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Quantum Effects in Multicolor Ultrafast Laser Processing: Broadening Boundaries of Classical Descriptions

Periodic Reporting for period 1 - QuantumLaP (Quantum Effects in Multicolor Ultrafast Laser Processing: Broadening Boundaries of Classical Descriptions)

Reporting period: 2015-09-01 to 2017-08-31

One of the key challenges in material laser processing is extending the technological limits in controllable modification of material properties down to nanometer scale. Surfaces of different materials, being modified with creation of nanoscale structures, possess unique electronic, optical, mechanical, and catalytic properties. Achieving the control over laser production of such functional surfaces at low cost would lead to tremendous progress in information technologies, health care, public security, and smart energy solutions. However, the trial-and-error approach based on repeated attempts until success, which are usually used for optimization of laser surface processing is inefficient. In-depth understanding of the underlying physics of laser-induced surface modification can push this field from empirical methods to a smart computer-predicting technique. This can be realized via developing adequate theoretical models of laser-induced phenomena in solids, which is the aim of this ambitious project.
The EU funded project “Quantum effects in multicolor ultrafast laser processing: broadening boundaries of classical descriptions” (QuantumLaP) was focused on the interaction of ultrashort laser pulses with semiconductors (in the first turn, with silicon as one of the most demanded materials in real-life applications). It aimed at investigating laser-induced processes based on a new approach by taking into account quantum effects, which can influence the behavior of laser-irradiated materials. The impact of quantum phenomena on laser-induced dynamics of material excitation was studied for mono- and bi-chromatic irradiation regimes; the latter can be more efficient for various applications, compared to monochromatic light. The final goal was the development of a predictive model for material functionalization, allowing a fast assessment of final morphology of laser-treated surfaces.
To achieve the project goals, two models have been developed, a quantum-level, first-principles model describing the action of mono- and bi-chromatic laser light on semiconductors and a continuum model for direct comparison with quantum simulations. This enabled to verify and improve the existing theory of photoionization of solids. Furthermore, quantum simulations overturned existing concepts of bandgap material photoionization via insight into temporal dynamics of free charge-carrier excitation.
Finally, a predictive theory of laser-induced periodic surface structure (LIPSS) formation was developed, which explains the LIPSS regularities on different materials and enables selection of materials for ultrafast fabrication of high-regular structures. A numerical code developed for calculations of the LIPSS regularity can be accessed by external users. A database on the LIPSS regularities has been created for more than 60 materials and their combinations, opening ways for synthesis of new plasmonic materials, which could be cheaper than presently existing ones.
QuantumLaP successfully led to broadening the available knowledge on ultrafast laser excitation of bandgap materials. Project findings are opening new opportunities for advanced fabrication of functional surfaces with nanofeatures.
During the contract period, a comprehensive analysis of ultrafast laser excitation of semiconductors was performed in the regimes of modification of material properties. For this aim, two mathematical descriptions were elaborated, a most advanced continuum modelling of ultrafast laser action on material surfaces and a quantum modelling based on Time-Dependent Density Functional Theory (TD-DFT), and high performance computations were carried out. Following is an outline of the main project results:
(1) A most complete, up-to-date 2D model of ultrashort laser action on semiconductors has been developed, which enables to investigate temporal evolution of material parameters and simulate the reflectivity of semiconductor surfaces in dynamics for direct comparison with experimental data. The contribution to the reflectivity variation due to lattice heating and melting (including non-thermal melting) has been accounted for. A good quantitative agreement of the modelling and experimental data on time-dependent reflectivity has been achieved for silicon. The developed 2D code is flexible in respect of application to different semiconductors. Its 3D version has been elaborated, being though less flexible due to a high mathematical complexity. The developed models can serve as predictive tools for achieving a better control over laser-induced modification of semiconductor surfaces.
(2) Single- and bi-color photoionization rates for silicon have been computed from the first principles based on TD-DFT. For single-color ionization rates, the calculated data match the Keldysh theory relatively well if to account for imperfectness in the latter. The limits of the Keldysh model application were identified. An analytical Keldysh-like theory was extended to bi-color laser beams. The database of single- and bi-color ionization rates as a function of laser intensity for several wavelengths was built for silicon, which is of high demand for simulations of laser-matter interaction.
(3) A novel model was proposed and the corresponding code was developed in close collaboration with the secondment institution. In the model, the most important processes responsible for localized energy absorption by silicon were accounted for. An approach based on the Kohn–Sham equation enabled to account for the quasi-particle concept that is of paramount importance for laser-matter interaction. Although this has led to an increase of mathematical complexity, the results have exceeded expectations. The model has enabled to gain insight into of charge-carrier generation in dynamics, leading to a revision of the accepted views on laser-induced excitation of bandgap materials.
(4) A new theory of LIPSS formation was developed, which for the first time enabled to explain why LIPSS are highly regular on surfaces of some materials and irregular on others. It predicts the structure regularity on various materials for specific irradiation conditions as verified experimentally. A code for calculations of the LIPSS regularity has been built, which addresses pure materials and multi-layered structures. A database on the LIPSS regularities has been created for more than 60 materials and their combinations. This study opens ways for synthesis of new cheaper plasmonic materials.
The results of QuantumLaP are disseminated in 5 journal papers, presentations in international conferences (including 6 Invited Talks), patent application and via the project web-site where the computer codes developed within the project can be accessed by request.
A new explanation was proposed for the origin of the LIPSS regularity. Direct correlations between the LIPSS regularities and the characteristic propagation distances of surface scattered electromagnetic waves were uncovered theoretically and verified experimentally. The regularity of laser-induced nanostructures was predicted for metals and semiconductors (~60 materials) in a wide range of laser wavelength (from UV to mid-IR). As a result, new possibilities are opened for nanostructuring of material surfaces.
A typical snapshot of the electron cloud distribution in thin silicon film under irradiation