The precise manipulation of nanoscale particles is essential for advancing applications in colloidal self-assembly, targeted sorting, and emerging paradigms such as colloidal memory. Here, we present a high-speed multiplane 3D imaging platform that enables direct visualization and quantification of dielectrophoretic (DEP) forces and electrokinetic flows acting on individual nanoparticles in real time. Using 200 nm dye-doped polystyrene particles, we map both positive and negative DEP regimes, capturing spatial force distributions with sub-50 nm axial resolution and frame rates exceeding 100 fps. Our approach reveals distinct particle behaviors across frequency regimes, including trapping near electrodes under positive DEP and levitation above the surface under negative DEP. In addition, we characterize electrokinetic flow fields in 3D, which play a critical role at higher voltages. This methodology provides a robust experimental framework for investigating DEP manipulation schemes, validating theoretical models, and selectively addressing particle mixtures, a key requirement for future molecular machines and frequency-controlled colloidal memory systems.

The dynamics and equilibrium configurations of immersed optically-bound particles are complex phenomena involving several physical mechanisms such as optical forces, electrostatic interactions, and fluid dynamics. In this work, we unravel, using experiments and numerical simulations, the key role played by short-range electrostatic forces. The repulsive interaction among gold nanoparticles is adjusted by changing the salt concentration. When the electrostatic interaction is reduced, near-field optical binding with particles oriented along the polarization direction is promoted, while, for low values of the salt concentration, inter-particle repulsion induces far-field (FF) optical binding configurations oriented perpendicular to the polarization. The importance of electrostatic force is confirmed by a theoretical model in which the repulsive effect is explicitly tuned. The numerical results reproduce the measured particle configurations and highlight the dominant role of electrostatic interactions, particularly in FF optical binding configurations.

Optical matter, a transient arrangement formed by the interaction of light with micro/nanoscale objects, provides responsive and highly tunable materials that allow for controlling and manipulating light and/or matter. A combined experimental and theoretical exploration of optical matter is essential to advance our understanding of the phenomenon and potentially design applications. Most studies have focused on nanoparticles composed of a single material (either metallic or dielectric), representing two extreme regimes, one where the gradient force (dielectric) and one where the scattering force (metallic) dominates. To understand their role, it is important to investigate hybrid materials with different metallic-to-dielectric ratios. Here, we combine numerical calculations and experiments on hybrid metal-dielectric core-shell particles (200 nm gold spheres coated with silica shells with thicknesses ranging from 0 to 100 nm). We reveal how silica shell thickness critically influences the essential properties of optical binding, such as interparticle distance, reducing it below the anticipated optical binding length. Notably, for silica shells thicker than 50 nm, we observed a transition from a linear arrangement perpendicular to polarization to a hexagonal arrangement accompanied by a circular motion. Further, the dynamic swarming assembly changes from the conventional dumbbell-shaped to lobe-like morphologies. These phenomena, confirmed by both experimental observations and dynamic numerical calculations, demonstrate the complex dynamics of optical matter and underscore the potential for tuning its properties for applications.

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.

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects, ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in the life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nano-particle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration.

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.

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.

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.

Active particles are capable of extracting energy from their environment and converting it into direct motion. This direct motion is usually attained by asymmetries, which induce a net force in the particle. In this article we show how a dimer made up of two nanoparticles can obtain this energy supply from the nonreciprocal force induced by an homogeneous and isotropic random electromagnetic field. We explicitly obtain the value of the force and find it to be different from zero only for sets of different particles in which at least one of them is absorbent. In addition, we prove that the nonreciprocity of the force is the condition for the interaction dynamics to be nonconservative. We further show how this mechanism can be used to induce active Brownian motion with persistent random walks, effective diffusion coefficients, and noticeable Peclet numbers.

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.

Lanthanide-based upconverting nanoparticles (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 we show how these problems can be elegantly solved using a thermoresponsive polymeric coating. Upon decorating the surface of NaYF4:Er,Yb 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 ºC). This optical force improvement is accompanied by a significant enhancement of the thermal sensitivity reaching a maximum value of 7 % °C-1 at 31.5 ºC caused by the temperature-induced collapse of PNIPAM.

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.

Focused laser beams allow controlling mechanical motion of objects and can serve as a tool for assembling complex micro and nano structures in space. While in a vast majority of cases small particles experience attractive gradient forces and repulsive radiation pressure, introduction of additional degrees of freedom into optomechanical manipulation suggests approaching new capabilities. Here we analyze optical forces acting on a high refractive index silicon sphere in a focused Gaussian beam and reveal new regimes of particles anti-trapping. Multipolar analysis allows separating an optical force into interception and recoil components, which have a completely different physical nature resulting in different mechanical actions. In particular, interplaying interception radial forces and multipolar resonances within a particle can lead to either trapping or anti-trapping scenarios, depending of the overall system parameters. At the same time, the recoil force generates a significant azimuthal component along with an angular-dependent radial force. Those contribution enable enhancing either trapping or anti-trapping regimes and also introduce bending reactions. These effects are linked to the far-field multipole interference resulting and, specifically, to its asymmetric scattering diagrams. The later approach is extremely useful, as it allows assessing the nature of optomechanical motion by observing far-field patterns. Multipolar engineering of optical forces, being quite general approach, is not necessarily linked to simple spherical shapes and paves a way to new possibilities in microfluidic applications, including sorting and micro assembly of nontrivial volumetric geometries.

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.

Fluctuating isotropic electromagnetic fields are gotten by thinking about a gathering of plane waves with wave vectors, polarizations and stages haphazardly conveyed and fluctuating on schedule. Because of the isotropic character of this electromagnetic field, the optical power instigated on a nanoparticle is, in normal, equivalent to nothing. Nonetheless, the elements of electric dipoles on these sorts of frameworks are a long way from being inconsequential.

The linear momentum of electromagnetic radiation is proportional to the Poynting vector. However, when light interacts with a dipole, the radiation pressure generated is no longer proportional to the Poynting vector: the so-called Belinfante’s momentum or equivalently, the curl of the spin density of the light field, must be considered. In this paper, we propose an optical configuration, based on two evanescent counter-propagating waves, perpendicularly polarized, capable of detecting Belinfante’s mechanical action. The two beams interact with a high-refractive-index particle like silicon. The direction of the radiation pressure exerted on the particle, proportional only to the curl of the spin density, depends on the electric and magnetic response of the particle and changes sign at the radiation wavelengths corresponding with the Kerker conditions.

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.

An optical system based on a photonic nanojet created by an optically trapped microsphere is proposed for enhanced trapping and remote manipulation of upconverting nanoparticles. The nanojet-induced enhancement in the optical forces acting on a 8 nm single upconverting nanoparticle are compared to those achievable by the traditional approach based on the use of high numerical aperture optics. The results presented in this work prove that this is a simple and effective method to remotely trap, manipulate, and detect single upconverting nanoparticles. This is an innovative approach to overcome the current challenges in the remote manipulation of probes in biosensing or in single particle studies.