Non-linear excitation of quantum emitters in two-dimensional hexagonal   boron nitride
  Two-photon absorption is an important non-linear process employed for high resolution bio-imaging and non-linear optics. In this work we realize two-photon excitation of a quantum emitter embedded in a two-dimensional material. We examine defects in hexagonal boron nitride and show that the emitters exhibit similar spectral and quantum properties under one-photon and two-photon excitation. Furthermore, our findings are important to deploy two-dimensional hexagonal boron nitride for quantum non-linear photonic applications. 
Andreas W. Schell, Toan Trong Tran, Hideaki Takashima, Shigeki   Takeuchi, and Igor Aharonovich
physics.optics cond-mat.mes-hall quant-ph