Flow-dependent optofluidic particle trapping and circulation

Microfluidics and fiber optics are integrated in-plane to achieve several flow-dependent particle trapping mechanisms on-chip. Each mechanism results from a combination of fluid drag and optical scattering forces. Parallel and offset fibers, orthogonally oriented to the flow, show cyclic cross-stream particle transit with flow-dependent particle trajectories and loss. Upstream-angled fibers with flow result in circulatory particle trajectories. Asymmetric angled fibers result in continuous particle circulation whereas symmetry with respect to the flow axis enables both stable trapping and circulation modes. Stable trapping of single particles, self-guided multi-particle arrays and particle assemblies are demonstrated with a single upstream-oriented fiber. Size tuning of trapped multiple particle assemblies is also presented. The planar interaction of fluid drag and optical forces results in novel possibilities for cost-effective on-chip diagnostics, mixing, flow rate monitoring, and cell analysis.     

On nonlinear translation-varying operators

In this work nonlinear translation-varying operators are analyzed and represented by means of a generalized impulse response. This is the response of the transpose operator to the family of shifted impulse functionals. Continuous operators from a topological vector space into the space of functions on Rⁿ, as well as $\text{A}$-bounded operators, are investigated.     

A Linear Goal Programming Model for Planning the Exports of Emerging Countries

A linear goal programming model for planning the exports of emerging countries is formulated in this paper. The model includes some economic factors which may affect the exports, e.g. industrial structure, labour force, value added in exports, capital efficiency, imports of raw material used for manufacturing goods designated for exports, investment plan, etc. An illustrative example is presented. A sensitivity analysis was performed in order to examine the effect of various changes made in the model on the results. It is felt that the model presented in this study would be useful to economic planners in emerging countries.     

Propagation of the two-frequency coherence function in an inhomogeneous background random medium

The spatial and temporal structures of time-dependent signals can be appreciably affected by random changes of the parameters of the medium characteristic of almost all geophysical environments. The dispersive properties of random media cause distortions in the propagating signal, particularly in pulse broadening and time delay. When there is also spatial variation of the background refractive index, the observer can be accessed by a number of background rays. In order to compute the pulse characteristics along each separate ray, there is a need to know the behaviour of the two-frequency mutual coherence function. In this work, we formulate the equation of the two-frequency mutual coherence function along a curved background ray trajectory. To solve this equation, a recently developed reference-wave method is applied. This method is based on embedding the problem into a higher dimensional space and is accompanied by the introduction of additional coordinates. Choosing a proper transform of the extended coordinate system allows us to emphasize 'fast' and 'slow' varying coordinates which are consequently normalized to the scales specific to a given type of problem. Such scaling usually reveals the important expansion parameters defined as ratios of the characteristic scales and allows us to present the proper ordering of terms in the desired equation. The performance of the main order solution is demonstrated for the homogeneous background case when the transverse structure function of the medium can be approximated by a quadratic term.     

Optimizing the resolution of nanohole arrays in metal films for refractive-index sensing

We optimized the resolution of nanohole arrays in metal films for refractive-index sensing by increasing the sensitivity with modifications to the hole-array parameters and by increasing the signal to noise ratio of the sensor system. The nanohole-array parameters (including film thickness, periodicity and diameter) were first optimized by finite-difference time-domain simulations, and then the arrays were fabricated and tested, showing good agreement between the two cases (theory and experiment) in terms of optimal parameters. To improve the sensitivity and to reduce the noise, the laser source wavelength was optimized (including the efficiency of the camera for detection) and the intensity was increased. A?bulk resolution of 6×10^?7 refractive-index units was demonstrated. Due to the collinear microscope geometry and potential for multiplexing of nanohole arrays, these results are encouraging for future biosensing applications.     

Application of the cross-entropy method to clustering and vector quantization

We apply the cross-entropy (CE) method to problems in clustering and vector quantization. The CE algorithm for clustering involves the following iterative steps: (a) generate random clusters according to a specified parametric probability distribution, (b) update the parameters of this distribution according to the Kullback–Leibler cross-entropy. Through various numerical experiments, we demonstrate the high accuracy of the CE algorithm and show that it can generate near-optimal clusters for fairly large data sets. We compare the CE method with well-known clustering and vector quantization methods such as K-means, fuzzy K-means and linear vector quantization, and apply each method to benchmark and image analysis data.     

Surface plasmon-quantum dot coupling from arrays of nanoholes

The coupling of semiconductor quantum dots (QDs) to the surface plasmon (SP) modes of nanohole arrays in a metal film was demonstrated for the first time, showing enhancement in the spontaneous emission by 2 orders of magnitude. The SP-enhanced transmission resonances of the nanohole arrays were tuned around the photoluminescence (PL) peak of polystyrene-b-poly(acrylic acid) (PS-b-PAA)-stabilized cadmium sulfide (CdS) quantum dots (QDs) in contact with the arrays. As a result the overall PL from the SP-QD system was enhanced by 2 orders of magnitude, even after excluding the enhanced transmission of the nanohole array without the QDs. The maximum enhancement occurred when the resonance from the nanohole array matched the QD PL spectrum. Time-resolved PL measurements were used to estimate the relative contribution of different physical mechanisms to the enhanced spontaneous emission. The increased spontaneous emission in the SP-QD system is promising for prospective plasmonic light-emitting devices incorporating QDs.