Feasibility Study for Near-Field Optical Sum-Frequency Generation Spectroscopy Using a Metal-Coated Fiber

Abstract

A near-field optical spectroscopy technique has been developed that collects the nonlinear sum-frequency signal from the surface adsorbed monolayer sample. The sum-frequency signal generated at the surface was collected with a silver-coated multi-mode fiber probe. The metal coating of the fiber effectively eliminates the far-field stray light that enters the core fiber from the outer edge, passing through the clad region. As the tip was pulled away from the sample surface, the sum-frequency signal from the GaAs crystal sample decreased rapidly with increasing tip-sample distance, indicating that the signal is essentially due to the near-field generated by the sum-frequency process. By using the metal-coated multi-mode fiber probe, information about the vibrational structure of the adsorbed molecules at the surfaces is studied.