Phase-resolved near-field optical microscopy approaching the shot-noise limit

Abstract

Scattering-scanning near-field optical microscopy (s-SNOM) is a powerful probe of molecular and quantum systems, using optical field enhancement at the apex of a scanning probe to provide few-nanometer resolution and sub-attomolar sensitivity. However, small sample volumes and associated small signals remain challenging. Established detection schemes rely on homodyne amplification combined with mechanical modulation of the local oscillator to remove background signals, leading to long acquisition times and increased noise. We demonstrate a detection scheme based on a balanced Mach–Zehnder interferometer that instead optically subtracts the background interference in both imaging and spectroscopy. This approach relies on subtraction of common-mode signals and also reduces optical noise in the measurement. We observe noise reduction by two orders of magnitude in the far-field, reaching shot-noise-limited detection with a low-noise light source. We demonstrate noise reduction in both near-field imaging and spectroscopy. Our new approach represents an important step toward low-noise background-free IR s-SNOM, with implications toward improved sensitivity to the weak optical response of nanoscale molecular systems.