Multipole Approach to the Dynamical Casimir Effect with Finite-Size Scatterers.

A mirror subjected to a fast mechanical oscillation emits photons out of the quantum vacuum-a phenomenon known as the dynamical Casimir effect (DCE). The mirror is usually treated as an infinite metallic surface. Here, we show that, in realistic experimental conditions (mirror size and oscillation frequency), this assumption is inadequate and drastically overestimates the DCE radiation. Taking the opposite limit, we use instead the dipolar approximation to obtain a simpler and more realistic treatment of DCE for macroscopic bodies. Our approach is inspired by a microscopic theory of DCE, which is extended to the macroscopic realm by a suitable effective Hamiltonian description of moving anisotropic scatterers. We illustrate the benefits of our approach by considering the DCE from macroscopic bodies of different geometries.

Time-Dependent Effective Hamiltonians for Light-Matter Interactions.

In this paper, we present a systematic approach to building useful time-dependent effective Hamiltonians in molecular quantum electrodynamics. The method is based on considering part of the system as an open quantum system and choosing a convenient unitary transformation based on the evolution operator. We illustrate our formalism by obtaining four Hamiltonians, each suitable to a different class of applications. We show that we may treat several effects of molecular quantum electrodynamics with a direct first-order perturbation theory. In addition, our effective Hamiltonians shed light on interesting physical aspects that are not explicit when employing more standard approaches. As applications, we discuss three examples: two-photon spontaneous emission, resonance energy transfer, and dispersion interactions.

Universal Casimir Interaction between Two Dielectric Spheres in Salted Water.

We study the Casimir interaction between two dielectric spheres immersed in a salted solution at distances larger than the Debye screening length. The long distance behavior is dominated by the nonscreened interaction due to low-frequency transverse magnetic thermal fluctuations. It shows universality properties in its dependence on geometric dimensions and independence of dielectric functions of the particles, with these properties related to approximate conformal invariance. The universal interaction overtakes nonuniversal contributions at distances of the order of or larger than 0.1 µm, with a magnitude of the order of the thermal scale k_{B}T such as to make it important for the modeling of colloids and biological interfaces.

Plane-wave approach to the exact van der Waals interaction between colloid particles.

The numerically exact evaluation of the van der Waals interaction, also known as Casimir interaction when including retardation effects, constitutes a challenging task. We present a new approach based on the plane-wave basis and demonstrate that it possesses advantages over the more commonly used multipole basis. The rotational symmetry of the plane-sphere and sphere-sphere geometries can be exploited by means of a discrete Fourier transform. The new technique is applied to a study of the interaction between a colloid particle made of polystyrene or mercury and another polystyrene sphere or a polystyrene wall in an aqueous solution. Special attention is paid to the influence of screening caused by a variable salt concentration in the medium. It is found that, in particular for low salt concentrations, the error implied by the proximity force approximation is larger than usually assumed. For a mercury droplet, a repulsive interaction is found for sufficiently large distances, provided that screening is negligible. We emphasize that the effective Hamaker parameter depends significantly on the scattering geometry on which it is based.

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.

Enantioselective manipulation of single 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 one 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.

Casimir interaction between plane and spherical metallic surfaces.

We give an exact series expansion of the Casimir force between plane and spherical metallic surfaces in the nontrivial situation where the sphere radius R, the plane-sphere distance L and the plasma wavelength lambda(P) have arbitrary relative values. We then present numerical evaluation of this expansion for not too small values of L/R. For metallic nanospheres where R, L and lambda(P) have comparable values, we interpret our results in terms of a correlation between the effects of geometry beyond the proximity force approximation and of finite reflectivity due to material properties. We also discuss the interest of our results for the current Casimir experiments which are performed with spheres of large radius R>>L.