# Dynamics of excited-state nonlinear optical processes in conjugated linear chains

#### Abstract

The enhancement of the third order nonlinear optical susceptibility $\chi\sp{(3)}({-}\omega;\omega,\omega,{-}\omega)$ in the conjugated linear chain, diphenylhexatriene (DPH), when the system is optically pumped to electronic excited states, has been studied theoretically and experimentally. Enhancements of $\chi\sp{(3)}({-}\omega;\omega,\omega,{-}\omega$) as high as 1000 times have been achieved. In this dissertation, the time evolution of the excited state enhanced $\chi\sp{(3)}({-}\omega;\omega,\omega,{-}\omega$) of DPH is experimentally studied using nonresonant picosecond degenerate four wave mixing (DFWM) measurements when the first one-photon allowed, $\pi$-electron excited state is populated by a 355 nm, 30 ps pump beam. The time evolved $\chi{\sbsp{xyyx}{(3)}}({-}\omega;\omega,\omega,{-}\omega$) is observed by varying the time delay between the pump beam and the DFWM probe beams. $\chi{\sbsp{xyyx}{(3)}}({-}\omega;\omega,\omega,{-}\omega$) exhibits two-component exponential decay with two characteristic lifetimes. These two lifetimes, which are also predicted by the dynamical model describing multilevel populations, are in the picosecond and nanosecond timescales, respectively. The short lifetime, in the picosecond timescale, corresponds to the time needed to reach thermal equilibrium between two excited states, $S\sb1\ (2\sp1A\sb{g}$) and $S\sb2\ (1\sp1B\sb{u}$). The long lifetime, in the nanosecond timescale, corresponds to the time for both excited states, in thermal equilibrium, to recover to the ground state $S\sb0\ (1\sp1A\sb{g}$). The study of the dynamics of the enhancement of $\chi{\sbsp{xyyx}{(3)}}({-}\omega;\omega,\omega,{-}\omega$) by DFWM measurements has been carried out for DPH samples with different concentrations in several solvents. The magnitudes of the time evolved $\chi{\sbsp{xyyx}{(3)}}({-}\omega;\omega,\omega,{-}\omega$) are also affected by the energy gap between the $S\sb1\ (2\sp1A\sb{g}$) and $S\sb2\ (1\sp1B\sb{u}$) excited states, which is strongly dependent on the solvent. This agrees very well with predictions from the dynamical model. The studies done on the DPH sample by transient pump-probe saturable absorption (TPSA) experiments, which are dependent only on the dynamics of the excited state populations, also reveal the same results. This confirms that the dynamics of $\chi{\sbsp{xyyx}{(3)}}({-}\omega;\omega,\omega,{-}\omega$) can be accurately described by the population dynamics of a multilevel system. Analysis of the magnitudes of the time evolved $\chi\sbsp{xyyx}{(3)}({-}\omega;\omega,\omega,{-}\omega$) shows the energy gaps of DPH in various solvents to be comparable to the thermal energy. Large values of the microscopic isotropically third order nonlinear optical susceptibilities of the excited states of DPH, as high as 60,900 $\pm$ 4,500 $\times$ 10$\sp{-36}$esu for $\left\langle\gamma\sp{S\sb2}({-}\omega;\omega,\omega,{-}\omega)\right\rangle$ and 22,500 $\pm$ 3,000 $\times$ 10$\sp{-36}$esu for $\left\langle\gamma\sp{S\sb1}({-}\omega;\omega,\omega,{-}\omega)\right\rangle$, compared to the ground state value of $\left\langle\gamma\sp{S\sb0}({-}\omega;\omega,\omega,{-}\omega)\right\rangle$ less than 200 $\times$ 10$\sp{-36}$ esu, are presented.

#### Subject Area

Optics|Condensation

#### Recommended Citation

Arayathanitkul, Kwan, "Dynamics of excited-state nonlinear optical processes in conjugated linear chains" (1996). *Dissertations available from ProQuest*. AAI9627875.

https://repository.upenn.edu/dissertations/AAI9627875