Holographic optical elements — State-of-the-art review:Part I

A state-of-the-art review on holographic optical elements (HOE) is presented in two parts. In Part I a conceptual overview and an assessment of the current status on the design of HOE have been included. It is pointed out that HOE development based on the use of squeezed light, speckle, non-linear recording, comparative studies between optics and communication approaches, are some of the promising directions for future research in this vital area of photonics.

Computationally efficient optical tomographic reconstructions through waveletizing the normalized quadratic perturbation equation - art. no.61640R

In this paper, we present a wavelet - based approach to solve the non-linear perturbation equation encountered in optical tomography. A particularly suitable data gathering geometry is used to gather a data set consisting of differential changes in intensity owing to the presence of the inhomogeneous regions. With this scheme, the unknown image, the data, as well as the weight matrix are all represented by wavelet expansions, thus yielding the representation of the original non - linear perturbation equation in the wavelet domain. The advantage in use of the non-linear perturbation equation is that there is no need to recompute the derivatives during the entire reconstruction process. Once the derivatives are computed, they are transformed into the wavelet domain. The purpose of going to the wavelet domain, is that, it has an inherent localization and de-noising property. The use of approximation coefficients, without the detail coefficients, is ideally suited for diffuse optical tomographic reconstructions, as the diffusion equation removes most of the high frequency information and the reconstruction appears low-pass filtered. We demonstrate through numerical simulations, that through solving merely the approximation coefficients one can reconstruct an image which has the same information content as the reconstruction from a non-waveletized procedure. In addition we demonstrate a better noise tolerance and much reduced computation time for reconstructions from this approach.

Elimination of cross-talk in diffuse optical tomographic reconstructions through a weighted update procedure: results of simulation study - art. no. 61640S

Reconstructions in optical tomography involve obtaining the images of absorption and reduced scattering coefficients. The integrated intensity data has greater sensitivity to absorption coefficient variations than scattering coefficient. However, the sensitivity of intensity data to scattering coefficient is not zero. We considered an object with two inhomogeneities (one in absorption and the other in scattering coefficient). The standard iterative reconstruction techniques produced results, which were plagued by cross talk, i.e., the absorption coefficient reconstruction has a false positive corresponding to the location of scattering inhomogeneity, and vice-versa. We present a method to remove cross talk in the reconstruction, by generating a weight matrix and weighting the update vector during the iteration. The weight matrix is created by the following method: we first perform a simple backprojection of the difference between the experimental and corresponding homogeneous intensity data. The built up image has greater weightage towards absorption inhomogeneity than the scattering inhomogeneity and its appropriate inverse is weighted towards the scattering inhomogeneity. These two weight matrices are used as multiplication factors in the update vectors, normalized backprojected image of difference intensity for absorption inhomogeneity and the inverse of the above for the scattering inhomogeneity, during the image reconstruction procedure. We demonstrate through numerical simulations, that cross-talk is fully eliminated through this modified reconstruction procedure.

Studies on optical absorption and photoluminescenceproperties of Pr3+ and Nd3+ doped mixed alkali chloroborate glasses

This article presents the optical absorption and emission properties of Pr3+ and Nd3+ doped two different mixed alkali chloroborate glass matrices of the type 70B(2)O(3)center dot xLiCl center dot(30 - x)NaCl and 70B(2)O(3)center dot xLiCl center dot(30 - x)KCl (x = 5, 10, 15.20 and 25). The variation of Judd-Ofelt parameters (Omega(2), Omega(4) and Omega(6)), total radiative transition probabilities (A(T)), radiative lifetimes (tau(R)) and emission cross-sections (sigma(p)) with the variation of alkali contents in the glass matrix have been discussed in detail. The changes in the peak wavelengths of the hypersensitive transition and intensity parameters with x are correlated to the structural changes in the host matrix. The estimated radiative lifetimes of certain excited states of Pr3+ and Nd3+ in these two glass matrices are reported. Peak stimulated emission cross-sections (sigma(p)) are reported for the observed emission transitions of Pr3+ and Nd3+ ions. Branching ratios (beta) of the observed emission transitions obtained from the Judd-Ofelt theory are compared with the values obtained from the emission spectra. (C) 2010 Elsevier B.V. All rights reserved.

Photoacoustic investigation of the optical absorption and thermal diffusivity in SixTe100−x glassesstar

The photoacoustic technique is used to determine the optical energy gap E0 of bulk SixTe100−x glasses in the glass-forming region 10 ≤ x ≤ 28. The thermal diffusivity α of these samples has also been measured. The variation of E0 and α with x is reported. It is found that E0 increases with x nearly linearly with a sharp decrease in the rate of increase beyond x = 20. The thermal diffusivity also increases with x up to x = 20 but decreases for compositions with higher values of x. The observed behaviour is explained on the basis of a chemical bond approach. It is accounted for in terms of the increase in the number of Te---Te bonds and formation of SiTe4 tetrahedra with an increase in the chalcogen content.

Transcriptional switching in Escherichia coli during stress and starvation by modulation of Sigma 70 activity

During active growth of Escherichia coli, majority of the transcriptional activity is carried out by the housekeeping sigma factor (Sigma 70), whose association with core RNAP is generally favoured because of its higher intracellular level and higher affinity to core RNAP. In order to facilitate transcription by alternative sigma factors during nutrient starvation, the bacterial cell uses multiple strategies by which the transcriptional ability of Sigma 70 is diminished in a reversible manner. The facilitators of shifting the balance in favour of alternative sigma factors happen to be as diverse as a small molecule (p)ppGpp (represents ppGpp or pppGpp), proteins (DksA, Rsd) and a species of RNA (6S RNA). Although 6S RNA and (p)ppGpp were known in literature for a long time, their role in transcriptional switching has been understood only in recent years. With themelucidation of function of DksA, a new dimension has been added to the phenomenon of stringent response. As the final outcome of actions of (p)ppGpp, DksA, 6S RNA and Rsd is similar, there is a need to analyse hese mechanisms in a collective manner. We review the recent trends in understanding the regulation of Sigma 70 by (p)ppGpp, DksA, Rsd and 6S RNA and present a case for evolving a unified model of RNAP redistribution during starvation by modulation of Sigma 70 activity in E. coli.

A novel two sources ultrasound modulated optical tomographic system for screening breast cancer through elasticity characterization

The study of non-invasive characterization of elastic properties of soft biological tissues has been a focus of active researches since recent years. Light is highly scattered by biological tissues and hence, sophisticated reconstruction algorithms are required to achieve good imaging depth and a reasonable resolution. Ultrasound (US), on the otherhand, is less scattered by soft tissues and it has been in use for imaging in biomedical ultrasound systems. Combination of the contrast sensitivity of light and good localization of ultrasound provides a challenging technique for characterization of thicker tissues deep inside the body non-invasively. The elasticity of the tissues is characterized by studying the response of tissues to mechanical excitation induced by an acoustic radiation force (remotely) using an optical laser. The US modulated optical signals which traverse the tissue are detected by using a CCD camera as detector array and the pixel map formed on the CCD is used to characterize the embedded inhomogeneities. The use of CCD camera improves the signal-noise-ratio (SNR) by averaging the signals from all of the CCD pixels.

Influence of substrate temperature and post-deposition heat treatment on the optical properties of SiO2 films

Silicon dioxide films are extensively used as protective, barrier and also low index films in multilayer optical devices. In this paper, the optical properties of electron beam evaporated SiO2 films, including absorption in the UV, visible and IR regions, are reported as a function of substrate temperature and post-deposition heat treatment. A comparative study of the optical properties of SiO2 films deposited in neutral and ionized oxygen is also made.

Contribution of sea-salt to aerosol optical depth over the Arabian Sea derived from MODIS observations

Making use of aerosol optical depths (AOD) derived from MODIS (onboard TERRA satellite) and winds from NCEP, and the fact that sea-salt optical depth over ocean is determined primarily by sea-surface wind speed, we examine the contribution of sea-salt to the composite aerosol optical depth ( AOD) over Arabian Sea ( AS), by developing empirical models for characterizing wind-speed dependence of sea-salt optical depth. We show that at high wind speeds, sea-salt contributes 81% to the coarse mode and 42% to the composite AOD in the southern AS. In contrast to this, over the northern AS, share of sea-salt to coarse mode and composite optical depth is only 35% and 16% respectively. Comparison of the sea-salt optical depth and coarse mode optical depth ( MODIS) showed excellent agreement. The sea-salt optical depth over AS at moderate to high wind speed is comparable to the anthropogenic AOD reported for this region during winter.

Influence of crossed electric and quantizing magnetic fields on the Einstein relation in nonlinear optical, optoelectronic and related materials: Simplified theory, relative comparison and suggestion for experimental determination

An attempt is made to study the Einstein relation for the diffusivity-to-mobility ratio (DMR) under crossed fields' configuration in nonlinear optical materials on the basis of a newly formulated electron dispersion law by incorporating the crystal field in the Hamiltonian and including the anisotropies of the effective electron mass and the spin-orbit splitting constants within the framework of kp formalisms. The corresponding results for III-V, ternary and quaternary compounds form a special case of our generalized analysis. The DMR has also been investigated for II-VI and stressed materials on the basis of various appropriate dispersion relations. We have considered n-CdGeAs2, n-Hg1-xCdxTe, n-In1-xGaxAsyP1-y lattice matched to InP, p-CdS and stressed n-InSb materials as examples. The DMR also increases with increasing electric field and the natures of oscillations are totally band structure dependent with different numerical values. It has been observed that the DMR exhibits oscillatory dependences with inverse quantizing magnetic field and carrier degeneracy due to the Subhnikov-de Haas effect. An experimental method of determining the DMR for degenerate materials in the present case has been suggested. (C) 2010 Elsevier B.V. All rights reserved.

Photo induced optical changes in Sb/As2S3 multilayered film and (As2S3)(0.93)Sb-0.07 film of equal thickness

The increase in optical band gap (photo bleaching) due to light illumination was studied at room temperature as well as at low (4.2 K) temperature for Sb/As2S3 multilayered film of 640 nm thickness by Fourier Transform Infrared Technique. The interdiffusion of Sb into As2S3 matrix results the formation of Sb-As2S3 ternary solid solutions which is explained by the change in optical band gap (E-g), absorption coefficient (alpha), Tauc parameter (B-1/2), Urbach edge (E-e). At the same time, photo darkening phenomena was observed in (As2S3)(0.93)Sb-0.07 film of same thickness both at low and room temperatures. From our X-ray Photoelectron Spectroscopy measurements,we are able to show that some of the As-As, S-S and Sb-Sb bonds are converted into As-S and S-Sb bonds in case of multilayers. We found that the energetically favoured heteropolar bond formation take place by a phonon-assisted mechanism using the lone pair pi electrons of S-2(0). But in case of (As2S3)(0.93)Sb-0.02 film, the homopolar bonds are playing a major role. (C) 2010 Elsevier B.V. All rights reserved.

ESR and optical studies of Mo5+ and W5+ ions in phosphomolybdate and phosphotungstate glasses

ESR and optical studies of phosphomolybdate and phosphotungstate glasses are discussed. Both the ESR and optical results indicate that molybdenum or tungsten ions are present in distorted octahedral environments in these glasses. In addition, ESR spectra of Mo5+ and W5+ ions show that the d electrons are localized on molybdenum and tungsten sites respectively. The variation of gperpendicular and gshort parallel values has been examined using appropriate structural models of these glasses.