Light on the Tip of a Needle: Plasmonic Nanofocusing for Spectroscopy on the Nanoscale

Abstract

The efficiency of plasmonic nanostructures as optical antennas to concentrate optical fields to the nanoscale has been limited by intrinsically short dephasing times and small absorption cross sections. We discuss a new optical antenna concept based on surface plasmon polariton (SPP) nanofocusing on conical noble metal tips to achieve efficient far- to near-field transformation of light from the micro- to the nanoscale. The spatial separation of the launching of propagating SPPs from their subsequent apex confinement with high energy concentration enables background-free near-field imaging, tip-enhanced Raman scattering, and nonlinear nanospectroscopy. The broad bandwidth and spectral tunability of the nanofocusing mechanism in combination with frequency domain pulse shaping uniquely allow for the spatial confinement of ultrashort laser pulses and few-femtosecond spatiotemporal optical control on the nanoscale. This technique not only extends powerful nonlinear and ultrafast spectroscopies to the nanoscale but can also generate fields of sufficient intensity for electron emission and higher harmonic generation.

Plasmonic mode converter for controlling optical impedance and nanoscale light-matter interaction

“…For guided TE mode, the opposite charges on the two wires result in a highly confined and enhanced field in the dielectric nanogap. The extremely concentrated field in the gap exhibits a well-defined polarization transverse to the propagation direction, which is of great potential for the enhancement of light-matter interaction at the nanoscale [6,14,42]. The guided TM mode, on the other hand, shows very low field intensity in the gap and, consequently, does not interact significantly with nanomatter in the gap.…”

“…Recently, prototype nanocircuits using gap nanoantennas [5,6] in connection with plasmonic two-wire transmission lines (TWTLs) have been theoretically proposed [7][8][9] and experimentally studied [10][11][12], showing a realizable approach to the manipulation of electromagnetic field for enhanced spectroscopy [13,14].…”

Plasmonic nano-antenna a-Si:H solar cell

“…Although no significant effect is observed here, energy transport via SPPs to the bottom of the solar cell is in principle possible [39,45]. How far a plasmon is able to travel is determined by the sharpness of the nanoneedle [46].…”

Femtosecond optical control on the nanoscale

“…This approach utilizes surface plasmon polarition (SPP) nanofocusing into the apex of 3D noble metal tips [2,3], which we have previously demonstrated to allow for nanometer spatial confinement of optical fields at the apex [4]. As shown in Figure 1a), this effect arises from the increase in index of refraction with decreasing tip radius experienced by propagating SPP's, which in combination with the associated decreasing group velocity and improved mode confinement leads to the continuous transformation of quasi-planar modes into a nanoconfined excitation at the tip apex [5,6].…”