Speaker
Description
Scattering-type scanning near-field optical microscopy (s-SNOM) allows optical imaging and spectroscopy far below the diffraction limit. Standard s-SNOM detects light scatter¬ed from the apex of a metallic tip in a tapping-mode atomic-force microscope (AFM), where background-free detection of the near-field signal is achieved by high-harmonic demodulation and interferometry. The spatial resolution given by the field localization is typically limited to 10-20 nm, which is determined by the geometry and stability of the SNOM tip. Recently, it has been demonstrated that optical fields can be confined to the atomic scale inside plasmonic pico-cavities. At low temperatures, stable atomistic protrusions at the apex of plasmonic tips exist leading to a strongly localized enhance¬ment. To disentangle this atomistically localized light from spatially more extended nano- and mesoscale scattering signals, Ångstrom-scale tapping amplitudes and strong plasmonic enhancements are required.
Here we report on the implementation of plasmonic ‘pico-SNOM‘ using noncontact-mode AFM (ncAFM) and z-modulated scanning tunneling microscopy (STM) at low temperatures. For ncAFM-based pico-SNOM, a quartz tuning fork sensor enables the tip oscillation at low temperatures with small (50 pm) amplitudes. We demonstrate precise detection of the scattered light from a sub-nanoscale gap between a Ag tip and Ag(111) surface. As a complementary approach, we demonstrate STM-based pico-SNOM by Å-scale modulation of the STM tip-sample gap. Despite the slow tapping frequencies below 1 kHz, efficient high-harmonic detection in the scattering signal is possible due to the strong plasmon enhancement and precise focusing and collection of the laser light in our STM. First results of STM-based pico-SNOM on ultrathin ZnO films yield a spatial resolution of 1 nm, provided that the excitation wavelength spectrally overlaps with the broadband plasmonic tip response, which can be characterized in-situ by STM-luminescence. While the electronic transition from the Ag(111)/ZnO interface state to the ZnO conduction band resonantly enhances the TERS intensity [1], we do not find significant resonant enhancement in the s-SNOM contrast.
With further increased the stability and implementation of interferometric detection methods, we anticipate that this approach will provide a new spectroscopic access to atomic-scale chemical structures and dynamics to characterize physical properties and functions of material surfaces.
References
[1] S. Liu, M. Müller, Y. Sun, I. Hamada, A. Hammud, M. Wolf, T. Kumaga,
Nano Lett. 19, 8, 5725–5731 (2019).
| Abstract Number (department-wise) | PC 15 |
|---|---|
| Department | PC (Wolf) |
