Spectroscopic analysis of Nd 3+ (4f 3 ) absorption intensities in a plastic host (HEMA)

Transcription

1 Polymer International DOI: /pi.1708 Spectroscopic analysis of Nd 3+ (4f 3 ) absorption intensities in a plastic host (HEMA) Dhiraj K Sardar, 1 Raylon M Yow, 1 Cody H Coeckelenbergh 1 Anthony Sayka 1 and John B Gruber 2 1 Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX , USA 2 Department of Physics, San José State University, San José, CA , USA Abstract: Spectroscopic and laser properties of Nd 3+ embedded in a 2-hydroxyethyl methacrylate (HEMA) solid plastic host are characterized. The Judd Ofelt model has been applied to the room temperature absorption intensities of Nd 3+ (4f 3 ) transitions in HEMA to determine the three phenomenological intensity parameters: 2, 4 and 6. The intensity parameters are then utilized to determine the radiative decay rates (emission probabilities of transitions) and branching ratios of the Nd 3+ (4f 3 ) transitions from the upper manifold state 4 F 3/2 to the lower-lying multiplet manifolds 4 I J. The predicted decay rates and branching ratios of the Nd 3+ (4f 3 ) inter-manifold transitions in HEMA are compared with those in other laser host media. From the radiative decay rates, the radiative lifetime of the Nd 3+4 F 3/2 excited state is determined Society of Chemical Industry Keywords: absorption/emission spectra; spectral intensity; rare-earth; plastic host INTRODUCTION Over the past four decades, trivalent neodymium Nd 3+ has been considered to be one of the most important laser activators in a large number of nearinfrared solid-state lasers. 1 3 The Judd Ofelt 4,5 (J O) 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 host media. In recent years, there has been significant interest in the development of tunable solid-state dye lasers, where the dye molecules are embedded in plastic hosts. 6 8 Hermes et al 6 have reported high efficiency of pyrromethene-doped solid-state dye lasers. The physical and optical properties of several plastic hosts, such as poly(methyl methacrylate) (PMMA) and copolymers of methyl methacrylate (MMA) with 2-hydroxyethyl methacrylate (HEMA), have been thoroughly investigated by Wadsworth et al. 9 In the present study, standard J O analysis is applied to the room-temperature absorption spectrum of Nd 3+ in a HEMA plastic host to determine the optical intensity parameters: 2, 4 and 6.These phenomenological intensity parameters in turn are utilized to determine the radiation transition probabilities (decay rates), radiative lifetimes, and branching ratios of Nd 3+ manifold-to-manifold transitions in HEMA. Since the J O intensity parameters used to determine the radiative probabilities depend on the particular rare-earth ion and its environment, a precise knowledge of these phenomenological parameters is needed in order to characterize the spectroscopic properties of Nd 3+. We have performed an in-depth J O analysis on Nd 3+ (4f 3 ) transitions in HEMA plastic host, using the room-temperature absorption spectrum. The intensity parameters obtained by the J O formalism are used to determine the radiative decay rates (Einstein A coefficients) and branching ratios of the Nd 3+ (4f 3 ) transitions from the upper manifold 4 F 3/2 to the lowerlying multiplet 4 I J manifolds. The predicted decay rates and branching ratios of the Nd 3+ (4f 3 ) intermanifold transitions have been compared with those in other laser hosts. From the radiative decay rates, the radiative lifetime of the Nd 3+ 4 F 3/2 excited state is determined. EXPERIMENTAL Solid-state plastic HEMA preparation Sample containers were prepared using standard glass slides that were cut to form a reservoir of Correspondence to: Dhiraj K Sardar, Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX , USA dsardar@utsa.edu Current address: Intel Corporation, Rio Rancho, New Mexico, USA Contract/grant sponsor: National Science Foundation; contract/grant number: DMR (Received 7 June 2004; revised version received 13 July 2004; accepted 10 August 2004) 2004 Society of Chemical Industry. Polym Int /2004/$30.00

2 DK Sardar et al 25 mm 25 mm 1 mm. Three sides of the container made of slides were carefully epoxied together and one side was left open for pouring liquid plastic. The container was left for 24 h to completely cure at room temperature. Meanwhile, we prepared the sample in liquid form from the HEMA monomer (obtained from Sigma-Aldrich) containing 300 ppm of monomethyl ether of hydroquinone (MEHQ), a polymerization inhibitor. First, the HEMA was uninhibited by running the liquid through an uninhibitor column, which is a hollow glass rod mounted vertically on a table. The inhibited HEMA was allowed to flow through the un-inhibitor column slowly over a period of 48 h. The uninhibited HEMA was collected in a large beaker placed directly below the un-inhibitor column. During the entire procedure, both the inhibited HEMA and uninhibited HEMA were kept in a cold, dark environment in order to prevent polymerization. After the above process was completed, 10 ml of uninhibited HEMA was poured into a glass test tube, to which was added 100 mg of benzoyl peroxide, an oxidizing agent to cure the HEMA. The benzoyl peroxide, a curing agent, initiated free radical polymerization. The required amount of neodymium nitrate salt was added to the mixture of uninhibited HEMA and benzoyl peroxide in order to obtain 2 wt% of trivalent neodymium Nd 3+ ; the corresponding Nd 3+ concentration was determined to be cm 3. 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 that was prepared earlier for the final polymerization process. The reservoir containing the HEMA solution was then baked in a rarefied atmosphere in an oven at 60 C for45minandthen75 C for 1 h in a beaker of water to allow consistent heating over the entire mass of the sample; this also 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. Finally, the glass slides were carefully broken and the final product was a 1 mm thick clear solid plastic doped with trivalent neodymium (Nd 3+ :HEMA). Symmetry of Nd 3+ in HEMA Okamoto and co-workers have performed studies on polymeric materials formed by reacting the salts of trivalent rare-earth ions (Ln 3+ ) with polymers to complex them with various functional groups such as COO, a carboxylate group attached to a polymeric backbone. 10 They observed that the fluorescence intensity decreases with increasing Ln 3+, 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, 10 the fluorescence quenching can be attributed to the clustering of rare-earth ions in solid polymer matrices. Nevertheless, the infrared spectrum of the salt in extremely dilute conditions showed that % of the COO groups were neutralized with Ln 3+. Miniscalco 11 has reported that Ln 3+ aggregated in glasses, giving rise to energy up-conversion. The energy transfer observed by Miniscalco due to aggregation of Ln 3+ could result in such cooperative energy up-conversion. Furthermore, Zhang et al 12 have concluded that the absorption spectrum of Nd 3+ in PMMA was similar to that of Nd 3+ in silica glass, thereby suggesting that Nd 3+ symmetry in PMMA would be similar to that of Nd 3+ in glass. Since the absorption spectrum of Nd 3+ :HEMA is also similar to that of Nd 3+ :glass, we can argue that Nd 3+ symmetry in HEMA plastic matrix may be similar to that of Nd 3+ in glass. Index of refraction measurement The refractive indices of the plastic host HEMA doped with Nd 3+ ions were measured by the method of minimum deviation using a polished prism that was cut from a solid cylindrical sample. The apex angle of the prism was measured to be 55.Inthis procedure, the solid prism was firmly mounted at the center of a goniometer table. By slowly rotating the prism, the minimum deviation was determined. Taking the measurements of the distances between the prism and the projection screen, as well as the distance between the incident beam s location (in the absence of the prism) and its location at the minimum angle of deviation on the screen, the angle of minimum deviation was determined using simple trigonometry. The index of refraction was calculated using the following equation: ( ) α + δm sin 2 n = ( α ) (1) sin 2 where α is the apex angle of the prism and δ m is the angle of minimum deviation. This method was chosen for its simplicity and accuracy. The indices of refraction were measured at five laser wavelengths. These values are listed in Table 1. The experimental values of n were subsequently least-squares fitted to the Sellmeier dispersion equation: n 2 (λ) = 1 + Sλ2 λ 2 λ 2 0 Table 1. Experimental and calculated index of refraction for Nd 3+ :HEMA Index of refraction Wavelength λ (nm) n exp n calc (2)

3 Spectroscopic analysis of Nd 3+ :HEMA Values of the Sellmeier constants, S = and λ 0 = nm, were obtained from the least-squares fit of the experimental data to eqn (2) and were then used to recalculate the values of the index of refraction at all wavelengths of interest. Absorption measurement The room-temperature absorption spectrum of Nd 3+ ions in HEMA was recorded using an upgraded Cary model 14R spectrophotometer controlled by a desktop computer with data acquisition software provided by Online Instrumentation Systems (OLIS). The spectrum, ranging from 400 to 900 nm, is shown in Fig 1. All spectra were taken at 0.2 nm intervals. The spectral bandwidth was about 0.5 nm for all measurements. The Cary model 14R spectrophotometer is equipped with two light sources and two detectors. A deuterium lamp is used for wavelengths below 350 nm and a tungsten lamp for wavelengths above 350 nm. A photomultiplier tube (PMT) is used for wavelengths below 900 nm with a fixed high voltage of 400 V, and a PbS detector is 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, thereby giving rise to the variable bandwidths in different spectral ranges. Fluorescence measurement The room-temperature fluorescence spectrum of Nd 3+ in HEMA was obtained by exciting the sample into the Nd 3+ absorption band of 2 K 13/2 + 4 G 7/2 + 4 G 9/2 with nm light at 1.0 W from a Spectra- Physics argon-ion laser (model 2005). Since the multiplet manifolds 2 K 13/2, 4 G 7/2 and 4 G 9/2 have nearly the same energy, strong J-mixing due to the ligand field results in states whose wavefunctions are mixtures of J, M J from each manifold. Therefore, the absorption band is characterized by the three multiplets. The fluorescence is likewise obtained from a manifold consisting of strongly mixed multiplets 4 G 5/2 and 2 G 7/2. The fluorescence spectrum was thus obtained for the 4 G 5/2 + 2 G 7/2 4 I 9/2, transition, Absorption coefficient (arbitrary uints) K15/2 + 2 G 9/2 + 4 G 11/2 +( 2 D, 2 P) 3/2 4 G9/2 2 K13/2 + 4 G 7/2 4 G5/2 + 2 G 7/ Wavelength (nm) Figure 1. Absorption spectrum of Nd 3+ :HEMA at room temperature. 4 F9/2 4 F7/2 + 2 S 3/2 4 F5/2 4 F3/2 Fluorescence intensity (arbitrary units) Nd 3+ :HEMA HEMA Wavelength (nm) Figure 2. Fluorescence spectrum of the 4 G 5/2 + 2 G 7/2 4 I 9/2 transition in Nd 3+ :HEMA at room temperature. and is shown in Fig 2. The fluorescence from the sample was collected at a right angle with respect to the direction of the excitation light, and focused onto the entrance slit of a SPEX scanning monochromator (model 340E) equipped with a 1200 grooves/mm reflection grating blazed at 0.5 µm. The fluorescence signals were detected by a SPEX PMT (model 1911F) attached to the exit slit of the monochromator and powered by a Bertan high-voltage power supply (Series 230). The signal from the detector was passed to a SPEX Datascan (model DS 1000) equipped with a RS- 232 interface card. The scanning rate of 0.1nms 1 was set and the spectral resolution was better than 0.2 nm for all measurements. The wavelength reproducibility of the monochromator is less than 0.01 nm. A desktop computer was employed to control the monochromator and to acquire and analyze the experimental data. The fluorescence lifetime of the Nd 3+4 G 5/2 + 2 G 7/2 4 I 9/2 transition was measured by exciting the Nd:HEMA sample with nm light from an argon-ion laser; the laser light was pulsed with a chopper at 220 Hz. The fluorescence from the 4 G 5/2 + 2 G 7/2 4 I 9/2 transition (centered at about 569 nm) was detected by the PMT attached to the exit slit of the monochromator and analyzed by a 150 MHz Tektronix oscilloscope (model 2445A) having a resolution of about 0.2 ns. The fluorescence lifetime of the 4 G 5/2 + 2 G 7/2 state of Nd 3+ in HEMA was measured as 14 ± 2 µs and found to be single exponential. RESULTS AND DATA ANALYSIS Eight absorption bands identified in the roomtemperature absorption spectrum between 400 and 900 nm displayed in Fig 1 were chosen to determine the phenomenological J O intensity parameters for Nd 3+ :HEMA. A brief outline of the J O analysis is given below. More detailed theory and applications can be found in the literature. Measured line

4 DK Sardar et al strengths, S meas (J J ), of the chosen bands are determined using the following equation: [ ] S meas (J J 3ch(2J + 1)n 9 ) = 8π 3 λe 2 N 0 (n 2 + 2) 2 Ɣ (3) where J and J are the total angular momentum quantum numbers of the initial and final states, respectively, n is the refractive index, N 0 is the Nd 3+ ion concentration, λ is the mean wavelength of the specific absorption band, Ɣ = α(λ)dλ is the integrated absorption coefficient as a function of λ, and c and h have their usual meanings. The factor [9/(n 2 + 2) 2 ] in eqn (3) represents the Lorentz field correction for the ions in the host medium. The measured line strengths were then used to obtain the J O parameters 2, 4 and 6 by solving a set of ten equations for the corresponding transitions between J and J manifolds in the following form: S calc (J J ) = t (S, L)J U (t) (S, L )J 2 t=2,4,6 (4) where the matrix elements U (t) are doubly reduced unit tensor operators of rank t calculated in the intermediate-coupling approximation and are independent of the crystal host. However, the parameters 2, 4 and 6 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 Nd 3+ bands are taken from Kaminskii. 24 When two or more absorption manifolds overlapped, the squared matrix element was taken to be the sum of the corresponding squared matrix elements, and are tabulated in Table 2. For these calculations of absorption line strengths and other spectroscopic parameters, the indices of refraction were calculated using eqn (2) for the appropriate wavelength: these values are given in Table 3. The values of the measured absorption line strengths, S meas, are also tabulated in Table 3. A leastsquares fitting of S meas to S calc provides the following values for the three J O intensity parameters for Nd 3+ :HEMA: 2 = cm 2 4 = cm 2 6 = cm 2 Table 2. Values of reduced matrix elements for the absorption transitions of Nd 3+ :HEMA at 300 K Transition (from 4 I 9/2 ) λ (nm) [U (2) ] 2 [U (4) ] 2 [U (6) ] 2 4 F 3/ F 5/2 + 2 H 9/ F 7/2 + 4 S 3/ F 9/ G 5/2 + 2 G 7/ K 13/2 + 4 G 7/2 + 4 G 9/ The spectroscopic quality factor X = 4 / 6 for the Nd 3+ in HEMA plastic host was found to be 0.59, lying within the acceptable range of The values of the intensity parameters can be used to recalculate the transition line strengths of the absorption bands using eqn (3). The calculated line strengths, S calc,are tabulated in Table 3. A measure of the accuracy of the fit is given by the rms deviation: S rms = [(q p) 1 ( S) 2] 1/2 (5) where S rms = S calc S meas is the deviation, q is the number of spectral bands analyzed, and p is the number of parameters sought, which in this case is three. The values in Table 3 provide an rms deviation of cm 2. The rms line strength is cm 2. The ratio of these two values yields an rms error of about 5.6 %, which is the relative error of fit: this falls within the 5 % to 25 % range obtained for other laser systems when the J O theory was applied. 24,25 The J O parameters can now be applied to eqn (4) to calculate the line strengths corresponding to the transitions from the upper states 4 G 5/2 + 2 G 7/2 and 4 F 3/2 to the lower lying manifolds 4 I J (J = 15/2, 13/2, 11/2, 9/2) of Nd 3+ :HEMA. These values are given in Table 4. Using these line strengths, the radiative transition rates for electric dipole transitions between an excited state and the lower lying terminal levels, A(J J ), can be calculated using the following equation: A(J J ) = 64π 4 e 2 n(n 2 + 2) 2 3h(2J + 1)λ 3 S calc (J J ) 9 (6) Table 3. Measured and calculated absorption line strengths of Nd 3+ :HEMA at 300 K Transition (from 4 I 9/2 ) λ (nm) n(λ) S meas S calc ( S) 2 (10 40 cm 4 ) 4 F 3/ F 5/2 + 2 H 9/ F 7/2 + 4 S 3/ F 9/ G 5/2 + 2 G 7/ K 13/2 + 4 G 7/2 + 4 G 9/

5 Spectroscopic analysis of Nd 3+ :HEMA Table 4. Predicted fluorescence line strengths, radiative decay rates and branching ratios Nd 3+ :HEMA at 300 K Transition [U (2) ] 2 [U (4) ] 2 [U (6) ] 2 (nm) λ S calc A JJ (s 1 ) β JJ 4 F 3/2 4 I 15/ F 3/2 4 I 13/ F 3/2 4 I 11/ F 3/2 4 I 9/ G 5/2 + 2 G 7/2 4 I 15/ G 5/2 + 2 G 7/2 4 I 13/ G 5/2 + 2 G 7/2 4 I 11/ G 5/2 + 2 G 7/2 4 I 9/ Table 5. Comparison of values and spectroscopic parameters of HEMA with other hosts 21 Crystal β( 4 F 3/2 4 I 11/2 )/ Intensity parameters host β( 4 F 3/2 4 I 13/2 ) / 6 HEMA a Y 3 Al 5 O 12 YAG YAlO 3 (YALO) Y 2 O YLiF 4 (YLF) Gd 3 Sc 2 Ga 3 O 12 (GSGG) a Host for current work. The radiative lifetime τ r for an excited state (J) is calculated by: τ r = 1 A(J J ) (7) where the sum runs over all final lower lying states J. The radiative rates given in Table 4 are added to obtain the total radiative rates of 1777 s 1 and s 1 of the 4 G 5/2 + 2 G 7/2 and 4 F 3/2 upper manifold states, respectively. The radiative lifetimes of these levels are therefore determined to be 563 µs and 9.47 ms, respectively. The room-temperature fluorescence branching ratios, β(j J ), can be determined from the radiative decay rates by: β(j J ) = A(J J ) A(J J ) = A(J J )τ r (8) where the sum runs over all final states J. The values of the branching ratios and the radiative lifetimes are given in Table 5. The luminescence branching ratio is a critical parameter to the laser designer, because it characterizes the possibility of attaining stimulated emission from any specific transition. The fluorescence spectrum given in Fig 2 shows a strong peak around 569 nm arising from the 4 G 5/2 + 2 G 7/2 4 I 9/2 transition, and it resembles the emission spectrum of Nd 3+ doped in PMMA fiber, as reported by Zhang et al.12 CONCLUSION An in-depth spectroscopic analysis of Nd 3+ in HEMA, a plastic host, has been performed utilizing the standard Judd Ofelt theory. The three phenomenological intensity parameters were found to be 2 = cm 2, 4 = cm 2 and 6 = cm 2. These values are higher than those reported for other Nd 3+ -based laser crystals. The spectroscopic quality factor, X = 4 / 6,of this sample is An attempt was made to measure the fluorescence spectra from the 4 F 3/2 4 I J transitions. However, the intensity was too weak to measure. The weakness of the fluorescence spectra in that region can be attributed to the fact that selfquenching was prevalent due to the Nd 3+ clustering in HEMA matrix. It is also possible that a significant amount of the Nd 3+ did not dissociate from the (NO 3 ) 3, thereby precluding fluorescence. 26 The lifetime (τ f ) of the 4 G 5/2 + 2 G 7/2 4 I 9/2 transition was measured to be approximately 14 µs. The fluorescence lifetime for this transition is considerably shorter than predicted and this could be due to two factors: (1) the actual Nd 3+ concentration is much lower than what is calculated based on total dissociation of Nd(NO 3 ) 3 salt in HEMA, and (2) there may be significant channels of non-radiative decays. In conclusion, this study shows that Nd 3+ : HEMA possesses some competitive spectroscopic characteristics. Therefore, this material should be considered a potential candidate as an efficient laser system that can be pumped with powerful diode lasers whose pump wavelength overlaps the strong and broad Nd 3+ absorption band that peaks around 800 nm the

6 DK Sardar et al absorption band is about four times broader than that of Nd:YAG. The major advantage of diode-pumping over flash-lamp pumping is that the harmful thermal stress produced by the unwanted UV radiation present in the flash lamp can be remedied by directly pumping the Nd 3+ ions in HEMA with near-infrared diode lasers, thereby achieving a more efficient laser system. ACKNOWLEDGEMENT The authors would like to acknowledge the support for this research from grant DMR from the National Science Foundation. REFERENCES 1 Meyn JP, Jensen T and Huben G, IEEE J Quantum Electron 30:913 (1994). 2 Kutsvoi SA, Laptev VV, Lebedev VA, Mazner SY, Pisarenpo VF and Chuev YM, Zh Prikl Spektrosk 53:370 (1990). 3 Gruber JB, Reynolds TA, Keszler DA and Zandi B, J Appl Phys 87:7159 (2000). 4 Judd BR, Phys Rev 127:750 (1962). 5 Ofelt GS, J Chem Phys 37:511 (1962). 6 Hermes RE, Allik TH, Chandra S and Hutchinson JA, Appl Phys Lett 63:877 (1993). references therein. 7 Allik TH, Chandra S, Hermes RE, Hutchinson JA, Soong ML and Boyer JH, OSA Proceedings on Advanced Solid-State Lasers, ed by Pinto AA and Fan TY, vol 15. Optical Society of America, Washington, DC, pp (1993). 8 Rahn MD and King TA, JModOptics63:877 (1993). 9 Wadsworth WJ, Giffin SM, McKinnie IT, Sharpe JC, Woolhouse AD, Haskell TG and Smith GJ, Appl Optics 38:2504 (1999). references therein. 10 Okamoto Y, J Macromol Sci Chem A24: (1987). 11 Miniscalco WJ, Rare Earth Doped Fiber Lasers and Amplifiers, Marcel Dekker Inc., New York (1993). 12 Zhang Q, Ming H and Zhai Y, Polym Int 41: (1996). 13 Krupke WF, Shinn MD, Marion JE, Caird JA and Stokowski SE, JOptSocAmerB3:102 (1986). references therein. 14 Sardar DK, Gruber JB, Zandhi B, Hutchinson JA and Trussell CW, J Appl Phys 93:2041 (2003). 15 Krupke WF, IEEE J Quantum Electron 10:45 (1974). 16 Krupke WF, IEEE J Quantum Electron 7:153 (1971). 17 Krupke WF, Phys Rev 145:325 (1966). 18 Krupke WF and Gruber JB, Phys Rev 139:A2008 (1965). 19 Weber MJ, J Chem Phys 48:4771 (1968). 20 Weber MJ, Phys Rev 171:283 (1968). 21 Weber MJ, Phys Rev 157:262 (1967). 22 Weber MJ and Varitimos TE, J Appl Phys 42:4996 (1971). 23 Ryan JR and Beach R, JOptSocAmerB9:1883 (1992). 24 Kaminskii AA,Laser Crystals, vol 14, Springer-Verlag, New York (1981). 25 Powell RC, Physics of Solid-State Laser Materials, Springer- Verlag, New York (1998). 26 Johnson D, Department of Chemistry, University of Texas at San Antonio, USA; private communication.