title: Magnitude and phase-resolved infrared vibrational nanospectroscopy with a swept quantum cascade laser.
abstract: We demonstrate a method of rapidly acquiring background-free infrared near-field spectra by combining magnitude and phase resolved scattering-type scanning near-field optical microscopy (s-SNOM) with a wavelength-swept quantum cascade laser (QCL). Background-free measurement of both near-field magnitude and phase allows for direct comparison with far-field absorption spectra, making the technique particularly useful for rapid and straightforward nanoscale material identification. Our experimental setup is based on the commonly used pseudo-heterodyne detection scheme, which we modify by operating the interferometer in the white light position; we show this adjustment to be critical for measurement repeatability. As a proof-of-principle experiment we measure the near-field spectrum between 1690 and 1750 cm(-1) of a PMMA disc with a spectral resolution of 1.5 cm(-1). We finish by chemically identifying two fibers on a sample surface by gathering their spectra between 1570 and 1750 cm(-1), each with a measurement time of less than 2.5 minutes. Our method offers the possibility of performing both nanoscale-resolved point spectroscopy and monochromatic imaging with a single laser that is capable of wavelength-sweeping.
authors.forename: Edward
authors.initials: E
authors.lastname: Yoxall
authors.forename: Martin
authors.initials: M
authors.lastname: Schnell
authors.forename: Stefan
authors.initials: S
authors.lastname: Mastel
authors.forename: Rainer
authors.initials: R
authors.lastname: Hillenbrand