5-1 Conclusions
Optical nonlinearities (including two-photon absorption coefficient, bound-electronic and free-carrier nonlinearities) of ZnO thin film growth by Laser-MBE is first observation near two-photon resonance. This ZnO thin film is inspected by XRD which indicated this film is a highly c-axis orientation quality and the result of PL reveals a strong free-exciton luminescence at room temperature. The measured result of visible light’s range of transmission spectrum reveals it has a high transparency of visible light’s range. Because of this high c-axis orientation quality, the biggest two-photon absorption coefficient that we measured at 748nm-wavelegth of incident laser light is ≈1905 cm/GW. This strong two-photon absorption is about 400 times larger than that of ZnO bulk measured at 532 nm with 25-ps pulse-width. The two-photon absorption coefficient is disappeared above 780nm-wavelength of incident laser light source. Moreover, there is a drastic downgrade near 762nm which is equal the half of band-edge energy plus exciton bind energy. It is a resonance behavior of excitonic enhancement near two-photon resonance. The measured biggest bound-electronic nonlinearity of ZnO thin film that we measured is at 748nm-wavelength of incident laser light. It’s about 2000 times larger than theoretical value. From the wavelength-dependent Z-scan, the energy of incident laser light is smaller than half band-gap energy, the measured bound-electronic nonlinearity become smaller.
The biggest bound-electronic nonlinearity of ZnO thin film ranging from 748-780 nm that we measured is about 2000 times larger than theoretical value. Compared with the results of DFWM of ZnO microcrystalline thin film, that measured third-order optical nonlinearity is about 100 times larger than us. This discrepancy of result of DFWM and Z-scan is due to the absorption strength of near excitonic resonance, band-to-band transition and different
wavelength of incident laser light source. Thus 2000 times larger than theoretical value of bound-electronic nonlinearity is due to two-photon resonance with excitonic enhancement.
The free-carrier nonlinearity of ZnO thin film at 748 nm is about 40 times larger than that of ZnO bulk measured at 532 nm with 25-ps pulse-width. Its two-photon absorption strength near two-photon resonance. The wavelength-dependent Z-scan reveals the resonance behavior near (Eg-Eb)/2.
5-2 Perspectives
Free-excitons dominate near band-edge emission at room temperature and bound-excitons will dominate at low temperature. It’s a very important issue to understand the nonlinearities of bound-excitonic enhancements at low temperature. A temperature-dependent Z-scan is needed to improve. At higher temperature, the free-exciton binding energy will be covered by thermal energy. At this point, the nonlinearities of ZnO thin film will decay if those are dominated by free-excitons. This temperature-dependent Z-scan will provide additional evidence that excitons enhanced and dominated optical nonlinearities near two-photon resonance at low temperature and room temperature. Besides, a tunable light source with more wide range is needed to measure these nonlinearities of ZnO thin film without two-photon resonance. A second-harmonic-generation of Ti:Sapphire laser is also needed to study these nonlinearities of ZnO thin film with near excitonic resonance and compare with the results of DWFM of a 55-µm thickness ZnO microcrystalline thin film. Theoretically, dispersion of bound-electronic nonlinearity of ZnO thin fill will be observed near excitonic resonance. In other words, the bound-electronic nonlinearity of ZnO thin film with UV light pumping will be negative. Moreover, the influence of thickness to third-order optical nonlinearity are the same with second-order one. Furthermore, the effect of Fabry-Perot interferometer is significant with a normal incident to surface of sample. To find out the carrier dephasing time of ZnO by time-resolved Z-scan is essentially needed. With
time-resolved two-color Z-scan, nondegenerate optical nonlinearities will be study. These techniques will facilitate us to comprehensive study the origins of optical nonlinearities of ZnO thin film which is a highly c-axis orientation quality.
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