Laboratory of Ultrafast Dynamics       

Laboratory Featured Article

Frequency comb generation at terahertz frequencies by coherent phonon excitation in silicon - M Hase, M Katsuragawa, AM Constantinescu, H Petek - Nature Photonics, doi: 10.1038/nphoton.2012.35

High-order nonlinear light-matter interactions in gases enable the generation of X-ray and attosecond light pulses, metrology and spectroscopy 1.  Optical nonlinearities in solid-state materials are particularly interesting for combining optical and electronic functions for high-bandwidth information processing 2.  Third-order nonlinear optical processes in silicon have been used to process optical signals with bandwidths greater than 1 GHz (ref. 2).  However, fundamental physical processes for a silicon-based optical modulator in the terahertz bandwidth range have not yet beeen explored.  Here, we deomonstrate ultrafast phononic modulation of the optical index of silicon by irradiation with intense few-cycle femtosecond pulses.  The anisotropic reflectivity modulation by the resonant Raman susceptibility at the fundamental frequency of the longitudinal optical phonon of silicon (15.6 THz) generates a frequency comb up to seventh order.  All-optical >100 THz frequency comb generation is realized by harnessing the coherent atomic motion of the silicon crystalline lattice at its highest mechanical frequency.

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Frequency comb observed in the Fourier transform spectrum