Magnitude and phase-resolved infrared vibrational nanospectroscopy with a swept quantum cascade laser

E. Yoxall, M. Schnell, S. Mastel, and R. Hillenbrand

Optics Express Vol. 23, Issue 10, pp. 13358-13369 (2015)
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