Under the framework of generalized Lorenz-Mie theory, we calculate the radiation force and torque exerted on a chiral sphere by a Gaussian beam. The theory and codes for axial radiation force are verified when the chiral sphere degenerates into an isotropic sphere. We discuss the influence of a chirality parameter on the radiation force and torque. Linearly and circularly polarized incident Gaussian beams are considered, and the corresponding radiation forces and torques are compared and analyzed. The polarization of the incident beam considerably influences radiation force of a chiral sphere. In trapping a chiral sphere, therefore, the polarization of incident beams should be chosen in accordance with the chirality. Unlike polarization, variation of chirality slightly affects radiation torque, except when the imaginary part of the chirality parameter is considered.
[Show abstract][Hide abstract] ABSTRACT: According to the electromagnetic scattering of two spheres, the incident on-axis Gaussian beam is expanded in terms of spherical vector wave functions (SVWFs), and the beam shape coefficients are obtained by applying the localized approximation method. Using the addition theorem, the interaction scattering fields of two chiral spheres and the internal fields are also expanded in terms of SVWFs. Based on the continuous tangential boundary conditions, the scattered field coefficients are derived analytically. Utilizing the Maxwell's stress tensor integration technique, the optical binding force between two chiral spheres is formulated explicitly. Numerical simulations of the binding force are carried out. The effects of the beam width and the radius of the sphere on the force are analyzed. The numerical results are compared with the results from references.
"The latter transition moments can provide an extra contribution to the local position-dependent energies and, hence, an associated optical force. Recently there has been a resurgence of interest in these and other forces specifically associated with chirality          . It emerges that a chiral molecule irradiated with circularly polarized light displays a discriminatory optomechanical response between leftand right-handed input polarizations. "
[Show abstract][Hide abstract] ABSTRACT: When circularly polarized light interacts with chiral molecules or nanoscale particles powerful symmetry principles determine the possibility of achieving chiral discrimination, and the detailed form of electrodynamic mechanisms dictate the types of interaction that can be involved. The optical trapping of molecules and nanoscale particles can be described in terms of a forward-Rayleigh scattering mechanism, with trapping forces being dependent on the positioning within the commonly non-uniform intensity beam profile. In such a scheme, nanoparticles are commonly attracted to local potential energy minima, ordinarily towards the centre of the beam. For achiral particles the pertinent material response property usually entails an electronic polarizability involving transition electric dipole moments. However, in the case of chiral molecules, additional effects arise through the engagement of magnetic counterpart transition dipoles. It emerges that, when circularly polarized light is used for the trapping, a discriminatory response can be identified between left- and right-handed polarizations. Developing a quantum framework to accurately describe this phenomenon, with a tensor formulation to correctly represent the relevant molecular properties, the theory leads to exact analytical expressions for the associated energy landscape contributions. Specific results are identified for liquids and solutions, both for isotropic media and also where partial alignment arises due to a static electric field. The paper concludes with a pragmatic analysis of the scope for achieving enantiomer separation by such methods.
New Journal of Physics 10/2014; 16(10):103021. DOI:10.1088/1367-2630/16/10/103021 · 3.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Optical forces take on a specific form when involving chiral light fields
interacting with chiral objects. We show that optical chirality density and
flow can have mechanical effects through reactive and dissipative components of
chiral forces exerted on chiral dipoles. Remarkably, these force components are
directly related to standard observables: optical rotation and circular
dichroism, respectively. As a consequence, resulting forces and torques are
dependent on the enantiomeric form of the chiral dipole. This leads to
promising strategies for the mechanical separation of chiral objects using
chiral light forces.
New Journal of Physics 06/2013; 15(12). DOI:10.1088/1367-2630/15/12/123037 · 3.56 Impact Factor
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