Infrared correlation nanoscopy with unprecedented spectral coverage

We demonstrate scattering near-field microsocpy (s-SNOM) imaging and spectroscopy based on a fully integrated and automated commercial OPO laser source covering the spectral range from 1.5−18.2μm (ca. 7100 – 540 cm^-1) with narrow linewidth <4 cm^-1 in the entire tuning range. To illustrate the capabilities of this light source, a thin hBN flake on a SiO substrate is imaged by s-SNOM with interferometric detection to obtain nanoscale resolved amplitude and phase images of the propagating surface polariton mode. Near-field imaging at 810 cm-1 reveals the characteristic fringe pattern at the edge of the flake, stemming from propagating Surface Phonon-Polaritons (SPhP) in the lower Reststrahlenband, launched by the AFM probing tip. Accessing the lower Reststrahlenband in hBN for nanoscale resolved far-IR s-SNOM imaging and spectroscopy is unique and unprecedented, and is a first demonstration of the capabilities of the presented light source for s-SNOM experiments to study polaritons in different material systems and devices.

Further, the methodology was successfully applied to selectively map the nanoscale spatial distribution of PVAC in a PS polymer matrix based on a 603cm^-1 absorption line, allowing to study characteristic material chemical heterogeneity and interfaces in the far-IR spectral range. It is worth mentioning that sweeping the laser frequency also allows to measure spectroscopic signatures of materials and other nanostructures with unprecedented spectral coverage, enabling studies of fundamental molecular resonances and quantum states in the long wavelength IR spectral range, which until now was not possible.