We report a technique to measure the mid-infrared photothermal response induced by a tunable quantum cascade laser in the neat liquid crystal 4-octyl-4 0 -cyanobiphenyl (8CB), without any intercalated dye. Heterodyne detection using a Ti:sapphire laser of the response in the solid, smectic, nematic and isotropic liquid crystal phases allows direct detection of a weak mid-infrared normal mode absorption using an inexpensive photodetector. At high pump power in the nematic phase, we observe an interesting peak splitting in the photothermal response. Tunable lasers that can access still stronger modes will facilitate photothermal heterodyne mid-infrared vibrational spectroscopy. Photothermal spectroscopy has rapidly emerged as the most sensitive label-free optical spectroscopic method, rivaling even fluorescence spectroscopy. The method has been shown to be remarkably sensitive in the visible region of the spectrum with reports of yoctomole sensitivity, eventually culminating in the observation of single molecule response 1,2,28 at room temperature. This unexpected sensitivity has led to rapid development of photothermal methods in the visible region, both for spectroscopy 3-5 and for imaging nanoparticles and organelles with high signal-to-noise ratio. 6,7 Extension of the photothermal technique to the midinfrared region is particularly attractive because the presence of a large number of characteristic normal modes of molecules in the so-called "fingerprint" region of the electromagnetic spectrum allows for spectroscopy and imaging without requiring a perturbing label. The standard instrument of choice for vibrational infrared spectroscopy remains Fourier transform infrared spectroscopy (FTIR) using cryogenically cooled detectors along with a $1200 K Globar blackbody source. But the lack of table-top stable high brightness sources and a fundamental quantum limit on the detectivity of broadband cryogenic mid-infrared detectors has translated to a lack of progress: the state-of-the-art 8 has not advanced significantly in several decades. Detection of the absorption of infrared radiation is still performed using narrow band-gap cryogenically cooled detectors made of indium antimonide (InSb) or mercury-cadmium-telluride (MCT), 8 which both are intrinsically less sensitive than the best available visible photodetectors. With the advent of tunable quantum cascade lasers (QCLs) as table-top high brightness sources, there is now hope of a rapid transformation in the field of midinfrared spectroscopy.9,10 The spectral brightness of these table-top QCL sources actually can exceed that of synchrotrons and other large relativistic electron-accelerator-based sources.
