Flow cytometric determination of size and complex refractive index for marine particles: comparison with independent and bulk estimates

We advance a method to determine the diameter D and the complex refractive index (n + n'i) of marine particles from flow cytometric measurements of forward scattering, side scattering, and chlorophyll fluorescence combined with Mie theory. To understand better the application of Mie theory with its assumptions to flow cytometry (FCM) measurements of phytoplankton cells, we evaluate our flow cytometric-Mie (FCM-Mie) method by comparing results from a variety of phytoplankton cultures with independent estimates of cell D and with estimates of n and n' from the inversion of bulk measurements. Cell D initially estimated from the FCM-Mie method is lower than independent estimates, and n and n' are generally higher than bulk estimates. These differences reflect lower forward scattering and higher side scattering for single-cell measurements than predicted by Mie theory. The application of empirical scattering corrections improves FCM-Mie estimates of cell size, n, and n'; notably size is determined accurately for cells grown in both high- and low-light conditions, and n' is correlated with intracellular chlorophyll concentration. A comparison of results for phytoplankton and mineral particles suggests that differences in n between these particle types can be determined from FCM measurements. In application to natural mixtures of particles, eukaryotic pico/nanophytoplankton and Synechococcus have minimum mean values of n' in surface waters, and nonphytoplankton particles have higher values of n than phytoplankton at all depths.     

Discrete-dipole-approximation-based light-scattering calculations for particles with a real refractive index smaller than unity

To assess the efficiency and accuracy of light-scattering calculations based on the discrete dipole approximation (DDA) for particles with a real relative refractive index smaller than unity, differential scattering cross sections and scattering efficiency factors were calculated for spherical particles. We performed the calculations for oxide particles and voids embedded in glass and silicon, using the exact scattering theory (Mie scattering) and the DDA. A comparison of the results shows that the DDA is applicable in the above refractive-index regime, and the conditions under which DDA-based calculations can provide scattering data with good accuracy are discussed.     

Photometric immersion refractometry: a method for determining the refractive index of marine microbial particles from beam attenuation

Photometric immersion refractometry is a technique for determining the refractive index of particulate material. In this technique, the attenuation of light by a suspension of particles is measured as a function of the refractive index of the immersion medium. A minimum attenuation occurs at the refractive index of the medium equal to that of the particles. This technique can serve as a benchmark method for the refractive index determination because it is independent of assumptions invoked by other techniques, such as those based on the inversion of the spectral attenuation data. We present a rigorous model of the photometric immersion refractometry based on the anomalous diffraction approximation for the attenuation efficiency of particles. This model permits one to determine the average value of the real part of the refractive index of the particles, its variance, and the average imaginary part of the refractive index of the particles. In addition, the fourth moment of the particle size distribution can be determined if the concentration and shape of the particles are known. We analyze the sensitivity of this model to experimental errors and discuss the applicability of photometric immersion refractometry to marine microbial particles. We also present experimental results of this technique as applied to heterotrophic marine bacteria. The results indicate that the refractive index of these bacteria was narrowly distributed about the average value of 1.3886.     

Shadow photoelectric method for determining the refractive index of a gas flow behind a shock wave

A quantitative shadow photoelectric method for the investigation of transient processes is described. The capabilities of the method are illustrated in the example of investigations of high-temperature gas flows behind strong shock waves in shock tubes and the determination of the refractive index and its gradient in the zone of nonequilibrium processes following a shock wave.Translated from Izmeritel'naya Tekhnika, No. 7, pp. 21–23, July, 1994.     

一种液体折射率的薄膜干涉测量方法

理论分析表明在镀有金属分光膜的玻璃介质和金属介质构成的几何楔形成的斐索干涉条纹中,相长干涉并不位于相邻的两相消干涉的中央,而是存在一明显偏移,这一偏移与引入几何楔中待测液体的折射率相关.基于该原理,提出了一种新的折射率测量的方法,并对该方进行了实验验证.该方法不需要利用标准液对光学几何量进行标定,测量范围广.     

Tightly focused epimicroscope technique for submicrometer-resolved highly sensitive refractive index measurement of an optical waveguide

We introduce a very sensitive new configuration, to the best of our knowledge, in an optical microscope system that utilizes two detectors: one is to measure the power of a low reflected signal from a sample, and the other is only to monitor the confocal geometry of the system. With this new configuration, we could effectively remove measurement noise associated with small perturbation in measurement conditions such as surface curvature, tilt, and vibration in a microscope system. We have obtained a high-resolution relative index precision of 9x10(-5) by employing this novel technique with two detectors.     

Tomographic reconstruction of three-dimensional refractive index fields by use of a regularized phase-tracking technique and a polynomial approximation method

We present a complete data-processing procedure for quantitative reconstruction of three-dimensional (3D) refractive index fields from limited multidirectional interferometric data. The proposed procedure includes two parts: (1) extraction of the projection data from limited multidirectional interferograms by a regularized phase-tracking technique and wavefront fitting and (2) reconstruction of the 3D refractive index fields by a modified polynomial approximation method. It has been shown that the procedure gives a satisfactory solution to the reconstruction issue in interferometric tomography, from the initial projection data extraction to the final image reconstruction. Computer simulation and experimental results are both presented.     

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.     

Radiative transfer equations with varying refractive index: a mathematical perspective

Established mathematical techniques to model the energy density of high-frequency waves in random media by radiative transfer equations and to model the small mean-free-path limit of radiative transfer solutions by diffusion equations are reviewed. These techniques are then applied to the derivation of radiative transfer and diffusion equations for the radiance, also known as specific intensity, of electromagnetic waves in situations where the refractive index of the underlying structure varies smoothly in space.     

Resonant enhancement of refractive index in a cascade scheme

The refractive index of coherently driven rubidium vapour is experimentally investigated in a three-level cascade configuration using a selective reflection technique. The maximum measured resonant change in the refractive index is Δn ? 0.1. The selective reflection is accompanied by a four-wave-mixing process that can reach ~90% efficiency.     

Radiative transfer in a turbid medium with a varying refractive index: comment

In the literature one can encounter at least two different radiative transfer equations for media with spatially varying refractive indices. These are the results of Ferwerda [J. Opt. A Pure Appl. Opt. 1, L1 (1999)] and Tualle and Tinet [Opt. Commun. 228, 33 (2003)]. Accordingly, two different diffusion approximations are derived from these two radiative transfer equations. I reconsider the derivation of the radiative transfer equation in a medium with an inhomogeneous refractive index and confirm the result of Tualle and Tinet. In the diffusion approximation, a simple analytical solution has been found for the steady-state illumination of a non-absorbing turbid medium with a varying refractive index.     

Circular harmonic filters for the recognition of marine microorganisms

We present an application of circular-harmonic filters (CHF's) for the recognition of planktonic microorganisms. CHF's discriminated both genera Acartia and Calanus. The symmetry of genus Acartia permitted discrimination to the species and sex levels, whereas the asymmetry of the genus Calanus permitted discrimination only to the generic level. The differences among organisms of different sex of the genus Calanus could not be detected by these particular CHF's. More research needs to be carried out with more complex CHF's to enhance their performance and to permit the implementation of an automated optodigital system to identify and count marine microorganisms.     

Synthesis of high refractive index composites for photonic applications

Composites with enhanced refractive index have been synthesised through a sol–gel process. A refractive index of approximately 1.70 has been achieved at a concentration of 75 wt% TiO₂. The refractive index increases with the concentration of TiO₂. The influence of the concentration of TiO₂ on the mechanical properties has been studied. The potential application of these materials in photonics area has been demonstrated by fabricating multi-layer spherical voids in them through a femtosecond laser-driven micro-explosion method.     

Synthesis of high refractive index composites for photonic applications

Composites with enhanced refractive index have been synthesised through a sol–gel process. A refractive index of approximately 1.70 has been achieved at a concentration of 75 wt% TiO₂. The refractive index increases with the concentration of TiO₂. The influence of the concentration of TiO₂ on the mechanical properties has been studied. The potential application of these materials in photonics area has been demonstrated by fabricating multi-layer spherical voids in them through a femtosecond laser-driven micro-explosion method.     

Measurements of a nonlinear refractive index with a single laser pulse

We demonstrate a theoretical model and the experimental proof of a new method for n(2) measurements using single pulses. The advantage of the method is fast measurement, a simple setup, and independence from small nonlinear absorptions and thermal effects. We report on first n(2) measurements with the new technique on carbon disulfide, toluene, and a polymer dissolved in toluene to demonstrate the measurement of positive and negative third-order nonlinearities. We achieved a sensitivity of greater than lambda/200 phase-front distortion using picosecond Nd:YLF laser pulses. The given theory shows that attainment of higher sensitivities is possible.     

Frequency-domain methods for a vibration-fatigue-life estimation – Application to real data

The characterization of vibration-fatigue strength is one of the key parts of mechanical design. It is closely related to structural dynamics, which is generally studied in the frequency domain, particularly when working with vibratory loads. A fatigue-life estimation in the frequency domain can therefore prove advantageous with respect to a time-domain estimation, especially when taking into consideration the significant performance gains it offers, regarding numerical computations. Several frequency-domain methods for a vibration-fatigue-life estimation have been developed based on numerically simulated signals. This research focuses on a comparison of different frequency-domain methods with respect to real experiments that are typical in structural dynamics and the automotive industry. The methods researched are: Wirsching–Light, the α_0.75 method, Gao–Moan, Dirlik, Zhao–Baker, Tovo–Benasciutti and Petrucci–Zuccarello. The experimental comparison researches the resistance to close-modes, to increased background noise, to the influence of spectral width, and multi-vibration-mode influences. Additionally, typical vibration profiles in the automotive industry are also researched. For the experiment an electro-dynamic shaker with a vibration controller was used. The reference-life estimation is the rainflow-counting method with the Palmgren–Miner summation rule. It was found that the Tovo–Benasciutti method gives the best estimate for the majority of experiments, the only exception being the typical automotive spectra, for which the enhanced Zhao–Baker method is best suited. This research shows that besides the Dirlik approach, the Tovo–Benasciutti and Zhao–Baker methods should be considered as the preferred methods for fatigue analysis in the frequency domain.