Photothermal Mid-Infrared Microscopy: a new tool for hyperspectral chemical imaging

We describe a method for label free microscopy in the mid-infrared region of the electromagnetic spectrum based on the photothermal effect. A Quantum Cascade Laser (QCL) tuned to an infrared active vibrational molecular normal mode is used as the pump laser. A low-phase noise Erbium-doped fiber (EDF) laser (1.5$\mu$m) is used as the probe. We demonstrate the method using a patterned image target with liquid crystal 4-cyano-4'-octylbiphenyl (8CB) as the mid-infrared absorber. The QCL is tuned across the C-H scissoring band, with a peak absorption at 1607cm$^{-1}$. Absorption of the modulated pump beam results in a change in the dielectric function and the refractive index at the probe beam frequency. The resultant scatter of the probe is observed in heterodyne lock-in detection. The combination of heterodyne detection, high brightness mid-infrared QCLs and low-phase noise stable EDF lasers provides an ultra-sensitive method for obtaining mid-infrared microscope images using short-wavelength optical detectors, whose performance far exceeds those of cryogenically cooled broadband mid-infrared detectors. The method provides a powerful new tool for hyperspectral label-free mid-infrared imaging.