Coherent far-infrared excitation processes with ultrafast pulses in multilevel systems

Ilya Sh. Averbukh, V.A. Kovarsky, N. F. Perel'Man

Alexander A. Makarov

H. Nishioka, H. Koutaka, Ken-ichi Ueda

Coherence control in a _ -type three-level system having an ultrafast (<10 fs) dephasing time has been demonstrated. Coherences in the multilevel system were excited by an intense supercontinuum yielding terahertz Rabi frequencies. The coherence was controlled by changing the interaction phase between the excited coherences and the broadband light field in the double-pulse pumping scheme.

John N. Sweetser, Ian A. Walmsley

The effects of coherent propagation of an ultrashort pulse through an extended collection of multilevel systems is examined. It is found that the transmitted field shows significant deviation from both Beer’s law absorption and the conventional area theorem. In addition, when the system is in a nonstationary state it is found that additional frequencies can appear in the transmitted pulse spectrum. Such effects may be important in several areas of current interest, such as ultrafast pump–probe, coherent control, and inversionless amplification experiments in extended, multilevel media as well as in ultrafast-laser development.

Cyrus D. Cantrell, W. H. Louisell, Jenny Lam

Gabriel Voith, Alexander A. Milner, Michael J. Desrochers +2 more

Ultrafast dynamics of collective excitations in superfluids remains largely unexplored beyond the roton region of the Landau excitation spectrum, despite the importance of such dynamics for understanding nonequilibrium processes in these systems. Here, we employ ultrafast coherent control with sequences of femtosecond pulses to perform spectroscopy of multiple quasiparticles in superfluid helium far from equilibrium. By measuring the time-resolved optical birefringence, we track the nonequilibrium dynamics of quasiparticle pairs associated with rotons, maxons and the Pitaevskii plateau region. The spectral lineshape of the roton peak is explained by an ab initio theoretical analysis of the roton-roton interaction. We also reveal strong energy shifts and short lifetimes of both maxon and Pitaevskii-plateau pairs, as well as an influence of the quasiparticle effective mass on the phase of their coherent response to laser pulses. These results demonstrate the ability to extract previously inaccessible information about collective excitations in a strongly interacting quantum fluid by probing its nonequilibrium dynamics on picosecond and sub-picosecond timescales.