title: Near-field localization in plasmonic superfocusing: a nanoemitter on a tip.
abstract: Focusing light to subwavelength dimensions has been a long-standing desire in optics but has remained challenging, even with new strategies based on near-field effects, polaritons, and metamaterials. The adiabatic propagation of surface plasmon polaritons (SPP) on a conical taper as proposed theoretically has recently emerged as particularly promising to obtain a nanoconfined light source at the tip. Employing grating-coupling of SPPs onto gold tips, we demonstrate plasmonic nanofocusing into a localized excitation of approximately 20 nm in size and investigate its near- and far-field behavior. For cone angles of approximately 10-20 degrees , the breakdown of the adiabatic propagation conditions is found to be localized at or near the apex region with approximately 10 nm radius. Despite an asymmetric side-on SPP excitation, the apex far-field emission with axial polarization characteristics representing a radially symmetric SPP mode in the nanofocus confirms that the conical tip acts as an effective mode filter with only the fundamental radially symmetric TM mode (m = 0) propagating to the apex. We demonstrate the use of these tips as a source for nearly background-free scattering-type scanning near-field optical microscopy (s-SNOM).
authors.affiliation: Department of Chemistry, University ofWashington, Seattle, Washington 98195, USA.
authors.forename: Catalin C
authors.initials: CC
authors.lastname: Neacsu
authors.forename: Samuel
authors.initials: S
authors.lastname: Berweger
authors.forename: Robert L
authors.initials: RL
authors.lastname: Olmon
authors.forename: Laxmikant V
authors.initials: LV
authors.lastname: Saraf
authors.forename: Claus
authors.initials: C
authors.lastname: Ropers
authors.forename: Markus B
authors.initials: MB
authors.lastname: Raschke
mesh_terms: D004867:Equipment Design; D007834:Lasers; D008027:Light; D008422:Materials Testing; D053758:Nanoparticles / Q000737:chemistry; D049329:Nanostructures / Q000737:chemistry*; D036103:Nanotechnology / Q000379:methods*; D055095:Optics and Photonics; D010316:Particle Size; D045663:Quantum Dots; D012666:Semiconductors; D020349:Surface Plasmon Resonance / Q000295:instrumentation*