Switchable Lensed Linear Micro Axicon in Plasmonic Structures for All Optical Light Processing

We propose a simple route to build up Fresnel bi-mirror microaxicons based on the localized absorption of an astigmatic pump laser beam by a plasmonic metal foil. The corresponding local overheating produces two main effects: the thermal swelling of the glass and the local increase of its refractive index. As a result, a two-mirrors axicon-like element coupled to a gradient index microlens is produced. We modelled the optothermal formation of this microelement by Finite Element Modeling and tested the predicted temperature raise by Scanning Thermal Microscopy (SThM). We calculated and measured the far field distribution of an impinging probe beam that passes through the lensed microaxicon, finding in both cases a noticeable change in the far field pattern when the pumping light is ON evidencing at the same time the good modulation capability of this opto-thermal element. A similar, even if smaller, effect was also found when the probe light impinges from the air side, where no gradient index region is present. This configuration allowed us to measure and confirm the theoretically expected angular deflection and, indirectly, the high value of the vertical thermomechanical deformation induced by the tight astigmatic focusing of the pump beam.