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IEEE Lasers and Electro-Optics Society Annual Meeting (LEOS)

Femtosecond light source for phase-controlled atomic processes

Stephen E. Harris, Alexei V. Sokolov, D. R. Walker, Deniz D. Yavuz, G. Y. Yin

Published Nov 13, 2002• 0 citations
Publisher

Abstract

Recent experiments have shown that coherent molecular oscillations may generate a comb of coherent collinear sidebands with 50,000 cm/sup -1/ of spectral bandwidth. The essence of this technique is the use of a Raman transition with a sufficiently large coherence that the generation and phase slip length are of the same order. The coherence is established by driving the molecular transition with two single-mode laser fields slightly detuned from the Raman resonance, so as to excite a single molecular superposition state. This talk will summarize the characteristics of this light source and describe two recent experiments which show the mutual coherence of the generated sidebands.

DOI
10.1109/leos.2001.968991

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Recent experiments have shown that coherent molecular oscillations may generate a comb of coherent collinear sidebands with 50,000 cm/sup -1/ of spectral bandwidth. The essence of this technique is the use of a Raman transition with a sufficiently large coherence that the generation and phase slip length are of the same order. The coherence is established by driving the molecular transition with two single-mode laser fields slightly detuned from the Raman resonance, so as to excite a single molecular superposition state. This talk will summarize the characteristics of this light source and describe two recent experiments which show the mutual coherence of the generated sidebands.
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 use molecular deuterium and two driving lasers to demonstrate collinear generation of mutually coherent equidistant sidebands, covering $50000{\mathrm{cm}}^{\ensuremath{-}1}$ of spectral bandwidth and ranging from $2.94\ensuremath{\mu}\mathrm{m}$ to 195 nm in wavelength. The essential idea is the adiabatic preparation of a single, highly coherent ( $|{\ensuremath{\rho}}_{\mathrm{ab}}|\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.33$) molecular eigenstate.
Previous work has shown that a new light source consisting of mutually coherent spectral sidebands ranging from 2.94 μm to 195 nm can be obtained by adiabatic preparation of a molecular ensemble in a single vibrational superposition state. Molecular motion modulated the refractive index and thus led to the frequency modulation of the driving beat-note. The resulting sidebands were equidistant and separated by a frequency equal to the modulation frequency. In the present work we extend this idea by applying more input fields to the molecular ensemble. We take two input fields separated by one half of the modulation frequency, such that their second harmonics drive the molecular ensemble. The proposed approach results in generating an equidistant comb of frequencies separated by a fraction (1/4) of the modulation frequency. Moreover, the intensity of the generated train of pulses increases by the inverse of the same factor. An important feature of the generated comb is that it reaches zero frequency, and as a consequence allows for control of the absolute phase, or the phase of the carrier with respect to the envelope. Since many physical processes, for example photoionization of molecules by intense laser pulses, are influenced by the time dependence of the electric field (and not the envelope), control of absolute phase will become an important issue for few-cycle pulses.
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 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.
We study the light evolution through a Raman-active medium, where a pair of fields pump molecules near-resonantly and excite the molecular coherence. A synthetic lattice including multiple frequency sidebands via the coherent Raman process is constructed. By varying the density of molecules in the light propagation direction, the system can present a lattice model undergoing dynamic modulations. Further considering the frequency-dependent couplings between synthetic lattice sites, we explore intriguing phenomena, including spectral Bloch oscillation and super Bloch oscillation with asymmetric patterns, which leads to interesting frequency-conversion phenomena with the Raman sideband generations. Our work could be useful for future experiments in coherent Raman sideband processes towards ultrashort pulse generations and higher-frequency conversion.