The role of avalanche ionization in optical breakdown of monolayer two-dimensional materials

Understanding the basic process by which carriers are generated in two-dimensional materials by the high-field laser is important for their applications in non-perturbative nonlinear optics as well as light emitting diode and detectors under strong injections. However, even the fundamental question on the relative role of multiphoton and avalanche ionizations is still not clear in these materials. In this report, we investigate this question by studying the optical breakdown threshold of monolayer MoSe₂, MoS₂, WS₂, and hBN by a single 800-nm 150-fs laser pulse. We found the breakdown threshold fluence of these materials scales linearly with respect to their electrical bandgap energy. We also found their breakdown threshold has little dependence on the state of polarization (i.e., linear vs. circular) and the crystalline axes (i.e., zigzag vs. armchair). These observations are consistent with a picture that avalanche ionization is primarily responsible for optical breakdown in monolayer two-dimensional materials. This indicates that impact ionization, which is the reverse Auger process, is very efficient in these materials, most likely due to enhanced Coulomb interaction originated from quantum confinement and reduced dielectric screening.