Superfocusing of electric or magnetic fields using conical metal tips: effect of mode symmetry on the plasmon excitation method

Abstract

We compare single- and double-sided excitation methods of adiabatic surface plasmon polariton (SPP) wave superfocusing for scattering-type metallic near-field scanning optical microscopy (s-NSOM). Using the results of full 3D finite difference time domain analyses, the differences in field enhancement factors are explained and reveal the mode selectivity of a conical NSOM tip for adiabatic SPP superfocusing. Exploiting the mode-symmetric nature of the tip further, we also show that it is possible to selectively confine either the electric or magnetic field at the NSOM tip apex, by simply adjusting the relative phase between the SPP waves in the double-sided excitation approach.

Plasmonic nanofocusing – grey holes for light

“…This bandwidth may in principle be further enhanced by using more sophisticated grating designs such as fan gratings [111]. The coupling efficiency of the grating can be tailored by using non-rectangular slit shapes or chirped gratings [81] or by adapting the wavefront of the laser radiation that is incident on the grating [85].…”

“…In principle, polarization control may be achieved by attaching nanoresonators with defined geometry or by using phase-locked pairs of excitation pulses, as illustrated, e.g. in [81]. Alternatively, conventional, dielectric scanning probe tapers may of course be used to generate optical near fields with arbitrary polarization state.…”

“…When coupling ultrafast laser pulses to propagating SPPs at the shaft of those tapers, nanometre-localized light spots with pulse durations as short as 10 fs are created at the taper apex with high efficiency [42,76]. Consequently, the optical properties of such antennas have been studied in considerable detail, both experimentally and theoretically, during the past several years [42,63,64,70,[75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90][91].…”

Novel SNOM Probes Based on Nanofocusing in Asymmetric Structures

“…Several theoretical studies have shown such a phenomenon [32][33][34][35]. The TM SPP mode can be excited using an axially symmetric grating under radially polarized excitation [36,37] or double-sided E-symmetric excitation [38] or asymmetric excitation via grating coupling on just one side of the tip [39].…”

“…Tip on aperture probes and probes based on a monopole antenna can also be included in this category [53,54]. An asymmetric illumination scheme can be used as an alternative to a structural asymmetry [55]: surface plasmons are excited only on one side of an apertureless probe via an opening close to the probe base in the offset aperture probe [56], while asymmetric single-sided SPP excitation results in field localization in an axisymmetric apertureless probe [38].…”