Estimation of complex refractive index of polydisperse particulate systems from multiple-scattered ultraviolet-visible-near-infrared measurements

A method to extract the complex refractive index of spherical particles from a polydisperse suspension at concentrations where multiple light-scattering effects are significant is presented. The optical constants are estimated from total diffuse reflectance and transmittance measurements and inverting the measurements using the radiative transfer equation (RTE) and the Mie theory for scattering by polydisperse spherical particles. The method is tested by applying it to three different polydisperse polystyrene suspensions and extracting the optical constants of polystyrene particles in the wavelength range of 450-1200 nm. The effect of particle size, concentration, and polydispersity on the estimated values of the optical constants is also discussed.     

Monte Carlo calculation of backscattered light intensity by suspension: comparison with experimental data

An optical device for measuring high particle concentrations is presented. The sensor consists of two optical fibers used for the emission and reception of the light scattered by particles in suspension. To describe the light path in this medium, we developed a Monte Carlo calculation including a mean free path, a phase function for particle scattering, and an absorption rate. A similarity law links concentration variations to a homothetic space transformation. A comparison between our calculation and experimental data is given for well-stirred, dense suspensions of silica particles with different sizes. A good fit is found when the model parameters (mean free path and phase function) are chosen according to the data given by a particle sizer.     

Labeling detonation nanodiamond suspensions using the optical methods

The percentage of large particles in the water suspension of diamond nanoparticles to a large extent determines the potential of using these suspensions in technology and medicine. It is demonstrated that there is an error in determining the percentage of large diamond nanoparticles in water suspensions using the dynamic light scattering (DLS) method to measure the size distribution of the particles. A method for labeling these suspensions based on supplementing the DLS measurements with recording of an optical spectrum in the visible region is proposed. The labeling allows one to perform more reliable determination of the percentage of large particles in a suspension.     

Particle sizing in strongly turbid suspensions with the one-beam cross-correlation dynamic light-scattering technique

The utility of the one-beam cross-correlation dynamic light-scattering system for sizing small particles in suspension was previously limited by its small-intensity signal-to-baseline ratio for strongly turbid suspensions. We describe three improvements in the optical system and sample cell that raise the ratio to a value comparable with that of other cross-correlation dynamic light-scattering systems. These improvements are (i) using a square cross-sectional sample cell to minimize the attenuation of the incident beam and singly scattered light, (ii) placing a 200-microm-wide slit between the sample cell and the detector fibers to mask off the region of weak single scattering and strong multiple scattering from the detectors' field of view, and (iii) aligning the center of the detectors' field of view with the region of strongest single scattering. We analyze a number of suspensions of polystyrene latex spheres with a diameter between 65 and 562 nm in water using this improved one-beam instrument and find that the measured radius is determined in a 2-min data collection time to better than +/-10% for volume fractions of the suspended polystyrene latex spheres up to a few percent.     

Spontaneous density grating formation in suspensions of dielectric nanoparticles

Experimental evidence for nonlinear optical behaviour due to the spontaneous formation of wavelength-scale density modulations or gratings in suspensions of dielectric particles is presented. A collection of dielectric particles pumped by a coherent radiation field may simultaneously form a density grating on the scale of the radiation wavelength and a coherently backscattered radiation field. The particle density grating is generated as a result of a periodic ponderomotive potential formed by the interference of the pump and backscattered fields. The experiment used a water suspension of latex microspheres (radius ≈ 56nm) pumped by a green CW laser (532nm, power ≤ 5W). A theoretical model of collective scattering of light from dielectric particles has been extended to include the effects of viscous and Brownian forces on the particles. This model predicts a small degree of particle bunching from which coherent backscattering of the pump occurs. The results of the theoretical model compare favourably with the experimental evidence. The relation between the results presented here and the phenomenon of Collective Rayleigh Scattering (CRS) is discussed.     

Spectroscopic characterization of coumarin-stained beads: quantification of the number of fluorophores per particle with solid-state 19F-NMR and measurement of absolute fluorescence quantum yields

The rational design of nano- and micrometer-sized particles with tailor-made optical properties for biological, diagnostic, and photonic applications requires tools to characterize the signal-relevant properties of these typically scattering bead suspensions. This includes methods for the preferably nondestructive quantification of the number of fluorophores per particle and the measurement of absolute fluorescence quantum yields and absorption coefficients of suspensions of fluorescent beads for material performance optimization and comparison. Here, as a first proof-of-concept, we present the first time determination of the number of dye molecules per bead using nondestructive quantitative ((19)F) NMR spectroscopy and 1000 nm-sized carboxylated polystyrene particles loaded with varying concentrations of the laser dye coumarin 153 containing a CF(3) group. Additionally, the signal-relevant optical properties of these dye-loaded particles were determined in aqueous suspension in comparison to the free dye in solvents of different polarity with a custom-built integrating sphere setup that enables spectrally resolved measurements of emission, transmission, and reflectance as well absolute fluorescence quantum yields. These measurements present an important step toward absolute brightness values and quantitative fluorescence analysis with particle systems that can be exploited, for example, for optical imaging techniques and different fluorescence assays as well as for the metrological traceability of fluorescence methods.     

Small-angle light scattering spectra of disperse particles suspended in absorbing liquids

The spectra of small-angle light scattering (SALS) by disperse particles suspended in an absorbing liquid have been numerically simulated using the Mie theory and measured in a wavelength range from 190 to 1100 nm. It is established that the SALS spectra contain regions sensitive to the size and polydispersity of particles with radii ranging from 0.4 to 30 μm. Using the joint processing of SALS and transmission spectra measured using a double-beam spectrophotometer with a modified optical scheme, it is possible to determine the parameters of particles suspended in the absorbing liquid. Data for model suspensions of polystyrene latex particles are presented.     

Measuring the optical parameters of weakly absorbing, highly turbid suspensions by a new technique: photoacoustic detection of scattered light

We present and apply a novel method, the scattering photoacoustic (SPA) technique, for measuring optical parameters in weakly absorbing, highly scattering suspensions. In this method, a solid absorber is in contact with a suspension sample to permit the photoacoustic detection of the sample's light-scattering properties. We conducted measurements conducted to determine the reduced scattering coefficients of Intralipid suspensions with a concentration range of 0.1-5%, and the results are in good agreement with those achieved by other researchers. Moreover, we also illustrate the relationship between the amplitude of the SPA signal and absorption, scattering, and detection distance. Through a study of Intralipid-ink mixes, we demonstrate that the SPA technique has the ability to determine simultaneously the absorption and reduced scattering coefficients of turbid media. This new technique has low cost and is noninvasive, and it enables on-line measurements to be made.     

Effect of particles on ultraviolet light penetration in natural and engineered systems

The effect of light scattering on measurement of UV absorbance and penetration of germicidal UVC irradiance in a UV reactor were studied. Using a standard spectrophotometer, absorbance measurements exhibited significant error when particles that scatter light were present but could be corrected by integrating sphere spectroscopy. Particles from water treatment plants and wastewater effluents exhibited less scattering (20%-30%) compared with particles such as clay (50%) and alumina (95%-100%). The distribution of light intensity in a UV reactor for a scattering suspension was determined using a spherical chemical actinometry method. Highly scattering alumina particles increased the fluence rate in the reactor near the UV lamp, whereas clay particles and absorbing organic matter reduced the fluence rate. A radiative transfer fluence rate model reasonably predicted the fluence rate of absorbing media and highly scattering suspensions in the UV reactor.     

Study of optical parameters of polystyrene spheres in dense aqueous suspensions

We investigated the dependence of the scattering and absorption coefficients of particles in dense suspensions by the low-coherence fiber optic dynamic light scattering (FODLS) technique. The estimated particle size was used to calculate the scattering coefficient of particles suspended in dense suspensions. The path-length resolved intensity distributions of light backscattered from absorbing dense suspensions were investigated experimentally. The absorption coefficient can be obtained by applying the measured path-length resolved intensity distributions to the modified Lambert-Beer law. As a result, the low-coherence FODLS technique can simultaneously measure the scattering and absorption coefficients of particles in absorbing dense suspensions, and the scattering and absorption coefficients are independent of each other in dense suspensions in the low-scattering regime of 2l(d) < 10?*.     

Investigation of an optical limiting mechanism in multiwalled carbon nanotubes

We report our investigation of the mechanism that is responsible for the optical limiting behavior in multiwalled carbon nanotubes. We conducted energy-dependent transmission measurements, picosecond time-resolved pump-probe experiment, and nonlinear scattering experiments at 532-nm wavelength on multiwalled carbon nanotube suspension. For comparison, C(60)-toluene solutions and carbon black suspensions were also studied in the same experiments. The similarities that we observed between the multiwalled carbon nanotubes and carbon black suspension suggest that nonlinear scattering, which is known to be responsible for the limiting action in carbon black suspension, should play an important role in the limiting effect in multiwalled carbon nanotubes.     

Low-coherence heterodyne photon correlation spectroscopy

Photon correlation spectroscopy (PCS) is routinely used to investigate the dynamics of colloidal particles undergoing Brownian motion. This technique is applicable to low-density colloidal suspensions in which the effects of multiple light scattering are minimal. We introduce a new low-coherence heterodyne PCS technique that allows direct investigation of colloidal suspensions of higher concentration than previously accessible with standard PCS. In this technique, low-coherence optical heterodyne interferometry is used tosuppress multiple light scattering, allowing preferential detection of single-scattering events.     

Effect of laser-radiation polarization on the nonlinear scattering of light in nanodiamond suspensions

The effect of laser radiation polarization on the nonlinear scattering of light in aqueous suspensions of detonation nanodiamonds (DNDs) in a regime of optical power limiting (OPL) has been studied. It is established that the nonlinear transmission coefficient of DND suspension in the OPL regime in a field of nanosecond laser pulses with a wavelength of 532 nm is independent of the polarization of incident radiation. The nonlinear scattering of light observed at an angle of 90° in the plane perpendicular to the plane of polarization of the incident radiation depends on the polarization angle in accordance with a trigonometric law. It is shown that the ratio of the signals of scattered radiation for the vertical and horizontal polarizations exhibits nonmonotonic dependence on the laser-beam power density. The results are explained by the Rayleigh-Mie scattering and a change in the size of scattering centers as a result of the effect of a laser upon the DND suspension.     

Application of the small-angle approximation to ocean water types

The small-angle approximation to the radiative transport equation is applied to particle suspensions that emulate ocean water. A particle size distribution is constructed from polystyrene and glass spheres with the best available data for particle size distributions in the ocean. A volume scattering function is calculated from the Mie theory for the particles in water and in oil. The refractive-index ratios of particles in water and particles in oil are 1.19 and 1.01, respectively. The ratio 1.19 is comparable to minerals and nonliving diatoms in ocean water, and the ratio 1.01 is comparable to the lower limit for microbes in water. The point-spread functions are measured as a function of optical thickness for both water and oil mixtures and compared with the point-spread functions generated from the small-angle approximation. Our results show that, under conditions that emulate ocean water, the small-angle approximation is valid only for small optical thicknesses. Specifically, the approximation is valid only for optical thicknesses less than 3.     

Measurement of particle-size distribution and concentration in heterogeneous turbid media with multispectral diffuse optical tomography

We present a method that is capable of extracting particle-size distribution (PSD) and concentration in heterogeneous turbid media by use of multispectral diffuse optical tomography (MSDOT). After the spectroscopic scattering images of the heterogeneous turbid media are obtained with MSDOT, the morphologic information of particles in the heterogeneities is recovered with an iterative regularized reconstruction algorithm based on Mie scattering theory when a particular form of PSD is assumed (Gaussian distribution is used in this study). The method described is tested and evaluated with both simulated and experimental data. The simulations are intended to test the sensitivity of the overall approach to noise effect. A series of phantom experiments are conducted with our newly developed ten-wavelength MSDOT system. Polystyrene microsphere suspensions contain particles of varying size from 2 to 6 microm as targets are embedded in a scattering background medium in these experiments. To achieve optimized results from experimental data, we developed a data preprocessing method for MSDOT as well as a scheme for calibrating scattering spectra. The results from both simulations and experiments show that the particle mean size and concentration can be reconstructed with acceptable accuracy, whereas the recovery of the standard deviation is sensitive to noise effect and can be as large as 86% from the experimental data.     

Precise measurement of the refractive index and optical rotatory power of a suspension by a delayed optical heterodyne technique

By using an optical heterodyne technique, we have demonstrated the detection of ballistic photons traveling through a suspension. We measured the propagation time of light in a sample with a variable optical delay line in the reference arm of an interferometer, using a superluminescent diode as a light source. The resolution and accuracy in propagation time measurement were 400 and 3 fs, respectively. The minimum detectable rotation angle of 1/10,000 deg was achieved with an integration time of 5 s. This system can measure the refractive index and optical rotatory power of the suspension in a solvent without disturbing heavy scattering from particles in the suspension, because ballistic photons are detected.     

Orientation dependence in near-field scattering from TiO(2) particles

This scattering of light by small particles embedded in a continuous transparent medium is influenced not only by the bulk optical properties of the particles and the medium but also by the size, shape, and spatial arrangement of the particles-that is, by the microstructure. If the particles are close together, as in agglomerated coatings or stereolithographic suspensions, interactions between the radiation fields of adjacent particles can lead to variations in the magnitude and spatial arrangement of the scattered light in the near and the far field, which can affect the color and hiding power of a coating, the cure depth and homogeneity in stereolithography, and the threshold intensity for stimulated emission in random lasers. Our calculations of the near- and the far-field scattering distribution for 200-nm TiO(2) spheres in pairs of various orientations and in an ordered array of five particles show that, depending on the orientation of the particles with respect to the incident light, these interactions can either increase or decrease the scattering efficiency, the isotropy of the scattering, and the magnitude of the electric field strength within the matrix and the particles. In the mid-visible range, two particles in line increase the backscattering fraction by 28% and the scattering strength by 38% over that of a single particle, whereas if the particles are in the diagonal configuration the backscattering fraction and scattering strength are actually reduced by addition of the second particle. At shorter or longer wavelengths the backscattering fraction is reduced regardless of the location of the second particle, by as much as 60% when five particles are arranged in the zigzag configuration. These results are surprising in that it is generally assumed that multiple scattering enhances backscattering. Simple models of multiple scattering or scattering of two particles as a single, larger particle are inadequate to explain these results.     

Validating the assumption to the interference approximation by use of measurements of absorption efficiency and hindered scattering in dense suspensions

Frequency domain photon migration (FDPM) measurements were employed to accurately quantify optical properties of both the suspending fluid and particles within dense polystyrene suspensions of 143- or 226-nm mean diameter at varying concentrations (5-30% by volume). The measured absorption coefficients varied linearly with particle volume fraction whereas the isotropic scattering coefficients varied nonlinearly in agreement with the prediction that utilizes the hard-sphere structure factor model. These results validate the interference approximation of light scattering to describe light propagation accurately within dense suspensions. Furthermore, owing to the accuracy of FDPM absorption measurements, the imaginary refractive indices for both particles and their suspending fluid were determined and were found to compare favorably with literature values.     

Synthesis and nonlinear optical characterization of nanostructured gold/polymer composites and suspensions

Nanometric gold particles were synthesized by a liquid/liquid phase-transfer reaction. Composites of these particles were prepared by free-radical polymerization of suspensions of the particles in styr ene and methyl methacrylate monomer. Concentrations of 1.0, 1.3, and 4.4 mg/ml were prepared,with no noticeable agglomeration of particles during processing. These composites and toluene suspensions (0.18 mg/ml) showed a linear absorption peak at 530 nm, which is characteristic of nanosized gold. High-resolution electron microscopy measurement showed that the particle diameters varied from 5 to 10 nm. Degenerate four-wave mixing experiments at 532 nm yielded, on 10-micron films, a maximum value of 1.0 × 10⁻¹⁰ e.s.u. for the third-order nonlinear optical susceptibility. The toluene suspension had a susceptibility of 7.7 × 10⁻¹² e.s.u.     

Synthesis of highly stable magnesium fluoride suspensions and their application in the corrosion protection of a Magnesium alloy

This study presents a new approach to enhance the corrosion resistance of tungsten inert gas (TIG) welded AZ31 magnesium alloys by using nanocrystalline magnesium fluoride suspensions in a suspension plasma spray (SPS) process. We have developed a synthesis for the preparation of nanocrystalline magnesium fluoride suspensions, which delivers nearly monodisperse nanoparticles in a gram scale yield. The particles were analyzed with transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD). Stable suspensions of magnesium fluoride nanoparticles in water were characterized by dynamic light scattering (DLS), zeta-potential, and viscosity measurements. Such suspensions were deposited with an SPS torch onto TIG welded seams of the magnesium alloy AZ31, thus producing a protective magnesium fluoride layer. Magnesium fluoride covered welded seams were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDXS). In order to introduce a simple method for sensoring the deposited magnesium fluoride coatings, the magnesium fluoride nanoparticles can also be fluorescence-labeled by co-doping with cerium(III) and terbium(III), the respective optical properties were characterized by reflection and luminescence spectroscopy. The deposited layers can, thus, be inspected by illumination with an UV lamp, because of their bright green emission. The corrosion properties of the magnesium fluoride layer on the welded seams were studied by means of potentiodynamic potential measurements.