| As an important optical crystal, LiNbO3crystal has high stability and excellent optical integration. In this thesis, LiNbO3crystal is given upconversion white-light property by doping rare-earth ions, and the upconversion mechanism of white-light emission is analyzed in detail. The regulating effects of metal ions on intensity and chromaticity of upconversion emission are studied from the angles of defect structure and energy transfer, which can provide theoretical basis and experimental guidance to obtain upconversion white-light emission with high efficiency.Ho3+/Yb3+/Tm3+/LiNbO3crystals are grown by Czochralski method. Effects of rare-earth ions on defect structure and upconversion emission property are studied by XRD, OH-infrared absorption spectrum, UV-visible absorption spectrum and upconversion emission spectrum. The relations between the defect structure and upconversion emission property are established. The research results indicate that the formation of rare-earth ions clusters in LiNbO3crystal is the main reason for the change of upconversion emission property. Yb3+ion has a remarkable function on the enhancement of upconversion emission intensity, and Ho3+ion and Tm3+ion play an important role in the regulation of upconversion emission chromaticity.The upconversion mechanism of white-light in Ho3+/Yb3+/Tm3+/LiNbO3crystal is analyzed through dependences of upconversion emission intensities on pump powers, decay dynamics of upconversion emissions and fluorescent steady rate equations. The results indicate that the upconversion blue emission is a three photon process, and the upconversion green and red emission are two photon processes in Ho3+/Yb3+/Tm3+/LiNbO3crystal under980nm excitation. The longer energy level lifetimes of intermediate states result that the photon numbers needed by upconversion emissions are less than their integer values. Besides, it is observed that the upconversion red emission in Ho3+/Yb3+/Tm3+/LiNbO3crystal mainly originates from the nonradiative relaxation of green emission.Effects of pump powers on upconversion white-light are studied. Upconversion emission intensities increase gradually with the increase of pump power, and color coordinates (CIE values) trend to shift from white via green to blue region. It is obtained that the changes of CIE values for upconversion emissions with pump powers are determined by the photon numbers needed by upconversion emissions. Higher pump power is more beneficial to the generation of upconversion emission with high photon process. Effects of temperature on upconversion white-light emission in Ho3+/Yb3+/Tm3+/LiNbO3crystal are explored. With the increase of temperature, the upconversion emission intensities are decreased and the color coordinates shift from blue via green to red region. It is believed that the increase of nonradiative rate is the main reason for the decrease of emission intensity with temperature increase, a small fluctuation around550K is caused by the exothermic phenomenon of LiNbO3crystal. The thermally coupled behavior of3F2/3and3H4energy levels for Tm3+ion is discussed in LiNbO3crystal. The results indicate that the high sensitivity of optical thermometry based on Yb3+/Tm3+co-doped LiNbO3single crystal is attributed to the much larger energy gap between the3F2/3and3H4energy levels of Tm3+ion and the higher crystallinity of host material. This kind of temperature sensor is very suitable to high temperature detection.Effects of metal ions on upconversion white-light in Ho3+/Yb3+/Tm3+/LiNbO3crystal are studied. The results show that Li+and Mg2+ions with smaller ionic radius can regulate the defect structure and crystal field environment around rare-earth ion well. In LiNbO3crystal, the increment of energy level lifetime of intermediate state and reduction of energy level lifetime of luminescent state for Ho3+and Tm3+ions are in favor of the enhancement of emission intensity, which can be realized by decreasing the defect concentration of crystal and geometry symmetry of crystal field, respectively. The intermediate energy transfer process occurs by utilizing the exposed d orbit of Mn2+ion, which increases the output probability of upconversion red emission. The excellent upconversion white-light is realized by adjusting the ratio of upconversion green and red emission.The location situations of Yb, Mg and Mn are predicted by first principle theory. The calculation results show that the Yb3+, Mg2+and Mn2+ions occupy Li site first, and then occupy Nb site. The electronic structures and optical properties are studied. Pure, Yb and Mg-doped LiNbO3crystals are p-type semiconductors, and Mn-doped LiNbO3crystal is an n-type semiconductor. The band gap of Yb-doped LiNbO3crystal becomes narrow, which is caused by the4f orbit of Yb. In contrast, no obvious change of band gap is found in Mg-doped LiNbO3crystal. The doped Mn makes the Fermi energy level of LiNbO3crystal shift to the bottom of conduction band, and the3d orbit of Mn passes through the Fermi energy level. Combining the calculation with experiment results, it is obtained that p-type semiconductor is more favorable to increase the upconversion emission intensity. Furthermore, the calculation results of refractive indexes indicate the Mg2+and Mn2+ions can be used as optical refractive resistant ion and optical refractive sensitive ion, respectively. |
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