  We describe and demonstrate the use of an adaptive wave front optimization
scheme for enhancing the efficiency of adiabatic nanofocusing of surface
plasmon polariton (SPP) waves along an ultrasharp conical gold taper. Adiabatic
nanofocusing is an emerging and promising scheme for controlled focusing of far
field light into nanometric volumes. It comprises three essential steps: SPP
excitation by coupling far field light to an SPP waveguide, SPP propagation
along the waveguide and adiabatic SPP nanofocusing towards a geometric
singularity. For commonly used complex waveguide geometries, such as, e.g.,
conical metal tapers, a realistic modeling and efficiency optimization is
challenging. Here, we use a deformable mirror to adaptively control the wave
front of the incident far field light. We demonstrate an eight-fold enhancement
in nanofocusing efficiency and analyze the shape of the resulting optimized
wave front. The introduced wave front optimization scheme is of general
interest for guiding and controlling light on the nanoscale.
