Generation of single-cycle pulses spaced by an integer number of molecular periods
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Previous work has shown that a new light source consisting of mutually coherent spectral sidebands ranging from 2.94 m 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. 2004 Taylor & Francis Group, LLC.
