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Optical studies of Dy3+ doped tellurite glass: Observation of yellow-green upconversion
Optics Communications 257 (2006) 112–119 www.elsevier.com/locate/optcom
Optical studies of Dy3+ doped tellurite glass: Observation of yellow-green upconversion Vineet Kumar Rai *, S.B. Rai, D.K. Rai Laser and Spectroscopy Laboratory, Department of Physics, B.H.U. Varanasi 221 005, India Received 11 February 2005; received in revised form 27 June 2005; accepted 6 July 2005
Abstract Optical absorption and fluorescence spectra of Dy3+ doped tellurite glass have been studied and Judd–Ofelt theory has been used to derive its optical properties. The effect of temperature on the fluorescence yield and fluorescence quenching for higher concentrations of Dy3+ has been studied and the mechanism involved discussed. It is found that the fluorescence intensity is larger at lower temperature. The lifetime of the 4F9/2 level has been measured and found to decrease with concentration as well as temperature. Intense upconversion has also been observed in the yellow-green region when pumped with NIR (862 nm) radiation. An explanation of the same involving energy transfer is offered. 2005 Elsevier B.V. All rights reserved. PACS: 42.70 Ce Keywords: Judd–Ofelt theory; Concentration quenching; Quantum efficiency; Upconversion
1. Introduction Solid-state lasers operating in the yellow-orange region (570–590 nm) have great technological applications in military, telecommunication, commercial displays, etc. Dy3+ doped in glass/crystal gives strong discrete emission in this wavelength * Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 368 468. E-mail addresses: [email protected] (V.K. Rai), [email protected] (S.B. Rai).
region while Dy3+-garnets have been used as a potential saturable absorber in the 2.8 lm region [1]. The luminescence properties of Dy3+ doped in different glass lattices have been studied by several workers [2–18]. It has been found that the optical properties of Dy3+ ion doped in fluoride lattices are very different from that in oxide lattices [5,7,11]. Kaminskii [13] has observed laser action in Dy3+ in the NIR region (1.35 and 3.0 lm) in inorganic solids (fluoride hosts). Laser action in Dy3+ doped tungstate glass in the visible region due to
0030-4018/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.07.022
V.K. Rai et al. / Optics Communications 257 (2006) 112–119 4
F9/2 ! 6H13/2 (570 nm) and 4F9/2 ! 6H11/2 (660 nm) transitions at liquid nitrogen temperature under excitation with Xe-flash lamp has also been reported [14]. In this paper, we have investigated the spectroscopic properties of Dy3+ ion doped in tellurite glass. Absorption and the fluorescence spectra of the glass have been studied for different concentrations of Dy3+. Judd–Ofelt [19,20] theory has been used to analyze and explain the experimental observations. Concentration quenching of fluorescence has been studied and the mechanism of the quenching discussed. The effect of pump laser power and temperature on the fluorescence yield and on the lifetime of the 4F9/2 level have been studied. Intense upconversion has been observed in the yellow-orange region on NIR (862 nm) pumping.
2. Experimental In order to form Dy3+ doped tellurite glass we used the chemicals TeO2 (BDH), Li2CO3 (BDH) and Dy2O3 (rare-earth chemicals) with reported purity of 99.9%, 97% and 99%, respectively, in the following proportions: ð80 xÞTeO2 þ 20Li2 CO3 þ xDy2 O3 where x = 0.5, 1.0, 2.0 and 3.0 mol%. All these compounds were made into fine powder in a ceramic mortar and finally mixed properly to get a homogeneous mixture. The sample was then placed in a platinum crucible and heated up to 900 C for one and half-hours. The melt was constantly stirred for homogeneous mixing and then suddenly poured into a steel cast kept at 460 C. The semisolid was then pressed with a flat disc to get flat glass of about 1.0 mm thickness. Several pieces of glasses were prepared for each concentration of the rare earth. The glasses were then cleaned and polished for further studies. The densities of the glass samples were measured using a single pan balance with Xylene as the immersion liquid. For refractive index measurements BrewsterÕs angle polarization method has been used. A 2 mW He–Ne polarized laser at 632.9 nm was used as light source for this purpose. The density is 5.74 g/cm3 and refractive index is 2.36 for glass
113
containing 1 mol% Dy3+. The absorption spectrum of Dy3+ (1.0 mol%) doped tellurite glass was recorded using a Lambda-19-UV–Vis–NIR double beam spectrophotometer in the region 400–2500 nm. For the fluorescence spectrum we employed the 457.9 nm line with 100 mW power from an Ar+ laser as excitation source. The glass was observed to fluoresce with a weak yellowish color. The fluorescence spectra were measured using a 0.5 m Spex monochromator attached with a IP21 PMT in the region 461.0–691.0 nm at room temperature (22 C). The lifetime of the 4F9/2 level had been measured for all the four samples using a KrF laser emitting at 248 nm with a spectral bandwidth of 0.05 nm. The incident laser light was focused on the glass and the fluorescence decay for the yellow band was seen on oscilloscope. The lifetime of the 4F9/2 level could be calculated from the decay curves. For upconversion measurements a Ti-Sapphire laser (emitting at 862 nm) pumped by the second harmonic of a diode-pumped Nd: yVO4 laser was used.
3. Results and discussion 3.1. Absorption spectrum The absorption spectrum of the triply ionized Dy doped in tellurite glass shows several absorption peaks in the 400–2500 nm region (see Fig. 1). The electronic configuration of Dy3+ is 4f95s25p6, which gives 6H15/2 as the ground state of Dy3+ with a large number of other low-lying excited states. It is found that the presently observed absorption peaks are only slightly shifted from their corresponding positions in other hosts. The wavelengths of these peaks and their assignments are given in Table 1. The oscillator strength (Fexp) for the different peaks (assumed to have a Gaussian shape) was obtained from the expression Z mc2 F exp ¼ 2 aðkÞ dk=k2 ; ð1Þ pe N where Ôa(k)Õ is the measured absorption coefficient at wavelength ÔkÕ and ÔNÕ is the number of rareearth ions per unit volume. The oscillator strength
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Fig. 1. Absorption spectrum of 1.0 mol% Dy3+ doped in tellurite glass.
Table 1 Observed absorption bands, their wavelengths, energies, assignments, measured and calculated values of the oscillator strength for Dy3+ doped in tellurite glass Transition
Wavelength (nm)
Energy (cm1)
Fm · 106
Fc · 106
4
6
6
6
490.9 757.6 810.6 896.8 1090.9 1280.3 1691.5
20365.1 13195.9 12333.1 11123.0 9164.2 7808.5 5911.9
1.3342 2.2665 0.5964 0.4594 0.2087 1.5913 0.2557
1.5342 2.6000 0.5474 0.6081 0.2471 1.1143 0.2851
F9/2 F3/2 6 F5/2 6 F7/2 6 H7/2 6 H9/2 6 H11/2
H15/2 H15/2 6 H15/2 6 H15/2 6 H15/2 6 H15/2 6 H15/2
thus obtained is given in Table 1. The oscillator strength according to the Judd–Ofelt theory [19,20] for an electric dipole allowed transition J ! J 0 is given as
The spontaneous emission transitions probability for an electric dipole allowed transition is represented in J–O theory by
8p2 mm F cal ðaJ ; bJ Þ ¼ ½fðn2 þ 2Þ2 =9ngS ed þ nS md ; 3hð2J þ 1Þ ð2Þ
Arad ðaJ ; bJ 0 Þ ¼
0
where ÔJÕ is the total quantum number for the ground level and J 0 for the upper level. ÔSedÕ and ÔSmdÕ are the electric and magnetic dipole line strengths, respectively. The values of Smd for these transitions are very small and are usually neglected. The electric dipole line strength is given by S ed ½ðS; L; J Þ; ðS 0 ; L0 ; J 0 Þ X 2 Xk jhðS; L; J ÞkU k kðS 0 ; L0 ; J 0 Þij . ¼
64p4 m3 nðn2 þ 2Þ2 e2 3hc3 ð2J þ 1Þ9 hX i Xk jhðS; L; J ÞkU k kðS 0 ; L0 ; J 0 Þij2 ð4Þ
and the total spontaneous transition probability by X AT ¼ Arad ðaJ ; bJ 0 Þ. ð5Þ The fluorescence branching ratio ÔbRÕ is obtained using the relation
ð3Þ
k¼2;4;6
The value of Fcal(aJ, bJ 0 ) thus obtained is given in Table 1. We have used the reduced matrix element iUki given by Carnall et al. [15] in our calculations.
bR ¼
AðaJ ; bJ 0 Þ . AT
ð6Þ
The radiative lifetime ÔsRÕ of the fluorescing level is given as
V.K. Rai et al. / Optics Communications 257 (2006) 112–119
sR ¼ 1=AT .
ð7Þ
Again, the stimulated emission cross-section of the fluorescing level is given as rEP ¼
k4 8p2 cn2 Dk
½AðaJ ; bJ 0 Þ;
ð8Þ
where ÔkÕ is the wavelength of the peak and ÔDk Õ is its effective bandwidth. Using the experimentally measured values of the oscillator strength for the different transitions, Xk parameters have been determined. The ÔXkÕ parameters thus obtained for Dy3+ doped in tellurite glass are compared with the corresponding values in other glass hosts in Table 2. These values of ÔXkÕ have been utilized to calculate the transition probability, branching ratio, radiative lifetime, stimulated emission cross-section, etc. for different transitions and these are given in Table 3. It is interesting to note that the branching ratio for the 4F9/2 ! 6H13/2 Table 2 Comparison of Judd–Ofelt intensity parameters (Xk · 1020 cm2) for Dy3+ in different host lattices Host
X2
X4
X6
References
Na2O + P2O5 Oxyfluoroborate Fluorozirconate 20Li2O + 80 TeO2
1.46 2.68 2.70 1.46
1.16 2.56 1.80 2.32
1.97 0.89 2.00 3.60
[25] [22] [12] Present work
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transition is much higher (0.548) compared to other transitions. The stimulated emission crosssection for this transition is also large indicating that the 4F9/2 ! 6H13/2 transition of Dy3+ in tellurite glass may have a potential for laser emission. In fact, laser emission corresponding to this transition has already been reported [14]. We also recorded the infrared spectrum of the glass. A strong absorption peak is observed at 650 cm1 due to TeO2. 3.2. Fluorescence spectrum There are three fluorescence peaks observed at 481.5, 575.7 and at 661.9 nm in Dy3+ doped tellurite glass when excited with 457.9 nm radiation. The peak at 575.7 nm is very strong and sharp whereas the others are broad and weak. The assignments of these peaks are given in Table 4. We have calculated the bandwidth and the stimulated emission cross-section for the peaks at 481.5 and 575.7 nm and find values of these as 15.1 nm; 0.57 · 1022 cm2 and 13.2 nm; 0.86 · 1022 cm2, respectively (see Table 4). A comparison of the fluorescence spectrum of Dy3+ doped in different hosts (see Fig. 2) shows that the transition probability and branching ratio for 4F9/2 ! 6H13/2 transition is much larger in tellurite host [21,22]. Also, the observed intensity of this peak in the tellurite
Table 3 Electric dipole line strength, transition probability, branching ratios for different transitions and radiative lifetime of 4F9/2 state of Dy3+ in tellurite glass SLJ S 0 L 0 J 0 4
Energy
Sed · 1020
A (s1)
br = A/AT
7283.0 7845.0 8650.0 10082.0 10892.0 11955.0 12039.0 13361.0 13390.0 15102.9 17363.8 20764.5
0.0009 0.0011 0.0127 0.0217 0.0129 0.0462 0.0162 0.0227 0.0211 0.0311 0.3209 0.1204
0.0297 0.0432 0.6853 1.8501 1.3810 6.5472 2.3405 4.4897 4.2087 8.8632 139.1900 84.2800
0.0001 0.0002 0.0027 0.0073 0.0054 0.0258 0.0092 0.0177 0.0165 0.0349 0.5482 0.3319
F9/2! 6
F1/2 F3/2 6 F5/2 6 F7/2 6 H5/2 6 H7/2 6 F9/2 6 F11/2 6 H9/2 6 H11/2 6 H13/2 6 H15/2 6
AT = 253.91, s = 3.94 ms
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Table 4 Assignment of the bands, their wavelengths, energies, half bandwidth and the stimulated emission cross-section observed in fluorescence spectra of Dy3+ doped in tellurite glass Transition
Wavelength (nm)
Energy (cm1)
Band width (Dk) (nm)
Stimulated emission cross-section ðrEP 1022 Þ cm2
4
481.5 575.7 661.9
20764.5 17363.8 15102.9
15.1 13.2
0.57 0.86
6
F9/2 ! H15/2 F9/2 ! 6H13/2 4 F9/2 ! 6H11/2 4
3.3. Effect of concentration of rare-earth ion on the fluorescence intensity Changing the concentration of Dy3+ in tellurite glass host shows that fluorescence starts getting quenched when the concentration of the rare-earth ion is increased beyond 1.0 mol%. This is attributed to energy transfer from an excited Dy3+ ion to a nearby unexcited Dy3+ ion. The lifetime of the 4F9/2 level of Dy3+ was measured for different concentrations of Dy3+. The results are listed in Table 5 and it is seen that the lifetime of the 4F9/2 level decreases for concentrations >1.0 mol%. Inokuti and Hirayama [24] proposed a model according to which the fluorescence intensity at a time ÔtÕ is given as IðtÞ ¼ Ið0Þ exp½t=s0 Cð1 3=SÞC=C 0 ðt=s0 Þ
3=S
;
ð10Þ
Fig. 2. Fluorescence spectrum of Dy3+ (1.0 mol%) doped in different hosts (tellurite, oxyfluoroborate and calibo).
host is larger and the FWHM much smaller compared to other hosts [21,22]. The intensity of this peak is seen to vary linearly with increasing incident laser power (within the range of our measurements). These parameters indicate that tellurite glass is a promising host for a laser using Dy3+. The luminescence quantum efficiency ÔgÕ may be defined by the relation [23], g ¼ emitted light power=absorbed radiation power ¼ sexp =sr . ð9Þ For Dy concentration of 1.0 mol% in tellurite glass the value of g is found to be 16%.
where ÔCÕ is the actual concentration of the emitting ion and ÔC0Õ is critical concentration. The value of ÔC0Õ is determined from a plot of fluorescence intensity versus the concentration of Dy3+ in the glass. Its value is found to be 1.69 mol%. A fitting of the above expression to the measured intensity results in a value for S = 6 which
Table 5 Lifetime of 4F9/2 level of Dy3+ doped in different glass hosts Host Tellurite glass
Concentration Lifetime (ms) Reference
0.5 mol% 1.0 mol% 2.0 mol% 3.0 mol% Borate glass 1.0 wt% Fluorozirconate 1.0 wt% Phosphate glass 1.0 wt%
0.52 0.66 0.61 0.58 1.4 1.2 1.0
Present work
[6] [12] [6]
V.K. Rai et al. / Optics Communications 257 (2006) 112–119
117
indicates that the process involved in energy transfer is based on dipole–dipole interactions.
The non-radiative relaxation rate at room temperature is estimated as 1.24 · 103 s1.
3.4. Temperature dependence of the fluorescence yield and the lifetime
3.5. Upconversion
The lifetime of the 4F9/2 level and the fluorescence yield corresponding to 4F9/2 ! 6H13/2 transition at different temperatures between 257 and 533 K were measured. It is noted that both the fluorescence yield and the lifetime decrease when the temperature is raised. It is expected that any increase in temperature would cause increased excitation of the lattice vibrations. The increased availability of higher energy phonons would help in a quicker deexcitation of the excited atoms through non-radiative decay. This is surmised as the cause of the decreased fluorescence yield and the lifetime of the fluorescing level. The observation of laser emission due to 4F9/2 ! 6H13/2 transition of Dy3+ at liquid nitrogen temperatures [14] is not inconsistent with this surmise. The non-radiative relaxation rate can be estimated by the relation X W p ðT Þ ¼ s1 A ¼ 1=sobs 1=srad . ð11Þ obs
NIR to visible upconversion luminescence observed for Dy3+ doped in tellurite glass when excited with 862 nm (11600 cm1) radiation from a Ti-Sapphire laser is shown in Fig. 4. The intense yellow fluorescence at 575.7 nm attributed to 4 F9/2 ! 6H13/2 transition is seen at incident power levels >20 mW. A log–log plot of the observed fluorescence intensity versus incident laser power is a straight line (see Fig. 5) with a slope of 1.8. This is indicative of the involvement of two photons in the excitation process. This value is less than 2 due to energy mismatch and the nonradiative relaxation involved in populating the upper 4F9/2 level. The energy levels of Dy3+ and the steps in the upconversion process are shown in Fig. 6. The optical radiation from the TiSapphire laser excites Dy3+ ions to the 6F7/2 state
J
The values of Wp(T) for different T were determined using the measured values of the lifetime at different temperatures and A value obtained from the absorption measurements. A plot of Wp(T) versus T is shown in Fig. 3 which is nearly a straight line.
Fig. 3. Variation of non-radiative relaxation rate [Wp(T)] versus temperature (T).
Fig. 4. Upconversion luminescence from Dy3+ (1.0 mol%) doped in tellurite glass (pump wavelength is 862 nm).
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ion, which receives the additional energy, is excited to the 4I15/2 state. Dy3+ ions in this excited state relax non-radiatively to the 4F9/2, which is the upper level of the emission in the yellow-green region. The upconversion luminescence in the glass shows an increasing luminescence with increasing rareearth concentration. This suggests that energy transfer amongst the excited ions is the most probable mechanism for upconversion. The above upconversion process can be represented as
Fig. 5. A plot for log of luminescence intensity [ln I] versus log of pump power [lnP] for upconversion luminescence observed in Dy3+ doped in tellurite glass.
(11,123 cm1). One of the excited Dy3+ ions transfers its excitation energy to another excited ion and comes to the ground state (Fig. 6). Other
6
F7=2 þ 6 F7=2 ! 4 I15=2 þ 6 H15=2 ! 4 F9=2 þ DE
4
F9=2 ! 6 H 13=2 þ hm
Another possible channel for upconversion may be that the Dy3+ ions excited by the incident photon to the 6F7/2 level relax to 6H7/2 level (9164 cm1). The ions in this latter level absorb another incident photon to populate resonantly the 4F9/2 level. It is not possible for the present experiments to eliminate this second process though it seems less likely as many phonons have to be involved.
4. Conclusions The absorption and fluorescence spectra of Dy3+ doped in tellurite glass have been recorded and analyzed using the Judd–Ofelt theory. The variation of the lifetime of 4F9/2 level and the fluorescence yield for the 4F9/2 ! 6H13/2 transition with concentration of the rare earth and with the temperature of the glass has been studied. It is found that for concentrations >1.25 mol% the two values decrease due to quenching. The quenching mechanism involves dipole–dipole interaction. The decrease with increasing temperature is attributed to increased non-radiative relaxation. Strong upconversion in the yellow region at 575.7 nm due to the 4F9/2 ! 6H13/2 transition on excitation with 862 nm radiation is seen and explained as involving excitation transfer between rare-earth ions.
Acknowledgment Fig. 6. The energy level diagram for upconversion in Dy3+ doped in tellurite glass.
AuthorÕs are grateful to D.S.T. New Delhi for financial assistance.
V.K. Rai et al. / Optics Communications 257 (2006) 112–119
References [1] M.D. Seltzer, A.O. Wright, C.A. Morrison, D.E. Wortman, J.B. Gruber, E.D. Filer, J. Phys. Chem. Solids 57 (1996) 1175. [2] R. Reisfeld, A. Honigbaum, G. Michach, L. Harel, M. Shalom, Israel J. Chem. 7 (1969) 613. [3] C.W. Struck, W.H. Fonger, J. Appl. Phys. 42 (1971) 4515. [4] R.H. Alberda, G. Blasse, J. Lumin. 12&13 (1976) 687. [5] R. Reisfeld, G. Kartz, N. Spector, C.K. Jorgensen, C. Jacoboni, R. De Pape, J. Solid-State Chem. 41 (1982) 253. [6] G.S. Raghuvanshi, H.D. Bist, H.C. Khandpal, J. Phys. Chem. Solids 43 (1982) 81. [7] M.D. Shinn, W.A. Sibley, M.G. Drexhage, R.N. Brown, Phys. Rev. B 27 (1983) 6635. [8] R. Reisfeld, G. Kartz, C. Jacoboni, R. De Pape, M.G. Drexhage, R.N. Brown, C.K. Jorgensen, J. Solid State Chem. 48 (1983) 32. [9] R. Reisfeld, E. Greenberg, R.N. Brown, M.G. Drexhage, C.K. Jorgensen, Chem. Phys. Lett. 95 (1983) 91. [10] K. Tanimura, M.D. Shinn, W.A. Sibley, M.G. Drexhage, R.N. Brown, Phys. Rev. B 30 (1984) 2429. [11] J.L. Adams, W.A. Sibley, J. Non-Cryst. Solids 76 (1985) 267. [12] V.M. Orera, P.J. Alonso, R. Cases, R. Alcala, Phys. Chem. Glasses 29 (1988) 59.
119
[13] A.A. Kaminskii, Crystalline Lasers: Physical Processes and Operating Schemes, Chemical Rubber Company, Boca Raton, FL, 1996. [14] A.A. Kaminskii, U. Hommerich, D. Temple, J.T. Seo, K. Ueda, S. Bagayev, A. Pavlyuk, Japan J. Appl. Phys. L 39 (2000) 208. [15] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4412. [16] E. Cavalli, E. Bovero, A. Belletti, J. Phys. Condens.: Matter 14 (2002) 5221. [17] A.V. Chandrasekhar, A. Radhapathy, B.J. Reddy, Y.P. Reddy, L. Ramamoorthy, R.V.S.S.N. Ravikumar, Opt. Mater. 22 (2003) 215. [18] P. Srivastava, S.B. Rai, D.K. Rai, Spectrochim. Acta A 59 (2003) 3303. [19] B.R. Judd, Phys. Rev. 127 (1962) 750. [20] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [21] B.C. Joshi, J. Non-Cryst. Solids 45 (1981) 39. [22] K.K. Mahato, Anita Rai, S.B. Rai, Spectrochim. Acta A 61 (2005) 431. [23] Z. Pan, S.H. Morgan, K. Dyer, A. Ueda, H. Liu, J. Appl. Phys. 79 (1996) 8906. [24] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978. [25] J. Hormadaly, R. Reisfeld, J. Non-Cryst. Solids 30 (1979) 337.
Optical studies of Dy3+ doped tellurite glass: Observation of yellow-green upconversion Vineet Kumar Rai *, S.B. Rai, D.K. Rai Laser and Spectroscopy Laboratory, Department of Physics, B.H.U. Varanasi 221 005, India Received 11 February 2005; received in revised form 27 June 2005; accepted 6 July 2005
Abstract Optical absorption and fluorescence spectra of Dy3+ doped tellurite glass have been studied and Judd–Ofelt theory has been used to derive its optical properties. The effect of temperature on the fluorescence yield and fluorescence quenching for higher concentrations of Dy3+ has been studied and the mechanism involved discussed. It is found that the fluorescence intensity is larger at lower temperature. The lifetime of the 4F9/2 level has been measured and found to decrease with concentration as well as temperature. Intense upconversion has also been observed in the yellow-green region when pumped with NIR (862 nm) radiation. An explanation of the same involving energy transfer is offered. 2005 Elsevier B.V. All rights reserved. PACS: 42.70 Ce Keywords: Judd–Ofelt theory; Concentration quenching; Quantum efficiency; Upconversion
1. Introduction Solid-state lasers operating in the yellow-orange region (570–590 nm) have great technological applications in military, telecommunication, commercial displays, etc. Dy3+ doped in glass/crystal gives strong discrete emission in this wavelength * Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 368 468. E-mail addresses: [email protected] (V.K. Rai), [email protected] (S.B. Rai).
region while Dy3+-garnets have been used as a potential saturable absorber in the 2.8 lm region [1]. The luminescence properties of Dy3+ doped in different glass lattices have been studied by several workers [2–18]. It has been found that the optical properties of Dy3+ ion doped in fluoride lattices are very different from that in oxide lattices [5,7,11]. Kaminskii [13] has observed laser action in Dy3+ in the NIR region (1.35 and 3.0 lm) in inorganic solids (fluoride hosts). Laser action in Dy3+ doped tungstate glass in the visible region due to
0030-4018/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.07.022
V.K. Rai et al. / Optics Communications 257 (2006) 112–119 4
F9/2 ! 6H13/2 (570 nm) and 4F9/2 ! 6H11/2 (660 nm) transitions at liquid nitrogen temperature under excitation with Xe-flash lamp has also been reported [14]. In this paper, we have investigated the spectroscopic properties of Dy3+ ion doped in tellurite glass. Absorption and the fluorescence spectra of the glass have been studied for different concentrations of Dy3+. Judd–Ofelt [19,20] theory has been used to analyze and explain the experimental observations. Concentration quenching of fluorescence has been studied and the mechanism of the quenching discussed. The effect of pump laser power and temperature on the fluorescence yield and on the lifetime of the 4F9/2 level have been studied. Intense upconversion has been observed in the yellow-orange region on NIR (862 nm) pumping.
2. Experimental In order to form Dy3+ doped tellurite glass we used the chemicals TeO2 (BDH), Li2CO3 (BDH) and Dy2O3 (rare-earth chemicals) with reported purity of 99.9%, 97% and 99%, respectively, in the following proportions: ð80 xÞTeO2 þ 20Li2 CO3 þ xDy2 O3 where x = 0.5, 1.0, 2.0 and 3.0 mol%. All these compounds were made into fine powder in a ceramic mortar and finally mixed properly to get a homogeneous mixture. The sample was then placed in a platinum crucible and heated up to 900 C for one and half-hours. The melt was constantly stirred for homogeneous mixing and then suddenly poured into a steel cast kept at 460 C. The semisolid was then pressed with a flat disc to get flat glass of about 1.0 mm thickness. Several pieces of glasses were prepared for each concentration of the rare earth. The glasses were then cleaned and polished for further studies. The densities of the glass samples were measured using a single pan balance with Xylene as the immersion liquid. For refractive index measurements BrewsterÕs angle polarization method has been used. A 2 mW He–Ne polarized laser at 632.9 nm was used as light source for this purpose. The density is 5.74 g/cm3 and refractive index is 2.36 for glass
113
containing 1 mol% Dy3+. The absorption spectrum of Dy3+ (1.0 mol%) doped tellurite glass was recorded using a Lambda-19-UV–Vis–NIR double beam spectrophotometer in the region 400–2500 nm. For the fluorescence spectrum we employed the 457.9 nm line with 100 mW power from an Ar+ laser as excitation source. The glass was observed to fluoresce with a weak yellowish color. The fluorescence spectra were measured using a 0.5 m Spex monochromator attached with a IP21 PMT in the region 461.0–691.0 nm at room temperature (22 C). The lifetime of the 4F9/2 level had been measured for all the four samples using a KrF laser emitting at 248 nm with a spectral bandwidth of 0.05 nm. The incident laser light was focused on the glass and the fluorescence decay for the yellow band was seen on oscilloscope. The lifetime of the 4F9/2 level could be calculated from the decay curves. For upconversion measurements a Ti-Sapphire laser (emitting at 862 nm) pumped by the second harmonic of a diode-pumped Nd: yVO4 laser was used.
3. Results and discussion 3.1. Absorption spectrum The absorption spectrum of the triply ionized Dy doped in tellurite glass shows several absorption peaks in the 400–2500 nm region (see Fig. 1). The electronic configuration of Dy3+ is 4f95s25p6, which gives 6H15/2 as the ground state of Dy3+ with a large number of other low-lying excited states. It is found that the presently observed absorption peaks are only slightly shifted from their corresponding positions in other hosts. The wavelengths of these peaks and their assignments are given in Table 1. The oscillator strength (Fexp) for the different peaks (assumed to have a Gaussian shape) was obtained from the expression Z mc2 F exp ¼ 2 aðkÞ dk=k2 ; ð1Þ pe N where Ôa(k)Õ is the measured absorption coefficient at wavelength ÔkÕ and ÔNÕ is the number of rareearth ions per unit volume. The oscillator strength
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V.K. Rai et al. / Optics Communications 257 (2006) 112–119
Fig. 1. Absorption spectrum of 1.0 mol% Dy3+ doped in tellurite glass.
Table 1 Observed absorption bands, their wavelengths, energies, assignments, measured and calculated values of the oscillator strength for Dy3+ doped in tellurite glass Transition
Wavelength (nm)
Energy (cm1)
Fm · 106
Fc · 106
4
6
6
6
490.9 757.6 810.6 896.8 1090.9 1280.3 1691.5
20365.1 13195.9 12333.1 11123.0 9164.2 7808.5 5911.9
1.3342 2.2665 0.5964 0.4594 0.2087 1.5913 0.2557
1.5342 2.6000 0.5474 0.6081 0.2471 1.1143 0.2851
F9/2 F3/2 6 F5/2 6 F7/2 6 H7/2 6 H9/2 6 H11/2
H15/2 H15/2 6 H15/2 6 H15/2 6 H15/2 6 H15/2 6 H15/2
thus obtained is given in Table 1. The oscillator strength according to the Judd–Ofelt theory [19,20] for an electric dipole allowed transition J ! J 0 is given as
The spontaneous emission transitions probability for an electric dipole allowed transition is represented in J–O theory by
8p2 mm F cal ðaJ ; bJ Þ ¼ ½fðn2 þ 2Þ2 =9ngS ed þ nS md ; 3hð2J þ 1Þ ð2Þ
Arad ðaJ ; bJ 0 Þ ¼
0
where ÔJÕ is the total quantum number for the ground level and J 0 for the upper level. ÔSedÕ and ÔSmdÕ are the electric and magnetic dipole line strengths, respectively. The values of Smd for these transitions are very small and are usually neglected. The electric dipole line strength is given by S ed ½ðS; L; J Þ; ðS 0 ; L0 ; J 0 Þ X 2 Xk jhðS; L; J ÞkU k kðS 0 ; L0 ; J 0 Þij . ¼
64p4 m3 nðn2 þ 2Þ2 e2 3hc3 ð2J þ 1Þ9 hX i Xk jhðS; L; J ÞkU k kðS 0 ; L0 ; J 0 Þij2 ð4Þ
and the total spontaneous transition probability by X AT ¼ Arad ðaJ ; bJ 0 Þ. ð5Þ The fluorescence branching ratio ÔbRÕ is obtained using the relation
ð3Þ
k¼2;4;6
The value of Fcal(aJ, bJ 0 ) thus obtained is given in Table 1. We have used the reduced matrix element iUki given by Carnall et al. [15] in our calculations.
bR ¼
AðaJ ; bJ 0 Þ . AT
ð6Þ
The radiative lifetime ÔsRÕ of the fluorescing level is given as
V.K. Rai et al. / Optics Communications 257 (2006) 112–119
sR ¼ 1=AT .
ð7Þ
Again, the stimulated emission cross-section of the fluorescing level is given as rEP ¼
k4 8p2 cn2 Dk
½AðaJ ; bJ 0 Þ;
ð8Þ
where ÔkÕ is the wavelength of the peak and ÔDk Õ is its effective bandwidth. Using the experimentally measured values of the oscillator strength for the different transitions, Xk parameters have been determined. The ÔXkÕ parameters thus obtained for Dy3+ doped in tellurite glass are compared with the corresponding values in other glass hosts in Table 2. These values of ÔXkÕ have been utilized to calculate the transition probability, branching ratio, radiative lifetime, stimulated emission cross-section, etc. for different transitions and these are given in Table 3. It is interesting to note that the branching ratio for the 4F9/2 ! 6H13/2 Table 2 Comparison of Judd–Ofelt intensity parameters (Xk · 1020 cm2) for Dy3+ in different host lattices Host
X2
X4
X6
References
Na2O + P2O5 Oxyfluoroborate Fluorozirconate 20Li2O + 80 TeO2
1.46 2.68 2.70 1.46
1.16 2.56 1.80 2.32
1.97 0.89 2.00 3.60
[25] [22] [12] Present work
115
transition is much higher (0.548) compared to other transitions. The stimulated emission crosssection for this transition is also large indicating that the 4F9/2 ! 6H13/2 transition of Dy3+ in tellurite glass may have a potential for laser emission. In fact, laser emission corresponding to this transition has already been reported [14]. We also recorded the infrared spectrum of the glass. A strong absorption peak is observed at 650 cm1 due to TeO2. 3.2. Fluorescence spectrum There are three fluorescence peaks observed at 481.5, 575.7 and at 661.9 nm in Dy3+ doped tellurite glass when excited with 457.9 nm radiation. The peak at 575.7 nm is very strong and sharp whereas the others are broad and weak. The assignments of these peaks are given in Table 4. We have calculated the bandwidth and the stimulated emission cross-section for the peaks at 481.5 and 575.7 nm and find values of these as 15.1 nm; 0.57 · 1022 cm2 and 13.2 nm; 0.86 · 1022 cm2, respectively (see Table 4). A comparison of the fluorescence spectrum of Dy3+ doped in different hosts (see Fig. 2) shows that the transition probability and branching ratio for 4F9/2 ! 6H13/2 transition is much larger in tellurite host [21,22]. Also, the observed intensity of this peak in the tellurite
Table 3 Electric dipole line strength, transition probability, branching ratios for different transitions and radiative lifetime of 4F9/2 state of Dy3+ in tellurite glass SLJ S 0 L 0 J 0 4
Energy
Sed · 1020
A (s1)
br = A/AT
7283.0 7845.0 8650.0 10082.0 10892.0 11955.0 12039.0 13361.0 13390.0 15102.9 17363.8 20764.5
0.0009 0.0011 0.0127 0.0217 0.0129 0.0462 0.0162 0.0227 0.0211 0.0311 0.3209 0.1204
0.0297 0.0432 0.6853 1.8501 1.3810 6.5472 2.3405 4.4897 4.2087 8.8632 139.1900 84.2800
0.0001 0.0002 0.0027 0.0073 0.0054 0.0258 0.0092 0.0177 0.0165 0.0349 0.5482 0.3319
F9/2! 6
F1/2 F3/2 6 F5/2 6 F7/2 6 H5/2 6 H7/2 6 F9/2 6 F11/2 6 H9/2 6 H11/2 6 H13/2 6 H15/2 6
AT = 253.91, s = 3.94 ms
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Table 4 Assignment of the bands, their wavelengths, energies, half bandwidth and the stimulated emission cross-section observed in fluorescence spectra of Dy3+ doped in tellurite glass Transition
Wavelength (nm)
Energy (cm1)
Band width (Dk) (nm)
Stimulated emission cross-section ðrEP 1022 Þ cm2
4
481.5 575.7 661.9
20764.5 17363.8 15102.9
15.1 13.2
0.57 0.86
6
F9/2 ! H15/2 F9/2 ! 6H13/2 4 F9/2 ! 6H11/2 4
3.3. Effect of concentration of rare-earth ion on the fluorescence intensity Changing the concentration of Dy3+ in tellurite glass host shows that fluorescence starts getting quenched when the concentration of the rare-earth ion is increased beyond 1.0 mol%. This is attributed to energy transfer from an excited Dy3+ ion to a nearby unexcited Dy3+ ion. The lifetime of the 4F9/2 level of Dy3+ was measured for different concentrations of Dy3+. The results are listed in Table 5 and it is seen that the lifetime of the 4F9/2 level decreases for concentrations >1.0 mol%. Inokuti and Hirayama [24] proposed a model according to which the fluorescence intensity at a time ÔtÕ is given as IðtÞ ¼ Ið0Þ exp½t=s0 Cð1 3=SÞC=C 0 ðt=s0 Þ
3=S
;
ð10Þ
Fig. 2. Fluorescence spectrum of Dy3+ (1.0 mol%) doped in different hosts (tellurite, oxyfluoroborate and calibo).
host is larger and the FWHM much smaller compared to other hosts [21,22]. The intensity of this peak is seen to vary linearly with increasing incident laser power (within the range of our measurements). These parameters indicate that tellurite glass is a promising host for a laser using Dy3+. The luminescence quantum efficiency ÔgÕ may be defined by the relation [23], g ¼ emitted light power=absorbed radiation power ¼ sexp =sr . ð9Þ For Dy concentration of 1.0 mol% in tellurite glass the value of g is found to be 16%.
where ÔCÕ is the actual concentration of the emitting ion and ÔC0Õ is critical concentration. The value of ÔC0Õ is determined from a plot of fluorescence intensity versus the concentration of Dy3+ in the glass. Its value is found to be 1.69 mol%. A fitting of the above expression to the measured intensity results in a value for S = 6 which
Table 5 Lifetime of 4F9/2 level of Dy3+ doped in different glass hosts Host Tellurite glass
Concentration Lifetime (ms) Reference
0.5 mol% 1.0 mol% 2.0 mol% 3.0 mol% Borate glass 1.0 wt% Fluorozirconate 1.0 wt% Phosphate glass 1.0 wt%
0.52 0.66 0.61 0.58 1.4 1.2 1.0
Present work
[6] [12] [6]
V.K. Rai et al. / Optics Communications 257 (2006) 112–119
117
indicates that the process involved in energy transfer is based on dipole–dipole interactions.
The non-radiative relaxation rate at room temperature is estimated as 1.24 · 103 s1.
3.4. Temperature dependence of the fluorescence yield and the lifetime
3.5. Upconversion
The lifetime of the 4F9/2 level and the fluorescence yield corresponding to 4F9/2 ! 6H13/2 transition at different temperatures between 257 and 533 K were measured. It is noted that both the fluorescence yield and the lifetime decrease when the temperature is raised. It is expected that any increase in temperature would cause increased excitation of the lattice vibrations. The increased availability of higher energy phonons would help in a quicker deexcitation of the excited atoms through non-radiative decay. This is surmised as the cause of the decreased fluorescence yield and the lifetime of the fluorescing level. The observation of laser emission due to 4F9/2 ! 6H13/2 transition of Dy3+ at liquid nitrogen temperatures [14] is not inconsistent with this surmise. The non-radiative relaxation rate can be estimated by the relation X W p ðT Þ ¼ s1 A ¼ 1=sobs 1=srad . ð11Þ obs
NIR to visible upconversion luminescence observed for Dy3+ doped in tellurite glass when excited with 862 nm (11600 cm1) radiation from a Ti-Sapphire laser is shown in Fig. 4. The intense yellow fluorescence at 575.7 nm attributed to 4 F9/2 ! 6H13/2 transition is seen at incident power levels >20 mW. A log–log plot of the observed fluorescence intensity versus incident laser power is a straight line (see Fig. 5) with a slope of 1.8. This is indicative of the involvement of two photons in the excitation process. This value is less than 2 due to energy mismatch and the nonradiative relaxation involved in populating the upper 4F9/2 level. The energy levels of Dy3+ and the steps in the upconversion process are shown in Fig. 6. The optical radiation from the TiSapphire laser excites Dy3+ ions to the 6F7/2 state
J
The values of Wp(T) for different T were determined using the measured values of the lifetime at different temperatures and A value obtained from the absorption measurements. A plot of Wp(T) versus T is shown in Fig. 3 which is nearly a straight line.
Fig. 3. Variation of non-radiative relaxation rate [Wp(T)] versus temperature (T).
Fig. 4. Upconversion luminescence from Dy3+ (1.0 mol%) doped in tellurite glass (pump wavelength is 862 nm).
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V.K. Rai et al. / Optics Communications 257 (2006) 112–119
ion, which receives the additional energy, is excited to the 4I15/2 state. Dy3+ ions in this excited state relax non-radiatively to the 4F9/2, which is the upper level of the emission in the yellow-green region. The upconversion luminescence in the glass shows an increasing luminescence with increasing rareearth concentration. This suggests that energy transfer amongst the excited ions is the most probable mechanism for upconversion. The above upconversion process can be represented as
Fig. 5. A plot for log of luminescence intensity [ln I] versus log of pump power [lnP] for upconversion luminescence observed in Dy3+ doped in tellurite glass.
(11,123 cm1). One of the excited Dy3+ ions transfers its excitation energy to another excited ion and comes to the ground state (Fig. 6). Other
6
F7=2 þ 6 F7=2 ! 4 I15=2 þ 6 H15=2 ! 4 F9=2 þ DE
4
F9=2 ! 6 H 13=2 þ hm
Another possible channel for upconversion may be that the Dy3+ ions excited by the incident photon to the 6F7/2 level relax to 6H7/2 level (9164 cm1). The ions in this latter level absorb another incident photon to populate resonantly the 4F9/2 level. It is not possible for the present experiments to eliminate this second process though it seems less likely as many phonons have to be involved.
4. Conclusions The absorption and fluorescence spectra of Dy3+ doped in tellurite glass have been recorded and analyzed using the Judd–Ofelt theory. The variation of the lifetime of 4F9/2 level and the fluorescence yield for the 4F9/2 ! 6H13/2 transition with concentration of the rare earth and with the temperature of the glass has been studied. It is found that for concentrations >1.25 mol% the two values decrease due to quenching. The quenching mechanism involves dipole–dipole interaction. The decrease with increasing temperature is attributed to increased non-radiative relaxation. Strong upconversion in the yellow region at 575.7 nm due to the 4F9/2 ! 6H13/2 transition on excitation with 862 nm radiation is seen and explained as involving excitation transfer between rare-earth ions.
Acknowledgment Fig. 6. The energy level diagram for upconversion in Dy3+ doped in tellurite glass.
AuthorÕs are grateful to D.S.T. New Delhi for financial assistance.
V.K. Rai et al. / Optics Communications 257 (2006) 112–119
References [1] M.D. Seltzer, A.O. Wright, C.A. Morrison, D.E. Wortman, J.B. Gruber, E.D. Filer, J. Phys. Chem. Solids 57 (1996) 1175. [2] R. Reisfeld, A. Honigbaum, G. Michach, L. Harel, M. Shalom, Israel J. Chem. 7 (1969) 613. [3] C.W. Struck, W.H. Fonger, J. Appl. Phys. 42 (1971) 4515. [4] R.H. Alberda, G. Blasse, J. Lumin. 12&13 (1976) 687. [5] R. Reisfeld, G. Kartz, N. Spector, C.K. Jorgensen, C. Jacoboni, R. De Pape, J. Solid-State Chem. 41 (1982) 253. [6] G.S. Raghuvanshi, H.D. Bist, H.C. Khandpal, J. Phys. Chem. Solids 43 (1982) 81. [7] M.D. Shinn, W.A. Sibley, M.G. Drexhage, R.N. Brown, Phys. Rev. B 27 (1983) 6635. [8] R. Reisfeld, G. Kartz, C. Jacoboni, R. De Pape, M.G. Drexhage, R.N. Brown, C.K. Jorgensen, J. Solid State Chem. 48 (1983) 32. [9] R. Reisfeld, E. Greenberg, R.N. Brown, M.G. Drexhage, C.K. Jorgensen, Chem. Phys. Lett. 95 (1983) 91. [10] K. Tanimura, M.D. Shinn, W.A. Sibley, M.G. Drexhage, R.N. Brown, Phys. Rev. B 30 (1984) 2429. [11] J.L. Adams, W.A. Sibley, J. Non-Cryst. Solids 76 (1985) 267. [12] V.M. Orera, P.J. Alonso, R. Cases, R. Alcala, Phys. Chem. Glasses 29 (1988) 59.
119
[13] A.A. Kaminskii, Crystalline Lasers: Physical Processes and Operating Schemes, Chemical Rubber Company, Boca Raton, FL, 1996. [14] A.A. Kaminskii, U. Hommerich, D. Temple, J.T. Seo, K. Ueda, S. Bagayev, A. Pavlyuk, Japan J. Appl. Phys. L 39 (2000) 208. [15] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4412. [16] E. Cavalli, E. Bovero, A. Belletti, J. Phys. Condens.: Matter 14 (2002) 5221. [17] A.V. Chandrasekhar, A. Radhapathy, B.J. Reddy, Y.P. Reddy, L. Ramamoorthy, R.V.S.S.N. Ravikumar, Opt. Mater. 22 (2003) 215. [18] P. Srivastava, S.B. Rai, D.K. Rai, Spectrochim. Acta A 59 (2003) 3303. [19] B.R. Judd, Phys. Rev. 127 (1962) 750. [20] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [21] B.C. Joshi, J. Non-Cryst. Solids 45 (1981) 39. [22] K.K. Mahato, Anita Rai, S.B. Rai, Spectrochim. Acta A 61 (2005) 431. [23] Z. Pan, S.H. Morgan, K. Dyer, A. Ueda, H. Liu, J. Appl. Phys. 79 (1996) 8906. [24] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978. [25] J. Hormadaly, R. Reisfeld, J. Non-Cryst. Solids 30 (1979) 337.