Pseudoheterodyne detection for background-free near-field spectroscopy

The authors present a detection technique for scattering-type near-field optical microscopy capable of background interference elimination in the entire near-UV to far-IR spectral range. It simultaneously measures near-field optical signal amplitude and phase by interferometric detection of scattered light utilizing a phase-modulated reference wave. They compare its background suppression efficiency to other known methods and experimentally show that it provides a reliable near-field optical material contrast even in the case where both noninterferometric and homodyne interferometric detection methods fail.