Ultrafast photophysics of conducting polymers

In this research we studied the ultrafast optical properties of {dollar}pi{dollar}-conjugated polymers, using a variety of time-resolved ultrafast measurement techniques and several different laser systems. The transient absorption and emission of photoexcitations were measured employing the time-resolved pump-and-probe correlation technique and picosecond streak camera imaging. The studies were conducted on thin films of both nonluminescent (NLCP) and luminescent conducting polymers (LCP); however, this work was focused on the latter group.; We found that the primary photoexcitations in LCP are free intrachain excitons. Their relaxation dynamics were measured. For the first time it was determined that their absorption spectrum consists of two strong PA bands. In addition, we found that excitons in LCP can provide a very strong optical gain, characterized by a wide spectrum spanning the visible optical range. The optical properties of excitons in poly-(2,5-dioctyloxy)-para-phenylenevinylene are optimized for laser action. They include high optical gain, minimal absorption loss and long excitonic lifetimes of 240ps in films and 480ps in solution.; Based on the results of transient photomodulation, two-photon absorption and electroabsorption spectroscopies, we developed a model of excitonic transitions in LCP. We also designed a new experimental method to probe LCP-three-beam technique. This novel technique enabled us to confirm the developed excitonic model. Furthermore, on its basis an ultrafast excitonic switch with variable switching times was developed.; We found that in some LCP at high exciton densities, exciton-exciton interaction through the common radiation field can lead to cooperative excitonic emission. Under these conditions, a drastic change in the emission spectrum was observed, where the initially broad photoluminescence spectrum collapses into a narrow emission band. Several different characteristic properties of this narrow band emission have been measured, determining that this phenomenon is due to excitonic cooperative emission or superfluorescence. This is the first confirmed observation of superfluorescence from an ensemble of excitons in {dollar}pi{dollar}-conjugated polymers at room temperature.; Our studies of NLCP showed that the ultrafast photoexcitation relaxation, which in NLCP is accompanied by a fast release of excess energy into the phonon modes, is generally followed by thermo-optically generated strain waves. Based on the effect of the strain waves on the polymer's absorption, we developed a novel transient strain spectroscopy, a method to detect hidden optically-forbidden electronic states in NLCP. We also determined that acoustic waves at high frequencies do not propagate through the polymer film, which was attributed to the localization of high frequency acoustical phonons. We investigated the nature of long-lived photoexcitations in NLCP and were able to distinguish between electronic and nonelectronic excitations.