Excited state enhancement of nonresonant degenerate four wave mixing in conjugated linear chains

A new enhancement mechanism for nonlinear optical processes originating from real population of electronic excited states in conjugated linear chains has been presented in previous theoretical studies. Compared to the ground state the calculated nonresonant third-order optical susceptibility \gamma\sp{S\sb{n}}({-}\omega\sb4;\omega\sb1, \omega\sb2, \omega\sb3) of linear chain molecules can be enhanced by orders of magnitude, or even change sign, when the first (S\sb1) or second (S\sb2) electronic excited state is optically pumped and then populated for times suitably long to perform nonresonant measurements of \gamma\sp{S\sb{n}}({-}\omega\sb4; \omega\sb1, \omega\sb2, \omega\sb3) at frequencies different from the resonant pump frequency. In this study, I report the first experimental observation of excited state enhancement of the degenerate four wave mixing (DFWM) susceptibility \gamma\sp{S\sb{n}} ({-}\omega; \omega, \omega,-\omega) of a conjugated linear chain, diphenylexatriene (DPH), when the first $\pi$-electron excited state is populated for nanosecond timescales and then probed nonresonantly through picosecond DFWM. A large increase in the completely nonresonant DFWM signal from DPH is observed when the 355nm pump beam saturates the S\sb0 \to S\sb2 absorption of DPH as compared to when no pump beam is present. The unpumped DFWM signal is independent of the concentration, demonstrating that the ground state \gamma\sp{S\sb0} ({-}\omega; \omega, \omega,{-}\omega) for DPH is relatively small. The pumped DFWM signal, however, increases strongly with increased DPH concentration showing an enhancement of the excited state \gamma\sp{S\sb2}(-\omega; \omega, \omega,-\omega) that is orders of magnitude larger than \gamma\sp{S\sb0}(-\omega; \omega, \omega,-\omega). Orthogonal polarization of the probe pulses eliminates the potential thermal and population gratings that can make large contributions in a parallel DFWM, ensuring that we observe a purely nonresonant electronic effect. The enhancement mechanism demonstrated in this study is generalizable to second order and other third order nonlinear optical processes and to other material structures, compositions, and phases. The experimental observations reported here as a demonstration of principle were carried out on a prototype molecule with a small ground state \gamma\sp{S\sb0} ({-}\omega\sb4; \omega\sb1, \omega\sb2, \omega\sb3). Furthermore, the measurements were made in solutions such that the relatively small number density of excited state molecules with large optical nonlinearity results in a much smaller \chi\sp{(3)}({-}\omega\sb4; \omega\sb1,\omega\sb2,\omega\sb3) than would be observed in a pure, single substance. Studies are underway on pure polymer thin films where typical nonresonant ground state values of \chi\sp{(3)}({-}\omega\sb4; \omega\sb1, \omega\sb2, \omega\sb3) on the order of 10\sp{-11} to 10\sp{-10} esu are expected to be also enhanced, leading to figures of merit sufficient for practical photonics devices