Solution-processed hybrid perovskite of CH3NH3PbI3 (MAPbI3) exhibits an abnormal luminescence behavior at around the tetragonal-orthorhombic phase transition temperature. The combination of time resolved photoluminescence (PL), variable excitation power PL, and variable-temperature X-ray diffraction (XRD) allows us to clearly interpret the abnormal luminescence features in the phase transition region of MAPbI3. Both PL and XRD results unambiguously prove the coexistence of the tetragonal and orthorhombic phases of MAPbI3 in the temperature range of 150 to 130 K. The two luminescence features observed in the orthorhombic phase at T < 130 K originate from free excitons and donor-acceptor-pair (DAP) transitions, respectively. The comprehensive understanding of optical properties upon phase transition in MAPbI3 will benefit the development of new optoelectronic devices.

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We study nonlinear Cerenkov radiation generated from a nonlinear photonic crystal waveguide where the nonlinear susceptibility tensor is modulated by the ferroelectric domain. Nonlinear polarization driven by an incident light field may emit coherently harmonic waves at new frequencies along the direction of Cerenkov angles. Multiple radiation spots with different azimuth angles are simultaneously exhibited from such a hexagonally poled waveguide. A scattering involved nonlinear Cerenkov arc is also observed for the first time. Cerenkov radiation associated with quasi-phase matching leads to these novel nonlinear phenomena.