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Physical Review A

Generation of subfemtosecond pulses by beating a femtosecond pulse with a Raman coherence adiabatically prepared in solid hydrogen

Fam Le Kien, Nguyen Hong Shon, K. Hakuta

Published Oct 8, 2001• 32 citations
Publisher

Abstract

We show that the beating of a nanojoule femtosecond pulse with a Raman coherence adiabatically prepared by a pair of millijoule nanosecond pulses in solid hydrogen leads to an efficient generation of a triplet, doublet, or even singlet of subfemtosecond pulses. The energy conversion efficiency of the subfemtosecond pulse generation can be larger than unity due to the transfer of energy from the driving pulses.

DOI
10.1103/physreva.64.051803

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We show that the beating of a nanojoule femtosecond pulse with a Raman coherence adiabatically prepared by a pair of millijoule nanosecond pulses in solid hydrogen leads to an efficient generation of a triplet, doublet, or even singlet of subfemtosecond pulses. The energy conversion efficiency of the subfemtosecond pulse generation can be larger than unity due to the transfer of energy from the driving pulses.
We present a numerical model for the interaction of a femtosecond probe pulse with a Raman medium excited by two nanosecond laser pulses in solid hydrogen. The model consists of a solution of a system of Bloch equations (for the medium state) and of the two-dimensional wave equation (for the field propagation). It is shown that by optimizing the tilt angle between the two driving pulses and the interaction length, the power of the probe pulse can be converted either into a quasicontinuous broadband spectrum or into a selected high-order sideband. The first situation can be used for generation of subfemtosecond pulses. The second situation can be used for the wavelength conversion of a femtosecond probe pulse from the infrared to the ultraviolet region. Due to the transfer of energy from the driving pulses, the energy conversion efficiency of these processes (with respect to the probe pulse) can be larger than unity.
We demonstrate the efficient modulation of an approximately 100-femtosecond pulse in a Raman medium coherently prepared by nanosecond pulses. Raman sidebands of the ultrashort pulse spanning 360 THz are generated with an efficiency of >5%. We show that the mechanism permitting the sidebands to be generated is the preparation of a significant vibrational coherence in the medium that is robust to disturbance by an intense short pulse. If the observed sidebands were phase compensated, they would form a short train of approximately ten 3-fs pulses. Focusing would permit the realization of a peak intensity of >10(13) W cm(-2).
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.
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 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.