Fig. 1

a The nondegenerate (ND) CARS process indicated on the energy diagram of bulk hexagonal boron nitride (hBN), where ω₁ is 910 nm, ω₂ is 1040 nm, and ω₃ is 455 nm. b The spectrum of ND-CARS from hBN flakes. c The power dependence of ND-CARS from hBN flakes. d The polarization dependence of ND-CARS from hBN flakes. e Four-wave mixing (ω₁, 810 nm; ω₂, 1040 nm; ω₃, 405 nm; black) and ND-CARS (ω₁, 910 nm; ω₂, 1040 nm; ω₃, 455 nm; red) intensity as a function of the thickness of hBN flakes