Title: Ultrafast Optical Studies of Amplified Spontaneous Emission in Hybrid Organic-Inorganic Perovskites

Abstract: Lead-halide perovskites have exhibited promising properties for optoelectronic applications. One of the stars of this category is methylammonium lead iodide (MPI), which has been found to have paradoxically high carrier mobilities and lifetimes, in addition to highly efficient exciton dissociation into free carriers due in part to the methylammonium cation, which is able to rotate to screen carriers. Amplified spontaneous emission (ASE) has also been observed in MPI, with chemical substitutions allowing for easy tunability of ASE energy, and with much work being devoted to the study of various microstructures with inherent resonances that enhance ASE output. One such microstructure forms spontaneously when the material temperature is lowered below the phase transition temperature at 150K. A phase transition from tetragonal phase (TP) to orthorhombic phase (OP) occurs below 150K, but is incomplete, with small TP inclusions remaining. As TP has a lower bandgap than OP, carriers are drawn to these TP inclusions, where they become concentrated, thus resulting in very low effective ASE threshold and allowing for sustainable continuous lasing. Such low threshold ASE is necessary for high efficiency lasing applications, such as nanoscale on-chip lasers, but little is known about the species and timescale of ASE in this system. I will give an overview of our work on this material, in which we have used ultrafast transient absorption and time-domain terahertz spectroscopy to study carrier dynamics in MPI in the low temperature (TP inclusion) regime, and uncovered the timescale and underlying carrier mechanisms of ASE.