Coherent Surface Research Articles

Toward broadband, dynamic structuring of a complex plasmonic field.

The ability to tailor a coherent surface plasmon polariton (SPP) field is an important step toward many new opportunities for a broad range of nanophotonic applications. Previously, both scanning a converging SPP spot and designing SPP profiles using an ensemble of spots have been demonstrated. SPPs, however, are normally excited by intense, coherent light sources, that is, lasers. Hence, interference between adjacent spots is inevitable and will affect the overall SPP field distributions. We report a reconfigurable and wavelength-independent platform for generating a tailored two-dimensional (2D) SPP field distribution by considering the coherent field as a whole rather than as individual spots. With this new approach, the inherent constraints in a 2D coherent field distribution are revealed. Our design approach works not only for SPP waves but also for other 2D wave systems such as surface acoustic waves.

Powder Metallurgy Fabrication of Porous 51(at.%)Ni–Ti Shape Memory Alloys for Biomedical Applications

The effect of time and temperature on the microwave sintering of 51(at.%)Ni–Ti shape memory alloys (SMAs) was investigated in the current research. Furthermore, the microstructure, mechanical properties, and bio-corrosion properties were analyzed based on the sintering conditions. The results revealed that the sintering condition of 700 °C for 15 min produced a part with coherent surface survey that does not exhibit gross defects. Increasing the sintering time and temperature created defects on the outer surface, while reducing the temperature to 550 °C severely affected the mechanical properties. The microstructure of these samples showed two regions of Ni-rich region and Ti-rich region between them Ti2Ni, NiTi, and Ni3Ti phases. The differential scanning calorimeter (DSC) curves of Ni–Ti samples exhibited a multi-step phase transformation B19′–R–B2 during heating and cooling. An increase in the sintering temperature from 550 to 700 °C was found to increase the fracture strength significantly and decreased the fracture strain slightly. Reducing the sintering temperature from 700 to 550 °C severely affected the corrosion behaviors of 51%Ni–Ti SMAs. This research aims to select the optimum parameters to produce Ni–Ti alloys with desired microstructure, mechanical properties, and corrosion behaviors for biomedical applications.

Partially coherent axiconic surface plasmon polariton fields

We introduce a class of structured polychromatic surface electromagnetic fields, reminiscent of conventional optical axicon fields, through a judicious superposition of partially correlated surface plasmon polaritons. We show that such partially coherent axiconic surface plasmon polariton fields are structurally stable and statistically highly versatile with regard to spectral density, polarization state, energy flow, and degree of coherence. These fields can be created by plasmon coherence engineering and may prove instrumental broadly in surface physics and in various nanophotonics applications.

The role of nearshore slope on cross-shore surface transport during a coastal upwelling event in Gulf of Finland, Baltic Sea

The wind induced coastal upwelling process often contains a mid-phase that involves coherent long-living cross-shore surface jets of cooler water. These jets extend to 40–45 km from the coast and tend to start from particular coastal locations. We develop a simple method for evaluating the depth in the water column from where the upwelled water originates. We also test the hypothesis that the development of cross-shore surface jets may be triggered by some permanent characteristics such as the bathymetry or the slope of the nearshore seabed. The analysis is performed for a strong and well-documented upwelling in the Gulf of Finland, the Baltic Sea, using high resolution bathymetry data, satellite derived sea surface temperature, surface currents measured by in situ drifters, and properties of water masses in two sampling locations. The results indicate that the cross-shore jets originate exclusively from the shore sections with much steeper slopes (>0.0075) than in the rest of the study area. The cooler water most likely origins from intermediate water masses at depths between 15 and 30 m. The resulting identification of the source depth of the upwelled water and its spatial location assists in understanding the flux of nutrients during upwelling events and their link to the onset of cyanobacteria blooms.

Coherent pseudo-mode representation of partially coherent surface plasmon polaritons.

We develop a general coherent (pseudo)-mode representation of partially coherent surface plasmon polaritons in the Kretschmann excitation geometry. We obtain explicit analytical results for coherent pseudo-modes of narrowband SPPs with Gaussian spectra and spectral correlations. We also evaluate the global degree of coherence of such SPPs which quantifies their overall coherence state within a given spatial domain. Not only will the developed coherent pseudo-mode representation facilitate SPP coherence state characterization, but it will also substantially reduce the computational complexity of the studies of partially coherent SPP interactions with nanoparticles.

Characterization of pinna motion patterns in hipposiderid bats

Hipposiderid bats have a set of highly differentiated pinna muscles that support different pinna motion patterns during biosonar behaviors. It has been shown that these bats' pinna motions fall into two separate categories: Rigid motions that change only pinna orientation and non-rigid motions that also change pinna shape. It remains unclear if further subdivisions of these two categories (e.g., different orientations for rigid motions and different deformation patterns for non-rigid motions) can be made. In order to investigate this question, experiments have been conducted where the positions of a dense set of landmark points (50–80 per pinna) were reconstructed based on footage from an array of four high-speed video cameras. The resulting point trajectory data (positions as a function of time) was clustered to facilitate the detection of patterns. Clustering revealed a structure of coherent surface patches for rigid as well as non-rigid motion. For the rigid motion pilot data set, these patches followe...

Quantitative disentanglement of coherent and incoherent laser-induced surface deformations by time-resolved x-ray reflectivity

We present time-resolved x-ray reflectivity measurements on laser excited coherent and incoherent surface deformations of thin metallic films. Based on a kinematical diffraction model, we derive the surface amplitude from the diffracted x-ray intensity and resolve transient surface excursions with sub-Å spatial precision and 70 ps temporal resolution. The analysis allows for decomposition of the surface amplitude into multiple coherent acoustic modes and a substantial contribution from incoherent phonons which constitute the sample heating.

Submesoscale CO<sub>2</sub> variability across an upwelling front off Peru

Abstract. As a major source for atmospheric CO2, the Peruvian upwelling region exhibits strong variability in surface fCO2 on short spatial and temporal scales. Understanding the physical processes driving the strong variability is of fundamental importance for constraining the effect of marine emissions from upwelling regions on the global CO2 budget. In this study, a frontal decay on length scales of 𝒪(10 km) was observed off the Peruvian coast following a pronounced decrease in down-frontal (equatorward) wind speed with a time lag of 9 h. Simultaneously, the sea-to-air flux of CO2 on the inshore (cold) side of the front dropped from up to 80 to 10 mmol m−2 day−1, while the offshore (warm) side of the front was constantly outgassing at a rate of 10–20 mmol m−2 day−1. Based on repeated ship transects the decay of the front was observed to occur in two phases. The first phase was characterized by a development of coherent surface temperature anomalies which gained in amplitude over 6–9 h. The second phase was characterized by a disappearance of the surface temperature front within 6 h. Submesoscale mixed-layer instabilities were present but seem too slow to completely remove the temperature gradient in this short time period. Dynamics such as a pressure-driven gravity current appear to be a likely mechanism behind the evolution of the front.

Generalization of fewest-switches surface hopping for coherences.

Fewest-switches surface hopping (FSSH) is perhaps the most widely used mixed quantum-classical approach for the modeling of non-adiabatic processes, but its original formulation is restricted to (adiabatic) population terms of the quantum density matrix, leaving its implementations with an inconsistency in the treatment of populations and coherences. In this article, we propose a generalization of FSSH that treats both coherence and population terms on equal footing and which formally reduces to the conventional FSSH algorithm for the case of populations. This approach, coherent fewest-switches surface hopping (C-FSSH), employs a decoupling of population relaxation and pure dephasing and involves two replicas of the classical trajectories interacting with two active surfaces. Through extensive benchmark calculations of a spin-boson model involving a Debye spectral density, we demonstrate the potential of C-FSSH to deliver highly accurate results for a large region of parameter space. Its uniform description of populations and coherences is found to resolve incorrect behavior observed for conventional FSSH in various cases, in particular at low temperature, while the parameter space regions where it breaks down are shown to be quite limited. Its computational expenses are virtually identical to conventional FSSH.

Quasiparticle Interference on Cubic Perovskite Oxide Surfaces.

We report the observation of coherent surface states on cubic perovskite oxide SrVO_{3}(001) thin films through spectroscopic-imaging scanning tunneling microscopy. A direct link between the observed quasiparticle interference patterns and the formation of a d_{xy}-derived surface state is supported by first-principles calculations. We show that the apical oxygens on the topmost VO_{2} plane play a critical role in controlling the coherent surface state via modulating orbital state.

Statistical Comparison and Combination of GPS, GLONASS, and Multi-GNSS Multipath Reflectometry Applied to Snow Depth Retrieval

Global navigation satellite system (GNSS) multipath reflectometry (MR) has emerged as a new technique that uses signals of opportunity broadcast by GNSS satellites and tracked by ground-based receivers to retrieve environmental variables such as snow depth. The technique is based on the simultaneous reception of direct or line-of-sight (LOS) transmissions and corresponding coherent surface reflections (non-LOS). Until recently, snow depth retrieval algorithms only used legacy and modernized GPS signals. Using multiple GNSS constellations for reflectometry would improve GNSS-MR applications by providing more observations from more satellites and independent signals (carrier frequencies and code modulations). We assess GPS and GLONASS for combined multi-GNSS-MR using simulations as well as field measurements. Synthetic observations for different signals indicated a lack of detectable interfrequency and intercode biases in GNSS-MR snow depth retrievals. Received signals from a GNSS station continuously operating in France for a two-winter period are used for experimental snow depth retrieval. We perform an internal validation of various GNSS signals against the proven GPS-L2-C signal, which was validated externally against in situ snow depth in previous studies. GLONASS observations required a more complex handling to account for topography because of its particular ground track repeatability. Signal intercomparison show an average correlation of 0.922 between different GPS snow depths and GPS-L2-CL, while GLONASS snow depth retrievals have an average correlation that exceeds 0.981. In terms of precision and accuracy, legacy GPS signals are worse, while GLONASS signals and modernized GPS signals are of comparable quality. Finally, we show how an optimal multi-GNSS combined daily snow depth time series can be formed employing variance factors with a ~59%–90% precision improvement compared to individual signal snow depth retrievals, resulting in snow depth retrieval with uncertainty of 1.3 cm. The developed combination strategy can also be applied for the European Galileo and the Chines BeiDou navigation systems.

Imaging the dark emission of spasers.

Spasers are a new class of laser devices with cavity sizes free from optical diffraction limit. They are an emergent tool for various applications, including biochemical sensing, superresolution imaging, and on-chip optical communication. According to its original definition, a spaser is a coherent surface plasmon amplifier that does not necessarily generate a radiative photon output. However, to date, spasers have only been studied with scattered photons, and their intrinsic surface plasmon emission is a "dark" emission that is yet to be revealed because of its evanescent nature. We directly image the surface plasmon emission of spasers in spatial, momentum, and frequency spaces simultaneously. We demonstrate a nanowire spaser with a coupling efficiency to plasmonic modes of 74%. This coupling efficiency can approach 100% in theory when the diameter of the nanowire becomes smaller than 50 nm. Our results provide clear evidence of the surface plasmon amplifier nature of spasers and will pave the way for their various applications.

Extremely stable temperature characteristics of 1550-nm band, p-doped, highly stacked quantum-dot laser diodes

We fabricated 1.55-µm band, broad-area, p-doped, 30-layer stacked quantum-dot (QD) laser diodes (LDs) grown on an InP(311)B substrate via a delta-doping method employing a strain compensation technique. We doped Be atoms to a depth of 5 nm from the bottom of each QD layer. The concentration of Be atoms doped in the InGaAlAs spacer layer was 1 × 1018 cm−3. We observed a strong photoluminescence emission and a relatively coherent surface of QDs using atomic force microscopy. In addition, we observed that the fabricated QD-LDs had extremely stable temperature characteristics, and a characteristic temperature T0 of more than 2156 K was obtained.

Effects of spin excitons on the surface states of SmB6 : A photoemission study

We present the results of high-resolution valence-band photoemission spectroscopic study of SmB6 which shows evidence for a V-shaped density of states of surface origin within the bulk gap. The spectroscopy data is interpreted in terms of the existence of heavy 4f surface states, which may be useful in resolving the controversy concerning the disparate surface Fermi-surface velocities observed in experiments. Most importantly, we find that the temperature dependence of the valence-band spectrum indicates that a small feature appears at a binding energy of about -9 meV at low temperatures. We attribute this feature to a resonance caused by the spin-exciton scattering in SmB6 which destroys the protection of surface states due to time-reversal invariance and spin-momentum locking. The existence of a low-energy spin-exciton may be responsible for the scattering which suppresses the formation of coherent surface quasi-particles and the appearance of the saturation of the resistivity to temperatures much lower than the coherence temperature associated with the opening of the bulk gap.

A new method for reconstructing medium inhomogeneities using Rayleigh waves: Case studies

The problem of reconstructing local low-contrast inhomogeneities in the Earth’s surface layers by means of coherent Rayleigh surface waves is considered. It is shown that analysis of the frequency characteristics of shear projections in this wave on the surface allows construction of the function of inhomogeneity distribution in a specified depth range. The results from seismoacoustic reconstruction of inhomogeneities correlate with data obtained using standard geophysical methods and are confirmed by direct observations.

Initiating and imaging the coherent surface dynamics of charge carriers in real space.

The tip of a scanning tunnelling microscope is an atomic-scale source of electrons and holes. As the injected charge spreads out, it can induce adsorbed molecules to react. By comparing large-scale ‘before' and ‘after' images of an adsorbate covered surface, the spatial extent of the nonlocal manipulation is revealed. Here, we measure the nonlocal manipulation of toluene molecules on the Si(111)-7 × 7 surface at room temperature. Both the range and probability of nonlocal manipulation have a voltage dependence. A region within 5–15 nm of the injection site shows a marked reduction in manipulation. We propose that this region marks the extent of the initial coherent (that is, ballistic) time-dependent evolution of the injected charge carrier. Using scanning tunnelling spectroscopy, we develop a model of this time-dependent expansion of the initially localized hole wavepacket within a particular surface state and deduce a quantum coherence (ballistic) lifetime of ∼10 fs.

Low temperature processed ZnO thin film as electron transport layer for efficient perovskite solar cells

Organic inorganic lead halide Perovskite photovoltaic devices are promising candidates for commercial application because of their high efficiency and low production cost. One integral part of these high efficiency solar cells is electron transport layer that provides the electron contact selectivity and mitigates recombination phenomena for enhanced device performance. However, high temperature sintering process of most widely used Titanium oxide electron transport layer or the sophisticated, time consuming processing with nanostructured electron extraction material is a fundamental barrier to mass production of Perovskite solar cell with roll-to-roll process. In this work, we have reported the application of simple, low temperature processed (<150 °C) sol-gel ZnO thin film as electron transport layer with efficient (PCE: 8.77%), highly reproducible Perovskite solar cell. Consecutive spin coating process has been implemented to find a multi-layer ZnO film that ensures high optical absorption in photoactive Perovskite layer by acting as a highly transmitting, less reflective, transparent layer. The optimized ZnO film also provides coherent surface morphology for the proper crystalline growth of overlying Perovskite layer and suppresses the deep trap states existing at the ZnO/perovskite interface. A systematic impedance spectroscopy study has been presented in this work to comprehend the improved device performance with the optimized multilayer electron transport material. The electronic properties like contact resistance, recombination resistance, flat-band potential and depletion width of the best performing device have been investigated. The interfacial charge transfer characteristics between methyl ammonium lead triiodide perovskite and low temperature solgel ZnO have also been elaborated based on the interface electronic properties.

Interaction between a surface quasi-geostrophic buoyancy filament and an internal vortex

This paper focuses on the nonlinear interaction between a surface quasi-geostrophic buoyancy filament and an internal vortex. We first revisit the stability of an isolated buoyancy filament. The buoyancy profile considered is continuous and leads to a continuous velocity field, albeit one with infinite shear just outside its edge. The stability properties of an isolated filament help to interpret the unsteady interaction with a sub-surface (internal) vortex studied next. We find that, in all cases, the filament breaks into billows, analogous in form to those occurring in Kelvin–Helmholtz shear instability. For intense buoyancy filaments, the vortex itself may undergo strong deformations, including being split into several pieces. Generally, the nonlinear interaction causes both the filament and the vortex to lose their respective “self”-energies to the energy of interaction. The flow evolution depends sensitively on whether the vertical vorticity of the filament and the vortex have the same or opposite signs – termed “cooperative” and “adverse” shear respectively. In cooperative shear, the filament rolls up into a coherent surface eddy above a vortex initially placed below it, whereas in adverse shear, buoyancy is expelled above the vortex. Although sufficiently great shear induced by the buoyancy filament may split the vortex in both cases, adverse shear is significantly more destructive.

Stereoscopic surface interpolation from illusory contours

Stereoscopic Kanizsa figures are an example of stereoscopic interpolation of an illusory surface. In such stimuli, luminance-defined disparity signals exist only along the edges of inducing elements, but observers reliably perceive a coherent surface that extends across the central region in depth. The aim of this series of experiments was to understand the nature of the disparity signal that underlies the perception of illusory stereoscopic surfaces. I systematically assessed the accuracy and precision of suprathreshold depth percepts using a collection of Kanizsa figures with a wide range of 2D and 3D properties. For comparison, I assessed similar perceptually equated figures with luminance-defined surfaces, with and without inducing elements. A cue combination analysis revealed that observers rely on ordinal depth cues in conjunction with stereopsis when making depth judgements. Thus, 2D properties (e.g. occlusion features and luminance relationships) contribute rich information about 3D surface structure by influencing perceived depth from binocular disparity.

Surface mechanics implications for a nanovoided metallic thin-plate under uniform boundary loading

The goal of this work is to shed some light on the problem of stress concentrations near nanoscale pores (nanovoids) in metallic thin-plates subjected to mechanical load. The limitations of classical elasticity at the nanoscale can be mitigated by the incorporation of a coherent surface model. The disturbance of the elastic field due to a nanovoid in an elastic thin-plate can be determined using a three-dimensional displacement formulation. Numerical results suggest that the surface energy and corresponding surface stress of the nanovoid significantly alter the local stress distribution and the relevant stress concentrations. The magnitude of this effect depends on parameters like the void size, film thickness, applied load, and material properties of the thin-plate and the void surface. The results of the study suggest that nanoporous thin-plates could be optimized for lower stress concentrations and might be less vulnerable to fracture, at least when subjected to uniaxial tensile loads.