title: Discrete frequency infrared microspectroscopy and imaging with a tunable quantum cascade laser.
abstract: Fourier-transform infrared (FT-IR) imaging is a well-established modality but requires the acquisition of a spectrum over a large bandwidth, even in cases where only a few spectral features may be of interest. Discrete frequency infrared (DF-IR) methods are now emerging in which a small number of measurements may provide all the analytical information needed. The DF-IR approach is enabled by the development of new sources integrating frequency selection, in particular of tunable, narrow-bandwidth sources with enough power at each wavelength to successfully make absorption measurements. Here, we describe a DF-IR imaging microscope that uses an external cavity quantum cascade laser (QCL) as a source. We present two configurations, one with an uncooled bolometer as a detector and another with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector and compare their performance to a commercial FT-IR imaging instrument. We examine the consequences of the coherent properties of the beam with respect to imaging and compare these observations to simulations. Additionally, we demonstrate that the use of a tunable laser source represents a distinct advantage over broadband sources when using a small aperture (narrower than the wavelength of light) to perform high-quality point mapping. The two advances highlight the potential application areas for these emerging sources in IR microscopy and imaging.
authors.affiliation: Department of Bioengineering, University of Illinois at Urbana-Champaign, 61801, United States.
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Kole
authors.forename: Rohith K
authors.initials: RK
authors.lastname: Reddy
authors.forename: Matthew V
authors.initials: MV
authors.lastname: Schulmerich
authors.forename: Matthew K
authors.initials: MK
authors.lastname: Gelber
authors.forename: Rohit
authors.initials: R
authors.lastname: Bhargava
mesh_terms: D019187:Cadmium Compounds / Q000737:chemistry*; D004553:Electric Conductivity; D004867:Equipment Design; D007091:Image Processing, Computer-Assisted; D054023:Lasers, Semiconductor; D017669:Mercury Compounds / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared / Q000295:instrumentation* / Q000379:methods
chemical_list: D019187:Cadmium Compounds; D017669:Mercury Compounds; C104191:mercury cadmium telluride