Abstract
Orbital angular momentum carrying light beams are usedfor optical trapping and manipulation. This emerging trend provides new challenges involving device miniaturization for improved performance and enhanced functionality at the microscale. Here we discus a new fabrication method based on combining the additive 3D structuring capability laser photopolymerization and the substractive sub-wavelength resolution patterning of focused ion beam lithography to produce micro-optical elements capable of compound functionality. As a case in point of this approach binary spiral zone pattern based high numerical aperture micro-lenses capable of generating topological charge carrying tightly focused vortex beams in a single wavefront transformation step are presented. The devices were modelled using finite-difference time-domain simulations, and the theoretical predictions were verified by optically characterizing the propagation properties of light transmitted through the fabricated structures. The resulting devices had focal lengths close to the predicted values of f = 18 µm and f = 13 µm as well as topological charge l dependent vortex focal spot sizes of ~ 1:3 µm and ~ 2:0 µm for l = 1 and l = 2 respectively.
Highlights
Orbital angular momentum (OAM) of an optical vortex presents a new dimension for controlling light-matter interaction, which confers substantial benefits to a multitude of application areas
Recent efforts to achieve optical trapping and manipulation at the microscale resulted in forgoing these bulky setups in favor of miniaturized structures, such as spiral zone plates (SZP) of both diffractive [8] and plasmonic [9] varieties, topological nanoslits [10], photopolymerized structures [11] as well as metasurface-based devices [12]
Fabrication of the 3D SZP structures involved the photopolymerization of the disk or cone shaped pedestal, followed by coating it with an optically opaque layer of Au, into which the spiral zone patterns were subsequently milled by means of ion beam lithography
Summary
Orbital angular momentum (OAM) of an optical vortex presents a new dimension for controlling light-matter interaction, which confers substantial benefits to a multitude of application areas. Optical tweezers/spanners that use vortices to trap particles can do so with higher axial trapping efficiency compared to a Gaussian beam of equivalent intensity, which is of special importance in biological applications where reduction in the laser power used diminishes the risk of sample damage [1] They provide additional functionality by enabling orbital rotation on top of typical spinning motion as well as stable trapping of absorbing particles [2]. Optical vortex phase masking is a technique of significant interest in astronomy, where it can be used to distinguish faint objects close to bright coherent sources for producing coronagraphs and searching of exoplanets [3, 4] Optical communications is another field where the theoretically unlimited range of topological charge (where indicates the winding number of helical wavefront) in OAM carrying beams shows great promise [5]. We outline the steps involved in creating such structures, starting by detailing preliminary numerical investigation by means of finite-difference timedomain (FDTD) simulations, describing of the fabrication procedure, as well as discussing the optical characterization results
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