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Physical Review Letters

Subfemtosecond Pulse Generation by Molecular Modulation

Stephen E. Harris, Alexei V. Sokolov

Published Oct 5, 1998• 306 citations
Publisher

Abstract

We suggest a technique for producing subfemtosecond pulses of radiation. The technique is based on using electromagnetically induced transparency to produce a strongly driven molecular coherence. This coherence results in a Raman spectrum with Bessel function amplitudes and phases corresponding to a frequency modulated signal, thereby allowing compression by the group velocity dispersion of the same medium.

DOI
10.1103/physrevlett.81.2894

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We suggest a technique for producing subfemtosecond pulses of radiation. The technique is based on using electromagnetically induced transparency to produce a strongly driven molecular coherence. This coherence results in a Raman spectrum with Bessel function amplitudes and phases corresponding to a frequency modulated signal, thereby allowing compression by the group velocity dispersion of the same medium.
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.
Subfemtosecond pulses can be generated in a Raman generator by using electromagnetically induced transparency if ultrabroad Raman sidebands are generated and then propagate in such a way that the generated sidebands at higher frequencies are temporally advanced with regard to those at lower frequencies. However, it was recently demonstrated by experiments [1] that above-resonance Raman excitations ensured the establishment of anti-phased Raman coherence for pulse compression but only very limited number of Raman sidebands could be generated due to Raman self-defocusing [2], while broadband sidebands could be generated under below-resonance Raman excitations, which supported no pulse compression in the same Raman media. In order to establish anti-phased Raman coherence and simultaneously overcome Raman self-defocusing to benefit efficient generation of broadband sidebands, we suggest in this report a new technique to generate sub-fs 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 nonadibatic processes. The simulation on the basis of the fundamental rotational Raman transitions in H2 shows that sub-fs pulse can be generated by above-resonance or below-resonance Raman excitations, or even Raman excitations with opposite detunings, as is shown in Fig.1. This opens a new way for experiments to observe enhanced generation of broadband Raman sidebands, together with compression of the phase-locked Raman sidebands into sub-fs pulse trains in the same Raman media
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.
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 propose a scheme for periodic generation of subfemtosecond pulses on the basis of modulated Raman coherence. The detailed theoretical analyses and numerical simulations show that an oscillatory Raman coherence with periodically changed phase is established. It leads to periodic compression of the generated broadband sidebands in the same medium. As compared with the scheme by using nonmodulated driving fields where subfemtosecond pulses can be generated only with negative Raman detunings, the modulated Raman coherence scheme supports subfemtosecond pulse generation with positive, negative, or even opposite Raman detunings around the Raman resonance.
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.