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 similar to 1 nm width with more than 20 times increase in the plasmon energy density is demonstrated in a silver-vacuum taper of similar to 1 mu m long. Comparison with the approximate theory based on the geometrical optics approximation is conducted.
This paper presents the results of the numerical finite-difference time-domain analysis of a strongly localized antisymmetric plasmon, coupled across a nanogap between two identical metal wedges. Dispersion, dissipation, field structure, and existence conditions of such coupled wedge plasmons are determined and investigated on an example of the fundamental coupled mode. It is shown that in the general case there exist three critical wedge angles and a critical gap width (separation between the wedge tips). If the gap width is larger than the critical separation, then the antisymmetric wedge plasmons can exist only in the ranges between the first and the second critical angles, and between the third critical angle and 180 degrees. If the gap width is smaller or equal to the critical separation, then the third and the second critical angles merge, leaving only one interval of wedge angles within which the antisymmetric coupled wedge plasmons can exist. The effect of rounded edge tips is also investigated and is shown to be similar to that of different wedge angles. Feasibility of using these plasmons for the design of efficient subwavelength waveguides is discussed.
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