Design of nonlinear optical response of multipole-type excitons by film thickness and incident pulse width

We theoretically investigate the nonlinear optical pulse responses of excitons in a thin film where the excitonic center-of-mass motion is confined. A large interaction volume between excitons and radiation yields particular coupled states with radiative decay times reaching several femtoseconds. By considering two polarization directions of light, we reveal that these fast-decay modes dominantly survive in an optical Kerr spectra even under a massive nonradiative damping $\mathrm{\Gamma }=30$ meV. The results clearly show that there is an optimal combination of the incident pulse width and the film thickness for maximizing the integrated intensity of nonlinear signals.

Figure 1

Calculation model for optical Kerr response.

Figure 2

(a) ${\mathrm{\Gamma }}_{\sigma }$ dependence of calculated optical Kerr spectra $I_y\left(\omega \right)$ normalized by the peak intensity of the input probe pulse (the dotted line indicates spectrum of input 120-fs probe pulse), and (b) eigenmodes of exciton-radiation-coupled system (radiative width vs eigenenergy) for a film thickness of 291 nm. Serial numbers from 1 to 5 are assigned to eigenmodes which correspond to ones in Fig. 4.

Figure 3

Pulse-width dependence of nonlinear efficiency $\eta$ for a film thickness of 291 nm.

Figure 4

Film-thickness dependence of (a) the radiative decay time ${\tau }_{\xi }$ of exciton-radiation-coupled modes, and (b) the nonlinear efficiency $\eta$ with different pulse widths at ${\mathrm{\Gamma }}_{\sigma }=30$ meV.