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Indirect absorption spectroscopy using quantum cascade lasers: Mid-infrared refractometry and photothermal spectroscopy

: Pfeifer, Marcel; Ruf, Alexander; Fischer, Peer

Postprint (PDF; )

Optics Express 21 (2013), No.22, pp.25643-25654
ISSN: 1094-4087
Journal Article, Electronic Publication
Fraunhofer IPM ()
spectroscopy; infrared; total internal reflection; absorption; photothermal effect

We record vibrational spectra with two indirect schemes that depend on the real part of the index of refraction: mid-infrared refractometry and photothermal spectroscopy. In the former, a quantum cascade laser (QCL) spot is imaged to determine the angles of total internal reflection, which yields the absorption line via a beam profile analysis. In the photothermal measurements, a tunable QCL excites vibrational resonances of a molecular monolayer, which heats the surrounding medium and changes its refractive index. This is observed with a probe laser in the visible. Sub-monolayer sensitivities are demonstrated.