Optical forces in lossless arbitrary refractive index optical trapping and micromanipulation

This paper shows, using both a ray optics approach and in the framework of the generalized Lorenz–Mie theory (GLMT), what happens to the optical forces exerted on a lossless spherical particle with an arbitrary (positive or negative) relative refractive index, allowing the external medium also to be metamaterial. It is shown that the anti-parallelism between the linear momentum p of each photon and the Poynting vector S associated with the propagating wave, observed in negative refractive index media, leads to shifts in the direction of the optical force of a single ray and, consequently, to the total optical force exerted by an arbitrary-shaped laser beam. This extends the possible realizable traps and reveals how arbitrary-shaped laser beams can be used to trap particles with arbitrary refractive indices.

► We introduce the concept of optical trapping for arbitrary refractive index media. ► Both the host medium and the particle may have a negative refractive index. ► They may be exploited in the future as auxiliary entities for trapping purposes. ► Extending the range of the refractive index extends the trapping possibilities. ► Interesting force profiles arise when the refractive index becomes negative.