Raman Scattering Induced by Strongly Nonequilibrium Phonons

Intrapulse Correlated Dynamics of Self-Phase Modulation and Spontaneous Raman Scattering in Synthetic Diamond Excited and Probed by Positively Chirped Ultrashort Laser Pulses

“…The latter effect is observed even for ultrashort laser pulses with a duration of 0.3 ps (weak dispersive chirping upon passage through diamond) but reaches saturation for the positively chirped ultrashort laser pulses with τ=1.3 ps, simultaneously weakening the electronic contribution and manifesting stronger on the leading "red" edge of the ultrashort laser pulse. For the ultrashort laser pulses with τ=2.4 ps, due to electron-hole plasma thermalization with the crystal lattice (characteristic time -1-2 ps [2]) and weakened electronic contribution, the "delayed" nonlinearity manifests more strongly in SPM on the leading ("red") edge of the ultrashort laser pulse, while on the trailing edge, thermal filling of low-frequency acoustic phonon modes should weaken lattice polarization on Raman-active zone-center optical phonons due to acceleration of their symmetric decay into acoustic phonons [19,20]. Finally, for the long, low-intensity ultrashort laser pulses with τ=9.5 ps, the electronic and phonon components of Kerr non-linearity will be equally weakly expressed for heated or even melted material.…”

“…4). Effective Raman generation is predominantly realized on the "red" wing, i.e., at the beginning of the chirped ultrashort laser pulses, where it is not yet weakened by thermally accelerated symmetric decay of Raman-active zone-center optical phonons into low-frequency acoustic phonons [19,20]. Despite clear filamentation of ultrashort laser pulses manifested as luminescent micro-tracks and pronounced self-phase modulation broadening and modulation of the laser spectrum at energies above 200 nJ, a linear output of the Raman signal is observed in the entire range of E, with a very narrow main peak (≈15 cm⁻¹) shifted to ≈1294 cm⁻¹.…”

Intrapulse Correlated Dynamics of Self-Phase Modulation and Spontaneous Raman Scattering in Synthetic Diamond Excited and Probed by Positively Chirped Ultrashort Laser Pulses

“…At the "red" wing, this may indicate the rapidly increasing Bose-like population of the optical-phonon mode and the rise of the related phonon-based nonlinear polarization with the characteristic 1-ps timescale. In contrast, at the "blue" pulse edge, the optical-phonon Raman mode could be depopulated yet by thermally accelerated symmetric decay of the optical phonons into two lower-frequency counterpropagating acoustic phonons with equal energies (half of the optical phonon energy) and wavenumbers [30,31]. This process is supported by the fast electron-hole plasma thermalization with the crystalline lattice (characteristic time: 1-2 ps [2]).…”

“…During such picosecond pulses, due to fast electron-hole plasma thermalization with the crystalline lattice (characteristic time-1-2 ps [2]) and pulsewidth-weakened electronic contribution, the delayed Raman-Kerr nonlinearity could be damped by phonon-phonon anharmonicity at the leading ("red") edge of the ultrashort laser pulses. Specifically, thermal filling of low-frequency acoustic phonon modes should weaken the lattice polarization based on Raman-active zone-center optical phonons due to the acceleration of their symmetric decay into acoustic phonons [30,31]. Finally, for the long, low-intensity ultrashort laser pulses with τ = 9.5 ps, the electronic Kerr contribution to the nonlinear polarization will be weakly expressed but it holds for the heated or even melted material in the focal region, compared to the negligible Raman-Kerr contribution.…”

Level broadening effects in quantum kinetic equations: Hot luminescence from a quantum wire near the optical-phonon-emission threshold

“…In order to obtain equations for Γ α (t, ǫ) and ∆ α (t, ǫ) one has to substitute an expression for the retarded self energy function, in terms of the one particle Green functions, into the definitions of Γ and ∆ (8). These equations are in fact the imaginary and the real parts of equation (17).…”