WAVE MATCHING OF FEMTOSECOND AND PICOSECOND PHOTON-ECHOES IN DYE-DOPED POLYMER-FILMS

We study experimentally two-pulse photon echo (2PPE) and the photochem ically accumulated stimulated photon echo (PASPE) in organic dye-doped polymer films at low temperature as a function of the angle between t he excitation pulses in transmission and reflection geometry. In the t ransmission geometry, by increasing the angle between two high-intensi ty femtosecond excitation pulses, we observe a blue shift of the 2PPE spectral profile, which we explain as wave-matching condition between the ultrashort excitation pulses and the emitted nonlinear coherent tr ansient response. In the case of PASPE with low-intensity picosecond p ulses, no spectral shift is observed, which is the result of the linea r nature of the coherent transient response at small excitation intens ities.