Optimization of plasmon nano-focusing in tapered metal rods

Analysis of adiabatic and non-adiabatic nano-focusing in tapered metal nano- rods leads to the determination of optimal taper angles and rod lengths as functions of material parameters (for gold, silver, and aluminum) at frequencies from the optical and near infra-red ranges. The considered nano-focusing structures appear to be highly toler- ant to such structural and fabrication imperfections as variations of length of the rod and taper angle around their optimal values. However, the major parameter that tends to sig- nificantly affect the nano-focusing capabilities of the rods is the radius of the tip, and this is the parameter that should be carefully reproduced in the experiments. Comparison of the numerical results with the adiabatic theory of nano-focusing for different metals and different wavelengths demonstrates the validity of the adiabatic theory in a much wider range of taper angles (up to tens of degrees) than it was previously expected. Major pre- dicted local field enhancements of up to ~ 2,500 times in the considered structures within nano-scale regions as small as a few nanometers will make tapered metal rods highly promising for single molecule detection and development of a new generation of sensors, measurement and nano-manipulation techniques.