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Physical Review A

Raman technique for single-cycle pulses

Stephen E. Harris, D. R. Walker, Deniz D. Yavuz

Published Jan 10, 2002• 31 citations
Publisher

Abstract

We show that two Raman generators which have different Raman transition frequencies and are used in series result in a multiplicative increase in the total number of generated sidebands. When phase-corrected, these sidebands synthesize to a train of randomly timed single-cycle pulses. For series generators on the rotational transitions of molecular ${\mathrm{D}}_{2}$ $(J=\stackrel{\ensuremath{\rightarrow}}{0}2)$ and ${\mathrm{H}}_{2}$ $(J=\stackrel{\ensuremath{\rightarrow}}{1}3),$ each with 40 modes, the average peak power enhancement is 1600 and the average duration between random pulses is 1.74 ps.

DOI
10.1103/physreva.65.021801

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We show that two Raman generators which have different Raman transition frequencies and are used in series result in a multiplicative increase in the total number of generated sidebands. When phase-corrected, these sidebands synthesize to a train of randomly timed single-cycle pulses. For series generators on the rotational transitions of molecular D2{\mathrm{D}}_{2}D2​ (J=0\ensuremath→2)(J=\stackrel{\ensuremath{\rightarrow}}{0}2)(J=0\ensuremath→​2) and H2{\mathrm{H}}_{2}H2​ (J=1\ensuremath→3),(J=\stackrel{\ensuremath{\rightarrow}}{1}3),(J=1\ensuremath→​3), each with 40 modes, the average peak power enhancement is 1600 and the average duration between random pulses is 1.74 ps.
It has been proposed that single-cycle pulses with an average spacing between two random pulses as large as 1.74 ps can be generated when two cascaded Raman medium are used [1]. In this report, we propose a new technique to generate of subfemtosecond (sub-fs) pulse train with large pulse spacing, on the basis of three correlated Raman generators within three involved molecular levels in a Raman medium. Each Raman generator comes from the corresponding Raman transition between two of the three involved molecular levels, which are coupled by two of three single-mode laser beams of sufficient intensities in such a way that correlation between the Raman generators are established. A large enough pulse spacing can be possibly achieved if proper Raman transitions are selected so that the frequency difference between two adjacent Raman sidebands is small enough.
We have investigated the behavior of high-order stimulated Raman scattering in a highly-transient regime. We demonstrate efficient collinear generation of vibrational sidebands in molecular hydrogen and methane using two-color pumping with pulses of duration $\ensuremath{\tau}\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ tuned to a vibrational Raman transition. A Raman spectrum with a large bandwidth was observed, ranging from the IR to the UV. Under some conditions strong pump depletion was observed and up to five anti-Stokes sidebands were observed to have energies exceeding 10% of the transmitted pump pulse energies. A numerical simulation reproduces qualitatively and quantitatively the experimental results and allows us to explain key experimental features. The simulation also confirms that the molecular coherence in the medium is substantially increased by the two-color pumping and allows us to deduce values for the degree of material excitation. The use of this technique looks promising for efficient subfemtosecond pulse generation.
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 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.