Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs) have been extensively studied due to their extraordinary electronic and optoelectronic properties. In particular, monolayer semiconductor TMDs such as MoS2, WS2, and MoSe2 are of great interest due their direct band gap and strong excitonic resonances. Due to its reduced dimensionality, MoS2 exhibits many strong optical nonlinearities including high harmonic generation and giant multiphoton absorption, making it an excellent candidate for attosecond photonics, mode locking, optical limiting, and multi-photon detectors. Given the multitude of applications, understanding the optical limitations of MoS2 under intense excitation is essential to optimize its performance. Accordingly, we investigate the femtosecond laser induced breakdown of monolayer MoS2 with a variety of techniques. In this study, the substrate is discovered to have a profound effect where the ablation threshold itself can vary by more than one order of magnitude due to a simple interference phenomenon within the monolayer. Via substrate engineering, the ablation threshold can be reduced such that laser patterning using pulse energies less than 100 pJ is possible. Similar to many other optical nonlinearities, absorption measurements and theoretical modeling reveal that avalanche ionization is also enhanced where more than 75% of the generated free carriers at breakdown are due to avalanche ionization. Finally, multishot studies demonstrate that MoS2 is one of the most optically robust materials with very weak incubation effects. Notably, the onset of optical damage results in the formation of nano-voids where clusters of atoms are removed while the overall integrity of the monolayer remains intact. These nano-voids are found to strongly influence the optical properties of the MoS2 monolayer due to the presence of mid-gap states introduced by dangling bonds. The study shows that these nano-voids only form for fluences within 80% of the ablation threshold. All of these findings help establish MoS2 as a promising candidate for strong field devices and provides foundational knowledge regarding the strong field physics of two-dimensional materials.
