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Physical Review A

Subfemtosecond-pulse generation by rotational molecular modulation and pulse-spacing increase

Jian Wu, Heping Zeng

Published Jul 9, 2003• 12 citations
Publisher

Abstract

A technique is proposed to increase multiplicatively the pulse spacing of a periodic subfemtosecond-pulse train based on molecular modulation, with more pairs of driving lasers to control the higher-order stimulated Raman scattering. We demonstrate that the pulse spacing can be readily doubled, tripled, or even quadrupled by using pairs of off-resonant two-photon Raman transitions driven by laser beams of sufficient intensities and relatively shifted carrier frequencies. A periodic subfemtosecond-pulse train with a pulse width down to 0.53 fs and pulse spacing up to 376 fs can be generated with appropriate molecular coherence control of the fundamental rotational transition in molecular hydrogen.

DOI
10.1103/physreva.68.015802

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A technique is proposed to increase multiplicatively the pulse spacing of a periodic subfemtosecond-pulse train based on molecular modulation, with more pairs of driving lasers to control the higher-order stimulated Raman scattering. We demonstrate that the pulse spacing can be readily doubled, tripled, or even quadrupled by using pairs of off-resonant two-photon Raman transitions driven by laser beams of sufficient intensities and relatively shifted carrier frequencies. A periodic subfemtosecond-pulse train with a pulse width down to 0.53 fs and pulse spacing up to 376 fs can be generated with appropriate molecular coherence control of the fundamental rotational transition in molecular hydrogen.
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.
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 suggest that subfemtosecond pulses can be generated by using high-order stimulated Raman scattering when a Raman transition is driven by three intense single-mode laser beams with two different modulation frequencies. An oscillatory strong Raman coherence is established by the modulated molecular modulation. Numeric simulations based on the first fundamental vibrational Raman transition of gaseous and solid deuterium indicate that subfemtosecond pulses can be generated by Raman excitations with above, below, or opposite Raman detunings.
Subfemtosecond (sub-fs) pulses can be generated by cascade-stimulated Raman scattering in a Raman medium with modulated Raman excitations, driven by two sufficiently intense laser beams, one of which is amplitude modulated. The nonadiabatic Raman interaction establishes a strong modulated Raman coherence, which supports compression of the generated broadband Raman sidebands to a train of sub-fs pulses regardless of whether the carrier frequencies of the driving lasers are tuned above, below or on two-photon Raman resonance.
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 study the generation of subfemtosecond pulses with the molecular coherence control in stimulated Raman scattering. We show analytically that the antiphased state temporally advances the higher frequencies with respect to the lower frequencies during a beating cycle. After some propagation distance, due to the dispersion and the difference between the antiphased and phased states in advancing high or low frequencies, the coherence is highest on the negative side of the detuning, when the two-photon Rabi frequency is about equal to the detuning. This asymmetry of the coherence magnitude with respect to the negative and positive sides of the Raman detuning is reflected in the behavior of the tuning characteristics of the high-order frequency components. When the Raman detuning is small, although the process is nonadiabatic, the subfemtosecond pulse generation may occur for both negative and positive sides of the detuning.