Picosecond two-color photon echoes in doped molecular solids

Abstract

The three pulse stimulated echo, long known in magnetic resonance (1), has more recently been demonstrated on optical transitions(2). An interesting way of looking at the stimulated-echo-formation relies on the fact that the first two excitation pulses create an "ordered" population as a function of detuning .This ordered population can also be looked upon as a " grating " in frequency space from which a probe pulse can "reflect" an echo. Since a grating in only one of the levels suffices, a corollary is that the echo may be stimulated at some entirely different frequency ω2 than the one ω1, at which the grating was prepared. We report the observation of such a connected two-color stimulated echo(C2CSE) in a solid and use it to confirm the relaxation out of the 747 cm−1 vibrational state of the S1 manifold of pentacene in naphthalene. It has also been realized (3) that after redistribution of the initially excited population grating, that the echo may be stimulated from any state into which the grating has relaxed. We report here the first observation of such a relaxed two-color stimulated echo(R2CSE) and note that this echo can be seen as the optical analogue of the cross-relaxation peak in a two-dimensional nmr spectrum. The rise and fall of this relaxed echo can be used as a probe for the vibrational deactivation pathway(s) subsequent to excitation of various vibrational levels in the S1 manifold. Prior to discussing briefly some of the results we note that from an analysis of this echo effect it is clear that a necessary prerequiste to observation of such two-color photon echoes is that the inhomogeneous broadening on the selected transitions is strongly correlated. Furthermore we note that in these echo effects interference with stimulated emission and super -fluorescence must be avoided by using small-angle excitation pulses. Some of the results are shown in Fig.1. In (A) the decay of the C2CSE is presented with the solid line being a fit to a population decay parameter of the 747 cm-1 level of 33 psec. In (B) the rise of the R2CSE is shown where the probe wavelength is at the pure electronic transition. The dashed line is the expected echo-intensity rise if the initially excited vibration would directly decay into the vibrationless state. The mismatch between the dashed curve and the experimental one shows that intermediate levels in the relaxation pathway must exist. The solid line in (B) is a fit to a model where one intermediate bottleneck level is assumed with a lifetime of 16±2 psec. Other examples will be presented at the meeting.