arXiv subjects

R. S. Dutra

Publications and source records attributed to R. S. Dutra.

11 recordsLinked to original sources

arXiv2023

Enantioselective optical forces in gain-functionalized single core-shell chiral nanoparticles

We propose a gain-assisted enantioselective scheme in dye-doped chiral particles, demonstrating optical pulling and pushing forces that can be tuned using externally controllable parameters. By controlling the concentration of dye molecules and pumping rate, we achieve all-optical chiral resolution of racemic mixtures and enantioselection of small dipolar chiral particles without relying on interference. This scheme is applicable to both lossless and lossy plasmonic chiral particles, making it a promising approach for chiral sensing, drug discovery, and molecular separation.

physics.optics↗

arXiv2022

Tailoring radiation pressure on infinite slab using pair of non-collinear plane waves

The electromagnetic field exerts radiation pressure on the matter and tends to move it either in the backward or forward direction due to net optical pulling or pushing force, respectively. In this work, we reveal an interesting phenomenon of a local positive and negative radiation pressure on a dielectric (chiral) slab by using two linearly (circularly) polarized plane waves. In this regard, we develop for the first time, a theory to describe the local radiation pressure appearing due to the interference between the two obliquely impinging (non-collinear) light sources. Under this {situation}, the radiation pressure strongly depends on the angle of incidence, the polarization of the electromagnetic field and the chirality parameters of the slab (in the case of chiral medium). Our numerical analysis shows that the radiation pressure, exerted on a dielectric or a chiral slab due to the two incident plane waves, is constant over the slab for normal incidence, and it varies locally for an oblique incidence, which indeed follows the conservation laws at all incident angles. It is expected that the results may find fruitful applications in optical manipulation of soft matters, for instance, cell membranes, chiral surfaces and other soft materials

physics.optics↗

arXiv2021

Gain-assisted optical tweezing of plasmonic and large refractive index microspheres

We have theoretically investigated optical tweezing of gain-functionalized microspheres using a highly focused single beam in the nonparaxial regime. We employ the Mie-Debye theory of optical tweezers to calculate the optical force acting on homogeneous and core-shell Mie microspheres with gain. We demonstrate that the optical gain plays a crucial role in optical manipulation, especially to optimize the restoring force and thus allowing for trapping of large refractive index and plasmonic particles. Indeed we demonstrate that one can trap such particles, which is usually not possible in the case of passive media, by functionalizing them with an optical gain material. We show that by varying the value of the gain, which can be realized by changing the pump power, one can not only achieve trapping but also manipulate the equilibrium position of the tweezed particle. Altogether our findings open new venues for gain-assisted optomechanics, where gain functionalized systems could facilitate optical trapping and manipulation of plasmonic nanoparticles in particular, with potential applications in self-assembling of nanoparticle suspensions and on a chip.

physics.optics↗

arXiv2021

Probing the screening of the Casimir interaction with optical tweezers

We measure the colloidal interaction between two silica microspheres in aqueous solution in the distance range from $0.2\,μ$m to $0.5\,μ$m with the help of optical tweezers. When employing a sample with a low salt concentration, the resulting interaction is dominated by the repulsive double-layer interaction which is fully characterized. The double-layer interaction is suppressed when adding $0.22\,$M of salt to our sample, thus leading to a purely attractive Casimir signal. When analyzing the experimental data for the potential energy and force, we find good agreement with theoretical results based on the scattering approach. At the distance range probed experimentally, the interaction arises mainly from the unscreened transverse magnetic contribution in the zero-frequency limit, with nonzero Matsubara frequencies providing a negligible contribution. In contrast, such unscreened contribution is not included by the standard theoretical model of the Casimir interaction in electrolyte solutions, in which the zero-frequency term is treated separately as an electrostatic fluctuational effect. As a consequence, the resulting attraction is too weak in this standard model, by approximately one order of magnitude, to explain the experimental data. Overall, our experimental results shed light on the nature of the thermal zero-frequency contribution and indicate that the Casimir attraction across polar liquids has a longer range than previously predicted.

quant-ph↗

arXiv2021

Enantioselection and chiral sorting of single microspheres using optical pulling forces

We put forward a novel, twofold scheme that enables at the same time all-optical enantioselection and sorting of single multipolar chiral microspheres based on optical pulling forces exerted by two non-collinear, nonstructured, circularly-polarized light sources. Our chiral resolution method can be externally controlled by varying the angle between their incident wavevectors, allowing for a fine-tuning of the range of chiral indices for enantioselection. Enantioselectivity is achieved by choosing angles such that only particles with the same handedness of the light sources are pulled. This proposal allows one to achieve all-optical sorting of chiral microspheres with arbitrarily small chiral parameters, thus outperforming current optical methods.

physics.optics↗

arXiv2020

Theory of optical tweezing of dielectric microspheres in chiral host media and its applications

We report for the first time the theory of optical tweezers of spherical dielectric particles embedded in a chiral medium. We develop a partial-wave (Mie) expansion to calculate the optical force acting on a dielectric microsphere illuminated by a circularly-polarized, highly focused laser beam. When choosing a polarization with the same handedness of the medium, the axial trap stability is improved, thus allowing for tweezing of high-refractive-index particles. When the particle is displaced off-axis by an external force, its equilibrium position is rotated around the optical axis by the mechanical effect of an optical torque. Both the optical torque and the angle of rotation are greatly enhanced in the presence of a chiral host medium when considering radii a few times larger than the wavelength. In this range, the angle of rotation depends strongly on the microsphere radius and the chirality parameter of the host medium, opening the way for a quantitative characterization of both parameters. Measurable angles are predicted even in the case of naturally occurring chiral solutes, allowing for a novel all-optical method to locally probe the chiral response at the nanoscale.

physics.optics↗

arXiv2020

Tailoring optical pulling forces with composite microspheres

Optical pulling forces or tractor beams can pull particles against light propagation by redirecting the incident photons forward. This is typically achieved using Bessel beams with very small half-cone angles, which considerably limits its applicability. One can circumvent such issue by using a superposition of plane waves. In order to investigate optical pulling forces exerted by a pair of non-colinear plane waves, we develop a theoretical framework based on Mie theory, Debye potentials and Wigner rotation matrices. We apply this framework to calculate the optical pulling force on metallo-dielectric composite particles, which we put forward as an alternative material platform to optimize and tailor tractor beams. Indeed we demonstrate that by adding a few plasmonic inclusions to low-refractive index dielectric particles of arbitrary sizes, we are able to produce polarization dependent optical pulling forces that cannot occur in the corresponding homogeneous particles. Altogether our findings not only provide innovative theoretical methods to compute optical pulling forces, but also provide new strategies to tailor and optimize them, paving the way to increase their applicability.

physics.optics↗

arXiv2020

Probing the optical chiral response of single nanoparticles with optical tweezers

We propose an enantioselective scheme to sort homogeneous chiral particles using optical tweezers. For a certain range of material parameters, we show that a highly focused circularly-polarized laser beam traps particles of a specific chirality selected by the handedness of the trapping beam. Furthermore, by applying a transverse Stokes drag force that displaces the trapped particle off-axis, we allow for the rotation of the particle center-of-mass around the trapping beam axis. The rotation angle is highly dependent on the handedness of the trapped particle and is easily measurable with standard video-microscopy techniques, allowing for an alternative mechanism for chiral resolution. Our platform not only allows for enantioselection of particles dispersed in solution but also paves the way to the characterization of the chiral parameter of individual, homogeneous chiral microspheres using optical tweezing.

physics.optics↗

arXiv2019

Enantioselective manipulation of chiral nanoparticles using optical tweezers

We put forward an enantioselective method for chiral nanoparticles using optical tweezers. We demonstrate that the optical trapping force in a typical, realistic optical tweezing setup with circularly-polarized trapping beams is sensitive to the chirality of core-shell nanoparticles, allowing for efficient enantioselection. It turns out that the handedness of the trapped particles can be selected by choosing the appropriate circular polarization of the trapping beam. The chirality of each individual trapped nanoparticle can be characterized by measuring the rotation of the equilibrium position under the effect of a transverse Stokes drag force. We show that the chirality of the shell gives rise to an additional twist, leading to a strong enhancement of the optical torque driving the rotation. Both methods are shown to be robust against variations of size and material parameters, demonstrating that they are particularly useful in (but not restricted to) several situations of practical interest in chiral plasmonics, where enantioselection and characterization of single chiral nanoparticles, each and every with its unique handedness and optical properties, are in order. In particular, our method could be employed to unveil the chiral response arising from disorder in individual plasmonic raspberries, synthesized by close-packing a large number of metallic nanospheres around a dielectric core.

physics.optics↗

arXiv2019

Negative optical torque on a microsphere in optical tweezers

We show that the optical force field in optical tweezers with elliptically polarized beams has the opposite handedness for a wide range of particle sizes and for the most common configurations. Our method is based on the direct observation of the particle equilibrium position under the effect of a transverse Stokes drag force, and its rotation around the optical axis by the mechanical effect of the optical torque. We find overall agreement with theory, with no fitting, provided that astigmatism, which is characterized separately, is included in the theoretical description. Our work opens the way for characterization of the trapping parameters, such as the microsphere complex refractive index and the astigmatism of the optical system, from measurements of the microsphere rotation angle.

physics.optics↗

arXiv2018

Optimizing optical tweezing with directional scattering in composite microspheres

Trapping of microspheres with a single focused laser beam is usually limited to materials with relative refractive indexes slightly larger than one. We show that directional light scattering can be employed to optically trap high-index materials. For this purpose, we propose a material platform to achieve zero backward scattering (ZBS), also known as the first Kerker condition, in a composite media containing spherical inclusions of silica embedded in a SiC microsphere. By tuning the volume filling fraction of inclusions and the microsphere radius, stable trapping can be achieved, provided that ZBS is combined with the condition for destructive interference between the fields reflected at the external and internal interfaces of the microsphere when located at the focal point. We show that our proposal also holds even in the presence of a significant amount of spherical aberration, which is a common condition in most optical tweezers setups. In this case, achieving ZBS is essential for trapping high-index materials.

physics.optics↗