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Chinese Physics Letters

Strengthened Hyper-Raman Lines with the Coherent Superposition State

Zhou Zhao-Yan, Yuan Jian-Min

Published Feb 19, 2007• 7 citations
Publisher

Abstract

High-order harmonic generations from a one-dimensional Coulomb potential atom are calculated with the initial state prepared as a coherent superposition between its ground and first excited states. When the energy difference of the two states is small, we can choose proper laser pulse such that the first excited state can be excited only to other bound states instead of being ionized. We show that only the hyper-Raman lines are observable instead of the harmonics. The energy difference of the ground and the first excited state can be deduced from the highest peak of the hyper-Raman lines. We further show that the similar results can be obtained by using a combination of two laser pulses with different frequencies interacting with the atom initially at the ground state.

DOI
10.1088/0256-307x/24/3/026

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High-order harmonic generations from a one-dimensional Coulomb potential atom are calculated with the initial state prepared as a coherent superposition between its ground and first excited states. When the energy difference of the two states is small, we can choose proper laser pulse such that the first excited state can be excited only to other bound states instead of being ionized. We show that only the hyper-Raman lines are observable instead of the harmonics. The energy difference of the ground and the first excited state can be deduced from the highest peak of the hyper-Raman lines. We further show that the similar results can be obtained by using a combination of two laser pulses with different frequencies interacting with the atom initially at the ground state.
Abstract Hyper-Raman lines (HRL) resulting from strong-field light–matter interaction have been predicted theoretically in the 1990s but never identified in high-order harmonic generation experiments. Here, we use a combination of 800 and 400 nm laser pulses to control independently the two processes required for the hyper-Raman emission: creation of a coherence between two electronic states and laser-dressing of these states. As a result we observe simultaneously high-order harmonics, XUV free induction decay and HRL. We investigate experimentally and numerically the properties of this novel emission source. It can be of high interest, amongst others, for high-resolution spatio-temporal spectroscopy of excited electronic states in the same fashion high-order harmonics generation provides it for ground state.
We investigate the coherent and incoherent contributions of the scattering spectrum of strongly driven two-level atoms as a function of the initial preparation of the atomic system. The initial ``phasing'' of the coherent superposition of the excited and ground states is shown to influence strongly the generation of both harmonics and hyper-Raman lines. In particular, we point out conditions under which harmonic generation can be inhibited at the expense of the hyper-Raman lines. Our numerical findings are supported by approximate analytical evaluation in the dressed state picture.
The photon emission spectrum of the hydrogen atom in an intense high-frequency laser pulse is simulated to show that regular fine structures appear in the two sides of both the odd and even order harmonics. The splitting of the fine structures around the normal harmonic line are determined by the energy differences of the eigenstates of the time averaged Kramers-Henneberger potential with the same parity. The lines around the even order harmonic positions are produced by the hyper-Raman transitions between the dressed atomic states with different parities. The hyper-Raman lines are predicted as long as stabilization occurs and are not so sensitive to the initial state of the atom as those appearing with the low-frequency and less strong laser field. The hyper-Raman lines can be strongly reduced with longer turn-on and turn-off times.