Nonlinear absorption properties and excited-state charge-transfer dynamics of Er doped ZnO films

Abstract

This work reports on the nonlinear optical properties of Er doped ZnO (EZO) films, measured by the Z-scan technique with different laser parameters (pulse width, wavelengths and energy). The nonlinear absorption mechanism of EZO films is analyzed by a singlet state three and four-level model, respectively. The effect of the direct current sputtering power of EZO films on the nonlinear optical properties was studied. From the results, the samples show the self-focusing effect and two photon absorption (TPA) induced excited state reverse saturable absorption (RSA) behavior. Under the different laser parameter excitation, it has been determined that the TPA induced saturable absorption (SA) or RSA properties for EZO films with 8 W dc sputtering power. Moreover, these findings indicate that free carrier density increases with Er doping and this behavior leads to excited state absorption. In addition, the ultrafast dynamics of EZO films have been investigated using the two-color pump-probe (TCPP) technique with 325, 380 and 400 nm wavelength excitation and probing over a broad range in the visible region. The decay of the positive signal is found to be biexponential, which we have assigned to the pump-induced deep-level (DL) or excited state absorption. We also present experimental results that the few decade picosecond component has been assigned to vibrational relaxation in the excited electronic states, and the slow components represent the decay from singlet excited state to the ground state ( 1.2 ns). Moreover, these findings indicate that the excitation wavelength increases and the detection wave band widens. Our results show that EZO films are a promising candidate in further optoelectronic device applications.