Enhanced nonresonant nonlinear optical processes from populated electronic excited states

Abstract

A general enhancement mechanism is reported for nonlinear optical processes originating from real population of electronic excited states. The calculated nonresonant third-order optical susceptibility ${\ensuremath{\gamma}}_{\mathit{i}\mathit{j}\mathit{k}\mathit{l}\mathit{n}}^{\mathit{S}}$(-${\mathrm{\ensuremath{\omega}}}_{4}$;${\mathrm{\ensuremath{\omega}}}_{1}$,${\mathrm{\ensuremath{\omega}}}_{2}$,${\mathrm{\ensuremath{\omega}}}_{3}$) of quasi-one-dimensional conjugated linear chains can be enhanced by orders of magnitude, or even change sign, when the first (${\mathit{S}}_{1}$) or second (${\mathit{S}}_{2}$) electronic excited state is optically pumped and then populated for times suitably long to perform nonresonant measurements of ${\ensuremath{\gamma}}_{\mathit{i}\mathit{j}\mathit{k}\mathit{l}\mathit{n}}^{\mathit{S}}$(-${\mathrm{\ensuremath{\omega}}}_{4}$;${\mathrm{\ensuremath{\omega}}}_{1}$,${\mathrm{\ensuremath{\omega}}}_{2}$,${\mathrm{\ensuremath{\omega}}}_{3}$) at frequencies different from the resonant pump frequency.