QCL-assisted infrared chemical imaging

Micro- and nano-technologies (MNT) have opened up the mid-infrared (IR) spectral region to room temperature techniques that are enabling new applications in chemical imaging. The mid-IR is rich in wavelength specific absorptions that can be used to identify different chemicals and materials. It is only in the last decade, however, that mid-IR cameras and tunable, intense mid-IR laser sources have become available that are compact and operate at room temperature. MNT has allowed these developments; micro-bolometer arrays are now routinely fabricated with 25 um resolution and the ability to sense mid-IR radiation from 7 to 14 um. Quantum cascade (QC) lasers fabricated with exquisite control of semiconductor layers and waveguides serve as the gain media for tunable mid-IR lasers, some even tuned with MEMS feedback elements. The current state of mid-IR imaging and illumination is discussed, and specific examples of its use in chemical imaging are presented. First, passive mid-IR imaging is considered, along with its ability to be used for chemical identification. The use of tunable mid-IR lasers as an illumination source is then considered. Technical aspects of illumination and detection paradigms are presented, with a consideration of how spectroscopic information gathered in both stimulated thermal emission and reflectance modes can be analyzed to determine chemical composition in an image. Finally, specific examples of QC laser assisted chemical imaging are presented.