Adiabatic Tip-Plasmon Focusing for Nano-Raman Spectroscopy

Abstract

True nanoscale optical spectroscopy requires the efficient delivery of light for a spatially nanoconfined excitation. We utilize adiabatic plasmon focusing to concentrate an optical field into the apex of a scanning probe tip of ∼10 nm in radius. The conical tips with the ability for two-stage optical mode matching of the surface plasmon polariton (SPP) grating-coupling and the adiabatic propagating SPP conversion into a localized SPP at the tip apex represent a special optical antenna concept for far-field transduction into nanoscale excitation. The resulting high nanofocusing efficiency and the spatial separation of the plasmonic grating-coupling element on the tip shaft from the near-field apex probe region allows for true background-free nanospectroscopy. As an application, we demonstrate tip-enhanced Raman spectroscopy (TERS) of surface molecules with enhanced contrast and its extension into the near-IR with 800 nm excitation.

Femtosecond Nanofocusing with Full Optical Waveform Control

“…By relying on the diverging index of refraction experienced by propagating SPP's with decreasing cone radius, the mode transformation into a nanoscale excitation at the tip apex remains continuous and impedance matched [7], while 6 the uniform taper surface prevents the otherwise typical scattering and reflection losses at structural discontinuities. Due to the radial symmetry and decreasing SPP group velocity, full nanoconfinement in all spatial dimensions is achieved [21,22,26].In addition, the tips allow for the desired broadband excitation necessary for true femtosecond optical control. As predicted theoretically, the nanofocusing efficiency is maximal in the 800 nm range for a tip cone angle of ∼14 • [23] as used in our experiment with previously demonstrated monochromatic nanofocusing efficiencies of up to 9% [26].…”

“…Due to the radial symmetry and decreasing SPP group velocity, full nanoconfinement in all spatial dimensions is achieved [21,22,26].…”

“…This light source with arbitrary waveform control at the nanoscale is of a fundamentally new quality compared to both conventional far-and near-field sources. It allows for the systematic extension of near background-free scanning probe microscopy [22,26,30] to the nanoscale implementation of many forms of nonlinear and ultrafast spectroscopies for spatio-temporal imaging [31].…”

Femtosecond optical control on the nanoscale

“…As previously shown, SPP nanofocusing can generate a nanoconfined excitation with 10's of nm spatial confinement where nanofocusing efficiencies as high as 9% enable efficient background-free imaging and spectroscopy [8,9]. Under sufficiently high illumination conditions SHG is generated locally at the tip apex that is used through a MIIPS algorithm to produce transform-limited pulses.…”

Remote excitation-tip-enhanced Raman scattering microscopy using silver nanowire

“…Indeed, lower background intensities have been observed with remote excitation relative to direct excitation, demonstrated by both our group and other researchers. [19][20][21][22][23] A notable example of this is given by Berweger et al, who created a grating structure as a light coupling point on a chemically etched gold tip. 22,23) As here, this resulted in a vertical separation of the excitation and detection spots, and led to a lower background intensity than when the laser was focused to the tip apex.…”