Dual-Wavelength Lasing Due to Second Phase Inclusions in MAPbCl ₃

We report for the first time dual-wavelength lasing at 78 K in a vertical cavity containing ultrasmooth MAPbCl₃ single crystals. To understand this unusual lasing behavior, the MAPbCl₃ single crystals were thoroughly investigated in terms of temperature-dependent optical experiments. Microreflectivity measurements reveal, besides the standard exciton feature at the ∼385 nm orthorhombic bandgap, an additional previously unreported excitonic peak at ∼412 nm. This second exciton feature aligns with the second lasing line, strongly suggesting the coexistence of a "second phase" within the primary orthorhombic lattice. We show that this second phase persists up to 300 K and is likely associated with the quantum dot-like nanostructures detected on the surface of the MAPbCl₃ crystals. The second phase depends on the growth method and crystal size but is likely traceable in all MAPbCl₃ systems. These insights offer enhanced understanding of the MAPbCl₃ system and open new pathways for blue-UV photonic devices.