Contrast and resolution enhancement of a near-field optical microscope by using a modulation technique

A new design of a tunneling near-field optical microscope (TNOM) combined with an atomic force microscope (AFM) is presented. This design can be used to generate three different images of the sample's surface: a non-contact (tapping mode) AFM image, a conventional TNOM and an image of a modulation signal of the conventional TNOM, which we call AC-TNOM. The images are obtained simultaneously, using a single light source. It is shown that the AC-TNOM has better resolution ( approximately 200A) and contrast compared to conventional TNOM ( approximately 400A).     

An instrument for fast acquisition of fluorescence decay curves at picosecond resolution designed for "double kinetics" experiments: Application to fluorescence resonance excitation energy transfer study of protein folding

The information obtained by studying fluorescence decay of labeled biopolymers is a major resource for understanding the dynamics of their conformations and interactions. The lifetime of the excited states of probes attached to macromolecules is in the nanosecond time regime, and hence, a series of snapshot decay curves of such probes might - in principle - yield details of fast changes of ensembles of labeled molecules down to sub-microsecond time resolution. Hence, a major current challenge is the development of instruments for the low noise detection of fluorescence decay curves within the shortest possible time intervals. Here, we report the development of an instrument, picosecond double kinetics apparatus, that enables recording of multiple fluorescence decay curves with picosecond excitation pulses over wide spectral range during microsecond data collection for each curve. The design is based on recording and averaging multiphoton pulses of fluorescence decay using a fast 13 GHz oscilloscope during microsecond time intervals at selected time points over the course of a chemical reaction or conformational transition. We tested this instrument in a double kinetics experiment using reference probes (N-acetyl-tryptophanamide). Very low stochastic noise level was attained, and reliable multi-parameter analysis such as derivation of distance distributions from time resolved FRET (fluorescence resonance excitation energy transfer) measurements was achieved. The advantage of the pulse recording and averaging approach used here relative to double kinetics methods based on the established time correlated single photon counting method, is that in the pulse recording approach, averaging of substantially fewer kinetic experiments is sufficient for obtaining the data. This results in a major reduction in the consumption of labeled samples, which in many cases, enables the performance of important experiments that were not previously feasible.     

A mixed formulation of the Bingham fluid flow problem: Analysis and numerical solution

In this paper we introduce a mixed formulation of the Bingham fluid flow problem. We consider both the original and a regularized version of the problem, where a parameter ε is introduced, forcing the entire domain to be formally a fluid region. In general, common solvers for the regularized problem experience a performance degradation when the parameter ε gets smaller. The method studied here introduces an auxiliary tensor variable and shows enhanced numerical properties for small values of ε. A good performance is also observed for the non-regularized case. The well posedness for the regularized problem and the equivalence – at the continuous level – between the original (primitive variables) and the mixed formulation are demonstrated. We analyze properties of linearized problems that are relevant for the convergence of numerical solvers. A finite element method for the mixed formulation is discussed. Numerical results confirm the predicted better performances of the mixed formulation when compared to the primitive variables formulation. A comparison to a non-regularized solver based on the augmented Duvaut–Lions–Glowinski formulation of the problem is carried out as well.     

Optical nanomanipulations of malignant cells: controlled cell damage and fusion

Specifically targeting and manipulating living cells is a key challenge in biomedicine and in cancer research in particular. Several studies have shown that nanoparticles irradiated by intense lasers are capable of conveying damage to nearby cells for various therapeutic and biological applications. In this work ultrashort laser pulses and gold nanospheres are used for the generation of localized, nanometric disruptions on the membranes of specifically targeted cells. The high structural stability of the nanospheres and the resonance pulse irradiation allow effective means for controlling the induced nanometric effects. The technique is demonstrated by inducing desired death mechanisms in epidermoid carcinoma and Burkitt lymphoma cells, and initiating efficient cell fusion between various cell types. Main advantages of the presented approach include low toxicity, high specificity, and high flexibility in the regulation of cell damage and cell fusion, which would allow it to play an important role in various future clinical and scientific applications.     

An Efficient Algorithm for Matrix-Valued and Vector-Valued Optimal Mass Transport

We present an efficient algorithm for recent generalizations of optimal mass transport theory to matrix-valued and vector-valued densities. These generalizations lead to several applications including diffusion tensor imaging, color image processing, and multi-modality imaging. The algorithm is based on sequential quadratic programming. By approximating the Hessian of the cost and solving each iteration in an inexact manner, we are able to solve each iteration with relatively low cost while still maintaining a fast convergence rate. The core of the algorithm is solving a weighted Poisson equation, where different efficient preconditioners may be employed. We utilize incomplete Cholesky factorization, which yields an efficient and straightforward solver for our problem. Several illustrative examples are presented for both the matrix and vector-valued cases.     

Attentional asymmetry in schizophrenia: Disengagement and inhibition of return deficits.

This research examined 2 components of visual orienting in medicated schizophrenia patients: the validity effect and the inhibition of return (IOR). In the 1st experiment, patients showed the expected asymmetry in orienting attention, that is, larger validity effect in the right visual field than in the left. However, this asymmetry was due to a deficit in facilitatory processes rather than a disengagement deficit. In addition, patients showed a deficit in IOR. In the 2nd experiment, a 2nd central cue for summoning attention, explicitly, back to the center was used. In this experiment, normal IOR in schizophrenia patients was found. Because it was shown that schizophrenia patients do not have a disengagement deficit, IOR possibly could not be observed because of the increased facilitation in that location. It was proposed that the abnormality in visual attention in schizophrenia is due to a deficit in inhibitory processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)