Abstract
Abstract Light’s orbital angular momentum (OAM) with inherent mode orthogonality has been suggested as a new way to the optical encryption. However, the dependence of annular intensity profiles on the topological charge complicates nanoscale light–matter interactions and hampers the ultra-secure encryption application. In this paper, we demonstrate ultra-secure image encryption by tightly focusing perfect optical vortex (POV) beams with controllable annular intensity profiles and OAM states. A simple scheme composed of single spatial light modulator to implement Fourier transform of an ideal Bessel mode with both amplitude and phase modulations is proposed to generate radius-controllable POV in tightly focused beams. Such focused POV beams with identical intensity profiles but varied local OAM density are applied to disorder-coupled gold nanorod aggregates to selectively excite electromagnetic hot spots for encoding information through photothermal deformation. As such, ultra-secure image encryption in OAM states of POV beams in combination with different polarizations can be achieved. Our results lay the ground for diverse nanophotonic applications harnessing the OAM division of POV beams.
Highlights
Optical vortices have attracted significant research interests owing to the well-defined on-axis orbital angular momentum (OAM) they may carry [1]
We demonstrate ultra-secure image encryption by tightly focusing perfect optical vortex (POV) beams with controllable annular intensity profiles and OAM states
These results indicate that ultra-secure optical image encryption by the generated POV beams with controllable OAM states and polarization is feasible
Summary
Optical vortices have attracted significant research interests owing to the well-defined on-axis orbital angular momentum (OAM) they may carry [1]. The interdependence complicates nanoscale light–matter interactions with varied both local intensity and OAM density and becomes problematic in applications that require to couple multiple OAM beams into fixed spatial modal distributions In this regard, the concept of perfect optical vortex (POV) whose annular intensity profiles of the generated beam are immune to the variation of topological charge has been introduced [18]. The complex field of Fourier transfer of the focused POV expressed as a diffraction-limited annular ring intensity profile superposed with a vortex phase is implemented through a single SLM approach. It allows the generated POV beam with controlled annular radius and arbitrary local OAM densities in the focal plane. The FT effect of the high-NA objective of the weighted field must be taken into considerations when implementing Eq (4) under tight focusing conditions [26]:
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