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Frequency comb generation at terahertz frequencies by coherent phonon excitation in silicon


High-order nonlinear light–matter interactions in gases enable the generation of X-ray and attosecond light pulses, metrology and spectroscopy1. Optical nonlinearities in solid-state materials are particularly interesting for combining optical and electronic functions for high-bandwidth information processing2. 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 been explored. Here, we demonstrate 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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Figure 1: Schematic of refractive index modulation in silicon.
Figure 2: Coherent phonon oscillations.
Figure 3: Fourier transform spectrum obtained from the transient anisotropic reflectivity.


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The authors acknowledge M. Kitajima for stimulating discussions. This work was supported in part by the National Science Foundation (grant no. CHE-0650756).

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M.H. and A.M.C. performed the experiments and analysed data. M.K. constructed the simulation model and M.H. carried out the model simulation. M.H., M.K. and H.P. discussed the results. M.H. and H.P. co-wrote the manuscript.

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Correspondence to Muneaki Hase or Hrvoje Petek.

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Hase, M., Katsuragawa, M., Constantinescu, A. et al. Frequency comb generation at terahertz frequencies by coherent phonon excitation in silicon. Nature Photon 6, 243–247 (2012).

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