  Metasurfaces have been extensively engineered to produce a wide range of
optical phenomena, allowing unprecedented control over the propagation of
light. However, they are generally designed as single-purpose devices without a
modifiable post-fabrication optical response, which can be a limitation to
real-world applications. In this work, we report a nanostructured planar fused
silica metalens permeated with a nematic liquid crystal (NLC) and gold
nanoparticle solution. The physical properties of embedded NLCs can be
manipulated with the application of external stimuli, enabling reconfigurable
optical metasurfaces. We report all-optical, dynamic control of the metalens
optical response resulting from thermo-plasmonic induced changes of the NLC
solution associated with the nematic-isotropic phase transition. A continuous
and reversible tuning of the metalens focal length is experimentally
demonstrated, with a variation of 80 um (0.16% of the 5 cm nominal focal
length) along the optical axis. This is achieved without direct mechanical or
electrical manipulation of the device. The reconfigurable properties are
compared with corroborating numerical simulations of the focal length shift and
exhibit close correspondence.
