Theory of correlation measurements of resonance light scattering

The boson nature of radiation is shown to give rise to a purely quantum mechanical exchange contribution to the intensity-intensity correlation function for resonant light scattering by an atomic or molecular system. The exchange contribution can be decomposed into three components, one involving the intensity correlation for a pair of coherently scattered photons (“resonant Raman” processes), another for a pair of incoherently scattered ones (“resonance fluorescence”), and the last involving the exchange correlation one of each. The intensity correlation measurements of Kimble et al., on optically pumped atomic beams of sodium atoms are interpreted with the theory, producing values of the decay rate of the excited sodium atoms and of the coherence time of the exciting radiation in good agreement with expectations.     

Anomalous light scattering in photonic cholesteric liquid crystals

ABSTRACT

We have carried out polarisation and angle-resolved measurements of the light scattered from photonic cholesteric liquid crystals. Both in samples doped with laser dyes and in inactive (non-doped) samples we have observed pronounced directional dependences of the scattered light, finding angular ranges where the scattering is greatly enhanced and regions where the effect is almost suppressed. Moreover, the total amount of scattered light has also been found to depend strongly on the polarisation and direction of the incident beam. All the results have been interpreted successfully in terms of a simple expression proposed for the scattering cross section, in which the density of states of the ingoing and outgoing beams plays a major role. The expression would be applicable not only to cholesteric liquid crystals but to any one-dimensional photonic material.     

Experimental observation of energy dependence of saturation thickness of multiply scattered gamma photons

The gamma photons continue to soften in energy as the number of scatterings increases in the target having finite dimensions both in depth and lateral dimensions. The number of multiply scattered photons increases with an increase in target thickness, and saturates at a particular value of the target thickness known as saturation thickness (depth). The present measurements are carried out to study the energy dependence of saturation thickness of multiply scattered gamma photons from targets of various thicknesses. The scattered photons are detected by a properly shielded NaI(Tl) gamma ray detector placed at 90° to the incident beam. We observe that the saturation thickness increases with increasing incident gamma photon energy. Monte Carlo calculations based upon the package developed by Bauer and Pattison [Compton scattering experiments at the HMI (1981), HMI-B 364, pp. 1–106] support the present experimental results.     

Bremsstrahlung from gas atom targets: Study of background processes

Recent absolute bremsstrahlung cross section experiments on gas targets of Ne, Ar, Kr and Xe at 28 and 50 keV have shown a significant polarization bremsstrahlung (PB) contribution, in contrast with previous thin-film experiments where no PB has been seen. Recently, Obolensky and Pratt have considered ways to improve the PB model, but the theory is still about 20% below the data. While a more complete theoretical calculation is certainly needed, we consider two additional background processes, not corrected in the experiment, that depend on the background photon spectrum in the beam line produced by electron interaction with collimators in the beam. We compare an estimate of both backgrounds with that from beam electrons elastically scattered by the gas into the cell window or wall and discuss initial efforts to measure these backgrounds.     

Experimental study of the potential use of diffusing wave spectroscopy to investigate the structural characteristics of blood under multiple scattering

The extension of the photon correlation spectroscopy (PCS) in multiple scattering regime, so-called diffusing wave spectroscopy (DWS) was employed to the study of blood samples. Multiple scattered light from a helium-neon (He-Ne) laser beam incident on the blood samples was detected by a photomultiplier, and both the temporal autocorrelation intensity functions g 2(tau) and power spectra S(omega) were measured by a spectrum analyzer. The potentials of using DWS for the qualitative and quantitative determination of the structural characteristics of the blood elements were studied experimentally. The experimental studies made, permits the use of DWS for blood cells monitoring in a multiple scattering regime. This paper describes our initial attempts at applying DWS to the study of the discrete blood samples of both healthy donors and patients with the cardiac ischemia. The subsequent experiments provide a verification of DWS of blood cells shape monitoring under multiple scattering.     

Determination of the sticking coefficient and scattering dynamics of water on ice using molecular beam techniques

The sticking coefficient for D(2)O impinging on crystalline D(2)O ice was determined for incident translational energies between 0.3 and 0.7 eV and for H(2)O on crystalline H(2)O ice at 0.3 eV. These experiments were done using directed molecular beams, allowing for precise control of the incident angle and energy. Experiments were also performed to measure the intensity and energy of the scattered molecules as a function of scattering angle. These results show that the sticking coefficient was near unity, slightly increasing with decreasing incident energy. However, even at the lowest incident energy, some D(2)O did not stick and was scattered from the ice surface. We observe under these conditions that the sticking probability asymptotically approaches but does not reach unity for water sticking on water ice. We also present evidence that the scattered fraction is consistent with a binary collision; the molecules are scattered promptly. These results are especially relevant for condensation processes occurring under nonequilibrium conditions, such as those found in astrophysical systems.     

Electron and photon emission accompanying deformation and fracture of polycarbonate

Electron and photon emission accompanying tensile loading and failure of polycarbonate show weak emissions during the onset of neck formation and intense emissions during the fracture event itself. These results are interpreted in terms of formation of active species by bond breaking followed by emission driven by energy released by recombination. Fast time scale measurements during fracture show that intense electron and photon emission typically begins about 50 μs prior to the completion of fracture and is most intense at the completion of fracture. The gradual onset reflects the final stages of growth of the failure-initiating defect. Defect growth was monitored by measuring the intensity of a light beam transmitted through the gauge length of the sample; the transmission is sensitive to scattering by surface and bulk defects. A marked decrease in transmission begins some tens of ms prior to fracture due to scattering from the fracture-initiating defect. These measurements allow accurate correlations of defect growth with the onset of the electron and photon signals. © 1993 John Wiley & Sons, Inc.     

QUANTITATIVE MICROPHOTOMETRY AT THE CELLULAR LEVEL: A SIMPLE TECHNIQUE FOR MEASURING CHLOROPLAST MOVEMENTS IN VIVO*

Abstract— A recording microphotometer was developed to permit quantitative analysis of light-induced chloroplast aggregation in single filaments of the alga Vaucheria. Optical fibers were used both to irradiate the cells and to collect the transmitted light. Light transmittance was examined as a function of time at a fixed point along an algal filament. The photoresponse was stimulated by combining an additional actinic beam with the measuring beams such that both actinic and measuring beams followed the same optical path. The quasi-opalescent properties of the optical fibers and dual-wavelength measurements compensated for light scattering and variations in sample geometry. Chloroplast aggregation, marked by a passive accumulation of organelles in the irradiated region of the cell, resulted in a decline of cell transmittance to 675 and 743 nm light. During stimulation with moderate intensities of actinic, 473 nm light the relative transmittance to 675 nm light (corrected for scattering, measured at 743 nm) fell below 0.1 within 5 min. The exponential form of this transmittance change as a function of time was consistent with changes in optical geometry of the aggregate during the photoresponse. It follows that the decay in algal transmittance can provide a measure of the rate of chloroplast accumulation in the light path.     

Dynamic light scattering from concentrated colloidal dispersions

From the scattered intensity as a function of scattering wave number, local ordering in concentrated latex dispersions in benzene is found. From the combination of intensity and photon correlation experiments follows the existence of strong hydrodynamic interactions between the dispersed particles.     

一氧化碳的双共振多光子电离研究

用两束脉冲染料激光进行了CO的光学双共振四光子电离, 获得了转动分辩的CO A~(1Π)←X~(1Σ~+)和B~(1Σ~+)←A~(1Π)跃迁的光谱。观察到了在此2+1光子相继吸收中特有的选择定则。动力学研究表明CO基态到A~(1Π)态的双光子吸收是速率决定步骤, 其吸收截面约为1×10~(-49) cm~4 s。还测定了A~(1Π)态的单态转动传能截面。对于宇称指数e守恒、△J×±1的过程的截面约为0.05 nm; 随△J值增大而减小。对相同的△J, e正负变号的过程的截面总是小于e不变过程的截面。     

Anomalous laser saturation resonances in NO2

We describe an unusual resonance in the total fluorescence intensity which appears when two unidirectional and spatially separated light beams intersect a beam of freely propagating NO₂ molecules. No conventional explanation of the observations seems to be possible. However the experimental results are in agreement with previously reported experiments and with the previously introduced assumption, that underlying the conventionally expected quantum states of the molecule is a substructure which evolves irreversibly in time.     

Elastic light scattering from nanoparticles by monochromatic vacuum-ultraviolet radiation

Elastic light scattering is reported using monochromatic vacuum-ultraviolet radiation to study free, spherical silica nanoparticles prepared by approaches from colloidal chemistry, with diameters between 100 and 240 nm. The colloidal nanoparticles of defined size are transferred from an aqueous solution into the gas phase using a particle beam experiment. After focusing of the particle beam by an aerodynamic lens, the scattered light from monochromatic synchrotron radiation is measured. Angle-resolved elastically scattered light is detected, showing a strong forward-scattering component. Additional evidence for the detection of elastically scattered light comes from plotting the scattered light intensity as a function of the dimensionless parameter qR, where q is the magnitude of the scattering wave vector and R is the particle radius. This yields different power-law regimes that are assigned to scattering from the surface and the bulk of the nanoparticles. Furthermore, there is evidence for modulations in the scattered light intensity as a function of scattering angle, which is clearly distinguished from the forward-scattering component. The experimental results are compared to Mie scattering simulations for isolated particles, yielding general agreement with the experimental results. Deviations from Mie simulations are observed for samples consisting of significant amounts of aggregates. The present results indicate that the optical properties of free nanoparticles are sensitively probed by vacuum-ultraviolet radiation.     

Static and dynamic light scattering study of fluorinated polymer colloids with a crystalline internal structure

The light scattering properties of polymer colloids made of polytetrafluoroethylene with added copolymers are described in some detail. The particles possess a partially crystalline internal structure which makes them optically anisotropic, and consequently gives rise to a strong depolarized component in the scattered light intensity. We present a complete theory of static and dynamic light scattering which includes the effects of polycrystallinity, of optical polydispersity, and of interparticle interactions. We also discuss applications to surfactant adsorption studies, to sedimentation experiments and to fractal aggregation processes     

Surface plasmon spectroscopy of metal/dielectric structures

Surface electromagnetic waves (i.e. plasmons) were excited in metal—oxide—metal (MOM) multi-layer thin film structures by the tunnel current from an external dc source. Surface plasmons decayed to photons in the visible region with typically 10⁻¹⁰−10⁻¹²W cm⁻² light intensity.A very sensitive single channel optical spectrometer was constructed to study both spectral and angular distributions of light emitted from aluminum—oxide—noble-metal (Ag, Au, Cu) thin film diodes mounted into a cold-finger type liquid nitrogen optical cryostat. When MOM structures were prepared on substrates with regular surface modulation of 300–900 nm periods, spectral and angular measurements of the emitted light allowed one to construct the energy—wave-vector dispersion diagram of surface plasmons. This was of help in identifying the top meta/vacuum fast mode plasmons as the source of photon emission.     

Photon level chemical classification using digital compressive detection

A key bottleneck to high-speed chemical analysis, including hyperspectral imaging and monitoring of dynamic chemical processes, is the time required to collect and analyze hyperspectral data. Here we describe, both theoretically and experimentally, a means of greatly speeding up the collection of such data using a new digital compressive detection strategy. Our results demonstrate that detecting as few as ∼10 Raman scattered photons (in as little time as ∼30 μs) can be sufficient to positively distinguish chemical species. This is achieved by measuring the Raman scattered light intensity transmitted through programmable binary optical filters designed to minimize the error in the chemical classification (or concentration) variables of interest. The theoretical results are implemented and validated using a digital compressive detection instrument that incorporates a 785 nm diode excitation laser, digital micromirror spatial light modulator, and photon counting photodiode detector. Samples consisting of pairs of liquids with different degrees of spectral overlap (including benzene/acetone and n-heptane/n-octane) are used to illustrate how the accuracy of the present digital compressive detection method depends on the correlation coefficients of the corresponding spectra. Comparisons of measured and predicted chemical classification score plots, as well as linear and non-linear discriminant analyses, demonstrate that this digital compressive detection strategy is Poisson photon noise limited and outperforms total least squares-based compressive detection with analog filters.