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Extreme-Light Seeded Control of Ultrafast Laser Material Modifications

Periodic Reporting for period 2 - EXSEED (Extreme-Light Seeded Control of Ultrafast Laser Material Modifications)

Reporting period: 2018-11-01 to 2020-04-30

High-peak power compact femtosecond lasers are the basis for high-precision laser micro-fabrication. They also create extreme conditions within the matter, leading to the generation of rainbow light used to produce even shorter pulses and new frequencies that can extend from the X-ray to the TeraHertz domain. However, due to the low conversion efficiences, these attractive light pulses remain unexploited in the context of laser nano-/micro-fabrication. The main objective of this project is to exceed the intrinsic limits of ultrafast laser material processing by developing novel seeded-control technologies with modest energy extreme light pulses. The project includes the study of interactions seeded with deep-ultraviolet, few-optical-cycle and mid-infrared ultrashort pulses.
By seeding interaction with infrared femtosecond pulses from new emerging sources, the nonlinear processes with these radiations open new and exciting opportunities to tailor material properties in the three dimensions (3D) for materials inside which the occurrence of breakdown is, today, inaccessible. On this front we concentrate on developing reliable solution for 3D writing inside materials as important as silicon. We envision to develop a new field of index engineering laser-induced transformation inside silicon. This must lead to the first demonstrations of rapid 3D prototyping by laser of silicon photonics microdevices.
By investigating material responses with shortest pulses and wavelengths (UV), we revisit the general question of resolution in the field of ultrafast laser surface processing. An important objective is to develop solution for routinely achieving machining with nanometer precision. This would open the door to nanosurgery of cells or of DNA, and to tailoring dielectric surfaces with unprecedented resolution. A long term objective is to open the door to the use of the most extreme ultrashort laser-induced radiations, including extreme-ultraviolet attosecond pulses that hold promises to reach the highest degree of control in the time and space of the interactions.
These and other ideas require investigations on ionization physics by ultrashort pulses at extreme wavelengths. They also require tight control of the ultrafast pulses, broadband manipulations and novel interaction diagnostics technologies that are developed as parts of the project.
The broadband spectral study of material response to ultrafast light in the context of this research program have already made possible important knowledge and conceptual advances accompanied with experimental proof-of-concept demonstrations.
As major achievement with infrared radiations, we have identified most physical fundamental aspects causing the strong limitations in space-time energy confinement inside Si with femtosecond lasers. Based on this knowledge, we revealed the first solution for three dimensional writing inside this important material for which the occurrence of ultrafast breakdown was, until today, inaccessible. After a proof-of-principle experimental demonstration with hyper-focused beams, our work now concentrates today on more practical methodologies with manipulation in the time domain due to new understanding of the dynamical aspects of these interaction. The target remains to achieve the first demonstrations of rapid 3D prototyping by laser writing of silicon photonics microdevices when appropriate transformations and performance will be reached. Adding the UV domain to the investigations and developing the methodologies for proper quantitative comparisons, we have revealed a general misconception for achieving ultra-high resolution in the field. These findings are today exploited for new designing and performing unique multi-pulse, multi-color experiments aiming at unprecedented control for material tailoring, constituting a core objective for the project.
Directly derived from the project, several novel methodologies have been proposed leading to results beyond the state. A solid immersion focusing strategy has been demonstrated as the sole identified solution, to date, for local transformation inside silicon in 3D space with femtosecond laser pulses (Nature Communications 8 (2017) 773). To overcome the complexity of this scheme, the limitations of conventional regimes using longer pulse process have been studied. This led to report new methods for optical functionalization (Optics Letters 43 (2018) 6069-6072) and contrasted chemical etching methodologies for silicon material structuring (Optics Letters 44 (2019) 1619-1622). Exploiting time-domain new control parameters for internal structuring, including pulse stretching (eg. Physical Review Applied 12 (2019) 024009, Applied Physics A 124 (2018) 302) and new burst modes of irradiation (submitted) we today look for practical solutions using ultrashort pulse for technology transfers. A complete metrology methology has been developped to acess full set of data on the wavelength dependence of femtosecond laser surface transformations and reliable comparisons between radiations (submitted). On this basis, we work on developments toward extreme-UV based solutions for solving resolution limits in femtosecond laser surface structuring (in progress). At this stage these important advances still allow us to target by the end of the project the first proof of concept experiment exploiting an extreme radiation (either toward extreme UV or THz) in the context of ultrafast laser material processing.
Artistic view of the energy flow inside silicon with tighly focused ultrashort pulses