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 particleComunidad de Madrid (SI1/PJI/2019-00052); Ministerio de Ciencia e Innovación (CEX2018-000805-M, PGC2018-095777-B-C21, PGC2018-095777-B-C22, PID2019-109905GA-C22

Fuerzas ópticas sobre partículas magneto-ópticas

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de Materiales. Fecha de Lectura: 13-09-2022Esta tesis tiene embargado el acceso al texto completo hasta el 13-03-2024The theoretical study of optical forces and torques has evolved during the last half century from models that described optical forces on a single dielectric or metal nanoparticle to more complex scenarios. Development has been made in models that study optical forces on magneto-dielectric, chiral or multilayer coated particles; models that account for more complex incident fields, such as Bessel beams, as well as the study of optical forces in the near-field, going beyond the Rayleigh limit. The study of these scenarios revealed rich phenomena, including negative forces on magneto-dielectric and chiral nanoparticles by structured beams, i.e. forces that drag these objects towards a light source; Non-conservative and negative torques as well as transversal forces. The modification of the optical properties of a material by the application of an external magnetic field is known as magneto-optical effect. This effect is of particular importance in hybrid metallic systems that support at the same time plasmon excitations (magnetoplasmonic systems), giving rise to plasmon enhanced magnetooptical activity. These structures have a special interest due to the possible applications in biology, sensing and telecommunications. Magneto-optics (MO) and optical forces are vibrant research areas with thousands of articles published every year in each domain separately. However, there is a serious lack of research combining both areas. The goal of this thesis is to theoretically model optical forces exerted on magneto-optical particles. As such, this thesis addresses the following subjects: First, the study of the optical force on a single magneto-optical small particle with only electric response. For this purpose we use previously developed models for optical forces on isotropic dipolar particles which we adapt to work with magnetooptical anisotropy. Using the dipole approximation, we have found new phenomena in magneto-optical forces. We have found a conservative MO force proportional to the gradient of the spin density of the incident light field and a non-conservative extinction force proportional to the helicity of that field. The conservative interaction allows for a spin-selective, magnetic field based Stern-Gerlach experiment, capable of differentiating between right and left circular polarizations. To isolate the magneto-optical effect on optical forces, we also study incident fields of non-interfering counter-propagating waves which do not exert any radiation pressure or gradient force on isotropic particles. These fields do exert optical forces on magneto-optical particles; and depending on the polarization of the incident fields the radiation pressure is proportional to either their spin or their helicity. Second, the study of the optical torque on a single MO particle where we used the dipole approximation. We find that a spinless linearly polarized plane wave, exerts a permanent nonreciprocal torque on a MO particle. Unlike any conservative and transient torque resulting from the transport of the linear momentum of incident waves to angular momentum, the magneto-optical torque is non-conservative and can continuously rotate particles. We show that the angular momentum "created" by rotating the particle is completely compensated by the angular momentum of the scattered field and interaction between the incident field and the scattered field. Third, still within the dipole approximation, we study the effect of electromagnetic radiation on the interaction between two magneto-optical particles. We find that contrary to common belief, it is possible to form a stable bound homo-dimer in the near-field. This could be done either with the help of the magneto-optical effect, by controlling the angle between the polarization plane and the dimer axis, or by using both. We showed that the equilibrium distance between the particles can be controlle to the nanometer scale by the external magnetic field and by the polarization angle and derived analytical formulas for those parameters. Fourth, beyond the dipole approximation, we solved the magneto-optical scattering problem (of a spherical particle) exactly. We used the method of expansion of the fields with spherical wave vector functions, finding the scattering coefficients from boundary conditions. We derive analytical expressions (in terms of the scattering coefficients) for scattering and extinction cross sections as well as for the force exerted on a MO particle. We use this model to calculate the force on a dipolar particle and we reach back the known formulas, previously reported, for force on a dipolar particle with both electric and magnetic respons

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.This work has been supported by the Spanish Ministerio de Ciencia e Innovación (MELODIA PGC2018-095777-B-C21 and C-22) and UAM-CAM project (SI1/PJI/2019-00052). MIM acknowledges also financial support from the Spanish Ministerio de Ciencia e Innovación, through the “María de Maeztu” Programme for Units of Excellence in R& D (CEX2018-000805-M). AGM acknowledges support from the Spanish Ministerio de Ciencia e Innovación through Grant No. PID2019-109905GA-C22

Magneto-optical Stern-Gerlach forces and nonreciprocal torques on small particles

In this paper we calculate the optical forces and torques caused by the presence of a sizable magneto-optical effect. We find a conservative force proportional to the gradient of the spin density of the light field and an extinction force proportional to the helicity of the light field. The conservative interaction allows for a spin-selective, magnetic field based Stern-Gerlach experiment, capable of differentiating between right and left circular polarizations. We also prove that by using a spinless linearly polarized plane wave, the magneto-optical effect allows for the existence of a permanent nonreciprocal torque, proportional to the intensity of the light field.This research was supported by the Spanish MICINN and European Regional Development Fund (ERDF) through Projects No. FIS2015-69295-C3-1-P, No. FIS2015-69295-C3-3-P, No. PGC2018-095777-B-C21, No. PGC2018-095777-B-C22, the Basque Departamento de Educación through Project No. PI-2016-1-0041, and the María de Maeztu Program No. MDM-2014-037