Mode selective excitation of terahertz vibrations in single crystalline rubrene.

Organic molecular crystals have a variety of low frequency vibrational modes composed of intra- and inter-molecular oscillations. They are mixed intricately in the terahertz (THz) region. We are interested in the controllability of the vibrational energy distribution among such THz vibrational modes based on the femtosecond double-pulse excitation scheme. Single crystalline rubrene is prepared by physical vapor transport. The optical response of vibrational modes in the electric ground state of rubrene is detected by the ultrafast pump-probe reflectivity measurement at 90 K. Three oscillation modes at 3.20, 3.67, and 4.18 THz are detected, and we demonstrate selective enhancement and depletion of each mode by properly tuning the double-pulse delay. The amplitude of the selected vibrational mode is modulated between 0.149 and 1.87, where 1.0 corresponds to the amplitude excited with a single pump pulse. The double-pulse delay dependence of the observed vibrational amplitude is simulated based on the classical driven harmonic oscillator model, and the results reasonably reproduce our experimental signals. Such selective manipulation of the vibrational amplitude can be a potential tool to investigate the vibronic and electron-phonon couplings which plays an important role for the charge transport characteristics and various optoelectronic properties in organic molecular crystals.