Second harmonic generation (SHG) in pentacene thin films grown by matrix assisted pulsed laser evaporation (MAPLE)

Thin films of pentacene, i.e. a polycyclic hydrocarbon consisting of five linearly-fused benzene rings, are grown on silicon and quartz (SiO2) substrates by matrix assisted pulsed laser evaporation (MAPLE) and by using toluene as a frozen matrix (melting point: -95 degrees C). The thin film samples are subsequently investigated for their optical, morphological and chemical structure, i.e. by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM), and spectroscopic-ellipsometry (SE). Due to its highly conjugated aromatic behaviour, pentacene exhibits semiconductor properties and has been shown to generate excitons upon absorption of ultra-violet (UV) or visible light. Here, we study the excited state dynamics of pentacene, in bulk and as thin films, after optical excitation, by using femtosecond (fs) time-resolved second harmonic generation (SHG). A pulsed femtosecond Ti:sapphire laser (80 MHz repetition rate, 800 nm, 100 fs) is employed to demonstrate the SHG potential of such polycyclic aromatic hydrocarbon simple molecule. The substrates used in studying the films are inert and non-interacting, known to deliver no noteworthy contribution to the SHG signal. Therefore, the pure response of pentacene to the electronic excitation can be resolved in contrast to the measurements on SiO2. Finally, a comparative discussion of our results is made with respect to literature. This study provides important insights on the excited states dynamics of pentacene, an essential step for understanding the photophysics of simple, linearly-fused arenes.