Photoinduced melting of the orbital order in La0.5Sr1.5MnO4 measured with 4-fs laser pulses

By measuring time-dependent optical birefringence with 4-femtosecond (fs) laser pulses, we determine the time scale for photoinduced melting of orbital order in the single-layer manganite La0.5Sr1.5MnO4. Such high time resolution is required to distinguish atomic motions that control the Jahn-Teller distortion from even faster electronic rearrangements. The experiment reveals an 18 fs bottleneck for the loss of orbital order, corresponding to about one-quarter period of the in-plane Jahn-Teller mode. Furthermore, we observe coherent oscillations of this Jahn-Teller mode. Both their amplitude and the birefringence drop exhibit a threshold in their fluence dependence, indicating cooperativity in the lattice response.