Detection of biological particles by the use of circular dichroism measurements improved by scattering theory

Light scattered from optically active spheres was theoretically analyzed for biodetection. The circularly polarized signal of near-forward scattering from circularly dichroic spheres was calculated. Both remote and point biodetection were considered. The analysis included the effect of a circular aperture and beam block at the detector. If the incident light is linearly polarized, a false signal would limit the sensitivity of the biodetector. If the incident light is randomly polarized, shot noise would limit the sensitivity. Suggested improvements to current techniques include a beam block, precise angular measurements, randomly polarized light, index-matching fluid, and larger apertures for large particles.     

Integrated light scattering as a function of wavelength in donor lenses.

The purpose of this work is to investigate the light-scattering properties of excised eye lenses with intact lens capsules--more specifically, to compare light scattering with light transmission at different wavelengths in aging and cataractous lenses. A lens is positioned at its focal-length distance from an optical fiber end and collimates the light from one of five laser lines (458-633 nm). By use of an integrating sphere with an extra circular port, the collimated directly transmitted light can be separated from the scattered light. For lenses with low light-scattering levels, integrated scattering showed a dependence on wavelength, but when light scattering increased the wavelength difference tended to level out. Despite the higher percentage of lens light scattering at lower wavelengths, when calculated as an "effective light scattering" (compensated for light transmission), more scattered light actually falls toward the retina at longer wavelengths.     

A method to measure the aperture field and experimentally determinethe near-field phase center of a horn antenna

A method whereby the aperture field distribution of a horn antenna can be measured is proposed. The method is based on the measurement of the scattered field of a small conducting sphere in the aperture. The position of the phase center of the aperture field is calculated using the measured phase distribution of the field in the aperture via a simple type of convolution. In principle, the method is suited to determine the field distribution of other types of apertures illuminated by an electromagnetic field     

Confocal theta fluorescence microscopy with annular apertures

In a confocal theta fluorescence microscope, two objective lenses with circular apertures are used, one to illuminate the sample and the other to detect the emitted light at an angle to the illumination axis. We show that annular illumination and detection apertures lead to a reduction in the extent of the point-spread function. A spatial resolution improved by more than 50% can be achieved with a central obstruction blocking the inner 80% of the diameter. For the limit of a very narrow annular aperture and a numerical aperture of 0.75, the volume at half-maximum of the point-spread function is reduced from 15to5 aL. Amixed setup with anannular illumination aperture and a circular detection aperture is also considered.     

Scattering of a tightly focused beam by an optically trapped particle

Near-forward scattering of an optically trapped 5-mum-radius polystyrene latex sphere by the trapping beam was examined both theoretically and experimentally. Since the trapping beam is tightly focused, the beam fields superpose and interfere with the scattered fields in the forward hemisphere. The observed light intensity consists of a series of concentric bright and dark fringes centered about the forward-scattering direction. Both the number of fringes and their contrast depend on the position of the trapping beam focal waist with respect to the sphere. The fringes are caused by diffraction that is due to the truncation of the tail of the trapping beam as the beam is transmitted through the sphere.     

Experimental investigation of the performance of an annular aperture and a circular aperture on the same very-small-aperture laser facet

A very-small-aperture laser (VSAL) with a circular aperture has a trade-off between the spot size and the output power. A nanometric annular aperture is fabricated to overcome this difficulty. The advantages of the annular aperture are demonstrated by measuring and comparing its near-field intensity distribution with that of a circular aperture. These apertures are fabricated on the same VSAL to ensure that they are under the same illumination conditions. The experimental results indicate that an annular aperture produces a smaller spot size and a higher peak intensity than a circular aperture. The confinement effect and the enhancement effect are attributed to the convergence of the power flow that passes through the annular aperture. The observed enhancement effect decreases when the distance from the VSAL facet is increased, but it does not vanish even when the distance is as large as 3.5 microm.     

Optical Measurement of Large Sphere Diameters by means of Scattering

A simple method is demonstrated to determine the diameters of dielectric spheres from 0.2 to 1.0 mm by observing the scattering of visible light. Theoretical calculations show that there is an approximately linear relationship between the size of the scattering sphere and the number of maxima and minima in the scattered field as a function of angle when the radius in the 200-1000-wavelength region.     

Light scattering by a coated sphere illuminated with a Gaussian beam

The intensity of light scattered by a coated sphere illuminated with an off-axis Gaussian beam is calculated. Results are shown for different beam positions with respect to the sphere. As the beam is shifted further away from the surface of the sphere, the higher-Q morphology-dependent resonances become increasingly important in the backscatter spectra, and the angular scattering intensity becomes smoother.

The scattered intensity depends on the beam position, the refractive indices of the core and coat, the radius of the core, and the thickness of the coat. As the beam is moved further away from the sphere, the effect of the core on the scattering intensity decreases. When the incident Gaussian beam is focused outside of a particle with a relatively small core, the scattering spectra and angular scattering patterns become similar to those of a homogeneous sphere having the refractive index of the coat. These calculated results suggest that measurements of spectral scattering and angular scattering patterns for several Gaussian beam positions could be useful for the characterization of coated spheres.

     

Quantification of glistenings in intraocular lenses using a ballistic-photon removing integrating-sphere method

An alternative method for quantification of glistenings in intraocular lenses (IOLs) using an integrating sphere with an adjustable back aperture to remove ballistic photons is presented. Glistenings in soft IOLs have been known for more than a decade; however, their severity and visual impact are still under investigation. A number of studies have been made to quantitatively describe glistenings in IOLs. Quantization and precise grading of IOLs will provide needed information to evaluate the severity and visual impact of glistenings in patients. We investigated the use of a simple modification of an integrating-sphere method to eliminate ballistic photons to quantitatively measure scattered light from glistenings in IOLs. The method described in this paper provides a simple and effective way to quantitatively characterize glistenings in vitro. It may be especially useful to quantify scattering associated with low-grade glistenings where the density of the scattering centers is low. Finally, the modified integrating-sphere method may also be generally applicable to quantitatively characterize scattering from other optical media.     

Patterns in Mie scattering: evolution when normalized by the Rayleigh cross section

An alternative to using the traditional scattering angle theta to describe light scattering from a uniform dielectric sphere is the dimensionless parameter qR, where R is the radius of the sphere, q = 2k sin(theta/2), and k is the wavenumber of the incident light. Simple patterns appear in the scattered intensity if qR is used in place of theta. These patterns are characterized by the envelopes approximating the scattered intensity distributions and are quantified by the phase-shift parameter rho = 2kR/m - 1/, where m is the real refractive index of the sphere. Here we find new patterns in these envelopes when the scattered intensity is normalized to the Rayleigh differential cross section. Mie scattering is found to be similar to Rayleigh scattering when p < 1 and follows simple patterns for p > 1, which evolve predictably as a function of p. These patterns allow us to present a unifying picture of the evolution of Mie scattering for changes in kR and m.     

Measurement of light scattering from a dielectric sphere behind a glass/air interface

Abstract

Experimental and theoretical light scattering data, for a microscopic sphere placed behind a glass/air interface and illuminated with p-polarized radiation, are compared. The intensity of light scattered from the sphere is measured as a function of scattering angle and these results are compared with established theory. This system is of interest in the field of optical biosensing.     

Monte Carlo study of solder paste microstructure and ultra-fine-pitch stencil printing

In this paper, we apply a Monte Carlo simulation technique to study the microstructure of solder pastes and investigate the influence of solder particle-size distribution on the ultra-fine-pitch stencil printing. First, the microstructures of bulk solder pastes with different particle-size distributions were generated using a random-packing model. Then a statistic model was applied to simulate the packing of solder paste inside the apertures. The numbers of solder particles and solid volume fraction embodied in the apertures were counted. Five particle-size distributions and two aperture shapes (circular and square) were investigated. Simulation results showed that the mean solid volume fraction of the solder particles inside the apertures is lower than that in the bulk solder paste. For the same aperture size and shape, as the particle size increases the mean solid volume fraction decreases and the standard deviation increases. This implies that to obtain consistent paste deposits in ultra-fine-pitch printing, the particle size must be proportionally reduced with the aperture size. The reasonable size ratio of the aperture to the solder particle was found to be around five. Excessive reduction in particle size could not improve the printing quality further, in contrast, it may lead to poor printability due to the increase in the paste viscosity and poor solder joints due to the generation of solder balls in the reflow soldering process.     

Light scattering from a sliced target through use of the internal field of infinite cylinders: comparison between Mie theory and a sliced sphere

Light scattering from a particle that can be sectioned into circular slices is calculated by performing a coherent integration of the internal field over the volume of the target. The internal field in each slice is taken to be the internal-field solution of an infinite cylinder of radius equal to the radius of the slice. It is shown that for a spherical scatterer with size parameters up to 1.4, the integration leads to results that are in good agreement with those predicted by the Mie theory. Thus, we show the remarkable result that the internal field from an infinite cylinder can reproduce scattering intensities for such a radically different shape as a sphere. This being the case, a wide variety of target shapes between a sphere and a cylinder should be open to evaluation by this technique. The approach also has the benefit of being computationally efficient, requiring a double integration of the internal field over a disk and then coherently adding these calculations. The computations demonstrated in this paper are performed relatively quickly on a computer such as the Macintosh Centris 650, and this efficiency allows us to obtain the scattered fields for many target shapes.     

Polarized light scattering by dielectric and metallic spheres on silicon wafers

The polarization and intensity of light scattered by monodisperse polystyrene latex and copper spheres, with diameters ranging from 92 to 218 nm, deposited on silicon substrates were measured with 442-, 532-, and 633-nm light. The results are compared with a theory for scattering by a sphere on a surface, originally developed by others [PhysicaA 137,209 (1986)], and extended to include coatings on the sphere and the substrate. The results show that accurate calculation of the scattering of light by a metal sphere requires that the near-field interaction between the sphere and its image be included in acomplete manner. The normal-incidence approximation does not suffice for this interaction, and the existence of any thin oxide layer on the substrate must be included in the calculation.     

Precision pointing and tracking through random media by exploitation of the enhanced backscatter phenomenon

The active illumination of a target through a turbulent medium with a monostatic transmitter-receiver results in a naturally occurring conjugate wave caused by reciprocal scattering paths that experience identical phase variations. This reciprocal path-scattering phenomenon produces an enhanced backscatter in the retroverse direction (precisely along the boresight of the pointing telescope). A dual aperture causes this intensity enhancement to take the form of Young's interference fringes. Interference fringes produced by the reciprocal path-scattering phenomenon are temporally stable even in the presence of time-varying turbulence. Choosing the width-to-separation ratio of the dual apertures appropriately and utilizing orthogonal polarizations to suppress the time-varying common-path scattered radiation allow one to achieve interferometric sensitivity in pointing accuracy through a random medium or turbulent atmosphere. Computer simulations are compared with laboratory experimental data. This new precision pointing and tracking technique has potential applications in ground-to-space laser communications, laser power beaming to satellites, and theater missile defense scenarios.     

Analysis and numerical computation of diffraction of an optical field by a subwavelength-size aperture in a thick metallic screen by use of a volume integral equation

Diffraction of an optical field by an aperture in a thick metallic screen is analyzed numerically by use of a three-dimensional volume integral equation together with a generalized conjugate residual method and fast Fourier transformation. Numerical results were validated by reciprocity and the independence of the results of the truncated discretized volume size used in numerical calculations. Near and far fields of square, circular, and triangular apertures in a thick screen are obtained numerically. Some of the numerical results obtained in the present study agree with previously reported experimental results. The surface plasmon polaritons excited on the sidewalls of the aperture can explain the basic characteristics of near-field distribution of apertures. The Bethe-Bouwkamp theory was found to be insufficient to explain the basic characteristics of the near field around the subwavelength aperture in a practical metallic screen.     

Finite-aperture wire grid polarizers

The transmission characteristics of wire grid polarizers fabricated in finite apertures are investigated by using a three-dimensional finite-difference time-domain formulation. Specifically, the optical transmissivity and extinction ratio are characterized for a wide variety of geometrical parameters including aperture size in both dimensions, conducting wire fill factor, and polarizer thickness. A dispersive material model is used to investigate the performance of polarizers fabricated by using realistic metals at infrared wavelengths. The results indicate that the aperture dimension significantly impacts the polarizer transmission behavior and that the extinction of the unwanted polarization is often limited by depolarizing scattering that is due to the finite aperture size.     

A Characterization of Light Scattering by Small Dielectric Spheres

Abstract

While using the standard Mie formulae to calculate the intensity distribution of light scattered by a small dielectric sphere, we have noticed some new qualitative results which suggest a boundary between a scattering regime qualitatively similar to the well known Rayleigh scattering distribution and that observed for rather larger particles. This may be of practical use when considering the position of photodetectors in light scattering experiments, for example particle velocimetry. The reason for such a transition has been discovered, using an unusual approximation in which the diameter of a sphere is made very small and its refractive index very large, with their product and the wavelength held constant.     

Electromagnetic fields for a spheroidal particle with an arbitrary embedded source

A spheroidal coordinate separation-of-variables solution has been developed for the determination of the internal, near-surface, and scattered fields of a spheroid (either prolate or oblate) with an embedded source of arbitrary type, location, and orientation. Presented results for (1,0) and (1,1) electric multipoles embedded in 2:1 axis ratio prolate and oblate spheroids (equal volume sphere size parameter equal to 20) illustrate that the presence of the spheroid interface can have a profound effect on the corresponding far-field scattering pattern. The calculation procedure could be used, for example, to model the emission of inelastic scattered light (Raman, fluorescence, etc.) from biological particles of appreciably elongated (prolatelike) or appreciably flattened (oblatelike) geometries.