Unraveling intra-pulse dynamics and fast energy transfer in silica glass - a pathway for smart processing using ultrafast lasers – INTRALAS

Nowadays micro/nano- technologies are critically dependent on the development of precise and controllable processing tools able to structure materials with utmost precision. Ultrashort laser processing appears as an ideal technology to take up this challenge, with intrinsic processing capabilities well into the nanoscale. To optimize structuring in terms of yield and scale the concept of smart laser material processing has emerged, based on the spatiotemporal design of irradiation to the material's response. Defining advanced processing strategies requires understanding the primary electronic processes governing laser energy deposition and relaxation paths (electronic vs vibrational) towards structural modifications. Little information is available at the moment. This pertains to processes occurring on the timescale of the pulse, notably material dynamics during the excitation phase. We target in this project functional glasses in view of their nonlinearities and fragile structures, and their potential for 3D design.
We propose a time-resolved introspection into electronic and structural evolution in fused silica upon ultrafast laser irradiation. The objective is to elucidate primary pathways of coupling and depositing energy during the timescale of the processing pulse. The choice of fused silica as a model material is justified by its technological interest and by the corpus of knowledge available.
We target two specific evolutions; band-gap dynamics and energy coupling to the matrix. The experimental procedures include original diagnostics methods based on time-resolved spectral interferometry with two specific approaches. Using time-resolved VUV ultrafast interferometry near the transmission cut-off (in the spectral region of the Urbach tail at the conduction edge) we aim to uncover optical bandgap dynamics during irradiation with an intense 50 fs laser pulse. The short VUV probe duration (2-3 fs) grants access to intrapulse dynamics, defining effects induced by the field and electronic population. This information can update existing scenarios of electron-hole plasma formation. Secondly, the dynamics of energy transfer will be interrogated. Using time-resolved vibrational spectroscopy of a marker embedded in the fused silica matrix (hydroxyl groups) and quantitative plasma imaging we illustrate the correlation of two mechanisms for energy transfer, strong molecular polarization coupling and collisional vibrational activation. This knowledge is essential to develop smart concepts for energy deposition and processing.

These experiments will be complemented by simulations of the photo-electronic processes and electronic structure evolution at quantum levels by ab initio density functional theory (DFT) and time-dependent DFT, with the ambition to enhance the current level of physical insights into a range of processes which are not considered by the current modeling approaches in laser processing.

Primary ionization: electronic population and band distortions
- Generation of the VUV probe pulses

Atomistic simulation of electronic effects:
- Determination of the band-gap dynamics

Primary ionization: electronic population and band distortions
-Time-resolved Fourier-Transform Spectral Interferometry

Energy transfer from electrons to the matrix
-Transient absorption spectroscopy in the Mid-IR spectral range
- Electronic decay

Atomistic simulation of electronic effects:
- Determination of the electron trapping nature, strength and lifetime
- Determination of the electron-phonon coupling time

1. E. Moreno, H. Nguyen, R. Stoian, J. P. Colombier, Full Explicit Numerical Modeling in Time-Domain for Nonlinear Electromagnetics Simulations in Ultrafast Laser Nanostructuring, Applied Sciences, vol. 11(16), pp. 7429 (2021)

2. Arshak Tsaturyan, Elena Kachan, Razvan Stoian, Jean-Philippe Colombier, submitted: «Ultrafast bandgap narrowing and cohesion loss of photoexcited fused silica«

Nowadays micro/nano- technologies are critically dependent on the development of precise and controllable processing tools able to structure materials with utmost precision. Ultrashort laser processing appears as an ideal technology to take up this challenge, with intrinsic processing capabilities well into the nanoscale. To optimize structuring in terms of yield and scale the concept of smart laser material processing has emerged, based on the spatiotemporal design of irradiation to the material's response. Defining advanced processing strategies requires understanding the primary electronic processes governing laser energy deposition and relaxation paths (electronic vs vibrational) towards structural modifications. Little information is available at the moment. This pertains to processes occurring ON the timescale of the pulse, notably material dynamics during the excitation phase. We target in this project functional glasses in view of their nonlinearities and fragile structures, and their potential for 3D design. We propose a time-resolved introspection into electronic and structural evolution in fused silica upon ultrafast laser irradiation. The objective is to elucidate primary pathways of coupling and depositing energy during the timescale of the processing pulse. The choice of fused silica as a model material is justified by its technological interest and by the corpus of knowledge available. We target two specific evolutions; band-gap dynamics and energy coupling to the matrix. The experimental procedures include original diagnostics methods based on time-resolved spectral interferometry with two specific approaches. Using time-resolved VUV ultrafast interferometry near the transmission cut-off (in the spectral region of the Urbach tail at the conduction edge) we aim to uncover optical bandgap dynamics during irradiation with an intense 50 fs laser pulse. The short VUV probe duration (sub-4 fs) grants access to intrapulse dynamics, defining effects induced by the field and electronic population. This information can update existing scenarios of electron-hole plasma formation. Secondly, the dynamics of energy transfer will be interrogated. Using time-resolved vibrational spectroscopy of a marker embedded in the fused silica matrix (hydroxyl groups) and quantitative plasma imaging we illustrate the correlation of two mechanisms for energy transfer, strong molecular polarization coupling and collisional vibrational activation. This knowledge is essential to develop smart concepts for energy deposition and processing. These experiments will be complemented by simulation of the photo-electronic processes and electronic structure evolution at quantum levels by ab-initio density functional theory (DFT) and time-dependent DFT, with the ambition to enhance the current level of physical insights into a range of processes which are not considered today by the current modeling approaches in laser processing.