Adiabatic and nonadiabatic nanofocusing of plasmons by tapered gap plasmon waveguides

Adiabatic and nonadiabatic nanofocusing of plasmons in tapered gap plasmon waveguides is analyzed using the finite-difference time-domain algorithm. Optimal adaptors between two different subwavelength waveguides and conditions for maximal local field enhancement are determined, investigated, and explained on the basis of dissipative and reflective losses in the taper. Nanofocusing of plasmons into a gap of ∼ 1 nm width with more than 20 times increase in the plasmon energy density is demonstrated in a silver-vacuum taper of ∼ 1 μ m long. Comparison with the approximate theory based on the geometrical optics approximation is conducted.