Synthesis Of Up-conversion Nano-materials Doped Micro-waveguides And Their Optical Properties

During the past decade,upconverting nanoparticles(UCNPs)doped with rare earth ions have become an important class of fluorescence contrast agents for thermal sensing,due to their unique properties.Their property of anti-Stokes luminescence,with both the excitation and emission wavelengths close to the optimal for thermal sensing,has been extensively explored in various UCNPs applications.The work described in this thesis is carried out research on the luminescence properties of microfibers which fabricated from a variety of powder phosphor materials doped with different rare earth ions after co-doping with PMMA solution.The proposed microfibers(UCNPs/PMMA)possess the advantages of easy fabrication,low cost,strong plasticity,and various emission bands,further supporting their use in application based on optical signal transmission,sensors and optical components.Moreover,it investigates the transmission lossess,coupling efficiencies of up-conversion luminescence microfibers to improve fiber's fluorescence,life time decay of photons with energy levels and sensitivities.Actually,UCNPs powder or rare earth doped bulk glass could make an effective optical thermometer in a large temperature range.For example,Na Yb F4:Tm3+@Si O2 coreshell micro-particles optical thermometry in the rage of 100-700 K was reported.Besides,optical temperature sensing characteristics based on Na Lu F4:Yb3+/Tm3+/Gd3+ microcrystals were validated in a temperaturerange of 298-523 K.However,the upper-limit of the temperature tolerance of PMMA based fiber is around 80 °C,higher ambient temperature destroyed the fiber structure.The aim of this work is also to develop a suitable method of characterizing a variety of rare earth ions doped nanomaterials and utilize it in microfibers to investigate the thermal sensors of UCNPs,and to optimize the excitation scheme in order to facilitate their use in broad sensing and UCNPs applications.Firstly,the structural and surface morphology of rare earth ions(RE3+)doped nanomaterials were analyzed by X-ray diffraction(XRD)and transmission electron microscopy(TEM)methods.From XRD results,the typical diffraction peak intensities matches with dopant concentration RE3+ ions.This property indicates proper crystallinity of nanomaterials due to substitute(Yb3+,Tm3+ and Er3+)ions.Moreover,the SEM,TEM and EDX of fabricated microfibers are observed.It shows that nanomaterials are distributed smoothly and uniformely in a fiber.The surface morphology analysis gives good results.The optical properties of RE3+ doped nanomaterials are studied by Ocean optics QE Pro spectrometer.But a 980 nm filter was used to overcome the reflection of the fiber laser.All RE3+ doped materials show high optical properties in visible region.Secondly,experiment was carried out at room temperature to assess the photoluminescence properties of microfibers including 10x(NA=0.25),20x(NA =0.35)and 40x(NA=0.65)objectives which were used to collect the signal of photoluminescence and emission spectra of fabricated microfibers,respectively.All microfibers exhibit strong emission with green,blue and red lights.Noteworthy,the luminescence emission of Ag co-doped fibers were increased as compared to without Ag fibers.The enhancement of green luminescence intensity attribute to presence of Ag nanoparticles in a fibers.Moreover,the luminescence characteristics of microfibers were analyzed using Lambert technique.Microfibers exhibit guiding performance under 980 nm excitation of laser source to prove that it could be used as an active and passive waveguides.Thirdly,the experiment about coupling of microfibers was carried to further understand the fiber's fluorescence and to discuss its efficiencies.Coupled microfibers were tuned to different angles and excited at different position under 980 nm laser sources.The measured transient emitted spectra were fitted by exponential decay function.The decay components attribute to absorption of optical light in coupled fibers whereas the brightness of fibers indicates the transmission of light process.Thus,we calculated the efficiencies of coupled microfibers.Fourthly,the nonlinear optical properties of RE3+ doped nanomaterials are investigated by using life time decay of photons via energy levels diagram.The excitation takes the form of successive absorption of pump photons by a single ground-state ion.When an ion is excited from the ground state to the E1 level,a second pump photon that promotes the ion from E1 to higher-lying state E2 results in up conversion emission,before it decays to the ground state.Consequently,up converted emission will occur from the E2 level.The lanthanide ions such as Er3+,Ho3+,Tm3+,and Nd3+ have such energy level Structures.This analysis provides the nonlinear optical response of the samples both the nonlinear optical response of the samples both in laser incident point region and surrounding of laser point on the surface of microfibers.From this analysis,nonlinear optical transitions of photons phenomena obtained under 980 nm laser sources.From the observed results,it is revealed that emitted photons have higher energies than absorbed photons because total energy = heat + luminescence energy,so total energy is conserved.Finally,the nonlinear optical thermal and sensing properties of RE3+ doped nanomaterials in microfibers were analyzed by using fluorescence intensity ratio(FIR)technique.This technique has been investigated using a relatively large number of sensing materials in a variety of forms including phosphor samples.Sensor materials are that they are compatible with a range of other fiber optic sensor schemes.The technical advances resulting from the development of optical fiber based amplifier and laser systems using rare earth-doped fibers have reduced costs and increased the availability of several of the components used in these sensors.Fluorescence based optical fiber temperature sensors have shown the ability to cover a relatively wide measurement range with reasonable measurement resolution.The fluorescence method of temperature sensing uses the temperature dependent change in the decay rate of fluorescence from a material upon removal of the excitation source.This temperature dependence has been utilized as a means of optical fiber-based temperature sensing for temperatures in the range of ~ 20 °C to 90 °C with measurement resolutions in the order of a degree Celsius.The work reported the measurement of temperature using the ratio of fluorescence intensities from different energy levels of the erbium(Er),thulium(Tm)and ytterbium(Yb)ions in different phosphor materials have shown near linear temperature dependencies.Optical fiber-based measurements have been made covering the 20 °C to 90 °C temperature range with a resolution of the order of a few degrees.Since this work the FIR technique has been investigated in microfibers using the energy levels of different of the rare earth ions doped into a wide variety of crystal host materials and codoped PMMA solution.The aim is to provide a summary of the work undertaken using the FIR method for temperature sensing.The reported research provides better synthesis technique of a variety of doped rare earth ions nano-materials as well as fabrication of microfibers after co-doping with PMMA solution.The photoluminescence behavior of microfibers,couping efficiencies of different diameters of fibers,life time decay of nanomaterials along with energy levels diagram,higher sensitivities in visible regions were discussed in details.The fabricated microfibers could fulfill the requirements of future applications of photovoltaics,optoelectronics,biomedicals,fluorescent,optical displays and sensors.Particularly,sensitizer possesses larger absorption cross section area as compared to activators.Thus,we used high quantity of sensitizer than activator during doping in nanomaterials.It highlights the importance of rare earth ions doping materials which improve the optical properties of nano-materials in microfibers.Hence,the innovations of this research on photoluminescence properties,nonlinear optical coupling efficiencies of microfibers by utilizing a variety of rare earth ions doped nanomaterials in microfibers and its thermal sensing behavior are valuable reference for future works and very promising candidate for future optoelectronics,photovoltaics,lighting,multiple color displays,biological and optical thermal sensing applications.