surfaces

The near fields of electromagnetic waves scattered from two-dimensional slightly rough surfaces are studied by using the stochastic functional approach. The correlation coefficient between the surface and the intensity of the scattered-wave field is investigated to estimate the fidelity of near-field intensity images. We show that the fidelity depends on both the polarization and the angle of incidence and that high fidelity can be obtained by a TM-polarized incident wave whose incident angle is not close to the critical angle of the total reflection.     

Theory of Light Scattering by Amorphous Absorbing Media

The present paper studies the determination of structural statistics of amorphous absorbing media from the distribution of intensity of scattered light. Assuming a gaussian distributed inhomogeneity, the form of the correlation function for absorbing materials is discussed. Furthermore, the relationship between the correlation function and the scattered intensity is established in both the Fresnel and Fraunhofer regions. The first Born approximation is discussed, as well as the difficulties in solving the inverse statistical problem due to the complex nature of the permittivity.     

Wave Scattering from Fractal Surfaces

Abstract

In this paper, a normalized band-limited Weierstrass function is presented for modelling 2D fractal rough surfaces. Some conventional statistical parameters, namely the root mean square and the correlation length of rough surfaces, are used to assess between fractal parameters and the roughness of surfaces. An analytic solution of the scattered light field from these fractal surfaces is derived based on Kirchhoff theory. Three statistical parameters, namely the average scattering coefficient, the average intensity of scattered field, and the root mean square of scattered field, are introduced to study the influence of various fractal parameters on the scattered field by theoretical analysis and numerical calculations.     

Light scattering from self-affine fractal silver surfaces with nanoscale cutoff: far-field and near-field calculations

We study the light scattered from randomly rough, one-dimensional, self-affine fractal silver surfaces with nanoscale lower cutoff illuminated by s- or p-polarized Gaussian beams a few micrometers wide. By means of rigorous numerical calculations based on the Green's theorem integral equation formulation (GTIEF), we obtain both the far- and near-field scattered intensities. The influence of diminishing the size of the fractal lower-scale irregularities (from approximately 50 nm to a few nanometers) is analyzed in the case of both single realization and ensemble-average magnitudes. For s polarization, variations are small in the far field, being significant only in the higher-spatial-frequency components of evanescent character in the near field. In the case of p polarization, however, the nanoscale cutoff has remarkable effects stemming from the roughness-induced excitation of surface-plasmon polaritons. In the far field, the effect is noticed both in the speckle pattern variation and in the decrease of the total reflected energy upon ensemble averaging, as a result of increased absorption. In the near field, more efficient excitation of localized optical modes is achieved with smaller cutoff, which in turn leads to huge surface electric field enhancements.     

Irradiance inversion algorithm for estimating the absorption and backscattering coefficients of natural waters: Raman-scattering effects

We modify an algorithm for retrieving the absorption (a) and backscattering (b(b)) coefficient profiles in natural waters by inverting profiles of downwelling and upwelling irradiance so as to include the presence of Raman scattering. For a given wavelength of interest, lambda, the light field at the appropriate Raman excitation wavelength lambda(e) is first inverted to obtain the Raman source function at lambda. Starting from estimates of the inherent optical properties at lambda, the contribution to the irradiances at lambda from Raman scattering is then estimated and subtracted from the total irradiances to obtain the elastically scattered irradiances. We then inverted the elastically scattered irradiances to find new estimates of a and b(b) using our original method [Appl. Opt. 37, 3886 (1998)]. The algorithm then operates iteratively: The new estimates are used with the Raman source function to derive a new estimate of the Raman contribution, etc. Sample results are provided that demonstrate the working of the algorithm and show that the absorption and scattering coefficients can be retrieved with accuracies similar to those in the absence of Raman scattering down to depths at which the light field is significantly perturbed by it, e.g., with ~90% of the upwelling light field originating from Raman scattering.     

Spectral Filtering of Light Possessing Non-gaussian Statistics

Abstract

We have used a heterodyne detection technique to observe the intensity fluctuations and statistics of non-Gaussian scattered light, formed by illuminating a rotating ground-glass screen with a tightly focused laser beam. This type of light field contains the characteristic ‘glints’ commonly observed with laser radar systems. It is demonstrated both theoretically and experimentally that the statistics of the fluctuating intensity are significantly altered by spectral filtering of the light. In the limit, when the filter bandwidth becomes much narrower than the overall spectral linewidth of the scattered light, the distribution of intensities tends to that expected for complex Gaussian field statistics (i.e. a negative exponential). We compare this behaviour with that obtained by other workers for spectral filtering within the linewidth of light from a single-mode laser.     

Directional emission from plasmonic Yagi-Uda antennas probed by angle-resolved cathodoluminescence spectroscopy

Optical nanoantennas mediate optical coupling between single emitters and the far field, making both light emission and reception more effective. Probing the response of a nanoantenna as a function of position requires accurate positioning of a subwavelength sized emitter with known orientation. Here we present a novel experimental technique that uses a high-energy electron beam as broad band point dipole source of visible radiation, to study the emission properties of a Yagi-Uda antenna composed of a linear array of Au nanoparticles. We show angle-resolved emission spectra for different wavelengths and find evidence for directional emission of light that depends strongly on where the antenna is excited. We demonstrate that the experimental results can be explained by a coupled point dipole model which includes the effect of the dielectric substrate. This work establishes angle-resolved cathodoluminescence spectroscopy as a powerful technique tool to characterize single optical nanoantennas.     

Experimental and theoretical study of Rayleigh-Lamb waves in a plate containing a surface-breaking crack

Scattering of Rayleigh-Lamb waves by a normal surface-breaking crack in a plate has been studied both theoretically and experimentally. The two-dimensionality of the far field, generated by a ball impact source, is exploited to characterize the source function using a direct integration technique. The scattering of waves generated by this impact source by the crack is subsequently solved by employing a Green's function integral expression for the scattered field coupled with a finite element representation of the near field. It is shown that theoretical results of plate response, both in frequency and time, are similar to those obtained experimentally. Additionally, implications for practical applications are discussed.