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

At a Glance

Metadata	Details
Publication Date	2023-05-29
Journal	Photonics
Authors	S. I. Kudryashov, П. А. Данилов, Jiajun Chen
Institutions	P.N. Lebedev Physical Institute of the Russian Academy of Sciences
Citations	1
Analysis	Full AI Review Included

Executive Summary

This research investigates the correlated dynamics of Self-Phase Modulation (SPM) and Spontaneous Raman Scattering (SRS) in synthetic diamond under excitation by positively chirped ultrashort laser pulses (USPs). The findings provide critical insights into the interplay between optical, electronic, and lattice effects during high-intensity laser processing.

Core Achievement: First direct investigation of the intrapulse-correlated dynamics of SPM and SRS in synthetic diamond using chirped USPs, revealing the temporal evolution of nonlinear polarization.
Raman-Kerr Contribution: The delayed phonon-based Raman-Kerr nonlinearity is confirmed to be a considerable contributor to the overall nonlinear polarization, dominating on the sub-picosecond timescale (τ < 1 ps).
Thermalization Dynamics: The efficiency of the Raman-Kerr effect is suppressed at longer pulse durations (greater than 1 ps) due to fast electron-lattice thermalization (characteristic time: 1-2 ps) and subsequent thermally enhanced symmetrical decay of optical phonons into acoustic modes.
Plasma Regime Confirmation: The nearly linear dependence of the integrated Raman signal intensity on pulse energy (angular slope 0.8-1.2) confirms that the interaction occurs within a characteristic near-critical dense electron-hole plasma regime.
SPM Asymmetry: Spectral broadening due to SPM is asymmetric: blue-wing asymmetry for short pulses (0.3 ps) is attributed to plasma shielding and delayed phonon effects, while red-wing asymmetry for intermediate pulses (1.3 ps) is linked to stronger phonon-based Raman-Kerr contribution at the leading pulse front.

Technical Specifications

Parameter	Value	Unit	Context
Material Type	Type IIa	N/A	Synthetic Diamond Cube
Sample Dimensions	2 x 2 x 2	mm	Polished facets
Laser Wavelength (λ)	515	nm	Second Harmonic of Yb-laser
Spectral FWHM (Initial)	1.3 (or ~63)	nm (cm^-1)	Unchirped pulse bandwidth
Pulse Duration (τ) Range	0.3 - 9.5	ps	Positively chirped
Repetition Rate	10	kHz	Laser operation frequency
Focusing Numerical Aperture (NA)	0.25	N/A	Micro-objective focusing
Focal Spot Radius (1/e)	<2	µm	Pre-filamentation regime
Peak Fluence (Pre-fil.)	~2	J/cm²	At 200 nJ threshold
Peak Intensity (Pre-fil.)	~7	TW/cm²	At 200 nJ threshold
Filamentation Threshold Energy (E_th)	210-230	nJ	Onset of nonlinear focusing
Raman Shift (Zone-Center Optical Phonon)	~1340	cm^-1	Spontaneous Raman signal
Raman Intensity Slope (vs. Energy)	0.8-1.2	N/A	Linear dependence in plasma regime
Electron-Lattice Thermalization Time	1-2	ps	Characteristic timescale in diamond

Key Methodologies

The experiment utilized a dynamic spectroscopic approach combining variable-chirp ultrashort pulses with tight focusing to probe nonlinear interactions in synthetic diamond.

Laser Source and Chirping: A Yb-laser system (Satsuma) provided 515 nm pulses. Pulse duration was varied (0.3-9.5 ps) via partial positive chirping (incomplete compression of stretched pulses).
Energy Control: Pulse energy was adjusted using a thin-film transmission attenuator across a range of 50-800 nJ.
Tight Focusing: Pulses were tightly focused within the diamond volume using a micro-objective (NA = 0.25), achieving a focal spot radius less than 2 µm, enabling both pre-filamentation and filamentation regimes.
Transmission and Collection: Transmitted radiation was collected by a fluorite microscope objective (NA = 0.2).
Spectroscopic Analysis: The collected light was guided to a spectrometer (ASP-190) to simultaneously measure:
- Self-Phase Modulation (SPM): Spectral broadening and modulation of the transmitted laser pulse.
- Spontaneous Raman Scattering (SRS): Intensity and spectral characteristics of the 1340 cm^-1 zone-center optical phonon signal.
Data Integrity: Spectra were accumulated over 10-s intervals at a 10 kHz repetition rate. The sample was translated 50 µm after each acquisition to ensure interaction with fresh, unexposed material.

Commercial Applications

The precise control and understanding of nonlinear dynamics in diamond under high-intensity USP excitation are critical for several high-value engineering sectors:

Ultrafast Laser Microfabrication:
- 3D Waveguide Inscription: Optimizing laser parameters (chirp, duration, energy) to precisely control the refractive index changes and structural features required for integrated photonic circuits in diamond.
- High-Density Data Storage: Developing robust, multi-layer optical data storage solutions by controlling the localized structural transformations induced by USPs.
Quantum Sensing and Computing:
- Color Center Encoding: Improving the efficiency and spatial resolution of laser writing techniques used to create optically active color centers (e.g., NV centers) in diamond for quantum sensing and single-photon sources.
- Thermal Management: Utilizing the determined electron-lattice thermalization time (1-2 ps) to select optimal pulse parameters that minimize thermal damage and maximize coherence times during defect creation.
Nonlinear Optics and Frequency Conversion:
- Raman Lasers and Amplifiers: Leveraging the understanding of the delayed Raman-Kerr nonlinearity to design highly efficient diamond-based stimulated Raman scattering devices, particularly those operating in the sub-picosecond regime.
High-Power Laser Components:
- Diamond’s superior properties make it ideal for high-power optics. This research provides fundamental limits and operational regimes related to plasma formation and nonlinear effects, ensuring component reliability under extreme optical loads.

View Original 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.

Tech Support

Original Source

DOI: https://doi.org/10.3390/photonics10060626

References

2014 - Ultrafast Lasers-Reliable Tools for Advanced Materials Processing [Crossref]
2022 - 100-Layer Error-Free 5D Optical Data Storage by Ultrafast Laser Nanostructuring in Glass [Crossref]
2007 - Femtosecond Filamentation in Transparent Media [Crossref]
2011 - Anatomy of a femtosecond laser processed silica waveguide [Crossref]
2015 - In the heart of femtosecond laser induced nanogratings: From porous nanoplanes to form birefringence [Crossref]
2010 - Peculiarities of Filamentation of Sharply Focused Ultrashort Laser Pulses in Air [Crossref]
2022 - High-NA Focusing of Ultrashort Laser Pulses in Bulk of ZnSe [Crossref]
2010 - Generation of Coherent Terahertz Phonons by Sharp Focusing of a Femtosecond Laser Beam in the Bulk of Crystalline Insulators in a Regime of Plasma Formation [Crossref]