Abstract
The coherent modulation of electronic and vibrational nonlinearities in atoms and molecular gases by intense few-cycle pulses has been used for high-harmonic generation in the soft x-ray and attosecond regime, as well as for Raman frequency combs that span multiple octaves from the terahertz to petahertz frequency regions. In principle, similar high-order nonlinear processes can be excited efficiently in solids and liquids on account of their high nonlinear polarizability densities. In this paper, we demonstrate the phononic modulation of the optical index of Si and GaAs for excitation and probing near their direct band gaps, respectively at ∼3.4 and ∼3.0 eV. The large amplitude coherent longitudinal optical (LO) polarization due to the excitation of LO phonons of Si (001) and LO phonon–plasmon coupled modes in GaAs (001) excited by 10 fs laser pulses induces effective amplitude and phase modulation of the reflected probe light. The combined action of the amplitude and phase modulation in Si and GaAs generates phonon frequency combs with more than 100 and 60 THz bandwidth, respectively.
Highlights
Experimental: ultrafast electro-optic sampling measurementsThe anisotropic transient reflectivity of n-doped (1.0 × 1015 cm−3 ) Si(001) and n-doped
Relaxation of the coherent LOPC modes in n-GaAs has been studied using pump–probe techniques [20, 21]
As with the spontaneous Raman spectra, the LO phonon signal is enhanced by resonance with the direct band gap of Si [32]
Summary
The anisotropic transient reflectivity of n-doped (1.0 × 1015 cm−3 ) Si(001) and n-doped Cm−3 ) GaAs(001) samples were measured in air at 295 K by the electro-optic (e-o) sampling technique [3, 8]. Pump and probe beams (397 nm (3.12 eV) central wavelength) were overlapped at a 7.2 × 10–7 cm spot on the sample. All measurements in GaAs were carried out at or below 10 mW, corresponding to a maximum of. After reflecting from the sample, the probe was analysed into polarization components parallel and perpendicular to that of the pump and each was detected with a photodiode. (1Rk − 1Rl )/R0 = (|E k |2 –|El |2 )/|E 0 |2 ] was recorded versus the pump–probe delay, as shown, where E k and E l are the components of the reflected probe light. In order to examine the resonant behaviour in Si, the laser wavelength was varied between 2.99 eV (415 nm) and 3.16 eV (392 nm), which is the maximum tuning range obtained with our 10 fs laser pulse by adjusting the phase matching angle of the 50 μm thick β-BaB2 O4 (BBO) crystal for second-harmonic generation of the laser fundamental
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