Biological microparticles, including bacteria, scatter light in all directions when illuminated. The complex scatter pattern is dependent on particle size, shape, refraction index, density, and morphology. Commercial flow cytometers allow measurement of scattered light intensity at forward and perpendicular (side) angles (28 y 1 208 and 708 y 2 1108, respectively) with a speed varying from 10 to 10,000 particles per second. The choice of angle is dictated by the fact that scattered light in the forward region is primarily dependent on cell size and refractive index, whereas side-scatter intensity is dependent on the granularity of cellular structures. However, these two-parameter measurements cannot be used to separate populations of cells of similar shape, size, or structure. Hence, there have been several attempts in flow cytometry to measure the entire scatter patterns. The published concepts require the use of unique custom-built flow cytometers and cannot be applied to existing instruments. It was also not clear how much information about patterns is really necessary to separate various populations of cells present in a given sample. The presented work demonstrates application of pattern-recognition techniques to classify particles on the basis of their discrete scatter patterns collected at just five different angles, and accompanied by the measurement of axial light loss. The proposed approach can be potentially used with existing instruments because it requires only the addition of a compact enhanced scatter detector. An analytical model of scatter of laser beams by individual bacterial cells suspended in a fluid was used to determine the location of scatter sensors. Experimental results were used to train the support vector machine-based pattern recognition system. It has been shown that information provided just by five angles of scatter and axial light loss can be sufficient to recognize various bacteria with 68-99% success rate. ' 2007 International Society for Analytical Cytology

Chiral metamaterials can have diverse technological applications, such as engineering strongly twisted local electromagnetic fields for sensitive detection of chiral molecules, negative indices of refraction, broadband circular polarization devices, and many more. These are commonly achieved by arranging a group of noble-metal nanoparticles in a chiral geometry, which, for example, can be a helix, whose chiroptical response originates in the dynamic electromagnetic interactions between the localized plasmon modes of the individual nanoparticles. A key question relevant to the chiroptical response of such materials is the role of plasmon interactions as the constituent particles are brought closer, which is investigated in this paper through theoretical and experimental studies. The results of our theoretical analysis, when the particles are brought in close proximity are dramatic, showing a large red shift and enhancement of the spectral width and a near-exponential rise in the strength of the chiroptical response. These predictions were further confirmed with experimental studies of gold and silver nanoparticles arranged on a helical template, where the role of particle separation could be investigated in a systematic manner. The "optical chirality" of the electromagnetic fields in the vicinity of the nanoparticles was estimated to be orders of magnitude larger than what could be achieved in all other nanoplasmonic geometries considered so far, implying the suitability of the experimental system for sensitive detection of chiral molecules. ■ INTRODUCTIONStrong light matter interactions in metallic nanoparticles (NPs), especially those made of noble metals such as gold and silver, is at the heart of much ongoing research in nanophotonics. Individual NPs can support collective excitations (plasmons) of the electron plasma at certain wavelengths, known as the localized surface plasmon resonance (LSPR), 1 which forms the basis of bright colors in colloidal solutions of Au and Ag and provides a powerful platform for various sensing, imaging, and therapeutic technologies. 2,3 It is interesting to note that the optical properties of a collection of NPs can be significantly different from isolated particles, an effect that originates in the electromagnetic interactions between the localized plasmon modes. This problem has been considered in one and two dimensions with periodic, 4−7 as well as with random, 8−10 arrays of NPs, and it was found that as the particles are brought closer, the plasmon resonances show distinct broadening, along with significant red or blue shifts in their spectral positions, where the sign of the shift depends on the direction of polarization with respect to the interparticle axes. With recent advances in nanotechnology, it has now been possible to develop wafer scale methods of fabricating 3-D arrays of NPs, 11,12 whose sizes and spacing could be engineered with high precision. Of great current interest is that of a chiral arrangement of NPs 13−16 that has been shown to give rise to strong chi...

The altered spontaneous emission of an emitter near an arbitrary body can be elucidated using an energy balance of the electromagnetic field. From a classical point of view it is trivial to show that the field scattered back from any body should alter the emission of the source. But it is not at all apparent that the total radiative and non-radiative decay in an arbitrary body can add to the vacuum decay rate of the emitter (i.e.) an increase of emission that is just as much as the body absorbs and radiates in all directions. This gives us an opportunity to revisit two other elegant classical ideas of the past, the optical theorem and the Wheeler-Feynman absorber theory of radiation. It also provides us alternative perspectives of Purcell effect and generalizes many of its manifestations, both enhancement and inhibition of emission. When the optical density of states of a body or a material is difficult to resolve (in a complex geometry or a highly inhomogeneous volume) such a generalization offers new directions to solutions.

We present experimental and theoretical results on monolayer colloidal cadmium selenide quantum dot films embedded with tiny gold nanoparticles. By varying the density of the embedded gold nanoparticles, we were able to engineer a plasmon-mediated crossover from emission quenching to enhancement regime at interparticle distances for which only quenching of emission is expected. This crossover and a nonmonotonic variation of photoluminescence intensity and decay rate, in experiments, is explained in terms of a model for plasmonmediated collective emission of quantum emitters which points to the emergence of a new regime in plasmonexciton interactions. The presented methodology to achieve enhancement in optical quantum efficiency for optimal doping of gold nanoparticles in such ultrathin high-density quantum dot films can be beneficial for new-generation displays and photodetectors.

1 Experimental Methods: a) Materials: Cadmium oxide (CdO) powder, Zinc oxide (ZnO), Trioctylphosphine (T OP ), Selenium powder, Sulphur, Oleic acid (OA), ODE, Chloroform and Toluene, were obtained from Sigma-Aldrich (Germany) for the synthesis of QDs. Dodecanethiol (DDT ), HAuCl 4 , N aBH 4 granules, N aOH, Hexene also procured from Sigma-Aldrich and Acetone,hexane, HCL from HDFCL for AuNP synthesis. Deionized water (18.2 Megaohms-cm, Milipore) was used for all Langmuir-Blodgett (LB) film transfer. b) Synthesis and characterization of Quantum Dot: The CdSe-ZnS core-shell quantum dots are prepared traditional hot-injection synthesis of CdSe cores followed by slow growth of a ZnS shell from solution. CdO and ZnO are mixed and dissolved in OA and ODE and the mixture is heated up to 310 deg C to get clear solution and then inject required amount of TOP-S and TOP-Se stock solution in the three neck flux containing hot Cadmium based growth solution. After injection, we maintain a constant temperature of 300 deg C for desired amount of time to get proper size of QD. This quantum dot has Cadmium selenide (CdSe) spherical core and it is covered by Zinc-Selenide (ZnS) shell. By keeping the temperature constant at 300 degree for 10 min, the formed QD has diameter around 6.7 nm and Photoluminescence emission at wavelength 570 nm. c)Synthesis and characterization of Gold Nano Praticle: Gold nanoparticles (AuNP) are made in water by borohydride reduction and phase-transferred to hexane. This dodecanethiolate-protected gold nanoparticles synthesis is a fast process and does not require a cleaning step. The stock solution of 50 mM gold chloride was made by dissolving HAuCl 4 − 3H 2 O with the same molar concentration of HCl in a glass vial and the aqueous stock solution of 50 mM borohydride was made by dissolving N aBH 4 granules with the same molar amount of N aOH. Now 100 µL of the AuCl 4 solution added with water and later injected 300 µL of the borohydride solution to that while stirring the mixture on a mechanical shaker for uniform * murugesh@iisc.ac.in † basu@iisc.ac.in