Propagation and nanofocusing of infrared surface plasmons on tapered transmission lines: Influence of the substrate

P. SARRIUGARTE; M. SCHNELL; P. ALONSO; L. ARZUBIAGA; F. GOLMAR; F. CASANOVA; L. HUESO; R. HILLENBRAND

OPTICS COMMUNICATIONS

ELSEVIER SCIENCE BV

Año: 2012 vol. 285 p. 3378 - 3382

0030-4018

We study the propagation of mid-infrared surface plasmons on non-tapered and tapered two-wire transmis- sion lines on Si and CaF2 substrates, the two materials representing substrates with large and small refractive index, respectively. A comparative numerical study predicts a larger effective wavelength and an increased propagation length (i.e. weaker damping) for the CaF2 substrate. By near-␣eld microscopy we image the near-␣eld distribution along the transmission lines and experimentally verify surface plasmon propagation. Amplitude- and phase-resolved near-␣eld images of a non-tapered transmission line on CaF2 reveal a stand- ing wave pattern caused by back-re␣ection of the surface plasmons at the open-ended transmission line. Cal- culated and experimental near-␣eld images of tapered transmission lines on Si and CaF2 demonstrate that for both substrates the mid-IR surface plasmons are compressed when propagating along the taper. Importantly, the nanofocus at the taper apex yields a stronger local ␣eld enhancement for the low-refractive index sub- strate CaF2. We assign the more ef␣cient nanofocusing on CaF2 to the weaker damping of the surface plasmons.