Periodic Reporting for period 4 - EXSEED (Extreme-Light Seeded Control of Ultrafast Laser Material Modifications)
Reporting period: 2021-11-01 to 2022-10-31
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 low conversion efficiencies, these attractive light pulses remain unexploited in the context of laser nano-/micro-fabrication. The main objective of this project was to exceed the intrinsic limits of ultrafast laser material processing by developing novel seeded-control technologies with modest energy extreme light pulses.
By investigating material responses with the shortest pulses and wavelengths (UV), we have revisited the general question of resolution in the field of ultrafast laser surface processing. A fulfilled objective is the development of solutions for routinely tailoring dielectric surfaces at the highest precision level. The demonstrations support the use of ultra-stable extreme UV light to reach the nanometer precision that would open the door to nanosurgery of cells or of DNA.
By seeding interaction with mid-infrared femtosecond pulses, the triggered nonlinear processes have confirmed new opportunities to tailor material properties in the three dimensions (3D) for materials inside which the occurrence of breakdown was, until today, inaccessible. On this front we have concentrated on developing reliable solutions for 3D writing inside silicon. We contributed to the advent of refractive index engineering by laser-induced transformation inside silicon with the first demonstrations of 3D silicon photonics microdevices. For the introduction of the most extreme radiation in this context, we developed a unique multi-beam experimental arrangement including a secondary THz radiation expected to shed light on the nature and functionality of the written structures.
These and other developments required investigations on ionization physics, tight control of the ultrafast pulses, broadband manipulations and novel interaction diagnostics technologies that constitute other important outputs from the project.
By investigating material responses with the shortest pulses and wavelengths (UV), we have revisited the general question of resolution in the field of ultrafast laser surface processing. A fulfilled objective is the development of solutions for routinely tailoring dielectric surfaces at the highest precision level. The demonstrations support the use of ultra-stable extreme UV light to reach the nanometer precision that would open the door to nanosurgery of cells or of DNA.
By seeding interaction with mid-infrared femtosecond pulses, the triggered nonlinear processes have confirmed new opportunities to tailor material properties in the three dimensions (3D) for materials inside which the occurrence of breakdown was, until today, inaccessible. On this front we have concentrated on developing reliable solutions for 3D writing inside silicon. We contributed to the advent of refractive index engineering by laser-induced transformation inside silicon with the first demonstrations of 3D silicon photonics microdevices. For the introduction of the most extreme radiation in this context, we developed a unique multi-beam experimental arrangement including a secondary THz radiation expected to shed light on the nature and functionality of the written structures.
These and other developments required investigations on ionization physics, tight control of the ultrafast pulses, broadband manipulations and novel interaction diagnostics technologies that constitute other important outputs from the project.
EXSEED researches primarily relied on the studying of femtosecond laser interactions at non-conventional driving wavelengths.
The range of nonlinear responses accessible by radiation tuning allowed revisiting questions as important as the achievable precision in laser surface machining technologies. In particular, we have established that the concept of nonlinear resolution is not applicable for femtosecond laser ablation [Optics Letters 45, 952-955 (2020)]. This is because any observable based on a threshold-based response simply ruins all potential benefits that could be expected on resolution from the nonlinear confinement of absorption. Another important consequence is that the achievable precision and repeatability can be directly derived from the level of determinism of the interaction. By comparing the results of a simple 'noise' model accounting for laser fluctuations, we have quantified the precision limits [Applied Physics Letters 117, 171604. (2020)] proposed an innovation to improve processing performances [Optics Letters 47 (2022) 993].
At the opposite side of the spectrum, ultrashort infrared laser pulses opened a way for internal structuring of semiconductor materials that are extremely challenging to process in the three dimensions (3D writing). Our first proposed solution used hyper-focused beams to demonstrate microplasmas up to permanent material modifications in the bulk of silicon with sub-100-fs pulses [Nature Communications 8, 773 (2017).]. For more practical alternatives, we worked on optimizations in the time domain. We investigated the picosecond regime limiting the nonlinearities and improving the process reliability [Physical Review Applied 12, 024009 (2019); Optics Express 28, 26623 (2020)]. We revealed that an even more promising approach is to rely on transient accumulation strategies. To this aim, we generated and applied ultrafast trains of pulses at up to Terahertz repetition-rates [Research 8149764 (2020), International Journal of Extreme Manufacturing 4 (2022) 045001]. An important aspect also addressed by our experiments is the unusually high sensitivity of 3D writing in semiconductors to the temporal contrast of the pulses [Physical Review Research 2, 033023 (2020)]. This causes laser-technology dependent results and represents an important finding for a comprehensive reading of the literature on this topic [Laser and Photonics Reviews (2021) 2100140]. Taken these approaches together, we recently introduced unique multi-timescale control parameters holding promises for future exploitation in the semiconductor industry. As an illustration of innovative process solutions deriving from this knowledge, we can note the first demonstration a transmission laser welding method applicable to semiconductors [Laser and Photonics Reviews 16 (2022) 2200208]. Among the general performance advances, this work culminates with a method for enhanced precision (incl. resolution and material change controllability) writing deep inside silicon chips. This holds promises for commercial exploitations and is today the subject of a patent application [EPEP21184898.1 R37868EP, PCT/EP2022/068835].
The range of nonlinear responses accessible by radiation tuning allowed revisiting questions as important as the achievable precision in laser surface machining technologies. In particular, we have established that the concept of nonlinear resolution is not applicable for femtosecond laser ablation [Optics Letters 45, 952-955 (2020)]. This is because any observable based on a threshold-based response simply ruins all potential benefits that could be expected on resolution from the nonlinear confinement of absorption. Another important consequence is that the achievable precision and repeatability can be directly derived from the level of determinism of the interaction. By comparing the results of a simple 'noise' model accounting for laser fluctuations, we have quantified the precision limits [Applied Physics Letters 117, 171604. (2020)] proposed an innovation to improve processing performances [Optics Letters 47 (2022) 993].
At the opposite side of the spectrum, ultrashort infrared laser pulses opened a way for internal structuring of semiconductor materials that are extremely challenging to process in the three dimensions (3D writing). Our first proposed solution used hyper-focused beams to demonstrate microplasmas up to permanent material modifications in the bulk of silicon with sub-100-fs pulses [Nature Communications 8, 773 (2017).]. For more practical alternatives, we worked on optimizations in the time domain. We investigated the picosecond regime limiting the nonlinearities and improving the process reliability [Physical Review Applied 12, 024009 (2019); Optics Express 28, 26623 (2020)]. We revealed that an even more promising approach is to rely on transient accumulation strategies. To this aim, we generated and applied ultrafast trains of pulses at up to Terahertz repetition-rates [Research 8149764 (2020), International Journal of Extreme Manufacturing 4 (2022) 045001]. An important aspect also addressed by our experiments is the unusually high sensitivity of 3D writing in semiconductors to the temporal contrast of the pulses [Physical Review Research 2, 033023 (2020)]. This causes laser-technology dependent results and represents an important finding for a comprehensive reading of the literature on this topic [Laser and Photonics Reviews (2021) 2100140]. Taken these approaches together, we recently introduced unique multi-timescale control parameters holding promises for future exploitation in the semiconductor industry. As an illustration of innovative process solutions deriving from this knowledge, we can note the first demonstration a transmission laser welding method applicable to semiconductors [Laser and Photonics Reviews 16 (2022) 2200208]. Among the general performance advances, this work culminates with a method for enhanced precision (incl. resolution and material change controllability) writing deep inside silicon chips. This holds promises for commercial exploitations and is today the subject of a patent application [EPEP21184898.1 R37868EP, PCT/EP2022/068835].
Directly derived from the project, several novel methodologies have been proposed leading to results beyond the state of the art. A solid immersion focusing strategy has been demonstrated as the sole identified solution, to date, for initiate local transformations inside silicon in 3D space with highly-contrasted femtosecond laser pulses [Nature Communications 8 (2017) 773]. To overcome the complexity of this scheme, the limitations of conventional regimes using longer pulse process has been studied. This led to the report of innovative methods for optical functionalization [Optics Letters 43 (2018) 6069-6072],as well as positive and negative manufacturing of semiconductor devices (incl. laser-assisted etching, dicing, welding [Optics Letters 44 (2019) 1619-1622, International Journal of Extreme Manufacturing 4 (2022) 045001, Laser and Photonics Reviews 16 (2022) 2200208] . With the introduction of new control parameters in the time domain have been revealed way to adjust individually the local plasma and thermal conditions inside semiconductors [Physical Review Research 2, 033023 (2020)]. This is currently the core of a patent application [EPEP21184898.1 R37868EP, PCT/EP2022/068835] and is expected to be the subject of important additional publications. In parallel, recent efforts have concentrated on making available strong-field THz radiations in these experiments. This brings extreme radiations in this context which was another major objective of the project. More generally, a unique methodology has been developed to generate full sets of data on the wavelength dependence of femtosecond laser material transformations [Open Research Europe (2021) 1:7 ]. On this basis, EXSEED worked on developments toward extreme-UV based solutions for solving resolution limits in femtosecond laser surface structuring [Optics Letters 47 (2022) 993]. Overall, these represent important advances to prepare the future of ultrafast laser material processing.