Dissipation Effect on Optical Force and Torque near Interfaces

Abstract

The Fresnel-Snell law, which is one of the fundamental laws in optics and gives insights on the behaviour of light at interfaces, is violated if there exists dissipation in the transmitting media. In order to overcome this problem, we extend the angle of refraction from a real number to a complex number. We use this complex-angle approach to analyse the behaviour of light at interfaces between lossy media and lossless media. We reveal that dissipation makes the wavenumber of the light exceed the maximum allowed at lossless interfaces. This is surprising because, in general, dielectric loss only change the intensity profiles of the light, so this excess wavenumber cannot be produced in the bulk even if there exists dielectric loss. Additionally, anomalous circular polarisation emerges with dissipation. The direction of the anomalous circular polarisation is transverse, whereas without dissipation the direction of circular polarisation has to be longitudinal. We also discuss how the excess wavenumber can increase optical force and how the anomalous circular polarisation can generate optical transverse torque. This novel state of light produced by dissipation will pave the way for a new generation of optical trapping and manipulation.