Coupled-Mode Theory of Field Enhancement in Complex Metal Nanostructures

We describe a simple yet rigorous theoretical model capable of analytical estimation of plasmonic field enhancement in complex metal structures. We show that one can treat the complex structures as coupled multi-pole modes with highest enhancements obtained due to superposition of high order modes in small particles. The model allows one to optimize the structures for the largest possible field enhancements, which depends on the quality factor Q of the metal and can be as high as Q^2 for two spherical particles. The "hot spot" can occur either in the nano-gaps between the particles or near the smaller particles. We trace the optimum field enhancement mechanism to the fact that the extended dipole modes of larger particles act as the efficient antennas while the modes in the gaps or near the smaller particles act as the compact sub-wavelength cavities. We also show how easily our approach can be extended to incorporate large numbers of particles in intricate arrangements.Comment: 23 pages, 7 figure

Strain-free Ge/GeSiSn quantum cascade lasers based on L-valley intersubband transitions

The authors propose a Ge/Ge0.76Si0.19Sn0.05 quantum cascade laser using intersubband transitions at L valleys of the conduction band which has a “clean” offset of150 meV situated below other energy valleys (Γ,X). The entire structure is strain-free because the lattice-matched Ge and Ge0.76Si0.19Sn0.05 layers are to be grown on a relaxed Ge buffer layer on a Si substrate. Longer lifetimes due to the weaker scattering of nonpolar optical phonons reduce the threshold current and potentially lead to room temperature operation

Observaton of tunneling of slow and fast electromagnetic modes in coupled periodic waveguides

We report the experimental observation of tunneling of slow and fast electromagnetic modes in coupled periodic waveguides shifted longitudinally by half of modulation period. According to the symmetry analysis, such a coupler supports two electromagnetic modes with exactly matched slow or fast group velocities but different phase velocities for frequencies close to the edge of the photonic band. We confirm the predicted properties of the modes by directly extracting their dispersion and group velocities from the near-field measurements using specialized Bloch-wave spectral analysis method.This work was supported by the Australian Research Council

Near-field interactions between metal nanoparticle surface plasmons and molecular excitons in thin-films: part I: absorption

In this and the following paper (parts I and II, respectively), we systematically study the interactions between surface plasmons of metal nanoparticles (NPs) with excitons in thin-films of organic media. In an effort to exclusively probe near-field interactions, we utilize spherical Ag NPs in a size-regime where far-field light scattering is negligibly small compared to absorption. In part I, we discuss the effect of the presence of these Ag NPs on the absorption of the embedding medium by means of experiment, numerical simulations, and analytical calculations, all shown to be in good agreement. We observe absorption enhancement in the embedding medium due to the Ag NPs with a strong dependence on the medium permittivity, the spectral position relative to the surface plasmon resonance frequency, and the thickness of the organic layer. By introducing a low index spacer layer between the NPs and the organic medium, this absorption enhancement is experimentally confirmed to be a near field effect In part II, we probe the impact of the Ag NPs on the emission of organic molecules by time-resolved and steady-state photoluminescence measurements

Concave Plasmonic Particles: Broad-Band Geometrical Tunability in the Near Infra-Red

Optical resonances spanning the Near and Short Infra-Red spectral regime were exhibited experimentally by arrays of plasmonic nano-particles with concave cross-section. The concavity of the particle was shown to be the key ingredient for enabling the broad band tunability of the resonance frequency, even for particles with dimensional aspect ratios of order unity. The atypical flexibility of setting the resonance wavelength is shown to stem from a unique interplay of local geometry with surface charge distributions

Thermal Links for the Implementation of an Optical Refrigerator

Optical refrigeration has been demonstrated by several groups of researchers, but the cooling elements have not been thermally linked to realistic heat loads in ways that achieve the desired temperatures. The ideal thermal link will have minimal surface area, provide complete optical isolation for the load, and possess high thermal conductivity. We have designed thermal links that minimize the absorption of fluoresced photons by the heat load using multiple mirrors and geometric shapes including a hemisphere, a kinked waveguide, and a tapered waveguide. While total link performance is dependent on additional factors, we have observed net transmission of photons with the tapered link as low as 0.04%. Our optical tests have been performed with a surrogate source that operates at 625 nm and mimics the angular distribution of light emitted from the cooling element of the Los Alamos solid state optical refrigerator. We have confirmed the optical performance of our various link geometries with computer simulations using CODE V optical modeling software. In addition we have used the thermal modeling tool in COMSOL MULTIPHYSICS to investigate other heating factors that affect the thermal performance of the optical refrigerator. Assuming an ideal cooling element and a nonabsorptive dielectric trapping mirror, the three dominant heating factors are (1) absorption of fluoresced photons transmitted through the thermal link, (2) blackbody radiation from the surrounding environment, and (3) conductive heat transfer through mechanical supports. Modeling results show that a 1 cm3 load can be chilled to 107 K with a 100Wpump laser. We have used the simulated steady-state cooling temperatures of the heat load to compare link designs and system configurations

Flat-band ferromagnetism in quantum dot superlattices

Possibility of flat-band ferromagnetism in quantum dot arrays is theoretically discussed. By using a quantum dot as a building block, quantum dot superlattices are possible. We consider dot arrays on Lieb and kagome lattices known to exhibit flat band ferromagnetism. By performing an exact diagonalization of the Hubbard Hamiltonian, we calculate the energy difference between the ferromagnetic ground state and the paramagnetic excited state, and discuss the stability of the ferromagnetism against the second nearest neighbor transfer. We calculate the dot-size dependence of the energy difference in a dot model and estimate the transition temperature of the ferromagnetic-paramagnetic transition which is found to be accessible within the present fabrication technology. We point out advantages of semiconductor ferromagnets and suggest other interesting possibilities of electronic properties in quantum dot superlattices.Comment: 15 pages, 7 figures (low resolution). High-resolution figures are available at http://www.brl.ntt.co.jp/people/tamura/Research/PublicationPapers.htm

Reconstructing Images from Projections Using the Maximum-Entropy Method. Numerical Simulations of Low-Aspect Astrotomography

The reconstruction of images from a small number of projections using the maximum-entropy method (MEM) with the Shannon entropy is considered. MEM provides higher-quality image reconstruction for sources with extended components than the Hogbom CLEAN method, which is also used in low-aspect astrotomography. The quality of image reconstruction for sources with mixed structure containing bright, compact features embedded in a comparatively weak, extended base can be further improved using a difference-mapping method, which requires a generalization of MEM for the reconstruction of sign-variable functions.We draw conclusions based on the results of numerical simulations for a number of model radio sources with various morphologies.Comment: 11 pages, 9 figure