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Ultrafast Laser–Material Interactions
Ultrafast Laser–Material Interactions
Lasers are coherent light sources that can be focused to a point on the surface or inside transparent materials to induce material modifications through various effects such as ionization, heating, and bond breaking. Ultrafast lasers feature pulsed outputs with durations on the scale of pico- to femto-seconds and extremely high peak power, enabling multiphoton absorption and precision control of thermal effects and, thus, the induced modifications in the exposed zone of materials.
At PULSE Lab, we strive to understand the mechanisms behind material ablation, densification, crosslinking, self-organization, and especially their combinations, which are critical for their applications in advanced microfabrication. We are building a workstation around a 10 W, 1 MHz femtosecond laser with advanced pulse controls such as compression, shaping, bursting, and delay, which enables versatile investigation of complex laser-material interactions. Solid transparent materials like glasses are of our particular interest, while we are always interested in playing with different materials.
A schematic explaining multiphoton absorption. Considering a material transparent to the original laser wavelength, i.e., the energy of one photon is not sufficient for causing material modifications, focusing of the ultrahigh peak power of an ultrafast laser to a small zone in the material can cause an ultrahigh local photon density. The photon density can be so high that the multiphoton absorption effect, i.e., the possibility that an electron absorbs two, three, or more photons at once, which, in total, provides sufficient energy for electron excitation, becomes significant.
Raman spectra of a glass-like precursor, hydrogen silsesquioxane (HSQ), before and after femtosecond laser exposure. It can be seen that the dominant Si-H bonds in the pristine HSQ was converted to features related to silica glass and Si-Si bonds. What is special about the conversion is that it indicates that HSQ was crosslinked by the exposure which can be used to create etching selectivity between exposed and pristine HSQ, useful for microfabrication. [Huang, P.-H., et al. ACS nano 18.43 (2024): 29748-29759., https://doi.org/10.1021/acsnano.4c09339]
Investigation of the evolution of the femtosecond laser-induced modification in HSQ with increasing input pulse numbers. E denotes the polarization of the laser beam. At one pulse, the structure was relatively homogeneous. As the input pulse number increased, polarization-dependent nanoplates began to appear and became sharper and shaper. What is special about the observation is that the structure we see here is crosslinked HSQ as it survived chemical etching, while the laser spot was not moved at all when making the structure in each sub-figure, which means that the nanoplates formed and self-organized inside the laser focal zone, showing a simultaneous occurrence of two distinct laser-material interactions. [Huang, P.-H., et al. ACS nano 18.43 (2024): 29748-29759., https://doi.org/10.1021/acsnano.4c09339]
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