Phase-Locking of Random Lasers by Cascaded Ultrafast Molecular Excitation Dynamics

There are conventionally coherent and incoherent types of random lasers, which are characteristic of broadband and randomly distributed narrow-band lasing spectrum, respectively. No fixed phase relationship has ever been observed between the lasing modes in such lasers. However, herein a new form of random lasers in patterned organic-inorganic hybrid perovskite MAPbBr(3) is discovered, where multiple simultaneously lasing modes are stably locked in phase so that lasing lines are achieved with equal spectral separations. This is the first observation of phase locking between random lasers, which is established by the cascaded molecular absorption-emission or cascaded excitation-injection processes between random laser modes. The fixed phase relationship is determined by the ultrafast relaxation of the excited molecules to the lower edge of the excitation band, correlating directly the absorption/excitation and emission/injection processes and producing a fixed phase shift. Thus, the bandwidth and separation of the lasing lines are dependent on the relaxation lifetime of the excited molecules and the number of cascading cycles. Such cascading dynamics has a timescale faster than 300 fs. These characteristics innovate the physics and explore new applications of both the MAPbBr(3) material and the random lasers. Such a strong phase-locking mechanism suggests new mode-locking schemes for ultrafast lasers.