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Physical Review A

Off-resonant preparation of a vibrational coherence for enhanced stimulated Raman scattering

Sarah Gundry, M. Anscombe, A M Abdulla, S. D. Hogan, E. Sali, John W. G. Tisch, J. P. Marangos

Published Sep 27, 2005• 9 citations

Abstract

We present a detailed experimental investigation into the off-resonant preparation of a vibrational coherence for multiorder Raman generation. In particular, the importance of the balance between the conflicting requirements for generating a strong coherence and for adiabatic evolution of the system are examined. For the parameters accessible in our experiment, we find that the strongest frequency modulation occurs under conditions for which there is some nonadiabatic behavior, and the coherence is less than the peak value obtainable. We investigate the modulation of a narrowband probe pulse, and an ultrashort probe pulse, in the coherently prepared medium, and demonstrate efficient modulation of both probe pulses resulting in the generation of $360\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$ bandwidth fields. This system therefore has an application in the field of subfemtosecond pulse generation.

DOI: 10.1103/physreva.72.033824

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We present a detailed experimental investigation into the off-resonant preparation of a vibrational coherence for multiorder Raman generation. In particular, the importance of the balance between the conflicting requirements for generating a strong coherence and for adiabatic evolution of the system are examined. For the parameters accessible in our experiment, we find that the strongest frequency modulation occurs under conditions for which there is some nonadiabatic behavior, and the coherence is less than the peak value obtainable. We investigate the modulation of a narrowband probe pulse, and an ultrashort probe pulse, in the coherently prepared medium, and demonstrate efficient modulation of both probe pulses resulting in the generation of

360\phantom{\rule{0.3em}{0ex}}\mathrm{THz}

bandwidth fields. This system therefore has an application in the field of subfemtosecond pulse generation.

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 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.

A model of two-level weakly interacting quantum systems is used to demonstrate that the phase of molecular vibrations excited through stimulated Raman scattering can be controlled by the relative phase of time-separated laser pulses, allowing synchronization and amplitude control of coherence excitation in a medium. For transients with a phase relaxation time less than the field cycle, the field half-cycle can serve as a subfemtosecond probe of coherent vibrations. In this regime, the high-frequency field-induced modulation of molecular dipole moments gives rise to a detectable modulation of the optical response of a medium at the frequency $2\ensuremath{\omega}$, $\ensuremath{\omega}$ being the frequency of the laser field.

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.