Upconversion Luminescence In Rare-Earth-Doped Lithium Niobate Crystals Under Femtosecond Laser Excitation

Frequency upconversion (UC) luminescences from near infrared into visible regionin rare-earth ions doped materials have extensive application perspective and huge devel-opment potential, thus questing for efficient UC luminescence mechanisms and openingout new materials attract much attention, and investigation on the UC luminescence ofrare-earth ions under femtosecond (fs) laser excitation can exploit new application of ma-terials in being. So under fs laser excitation, we have systematically investigated the UCluminescence mechanisms of various rare-earth ions and the energy transfer characteris-tics between rare-earth ions doped in lithium niobate (LN) crystals, which are the host ma-terial of waveguide UC lasers, by measuring UC ?uorescence spectra and time-resolved?uorescence spectra.Firstly, UC luminescence under fs laser excitation was investigated in Er3+-dopedLN, in which UC luminescence has been observed. The mechanism and process of UCluminescence are confirmed based on Judd-Ofelt analysis theory, and found that the ex-cited state absorption (ESA) UC luminescence under fs laser excitation owns much higherefficiency than that under continuous wave (cw) laser excitation. We measured the life-time of a near infrared level, which serves as an intermediate level of UC luminescence,using the dependence of ESA UC luminescence intensities on fs laser repetition frequen-cies. Although heavily codoping with MgO can increase the optical damage resistantcapability of Er:LN, but simultaneously facilitates Er3+ to form clustered sites, that isdisadvantageous to ESA UC luminescence.Secondly, UC luminescence under fs laser excitation was investigated in Dy3+-dopedLN, in which UC luminescence has not been reported up to now though excitation photoncan be absorbed. Yellow and blue UC luminescences show that the ultrahigh instanta-neous power density of fs laser can change the short-lived intermediate level into reservoirlevel of UC luminescence. We found that energy transfer can occur effectively betweenDy3+ and Er3+, this energy transfer can enhance the Er3+ green UC luminescence.Thirdly, UC luminescence under fs laser excitation was investigated in singly dopedLN with rare-earth ion absent excitation photon absorbable level. Two-photon UC lu-minescence under fs laser excitation was observed in Eu3+-doped LN, two-photon si- multaneously absorption mechanism was confirmed by the dependencies of LiNbO₃uorescencespectra on excitation light polarization and LiNbO₃uorescence intensities on temperature. It wasdisplayed by analyzing luminescence decay curve that there are two paths to populate5D0 level. In addition, two-photon UC luminescence under fs laser excitation was alsoachieved in Tb³⁺-doped LN, second harmonic generation (SHG) mechanism was con-firmed through the enhancement degree of LiNbO₃uorescence intensity after inserting a BBOcrystal into the experimental light path, and the UC luminescence processes are SHGin LN host, reabsorbing frequency-doubled light by NbO3LiNbO₃ , sensitizing Tb³⁺ by energytransfer from NbO3LiNbO₃ , and luminizing via Tb³⁺ transition. These experimental investiga-tions forcefully revealed that UC luminescence can be achieved in the materials, in whichUC luminescence can not be achieved under cw laser excitation, under fs laser excitation.In order to enhance the UC luminescence efficiency of singly doped systems, lastly,sensitized enhancement of UC luminescence under 800 nm laser excitation in triply dopedLN was investigated. Energy transfer of Nd³⁺→Yb³⁺ in Nd:Yb:LN was phonon-assisted transfer process accomplished by dipole-dipole interaction mechanism, and theenergy transfer efficiency can reach to 66%. The enhancements of Ho³⁺ and Tm3+ UCluminescence via Nd³⁺-Yb³⁺ ions pair were investigated, 15 times enhancement wasachieved in Ho³⁺ green UC luminescence, enhancement was also achieved in Tm3+ blueUC luminescence. Experiments showed that the function of Nd³⁺-Yb³⁺ ions pair in ETUluminescence processes under 800 nm fs laser excitation is similar to that of Yb³⁺ under980 nm cw laser excitation. For UC luminescence under fs laser excitation, we suggesteda method to distinguish ESA and energy transfer UC (ETU) mechanisms using the de-pendence of UC luminescence intensities on excitation pulse repetition frequency.