Interparticle Interactions: Energy Potentials, Energy Transfer, and Nanoscale Mechanical Motion in Response to Optical Radiation

2015

Opt. Lett.

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The optical trapping of molecules with an off-resonant laser beam involves a forward-Rayleigh scattering mechanism. It is shown that discriminatory effects arise on irradiating chiral molecules with circularly polarized light; the complete representation requires ensemble-weighted averaging to account for the influence of the trapping beam on the distribution of molecular orientations. Results of general application enable comparisons to be drawn between the results for two limits of the input laser intensity. It emerges that, in a racemic mixture, there is a differential driving force whose effect, at high laser intensities, is to produce differing local concentrations of the two enantiomers. Trapping of this type usually relates to the interaction of the laser field with a transition electric dipole, as shown by Fig. 1(a). Interactions between the irradiating beam and a transition magnetic dipole are also possible, but the associated coupling strength is usually much smaller in magnitude, and the effects are generally ignored. However, when considering the possibility of chiral discrimination (in which input light of left-handed circular polarization offers different observables compared to right-handed polarization), the conventional trapping mechanism involving only electric dipole transition moments has to be extended to accommodate transition magnetic dipoles [7]. In fact, the forward-Rayleigh scattering events most relevant in chiral discrimination involve a mixture of magnetic and electric dipole interactions, as illustrated by Fig. 1(b), with one kind of coupling involved in the input photon annihilation and the other in the output photon release.To understand the energetics in greater detail, we note that the underlying mechanism for the optical trapping of molecules is based on the variation of intensity within the optical beam, which produces a position-dependent lowering of energy for the molecules it encounters: the alternating electric field of the radiation may be interpreted as producing a dynamic Stark shift to the molecular ground state energy. In terms of quantum electrodynamics, the energy shift (which is quadratically dependent on the electric field of the light) has to originate in forward-Rayleigh scattering, i.e., the concerted annihilation and creation of photons with identical energy and wave-vector. In such a case, the optical wavelengths will lie in a region of transparency for the irradiated molecule-the throughput laser beam therefore emerges unchanged. The primary physical determinant of optical trapping is the potential energy U, whose evaluation from quantum theory requires that the initial and final states (here denoted by I) are identical [8]. An expression for U per molecule is determined from second-order time-dependent perturbation theory, i.e.,where S is an intermediate state of the system comprising both molecule and radiation, E is the energy of the state denoted by its subscript, and H int is the dipolar interaction Hamiltonian given byHere, μ and m are the electric and...

mentioning

confidence: 99%

Laser optical separation of chiral molecules

Bradshaw

2015

Opt. Lett.

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“…However, from a fundamental photonic perspective, such a system cannot relate to a chiral force (or a discriminatory energy) since the initial and final states are not identical. 26 This is readily demonstrated: since two beams are required for the proposed mechanism, and the process is elastic (no overall excitation or relaxation of molecular electronic state occurs) then any contributory mechanism has to involve a scattering event in which a photon is annihilated from one beam and a photon created into the other Since the initial and final radiation states of such a process are not identical, energy shifts cannot arise, and it becomes evident that there are no grounds for discriminatory forces to arise. Related chiral discriminatory systems have been proposed in the literature, although they lack the advantages of our optical trapping model.…”

Section: Discussionmentioning

confidence: 99%

On the viability of achieving chiral separation through the optical manipulation of molecules

2015

SPIE Proceedings

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Several different optical methods have recently been proposed for the potential separation of chiral molecules according to their intrinsic handedness. Applying fundamental symmetry and electrodynamical principles provides a perspective that casts doubt over the viability of some of the more extravagant claims. However there is a genuine basis for achieving chiral separation by using circularly polarized light to deliver chirally sensitive optical forces. The mechanism comes into play when molecules (or nanoscale particles) are optically trapped in a laser beam by forward Rayleigh scattering, as a result of trapping forces that depend on positioning within the beam profile. In such a setup, chiral molecules experience subtle additional forces associated with a combination of electric and magnetic transition dipoles; when circularly polarized light is used for the trapping, a discriminatory response can be identified that has the capacity to separate left-and righthanded molecular isomers. Here, clear differences can be observed between the behavior of isotropic liquids and poled solutions or liquid crystals. Detailed analysis provides an objective basis to assess new prospects for the recognition and differentiation of molecules with opposite chiral form, identifying and paving the way for future commercial applications.

“…Let us suppose that the dispersion interaction is negligible compared to the optical binding; although it is interesting that, in certain circumstances, the denominator of equation (7) may be negative if either nanoparticle is excited -denoting that the dispersion force is not always attractive under such conditions. 22 Unlike Section 2, where optical binding operates in isolation, we now consider the phenomenon in the context of resonant absorption or resonance energy transfer. The observables for these phenomena remain independent and do not -indeed cannot -together forge a new mechanism.…”

Section: Modifying Optical Bindingmentioning

confidence: 99%

Near-field manipulation of interparticle forces through resonant absorption, optical binding, and dispersion forces

2013

SPIE Proceedings

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The relative motions of two or more neutral particles, subject to optical trapping forces within a beam, are influenced by intrinsic inter-particle forces. The fundamental character of such forces is well-known and usually derives from dispersion interactions. However, the throughput of moderately intense (off-resonant) laser light can significantly modify the form and magnitude of these intrinsic forces. This optical binding effect is distinct from the optomechanical interactions involved in optical tweezers, and corresponds to a stimulated (pairwise) forward-scattering mechanism. In recent years, attention has begun to focus on optical binding effects at sub-micron and molecular dimensions. At this nanoscale, further manipulation of the interparticle forces is conceivable on the promotion of optically bound molecules to an electronic excited state. It is determined that such excitation may influence the intrinsic dispersion interaction without continued throughput of the laser beam, i.e. independent of any optical binding. Nevertheless, the forwardscattering mechanism is also affected by the initial excitation, so that both the optical binding and dispersion forces can be manipulated on input of the electromagnetic radiation. In addition, the rate of initial excitation of either molecule (or any energy transfer between them) may be influenced by an off-resonant input beam which, thus, acts as an additional factor in the modification of the interparticle force. A possible experimental set-up is proposed to enable the measurement of such changes in the interparticle coupling.