Abstract
The metasurface spiral focusing (MSF) generator has gained attention in high-speed optical communications due to its spatial orthogonality. However, previous MSF generators only can generate a single orbital angular momentum (OAM) mode for one polarized light. Here, a MSF generator with tunable OAM is proposed and it has the ability to transform linearly polarized light (LPL), circularly polarized light or Gaussian beams into vortex beams which can carry tunable OAM at near-infrared wavelength by controlling the phase transition of vanadium dioxide (VO2). Utilizing this MSF generator, the beams can be focused on several wavelength-sized rings with efficiency as high as 76%, 32% when VO2 are in the insulating phase and in the metallic phase, respectively. Moreover, we reveal the relationship between the reflective focal length and transmissive focal length, and the latter is 2.3 times of the former. We further demonstrate the impact of Gaussian beams with different waist sizes on MSF generators: the increase in waist size produces the enhancement in spiral focusing efficiency and the decrease in size of focal ring. The MSF generator we proposed will be applicable to a variety of integrated compact optical systems, such as optical communication systems and optical trapping systems.
Highlights
It is well known that electromagnetic waves can carry angular momentum
Beth [1] initially observed that circularly polarized light has spin angular momentum (SAM) of ±h per photon (h is reduced Planck’s constant) and SAM is associated with the polarization of electromagnetic waves
Dissimilar to the SAM, Allen [2] recognized that orbital angular momentum (OAM) is related to the spatial phase distribution of electromagnetic waves, and vortex beams carrying OAM are characterized by a rotating phase factor of exp(ilθ), where θ refers to the azimuthal angle and l is topological charge
Summary
It is well known that electromagnetic waves can carry angular momentum. Angular momentum comprises spin angular momentum (SAM) and orbital angular momentum (OAM) [1,2]. The devices for generating vortex beams have been extensively investigated, including spiral phase plates [13,14,15], antenna arrays [16,17,18], spatial light modulators [19,20], geometric mode converters [21], etc. These works usually fail to achieve adjustable topological charge, which means that, once it is manufactured, its topological charge is fixed Their effective operation range is mainly limited to the visible light or mid-infrared, and their light sources rely deeply on circularly polarized light or linearly polarized light (LPL). The MSF generator with tunable OAM has potential application in optical manipulation at nanoscale [45] and optical fiber communication of optical metasurfaces [46]
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