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

Abstract

In synthetic diamond plates, the intrapulse-correlated dynamics of self-phase modulation and spontaneous nonresonant Raman scattering by center-zone optical phonons were for the first time directly investigated for tightly focused (focusing numerical aperture NA = 0.25) positively chirped visible-range high-intensity laser pulses with variable durations (0.3–9.5 ps) and energies transmitted through the sample. The observed self-phase modulation broadening and modulation of the transmitted light and Stokes Raman spectra for the (sub)picosecond pulse durations indicate the considerable Raman–Kerr contribution to the nonlinear polarization. The latter appears through plasma emission of the optical phonons, which emerges on the (sub)picosecond timescale and dominates at ≈1 ps. Later, this phonon contribution is eventually suppressed in the material due to picosecond-scale electron-lattice thermalization and the related thermally enhanced symmetrical decay of optical phonons into lower-frequency acoustic ones.

Periodical Ultra-Modulation of Broadened Laser Spectra in Dielectrics at Variable Ultrashort Laser Pulsewidths: Ultrafast Plasma, Plasmonic and Nanoscale Structural Effects

“…In this study, optical transmittance spectra of ultrashort (0.3-12 ps) positively chirped laser pulses at 515 nm and 1030 nm wavelengths and variable pulse energy were acquired in the pre-and filamentation regimes in bulk natural and synthetic diamonds to search, for the first time, in situ for spectral signatures of dynamic electron-hole plasma density modulation related to non-local laser-plasmonic interactions. Compared to our previous spectral studies of ultrashort chirped-pulse laser self-transmittance in synthetic diamond [19], in the present work, low-intensity, low-frequency sideband spectral components were revealed and analyzed for the first time both in bulk natural and synthetic diamonds in order to identify such spectral signatures. The related bulk structural modifications produced in this laser irradiation regime in crystalline lithium niobate were visualized by atomic force microscopy cross-sectional analysis and for the first time correlated with the onset of such sideband components in the material.…”

“…Meanwhile, at later picosecond times, such non-linear polarization related to optical phonons should be damped by anharmonic decay of optical phonons into pairs of acoustic ones via the Klemens mechanism [27], being facilitated by electron-phonon thermalization after 2-3 ps [28]. These picosecond effects in spectral broadening of ultrashort laser pulses and related lattice effects were recently overviewed in [19]. However, due to the positive chirping of the pulses Previously, such pulsewidth-dependent broadening of ultrashort-laser pulses was related to Kerr-like self-phase modulation and EHP screening, enhanced for the longer (picosecond) pulsewidths by the Raman-Kerr effect [18,23,24] driven by prompt phonon excitations [19,[23][24][25][26].…”

“…Meanwhile, at later picosecond times, such non-linear polarization related to optical phonons should be damped by anharmonic decay of optical phonons into pairs of acoustic ones via the Klemens mechanism [27], being facilitated by electron-phonon thermalization after 2-3 ps [28]. These picosecond effects in spectral broadening of ultrashort laser pulses and related lattice effects were recently overviewed in [19]. However, due to the positive chirping of the pulses (leading "red" and trailing "blue" frequencies) for longer than 0.3 ps, one could observe not only the pulsewidth-dependent spectral changes, but also some temporal dynamics in the spectrally acquired laser-diamond interaction, e.g., more SPM broadening at the longer wavelengths (the leading pulse front, less plasma) and slightly stronger spectral modulation for the shorter wavelengths (the trailing pulse part, more photogenerated plasma) in Figures 2, 4 and 5.…”

“…These picosecond effects in spectral broadening of ultrashort laser pulses and related lattice effects were recently overviewed in [19]. However, due to the positive chirping of the pulses Previously, such pulsewidth-dependent broadening of ultrashort-laser pulses was related to Kerr-like self-phase modulation and EHP screening, enhanced for the longer (picosecond) pulsewidths by the Raman-Kerr effect [18,23,24] driven by prompt phonon excitations [19,[23][24][25][26]. The two former contributions are directly driven by pulse energy/pulsewidth ratio; however, the latter, Raman-Kerr contribution induced, e.g., by prompt excitation of coherent optical phonons [25,26], could appear on a picosecond timescale as a pulsewidth-invariant energy threshold value [23].…”