Design, Preparation And Optical Properties Of Novel Luminescent Materials For Super-Broadband Optical Amplification

With the speedy development of computer network and telecommunication technology, optical fiber transmission technology with high speed and high capacity is demanded. Recently, great progress has been achieved in the OH elimination of silica fibers, and as a result, the telecommunication transmission has been extended to the range from 1.2 to 1.7μm. Therefore, considerable effort has been devoted to the development of optical fiber amplifiers which can be used to produce optical gains at different communication bands. For examples, erbium (Er)-doped fiber amplifiers provide gain in the C band (1530-1565 nm), L band (1570-1605 nm) and S band (1450-1520 nm). Some other types of the amplifiers such as thulium (Tm)-doped amplifier in the S band (1450-1520 nm) and praseodymium (Pr)-doped amplifier in the O band (1260-1360 nm) were also developed. If we want to realize optical amplification in the whole telecommunication window by using rare-earth-ions-doped amplifiers, the only way is to combine different types of existing amplifiers. So the broadband amplification in the whole 1300-1600 nm region by using only a fiber would be expected to simplify the configuration of optical amplifiers and revolutionize the present telecommunication systems.This thesis provides a comprehensive review on the luminescence characteristics of rare-earth, transition-metal and main-group ions doped materials, gives an overview of the recent progress and problems, and puts forwards their future research directions. To develop the novel light source with the typical characteristics of "highly efficient", "wavelength tunable", "ultra-broadband", "multifunctional" and "micro-structured", several novel research ideas are presented, such as tuning optical properties of emission centers via tailoring the ligand-field, stabilizing multiple emission centers in a tolerant porous host and space-selective fabricating via ultra-short laser pulse. Based on these ideas, several new types of infrared luminescent materials have been developed successfully. Thermal analysis (DTA), X-ray diffraction (XRD), transmission electronic microscope (TEM), Raman scattering spectroscopy, electron spin resonance (ESR), absorption/transmission spectra, photoluminescence spectroscopy (PL) were used to study the structure and luminescence properties of the materials. A series of important conclusions and innovative results with practical significance were obtained.The infrared luminescence of Ni²⁺-doped ZnO-Al₂O₃-SiO₂ glass and glass-ceramics has been investigated. It is found that the infrared luminescence is originated from the ³T₂(F)→A₂(F) transition of octahedral Ni²⁺ ions doped into the nanocrystals. The result demonstrates that only octahedral Niv ions in crystalline hosts can act as infrared emission centers. On the above base, we designed two novel Ni²⁺-doped MgO-Al₂O₃-SiO₂ and Li₂O-Al₂O₃-SiO₂ glass-ceramics with broadband infrared luminescence and fabricated them successfully. The investigations on the optical properties of Li₂O-Al₂O₃-SiO₂ glass-ceramics treated at various conditions indicate that the luminescence characteristics of Ni²⁺ ions are very sensitive to the local environment The blue-shift of the peak position and the intensity decrease of the infrared luminescence could be attributed to the sites change ofNi²⁺ ions in the glass-ceramics.Two research ideas are proposed for designing the single ion doped materials which show tunable infrared luminescence: (1) tuning the infrared luminescence via controlling the size of the nanocrystals; (2) tuning optical properties of emission centers via tailoring the ligand field Transparent Ni²⁺-doped Li₂O-Ga₂O₃-SiO₂ glass-ceramics embeddedγ-Ga₂O₃ nanocrystals was prepared successfully and characterized. By controlling the size of the nanocrystals, the infrared luminescence is tunable. The result can be ascribed to the alteration of crystal field strength Dq of octahedral Ni²⁺, which is due the changes of the lattice parameters. Two novel M²⁺-doped glass-ceramics containing Mg₂SiO₄ and Ba_0.808(Al_1.71Si_2.29)O₈ nanocrystals were designed and fabricated successfully. The above-mentioned three types of glass-ceramics show interesting wavelength tunable luminescence from 1245,1450 to 1570 nm. The theoretical analysis suggests that the tunable luminescence is the result of fine controlling the internuclear distance between the active Ni²⁺ ion and the surrounding ligand. It is necessary to point out that the research idea can be potentially employed to design and fabricate novel soluble infrared luminescent nanocrystals with tunable characteristic as potential substitutes for the traditional Pb and Cd containing infrared-ernitting quantum dots with high toxicity.The research idea is proposed for designing the single ion doped materials which show ultra-broadband luminescence: controlling the luminescence from active Ni²⁺ ions occupied the multiple octahedral local environments. Transparent Ni²⁺-doped Na₂O-Ga₂O₃-GeO₂ glass-ceramics embedded (Ga₂O₃)₃(GeO₂)₂ nanocrystals were prepared successfully and characterized. According to EELS results, Ni²⁺ ions are incorporated into the multiple positions: one is in the regular octahedral position and the other is in the distorted one. Since the distortion of ligand may induce the weakening effect on the crystal field strength of central Ni²⁺ ion, the multiple centers may show different crystal field strength. Therefore, the material shows interesting luminescent characteristics with two emission bands at 1300 and 1450 nm originated from high crystal field Ni²⁺ ions and low crystal field ones, respectively. The mixing of these two bands presents ultrabroadband luminescence with the large full width at half maximum (FWHM) of about 400 nm. The corresponding photoluminescence excitation spectra and fluorescence decay curves suggest a possible energy transfer between above-mentioned two centers. As a result, infrared luminescence can also be tuned via changing pumping sources. Another interesting point observed in our experiment is related to the surprisingly effective doping. We suppose that the strong preference of active center for special coordination, octahedral position for Ni²⁺ here, might be the underling drive force for doping. The results may potentially provide useful reference for the present hot issue of doping in nanocrystals.Wide band gap semiconductorβ-Ga₂O₃ is selected as the host material for Ni²⁺ ion since the thermal-quenching effects are inversely proportional to the band gap of the hosts. Highly transparent glass-ceramics containingβ-Ga₂O₃:Ni²⁺ nanocrystals were synthesized successfully and characterized. Intense broadband luminescence centering at 1200 nm was observed when the sample was excited by a diode laser at 980 nm. The room-temperature fluorescent lifetime is 665μs, which is longer than the Ni²⁺-doped ZnAl₂O₄ and LiGa₅O₈ glass-ceramics and is also comparable to the Ni²⁺-doped LiGa₅O₈ single crystal. The intense infrared luminescence with long fluorescent lifetime may be ascribed to the high crystal field hold by Ni²⁺ ions according to the generalized single configurational coordinate diagram. In addition, the moderate lattice phonon energy ofβ-Ga₂O₃ may also favor the radiative transition of active Ni²⁺ ions. Furthermore, we demonstrated broadband optical amplification at 1.3 μm in glass-ceramics with 980 nm excitation for the first time. The optical gain efficiency is about 0.283 cm^-1 when the excitation power is 1.12 W. The optical gain shows similar wavelength dependence to luminescence spectrum. The obtained gain material has potential applications in broadband optical fiber amplifiers and tunable lasers.The research idea is proposed for designing Bi-doped materials which show ultra-broadband luminescence: tuning the composition of glass network former to favor the presence of multiple structural units around Bi centers. Bi activated germanium silicate glass was prepared successfully and characterized. The glass sample shows broadband and flat emission characteristics compared with germinate glass. Ultrabroadband optical amplification at 1272 and 1560 nm is observed simultaneously. It is possible that the presence of multiple structural units in the vitreous matrix in germanium silicate glass might provide more than one type of crystal field environment for Bi ions and as a result, inhomogeneous broadened emission and flatter spectrum are observed In comparison with the reported method such as adding glass modifiers (BaO and Li₂O) into the glass system, the presented way can give both considerations of tunable luminescence and high gain efficiency. Photoluminescence excitation spectra were employed to investigate the characteristic of excitation wavelength dependent luminescence in Bi doped germanium glass. Two active centers which occupy strong and weak crystal field environment are identified. The tunable and ultrabroadband luminescence properties are originated from electron transitions of these two active centers. The wavelength-dependent internal gains excited with 808 and 980 nm laser diodes show difFerent characteristics, and the relative flat optical amplification can be realized by choosing 980 nm pumping.A facile method is presented to realize multifunctional light source by stabilizing "active" centers (bismuth) in a "tolerant" host (nanoporous silica glass). Highly transparent materials, in which, unusual multiple bismuth centers were stabilized, were prepared successfully and characterized. The novel hybrid system shows multicolor luminescence from blue-green, orange, red, and white to the near-infrared region and it can be potentially applied as a wide-spectrum light source. The luminescence mechanism is also discussed and the results provide evidence that infrared luminescence is originated from the low-valence-state Bi. We have also found that nanoporous matrices can potentially be used as templates for in situ preparation of Bi nanocrystals. This might be attractive because of its multiple physical effects, such as thermal-optical switching, light-induced reflectivity tuning, and enhancement of second-harmonic generation. It is believed that the Bi-doped nanoporous glass thus obtained shows optical performance superior to that in other matrices. We propose that immobilizing active centers in a nanocage structure provides a new platform for design and fabrication of novel transparent photonic materials.The research idea is proposed for space-selective fabricating of active multifunctional light source: employing ultra-short laser pulse to induce multi emission centers in a "tolerant" host We have demonstrated space-selective fabrication of ultra-broadband light source by femtosecond laser in the active "tolerant" host (Bi-doped nanoporous silica glass). The acquired Bi²⁺ and multiple Bi⁺ centers show ultra-broadband luminescence covering from red to infrared region. Since the luminescence change are only limited in the focal spot, we believe the material can be fabricated into active micro or even nano-scale photonic components. In Bi-doped silicate glass, a novel photochromic phenomenon under irradiation of a femtosecond laser is reported. Not only broadband absorption in the visible but also infrared luminescence variations have been observed after irradiation. This behavior is related to the process of multiphoton absorption initiating local chemical reactions involving multiple active centers of Bi. The materials studied here pave the way to three-dimensional memory systems with ultra-high density, as well as micro-photonic components or devices operating in various spectral bands, including the technologically important low-loss telecommunication window.