Manuel I. Marqués

Giant Enhanced Diffusion of Gold Nanoparticles in Optical Vortex Fields

We study the diffusion of a metal nanoparticle in the nonconservative force field of an optical vortex lattice. Radiation pressure in the vortex array is shown to induce a giant enhancement over the free thermal diffusion. Langevin dynamics simulations show that the diffusion coefficient of (50 nm radius) gold particles at room temperature is enhanced by 2 orders of magnitude at power densities of the order or smaller than those used to trap nanoparticles with optical tweezers.

Light control of silver nanoparticle’s diffusion

The diffusion of silver nanoparticles in water at 298K inside an optical vortex lattice is analyzed in detail by numerical simulations. At power densities of the order of those used to trap nanoparticles with optical tweezers, the dynamic response shows three different regimes depending on the light wavelength. In the first one particles get trapped inside the light vortices following almost closed trajectories. In the second one, around the plasmon resonance, the diffusion constant is dramatically enhanced with respect to the Brownian motion. In the third one, at longer wavelengths, nanoparticles are confined during a few seconds in quasi-one-dimensional optical traps.

SELF-AVERAGING OF RANDOM AND THERMALLY DISORDERED DILUTED ISING SYSTEMS

Self-averaging of singular thermodynamic quantities at criticality for randomly and thermally diluted three dimensional Ising systems has been studied by the Monte Carlo approach. Substantially improved self-averaging is obtained for critically clustered (critically thermally diluted) vacancy distributions in comparison with the observed self-averaging for purely random diluted distributions. Critically thermal dilution, leading to maximum relative self-averaging, corresponds to the case when the characteristic vacancy ordering temperature is made equal to the magnetic critical temperature for the pure 3D Ising systems. For the case of a high ordering temperature, the self-averaging obtained is comparable to that in a randomly diluted system.

Beam configuration proposal to verify that scattering forces come from the orbital part of the Poynting vector

In this Letter, the optical forces on an electric dipole generated by a beam made up of two circularly polarized Hermite-Gaussian modes have been analyzed. When the intensity of the two modes is not the same, the mechanical action of the scattering force is completely different from the behavior of the Poynting vector. The dynamics of resonant metallic nanoparticles under this field have been calculated by means of Langevin molecular dynamic simulations. This configuration could be useful to experimentally verify how radiation pressure on a Rayleigh particle is due to the transfer of linear momentum coming solely from the orbital part of the Poynting vector.

On the existence of two states in liquid water: impact on biological and nanoscopic systems

This work reviews several properties of liquid water, including the dielectric constant and the proton-spin lattice relaxation, and draws attention to a bilinear behaviour defining a crossover in the temperature range 50 ± 10°C between two possible states in liquid water. The existence of these two states in liquid water plays an important role in nanometric and biological systems. For example, the optical properties of metallic (gold and silver) nanoparticles dispersed in water, used as nanoprobes, and the emission properties of CdTe quantum dots (QDs), used for fluorescence bioimaging and tumour targeting, show a singular behaviour in this temperature range. In addition, the structural changes in liquid water may be associated with the behaviour of biological macromolecules in aqueous solutions and in particular with protein denaturation.

Relationship between fragility, diffusive directions and energy barriers in a supercooled liquid

An analysis of diffusion in a supercooled liquid based solely in the density of diffusive directions and the value of energy barriers shows how the potential energy landscape (PEL) approach is capable of explaining the α and β relaxations and the fragility of a glassy system. We find that the β relaxation is directly related to the search for diffusive directions. Our analysis shows how in strong liquids diffusion is mainly energy activated, and how in fragile liquids the diffusion is governed by the density of diffusive directions. We describe the fragile-to-strong crossover as a change in the topography of the PEL sampled by the system at a certain crossover temperature T×.

Poling effect on distribution of quenched random fields in a uniaxial relaxor ferroelectric

The frequency dependence of the dielectric permittivitys maximum has been studied for poled and unpoled doped relaxor strontium barium niobate Sr0.61Ba0.39Nb2O6:Cr3 + (SBN-61:Cr). In both cases the maximum found is broad and the frequency dispersion is strong. The present view of random fields compensation in the unpoled sample is not suitable for explaining this experimental result. We propose a new mechanism where the dispersion of quenched random electric fields, affecting the nanodomains, is minimized after poling. We test our proposal by numerical simulations on a random field Ising model. Results obtained are in agreement with the polarizations measurements presented by Granzow et al. (Phys. Rev. Lett. 92 (2004) 065701).

Field emission magnetic sensors based on focusing devices

Abstract In the present article we analyze a vacuum microelectronics magnetic sensor based in a simple configuration with a double-gated wedge emitter with a split anode as current detector. The actuation of such device has been simulated using several numerical approaches including full determination of electrostatic potentials with accurate description of fields at the wedge region, electron beam propagation inside the device, and evaluation of the current density distributions at the anode detector screen. Calculated sensor sensitivities are larger than those found in conventional magnetic field sensors designed to detect similar magnetic field ranges. The focusing system provides a tuning mechanism able to change the sensor sensitivity and the magnetic field detection working range. The dependence of the sensitivity on geometrical parameters and control voltages is discussed.

Optical image contrast enhancement in near-field optics induced by water condensation.

In surface science, water adsorption on hydrophilic samples is usually invoked, addressing their nanoscale experimental effects in scanning probe microscopy, especially when water condensates between tip and sample. Here we study by means of a numerical hybrid method the effect of water bridge formation in near field imaging. We show how this nanometric water neck plays an important role not only in the optical image, producing a high contrast at hydrophilic patches, but also in the tip-sample distance control. This work contributes with a new methodology able to retrieve the original application of SNOM, using it as an instrument to study the optical properties of matter overcoming the diffraction limit. It extends the application of SNOM to study the hydrophilic character of polymeric and biological samples, taking advantage of ubiquitous effect of humidity when operating in ambient condition.

Scattering forces and electromagnetic momentum density in crossed circularly polarized standing waves

We analyze the forces on a small dipolar particle and the electromagnetic momentum density in a configuration consisting in two perpendicular circularly polarized stationary waves. The field distribution shows regions in which the electric and magnetic fields are parallel corresponding to a null Poynting vector. Although the average value of the momentum density, proportional to the Poynting vector, is zero in these regions, there are scattering forces acting on small particles due to lights spin force. The total scattering force suggests a new definition of the average value of the momentum density for free propagating electromagnetic fields.We analyze the forces on a small dipolar particle and the electromagnetic momentum density in a configuration consisting in two perpendicular circularly polarized stationary waves. The field distribution shows regions in which the electric and magnetic fields are parallel corresponding to a null Poynting vector. Although the average value of the momentum density, proportional to the Poynting vector, is zero in these regions, there are scattering forces acting on small particles due to lights spin force. The total scattering force suggests a new definition of the average value of the momentum density for free propagating electromagnetic fields.