Photo-thermal infrared nanoscopy (AFMIR) is a non-destructive analytical technique that allows to acquire mid-infrared (mid-IR) spectra and images at a spatial resolution ⩽50 nm. A pulsed tunable infrared laser is used to cause localized thermal expansion around infrared absorbers in the sample which are detected by an atomic force microscope (AFM) at high spatial resolution. External cavity - quantum cascade lasers (EC-QCLs) are a type of semiconductor lasers that combine several properties that are very favorable for AFMIR. Their main advantage is, that their repetition rate can be continuously tuned over the kilohertz range and thus can be adjusted to the contact resonance of the AFM cantilever. This allows to amplify the AFMIR signal by the Q-factor of the system. Additionally EC-QCLs oﬀer a broad tunability across several hundreds of wavenumbers in the mid-IR and a spectral resolution of about 1 cm-1. However, using EC-QCLs and the resonance enhancement introduces two important challenges into AFMIR: 1. The wavelength emitted by EC-QCLs is selected via the angle of the grating on one end of the external cavity. Inaccuracies in the re-positioning of the grating in combination with interferences inside the cavity lead to changes in intensity in the order of several percent. 2. The measurement signal depends on the mechanical amplification of the thermal expansion at the repetition rate of the laser. Hence, changes of the position of resonance or the magnitude of the Q-factor have to be compensated for. These problems can be overcome through data acquisition adapted to resonant AFMIR and chemometric treatment of the acquired spectra, as will be shown in this presentation.
For further information please contact Andrea Centrone, 301-975-8225, andrea.centrone [at] nist.gov (andrea[dot]centrone[at]nist[dot]gov)