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Free-carrier induced soliton fission unveiled by in-situ measurements in nanophotonic
Get PDFSolitons are localized waves formed by a balance of focusing and defocusing effects. These
nonlinear waves exist in diverse forms of matter yet exhibit similar properties including
stability, periodic recurrence and particle-like trajectories. One important property is soliton
fission, a process by which an energetic higher-order soliton breaks apart due to dispersive or
nonlinear perturbations. Here we demonstrate through both experiment and theory that
nonlinear photocarrier generation can induce soliton fission. Using near-field measurements,
we directly observe the nonlinear spatial and temporal evolution of optical pulses in situ in a nanophotonic semiconductor waveguide. We develop an analytic formalism describing the
free-carrier dispersion (FCD) perturbation and show the experiment exceeds the minimum
threshold by an order of magnitude. We confirm these observations with a numerical
nonlinear Schro¨dinger equation model. These results provide a fundamental explanation and
physical scaling of optical pulse evolution in free-carrier media and could enable improved
supercontinuum sources in gas based and integrated semiconductor waveguides
Calcium-mediated actin reset (CaAR) mediates acute cell adaptations
Get PDFActin has well established functions in cellular morphogenesis. However, it is not well understood how the various actin assemblies in a cell are kept in a dynamic equilibrium, in particular when cells have to respond to acute signals. Here, we characterize a rapid and transient actin reset in response to increased intracellular calcium levels. Within seconds of calcium influx, the formin INF2 stimulates filament polymerization at the endoplasmic reticulum (ER), while cortical actin is disassembled. The reaction is then reversed within a few minutes. This Calcium-mediated actin reset (CaAR) occurs in a wide range of mammalian cell types and in response to many physiological cues. CaAR leads to transient immobilization of organelles, drives reorganization of actin during cell cortex repair, cell spreading and wound healing, and induces long-lasting changes in gene expression. Our findings suggest that CaAR acts as fundamental facilitator of cellular adaptations in response to acute signals and stress
A New Cathodoluminescence System for Nanoscale Optics, Materials Science, and Geology
Get PDFCathodoluminescence (CL) spectroscopy is a technique that can be used to unravel optical and material properties at the nanoscale. This article describes a novel CL detection system. The system sensitivity, the modularity, and the capability to perform angle-resolved CL imaging enable many new research directions. We introduce the technique and give a description of the setup as well as the different CL emission processes. Subsequently, we show a number of practical examples where
the technique is used in the fields of nanophotonics, materials science, and geology
Combined electron energy-loss and cathodoluminescence spectroscopy on individual and composite plasmonic nanostructures
Get PDFWe systematically investigate the plasmonic "dolmen" geometry and its constituent elements using electron energy-loss spectroscopy and cathodoluminescence spectroscopy. In particular, we study the effects of the particle size and spacing on the resonant behavior and interparticle coupling. Because we apply both techniques on the same structures we can directly compare the results and investigate the radiative versus nonradiative character of the different modes. We find that the cathodoluminescence response is significantly lower than the electron energy-loss response for higher-energy modes because strong absorption reduces the scattering efficiency in this regime. Furthermore, we show that the overall resonant response roughly scales with size as expected for plasmonic structures but that the transverse resonant modes do become more dominant in larger structures due to a relative reduction in Ohmic dissipation. Using EELS and CL we can rigorously study coupling between the elements and show that the coupling diminishes for larger spacings
Rescaling of Spatio-Temporal Sensing in Eukaryotic Chemotaxis
Get PDFEukaryotic cells respond to a chemoattractant gradient by forming intracellular gradients of signaling molecules that reflect the extracellular chemical gradient D an ability called directional sensing. Quantitative experiments have revealed two characteristic input-output relations of the system: First, in a static chemoattractant gradient, the shapes of the intracellular gradients of the signaling molecules are determined by the relative steepness, rather than the absolute concentration, of the chemoattractant gradient along the cell body. Second, upon a spatially homogeneous temporal increase in the input stimulus, the intracellular signaling molecules are transiently activated such that the response magnitudes are dependent on fold changes of the stimulus, not on absolute levels. However, the underlying mechanism that endows the system with these response properties remains elusive. Here, by adopting a widely used modeling framework of directional sensing, local excitation and global inhibition (LEGI), we propose a hypothesis that the two rescaling behaviors stem from a single design principle, namely, invariance of the governing equations to a scale transformation of the input level. Analyses of the LEGI-based model reveal that the invariance can be divided into two parts, each of which is responsible for the respective response properties. Our hypothesis leads to an experimentally testable prediction that a system with the invariance detects relative steepness even in dynamic gradient stimuli as well as in static gradients. Furthermore, we show that the relation between the response properties and the scale invariance is general in that it can be implemented by models with different network topologies
Criticality in Vibrated Frictional Flows at a Finite Strain Rate
Get PDFWe evidence critical fluctuations in the strain rate of granular flows that are weakly vibrated. Strikingly, the critical point arises at finite values of the mean strain rate and vibration strength, far from the yielding critical point at a zero flow rate. We show that the global rheology, as well as the amplitude and correlation time of the fluctuations, are consistent with a mean-field, Landau-like description, where the strain rate and the stress act as conjugated variables. We introduce a general model which captures the observed phenomenology and argue that this type of critical behavior generically arises when self-fluidization competes with friction
Generalized antireflection coatings for complex bulk metamaterials
Get PDFWe present the optimized design of an antireflection coating to efficiently couple an incident plane wave into a metamaterial with a complex field profile. We show that such an antireflection coating must enable spatial engineering of the field profiles at the coating/metamaterial interface to achieve high transmission, and therefore it is required to be inhomogeneous. As a demonstration, we investigate theoretically a waveguide-based negative-index metamaterial, which under normal incidence cannot be excited due to the antisymmetric propagating eigenmode. Through careful engineering of the field profile, lateral position, and thickness of the coating layer, we enhance the transmission under normal incidence from 0% to 100%. This principle may generally be applied to overcome low coupling efficiency between incident plane waves and complex mode profiles in metamaterials
Harmonics generation by surface plasmon polaritons on single nanowires
Get PDFWe present experimental observations of visible wavelength second- and third- harmonic generation on single plasmonic nanowires of variable widths. We identify that near infrared surface plasmon polaritons, which are guided along the nanowire, act as the source of the harmonics generation. We discuss the underlying mechanism of this nonlinear process, using a combination of spatially resolved measurements and numerical simulations to show that the visible harmonics are generated via a combination of both local and propagating plasmonic modes. Our results provide the first demonstration of nanoscale nonlinear optics with guided, propagating plasmonic modes on a lithographically defined chip, opening up new routes towards integrated optical circuits for information processing
Backaction in metasurface etalons
Get PDFWe consider the response of etalons created by a combination of a conventional mirror and a metasurface composed of a periodic lattice of metal scatterers with a resonant response. This geometry has been used previously for hybridization of localized plasmons with Fabry-Perot resonances and for perfect absorption, in so-called Salisbury screens. The particular aspect we address is if one can assume an environment-independent reflectivity for the metasurface when calculating the reflectivity of the composite system, as in a standard Fabry-Perot analysis, or whether the fact that the metasurface interacts with its own mirror image renormalizes its response. Using lattice sum theory, we take into account all possible retarded dipole-dipole interactions of scatterers in the metasurface amongst each other and through the mirror. We show that while a layer-by-layer Fabry-Perot formalism captures the main qualitative features of metasurface etalons, in fact the mirror modifies both the polarizability and reflectivity of the metasurface in a fashion that is akin to Drexhage's modification of the radiative properties of a single dipole