Mollow triplet emission and high-harmonic generation involving isolated impurity atoms of semiconductors in a strong terahertz field

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This proposal asks for funding for a short research visit at the University of California at Santa Barbara (UCSB) in order to perform atomic-physics-type experiments on isolated dopant atoms in semiconductors with the university's free-electron laser (FEL). The proposal's two topics are the Mollow triplets-the frequency analogue of Rabi oscillations in a strong quasi-continuous terahertz field, and the generation of higher harmonics by charge carriers which, after ionization, are accelerated in the oscillating field and recollide with the dopant ions. These topics are strongly motivated by analog phenomena of atomic physics with gasses, where notably the observation of higher harmonics by the recollision phenomenon represents the foundation for the hot research field of attosecond physics. The embedding of isolated atoms in a host crystal has the consequence that nonlinear optical effects (with rather long coherence times) occur at much lower field strength and photon energy than in free atoms. Such phenomena could hardly be studied in the past for a lack of suitable radiation sources and measurement techniques. Building on our prior studies, we intend to take advantage of the UCSB-FEL's unique properties for the observation of the effects addressed above. These properties are the very small bandwidth of the FEL's long THz pulses which are favorable for the addressing of individual quantum states and the observation of newly generated sidebands, and the low radiation frequencies accessible (down to a few hundred GHz) which support the generation of high orders of the harmonics upon carrier-ion recollision.