Ultrafast Four‐Wave Mixing Phase‐Matched by Transient Nonlinear Phase Modulation in a MAPbBr ₃ Single Crystal

Abstract A degenerated four‐wave‐mixing (FWM) process in single‐crystal (CH 3 NH 3 )PbBr 3 (MAPbBr 3 ) is reported, where 150‐fs laser pulses at 1.33 µm are employed as the pump. Two pump photons interact with a single crystal, producing another two photons with higher and lower energies, respectively. One of the FWM‐generation sidebands is tuned from ∼1.23 to 1.21 µm and the other from ∼1.43 to 1.48 µm for the center wavelength, as the pump fluence is increased from 2.6 to 11.69 mJ cm −2 . The self‐phase modulation induced by the strong pump pulses through the optical Kerr effect is responsible for the tuning dynamics. Transient spectroscopy not only verifies the FWM scheme for the interacting waves but also reveals the interference dynamics between the FWM‐generated sidebands and the probe pulse. In particular, the angular dependence of the FWM generation supplies direct evidence for the phase‐matching geometry. Using experimental data, a nonlinear refractive index coefficient of 1.19 × 10 −14 cm 2 W −1 at 1.33 µm for single‐crystal MAPbBr 3 is determined.