Experimental verification of the "rainbow" trapping effect in plasmonic graded gratings

We report the first experimental observation of trapped rainbow1 in graded metallic gratings2-4, designed to validate theoretical predictions for this new class of plasmonic structures. One-dimensional tapered gratings were fabricated and their surface dispersion properties tailored by varying the grating period and depth, whose dimensions were confirmed by atomic force microscopy. Reduced group velocities and the plasmonic bandgap were observed. Direct measurements on graded grating structures show that light of different wavelengths in the 500-700nm region is "trapped" at different positions along the grating, consistent with computer simulations, thus verifying the "rainbow" trapping effect. The trapped rainbow effect offers exciting pathways for optical information storage and optical delays in photonic circuits at ambient temperature

Chemical Force Microscopy: Probing Chemical Origin of Interfacial Forces and Adhesion

Experimental methods of measuring intermolecular interactions have had several recent developments which have improved our understanding of chemical forces. First, they allowed direct exploration of the role that different functionalities, solvents and environmental variables play in shaping the strength of intermolecular interactions. Chemical force microscopy approach, in particular, became an extremely effective tool for exploring the contributions of each of these factors. Second, CFM studies clearly debunked the naive notion that intermolecular interaction strength is determined only by the nature of the interacting groups. These studies showed that the interaction strength between two chemical species must always considered in context of the environment surrounding these species. Third, CFM studies highlighted the critical role solvent plays in shaping intermolecular interactions in condensed phases. Emerging kinetic view of the intermolecular interactions introduced a completely new paradigm for understanding these interactions. Kinetic modeling showed that the measured interactions strength depends not only on the energy landscape of the system, but also on the loading history prior to the bond break-up. This new paradigm refocused our attention to the energy landscape as a fundamental characteristic of the interaction. Moreover, dynamic force spectroscopy, derived from kinetic models, allowed direct characterization of the geometry of the potential energy barrier, while some other methods attempt to probe the equilibrium energy landscape directly. Further investigations of the interactions in different systems, especially interactions between biomolecules, will uncover many interesting characteristics of intermolecular potentials. These studies have the potential to reveal, for the first time, a true picture of the energy landscapes of adhesion processes in complex chemical and biological systems

Brownian Dynamics Simulation of Peeling a Strongly-Adsorbed Polymer Molecule from a Frictionless Substrate

We used Brownian dynamics to study the peeling of a polymer molecule, represented by a freely jointed chain, from a frictionless surface in an implicit solvent with parameters representative of single-stranded DNA adsorbed on graphite. For slow peeling rates, simulations match the predictions of an equilibrium statistical thermodynamic model. We show that deviations from equilibrium peeling forces are dominated by a combination of Stokes (viscous) drag forces acting on the desorbed section of the chain and a finite rate of hopping over a desorption barrier. Characteristic velocities separating equilibrium and nonequilibrium regimes are many orders of magnitude higher than values accessible in force spectroscopy experiments. Finite probe stiffness resulted in disappearance of force spikes due to desorption of individual links predicted by the statistical thermodynamic model under displacement control. Probe fluctuations also masked sharp transitions in peeling force between blocks of distinct sequences, indicating limitation in the ability of single-molecule force spectroscopy to distinguish small differences in homologous molecular structures