Comparative Investigation of Plasmonic Properties between Tunable Nanoobjects and Metallized Nanoprobes for Optical Spectroscopy

In order to evaluate the optical efficiency of tip-based probes for future tip-enhanced optical spectroscopy applications, we developed an experimental setup based on the coupling of an achromatic inverted microscope equipped with a total internal reflection objective and an atomic force microscopy (AFM) head. This spectroscopic tool has been validated using individual nanofabricated antennas (gold nanodisks/nanocones) on a glass substrate which act as nanoresonators based on localized surface plasmons. Spectrally tunable transverse electric and magnetic plasmonic resonances are identified and are in excellent agreement with numerical calculations performed as a function of the nanoantenna geometry and size. We investigated a series of state-of-the-art gold-coated AFM probes, which are commonly used for tip-enhanced (Raman spectroscopy) optical experiments. Their scattering spectrum consists of resonances depending on the tip sharpness or granularity superimposed on a broad emission spectrum due to a semi-infinite metal layer acting as a nonresonant antenna. From the comparison between the plasmonic response of both types of optical antennas, a new generation of probes for tip-enhanced optical spectroscopy is proposed in which single plasmonic nanoantennas are engineered at the apex of a nonmetallic AFM tip. As from numerical simulation results, such tips would ensure a spectral tunability as a function of the material, size, and geometry, together with expected high enhancement factors. Such features would allow the design of spectrally tunable surface-enhanced Raman spectroscopy substrates and should be a reliable and efficient alternative to tips commonly used in tip-enhanced optical spectroscopy experiments such as tip-enhanced Raman spectroscopy.