Characterization of spectroscopic and laser properties of Pr 3 in Sr 5 PO 4 3 F crystal

Transcription

1 JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 3 1 FEBRUARY 2002 Characterization of spectroscopic and laser properties of Pr 3 in Sr 5 PO 4 3 F crystal Dhiraj K. Sardar and Francisco Castano Department of Physics and Astronomy, The University of Texas at San Antonio, San Antonio, Texas Received 27 August 2001; accepted for publication 20 October 2001 Spectroscopic and laser properties have been characterized for Pr 3 incorporated into Sr 5 PO 4 3 F crystal belonging to the apatite structure family. The standard Judd Ofelt analysis has been applied to the measured optical absorption intensities to determine the radiative decay rates, branching ratios, and emission cross sections of principal intermanifold transitions of Pr 3 from the 1 D 2 and 3 P 0 states to the lower-lying manifolds in the visible region. The measured room temperature fluorescence lifetimes of the 1 D 2 3 H nm and 3 P 0 3 F nm transition are 325 and 105 s, respectively, while the Judd Ofelt analysis predicts the radiative lifetimes for the 1 D 2 and 3 P 0 states to be 822 and 116 s, respectively. The room temperature emission cross sections of the 1 D 2 3 H 4 and 3 P 0 3 F 2 intermanifold transitions have been also determined to be and cm 2, respectively. DOI: / American Institute of Physics. I. INTRODUCTION Trivalent praseodymium ions (Pr 3 ) have potential for laser applications due to a large number of available transitions in the UV, visible, and near infrared regions. In the past few years, researchers have demonstrated laser actions of Pr 3 in a number of hosts. 1,2 Spectroscopic and laser properties of Pr 3 have also been investigated in different crystalline hosts. The fluorapatite crystals are important hosts for optically active trivalent rare earth ions because of their characteristics such as unusually high crystal field splittings and large transition cross sections. 3 5 Furthermore, improvements in crystal growth and optical pumping sources have sparked renewed interest in developing fluoroapatite hosts for optical devices. Wright et al. have presented spectroscopic data for Pr 3 in strontium fluoroapatite, Sr 5 PO 4 3 F S-FAP, crystal with a complete tabulation of experimental energy levels of Pr 3 in S-FAP. 6 In this article, we report a Judd Ofelt analysis of the spectroscopic and laser parameters of Pr 3 in S-FAP. A number of studies have been reported on the Judd Ofelt analysis of Pr 3 in various glass and other crystal hosts; these studies exhibit a remarkable variation in the determined values of the Judd Ofelt parameters A number of previous researchers have opted to substitute the measured oscillator strengths for the calculated ones in the Judd Ofelt formalism in order to determine the intensity parameters. 7,8,10 13 We have, however, applied the measured intensities of the room temperature absorption bands to the standard Judd Ofelt theoretical analysis to determine the intensity parameters, and a good agreement between the experimental results and theory is obtained. In our work, we have applied the standard Judd Ofelt analysis to the measured optical absorption intensities to determine the radiative decay rates, branching ratios, and emission cross sections of principal intermanifold transitions of the Pr 3 from the 1 D 2 and 3 P 0 states to the lower-lying manifolds. The room temperature fluorescence lifetimes of the 1 D 2 and 3 P 0 states are measured and compared with those predicted by the Judd Ofelt analysis. The room temperature emission cross sections of the 1 D 2 3 H nm and 3 P 0 3 F nm intermanifold transitions have also been determined. II. EXPERIMENTAL DETAILS A. Material The sample used in this study was a single crystal of Pr 3 :S-FAP grown by the standard Czochralski pulling method. The crystal was doped with nominal 1 at. % of Pr 3. The optical quality of the material was found to be excellent. The sample used for spectroscopic measurements was optically polished and the thickness of the sample was measured to be 1.09 cm. The S-FAP host has a well known structure belonging to the uniaxial, hexagonal crystal with a space group of P6 3 /m. 14 The apatite unit cells contain two molecules of M 5 PO 4 3 F and there are two nonequivalent M 2 positions; when doped with the trivalent rare earth ion, the apatite structure incorporates the ion into one of the two existing sites. 6,14 B. Optical absorption and fluorescence measurements The room temperature absorption spectrum of Pr 3 :S-FAP was recorded on an upgraded Cary model 14R spectrophotometer controlled by a desktop computer with a data acquisition software from OLIS. This spectrum, ranging from 400 to 2500 nm, consists of the Pr 3 absorption bands observed around 433, 457, 489, 588, 945, 1358, 1739, and 2192 nm; these bands are portrayed in Figs. 1 and 2. The spectral bandwidth was set at 0.2 nm for all measurements, and the spectrometer was internally calibrated to an accuracy of 0.3 nm /2002/91(3)/911/5/$ American Institute of Physics

2 912 J. Appl. Phys., Vol. 91, No. 3, 1 February 2002 D. K. Sardar and F. Castano TABLE I. Values of reduced matrix elements for the absorption transitions of Pr 3 in S-FAP. Transition from 3 H 4 nm U (2) 2 U (4) 2 U (6) 2 3 P I 6 3 P P D G F 3 3 F F H FIG. 1. Room temperature absorption spectrum of Pr 3 in S-FAP from 400 to 650 nm. The room temperature fluorescence spectra were measured on Pr 3 :S-FAP by exciting the sample with the nm line from a Spectra Physics Ar ion laser model The spectral bandwidth of the laser was 0.12 nm and the maximum laser power used in these measurements was maintained at 3.0 W. The laser beam with a Gaussian profile had a1/e 2 radius of 0.63 mm and the beam divergence was 0.69 mrad. The fluorescence from the sample was collected by a pair of positive lenses at 90 with respect to the exciting laser beam, and detected by a photomultiplier tube for the visible region. The detector was attached to the exit slit of a SPEX model 340E monochromator. A Hoya sharp-cut filter Y-52 was used at the entrance slit of the monochromator to prevent the exciting laser beam from entering the monochromator. A reflection grating with 1200 grooves/mm blazed at 500 nm was used for the measurements of the fluorescence spectrum in the visible region. The spectral resolution was better than 0.8 nm for all measurements. The signals were taken from the detector to a Spex Datascan model DS 1000 equipped with a RS-232 interface card. A personal computer was used to control the monochromator and acquire the data using a software known as spectramax from SPEX. Both the FIG. 2. Room temperature absorption spectrum of Pr 3 in S-FAP from 700 to 2500 nm. absorption and fluorescence spectra were analyzed and plotted by using the computer software, Sigma Plot. C. Lifetime measurements The Pr 3 kinetic measurements fluorescence lifetimes were performed by exciting the sample with the nm line from the argon ion laser. The exciting argon ion laser beam was chopped at 10 Hz with a mechanical chopper. The signals from the photomultiplier were fed into a 150 MHz Tektronix model 2445A oscilloscope. The fluorescence decay signals that were displayed on the oscilloscope were carefully recorded to determine the fluorescence lifetimes. The measurements were repeated five times for every transition. III. RESULTS AND DATA ANALYSIS Eight absorption bands identified in the room temperature absorption spectrum between 400 and 2500 nm portrayed in Figs. 1 and 2 were chosen to determine the phenomenological Judd Ofelt (J O) parameters for Pr 3 :S-FAP. Many researchers have applied the Judd Ofelt theoretical analyses to determine the important spectroscopic and laser parameters Below is given a brief description of the Judd Ofelt analysis. The measured line strengths, S meas (J J ), of the chosen bands are determined using the following expression: S meas J J 3ch 2J 1 n e 2 N 0 n 2 2 2, 1 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 Pr 3 ion concentration, is the mean wavelength of the specific absorption band, ( )d is the integrated absorption coefficient as a function of, and c and h have their usual meaning. The factor 9/(n 2 2) 2 in Eq. 1 represents the local field correction for the ion in the dielectric host medium. 17 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 2,4,6 t S,L J U t S,L J 2, 2

3 J. Appl. Phys., Vol. 91, No. 3, 1 February 2002 D. K. Sardar and F. Castano 913 TABLE II. Measured and calculated absorption line strengths of Pr 3 in S-FAP. Transition from 3 H 4 nm where 2, 4, and 6 are the Judd Ofelt intensity parameters, and U (t) are the doubly reduced matrix elements of rank t (t 2,4,6) between states characterized by the quantum numbers S,L,J and (S,L,J ). The matrix elements are independent of the crystal host and can be easily calculated from the tables of Nielson and Koster. 28 The Judd Ofelt parameters, however, exhibit the influence of the host on the transition probabilities since they contain the crystalfield parameters, interconfigurational radial integrals, and the interaction between the central ion and the intermediate environment. We have used the values of the reduced matrix elements for the chosen Pr 3 bands calculated by Carnall et al. 29 When two absorption manifolds overlapped, the squared matrix element was taken to be the sum of the corresponding squared matrix elements and tabulated in Table I. For these calculations of absorption line strengths and other spectroscopic parameters, the index of refraction was taken to be 1.64 independent of wavelength. The values of the measured absorption line strengths, S meas are tabulated in Table II. A least-squares fitting of S meas to S calc provides the following values for the three J O parameters for Pr 3 in SFAP host material: cm 2, cm 2, S meas (10 20 cm 2 ) S calc (10 20 cm 2 ) ( S) 2 (10 40 cm 4 ) 3 P I 6 3 P P D G F 3 3 F F H cm 2. The spectroscopic quality factor, X 4 / 6, for the Pr 3 :S-FAP laser host was found to be 1.64, which is slightly higher than that of the upper limit of the range of for Nd 3 in different hosts. 30 The values of the intensity parameters can then be used to recalculate the transition line strengths of the absorption bands using Eq. 2. The calculated line strengths S calc are tabulated in Table II. A measure of the accuracy of the fit is given by the rms deviation S rms q p 1 S 2 1/2, where S S calc S meas is the deviation, q is the number of spectral bands analyzed, and p is the number of the parameters sought, which in this case is three. The values in Table II 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 33%. However, if we exclude the 3 H 4 3 P 2 transition from the least square fit as suggested by other authors, 11,31 the intensity parameters are found to be still comparable, but a better fit between the theory and experiment is achieved with a rms error of only 21%. The J O parameters can now be applied to Eq. 2 to calculate the line strengths corresponding to the transitions from the metastable 1 D 2 state to the lower lying 3 F 4, 3 F 2, 3 H 6, 3 H 5, and 3 H 4, and from 3 P 0 to 1 G 4, 3 F 4, 3 F 2, and 3 H 6 manifolds of Pr 3. These values are given in Table III. 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 expression: A J J 64 4 e 2 n n S calc J J. 3h 2J The radiative lifetime r for an excited state J is calculated by 1 r A J J, where the sum is over all final lower-lying states J. The radiative rates for the transitions studied are given in Table III. The values of the radiative rates calculated by using Eq. 4 are added to obtain the total radiative rates of and s 1 for the 1 D 2 and 3 P 0 states, respectively. The radiative lifetimes of these levels are, therefore, determined to be 822 and 116 s, respectively TABLE III. Predicted radiative decay rates, branching rations, and fluorescence line strengths for Pr 3 in S-FAP. Transition nm U (2) 2 U (4) 2 U (6) 2 S calc (10 20 cm 2 ) A JJ s 1 JJ 1 D 2 3 F F H H H P 0 1 G F F H

4 914 J. Appl. Phys., Vol. 91, No. 3, 1 February 2002 D. K. Sardar and F. Castano TABLE IV. Predicted optical properties and principal intermanifold emission cross sections of Pr 3 in S-FAP. Radiative lifetimes ( 1 D 2 ) 822 s ( 3 P 0 ) 116 s Branching ratios ( 1 D 2 3 H 4 ) ( 3 P 0 3 F 2 ) Emission cross sections p ( 1 D 2 3 H 4 ) cm 2 p ( 3 P 0 3 F 2 ) cm 2 where I( ) is the intensity at. According to Eq. 8, the line shape functions for the 1 D 2 3 H 4 and 3 P 0 3 F 2 transitions are determined by dividing the intensities I( ) by the integrated areas of the fluorescence spectral bands. The values of g( ) determined from Eq. 8, the values of radiative lifetimes and intermanifold branching ratios from Table IV, and the values of and n are applied to Eq. 7 to determine the emission cross sections for the 1 D 2 3 H 4 and 3 P 0 3 F 2 transitions. These values are given in Table IV. The room temperature fluorescence branching ratios, (J J ), can be determined from the radiative decay rates by J J A J J A J J A J J r, 6 where the sum runs over all final states J. These values of the branching ratios and the radiative lifetimes are given in Table IV. 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 room temperature fluorescence spectrum representing the 1 D 2 3 H nm and 3 P 0 3 F nm transitions is shown in Fig. 3. The emission cross sections of these transitions can be determined from the following expression: 32 J,J ; 2 J,J 8 cn 2 g, 7 r where is the central peak wavelength, 1, (J,J ) is the florescence branching ratio from the upper manifold J to the lower manifold J, r is the radiative lifetime of the excited state manifold J, and g( ) is the line shape function. The line shape function is obtained from the fluorescence spectrum and using the following relation: 32 I g I d, FIG. 3. Fluorescence spectrum of Pr 3 in S-FAP for the 1 D 2 3 H 4 and 3 P 0 3 F 2 transitions at room temperature. 8 IV. CONCLUSIONS The spectroscopic analysis of Pr 3 in the S-FAP laser host has been performed following the standard Judd Ofelt theory. The three phenomenological parameters were found to be cm 2, cm 2, and cm 2. The spectroscopic quality factor X 4 / 6 of this sample is approximately 1.64, which is about 5.0 times larger than that of the most standard laser host Nd 3 : yttrium aluminum garnet YAG (X 0.3). 32 As a result, the major laser transitions for Pr 3 :S-FAP are likely to be very strong. The room temperature fluorescence lifetimes of the 1 D 2 3 H 4 and 3 P 0 3 F 2 transitions of Pr 3 :S-FAP are measured as 325 and 105 s, respectively, and the radiative lifetimes of the 1 D 2 and 3 P 0 states determined by the J O analysis were found to be 822 and 116 s, respectively. The fluorescence quantum efficiencies QE (QE f / r ) of these transitions are determined to be 40% and 91%, respectively. Due to the significant variation between the experimental and calculated values of the absorption line strengths as predicted by the rms error of 33%, the percentages of these quantum efficiencies are likely to vary by about 30%. It is imperative to mention that there are some situations in which the Judd Ofelt theory does not give predictions consistent with experimental observations because the 4 f N 5d configuration is at low energy; one such example is Pr Owing to this complexity, the previous researchers have removed the 3 H 4 3 P 2 transition to obtain a better fit, and thereby a lower rms error. 11,31 In this study, if we remove this transition, a better fit between theory and experimental results is achieved with a rms error of only 21%. The room temperature emission cross sections of the 1 D 2 3 H nm and 3 P 0 3 F nm intermanifold transitions have been determined to be and cm 2, respectively. The emission cross sections of these transitions are approximately 1 order of magnitude lower than that of the well-known laser transition ( 4 F 3/2 4 I 11/2 )ofnd 3 :YAG. Finally, this study shows that Pr 3 :S-FAP possesses important spectroscopic and laser properties that are favorable for this material to become a potential candidate as an efficient laser system. This study shows that the 3 P 0 3 F 2 intermanifold transition of Pr 3 in S-FAP is about ten times stronger than the 1 D 2 3 H 4 transition. Therefore, the 3 P 0 3 F 2 transition is considered to be the most viable laser transition for Pr 3 in S-FAP host.

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