Femtosecond laser pulses are extremely short bursts of laser light that last for a duration of one quadrillionth of a second (10^-15 seconds). These ultrafast laser pulses are created using femtosecond laser technology, which involves using a mode-locked laser to generate pulses with extremely short durations.
Researchers at the RIKEN Center for Advanced Photonics (RAP) who study laser applications have shown that special two-dimensional (2D) LIPSS on silicon substrates may be produced by GHz burst mode femtosecond laser pulses.
The GHz burst mode femtosecond laser pulses, which consist of a series of trains of extremely brief laser pulses with a pulse interval of several hundred picoseconds (ps), have previously been reported to significantly improve ablation efficiency and quality when compared to conventional femtosecond laser processing (single-pulse mode).
To demonstrate the capability of GHz burst for development of distinctive nanostructures, the scientists used the GHz burst mode to synthesize LIPSS on silicon substrates and published their findings in International Journal of Extreme Manufacturing (IJEM). They have demonstrated that the GHz burst mode femtosecond laser pulses create unique 2D LIPSS, that is distinct from the 1D structures fabricated by the conventional single-pulse mode of linearly polarized femtosecond laser.
In addition to the 1D LIPSS produced by the single-pulse mode, whose direction is parallel to the laser polarization, another periodic structure is produced by the GHz burst mode parallel to the laser polarization direction to produce a pattern resembling a lattice.
The results of this study have demonstrated that femtosecond laser processing in the GHz burst mode has distinct advantages for not only the ablation of materials but also for other forms of processing including LIPSS generation, opening up new possibilities for micro and nanofabrication.
Ultrashort pulse laser subtractive manufacturing is induced by three dominant phenomena of nonlinear optical absorption inside the material, energy transfer such as electron-electron scattering and electron-lattice scattering, and phonon excitation, and ablation. Here, the relaxation time of the excited state of the material is in a region of femtosecond to sub-microsecond. The conventional single-pulse mode induces ablation by the interaction process between femtosecond laser pulse and static material, while the GHz burst mode interacts with excited material in relaxation.
Professor Godai Miyaji
The team has further proposed a possible mechanism for the formation of 2D LIPSS formed by the GHz burst mode, which is regarded as the synergetic contribution of the electromagnetic and hydrodynamic mechanisms.
In particular, 2D LIPSS is produced by the localized surface plasmon resonance of following pulses in the bursts within the nanogrooves of 1D LIPSS established by the preceding pulses. These hotspots have greatly amplified electric fields.
Additionally, hydrodynamic instability including convection flow determines the final structure of 2D LIPSS. Based on this hypothesis, they successfully created well-defined 2D LIPSS by tailoring the envelope of the GHz burst.
The well-known phenomena of LIPSS formation can be achieved on a variety of solid surfaces by simply exposing the material surfaces to several pulses of a linearly polarized laser beam, even in the air.
Significantly, LIPSS may functionalize the material surfaces to provide surface coloring, friction reduction, surface wettability management, etc., which is piqueing interest in industrial applications.
The ability of the GHz burst mode enabling the fabrication of 2D LIPSS will offer the possibility of the formation of more functionalized surfaces and thereby diversify the application and accelerate the commercialization.
Corresponding author, Prof. Koji Sugioka, said that “The results that the GHz burst mode femtosecond laser pulses can enhance the ablation efficiency with improved ablation quality, reported by Ilday’s group in 2016, have overturned common sense and significantly impacted the community of laser materials processing. Immediately after that, some groups including our group started to carry out experiments on GHz burst mode ablation of different types of materials for more detailed investigation.”
“In the process of the GHz burst mode ablation study, we considered that more controlled energy deposition as compared with the single-pulse mode may also offer some advantages to other kinds of materials processing. Then, we applied the GHz burst mode to LIPSS formation and succeeded in showing interesting results.”
“The obtained results may offer a new possibility of GHz burst mode for processing other than ablation, including microbonding, crystallization, polishing, two-photon polymerization, and internal optical waveguide writing. Thus, we believe that GHz burst mode will open new paths to femtosecond laser processing.”
One of the co-authors, Prof. Godai Miyaji, said that “Ultrashort pulse laser subtractive manufacturing is induced by three dominant phenomena of nonlinear optical absorption inside the material, energy transfer such as electron-electron scattering and electron-lattice scattering, and phonon excitation, and ablation. Here, the relaxation time of the excited state of the material is in a region of femtosecond to sub-microsecond. The conventional single-pulse mode induces ablation by the interaction process between femtosecond laser pulse and static material, while the GHz burst mode interacts with excited material in relaxation.”
“It is expected to induce not only efficient and particular optical absorption by transiently changing in its dielectric constant but also the electromagnetic mechanical interactions under high-pressure and high-temperature conditions. This is a unique physical process that cannot be realized and controlled by the conventional single-pulse mode and is expected to open up new fields in science and technology, including the realization of novel ablation shapes and the creation of new materials through novel bonding structural changes.”
