Lower bound for the spatial extent of localized modes in photonic-crystal waveguides with small random imperfections

Light localization due to random imperfections in periodic media is paramount in photonics research. The group index is known to be a key parameter for localization near photonic band edges, since small group velocities reinforce light interaction with imperfections. Here, we show that the size of the smallest localized mode that is formed at the band edge of a one-dimensional periodic medium is driven instead by the effective photon mass, i.e. the flatness of the dispersion curve. Our theoretical prediction is supported by numerical simulations, which reveal that photonic-crystal waveguides can exhibit surprisingly small localized modes, much smaller than those observed in Bragg stacks thanks to their larger effective photon mass. This possibility is demonstrated experimentally with a photonic-crystal waveguide fabricated without any intentional disorder, for which near-field measurements allow us to distinctly observe a wavelength-scale localized mode despite the smallness (∼1/1000 of a wavelength) of the fabrication imperfections

Heat transfer enhancement by combination of chaotic advection and nanofluids flow in helically coiled tube

Abstract In this study, two passive techniques are simultaneously investigated for heat transfer improvement (i.e. chaotic advection and nanofluids) in coiled heat exchangers. Performance of these two different coils (one with normal configuration and another with chaotic configuration) is numerically analyzed and compared for both water and nanofluid as fluid. Effects of different parameters such as geometry, types of nanofluids, nanoparticle volumetric concentration and Reynolds number on heat transfer and pressure drop are studied. The CuO and Al2O3 base water nanofluids with different nanoparticle concentrations 1-3% were simulated. Equations of conservation of mass, momentum and energy were discretized using a finite element based technique and were solved using ANSYS software. Numerical results showed that heat transfer in the chaotic coil with water as fluid was higher than that in the normal coil with nanofluids at various volumetric concentrations and addition small amount of nanofluid in the chaotic coil flow resulted in significant enhancement of heat transfer

Strong Interactions in Multimode Random Lasers

Unlike conventional lasers, diffusive random lasers (DRLs) have no resonator to trap light and no high-Q resonances to support lasing. Due to this lack of sharp resonances the DRL has presented a challenge to conventional laser theory. We present a theory able to treat the DRL rigorously, and provide results on the lasing spectra, internal fields and output intensities of DRLs. Typically DRLs are highly multimode lasers, emitting light at a number of wavelengths. We show that the modal interactions through the gain medium in such lasers are extremely strong and lead to a uniformly spaced frequency spectrum, in agreement with recent experimental observations.Comment: 13 pages, 4 figures. Supplementary information available at arXiv:0805.449

A numerical study and design of multiple jet impingement in a PEMFC

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.Impinging jets are widely used in applications where high rates of heat and mass transfer are required. Similarly, an efficient operation of the Proton Exchange Membrane Fuel Cell (PEMFC) relies on high heat and mass transfer rates to and from the catalyst layers on the anode and cathode side, which raises the question of whether jet impingement could be employed for a PEMFC as well. To answer this question, a laminar non-isothermal gas-phase model for a PEMFC equipped with a porous flow field is solved numerically for five different cases: (i) single jet (cathode); (ii) double jet (cathode); (iii) triple jet (cathode); (iv) single jets (anode, cathode); (v) ordinary flow without jets. It is found that the jets reduce the size of the concentration boundary layers in the net at the flow field/gas diffusion layer interface (GDL), but do not penetrate significantly into the GDL for low permeabilies of around 10-12 m2. For macroporous layers with permeabilities of around 10-9 m2, the jets are able to penetrate deeply. For multiple jets, the risk of entrainment with oxygen depletion between jets is demonstrated, with a resulting loss in cell performance. Overall, this initial study indicates that jets can enhance cell performance, but care must be taken so as to avoid entrainment effects when employing multiple jets in a PEMFC.vk201

Drying kinetic analysis of municipal solid waste using modified page model and pattern search method

This work studied the drying kinetics of the organic fractions of municipal solid waste (MSW) samples with different initial moisture contents and presented a new method for determination of drying kinetic parameters. A series of drying experiments at different temperatures were performed by using a thermogravimetric technique. Based on the modified Page drying model and the general pattern search method, a new drying kinetic method was developed using multiple isothermal drying curves simultaneously. The new method fitted the experimental data more accurately than the traditional method. Drying kinetic behaviors under extrapolated conditions were also predicted and validated. The new method indicated that the drying activation energies for the samples with initial moisture contents of 31.1 and 17.2 % on wet basis were 25.97 and 24.73 kJ mol−1. These results are useful for drying process simulation and industrial dryer design. This new method can be also applied to determine the drying parameters of other materials with high reliability

Spectral correlations in a random distributed feedback fibre laser

Random distributed feedback fibre lasers belong to the class of random lasers, where the feedback is provided by amplified Rayleigh scattering on sub-micron refractive index inhomogenities randomly distributed over the fibre length. Despite the elastic nature of Rayleigh scattering, the feedback mechanism has been insofar deemed incoherent, which corresponds to the commonly observed smooth generation spectra. Here, using a real-time spectral measurement technique based on a scanning Fabry-Pérot interferometer, we observe long-living narrowband components in the random fibre laser's spectrum. Statistical analysis of the ∼104 single-scan spectra reveals a preferential interspacing for the components and their anticorrelation in intensities. Furthermore, using mutual information analysis, we confirm the existence of nonlinear correlations between different parts of the random fibre laser spectra. The existence of such narrowband spectral components, together with their observed correlations, establishes a long-missing parallel between the fields of random fibre lasers and conventional random lasers

Sequence-Dependent Fluorescence of Cyanine Dyes on Microarrays

Cy3 and Cy5 are among the most commonly used oligonucleotide labeling molecules. Studies of nucleic acid structure and dynamics use these dyes, and they are ubiquitous in microarray experiments. They are sensitive to their environment and have higher quantum yield when bound to DNA. The fluorescent intensity of terminal cyanine dyes is also known to be significantly dependent on the base sequence of the oligonucleotide. We have developed a very precise and high-throughput method to evaluate the sequence dependence of oligonucleotide labeling dyes using microarrays and have applied the method to Cy3 and Cy5. We used light-directed in-situ synthesis of terminally-labeled microarrays to determine the fluorescence intensity of each dye on all 1024 possible 5′-labeled 5-mers. Their intensity is sensitive to all five bases. Their fluorescence is higher with 5′ guanines, and adenines in subsequent positions. Cytosine suppresses fluorescence. Intensity falls by half over the range of all 5-mers for Cy3, and two-thirds for Cy5. Labeling with 5′-biotin-streptavidin-Cy3/-Cy5 gives a completely different sequence dependence and greatly reduces fluorescence compared with direct terminal labeling

Polyphenolic Contents and Antioxidant Potential of Stem Bark Extracts from Jatropha curcas (Linn)

We assessed the polyphenolic contents and antioxidant potential of the aqueous, ethanol and methanol stem bark extracts of Jatropha curcas. The total phenol, flavonoids, flavonols and proanthocyanidin contents of the extracts were evaluated to determine their effect on the antioxidant property of this plant, using standard phytochemical methods. The antioxidant and free radical scavenging activity of ethanol, methanol and aqueous extracts of the plant were also assessed against 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), ferric reducing, nitric oxide (NO), superoxide anion, (O2−) and hydrogen peroxide (H2O2) using spectroscopic methods and results were compared with that of butylated hydroxyl toluene (BHT) and ascorbic acid as standards. The concentrations of different classes of phenolic compounds were higher in methanol and ethanol extracts compared to aqueous extracts. There was correlation between total phenol, total flavonoids, total flavonol and total proanthocyanidins (r = 0.996, 0.978, 0.908, and 0.985) respectively. There was correlations between the amount of phenolic compounds and percentage inhibition of DPPH radicals scavenging activity of the extract (r = 0.98). Findings from the present study indicated that J. curcas is a potential source of natural antioxidants and may be a good candidate for pharmaceutical plant based products