We demonstrate the use of mid-infrared (mid-IR) photothermal spectroscopy as a sensitive and label-free technique for hyperspectral chemical imaging. In photothermal spectroscopy, a pump beam is absorbed by the sample causing a temperature-induced change in the local refractive index. The scattered probe beam then is detected in a heterodyne measurement. Here, a pulsed mid-IR quantum cascade laser pump (tunable around 6µm) and an erbium-doped fiber laser probe at eye-safe wavelengths (around 1.55µm) are used. Extending photothermal spectroscopy into the mid-IR is attractive due to the presence of a large number of characteristic infrared-active normal vibrational modes, particularly in the molecular fingerprint region, allowing bond-specific imaging of biological and chemical samples.
Photothermal studies of a liquid crystal, 4′-octyl-4-biphenylcarbonitrile (8CB), targeting the C-H scissoring band at 1607cm-1 were conducted. Spectral scans of 8CB were demonstrated with a good signal-to-noise ratio, an increase of over one order of magnitude from previous studies that used a titanium:sapphire probe laser. Contributions to the signal-to-noise ratio from several different probe laser configurations, including continuous-wave and mode-locked lasers, are investigated. Histopathological bird brain sections were studied and high contrast mid-IR photothermal images were obtained without the use of stains. Images of the bird brain sections were acquired by targeting the amide-I band centered at 1670cm-1 and raster-scanning the sample.
Photothermal spectroscopy with a fiber laser probe offers many advantages for imaging in the mid-IR. Fiber lasers can be used with commercially available and highly sensitive photodetectors that operate at room temperature, in contrast to other mid-IR spectroscopy techniques such as Fourier Transform Infrared Spectroscopy (FTIR) which commonly uses cryogenically cooled mid-IR detectors. Turn-key operation of compact and stable fiber lasers makes the photothermal system more robust and user-friendly. These studies show that high-contrast images of biological samples with good sensitivity can be obtained using mid-IR photothermal spectroscopy with a fiber laser probe.