  Hexagonal boron nitride (hBN) has emerged as a promising two-dimensional (2D)
material for many applications in photonics. Although its linear and nonlinear
optical properties have been extensively studied, its interaction with
high-intensity laser pulses, which is important for high-harmonic generation,
fabricating quantum emitters, and maskless patterning of hBN, has not been
investigated. Here we report the first study of dielectric breakdown in hBN
monolayers induced by single femtosecond laser pulses. We show that hBN has the
highest breakdown threshold among all existing 2D materials. This enables us to
observe clearly for the first time a linear dependence of breakdown threshold
on the bandgap energy for 2D materials, demonstrating such a linear dependency
is a universal scaling law independent of the dimensionality. We also observe
counter-intuitively that hBN, which has a larger bandgap and mechanical
strength than quartz, has a lower breakdown threshold. This implies carrier
generation in hBN is much more efficient. Furthermore, we demonstrate the clean
removal of hBN without damage to the surrounding hBN film or the substrate,
indicating that hBN is optically very robust. The ablated features are shown to
possess very small edge roughness, which is attributed to its ultrahigh
fracture toughness. Finally, we demonstrate femtosecond laser patterning of hBN
with sub-wavelength resolution, including an isolated stripe width of 200 nm.
Our work advances the knowledge of light-hBN interaction in the strong field
regime and firmly establishes femtosecond lasers as novel and promising tools
for one-step deterministic patterning of hBN monolayers.
