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Physical Review A

Stimulated rotational Raman generation controlled by strongly driven vibrational coherence in molecular deuterium

Andrea Burzo, A. V. Chugreev, Alexei V. Sokolov

Published Feb 26, 2007• 12 citations
Publisher

Abstract

We report an experimental observation of simultaneous rotational and vibrational collinear Raman generation in low pressure cooled deuterium gas, with only two narrow-band laser pulses applied at the input of the molecular cell. Only the fundamental vibrational transition ${Q}_{1}$ (0) is driven strongly in this experiment. However, in addition to efficient vibrational Raman generation we observe generation of a large number of rotational sidebands (more than 100) corresponding to ${S}_{0}$ (0) transition. We notice that fine-tuning of the frequency difference of the driving fields near the vibrational Raman resonance changes dramatically the aspects of the generated spectrum and the efficiency of rotational generation: from complete suppression of a self-starting stimulated rotational Raman generation to a strong enhancement of two orders of magnitude. We conduct a numerical analysis that allows us to attribute this behavior to quantum interference among the probability amplitudes of the three molecular states involved.

DOI
10.1103/physreva.75.022515

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We report an experimental observation of simultaneous rotational and vibrational collinear Raman generation in low pressure cooled deuterium gas, with only two narrow-band laser pulses applied at the input of the molecular cell. Only the fundamental vibrational transition Q1{Q}_{1}Q1​ (0) is driven strongly in this experiment. However, in addition to efficient vibrational Raman generation we observe generation of a large number of rotational sidebands (more than 100) corresponding to S0{S}_{0}S0​ (0) transition. We notice that fine-tuning of the frequency difference of the driving fields near the vibrational Raman resonance changes dramatically the aspects of the generated spectrum and the efficiency of rotational generation: from complete suppression of a self-starting stimulated rotational Raman generation to a strong enhancement of two orders of magnitude. We conduct a numerical analysis that allows us to attribute this behavior to quantum interference among the probability amplitudes of the three molecular states involved.
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We extend a recent suggestion for the generation of subfemtosecond pulses by molecular modulation [Phys. Rev. Lett. 81, 2894 (1998)] to the rotational spectrum of molecular hydrogen (H(2)) . When a rotational transition |a? ? |b? is strongly driven (|rho(ab) |=0.5) the generation and phase-slip lengths are of the same order and the Raman spectrum has approximately Bessel function sideband amplitudes. Numerical simulation predicts that this spectrum (generated in a 14-cm-long cell at 1-atm pressure of H(2)) will compress into a train of pulses with 94-fs pulse separation and a pulse length of 0.5 fs.
We suggest a technique to generate subfemtosecond pulse train in a Raman generator driven by three sufficiently intense single-mode laser beams whose carrier frequencies are tuned off the Raman resonance with two different two-photon detunings. The Raman interaction establishes a molecular modulation through a nonadiabatic process. The simulation based on the first fundamental vibrational Raman transition of solid hydrogen indicates that subfemtosecond pulse train can be generated when the signs of the two-photon detunings are opposite. This opens an experimentally feasible way to observe enhanced generation of broadband Raman sidebands as well as compression of the phase-locked Raman sidebands into subfemtosecond pulse trains in the same Raman media with above- and below-resonance Raman excitations.
We show that subfemtosecond pulse train with large pulse spacing can be generated by using three correlated Raman generators between three Raman levels with strong molecular modulation to control the generation and propagation of the cascade stimulated Raman sidebands. For three Raman generators based on the fundamental rotational and vibrational Raman transitions $(v=0,J=0\ensuremath{\leftrightarrow}v=0,J=2\ensuremath{\leftrightarrow}v=1,J=0\ensuremath{\leftrightarrow}v=0,J=0)$ in molecular hydrogen, a train of subfemtosecond pulses with a pulse spacing of 379.9 fs and pulse width down to 0.26 fs can be generated.