Investigation On Up-conversion Characteristic Of The Pr³⁺:Y₂SiO₅ Crystal And On The Detection Of OH & CH₃ Radicals With IRPS

Investigation on up-conversion characteristic of the Pr³⁺ : Y₂SiO₅ crystal and on the detection of OH & CH₃ radicals with IRPSIn recent years, investigations on the process of frequency up-conversion in rare earth ions doped laser crystals, including fluorescence emission, stimulated emission, and the governing mechanism of the process of frequency up-conversion, have become hot research subjects. One important reason is due to the fact that frequency up-conversion lasers have broadly applicable prospects in the infrared display, laser display, the memory of high-density data and the others. Its favored characters lie in the phase-matched-free condition, the simple structure, the entire solidification and the relatively low cost. Rare earth ions doped laser crystal or non-crystal is the main material for up-conversions. This thesis investigated experimentally the optical characteristics of the Pr³⁺:Y₂SiO₅ crystal and also the mechanism of the process of the visible-ultraviolet up-conversion. The aim of the thesis work has been three parts.The first part of the thesis, introduction and fundamental principle.The second part of the thesis, the investigation was focused on the absorption spectrum, fluorescent emission spectrum, excitation spectrum and the fluorescent lifetimes of the samples with different doped concentrations in each energy level. Regarding the fluorescent emission spectrum, we have measured that at the energylevel of 4f5d, ³P₀, and ¹D₂, respectively, and studied the corresponding transitions.Laser radiations at wavelengths of 260 nm, 460 nm, and 600 nm were used to excite the Pr³⁺:Y₂SiO₅ crystal and the fluorescent emission spectrum of three samples with different doped concentrations were detected and recorded in the range from 270 nm to 770 nm under room temperature. The peak values of the fluorescence emission spectrum while exciting the Pr³⁺:Y₂SiO₅ crystal at the energy level of 4f5d are 276 nm, 279 nm, 303 nm, 543 nm, 570 nm, 602 nm, 632 nm and the corresponding transitions are 4f5d→³H₅, 4f5d→³H₆, 4f5d→³F₂, ³P₀→³H₄, ³P₀→³H₅, ³P₀→³H₆,³P₀→³F₂. The peak values of the fluorescence emission spectrum by exciting the level ³P₀ are 280 nm,311 nm, 326 nm, 493 nm, 552 nm, 621 nm, 652 nm, 658 nm, 663 nm while the corresponding transitions are 4f5d→³H₅, 4f5d→³H₆, 4f5d→³F₄,³P₀→³H₄, ³P₀→³H₅, ³P₀→³H₆,³P₀→³F₂, ³P₀→³F₃,³P₀→³F₄. Similarly atthe energy level of ¹D₂ we get 607 nm as the peak value of the fluorescent spectrum and ¹D₂→¹H₄ .as the corresponding transition. Moreover, we have also measuredÏ„_if which is the fluorescent lifetime of the energy level of 4f5d,³P₀ and ¹D-2 with different doping concentrations.We calculates the fluorescence lifetimeÏ„_is, fluorescence quantum efficiencyη_i and the transition rate between different levels of 4f2 based on the Judd-Ofelt theory. Table 1From Table 1, we can see that the level lifetime value increases with the decreasing density of doping ion. The reason is that with the decreasing density, mutual action between the granule weakens, the cross decay weakens, and the lifetime therefore increases. The fluorescence quantum efficiency of 4f5d is approximate 90%, which is useful to produce the tunable ultraviolet laser.We studies the upconversion of Pr³⁺:Y₂SiO₅ crystal, including the ultraviolet up-conversion in the excitation of continual wave and pulse wave and the upconversions from infrared to visible light and from visible light to visible light.The upconversion phenomenon under the excitation of continual frequency: The ultraviolet fluorescence band between 280nm - 350nm is found with the stimulation of Ar⁺ ion laser (488nm). The upconversion mechanism is studied based on the velocity equation theory and experimental results. ETU is the main upconversion mechanism for high density samples (1at % and 0.5at %); ESA is the main upconversion mechanism for low density samples (0.02at %).The upconversion phenomenon was measured under the excitation of pulse light. The ultraviolet fluorescence band between 280nm - 350nm is found with the stimulation of 488nm pulse laser. It is proved that ESA process is the main up-conversion mechanism based on the relations measure between the fluorescence intensity and the pump power and the time resolution spectrum measure of ultraviolet fluorescence.The fluorescence band between 480nm - 515nm is found with the stimulation of 579nm laser. It is proved that ETU process is the main upconversion mechanism based on the relations measure between the fluorescence intensity and the pump power and the time resolution spectrum measure of up-conversion fluorescence.The fluorescence bands between 470nm - 530nm and 620nm - 630nm are found with the stimulation of 930nm laser. It is proved that ESA process is the main upconversion mechanism based on the relations measure between the fluorescence intensity and the pump power and the time resolution spectrum measure of upconversion fluorescence.The third part of the thesis demonstrated the application of mid-infrared polarization spectroscopy (mid-IRPS) in detecting and analyzing OH and CH₃ radicals which are the combustion intermediates in flames.Laser spectroscopy played crucial roles in the study of molecular species in harsh reactive environments like chemical reaction in combustion process. However, most of the existing works are done within the UV/visible spectral range, requiring atoms and molecules for specific electronic transmissions among different energy levels, which limits the candidate molecules and atoms to only a small number. It is consequently that many important small molecules, such as OH, CH₃, can't be quantitatively investigate due to the lack of proper electronic transitions in this spectral range. This part of the thesis demonstrated that the use of IRPS allows the detection of OH and CH₃ as the minor combustion intermediates in the mid-infrared spectral range by probing the vibrational and rotational levels. On designing and performing our experiments we have studied thoroughly the principle of IRPS as well as the spectrum of OH and CH₃ so as to achieve quantitative measurements. The detection of OH and CH₃ with high temporal and spatial resolution in combustion processes implies that IRPS can be regarded as a common tool to apply in general whenever the spectrally resolved and temporally resolved detection of small molecular hydrocarbons in harsh chemical reactions are involved.The main research results of the thesis:1. We studies the upconversion of Pr³⁺:Y₂SiO₅ crystal, including the ultraviolet up-conversion in the excitation of continual wave and pulse wave and the upconversions from infrared to visible light and from visible light to visible light. The mechanism of the up-conversion was investigated through temporal resolution laser spectroscopy. This part of result was published in Chem. Phys., 2006, 325:563-566; European Physical Journal D, 2006, 39:303-306. et.al.2. This thesis enriched the optical features of the Pr³⁺:Y₂SiO₅ single crystal, especially those in the ultraviolet region, and it also supplied more scientific data for the practical application of the crystal. This part of result was published in Chin. Phys. Lett., 2006, 23:1915-1918.3. The thesis demonstrated the use of IRPS for the detection of OH and CH₃ as minor combustion intermediate products in the mid-infrared spectral range by probing the vibrational and rotational levels of the molecules. The successful detection of OH and CH₃ with high temporal and spatial resolution in combustion processes implies that IRPS can be regarded as a common tool to apply in general whenever the spectrally resolved and temporally resolved detection of small molecular hydrocarbons in harsh chemical reactions are involved. This part of result was published in J. Chem. Phys., 2007,127:084310.