Magneto-optical Particles in Isotropic Spinning Fields Mimic Magnetic Monopoles.

In contrast with the typical electric currents accelerated under the influence of a Coulombic force, there are only a few condensed matter examples of particles experiencing a force proportional to a constant, external magnetic field. In this Letter, we present a new alternative, based on an isotropic radiation spinning field and the magneto-optical effect, in which a particle is propelled by a magnetic field just like a magnetic monopole will do. This is a purely nonreciprocal effect as the reciprocal equivalent (a chiral dipole), despite presenting a dichroic response, does not experience any force when illuminated by the spinning field. The "magnetic charge" induced by impinging radiation on the magneto-optical dipole is found to depend linearly on the helicity of the field. In addition, this artificial monopole experiences a dichroic permanent optical torque and does not interact with an external electric field.

Fano-Like Resonance from Disorder Correlation in Vacancy-Doped Photonic Crystals.

By preparing colloidal crystals with random missing scatterers, crystals are created where disorder is embodied as vacancies in an otherwise perfect lattice. In this special system, there is a critical defect concentration where light propagation undergoes a transition from an all but perfect reflector (for the spectral range defined by the Bragg condition), to a metamaterial exhibiting an enhanced transmission phenomenon. It is shown that this behavior can be phenomenologically described in terms of Fano-like resonances. The results show that the Fano's parameter q experiences a sign change signaling the transition from a perfect crystal exhibiting a reflectance Bragg peak, through a state where background scattering is maximum and Bragg reflectance reaches a minimum to a point where the system reenters a low scattering state recovering ordinary Bragg diffraction. A simple dipolar model considering the correlation between scatterers and vacancies is proposed and the reported evolution of the Fano-like scattering is explained in terms of the emerging covariance between the optical paths and polarizabilities and the effect of field enhancement in photonic crystal (PhC) defects.

Thermoresponsive Polymeric Nanolenses Magnify the Thermal Sensitivity of Single Upconverting Nanoparticles.

Lanthanide-based upconverting nanoparticles (UCNPs) are trustworthy workhorses in luminescent nanothermometry. The use of UCNPs-based nanothermometers has enabled the determination of the thermal properties of cell membranes and monitoring of in vivo thermal therapies in real time. However, UCNPs boast low thermal sensitivity and brightness, which, along with the difficulty in controlling individual UCNP remotely, make them less than ideal nanothermometers at the single-particle level. In this work, it is shown how these problems can be elegantly solved using a thermoresponsive polymeric coating. Upon decorating the surface of NaYF4 :Er3+ ,Yb3+ UCNPs with poly(N-isopropylacrylamide) (PNIPAM), a >10-fold enhancement in optical forces is observed, allowing stable trapping and manipulation of a single UCNP in the physiological temperature range (20-45 °C). This optical force improvement is accompanied by a significant enhancement of the thermal sensitivity- a maximum value of 8% °C+1 at 32 °C induced by the collapse of PNIPAM. Numerical simulations reveal that the enhancement in thermal sensitivity mainly stems from the high-refractive-index polymeric coating that behaves as a nanolens of high numerical aperture. The results in this work demonstrate how UCNP nanothermometers can be further improved by an adequate surface decoration and open a new avenue toward highly sensitive single-particle nanothermometry.

Circular dichroism in magneto-optical forces.

In this article we use an exact method to resolve the fields scattered by a spherical magneto-optical particle and calculate the optical forces exerted on it. The resulting force and the contributing components, i.e. magneto-optical gradient force and magneto-optical extinction force, are presented in an analytical form. We also derive analytical expressions for the scattering and extinction cross sections of a magneto-optical particle, expressions which intuitively demonstrate the effect of circular dichroism in magneto-optical scattering and forces. Finally, we demonstrate that the magneto-optical extinction force is the result of circular dichroism in magneto-optical scattering. We show that it is possible to completely cancel the scattering in the forward or in the backward direction, when the incident field is composed of a circularly-polarized reflected beam. Moreover, the directional scattering is interrelated to the direction of the force exerted on the particle.

Nanojet Trapping of a Single Sub-10 nm Upconverting Nanoparticle in the Full Liquid Water Temperature Range.

Upconverting nanoparticles (UCNPs) have been used as optical probes in a great variety of scenarios ranging from cells to animal models. When optically trapped, a single UCNP can be remotely manipulated making possible, for instance, thermal scanning in the surroundings of a living cell. When conventional optics is used, the stability of an optically trapped UCNP is very limited. Its reduced size leads to optical potentials comparable to thermal energy, and up to now, stable optical trapping of a UCNP has been demonstrated only close to room temperature. This fact limits their use above room temperature, for instance, the use to investigate protein denaturalization that occurs in the 40-50 °C range. In this work, stable optical trapping of a single UCNP in the 20-90 °C range has been demonstrated by using a photonic nanojet. The use of an optically trapped microsphere makes it possible to overcome the diffraction limit producing another optical trap of smaller size and enhanced strength. This simple strategy leads not only to an improvement in the thermal stability of the optical trap but also to an enhancement of the emission intensity generated by the optically trapped UCNP.

Modulated flipping torque, spin-induced radiation pressure, and chiral sorting exerted by guided light.

In recent years, optical forces and torques have been investigated in sub-wavelength evanescent fields yielding a rich phenomenology of fundamental and applied interest. Here we demonstrate analytically that guided modes carrying transverse spin density induce optical torques depending on the character, either electric or magnetic, of the dipolar particles. The existence of a nonzero longitudinal extraordinary linear spin momentum suitable to manipulate optical forces and torques modifies optical forces either enhancing or inhibiting radiation pressure. Hybrid modes supported by cylindrical waveguides also exhibit intrinsic helicity that leads to a rich distribution of longitudinal optical torques. Finally, we show that chiral dipolar particles also undergo lateral forces induced by transverse spin density, amenable to chiral particle sorting. These properties are revealed in configurations on achiral and chiral dipolar particles within confined geometries throughout the electromagnetic spectra.

Optical Forces at the Nanoscale: Size and Electrostatic Effects.

The reduced magnitude of the optical trapping forces exerted over sub-200 nm dielectric nanoparticles complicates their optical manipulation, hindering the development of techniques and studies based on it. Improvement of trapping capabilities for such tiny objects requires a deep understanding of the mechanisms beneath them. Traditionally, the optical forces acting on dielectric nanoparticles have been only correlated with their volume, and the size has been traditionally identified as a key parameter. However, the most recently published research results have shown that the electrostatic characteristics of a sub-100 nm dielectric particle could also play a significant role. Indeed, at present it is not clear what optical forces depend. In this work, we designed a set of experiments in order to elucidate the different mechanism and properties (i.e., size and/or electrostatic properties) that governs the magnitude of optical forces. The comparison between experimental data and numerical simulations have shown that the double layer induced at nanoparticle's surface, not considered in the classical description of nanoparticle's polarizability, plays a relevant role determining the magnitude of the optical forces. Here, the presented results constitute the first step toward the development of the dielectric nanoparticle over which enhanced optical forces could be exerted, enabling their optical manipulation for multiples purposes ranging from fundamental to applied studies.

Unveiling Molecular Changes in Water by Small Luminescent Nanoparticles.

Nowadays a large variety of applications are based on solid nanoparticles dispersed in liquids-so called nanofluids. The interaction between the fluid and the nanoparticles plays a decisive role in the physical properties of the nanofluid. A novel approach based on the nonradiative energy transfer between two small luminescent nanocrystals (GdVO4 :Nd3+ and GdVO4 :Yb3+ ) dispersed in water is used in this work to investigate how temperature affects both the processes of interaction between nanoparticles and the effect of the fluid on the nanoparticles. From a systematic analysis of the effect of temperature on the GdVO4 :Nd3+ → GdVO4 :Yb3+ interparticle energy transfer, it can be concluded that a dramatic increase in the energy transfer efficiency occurs for temperatures above 45 °C. This change is properly explained by taking into account a crossover existing in diverse water properties that occurs at about this temperature. The obtained results allow elucidation on the molecular arrangement of water molecules below and above this crossover temperature. In addition, it is observed that an energy transfer process is produced as a result of interparticle collisions that induce irreversible ion exchange between the interacting nanoparticles.

Optical Torques on Upconverting Particles for Intracellular Microrheometry.

Precise knowledge and control over the orientation of individual upconverting particles is extremely important for full exploiting their capabilities as multifunctional bioprobes for interdisciplinary applications. In this work, we report on how time-resolved, single particle polarized spectroscopy can be used to determine the orientation dynamics of a single upconverting particle when entering into an optical trap. Experimental results have unequivocally evidenced the existence of a unique stable configuration. Numerical simulations and simple numerical calculations have demonstrated that the dipole magnetic interactions between the upconverting particle and trapping radiation are the main mechanisms responsible of the optical torques that drive the upconverting particle to its stable orientation. Finally, how a proper analysis of the rotation dynamics of a single upconverting particle within an optical trap can provide valuable information about the properties of the medium in which it is suspended is demonstrated. A proof of concept is given in which the laser driven intracellular rotation of upconverting particles is used to successfully determine the intracellular dynamic viscosity by a passive and an active method.

Dynamics of a small particle in a fluctuating random light field.

The dynamics of an electric dipole in a light field consisting of electromagnetic plane waves with polarizations randomly distributed and fluctuating phases is theoretically analyzed. The expression for the optical random-force fluctuations is derived and found to be proportional to the scattering cross section and to the square of the intensity divided by the frequency of the electromagnetic field. Under these fluctuations, and in the absence of damping, the dipole behaves like a super-diffusive particle with a kinetic energy growing linearly with time. The analytic predictions are tested against numerical simulations for the particular case of a resonant dipole.

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.

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 light's spin force. The total scattering force suggests a new definition of the average value of the momentum density for free propagating electromagnetic fields.

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 light's spin force. The total scattering force suggests a new definition of the average value of the momentum density for free propagating electromagnetic fields.

Plasmonic nanoparticle chain in a light field: a resonant optical sail.

Optical trapping and driving of small objects has become a topic of increasing interest in multidisciplinary sciences. We propose to use a chain made of metallic nanoparticles as a resonant light sail, attached by one end point to a transparent object and propelling it by the use of electromagnetic radiation. Driving forces exerted on the chain are theoretically studied as a function of radiation's wavelength and chain's alignments with respect to the direction of radiation. Interestingly, there is a window in the frequency spectrum in which null-torque equilibrium configuration, with minimum geometric cross section, corresponds to a maximum in the driving force.

Opto-mechanically generated resonant field enhancement.

A link between the resonant cumulative field enhancement experienced by a chain of plasmonic nanoparticles in a light field and the orientation of the chain with respect to the field is obtained. We calculate analytically the optical torque and the equilibrium configuration and we show how stable orientations are triggered by the geometric resonance conditions. Analytical predictions are checked using numerical calculations based on the coupled dipoles method (CDA) for the particular case of a chain of silver nanoparticles. The reported resonance driven optical torque allows for a tuning of the orientation of the chain depending on radiation's wavelength.

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.

Magneto-optical binding in the near field.

In this paper we show analytically and numerically the formation of a near-field stable optical binding between two identical plasmonic particles, induced by an incident plane wave. The equilibrium binding distance is controlled by the angle between the polarization plane of the incoming field and the dimer axis, for which we have calculated an explicit formula. We have found that the condition to achieve stable binding depends on the particle's dielectric function and happens near the frequency of the dipole plasmonic resonance. The binding stiffness of this stable attaching interaction is four orders of magnitude larger than the usual far-field optical binding and is formed orthogonal to the propagation direction of the incident beam (transverse binding). The binding distance can be further manipulated considering the magneto-optical effect and an equation relating the desired equilibrium distance with the required external magnetic field is obtained. Finally, the effect induced by the proposed binding method is tested using molecular dynamics simulations. Our study paves the way to achieve complete control of near-field binding forces between plasmonic nanoparticles.

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.

Monte Carlo study of the competition between long-range and short-range correlated disorder in a second-order phase transition.

The influence of coexisting correlated and noncorrelated impurities on the critical behavior of the three-dimensional Ising model is studied using Monte Carlo numerical simulations and finite-size scaling. The amount of correlated and noncorrelated vacancies is modified and controlled during the simulations. The long-range correlated (LRC) critical behavior is always found for any value of the concentration of correlated vacancies. The smaller the amount of correlated vacancies the larger the system size needed to detect the LRC universality class. This result explains why critical values measured in xerogel liquid-vapor experiments, where the concentration of correlated vacancies is marginal, seem to correspond to a short-range correlated disorder.