Femtosecond radiative decay of two excitons coupled via radiation-induced interaction in ZnO thin films (Conference Presentation)

Abstract

Micro-resonators with small mode volume and high quality factor are widely used to enhance the interaction between light and matter. On the other hand, simple semiconductor thin films with high crystallinity, which provide long coherent length of the wave functions of center-of-mass motion of excitons, bring about size dependent enhancement of the interaction caused by nonlocal wave-wave coupling between light and exciton beyond long wavelength approximation. Actually, we found peculiar optical responses, e.g. large energy shift and sub-picosecond radiative decay of the weakly confined excitons in CuCl thin films with thickness of hundreds of nm [Phys. Rev. Lett. 103, 257401 (2009)]. Recently, thickness dependence of radiative decay rate was clearly observed in a single crystal Cu2O film with continuously varying thickness (16-1000 nm) [Phys. Rev. B 97, 205305 (2018)]. In the case of ZnO, where A and B excitons closely located, the radiation-induced interaction between the two excitons, which further enhances the energy shift and radiative decay rate, is theoretically predicted [Phys. Rev. B 94, 045441 (2016)]. Therefore, we investigated ZnO thin films by transient grating spectroscopy. The samples were fabricated by pulsed laser deposition on Al2O3 substrates. We confirmed that the measured spectra reflect the shapes of the calculated spectra based on the theory. Moreover, we found the temporal profile of the signal shows ultrafast decay faster than 100 fs at 5 K in the film with a thickness of 289 nm. The results show that strong spatial interaction between light and excitons is realized in multicomponent excitonic systems.