Abstract
We explore the coherent phonon induced refractive index modulation of a Si(001) surface upon the excitation in near-resonance with the direct band gap of Si. Through the anisotropic e-h pair generation and coherent Raman scattering, ~ 10-fs laser pulses exert a sudden electrostrictive force on Si lattice launching coherent LO phonon oscillations at 15.6 THz frequency. The concomitant oscillatory change in the optical constants modulates the reflected probe light at the fundamental LO phonon frequency, generating a broad comb of frequencies at exact integer multiples of the fundamental frequency extending to beyond 100 THz. On the basis of an analytical model, we show that the simultaneous amplitude and phase modulation of the reflected light by the coherent lattice polarization at 15.6 THz generates the frequency comb.
Highlights
The coherent modulation of Raman transition in gas phase atoms and molecules by intense few-cycle pulses has been used to generate Raman frequency combs that span multiple octaves [1, 2]
On the basis of an analytical model, we show that the simultaneous amplitude and phase modulation of the reflected light by the coherent lattice polarization at 15.6 THz generates the frequency comb
We demonstrate the phononic modulation of the optical index of Si for excitation and probing near its direct band gap ( ≈ 3.4 eV)
Summary
The coherent modulation of Raman transition in gas phase atoms and molecules by intense few-cycle pulses has been used to generate Raman frequency combs that span multiple octaves [1, 2]. Similar Raman frequency combs can be generated efficiently in solids on account of the high Raman polarizability densities. In the case of the Raman active phonon mode in solids, large amplitude coherent optical phonons could modulate light at multiple phonon frequencies in THz range. We demonstrate the phononic modulation of the optical index of Si for excitation and probing near its direct band gap ( ≈ 3.4 eV). The large amplitude coherent longitudinal optical (LO) phonon of Si (001) excited by 10-fs laser pulses induces effective amplitude and phase modulation on the reflected probe light, generating a phonon frequency comb with more than 100 THz bandwidth
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