Metal nanoparticles with sharp corners: Universal properties of plasmon resonances

We predict the simultaneous occurrence of two fundamental phenomena for metal nanoparticles possessing sharp corners: First, the main plasmonic dipolar mode experiences strong red shift with decreasing corner curvature radius; its resonant frequency is controlled by the apex angle of the corner and the normalized (to the particle size) corner curvature. Second, the split-off plasmonic mode experiences strong localization at the corners. Altogether, this paves the way for tailoring of metal nano-structures providing wavelength-selective excitation of localized plasmons and a strong near-field enhancement of linear and nonlinear optical phenomena

Selective excitation of plasmons superlocalized at sharp perturbations of metal nanoparticles

Sharp metal corners and tips support plasmons localized on the scale of the curvature radius -- superlocalized plasmons. We analyze plasmonic properties of nanoparticles with small and sharp corner- and tip-shaped surface perturbations in terms of hybridization of the superlocalized plasmons, which frequencies are determined by the perturbations shape, and the ordinary plasmons localized on the whole particle. When the frequency of a superlocalized plasmon gets close to that of the ordinary plasmon, their strong hybridization occurs and facilitates excitation of an optical hot-spot near the corresponding perturbation apex. The particle is then employed as a nano-antenna that selectively couples the free-space light to the nanoscale vicinity of the apex providing precise local light enhancement by several orders of magnitude

Asymmetry in shape causing absolute negative mobility

We propose a simple classical concept of nanodevices working in an absolute negative mobility (ANM) regime: The minimal spatial asymmetry required for ANM to occur is embedded in the geometry of the transported particle, rather than in the channel design. This allows for a tremendous simplification of device engineering, thus paving the way towards practical implementations of ANM. Operating conditions and performance of our model device are investigated, both numerically and analytically.Comment: 6 pages; accepted for publication in PR

Investigation of nonlinear absorption processes with femtosecond light pulses in lithium niobate crystals

The propagation of high-power femtosecond light pulses in lithium niobate crystals (LiNbO3) is investigated experimentally and theoretically in collinear pump-probe transmission experiments. It is found within a wide intensity range that a strong decrease of the pump transmission coefficient at wavelength 388 nm fully complies with the model of two-photon absorption; the corresponding nonlinear absorption coefficient is betap~=3.5 cm/GW. Furthermore, strong pump pulses induce a considerable absorption for the probe at 776 nm. The dependence of the probe transmission coefficient on the time delay Deltat between probe and pump pulses is characterized by a narrow dip (at Deltat~=0) and a long (on the picosecond time scale) lasting plateau. The dip is due to direct two-photon transitions involving pump and probe photons; the corresponding nonlinear absorption coefficient is betar~=0.9 cm/GW. The plateau absorption is caused by the presence of pump-excited charge carriers; the effective absorption cross section at 776 nm is sigmar~=8×10^–18 cm^2. The above nonlinear absorption parameters are not strongly polarization sensitive. No specific manifestations of the relaxation of hot carriers are found for a pulse duration of ~=0.24 ps

Femtosecond time-resolved absorption processes in lithium niobate crystals

emtosecond pump pulses are strongly attenuated in lithium niobate owing to two-photon absorption; the relevant nonlinear coefficient beta_p ranges from ~3.5 cm/GW for lambda_p = 388 nm to ~0.1 cm/GW for 514 nm. In collinear pump-probe experiments the probe transmission at the double pump wavelength 2lambda_p=776 nm is controlled by two different processes: A direct absorption process involving pump and probe photons (beta_r ~ or = 0.9 cm/GW) leads to a pronounced short-duration transmission dip, whereas the probe absorption by pump-excited charge carriers results in a long-duration plateau. Coherent pump-probe interactions are of no importance. Hot-carrier relaxation occurs on the time scale of < or ~0.1 ps

Highly tunable low-threshold optical parametric oscillation in radially poled whispering gallery resonators

Whispering gallery resonators (WGR's), based on total internal reflection, possess high quality factors in a broad spectral range. Thus, nonlinear optical processes in such cavities are ideally suited for the generation of broadband or tunable electromagnetic radiation. Experimentally and theoretically, we investigate the tunability of optical parametric oscillation in a radially structured WGR made of lithium niobate. With a 1.04 /mum pump wave, the signal and idler waves are tuned from 1.78 to 2.5 \mum - including the point of degeneracy - by varying the temperature between 20 and 62 {\deg}C. A weak off-centering of the radial domain structure extends considerably the tuning capabilities. The oscillation threshold lies in the mW-power range.Comment: 4 pages, 5 figure

Probing Pauli Blocking Factors in Quantum Pumps with Broken Time-Reversal Symmetry

A recently demonstrated quantum electron pump is discussed within the framework of photon-assisted tunneling. Due to lack of time-reversal symmetry, different results are obtained for the pump current depending on whether or not final-state Pauli blocking factors are used when describing the tunneling process. Whilst in both cases the current depends quadratically on the driving amplitude for moderate pumping, a marked difference is predicted for the temperature dependence. With blocking factors the pump current decreases roughly linearly with temperature until k_B T ~ \hbar\omega is reached, whereas without them it is unaffected by temperature, indicating that the entire Fermi sea participates in the electronic transport.Comment: 4 pages in RevTex4 (beta4), 6 figures; status: to appear in PR

Support of Rock Cuts at Washington-Dulles International Airport

Expansions at the Washington-Dulles International Airport since 1999 have required extensive vertical, open-cut rock excavations in Triassic age siltstone bedrock. These excavations have extended to depths of up to approximately 65 ft (20 m) adjacent to existing infrastructure for construction of new below-ground stations for the new Automated People Mover (APM) light rail system. The selection of design support pressures for the rock excavations was an important decision, balancing the projects’ risks and construction costs. At the center of this issue was the development of a geotechnical model of the rock mass and its primary failure mechanism. Thus, a comprehensive subsurface characterization was required. The rock mass characterization included observation and mapping of excavation faces, detailed logging of rock cores, use of optical and acoustic televiewer, testing of discontinuity samples for shear strength evaluation, groundwater monitoring, and inclinometer monitoring of supported faces. The televiewer data, combined with site observations, allowed for a more complete understanding of the engineering characteristics of the bedding plane and joint discontinuities within the siltstone rock mass. Based on the pattern of the predominant discontinuities, it was concluded that bedding planes dipping into the excavation at approximately 30 degrees intersecting near-vertical joints would present the greatest risk for rock cut failures. Extensive laboratory testing and field inspections at a variety of exposed cuts with varying bedding plane and joint orientations suggested that the potential for a large slide along a bedding plane was relatively low. This conclusion was based on observations of discontinuous clay seams of limited number, the first- and second-order roughness of joint and bedding plane surfaces, and the limited persistence of joint and bedding plane discontinuities. Previous design lateral pressures for permanent station walls had been based on an assumed potential failure model of a large, excavation-scale block failure. However, using the recent characterization data, the rock mass failure mechanism of a local joint- and bedding-controlled sliding block mechanism was considered more appropriate. The resulting design lateral pressure necessary to support a rock face using this mechanism and the shear strength of discontinuities and intact rock was significantly lower than the initial design values. Construction-phase observations and monitoring, which included detailed field mapping, automated instrumentation monitoring, and groundwater monitoring, have verified the rock characterization and design assumptions. The reduction in design pressures for the permanent below-grade walls for the APM station structures resulted in major cost savings for the projects now in design and construction. Based on the scale of future expansion plans at Dulles, the projected total cost savings resulting from the reduced design lateral rock pressures will be considerable