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

Self-phase modulation (SPM) broadening of prompt laser spectra was studied in a transmission mode in natural and synthetic diamonds at variable laser wavelengths (515 and 1030 nm), pulse energies and widths (0.3–12 ps, positively chirped pulses), providing their filamentary propagation. Besides the...

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Bibliographic Details
Published in:	Photonics Vol. 10; no. 12; p. 1316
Main Authors:	Kudryashov, Sergey, Danilov, Pavel, Gorevoy, Alexey, Kovalov, Volodymyr, Kosobokov, Mikhail, Akhmatkhanov, Andrey, Lisjikh, Boris, Turygin, Anton, Greshnyakov, Evgeny, Shur, Vladimir
Format:	Journal Article
Language:	English
Published:	Basel MDPI AG 01-12-2023
Subjects:	Atomic force microscopy Bragg gratings Cross-sectional studies Damping diamond Diamonds Dielectrics Energy Femtosecond pulsed lasers Femtosecond pulses Filaments Filtration Frequency dependence Frequency modulation Holes (electron deficiencies) Inscriptions Lasers Lithium Lithium niobate Lithium niobates Microfilaments Microscopy Nitrogen Periodicals Phase modulation Plasma Propagation Radiation self-phase modulation broadening Sidebands Spectra Topography transmission spectra ultrashort-pulse laser Wavelengths Russia Lithuania Japan
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Summary:	Self-phase modulation (SPM) broadening of prompt laser spectra was studied in a transmission mode in natural and synthetic diamonds at variable laser wavelengths (515 and 1030 nm), pulse energies and widths (0.3–12 ps, positively chirped pulses), providing their filamentary propagation. Besides the monotonous SPM broadening of the laser spectra versus pulse energy, which was more pronounced for the (sub)picosecond pulsewidths and more nitrogen-doped natural diamond with its intra-gap impurity states, periodical low-frequency modulation was observed in the spectra at the shorter laser pulsewidths, indicating dynamic Bragg filtering of the supercontinuum due to ultrafast plasma and nanoplasmonic effects. Damping of broadening and ultra-modulation for the longer picosecond pulsewidths was related to the thermalized electron-hole plasma regime established for the laser pulsewidths longer, than 2 ps. Unexpectedly, at higher pulse energies and corresponding longer, well-developed microfilaments, the number of low-intensity, low-frequency sideband spectral modulation features counterintuitively increases, thus indicating dynamic variation of the periods in the longitudinal plasma Bragg gratings along the filaments due to prompt secondary laser–plasmon interactions. The underlying sub- and/or near-wavelength longitudinal nanoscale Bragg gratings produced by femtosecond laser pulses in this regime could be visualized in less hard lithium niobate by atomic force microscopy cross-sectional analysis in the correlation with the corresponding sideband spectral components, supporting the anticipated Bragg filtering mechanism and envisioning the corresponding grating periods.
ISSN:	2304-6732 2304-6732
DOI:	10.3390/photonics10121316