Dielectric breakdown and sub-wavelength patterning of monolayer hexagonal boron nitride using femtosecond pulses

Hexagonal boron nitride (hBN) has emerged as a promising two-dimensional (2D) material for many applications in electronics and photonics. Although its linear and nonlinear optical properties have been extensively studied, the interaction of hBN with high-intensity laser pulses, which is important for realizing high-harmonic generation, creating deterministic defects as quantum emitters, and resist-free patterning in this material, has not been investigated. Here we report the first systematic study of dielectric breakdown in chemical vapor deposition (CVD)-grown hBN monolayers induced by single femtosecond laser pulses. We report a breakdown fluence of 0.7 J cm-2, which is at least 7x higher than that of other monolayer 2D materials. A clean removal of hBN without leaving traces behind or causing lateral damage is demonstrated. The ablation features exhibit excellent fidelity with very small edge roughness, which we attribute to its ultrahigh fracture toughness due to its heterogeneous nature with three-fold symmetry. Moreover, even though defects are known to be abundant in CVD-grown hBN, we show experimentally and theoretically that its nonlinear optical breakdown is nearly intrinsic as defects only marginally lower the breakdown threshold. On top of this, we observe that hBN monolayers have a 4-5x lower breakdown threshold than their bulk equivalent. The last two observations can be understood if the carrier generation in monolayers is intrinsically enhanced due to its 2D nature. Finally, we demonstrate laser patterning of array of holes and lines in hBN with sub-wavelength feature sizes. Our work advances the fundamental knowledge of light-hBN interaction in the strong field regime and firmly establishes femtosecond lasers as novel and promising tools for resist-free patterning of hBN monolayers with high fidelity.