EVALUATION OF CRYSTAL FIELD PARAMETERS OF NEODYMIUM ACTIVATED LASER GLASSES

Transcription

1 EVALUATION OF CRYSTAL FIELD PARAMETERS OF NEODYMIUM ACTIVATED LASER GLASSES By RUPASREE CHAKRABORTY Class Roll No Examination Roll No. M4LST13-03 Registration No of THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF LASER TECHNOLOGY IN THE FACULTY OF ENGINEERING AND TECHNOLOGY JADAVPUR UNIVERSITY SCHOOL OF LASER SCIENCE AND ENGINEERING JADAVPUR UNIVERSITY Raja S.C. Mallick Road, Kolkata , West Bengal, India 2013

2 JADAVPUR UNIVERSITY Raja S.C. Mallick Road, Kolkata , West Bengal, India FACULTY OF ENGINEERING AND TECHNOLOGY CERTIFICATE OF RECOMMENDATION Date: May, 2013 We hereby recommend that the thesis prepared under my supervision by Rupasree Chakraborty, entitled EVALUATION OF CRYSTAL FIELD PARAMETERS OF NEODYMIUM ACTIVATED LASER GLASSES be accepted in partial fulfilment of the requirements for the degree of Master of Laser Technology. To the best of my knowledge, the matter embodied in the thesis has not been submitted to any other University/Institute for the award of any degree or diploma. Dr. RADHABALLABH DEBNATH Thesis Supervisor Countersigned Shri Dipten Misra Director, School of Laser Science and Engineering

3 JADAVPUR UNIVERSITY Raja S.C. Mallick Road, Kolkata , West Bengal, India FACULTY OF ENGINEERING AND TECHNOLOGY CERTIFICATE OF APPROVAL * The foregoing thesis is hereby approved as a credible study of an engineering subject carried out and presented in a manner satisfactory to warrant its acceptance as a pre-requisite to the degree for which it has been submitted. It is understood that by this approval the undersigned do not endorse or approve any statement made, opinion expressed or conclusion drawn therein but approve the thesis only for the purpose for which it is submitted. Committee On Final examination For evaluation of the thesis Signature (s) of the examiners *Only in case the thesis is approved

4 Acknowledgement It is a pleasure to me to record my appreciation and thanks to individuals who have helped me both directly and indirectly in pursuing and completing the thesis. First, I would like to express my gratitude to my respected guide Dr. Radhaballabh Debnath and Saptasree Bose for their support and encouragement throughout my work. I am grateful to all my teachers particularly those of School of Laser Science and Engineering who have given me valuable suggestion time to time during the progress of the project. I thank Shri. Dipten Misra, Dr. Asish Bandyopadhyay, Dr. Pradip Kumar Pal, Dr. Souren Mitra, Dr. Arunangshu S. Kuar for their constant encouragement. I am very much thankful to my classmates, Aritra, Akshay, Nikhil, Subhraneel and Srinjoy who have given me moral support during this work. I would also like to thank Mr. Biswanath Maity, Mr. Kingshuke Pal for their help. My heartfelt thanks to Mr. S. Balaji(CGCRI, Kolkata) who helped me a lot in carrying out the project. RUPASREE CHAKRABORTY I

5 CONTENT Page No. Acknowledgement...I List of Figures...III-IV List of Tables...IV Content...II-IV Chapter 1: Introduction Introduction Solid State Laser Nd:YAG Laser Glass as host for laser activator ion Objective of the present work Chapter 2: Literature survey Literature Survey Chapter 3: Theory Crystal Field Theory Theoretical Background of Judd-Ofelt Calculation Chapter 4: Experimental Sample Preparation Composition of sample Melting Casting Annealing Polishing Instruments and measurements II

6 Measurements of Density of the sample Concentration of Nd +3 ions in the sample Recordings of Absorption Spectrum Emission process of upon excitation of the glass Chapter 5: Results Absorption Spectrum Analysis Absorption Spectrum Possible Emission transitions and their lifetimes Chapter 6: Discussion Discussion of results Chapter 7: Conclusion and Future Scope of the work Conclusion Future Scope of the work.. 50 Chapter 8: References Books and Journals List of Figures Figure1: Nd:YAG laser rod... 7 Figure2: Absorption spectrum of Nd:YAG crystal... 7 Figure3: Emission of Nd:YAG crystal Figure4: Nd:doped phosphate glass rods 10 Figure5: Nd doped Yttrium Lithium Fluoride glass rods Figure6: Alumina crucible..26 Figure7: Batch melting furnace...26 Figure8: Rectangular steel mould...27 Figure9: Annealing furnace Figure10: Base glass sample...28 III

7 Figure11: Nd +3 doped sample glass after annealing 29 Figure12: Grindings machine..29 Figure13: Rectangular transparent Nd +3 doped sample plate Figure14: Specific gravity bottle Figure15: Electronic balance...31 Figure16: Absorption spectrophotometer with display unit Figure17: Absorption spectra of base glass and that of Nd doped glass. 34 Figure18: Possible emission transmission levels 35 Figure19: Absoption spectrum of base glass...37 Figure20: Absorption spectrum of Nd +3 doped glass sample.37 List of Tables Table1: Composition of sample with their mole% and equivalent weight in gm...25 Table2: Temperature segment with respect to time and current consumption Table3: Electronic transitions from ground state to excited state and their corresponding wavelength and reduced square matrix value...39 Table4: Electronic transition from ground state to excited state and area under the curve of absorption band Table5: Electronic transition from ground state to excited state with corresponding absorption cross section, measured S ed, calculated S ed, measured oscillator strength and calculated oscillator strength...42 Table6: Possible emission transitions data of Nd +3 ion in present glass Table7: Judd-Ofelt parameters and spectroscopic quality parameter of Nd +3 ion in present glass compared with different Nd +3 doped host Table8: Possible emission transitions data of Nd +3 ion in present glass compared with other Nd +3 doped glass IV

8 Chapter 1: Introduction This chapter discusses the history of invention of laser, application of laser, solid state laser, Nd:YAG laser, glass as host for laser activator ions and objectives of the work. 1

9 1.1. Introduction In 1917, Albert Einstein first theorized about the quantum basis of laser physics, the stimulation emission effect. In 1954, Charles Townes and Arthur Schalow created the first maser (microwave amplification by stimulated emission of radiation). In December 1958, the same authors reported the possibility of maser action in the infrared and visible region of light, which Gordon Gould later referred to as laser (light amplification by stimulated emission of radiation). In 1960, Theodore Maiman created the first successful solid-state laser. Pulsed laser emission was obtained by using high energy flashlamps to stimulate a synthetic ruby crystal (Cr:Al 2 O 3 ). In the same year, Ali Javan created the first gas laser (He-Ne) which emitted a continuous beam after initiating the laser action by an electric discharge. The next 20 years were dominated by gas lasers even though first dye lasers and semi-conductor lasers were created in 1962 and In the mid-80s, cheap semi-conductor or diode lasers took over. Advances in diode lasers made a decisive contribution to the development of compact solid-state lasers (SSL). Laser light has many potential applications in our everyday life, because of its special characteristic. That is why immediate after invention of laser in 1960, it was commercialized for different applications of our everyday modern life. The first use of lasers in our daily lives was in the supermarket barcode scanner introduced in The laserdisc player, introduced in 1978, was the first successful consumer product, beginning in 1982 followed shortly by laser printers. Some other important uses are: Medicine: Bloodless surgery, laser healing, surgical treatment, kidney stone treatment, eye treatment, dentistry etc. Industry: Cutting, welding, material heat treatment, marking parts, noncontact measurement of parts etc. 2

10 Military: Marking targets, guiding munitions, missile defence, electrooptical countermeasures (EOCM) etc. Law-enforcement: Used for latent fingerprint detection in the forensic identification field. Scientific Research: Spectroscopy, laser ablation, laser annealing, laser scattering, laser interferometers, LIDAR, laser capture micro dissection, fluorescence microscopy etc. Product-development/commercial: Thermometers, laser-pointers, holograms etc. Laser lighting displays: Laser light shows. Cosmetic skin-treatments: acne treatment, cellulite and striae reduction, and hair removal Solid state lasers A solid-state laser (SSL) is generally made up of three components: a solid active medium, a pump source to excite the gain medium, and a laser cavity or laser resonator in which the laser radiation oscillates and pass through the gain medium to amplify the power of the emitted light. Active medium The active medium is composed of a host and active ions, which are able to generate and amplify light. The host should be thermodynamically and mechanically stable and transparent in both the absorption and emission wavelength ranges of the active ions. These active ions substitute some of the constituent ions of the solid host, e.g. a crystal or a glass, either partially or totally. So, the host carry the active ions as impurities or dopants or even as stoichiometric elements. 3

11 Lanthanide and transition-metal ions are most used as laser-active ions whose light amplification has been proved in more than 500 active media. Optical amplification takes place in an active medium as a result of stimulated emission of additional photons. Electronic transitions of laser-active ions from an excited level to a lower level are induced by an input light with the appropriate frequency of energy the Bohr s relation, E 2 -E 1 =hν. Therefore, electrons must be previously transferred to the excited level by external pumping in order to invert the electronic population between the energy levels. The generation of this inverted population is highly dependent on the energylevel scheme of the active ion. Here, there is a distinction between the four-level and three-level active media. In a four-level active medium, the lower laser level is quickly depopulated by nonradiative transitions e.g. multiphonon transitions, to lower lying levels. Thus, there can be no appreciable population in this level and re-absorption of the laser radiation is avoided. In a quasi-threelevel active medium, the lower laser level is the ground level so the population can be appreciable in thermal equilibrium at the operating temperature. Consequently, amplification is only possible above a particular threshold pump power, when the laser threshold has just been reached. Below the laser threshold, the excited ions return to the ground state by spontaneous emission. High-intensity sources are needed therefore to pump quasi-three-level active media and longer radiative lifetime of the fluorescent level to reach the population inversion. However, quasi-three-level active media present a small quantum defect because this scheme just enforces a small energy difference between the lower laser level and the ground state. The most prominent example of such media is neodymium-doped media. Pump source Generally, solid-state lasers are optically pumped. From the mid-80s, the most efficient pump sources are diode arrays. These diode panels emit radiation 4

12 at wavelengths that are close to the strong absorption bands of several active laser ions, so the diode pump output can lead to an efficient population inversion. This good spectral match improves the pumping efficiency and thus reduces the operating temperature of the laser, which alleviates the thermooptical effects and leads to better beam quality. The fact that the pumping efficiency of diodes is higher than that of lamps led to design of more efficient, simple, and compact SSL. Some very useful active media such as Nd:Y 3 Al 5 O 12 (YAG), Nd:LiYF 4 (YLF) can be of both lamp- and diode- pumped but they only rose to prominence with diode pumps. In spite of this, they continued to be lamp pumped for a long time because the price per watt of generated pump power was cheaper than that of diode lasers. Active ion neodymium, can be directly pumped with high-power diode arrays or diode lasers such as InGaAs or AlGaAs, respectively. Broad band tunable laser operation can be reached using the Ti:sapphire laser as a continuous-wave (cw) pump source. Wavelengths can be tuned because the strong coupling between titanium and the sapphire host causes broad and widely separated absorption and fluorescence bands, which in turn means that titanium has a very large gain bandwidth. A very important application of this laser is the generation and amplification of femtosecond mode-locked pulses. Ti:sapphire was introduced in 1986 and it quickly replaced most dye lasers which had dominated the ultrashort pulse generation field. Ti:sapphire is also suitable for pumping test setups of new SSL because it can be easily tuned to the required pump wavelength with a high pump brightness. Laser cavity The efficiency of the laser system heavily depends on the pumping efficiency. A laser resonator efficiently transfers radiation between the pump source and the active media. It also controls the pump density distribution in the active media and thus the uniformity, divergence, and optical distortions of the 5

13 output laser beam. The design of the pump geometry plays an important role in the laser system to optimize the mode matching, that is the overlap between the -pumped volume and the volume occupied by a low-order resonator mode. For low-power SSL pumped by a diode laser, the end-pumping geometry has become more common than face- or side-pumping geometries. In the endpumping technique, pump radiation is introduced horizontally to the active medium i.e. co-linear with the resonator axis, whereas in the side-pumping technique, it enters transverse to the resonator axis. However, there are an increasingly large number of cavity designs that has been employed in lasers such as Z-, X- or V-shaped resonators in which the sample is placed under Brewster angle to avoid pump reflections. The laser resonator requires mirrors in both sides: a high-reflective mirror (HR) in one of them and a partially transmissive output-coupler (OC) in the other. Output intensity will be maximum for a unique output-coupler reflectivity. The design of the laser cavity - basically comprising optical elements, angles of incidence, and the distance between components - determines the cavity modes, which can be longitudinal or transverse at resonance frequencies. The gain in a laser is basically determined by the active medium and the optical pump source. As far as the active medium is concerned, the laser gain is directly related to the absorption and emission cross-sections and to the population inversion, thus to the inversion dependent gain cross section. The spectral match with the pump source determines the population inversion besides the fluorescent level lifetime 1.3. Nd:YAG Laser Rare earth activated crystal and glasses are extensively used as laser medium. Many of rare earth ions have energy levels which are suitable for 6

14 creating laser action. Nd +3 ion is one of the most widely used rare earth ion for this purpose. It is used as activator both in crystalline and glassy host. Nd:YAG i.e Nd +3 doped YAG crystal is a representative example of this type of laser medium. Figure 1 shows a picture of Nd +3 doped YAG rod used in cavity to generate laser light. Figure 1: Nd +3 doped YAG laser rod Figure 2: Absorption Spectrum of Nd:YAG crystal 7

15 Figure 3 :Emission energy levels of Nd:YAG crystal Figure 2 shows absorption spectrum of Nd +3 in YAG; while Figure 3 shows the emission of Nd +3 in the same host. It is the 4 F 3/2 4 I 11/2 transition which gives most intense NIR emission. Actually this emission transition is exploited to created 1.06 µm m laser light in the case of Nd:YAG laser. In the case of glassy host the same NIR emission is little bit broadened and shifted depending on the structure of the glass. Nd +3 doped laser glasses are also used as laser medium particularly for generation of high power laser. Nd:YAG laser, at present finds the application in various field of our day to day life. Unfortunately in India till date no effective technology has yet been developed for preparation of Nd doped crystals or glasses on commercial basis. We have undertaken an initiative in the School of Laser Science and Engineering, Jadavpur University, Kolkata to meet this challenge. Since, at this moment we do not have proper instrumental facilities for growing large crystals, as a first step of our initiative; we have undertaken a project to develop Nd +3 doped glasses in the present work. 8

16 1.4. Glass as host for laser activator ions A crystal has regular arrangement of their lattices, and for this reason there is no site to site variation in its structure. When a rare earth ion is incorporated in a crystal, every ion in the crystal experiences equivalent crystal field environment and therefore the absorption and emission of the activator ion in the host becomes sharp. Although crystalline hosts have some advantages but they are not suitable for making high power laser. It is not easy to grow a defect free large crystal. It is not possible to incorporate a large concentration of rare earth ion in a crystal because it may cause change in the crystalline phase and hence crystal-field properties of the crystal. To generate a high power laser, we need large laser medium to get more and more activator ions involved in the emission process. The advantages of glass over the crystals are:- We can make large defect free activator-doped glass by casting and melting technique in any justified shape. Relatively large concentration of rare earth ion can be incorporated in a glass compared to that in crystal. However, because of random structure of glasses, they have site to site variation in their network. When a rare earth ion is incorporated in a glass, every ion in the glass does not experience equivalent crystal field environment and therefore the absorption and emission of the activator ion in the host becomes broad. Conventional inorganic glasses are of fluoride, silicate, phosphate, borate based. 9

17 Figure 4: Nd +3 doped phosphate glass rods Figure 5: Nd +3 doped YLF glass rods Figure 4 shows the Nd doped Phosphate glass rods. Figure 5 depicts the Nd doped Yttrium Lithium Fluoride glass rods. To prevent non radiative excitation loss of the excited rare earth ions a low phonon glass is preferred. For this reason a lot of studies have been made on rare earth doped fluoride glasses in the past. As fluoride glass has low chemical durability, difficult to prepare and poor resistance to moisture, they are no longer considered as most desired host. Considering high stability and durability, we have chosen silicate glass as the host in the present work. Pure silica has very high melting temperature (around C). However the melting temperature of silica can be brought down to almost C by adding fluxes like alkali and alkaline earth oxides boron oxides etc. After extensive survey of literatures we have chosen a composition of silicate glass which is as follows; 10

18 SiO 2, B 2 O 3, CaO, SrO, Li 2 O, Na 2 O and Al 2 O mol% of Nd 2 O 3 has been added to this composition of glass sample Objective of the present work Neodymium activated glasses of different compositions and neodymium activated various crystals are already in use as a laser medium commercially. Unfortunately, till date no technology has been developed in our country to prepare such glass or crystals on commercial basis. So in our laboratory we have undertaken a project to develop optical quality Nd +3 activated glass of suitable compositions to study the feasibility of use of such glass as a laser medium. So main objective of the present work was to develop a Nd +3 activated optical quality stable glass and judge the feasibility of use of the material as laser medium. In this program our plan was first to select a glass composition after intensive survey of various relevant literatures and then melt a glass. Our next plan was to polish the sample for studying the absorption properties and calculate the crystal field parameters using Judd-Ofellt theory. We also planned to analyse the emission spectrum of Nd +3 ion in the glass and then compare the obtained values of Nd +3 ion in the present glass with those of other Nd +3 ion activated different crystals and glasses in order to judge how good is our glass for use as practical laser medium. 11

19 Chapter 2: Literature survey In reference to the present work some relevant journals have been studied, the survey of some of those journals habe been given in this chapter. 12

20 2.1. Literature Review In reference to the current research work, we have made an extensive survey of relevant literatures. A review of that survey is given below. H. Li et al. [6] reported the optical spectroscopic study on Al 2 O 3 -B 2 O 3 -SiO 2 glasses containing Nd 2 O 3. They have studied the local chemical environment of Nd(III) ions in glass with the help of Jud-Ofelt (J-O) optical parameter Ω 2 and Ω 6 which are related to ligand field symmetry (LFS) and the degree of bond covalency (BC) respectively. Two transition points (TP) in terms of Ω λ versus Nd 2 O 3 concentration were shown. The first TP is defined by Nd:3(B + Al) and the second TP by Nd:3(B + Al + Si). Ω 2 and Ω 6 increased and then decreased between first and second TP. Comparing with the similar types of glasses reported in literatures they proposed that up to the first TP, Nd(III) preferentially dissolves in a borate rich environment. Further increasing the concentration of Nd 2 O 3, results in excess of Nd(III) cations, partitioning to a silicate rich environment, whereas the concentration of Nd cations in the borate sites is expected to be unchanged. Crystallization results from the saturation of Nd in the silicate rich environment, indirectly supporting the proposed Nd(III) dissolution mechanism via a partitioning process. L. Jyothi et al. [7] have extensively described the spectroscopic properties of tellurite glasses of composition TNKNd:(70-x)TeO 2-15Nb 2 O 5-15K 2 O-xNd 2 O 3 (x= 0.1, 1.0, 1.5, 2.0 and 2.5), TNLNd10:69TeO 2-15Nb 2 O 5-15Li 2 O-1.0 Nd 2 O 3 and lithium metaborate glass of composition LBNNd10:89LiBO 2-10 Nb 2 O 5-1.0Nd 2 O 3. They have investigated the absorption and emission spectra and analysed the decay curve. An energy level analysis has been carried out considering the experimental energy position of the absorption and emission bands using free-ion Hamiltonian model. They have estimated the spectral 13

21 intensities, radiative transition probabilities, emission cross-sections, branching ratios and radiative lifetimes using Judd-Ofelt theory. They found that the decay curve of the glasses is exponential at the lower concentration and non exponential for higher concentration (>=1.0 mol%) of Nd +3 ions. They have compared the results with Nd:doped other hosts like phosphate, fluorophosphates, lead borate, tellurite, germinate and silicate glasses, YAG ceramic and Ca 2 Nb 2 O 7 crystals. N.W. Jenkins et al. [8] presented a discussion of the low-phonon energy host material potassium lead chloride, KPb 2 Cl 5, doped with Nd. They have studied the crystal growth, spectroscopic properties and they have presented an analysis of the magnetic and electric dipole transitions based on Judd-Ofelt model. They have also examined the concentration quenching of the fluorescence. They suggested the possibility of a 3-for-1 cross-relaxation process on the basis of the spectroscopic data and the known resonances in energy spacing of the first three excited states in Neodymium. They have shown this process may efficiently populate the first excited state, 4 I 11/2, to be metastable with lifetime on the order of few milliseconds. Population inversion of this level should be possible enabling a laser based on 5.5 µm, 4 I 11/2 to 4 I 9/2 transition. K. Binnemans et al. [9] conducted a comparative study on the optical absorption spectra of Nd-doped fluorophosphate glasses of the type 75NaPO 24AF 1NdF (A=Li, Na, K) and of the type 75NaPO 24AF 1NdF (A=Ca, Sr, Ba, Zn, Cd). The dipole strengths are parameterised in terms of three phenomenological Judd-Ofelt intensity parameters Ω λ (λ=2, 4 and 6). They determined the value of these three intensity parameters, the probability for spontaneous emission, the branching ratios and the radiative lifetime are. They 14

22 discussed the relation between the spectral intensities and the glass composition. They compared the optical properties of the fluorophosphate glasses with those of phosphate and fluoride glasses. The authors concluded the spectral behaviour of the fluorophosphate glasses which are intermediate between the spectral behaviour of pure phosphate and fluoride glasses. G.N. Hemantha Kumar et al. [11] provided a survey of sodium potassium phosphate glass consisting different Nd 2 O 3 concentrations. They studied the effect of Nd 3+ concentration on optical absorption and fluorescence properties. From the absorption Judd Ofelt intensity parameters (Ω 2, Ω 4 and Ω 6 ) are evaluated and these parameters are used to study the covalency as a function of Nd 3+ concentration. Authors have shown that covalency decreases with the increase of Nd 3+ concentration. By using these three intensity parameters, total radiative transition probabilities, radiative lifetimes, branching ratios and integrated absorption cross-sections, they have calculated for certain excited states of Nd 3+ in these mixed alkali phosphate glasses for all the concentrations. From the fluorescence spectra, they have calculated the peak stimulated emission cross-sections, for the two transitions, 4 G 7/2 4 I 13/2 and 4 G 7/2 4 I 11/2 of Nd 3+ in all these glass matrices. They compared all these spectroscopic parameters for different Nd 3+ concentrations. K. Upendra Kumar et al. [12] investigated the optical absorption, fluorescence and decay curves for the 4 F 3/2 level of Nd 3+ ions in phosphate (P 2 O 5 K 2 O SrO Al 2 O 3 ) and fluorophosphates (P 2 O 5 K 2 O SrO Al 2 O 3 AlF 3 and P 2 O 5 K 2 O SrO Al 2 O 3 BaF 2 ) glasses doped with three concentrations (0.1, 1.0 and 2.0 mol%) of Nd 3+ ions. They applied the Judd Ofelt (JO) theory to the absorption spectra of 1.0 mol% Nd 3+- doped glasses to derive JO intensity parameters which are in turn used to calculate the radiative properties of the Nd 3+ ion in fluorescent 15

23 levels. They analyzed the energy level data of Nd 3+ (4f 3 ) ions in terms of a parameterized free-ion Hamiltonian model that consists of 20 interaction parameters of atomic nature. They calculated the stimulated emission cross section and branching ratios using the emission spectra. The relatively higher branching ratio for 4 F 3/2 4 I 11/2 transition showed the suitability of these glasses for laser application. A. Belykh et al. [13] measured the absorption and luminescence of chalcogenide glasses doped with neodymium. The concentrations of Neodymium were achieved <=2 wt%. They also measured the luminescence spectra and the excited state lifetime of Nd +3 ions. They have shown that spectral properties of Nd +3 are similar regardless of which neodymium compound was used. However, these spectral properties differ when the base glass is either an oxide or a fluoride. They found that the quantum yield of Nd+ 3 in the chalcogenide samples approaches 100%. They discussed the viability of chalcogenide matrixes as laser hosts. A. Renuka Devi et al. [14] investigated through their work the compositional dependence of optical properties of Nd +3 ions are in the following lithium borate (LBO) glasses: Li 2 CO 3 + H 3 B0 3 and MCO 3 + Li 2 CO 3 + H 3 BO 3 (M = Mg, Ca, Sr and Ba). They assigned the energy-level data of Nd 3+ (4f 3 ) in these borate glasses, as well as the data that are available for different systems in the literature, they have analysed in terms of a parametrized Hamiltonian model that includes 20 free-ion parameters. They determined the Judd-Ofelt intensity parameters which were used to predict various radiative parameters for fluorescent levels of Nd 3+ ions. They have discussed the dependence of spectroscopic properties of the glass composition and compared with similar results. 16

24 Qingguo Wang et al. [15] obtained a Nd-doped CaF 2 crystal with high optical quality by temperature gradient technique (TGT). They determined the energies of the crystal field levels of Nd 3+ multiplets relevant to laser operation based on optical spectra recorded at T = 10 K. They analyzed the absorption spectra in the room temperature. With the help of the Judd Ofelt theory they have calculated radiative transition rates and luminescence branching ratios for the 4 F 3/2 level. They have calculated the 4 F 3/2 radiative lifetime, 1295 µs whereas a monotonous decrease of the 4 F 3/2 luminescence lifetime value from 1.45 ms to 0.9 ms was observed when the temperature increased from 10 K to 300 K. They determined the stimulated emission cross-section cm 2 at 1061 nm, using the Fuchtbauer Ladenburg relation. All the results of their work has shown that Nd: CaF 2 would be a promising gain media in solid-state lasers. Artur Bednarkiewicz et al.[18] synthesized and characterized transparent, chloroform dispersed χ-nayf 4 nanocrystals doped with neodymium ions. XRD and TEM measurement confirmed the cubic structure of χ-nayf 4 of the nanoparticles (NPs). They measured the absorption and emission spectra as well as 4 F 3/2 level fluorescence decay curves in order to estimate the influence of Nd 3+ concentration in the matrix on the optical properties of the NPs. With the increase of Nd3+ doping level, the Judd Ofelt Ω 4 parameter as well as the spectroscopic Nd 3+ parameter χ Nd = Ω 4 / Ω 6 was growing. In the same time the Ω 4 and Ω 6 were decreasing. They also calculated the theoretical luminescence lifetimes of the 4 F 3/2 level equal to 300 µs and compared with the experimental values to quantify the concentration quenching. Based on this comparison, quantum efficiency, they found varying systematically between 100% and 4% for Nd 3+ content increasing from 2 up to 25%. 17

25 D.A. Ikonnikov et al. [19] through their work studied and measured the absorption spectra of the Nd 3+ ions in an orthorhombic δ-bib 3 O 6 single crystal in the spectral range 11,000 20,500 cm_1. The f f transition intensities were analyzed in terms of the Judd Ofelt theory, and the following parameters of the theory were obtained: Ω 2 = cm 2, Ω 4 = cm 2 and Ω 6 = cm 2. The strengths, spontaneous emission probabilities, branching ratios, spectroscopic quality factor and excited state radiative lifetime were calculated for laser transitions from the 4F 3/2 state to 4 I J manifold. They have shown that the spectroscopic properties of Nd 3+ :δ- BiB 3 O 6 crystal favour lasing at 1.3 lm, where this crystal possesses near non-critical phase matching for second harmonic generation. Guohua Jia et al. [20] successfully developed the Nd 3+ doped LaB 3 O 6 single crystal with the size up to ɸ20 35 mm by the Czochralski technique. The absorption and luminescence spectra of trivalent neodymium in LaB 3 O 6 crystal were measured at room temperature. They also measured, room temperature luminescence decay curve in correspondence with the emission line 4 F 3/2 4 I 11/2 centered at 1062 nm. In the framework of the Judd Ofelt (J O) theory, the intensity parameters were calculated to be: Ω 2 = cm 2, Ω 4 = cm 2, Ω 6 = cm 2. They calculated the radiative probabilities, radiative branching ratios and radiative lifetime for the emissions 4 F 3/2 from the level of Nd 3+ ions in LaB 3 O 6 single crystal. 18

26 Chapter 3: Theory The discussion of crystal field theory and Judd-Ofelt theory are given in this chapter to analyze the absorption and emission spectrum of Nd +3 ion in glass to find out the crystal field parameters, radiative transition probability, branching ratio, radiative lifetime and lasing efficiency. 19

27 3.1. Crystal Field Theory When an activator ion is incorporated in a host, the electric field effect on the activated ion varies with the variation of the geometry of the surrounding ions of the host. Therefore electric dipole mediated transition vary with the variation of the geometry i.e. crystal field of the surroundings, but magnetic dipole mediated transition does not vary with the geometry of the surroundings. The main approximation of crystal-field theory is associated with introduction of an effective Hamiltonian of the dopant ion with an unfilled electron (d or f) shell in a crystal. H=H 0 +W cr, Where H 0 =Hamiltonian of a free ion, and W cr =the energy of the ion interaction with the lattice, averaged over quantum-mechanical states of the latter so that W cr can be represented as the energy of electrons of the activator ion in an effective crystalline electric field. The centrally symmetric self-consistent potential determining one-electron energies having been selected in H 0,in order to find the energy levels of every electronic configuration governed by electron distribution over the states with fixed principal (n) and orbital (l) quantum numbers, it is necessary to calculate the Eigen value of the operator W=W ee +W so +W cr, This is treated as a perturbation of energy. Here W ee is the energy of the electrostatic interaction of the activator electrons and W so is the operator of the spin-orbit interaction. We know from spectroscopy that any (f-f) transition of n rare earth ion in the Free State is forbidden because of parity consideration, but when the ion is incorporated in a host i.e. in a crystal or glass, crystal field of the host influences the symmetry of the different electronic state of the incorporated rare earth ions. As a result these (f-f) transitions in a host become 20

28 partially allowed. The strength of any electric dipole mediated transition of a rare earth ion is strongly dependent on its surrounding crystal field, the more is the crystal field strength, better is the probability of transition of the rare earth ion Theoretical background of Judd-Ofelt calculation In the year 1962 Judd and Ofelt, working independently, developed the theory of electronic transition line intensities of Li +3 ions in crystals, which is universal in estimating the optical and laser properties of Li +3 ions in crystals and glasses. According to Judd-Ofelt theory the electric dipole line strength S ed of an electronic transition from an initial, state to the excited, state is given by the following expression:, =Ω,, +Ω,, + Ω,, (1) Where Ω,Ω,Ω are the Judd-Ofelt parameters of the concerned host which actually represent crystal field strength of the host. The term,,,,, and,, are the squares of the reduced-matrix elements for the related transitions. These values of matrix elements for different transitions of a rare earth ion are available in the literature. Since a rare earth ion in a host may exhibit number of electric dipole mediated transitions, for each transition we can write an expression relating its experimentally measured to its different square of reduce matrix element. The experimental absorption strengths of a transition can be determined by studying the absorption spectrum of the rare earth ions in the host. 21

29 , =. (2) Where, is the measured electric dipole line strength, λ is the mean wavelength of the absorption band, n is the refractive index of the host with respect to λ, d is the thickness of the host understudy, ρ is the concentration of Nd +3 ions(ions/cc) in the host, is the experimental integrated optical density in the wavelength range of the band ( which can be obtained by calculating the total area under the band ), e is the electronic charge, c is the velocity of light, h is the plank s constant. is the magnetic dipole line strength of the transition. is related to the magnetic dipole oscillator strength by the following equation: = (3) The values for different transitions of a series of rare earth ions have been reported in literature. By putting the experimental values of for a particular transition of Nd +3 ion in the case of our glass in equation (1) and finding the respective square of the reduce matrix element of the transition from the literature, we can have equation relating with experimental measured and Judd-Ofelt parameters. For each different transition we will get different equation of relating to Ω,Ω,Ω. These equations can be solved iteratively by using a least square fitting program. This solution will give us the values of Ω,Ω,Ω of the host/glass and hence we will have an idea of the emission cross section, radiative rate constant, radiative lifetime, branching ratio of Nd +3 ions in the glass. The radiative rate constant: 22

30 , = +..(4) The radiative branching ratio which defines a specific radiative transition from the excited state ( ) to a terminal state ( ), divided by the sum of all the radiative transitions from. =,, Radiative lifetime: =, (5), =,..(6) Hence and denote the total angular momentum quantum number of the initial and final states., is the non radiative transition rates, τ is the experimental luminescence lifetime. The luminescence quantum efficiency (η) of the system can be calculated by η = τ /τ, the lower the η is the greater amount of energy is being lost for the non-radiative process. So, after studying the absorption and luminescence spectra and compare them with the theoretical value, we may conclude whether the Nd-doped glass is suitable for laser medium. 23

31 Chapter 4: Experimental This chapter includes the preparation of sample : melting and casting technique, annealing and polishing of the sample, measurement of density of the sample and the concentration of Nd +3 ion in the sample. Recordings of absorption spectrum and energy diagram of all possible emission transition of Nd +3 ion are also given in this chapter. 24

32 4.1. Sample Preparation Glass Composition The compositions of glass chosen foe the present work are given in Table 1. The compositions were chosen stoichiometrically and were mixed vigorously to achieve a homogeneous batch mixture. in gm Table 1: Composition of sample with their mole% and equivalent weight Compositions of Glass % (mole) Equivalent weight in gm SiO B 2 O CaO SrO Li 2 O Na 2 O Al 2 O Nd 2 O In the case of each melting a batch of approximately 10gm was taken Melting The sample has taken in an alumina crucible shown in Figure 6 and kept inside a batch melting furnace. 25

33 Figure 6: Alumina crucible The temperature range of batch melting furnace is C. AC three phase and 440V line supply is necessary to operate the furnace. Figure 7: Batch melting furnace Figure 7 shows the picture of Batch Melting Furnace. There is a PID controller in the furnace where the temperature operation for melting of the glass sample was set. From the room temperature to C, the temperatures were segmented in eight part corresponding time and current consumption. They were set manually according to the program mentioned in Table 2. 26

34 Table 2:Temperature segment with respect to time and current consumption Segment Temperature( 0 C) Time Current consumption (%) hour 10 min hour 40 min min min hour 15 min min min Casting The melt was then cast in a steel mould shown in Figure 8 in the shape of a rectangular block outside the batch melting furnace. Figure 8: rectangular steel mould 27

35 Annealing The melted sample after casting has kept inside an annealing furnace. The temperature range of annealing furnace is C. AC three phase and 440V line supply is necessary to operate the furnace. Figure 9: Annealing furnace Figure 9 shows the picture of the Annealing furnace. In this furnace also, there is a PID controller which is used to fix the temperature and rate of cooling. The temperature of the Annealing furnace was set C manually. Foe annealing of our glass sample, we set a programme of cooling 10 0 per hour. The sample was checked for any residual engineering stress, fine crack in microstructure, and was found flawless. Figure 10: Base glass sample 28

36 Figure 11: Nd +3 doped sample glass after annealing Figure 10 and11 show the picture of base glass and that of Nd +3 doped sample glass after annealing Polishing The glass sample used for optical and luminescence measurements was subjected to three steps grinding and polishing. For grinding purpose a mesh of 320-, 400- and 600- SiC abrasive powder are used. For fine polishing a mesh of 300µs Ce 2 O 3 is used. Figure 12: Grindings machine 29

37 Figure 12 shows the picture of Grindings machine on which the sample was polished in a rectangular plate shaped. After polishing, the plate became optically transparent. The polished sample plate was cleansed in acetone. Figure 13: Rectangular transparent Nd +3 doped sample plate Figure 13 shows the optically transparent rectangular Nd +3 doped glass plate after polishing Instrument and Measurement Measurement of density of the sample The density of the glass sample has been measured using Archimedes s principle. This law of physics states that the upward buoyant force exerted on a body immersed in a fluid is equal to the weight of the fluid the body displaces. In other words, an immersed object is buoyed up by a force equal to the weight of the fluid it actually displaces. In this work we have measured the apparent weight loss of sample after it was immersed in fluid and then calculated the density by finding out the volume of the solvent displaced. Hence, we have chosen cyclo-hexane as the solvent. In an electronic balance using specific gravity bottle we have measured all data to find out the density of the Nd +3 doped glass. 30

38 Figure 14: Specific gravity bottle Figure 15: Electronic balance Methods Weight of empty bottle, W 1 = gm Weight of bottle + solvent, W 2 = gm Weight of bottle + sample, W 3 = gm Weight of bottle + solvent + sample, W 4 = gm Therefore, Weight of full solvent, (W 2 - W 1 ) = gm 31

39 Weight of solvent + sample, {(W 2 - W 1 ) (W 3 W 1 )}= gm Weight of solvent + sample after immersion, (W 4 - W 1 ) =8.5725gm So, Apparent weight loss of sample, M a,{ (W 2 - W 1 ) (W 3 W 1 ) }- (W 4 - W 1 ) =0.0542gm Hence, Density, D of the solvent, cyclohexane = gm/cc Now, Volume of the displaced solvent, V s = M a /D = g Density of the sample, {weight of the sample i.e (W 3 W 1 )}/Volume of displaced solvent i.e V s =2.715 gm/cc Concentration of Nd +3 ion in the sample 100.5mol contains gm Nd 2 O 3, i.e 1 mol Nd 2 O 3 is equivalent to, /336.5 = 0.005gm-mol 1 gm-mol Nd 2 O 3 contains 2 Avogadro s number i.e N ions of Nd +3 (N= ) 0.005gm-mol contains 2 N ions Volume of sample, weight of sample/density of sample = /2.715 = cc Hence, cc contains 2 N ions of Nd +3 32

40 So, 1 cc contains (2 N 0.005)/ = ions/cc The concentration of Nd +3 ions in the sample is ions/cc Recordings of Absorption Spectra Most materials absorb light to a degree that depends on the electronic structure of the material. In the near ultraviolet (UV), visible, and infrared (IR) spectral ranges of light, radiation is mainly absorbed by electrons in atoms, ions, or molecules. Thus, the absorption is due to electronic transitions. The Beer- Lambert law, also known as Beer's law, is an empirical relationship that relates the intensity of absorption of light to the electronic transition properties of the material through which it is travelling: I=I 0 e -αd where I 0 is the incident light intensity; α is the absorption coefficient; and d is the thickness of the sample. The absorption is manifested in the variation of the intensity of the electromagnetic radiation (I) as a function of the wavelength. To measure this absorption, we experimentally measured the optical density (OD). OD = log (I /I 0 ) Combining the two equations, we can correlate the absorption coefficient with the optical density: α = OD (1/dlog e) We can also correlate α with the absorption cross-section of every absorbing atom or ion as: σ = α/n 33

41 where N is the density of absorbent centres. In this expression α is given in cm -1 and N in atoms cm -3 ; thus, the units of σ are atoms cm -2. Absorption spectra of the base glass and that of Nd doped glass sample were recorded at 300K in a Shimadzu absorption spectrophotometer in UV- VIS-NIR spectral region. Figure 16: Absorption Spectrophotometer Figure 16 shows the picture of Absorption Spectrophotometer with a display unit where the spectra i.e Optical density versus Absorption wavelength of Nd +3 ion in is displayed shown in Figure 17. Figure 17: Absorption spectra of base glass and that of Nd doped glass 34

42 Emission process of upon excitation of the glass Optical emission, or luminescence, is the result of a radiative transition of an electron from an excited or higher energy level to a lower energy level. The luminescence spectra are determined by fixing the excitation wavelength and changing the detection wavelength. It is also interesting to determine the temporal evolution of the luminescence after excitation. Usually, the luminescence intensity declines exponentially. The time at which the intensity has decreased to 1/e of its initial value is called the radiative lifetime (τ). The radiative lifetime is an important parameter since it characterises how an electronic level is depopulated and what the thermalisation mechanisms of this excitation are. To determine the lifetime, a pulsed excitation source is needed. Moreover, the pulses must be shorter than the mean lifetime of the electronic level of the ion. Due to lack of facility we could not record the luminescence of Nd +3 in the present glass. However, in the case of Nd +3 activated sample glass possible emission transition are shown in the Figure below; 4 F 3/2 Laser Transition States 4 I 15/2 4 I 13/2 4 I 11/2 4 I 9/2 Figure18:Possible emission transition levels Figure 18 shows the possible emission transitions from excited 4 F 3/2 state to ground 4 I J states. 35

43 Chapter 5: Result The value of crystal field parameters of Nd +3 ion in glass have been found out by analysing the absorption spectrum using Judd-Ofelt equations. Radiative transition probability, branching ratio, radiative lifetime of possible emission transition of Nd +3 ion in glass are also given in this chapter. 36

44 5.1. Absorption Spectrum Absoprtion spectra of base glass and sample glass was monitored using Origin 6.1 software. We have extracted the absorption spectrum (shown in Figure 20) of Nd:doped sample glass from base glass(shown in Figure 19) and analysed for further calculation. Figure 19: Absoption spectrum of base glass Figure 20: Absorption spectrum of Nd +3 doped glass sample 37

45 The base glass and base glass corrected absorption spectrum of the Nd doped glass are shown in Figure 19 and Analysis of Absorption Spectrum The spectrum has a very strong absorption edge below 400nm and exhibits a number of very distinct sharp absorption bands in the VIS-NIR region e.g. at 527, 586, 750, 807 and 879 nm. Comparing the energy of these observed bands with those of Nd +3 in other similar hosts reported in the literature [7, 8], the corresponding electronic transitions can be defined as transition from ground 4 I 9/2 state to ( 2 K 13/2, 4 G 7/2, 4 G 9/2 ), ( 4 G 5/2, 4 G 7/2 ), ( 4 F 7/2, 4 S 3/2 ), ( 4 F 5/2, 2 H 9/2 ) and 4 F 3/2 states of Nd +3 ions of the sample glass. With help of Judd-Ofelt equations the absorption spectrum can be analyzed. The Equations are:, =Ω,, +Ω,, + Ω,,.(1), = =...(2).(3) Where Ω,Ω,Ω are the Judd-Ofelt parameters of the concerned host which actually represent crystal field strength of the host. The term,,,,, and,, are the squares of the reduced-matrix elements for the related transitions. These values of matrix elements for different transitions of a rare earth ion are available in the literature. Since a rare earth ion in a host may exhibit number of electric dipole mediated transitions, for each transition we can write an expression relating its experimentally measured to its 38

46 different square of reduce matrix element. The experimental absorption strengths of a transition can be determined by studying the absorption spectrum of the rare earth ions in the host. Where, is the measured electric dipole line strength, λ is the mean wavelength of the absorption band, n is the refractive index of the host with respect to λ, d is the thickness of the host understudy, ρ is the concentration of Nd +3 ions(ions/cc) in the host, is the experimental integrated optical density in the wavelength range of the band ( which can be obtained by calculating the total area under the band ), e is the electronic charge, c is the velocity of light, h is the plank s constant. is the magnetic dipole line strength of the transition. is related to the magnetic dipole oscillator strength eqn(3). The reduced square matrix corresponding to electronic transition from ground 4 I 9/2 is given in Table 3. The values of reduced square matrix were taken from Kaminiskii [1]. Table 3: Electronic transitions from ground state to excited state and their corresponding wavelength and reduced square matrix value, : 4 I 9/2 (U (2) ) 2 (U (4) ) (U (6) ) λ max (nm), 4 F 3/ F 5/2, 2 H 9/ F 7/2, 4 S 3/ G 5/2, 4 G 7/ K 13/2, 4 G 7/2, G 9/2 39