Partially-metal--coated Tips for Near-field Nanospectroscopy.*

Here, we test the possibility of a generic design scheme for wavelength-selective tip enhancement via finite-element numerical modeling. We employ a Si-based tip with various gold coating lengths on the top, yielding a tunable near-field field strength at the tip apex. Calculations show a wavelength-dependent enhancement factor of the metal-coated tip due to the geometrical antenna resonances, which can be precisely tuned throughout a broad spectral range from visible to terahertz frequencies by adjusting the length of the metal coating. By changing the coating pattern into a chiral helical structure on an achiral tip, we also demonstrate the importance of coating-length effect in designing high-performance enantiomeric near-field scanning. Our methods and findings offer interesting perspectives for developing near-field optical probes, pushing the detection and resolution limits of tip-enhanced near-field detections and nanospectroscopies.