Synergetic Enhancement of Light-Matter Interaction by Nonlocality and Band Degeneracy in ZnO Thin Films

This study aims to reveal the full potential of ZnO as an ultrafast photofunctional material. Based on nonlocal response theory to incorporate the spatially inhomogeneous quality of the samples coupled with experimental observations of linear and nonlinear optical responses, we establish the ultrafast radiative decay of excitons in ZnO thin films that reaches the speed of excitonic dephasing at room temperature in typical semiconductors at a couple tens of femtoseconds. The consistency between the observed delay-time dependence of the transient-grating signals and the theoretical prediction reveals that the ultrafast radiative decay is due to the synergetic effects of the giant light-exciton interaction volume and the radiative coupling between multicomponent excitons.

Figure 1

(left) Energy diagram and (right) wave functions of $A$ and $B$ excitons. $\sigma$ and $n$ are indices to label exciton components ($\sigma =A$ or $B$) and size-quantized states, respectively.

Figure 2

Fitting of the observed reflectance for (a) 289 and (b) 220 nm thick ZnO. (c) and (d) The calculation model of position dependence of the nonradiative damping ${\mathrm{\Gamma }}_{\sigma ,i}$ for the respective thicknesses.

Figure 3

(a) Fitting of the TG signal by a sum of the induced polarizations (a-1) without and (a-2) with component mixing of the $A$ and $B$ excitons for 289 nm thick ZnO. (b) The same for a 220 nm thick sample. (a-1’), (a-2’), (b-1’), and (b-2’) show enlarged figures of the respective figures above, where induced polarizations of $n=1,2,\dots ,10$th exciton states are displayed.

Figure 4

Observed delay-time dependence of the TG signal at (a) 3.3571 and (b) 3.3685 eV for a 289 nm thick sample, and at (c) 3.3705 eV for a 220 nm thick sample. The solid red lines represent fitting curves by a double-exponential function convoluted by the excitation laser profile.