Laser-induced forces between carbon nanotubes

Abstract

Carbon nanotubes are the focus of intense research interest because of their unique properties and applications potential. We present a study based on quantum electrodynamics concerning the optical force between a pair of nanotubes under laser irradiance. To identify separate effects associated with the pair orientation and laser beam geometry, two different systems are analyzed. For each, an analytical expression for the laser-induced optical force is determined, and the corresponding magnitude is estimated. © 2005 Optical Society of America OCIS codes: 020.5580, 140.7010, 220.4880, 260.2110, 270.5580, 290.5890. Carbon nanotubes are currently the subject of intense research because of their unique nanostructures and remarkable combination of conductive, steric, and mechanical characteristics. 1 Recent studies 2,3 have shown that nanotubes can be optically trapped and manipulated by optical tweezers, 4 exploiting the fact that the radiation pressure of inhomogeneous optical fields produces forces on neutral particles. 5 In a separate development, other work 6 has verified that significant optomechanical forces can be induced between particles through the application of an optical field, and differing methods of analysis have been utilized to derive this force with classical theory. 7,8 In this Letter we use a quantum electrodynamics (QED) approach to determine the general result for optically induced forces between molecules, representing a form of stimulated scattering that may afford a new means for optical nanomanipulation. Two single-walled carbon nanotube (SWCNT) systems are analyzed in detail, differing in the angular disposition of the nanotubes and the incoming laser light: (i) parallel nanotubes disposed at a variable angle to the electric field vector of the incident light and (ii) nanotubes with variable mutual orientation averaged with respect to the field vector.The laser-induced force F ind between chemically identical nanotubes A and B can be determined from the optically induced energy shift ⌬E ind . This energy shift has physical grounds similar to the well-known dispersion energy-an attractive interaction usually associated with the inductive coupling of fluctuating electric dipoles and that, in terms of molecular QED, involves the creation and annihilation of two virtual photons, 9 i.e., four matter-photon interactions. In contrast, the laser-induced interaction reflects a process involving the absorption of a real input photon at one component and the stimulated emission of a real photon at the other, with one virtual photon acting as a messenger between them (Fig. 1). The throughput radiation suffers no overall change in state. As with the dispersion energy, the involvement of four matter-photon interactions requires the application of fourth-order perturbation theory: ͑1͒Here all Dirac brackets relate to states of the system comprising both nanotubes and the radiation field; ͉i͘ is the unperturbed state in which both molecules are in their electronic ground state; ͉r͘, ͉s͘, and ͉t͘ are ...

Nanoparticle manipulation through inter-particle optical forces and torques

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