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Physical Review A

Vibrational molecular modulation in hydrogen

Shu Wei Huang, Wei-Jan Chen, A. H. Kung

Published Dec 26, 2006• 17 citations
Publisher

Abstract

Detailed numerical modeling of using the vibrational coherence of ${\mathrm{H}}_{2}$ for molecular modulation is presented. The focus of the calculation is on a strongly driven system aimed at producing many sidebands in the presence of Doppler broadening and the effects of collisions at room temperature. It is shown that Dicke narrowing that reduces the Doppler width plays a critical role in high order sideband generation in room temperature ${\mathrm{H}}_{2}$. In addition, the calculation shows that generation of many sidebands favors the phased state as has been reported in all gas phase experiments and is primarily a consequence of the Stark shifts that result from the applied high intensities. The influence of self-focusing in the gas medium that has been conjectured in previous studies is only secondary. The numerical results agree with experimental data obtained in our laboratory, where we have succeeded in generating collinearly propagating Raman sidebands with wavelengths that range from $2216\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in the infrared to $133\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in the vacuum ultraviolet. The frequencies covered by these sidebands span over four octaves for a total of more than $70\phantom{\rule{0.2em}{0ex}}600\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in the optical region of the spectrum.

DOI
10.1103/physreva.74.063825

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Detailed numerical modeling of using the vibrational coherence of H2{\mathrm{H}}_{2}H2​ for molecular modulation is presented. The focus of the calculation is on a strongly driven system aimed at producing many sidebands in the presence of Doppler broadening and the effects of collisions at room temperature. It is shown that Dicke narrowing that reduces the Doppler width plays a critical role in high order sideband generation in room temperature H2{\mathrm{H}}_{2}H2​. In addition, the calculation shows that generation of many sidebands favors the phased state as has been reported in all gas phase experiments and is primarily a consequence of the Stark shifts that result from the applied high intensities. The influence of self-focusing in the gas medium that has been conjectured in previous studies is only secondary. The numerical results agree with experimental data obtained in our laboratory, where we have succeeded in generating collinearly propagating Raman sidebands with wavelengths that range from 2216nm2216\phantom{\rule{0.3em}{0ex}}\mathrm{nm}2216nm in the infrared to 133nm133\phantom{\rule{0.3em}{0ex}}\mathrm{nm}133nm in the vacuum ultraviolet. The frequencies covered by these sidebands span over four octaves for a total of more than 70600cm\ensuremath−170\phantom{\rule{0.2em}{0ex}}600\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}70600cm\ensuremath−1 in the optical region of the spectrum.
We have demonstrated that coherent molecular modulation can result in the collinear generation of mutually-coherent spectral sidebands that extend in frequency from the infrared to the far ultraviolet. Our technique is based on adiabatic preparation of a highly coherent molecular superposition-state, which is achieved by using narrow-linewidth lasers slightly detuned from a Raman resonance. The phases of the resultant Stokes and anti-Stokes sidebands are adjusted in order to synthesize desired single-cycle pulse trains at the target. In this article we review recent improvements and developments in this area, including: techniques for increasing the number of generated sidebands; synchronization of the pulse trains with the molecular motion in the given molecular system; laser self-focusing and spatial soliton formation due to the coherent interaction of light with oscillating molecules. In the future, this Raman source may produce sub-cycle optical pulses, and allow synthesis of waveforms where the electric field is a predetermined function of time, not limited to a quasi-sinusoidal oscillation.
We report an experimental observation of simultaneous rotational and vibrational collinear Raman generation in low pressure cooled deuterium gas, with only two narrow-band laser pulses applied at the input of the molecular cell. Only the fundamental vibrational transition Q1{Q}_{1}Q1​ (0) is driven strongly in this experiment. However, in addition to efficient vibrational Raman generation we observe generation of a large number of rotational sidebands (more than 100) corresponding to S0{S}_{0}S0​ (0) transition. We notice that fine-tuning of the frequency difference of the driving fields near the vibrational Raman resonance changes dramatically the aspects of the generated spectrum and the efficiency of rotational generation: from complete suppression of a self-starting stimulated rotational Raman generation to a strong enhancement of two orders of magnitude. We conduct a numerical analysis that allows us to attribute this behavior to quantum interference among the probability amplitudes of the three molecular states involved.
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