Coupling Geometry Research Articles

Giant spectral transformations in plasmon-enhanced difference-frequency generation with polychromatic light

We theoretically study the generation of mid-infrared light through difference-frequency excitation using a single dual-wavelength coherent femtosecond laser source. The difference-frequency wave is generated from the surface of a thin gold film in the Kretschmann coupling geometry due to the surface nonlinearity of the film. We show a clear enhancement of the difference-frequency wave around the surface plasmon polariton coupling angle of the incident fundamental wave. We also show an enormous shift and modification of the difference-frequency spectrum near surface plasmon resonance. We discuss the discovered spectral change dependence on the source pulse duration and incidence angle of the fundamental wave. Our findings have an enormous potential for use in difference-frequency surface sensing and spectroscopy applications.

Twisted spin vortices in a spin-1 Bose-Einstein condensate with Rashba spin-orbit coupling and dipole-dipole interaction

We consider a spin-1 Bose-Einstein condensate with Rashba spin-orbit coupling and dipole-dipole interaction confined in a cigar-shaped trap. Due to the combined effects of spin-orbit coupling, dipole-dipole interaction, and trap geometry, the system exhibits a rich variety of ground-state spin structures, including twisted spin vortices. The ground-state phase diagram is determined with respect to the strengths of the spin-orbit coupling and dipole-dipole interaction.

Switching of transmission resonances in a two-channels coupler: A Boundary Wall Method scattering study

In this work, we study the transmission characteristics of a two-channels coupler model system using the Boundary Wall Method (BWM) to determine the solution of the corresponding scattering problem of an incident plane wave. We show that the BWM provides detailed information regarding the transmission resonances. In particular, we focus on the case of single channel input aiming to explore the energy switching performance of the coupler. We show that the coupler geometry can be tailored to allow for the first transmission resonances to be predominantly transmitted on specific output channels, an important characteristic for the realization of logical operations.

Integrative Inferences on Pattern Geometries of Grapes Grown under Water Stress and Their Resulting Wines.

Multiple datasets of two consecutive vintages of replicated grape and wines from six different deficit irrigation regimes are characterized and compared. The process consists of four temporal-ordered signature phases: harvest field data, juice composition, wine composition before bottling and bottled wine. A new computing paradigm and an integrative inferential platform are developed for discovering phase-to-phase pattern geometries for such characterization and comparison purposes. Each phase is manifested by a distinct set of features, which are measurable upon phase-specific entities subject to the common set of irrigation regimes. Throughout the four phases, this compilation of data from irrigation regimes with subsamples is termed a space of media-nodes, on which measurements of phase-specific features were recoded. All of these collectively constitute a bipartite network of data, which is then normalized and binary coded. For these serial bipartite networks, we first quantify patterns that characterize individual phases by means of a new computing paradigm called “Data Mechanics”. This computational technique extracts a coupling geometry which captures and reveals interacting dependence among and between media-nodes and feature-nodes in forms of hierarchical block sub-matrices. As one of the principal discoveries, the holistic year-factor persistently surfaces as the most inferential factor in classifying all media-nodes throughout all phases. This could be deemed either surprising in its over-arching dominance or obvious based on popular belief. We formulate and test pattern-based hypotheses that confirm such fundamental patterns. We also attempt to elucidate the driving force underlying the phase-evolution in winemaking via a newly developed partial coupling geometry, which is designed to integrate two coupling geometries. Such partial coupling geometries are confirmed to bear causal and predictive implications. All pattern inferences are performed with respect to a profile of energy distributions sampled from network bootstrapping ensembles conforming to block-structures specified by corresponding hypotheses.

Design of a hybrid silicon-plasmonic co-propagating coupler operating close to coherent perfect absorption

By hybrid integration of plasmonic and dielectric waveguide concepts, it is shown that nearly perfect coherent absorption can be achieved in a co-propagating coupler geometry. First, the operating principle of the proposed device is detailed in the context of a more general 2 × 2 lossy coupler formalism. Then, it is shown how to tune the device in a wide region of possible working points, its broadband operation, and the tolerance to fabrication uncertainties. Finally, a complete picture of the electromagnetic modes inside the hybrid structure is analyzed, shining light onto the potentials which the proposed device holds in view of classical and quantum signal processing, nonlinear optics, polarization control, and sensing.

Observation of localized surface plasmons and hybridized surface plasmon polaritons on self-assembled two-dimensional nanocavities.

Large-area patterning of periodic nanostructures using self-assembled nanospheres is of interest for fabricating low-cost plasmonic substrates, such as two-dimensional (2D) metallic gratings. Surface plasmon polaritons (SPPs) excited on metallic gratings have applications in biosensors, thin-film photovoltaics, photoelectrochemical cells, and photodetectors. Here we fabricated large-area metallic gratings using nanosphere lithography, and the geometry of gratings was controlled by the sphere size and distance between nanospheres. Both forward and backward propagating SPPs were observed using the grating coupling geometry. Furthermore, we reported the first observation of localized surface plasmons (LSPs) on this large-area metallic grating by both simulation and experimental studies. Such an LSP mode was confined in the 2D nanocavities and was not supported by dielectric gratings with the same 2D geometry.

Removing the Directional Degeneracy of LP11Mode in a Fused-Type Mode Selective Coupler

The removal of polarization and modal degeneracy in the excitation of higher order modes is realised in all-fiber mode-selective weakly-fused-couplers by controlling the coupler geometry. Experiments carried out with short couplers show strong polarization dependency, while long couplers with a slow varying cross-section have shown to remove polarization and spatial degeneracy in the first high order mode.

Plasmon induced transparency in the trimer of gold nanorods

The localized surface plasmon resonance can be generated on the surface of the nano-metamaterial by the interaction between the nano-metamaterial and the light field, and also many plasmon oscillation modes can occur in the process of the hybridization between many infinitesimal composite structures, which is widely used for adjusting the resonant frequency in the optical frequency domain. Recently, analogue of the electromagnetically induced transparency（EIT) has been realized in the low-loss nano-metamaterial, and is well known as the plasmon induced transparency（PIT). In atomic physics, EIT is an effect which originates from the destructive quantum interference of two different excitation pathways. A sharp dip of nearly ideal transmission can arise within the broad absorption profile, which indicates that the EIT can be used in the fields of slow slight, delay lines and low-loss metamaterial. In this paper, a trimer consisting of a vertical nanorod（serving as a dipole antenna) and two parallel nanorods（used as a quadrupole antenna) is employed to investigate the process mechanism of the PIT in detail. It is found that the vertical nanorod with a large broad linewidth can be strongly coupled with the light. However, the parallel nanorods are weakly coupled with the light and their narrow linewidths are almost from the intrinsic metal loss（Drude damping) that is much smaller than the radiative damping of the dipole antenna. These two antennas can be strongly coupled due to their close similarities. Moreover, the absorption spectra of the trimer obtained by using three-dimensional finite element method vary with its coupling distance and geometry size, and the dipole bright mode corresponding to the dipole antenna splits under the action of the dark mode for the quadrupole antenna. Thus, a fresh physical interpretation is given:the PIT is mainly due to the coherent superposition after the splitting of the dipole oscillation mode in the vertical nanorod, rather than the parallel nanorods. Taking into consideration the phase correlation associated with coupling process of two oscillators, we introduce a modified Lorentzian oscillator model to investigate the effects of the coupling phase factor on the modulation of the absorption spectra and the coherent superposition between the splitting bright modes on the PIT. These findings will provide theoretical references for the applications of artificial atom, optical switching and slow light devices designed in the nanosize range.

Thermoelectric Properties of Single Molecule Junction in Presence of Dephasing and Different Coupling Geometries

The thermoelectric properties of a benzene molecule coupled to three-dimensional metallic electrodes are examined in the presence of dephasing processes using Green function formalism in the linear response regime. The three different kind of configurations (ortho, meta and para) are taken into account for the coupling between the molecule and electrodes. Results show that the thermoelectric properties of the single molecular junction are strongly dependent on the coupling geometry. Our results predict that the thermoelectric efficiency is increased in specific geometry as a consequence of interference effects. It is found that the dephasing effects result in the reduction of the thermoelectric efficiency.

Analysis of the modal evolution in fused-type mode-selective fiber couplers.

Fused-type mode-selective fiber couplers exciting the LP(11) mode are fabricated by well-defined fiber cladding reduction, pretapering and fusion. At a wavelength of 905 nm 80 % of the injected power in the single-mode fiber was transmitted in the few-mode fiber selectively exciting the LP(11) mode. The coupling behavior was experimentally investigated for the case of strong as well as weak fusion. Numerical simulations based on the super-mode coupling approach were used to estimate fabrication parameters and to discuss the modal evolution in arbitrarily fused couplers. The influence of changes in the coupler geometry on the super-modes and their modal weighting are analyzed by calculations of the effective refractive index and by modal decomposition.

Photonic Crystal Microcavities in a Microelectronics 45-nm SOI CMOS Technology

We demonstrate the first monolithically integrated linear photonic crystal microcavities in an advanced SOI CMOS microelectronics process (IBM 45nm 12SOI) with no in-foundry process modifications. The cavities were integrated into a standard microelectronics design flow meeting process design rules, and fabricated alongside transistors native to the process. We demonstrate both 1520nm wavelength and 1180nm cavity designs using different cavity implementations due to design rule constraints. For the 1520nm and 1180nm designs, loaded quality factors of 2,000 and 4,000 are measured, and intrinsic quality factors of 100,000 and 60,000 are extracted. We also demonstrate an evanescent coupling geometry which decouples the cavity and waveguide-coupling design.

Data mechanics and coupling geometry on binary bipartite networks.

We quantify the notion of pattern and formalize the process of pattern discovery under the framework of binary bipartite networks. Patterns of particular focus are interrelated global interactions between clusters on its row and column axes. A binary bipartite network is built into a thermodynamic system embracing all up-and-down spin configurations defined by product-permutations on rows and columns. This system is equipped with its ferromagnetic energy ground state under Ising model potential. Such a ground state, also called a macrostate, is postulated to congregate all patterns of interest embedded within the network data in a multiscale fashion. A new computing paradigm for indirect searching for such a macrostate, called Data Mechanics, is devised by iteratively building a surrogate geometric system with a pair of nearly optimal marginal ultrametrics on row and column spaces. The coupling measure minimizing the Gromov-Wasserstein distance of these two marginal geometries is also seen to be in the vicinity of the macrostate. This resultant coupling geometry reveals multiscale block pattern information that characterizes multiple layers of interacting relationships between clusters on row and on column axes. It is the nonparametric information content of a binary bipartite network. This coupling geometry is then demonstrated to shed new light and bring resolution to interaction issues in community ecology and in gene-content-based phylogenetics. Its implied global inferences are expected to have high potential in many scientific areas.

INFLUENCE OF COUPLING GEOMETRY AND DEPHASING ON THERMOELECTRIC PROPERTIES OF A C60 MOLECULAR JUNCTION

Thermoelectric properties of a C60 molecule coupled to three-dimensional metallic electrodes are studied using Green function formalism in linear response regime. A tight-binding model is used to investigate the effects of the dimerization and coupling geometry on the electrical conductance, thermopower and figure of merit. Increase of the contact points between the molecule and electrodes results in decrease of the number of the peaks of the electrical conductance because of the interference effects. In addition, oscillation of the thermopower is reduced in the multiple contacts. Results show that the increase of the contact points leads to the reduction of the figure of merit. Furthermore, the effect of the phase breaking scattering on the figure of merit is analyzed using the Büttiker probe method.

Abnormal high-Q modes of coupled stadium-shaped microcavities.

It is well known that the strongly deformed microcavity with fully chaotic ray dynamics cannot support high-Q modes due to its fast chaotic diffusion to the critical line of refractive emission. Here, we investigate how the Q factor is modified when two chaotic cavities are coupled, and show that some modes, whose Q factor is about 10 times higher than that of the corresponding single cavity, can exist. These abnormal high-Q modes are the result of an optimal combination of coupling and cavity geometry. As an example, in the coupled stadium-shaped microcavities, the mode pattern extends over both cavities such that it follows a whispering-gallery-type mode at both ends, whereas a big coupling spot forms at the closest contact of the two microcavities. The pattern of such a "rounded bow tie" mode allows the mode to have a high-Q factor. This mode pattern minimizes the leakage of light at both ends of the microcavities as the pattern at both ends is similar to the whispering gallery mode.

Pulsed source of spectrally uncorrelated and indistinguishable photons at telecom wavelengths.

We report on the generation of indistinguishable photon pairs at telecom wavelengths based on a type-II parametric down conversion process in a periodically poled potassium titanyl phosphate (PPKTP) crystal. The phase matching, pump laser characteristics and coupling geometry are optimised to obtain spectrally uncorrelated photons with high coupling efficiencies. Four photons are generated by a counter-propagating pump in the same crystal and anlysed via two photon interference experiments between photons from each pair source as well as joint spectral and g((2)) measurements. We obtain a spectral purity of 0.91 and coupling efficiencies around 90% for all four photons without any filtering. These pure indistinguishable photon sources at telecom wavelengths are perfectly adapted for quantum network demonstrations and other multi-photon protocols.

The square-kagome quantum Heisenberg antiferromagnet at high magnetic fields: The localized-magnon paradigm and beyond

We consider the spin-1/2 antiferromagnetic Heisenberg model on the two-dimensional square-kagome lattice with almost dispersionless lowest magnon band. For a general exchange coupling geometry we elaborate low-energy effective Hamiltonians which emerge at high magnetic fields. The effective model to describe the low-energy degrees of freedom of the initial frustrated quantum spin model is the (unfrustrated) square-lattice spin-1/2 XXZ model in a z-aligned magnetic field. For the effective model we perform quantum Monte Carlo simulations to discuss the low-temperature properties of the square-kagome quantum Heisenberg antiferromagnet at high magnetic fields. We pay special attention to a magnetic-field driven Berezinskii-Kosterlitz-Thouless phase transition which occurs at low temperatures.

Stabilizing and Controlling Swimming Bacteria: Shaping a Turbulent Suspension into a Ferromagnetic State

Dense suspensions of swimming bacteria are famous for self-organizing into large and turbulent jets and swirls. Confining a dense Bacillus subtilis suspension into a flattened drop allows the stabilisation of a vortex (‘spin’), the structure of which results from a complex interplay between steric and hydrodynamic interactions [Wioland et al., PRL 110, 268102 (2013)]. We now couple up to 100 of such vortices inside microfluidic devices to create bacterial spin lattices. Depending on the vortex and coupling geometry, we were able to reproduce ‘ferromagnetic’ and ‘antiferromagnetic’ bacterial vortex states, that appear to be controlled by the subtle competition between bacterial boundary layer flows and bulk dynamics.

Saturable absorption in multicore fiber couplers

The saturable absorption characteristics of two-, three-, and five-core one-dimensional fiber coupler arrays and the seven-core hexagonal fiber coupler array are investigated. It is shown that the performance of all these saturable absorbers are comparable and not much is gained, if anything, by going from a two-core nonlinear coupler geometry to a higher number of cores. This observation is supported by the similarity of the saturable absorption curves, as well as comparable pulse characteristics obtained from the simulation of a generic mode-locked fiber laser cavity.

Silicon nanophotonic multitaper for efficient light coupling between fiber and silicon waveguide

A multitaper (MT) coupler is fabricated on a silicon chip for light coupling with high efficiency and resistance to fabrication errors. Other coupling techniques such as a single taper are less efficient and prone to fabrication errors. The dimensions of the MT can be optimized for a specific coupling geometry and can span sizes from several nanometers up to several microns.

Implications of local osteoporosis on the efficacy of anti-resorptive drug treatment: a 3-year follow-up finite element study in risedronate-treated women

The existence of local osteoporosis necessitates patient-specific analysis. Lower and higher ranges of local buckling ratio were found at femoral necks for adequate and inadequate drug response groups, respectively (grouped based on fracture loads). Management of hip fracture risk should be targeted at local geometric abnormalities causing instability. Hip fracture amongst the elderly is a growing concern especially with improvements in living standards and increasing lifespan. Approximately half of the total hip fractures result from those without osteoporosis. This escalates the need to observe local osteoporosis. By observing the local buckling ratio (BR) in the femoral neck in ten risedronate-treated subjects over 3years, we discovered that subjects with improved fracture loads, as predicted by finite element (FE) analysis, were associated with lower local BR and vice versa. The 3D models of the left proximal femurs were generated, and local BR values at 30° intervals were obtained from femoral neck slices by measuring the respective mean cortical thickness and mean outer radius. Following geometric analysis, structural strength was examined with FE analysis where critical fracture loads (F cr) were acquired from sideways fall load simulations. We classified subjects in three groups according to the change in F cr: adequate (+20%), inadequate (-22%) and indefinite (-2%) drug response groups. A common striking feature was that lower and higher ranges of local BR values (baseline year) were found for adequate (min = 2.14, max = 8.04) and inadequate (min = 1.72, max = 11.38) drug response groups, respectively. Subjects in the inadequate drug response group exhibited high local BR at the supero-anterior and supero-posterior regions. These high local BR values coincided with FE-predicted critical strain regions, whereas subjects from the adequate drug response group showed significantly reduced strain regions. The superiority of coupling geometry (BR) with structure (F cr) over bone mineral density measurements alone by monitoring local osteoporosis has been illustrated.