Optical Properties of Trivalent Praseodymium Doped In A Polymeric Plastic Environment By: Dhiraj K. Sardar, Ph.D., and Anthony Sayka June 2008

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1 Optical Properties of Trivalent Praseodymium Doped In A Polymeric Plastic Environment By: Dhiraj K. Sardar, Ph.D., and Anthony Sayka June 2008 A : HEMA based laser module has the potential for numerous biomedical and semiconductor applications because of its unique physical and optical properties. Trivalent praseodymium ion has long been considered a serious contender for laser applications. The Judd-Ofelt 1,2 theory has been one of the most successful theories in estimating the magnitude of the forced electric dipole transitions of rare-earth ions in a large variety of laser host media. Also, there has been significant interest in the development of tunable solid-state dye lasers, where the fluorescent dye molecules are embedded in polymeric plastic hosts suitably shaped for various photonic applications. 3,4 High efficiency of pyrromethene doped solid-state dye lasers have been reported by Hermes et al. 3 Wadsworth et al. 4 have investigated the physical and optical properties of such plastic hosts as polymethyl methacrylate (PMMA) and copolymers of methyl methacrylate (MMA) with 2-hydroxyethyl methacrylate (HEMA). Sardar et al. have reported the optical properties of in HEMA. 6 In this article, the Judd-Ofelt (J-O) theory is applied to the room temperature absorption spectrum of in HEMA plastic host to determine the intensity parameters: The intensity parameters are subsequently used to determine the radiative decay rates and branching ratios of the transitions from the upper to the lower energy levels. From the radiative decay rates, the radiative lifetimes of the excited states are also determined. Since the J-O intensity parameters used to determine the radiative probabilities depend on the particular rareearth ion and its environment, a precise knowledge of these phenomenological parameters is imperative to characterize the spectroscopic properties of ion. PLASTIC HEMA DOPED WITH The solid plastic sample of HEMA doped with trivalent praseodymium was fabricated in our laboratory from the liquid form of the monomer: 2-hydroxyethyl methacrylate, HEMA (obtained from Sigma-Aldrich), containing 300 ppm of monomethyl ether of hydro quinine, MEHQ, a polymerization inhibitor. An appropriate amount of praseodymium nitrate salt was added to the mixture of uninhibited HEMA and benzoyl peroxide to obtain a 2 wt.% of. The mixture was then stirred slowly until both compounds were completely dissolved in the HEMA. Stirring was done carefully and in a cool and dark environment to avoid rapid polymerization. The mixture was then transferred into the sample reservoir made of standard glass slides and epoxy. The reservoir containing the HEMA solution was then baked in a rarified atmosphere in an oven at 60 C for 45 minutes and then 75 C for 1 hour in a beaker of water to allow consistent heating over the entire mass of the sample. This also has helped suppress turbidity caused by air bubbles due to unwanted hot spots. The oven was then turned off and the sample was left in the oven overnight for slow cooling of the sample. Finally, the glass slides were carefully broken and the final product was a 1 mm thick clear solid plastic doped with tri valent praseodymium ( : HEMA). Modules formed of : HEMA are durable, biocompatible, and can be formed easily into any size and shape. This will allow for a diverse range of applications for the biomedical, semiconductor, and photonic industries. Okamoto et al. have studied polymeric materials formed by reacting the salts of trivalent rare earth ions with polymers to complex them with various functional groups such as COO-, a carboxylate group attached to a polymeric backbone. 7 They have reported that the fluorescence intensity decreases with increasing ion concentration, thereby suggesting that fluorescence quenching can occur due to energy transfer between rare earth ions that are within 1 mm of each other. According to Okamoto et al., the fluorescence quenching can be (1 of 6)6/10/2008 8:19:49 AM

2 attributed to the clustering of rare earth ions in solid polymer matrices. EXPERIMENT The room temperature absorption spectrum of ions in HEMA was recorded using an upgraded Cary model 14R spectrophotometer. This spectrum ranging from 400 to 2100 nm is shown in Figure 1. All spectra were taken at 0.2 nm intervals. The spectral bandwidth was about 0.5 nm for all measurements. The spectrophotometer is equipped with two light sources and two detectors. A deuterium lamp was used for wavelengths below 350 nm, and a tungsten lamp for wavelengths above 350 nm. A photomultiplier tube (PMT) was used for wavelengths below 900 nm with a fixed high voltage of 400 V, and a PbS detector was used for wavelengths above 900 nm with a fixed biasing voltage of 6.4 V. The slit width, therefore, automatically adjusts depending on the light intensity. RESULTS AND DISCUSSION Eight absorption bands identified in the room temperature absorption spectrum between 400 and 2100 nm displayed in Figure 1 were chosen to determine the intensity parameters for in HEMA. The intensity,, of the individual absorption band is determined by using the following equation: where and are the total angular momentum quantum numbers of the initial and final states, respectively, n is the refractive index of the host material, is the -concentration, is the mean wavelength of the specific absorption band, is the integrated absorption coefficient as a function of, and c and (2 of 6)6/10/2008 8:19:49 AM

3 h have their usual meaning. The measured intensities were then used to obtain the J-O parameters by solving a set of eight equations for the corresponding transitions between the manifolds in the following form: and where a and b denote other quantum numbers specifying the initial and final eigenstates, respectively, and the matrix elements are doubly reduced unit tensor operators of rank t calculated in the intermediatecoupling approximation and are independent of the crystal host. The J-O parameters that are treated as phenomenological parameters tend to vary with local environment due to changes in the crystalfield parameters associated with changes in the charges and positions of the ligand ions. However, the parameters exhibit influence of the host on the transition probabilities since they contain the crystal-field parameters, interconfigurational radial integrals, and the interaction between the central ion and the intermediate environment. Values of the reduced matrix elements for the chosen bands are taken from Kaminskii. 8 When two or more absorption manifolds overlapped, the squared matrix element was taken to be the sum of the corresponding squared matrix elements. The values of the measured absorption line strengths, Smeas are tabulated in Table I. A least-squares fitting of measured intensities to the intensities represented by the Judd-Ofelt theory provides the following values for the following three J-O intensity parameters for :HEMA: Values of the intensity parameters can be used to recalculate the transition line strengths of the absorption bands. These parameters were then used to determine the line strengths corresponding to the transitions from the upper metastable states to the corresponding lower lying manifold states of in HEMA. Using these line strengths, the radiative transition rates,, for electric dipole transitions between an excited state and the lower lying states can be calculated using the following expression: (3 of 6)6/10/2008 8:19:49 AM

4 The room temperature fluorescence branching ratios, rates by, can be determined from the radiative decay where the sum runs over all final states transitions studied are given in Table II.. The radiative transition rates and branching ratios for the The radiative lifetime for an excited state ( ) is calculated by where the sum is taken over all final states. The values of the radiative rates calculated by using Eq. (3) are added to obtain the total radiative rates for the states, respectively. Therefore, the radiative lifetime of the metastable states were determined to be 10.56, 3.68, and 0.69 ms, respectively. The values of these radiative lifetimes are tabulated in Table II. The fluorescence branching ratios are critical parameters to the laser designer, since they characterize the possibility of attaining stimulated emissions from any specific transition. These values are also given in Table II. (4 of 6)6/10/2008 8:19:49 AM

5 (Click Image For A Larger Version) CONCLUSION The spectroscopic analysis of in HEMA has been performed following the standard J-O model that predicts the three intensity parameters. Values of these parameters were then employed to obtain the radiative decay rates, radiative lifetimes, and branching ratios of the principal intermanifold transitions of from the manifold states to the lower-lying manifolds. The calculated line strengths for the forced electric dipole transitions were fitted to the previously measured absorption line strengths yielding an rms error of 3.2%. Owing to the high branching ratio (52.7%) for the transition (~ 630 nm), this material has the potential to become an excellent laser system. Furthermore, because the absorption spectrum of in HEMA is similar to that of in phosphate glass, 9 it can be suggested that the symmetry in HEMA is similar to that of in glass. The authors would like to thank David Stonestreet and Doug Dee for taking the spectroscopic data. This work was supported in part by the American Chemical Society- Petroleum Research Fund No. PRF B6 and the National Science Foundation Grant No. DMR References 1. B. R. Judd, Phys. Rev., 127, 750 (1962). 2. G. S. Ofelt, J. Chem. Phys., 37, 511 (1962). 3. R. E. Hermes, T. H. Allik, S. Chandra, and J. A. Hutchinson, Appl. Phys. Lett., 63, 877 (1993). (5 of 6)6/10/2008 8:19:49 AM

6 4. W. J. Wadsworth, S. M. Giffin, I. T. McKinnie, J. C. Sharpe, A. D. Woolhouse, T. G. Haskell, and G. J. Smith, Appl. Optics, 38, 2504 (1999). 5. D. K. Sardar, R. M. Yow, C. D. Coeckelenbergh, A. Sayka, and J. B. Gruber, Polym Int. 54, 412 (2005). 6. D. K. Sardar, R. M. Yow, and J. B. Gruber, Opt. Mat. (in press), (2007); references therein. 7. Y. Okamoto, J. Macromol. Sci. Chem., A24, (1987). 8. A. Kaminskii, Laser crystals, Vol. 14, (Springer-Verlag, New York, 1981). 9. D. K. Sardar and C. C. Russell III, J. Applied Phys. 95, 5334 (2004). Dhiraj K. Sardar, Ph.D. is a professor of physics at the University of Texas at San Antonio. He has more than 26 years of research experience in laser materials, and has supervised a large number of student research projects involving nanotechnology, semiconductor processing, and characterization of optical and laser properties of various solid-state laser materials. He can be contacted at dsardar@utsa.edu. Anthony Sayka received an M.B.A. degree and a B.S. degree in Physics from the University of Texas at San Antonio and is currently employed by Maxim Integrated Products, Inc. He has 18 years of experience in the semiconductor industry, including engineering positions with Intel, Advanced Micro Devices, VLSI Technology Inc., and Micron Technology Inc. He has published nine papers in the areas of semiconductor processing and laser materials, and holds numerous patents. He can be contacted at saykacr@hotmail.com. (6 of 6)6/10/2008 8:19:49 AM