Geometric optimization of radiation pressure in dielectric waveguides

Janderson R. Rodrigues and Vilson R. Almeida
Stimulated Brillouin scattering (SBS) processes have been enabling important technological breakthroughs in integrated photonics and nano-optomechanics by exploiting light-sound (photon-phonon) interactions at the nanoscale. These nonlinear processes are created by two main effects: radiation pressure and electrostriction; however, the former is the predominant one in high-index-contrast nanowaveguides. In this work, we derive a simple set of analytical expressions that can be used for optimizing the radiation pressure on the waveguide boundaries for any optical mode, polarization, and wavelength. We observe the very strong influence of waveguide geometric parameters on the optimal radiation pressure value. Furthermore, we explain how the existence of such optimal geometric dimensions is physically related to the minimization of the electromagnetic momentum flow in the propagation direction. This work provides a novel and robust yet simple method to optimize the radiation pressure in dielectric nanowaveguides, which may be of great relevance for designing integrated photonic-phononic devices.

DOI

Coarse-grained particle dynamics along helical orbit by an optical vortex irradiated in photocurable resins

Ryo Nagura, Tempei Tsujimura, Tetsuro Tsuji, Kentaro Doi, and Satoyuki Kawano

Optical vortices, which carry orbital angular momentum, have attracted much attention in various research fields, such as materials processing, chirality control, and particle manipulation. A recent study experimentally confirmed that twisted fibers of polymerized photocurable resins with a constant period can be formed via irradiation by an optical vortex. It is suspected that this phenomenon is caused by the projection of the angular momentum of an optical vortex to the photocurable resin. The detailed mechanism of the growth of such peculiar fibers has not yet been clarified. In this study, which focuses on one aspect of polymerized structure formation, we develop a coarse-grained particle model in which the particle dynamics in the framework of the Rayleigh scattering theory involving light absorption is theoretically simulated. The period of the twisted fibers expressed using the coarse-grained particles is found to be in reasonable agreement with experimental values and independent of the input power of the laser. In addition, the shape of the polymerized fibers can be controlled by modulating the degree of light absorption.

DOI

Ray optics analysis of optical forces on a microsphere in a (2 + 1)D Airy beam

Shuhe Zhang, Jinhua Zhou, and Yu-Xuan Ren

The optical forces of a (2 + 1)D Airy beam on a microsphere are studied in the ray optics regime. The ray model of a (2 + 1)D Airy beam is derived from its Fourier angular spectrum using a stable aggregate of the flexible elements theory. Numerical results demonstrate that the microsphere can be trapped by the transverse optical force and pulled towards the beam major lobe. Longitudinal optical forces further push the microsphere towards the positive z-direction. The trend for the movement of a microsphere in an Airy beam is clearly demonstrated as the stream line of optical forces, which is consistent with the observed phenomena in optical trapping experiments. In the meantime, both the transverse and the longitudinal optical forces increase when the relative refractive index of the trapped microsphere increases. Calculation of optical forces on microspheres with larger size reveals that the optical forces contributed by rays on each hemisphere are actually different due to the asymmetry of the Airy beam. The force difference could cause natural torque on the trapped objects if they are ovals or other asymmetric shapes.

DOI

Optical radiation force (per–length) on an electrically conducting elliptical cylinder having a smooth or ribbed surface

F. G. Mitri

The aim of this work is to develop a formal semi-analytical model using the modal expansion method in cylindrical coordinates to calculate the optical/electromagnetic (EM) radiation force-per-length experienced by an infinitely long electrically-conducting elliptical cylinder having a smooth or wavy/corrugated surface in EM plane progressive waves with different polarizations. In this analysis, one of the semi-axes of the elliptical cylinder coincides with the direction of the incident field. Initially, the modal matching method is used to determine the scattering coefficients by imposing appropriate boundary conditions and solving numerically a linear system of equations by matrix inversion. In this method, standard cylindrical (Bessel and Hankel) wave functions are used. Subsequently, simplified expressions leading to exact series expansions for the optical/EM radiation forces assuming either electric (TM) or magnetic (TE) plane wave incidences are provided without any approximations, in addition to integral equations demonstrating the direct relationship of the radiation force with the square of the scattered field magnitude. An important application of these integral equations concerns the accurate determination of the radiation force from the measurement of the scattered field by any 2D non-absorptive object of arbitrary shape in plane waves. Numerical computations for the non-dimensional radiation force function are performed for electrically conducting elliptic and circular cylinders having a smooth or ribbed/corrugated surface. Adequate convergence plots confirm the validity and correctness of the method to evaluate the radiation force with no limitation to a particular frequency range (i.e. Rayleigh, Mie, or geometrical optics regimes). Particular emphases are given on the aspect ratio, the non-dimensional size of the cylinder, the corrugation characteristic of its surface, and the polarization of the incident field. The results are particularly relevant in optical tweezers and other related applications in fluid dynamics, where the shape and stability of a cylindrical drop stressed by a uniform external electric/magnetic field are altered. Furthermore, a direct analogy with the acoustical counterpart is noted and discussed.

DOI

Vectorial motion of matter induced by light fueled molecular machines

Zouheir Sekkat

A theory of vectorial, photochemically-induced motion of matter is reported. Molecules become mobile when they are photo-selected in a gradient of light intensity. The motion occurs in the direction of the vector of the intensity gradient, and its efficiency depends on the respective orientations of the vectors of light polarization and intensity gradient. Directional motion is imparted into materials containing such smart molecules. The theory well describes experimental observations, and its application to different types of gradients and light polarization excitations is considered. The theory opens important perspectives for the transport of matter by light.

DOI

Electromagnetic binding and radiation force reversal on a pair of electrically conducting cylinders of arbitrary geometrical cross-section with smooth and corrugated surfaces

F. G. Mitri

The electromagnetic (EM) radiation force-per-length exerted on a pair of electrically-conducting cylindrical particles of circular and non-circular cross-sections is examined using a formal semi-analytical method based on boundary matching in cylindrical coordinates. Initially, the scattering coefficients of the particle pair are determined by imposing suitable boundary conditions leading linear systems of equations computed via matrix inversion and numerical integration procedures. Standard cylindrical (Bessel and Hankel) wave functions are used and closed-form expressions for the dimensionless longitudinal and transverse radiation force functions are evaluated assuming either magnetic (TE) or electric (TM) plane wave incidences. Particle pairs with smooth and corrugated surfaces are considered and numerical computations are performed with emphasis on the distance separating their centers of mass, the angle of incidence of the incident illuminating field and the surface roughness. Adequate convergence plots confirm the validity of the method to evaluate the radiation force functions, and the model is adaptable to any frequency range (i.e. Rayleigh, Mie or geometrical optics regimes). The results can find potential applications in optical tweezers and other related applications in fluid dynamics. In addition, the acoustical analogue is discussed.

DOI

Influence of higher modes on plasmonic force in a narrow slit

Alexander Tusnin and David Shapiro

The plasmonic force due to electromagnetic waves between two metallic walls has been studied earlier for a subwavelength slit taking into consideration only zero mode. In the present paper, the effects of the second mode are analyzed. The higher modes are shown to decrease the attractive force. The magnetic field of the p-wave is compared with the model of a perfect conductor. The difference occurs maximal at the threshold, where the second mode changes its behavior from evanescent to propagating. The effect of possibly changing the attractive force to the repulsive force for a relatively wide slit is found.

DOI