Ultrafast Phase Transitions in Solids

In this talk we present the coherent excitation and coherent control of the A1 phonon mode in Te. First, the underlying theory about the excitation of the A1 phonon mode, and only this, in a certain class of materials is discussed. The theory, called Displacive Excitation of Coherent Phonons (DECP), predicts the excitation of the A1 phonon mode as a result of electronic excitation following absorption of an ultrashort laser pulse by the material. Since there is no symmetry breaking mechanism in the electronic excitation through absorption the effect can only take place in materials which possess the symmetry preserving, breathing A1 phonon mode. Earlier experiments which demonstrate the effect on Te, Sb, Ti2O3 and Bi are presented. Next, our results on the excitation of large amplitude A1 phonons in Te are discussed. We excite Te with an amplified femtosecond laser pulse and therefore promote a significant amount of electrons from bonding to antibonding states. The A1 phonon mode is excited with large amplitude ionic oscillations and the bandstructure of the material is considerably modified. Our results exhibit a band crossing taking place in Te in hundreds of femtpseconds after the excitation. The indirect band gap of Te seems to collapse and the material seems to undergo a semiconductor to semimetal transition. As the ions tend to relax around their new equilibrium positions their oscillations exhibit a decreasing amplitude and the material recovers its bandgap. The oscillations have a frequency of about 3.6 THz, too fast to give time to the material to exhibit a metallic behaviour while it is in the crossed bands state. We have been working on a double pump experiment which will allow us to stabilize the bandstructure in the semimetallic state for times much larger than before and therefore study the response of the material under this trabsition. Our most recent results on this coherent control scheme are presented and discussed.