2011, Pashkin, Alexej, Kübler, Carl, Ehrke, Henri, Lopez, Ricardo, Halabica, Andrej, Haglund, Richard, Huber, Rupert, Leitenstorfer, Alfred

The ultrafast photoinduced insulator-metal transition in VO2 is studied at different temperatures and excitation fluences using multi-THz probe pulses. The spectrally resolved midinfrared response allows us to trace separately the dynamics of lattice and electronic degrees of freedom with a time resolution of 40 fs. The critical fluence of the optical pump pulse, which drives the system into a long-lived metallic state, is found to increase with decreasing temperature. Under all measurement conditions, we observe a modulation of the eigenfrequencies of the optical phonon modes induced by their anharmonic coupling to the coherent wave-packet motion of V-V dimers at 6.1 THz. Furthermore, we find a weak quadratic coupling of the electronic response to the coherent dimer oscillation resulting in a modulation of the electronic conductivity at twice the frequency of the wave-packet motion. The findings are discussed in the framework of a qualitative model based on an approximation of local photoexcitation of the vanadium dimers from the insulating state.

2007, Kübler, Carl

Ziel der vorliegenden Arbeit war die experimentelle Untersuchung von Vielteilchen-Korrelationen auf ultrakurzen Zeitskalen. Folgende beispielhafte und repräsentative Szenarien wurden mittels zeitaufgelöster ultrabreitbandiger THz-Spektroskopie studiert: - Im polaren Halbleitermaterial Indiumphosphid (InP), einem idealen Laborsystem dessen Grundzustand sehr gut verstanden ist, wurde der Aufbau von Korrelationen zwischen elektronischen und Gitterfreiheitsgraden in Echtzeit verfolgt. - In dem stark korrelierten Materialsystem Vanadiumdioxid (VO_{2}) wurde das Wechselspiel zwischen Gitterschwingungen und elektronischen Korrelationen während eines ultraschnellen photoinduzierten Isolator-Metall Übergangs beobachtet, in einem Regime jenseits der Gültigkeit der Born-Oppenheimer Näherung.

2009, Leitenstorfer, Alfred, Kübler, Carl, Lopez, Rene, Halabica, Andrej, Haglund, Richard F., Huber, Rupert

The multi-THz conductivity of VO₂ recorded during a photoinduced insulator-metal transition directly reveals the femtosecond dynamics of V-V stretching modes and electronic correlations. The strongly correlated character of the electronic system leads to a transient behaviour of the conductivity that cannot be understood in a Born-Oppenheimer picture. Based on our measurements with a time resolution of 12 fs and recent theoretical simulations for the electronic structure in steady state, we suggest a novel qualitative model for the non-thermal phase transition. The extremely high speed for the photoinduced phase transition is elegantly explained via a directed wave packet motion in the electronically excited state of the dimer that transfers the crystal structure of the insulator into the configuration of the metallic phase.

2007, Kübler, Carl, Ehrke, Henri, Lopez, Rene, Halabica, Andrej, Haglund, Richard F., Leitenstorfer, Alfred

We directly trace the multi-THz conductivity of VO2 during an insulator-metal transition triggered by a 12-fs light pulse. The femtosecond dynamics of lattice and electronic degrees of freedom are spectrally discriminated. A coherent wave packet motion of V-V dimers at 6 THz modulates the lattice polarizability for approximately 1 ps. In contrast, the electronic conductivity settles to a constant value already after one V-V oscillation cycle. Based on our findings, we propose a qualitative model for the nonthermal phase transition.

2009, Huber, Rupert, Kübler, Carl, Ehrke, Henri, Lopez, Ricardo, Halabica, Andrej, Haglund, Richard F., Leitenstorfer, Alfred

The multi-THz conductivity of VO₂ recorded during a photoinduced insulator-metal transition directly reveals the femtosecond dynamics of V-V stretching modes and electronic correlations. We suggest a novel qualitative model for the nonthermal phase transition.