Transient Aspects and Ultrafast Dynamical Processes of Amplified Spontaneous Emission in Conjugated Polymers

Experimental research has revealed that the stimulated emission in organic semiconductors is associated with lattice vibrations. To reveal the dynamical aspects of the excited state of the conjugated polymer, the electron-transition process has been incorporated into the molecular dynamics, making it possible to simulate in detail the whole ultrafast process of amplified spontaneous emission (ASE). A typical conjugated polymer, poly( p-phenylene vinylene), is chosen as a model for the research. When an external laser beam of 60 μJ/cm2 is applied to photoexcite the polymer, the energy levels begin to oscillate within 100 fs. Thanks to the prominent self-trapping effect of the conjugated polymer, the laser beam also drives the lattice of the polymer chain to strongly vibrate. Until about 200 fs, four energy levels are pulled to the center of the energy gap, resulting in a transient discrete four-energy-level electronic structure for ASE. Simultaneously, in this ultrafast dynamical process, within the first 1 ps, inversion of the electron population occurs along with the appearance of a localized electronic cloud and locally distorted alternating bonds of the polymer chain. This detailed description of the whole ultrafast process of ASE helps in the understanding of the microscopic dynamical evolution of excitation in conjugated polymers.