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Fluorescence and Spectroscopic Properties of Yb3+-Doped Phosphate Glasses
Available online at www.sciencedirect.com
Physics Procedia 29 (2012) 109 – 113
16th International Conference on Luminescence- (ICL`11)
Fluorescence and spectroscopic properties of Yb3+-doped phosphate glasses K. Venkata Krishnaiah1, K. Upendra Kumar2, V. Agarwal3, C.G. Murali4, S. Chaurasia4, L.J. Dhareshwar4, C.K. Jayasankar1*, Victor Lavin5 1
Department of Physics, Sri Venkateswara University, Tirupati - 517 502, Andhra Pradesh, India División de Ciencias e Ingenierías, Campus León, Universidad de Guanajuato, Lomas del Bosque 103, Col. Lomas del Campestre, C.P. 37150, León Guanajuato, Mexico 3 Centro de Investigacion en Ingenieria y Ciencias Aplicadas, Universidad Autonoma del Estado de Morelos, Avenida Universidad 1001,Colonia Chamilpa, C.P. 62210, Cuernavaca, Morelos, Mexico 4 Laser and Nutron Physics Division, Bhabha Atomic Research Centre, Mumbai - 400 085, India 5 Departamento de Física Fundamental y Experimental, Electrónica y Sistemas, Universidad de La Laguna, E-38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain 2
Received 25 July 2011; accepted 25 August 2011
Abstract The concentration dependent Yb3+: phosphate (P2O5 + K2O + MgO + Al2O3 + Yb2O3) glasses have been prepared and characterized their fluorescence and laser properties. The stimulated emission cross-section and laser performance parameters were determined from the measured absorption spectra using the method of reciprocity. The refractive index, absorption and emission cross-sections and fluorescence lifetimes varied with Yb3+ ion concentration. The higher emission cross-section was found to be 1.01 × 10-20 cm2 at the extraction wavelength of 1005 nm. The fluorescence lifetime of 2F5/2 level decreases from 1.04 ms to 0.28 ms with increase of Yb2O3 concentration from 0.05 to 6.0 mol %. The gain cross-section spectra can be obtained from the measured absorption and emission cross-sections with different population levels. The values of emission cross-section, fluorescence lifetime, minimum pulse duration, pump power, extraction efficiency and gain coefficients suggest that these glasses can be used as a laser gain media for the generation of ultrashort pulse and high power laser applications. © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the organizing committee © 2011 Published by Elsevier represented by Stephen C. RandB.V. and the guest editors.
Keywords: Ytterbium, phosphate glasses, fluorescence, laser parameters
1. Introduction In recent years, the advances of solid state lasers have witnessed tremendous progress in the development of both laser materials and the laser system engineering. The properties of laser host materials determine the performance of a laser system that should have a high gain, high laser induced material damage threshold, extremely good thermal properties and scalability of growth to large volumes. Among several laser glass materials, ytterbium doped glass materials are very important and interesting since they possess long fluorescence lifetime (~2 ms) and broad fluorescence bandwidth (~50 nm) [1]. The investigation on spectroscopic and laser properties of Yb3+-doped glasses are widely fascinating due to its special features: (i) a simple energy level scheme with the 2F7/2 ground and the 2F5/2 excited states, for which excited state absorption and up-conversion can be ignored, (ii) the absence of cross-relaxation mechanisms that can decrease the effective lifetime, (iii) a higher trivalent lanthanide (Ln3+) ion solubility is possible because of a weak concentration quenching, (iv) an excellent sensitizer for other Ln3+ (Er3+, Tm3+, Ho3+, etc.) ions through energy transfer and back-transfer efficient processes [2]. Among various glass hosts, Tel : + 91 877 2248033, fax: +91 877 2289472 * Corresponding author: [email protected] (C.K. Jayasankar) 1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the organizing committee represented by Stephen C. Rand and the guest editors. doi:10.1016/j.phpro.2012.03.700
110
K. Venkata Krishnaiah et al. / Physics Procedia 29 (2012) 109 – 113
phosphate glasses received much attention because they can store optical energy at greater densities and this energy can be efficiently extracted [3]. The phosphate glasses offer good laser and optical properties such as low nonlinear refractive index, high cross-section for stimulated emission, long fluorescence lifetime and thermo-mechanical properties. Moreover, these properties can be improved by chemical strengthening [4,5]. The selection of Al(PO3)3, Ba(PO3)2 and KPO3 as a raw materials for phosphate glass has its own advantages compared to other metaphosphates [6]. The drawback of phosphate glasses are that they have a lower mechanical strength than silicate glasses which do not allow one to use high average pump powers and a low resistance to atmospheric moisture. These draw backs are caused by low chemical bonds among the main structural elements of phosphate glasses (PO4 tetrahedrons) [7]. In the present work, the detailed study focuses on Yb3+: phosphate glasses which show very good fluorescence and lasing properties. 2. Experimental details Phosphate glasses of composition PKMAYb: (60-x/2) P2O5 - 15 K2O - (15-x/2) MgO - 10 Al2O3 - x Yb2O3, where x = 0.05, 0.5, 1.0, 2.0, 4.0 and 6.0 mol % referred as PKMAYb005, PKMAYb05, PKMAYb10, PKMAYb20, PKMAYb40 and PKMAYb60, respectively, have been prepared by melt quenching technique. After fabrication glasses were characterized through absorption, emission and decay rates. The high purity metaphosphates of KPO3, Al(PO3)3, Mg(PO3)2 and Yb2O3 are used as starting materials. The procedure of glass preparation, measurements and analysis of physical and spectral properties are similar to our earlier work [8]. All the glasses were prepared at ambient condition where no attempt has been made to control the OH groups in the glass matrix. 3. Results and discussion Figure 1 shows the absorption and emission cross-section spectra of PKMAYb glasses for different concentrations. Because of the simple energy level scheme and the effect of radiation trapping, the emission crosssection of Yb3+ is generally obtained by the reciprocity method (RM) [9]. The absorption and emission crosssections decrease with increase of Yb2O3 concentration. According to the McCumber theory [9], the emission crosssection is inversely proportional to the concentration. Higher the Yb3+ ion concentration, occupy different sites in the glass host, that can change the local field and also reabsorption takes place for Yb3+ ion concentration. The fluorescence trapping is also increases with increase of concentration. Hence, the radiation trapping depends on the concentration and occurs even for small doping levels. Therefore, decrease in emission cross-section with increase in Yb3+ ion concentration has been noticed. Table 1 presents the spectroscopic as well as laser properties of
1.5
Vem 1.0
Vab
0.5
0.0
2.0
1.5
Vem
1.0
Vab
0.5
0.0
860
910
960
1010
(c) PKMAYb10
2.5
2
PKMAYb05
-20
2.0
2.5
3.0
Cross-section ( x 10
2
cm )
-20
PKMAYb005
Cross-section ( x 10
2
cm )
-20
Cross-section (x 10
2.5
(b)
3.0
cm )
(a)
3.0
2.0
1.5
Vem
1.0
Vab 0.5
0.0
1060
860
Wavelength (nm)
910
960
1010
1060
860
910
960
Wavelength (nm)
1.5
Vemi
1.0
Vabs 0.5
1.5
Vem
1.0
Vab
0.0
860
910
960
1010
Wavelength (nm)
1060
PKMAYb60
2 -20
2.0
0.5
0.0
(f)
2.5
cm )
2.5
1060
3.0
PKMAYb40
Cross-section (x10
2
cm )
-20
PKMAYb20
2.0
Cross-section (x10
2
cm )
-20
Cross-section ( x 10
2.5
(e)
3.0
(d)
3.0
1010
Wavelength (nm)
860
2.0
1.5
Vem
1.0
Vab
0.5
0.0
910
960
1010
1060
860
Wavelength (nm)
Fig.1 Absorption and emission cross-section spectra of PKMAYb glasses.
910
960
1010
Wavelength (nm)
1060
111
K. Venkata Krishnaiah et al. / Physics Procedia 29 (2012) 109 – 113
PKMAYb glasses. They are compared with the Yb3+-doped fluorophosphate [10], heavy metal oxide (HMO) [11], oxy-fluoride silicate (SACF0.4) [12], lead fluoroborate (PbFB) [13], PNK25 (P2O5 + Nb2O5 + K2O) [14] and phospho-tellurite (35PT1Y) [15] glasses. The larger emission cross-section is obtained for the PKMAYb005, which is higher than the value found in HMO [11] and SACF0.4 [12] glasses as listed in the Table 1. No systematic trend has been observed in the effective bandwidth with increase of Yb2O3 concentration. The highest effective bandwidth of 111 nm is obtained for PKMAYb20 glass than those glasses shown in Table 1. Table 1. Glass lable, absorption cross-section (ıab, x 10-20 cm2), emission cross-section (ıem, x 10-20 cm2 at peak absorption (Ȝp) and extraction (Ȝ0) wavelengths), effective bandwidth (ǻȜeff, nm), fluorescence lifetime (IJf, ms), saturation pumping intensity (Isat, kWcm-2), minimum pumping intensity (Imin, kWcm-2) pump power (Usat, Jcm-2), minimum pulse duration (IJmin, fs) and gain coefficient (G (ıab(Ȝp) x IJf x ıem(Ȝo)), cm4 ms) of Yb3+:glasses. Parameters o
ıab
ıem
ıem
Glass lable p
(Ȝp)
(Ȝp)
(Ȝo)
ǻȜeff
IJf
Isat
Imin
Usat
IJmin
G
PKMAYb005
2.35
1.84
1.01
89
1.04
8.42
5.88
6.07
38
0.31
PKMAYb05
1.57
1.17
0.43
81
0.96
9.08
7.13
10.20
47
0.79
PKMAYb10
1.51
1.13
0.41
74
0.56
13.50
10.6
10.60
45
1.52
PKMAYb20
1.48
1.11
0.38
111
0.42
21.80
18.1
11.42
30
1.51
PKMAYb40
1.44
1.08
0.31
73
0.32
44.70
34.7
11.02
46
1.73
PKMAYb60
1.24
0.93
0.28
80
0.28
60.00
46.5
12.75
42
1.77
FP[10]
1.77
-
1.39
-
0.66
17.3
3.70
-
-
1.65
HMO [11]
2.20
-
0.75
86
0.40
-
3.40
22.60
-
-
SACF0.4[12]
-
1.17
0.61
42
1.43
-
1.51
11.20
-
-
PbFB [13]
-
-
1.07
60
0.81
-
1.69
-
-
-
PNK25 [14]
-
-
-
-
0.96
15.55
1.45
-
-
-
35PT1Y[15]
-
-
1.79
-
1.17
-
1.84
-
-
-
The decay rates for 2F5/2 level of Yb3+ ion in PKMAYb glasses exhibit single exponential (Figure 2) for all the concentrations. The lifetime decreases from 1.04 ms to 0.28 ms with increase of Yb2O3 concentation from 2
F5/2o
(a) PKMAYb005, W 1.04 ms (b) PKMAYb05, W 0.96 ms
Oex= 950 nm Oem= 975 nm
1.8
1.8
1.6
1.6
1.4
1.4
1.2
1.2
1.0
1.0
(c) PKMAYb10, W 0.56 ms (d) PKMAYb20, W 0.42 ms (f) PKMAYb60, W 0.28 ms
0.1 (f)
(e) (d)
(b)
(c)
(a)
0.01
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0
1
2
3
4
5
Time (ms)
Fig. 2. Decay rates of the 2F5/2 level of PKMAYb glasses
-2
0
2
4
6
8
10
12
14
0.0 16
Concentration (ions/cc)
Fig. 3. Variation of emission cross-section and the gain coefficient with Yb2O3 concentration
G
(e) PKMAYb40, W 0.32 ms
Vem(O0)
Normalised intensity (arb. units)
1
112
K. Venkata Krishnaiah et al. / Physics Procedia 29 (2012) 109 – 113
0.05 to 6.0 mol %. It is noticed that the value of IJf (1.04 ms) found to be longer for PKMAYb005 than that of other concentrations and higher than fluorophosphate [10], HMO [11], PbFB [13] and PNK25 [14] glasses. The product of ıab(Ȝp) x IJf is proportional to the stored energy[10].
2 Gain cross-section (x 10-20 cm )
The high stored energy and emission cross-section indicates good potential for laser host materials. Therefore, it has been suggested that the laser glass should have high gain coefficient, G, for laser applications. The emission cross-section decreases whereas the gain coefficient increases with increase of Yb3+ ion concentration as shown in Figure 3. The low pump fluence value of 6.07 Jcm-2 obtained for 0.05 mol % Yb2O3 doped glass is far better than HMO [11] and SACF0.4 [12] glasses. The IJmin and Usat were found to be 38 fs and 6.07 J cm-2, respectively, obtained for 0.05 mol % Yb2O3 doped glass suggested that it can be used for ultrashort pulse E and high power laser applications. The wavelength 1.2 PKMAYb10 dependent gain cross-section can be expressed as 1.0 ıg(Ȝ) = ȕıem(Ȝ)-(1-ȕ) ıab(Ȝ), where ȕ (0 to 1 with the 0.8 increments of 0.1) denotes that the ratio of inverted 0.6 ions to the total Yb3+ ion density [16]. The obtained 0.4 gain cross-section (ıg(Ȝ)) spectra of PKMAYb10 glass 0.2 is shown in Figure 4. The ıg(Ȝ) was calculated from 0.0 the measured emission cross-section (ıem) and absorption cross-section (ıab) determined after the -0.2 McCumber method and are given for the values ȕ=1 -0.4 and ȕ=0, respectively. The present glass shows the -0.6 largest gain at 975 nm and also broad emission is -0.8 promising for the generation of tunable and ultrashort E -1.0 pulse lasers. It is observed from the gain spectra, a 890 910 930 950 970 990 1010 1030 1050 wide tenable wavelength range from 980 to 1040 nm is Wavelength (nm) expected whenever the value of ȕ is larger than 0.4. From the point of view of laser operation, it is Fig. 4 Gain spectra of PKMAYb10 glass for generally desirable that the emission cross-section to different population levels be as large as possible to provide greater gain, the fluorescence lifetime to be long in order to permit high inversion densities and the absorption cross-section at the pump wavelength to be as large as possible to allow for efficient diode pumping. These considerations lead the laser parameters ((saturation pumping intensity (Isat) and minimum pumping intensity (Imin)) with low values. An efficient host for laser operation should also incorporate a high concentration of the trivalent rare-earth ion as the laser gain is proportional to the doping concentration. Conclusions Absorption and emission spectra and fluorescence decay rates of Yb3+-doped phosphate glasses with different concentrations were measured and analyzed. The systematic studies reveal that better spectroscopic and laser properties with large absorption (>1.50 x 10-20 cm2) and emission (>1.00 x 10-20 cm2 at 1005 nm) crosssections, effective band width (>70 nm), longer fluorescence lifetime (>1.00 ms) and higher gain coefficient (>1.5 x 10-20 cm2 ms) has been obtained for Yb3+-doped phosphate glasses, which can be used as gain media. The positive gain can be obtained when the population inversion is more than 40 %. The minimum pulse duration and pump fluence values, which are found to be relatively better for the Yb3+-doped phosphate glasses are useful to generate ultrashort pulse and high power lasers. Acknowledgements One of the authors (CKJ) is grateful to DAE-BRNS, Mumbai, Govt. of India for the sanction of major research project (No. 2007/34/25-BRNS/2415, dt. 18-01-2008). Dr. K. Upendra Kumar is gratefully acknowledged the partial financial support by FONCICYT PROJECT 94142.
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References 1. 2. 3. 4. 5. 6. 7. 8.
M.J. Weber, J.E. Lynch, D.H. Blackburn and D.J. Cronin, IEEE J. Quantum Electron QE-19 (10) (1983) 1600. G. Boulon J. Alloys Compd. 451 (2008) 1. W. Koechner, Solid State Laser Engineering, Springer, 6th Edition. J.E. Marion, Proc. Amer. Inst. Physics 146 (1986) 234. R.R. Jacobs and M.J. Weber, IEEE J. Qunt. Elect. QE-12 (1976) 102. R.P. Mauer, Appl. Opt. 3 (1964) 153. J.S. Hayden, Y.T. Hayden and J.H. Campbell, SPIE 1277 (1990) 121. V. Venkatramu, R. Vijaya, S.F. Leon-Luis, P. Babu, C.K. Jayasankar, V. Lavin and L.J. Dhareshwar, J. Alloys Compds. 509 (2011) 5084. 9. D.E. McCumber, Phys. Rev. 136 (1964) A954. 10. J.H. Choi, A. Margarayan and F.G. Shi, Mater. Res. Bull. 40 (2005) 2189. 11. L.C. Courrol, L.R.P. Kassab, A.S. Morais, C.M.S. Mendes, L. Gomes, N.U. Werrer, N.D. Vieira Jr., F.C. Cassanjes, Y. Messaddeq and S.L.J. Ribeiro, J. Lumin. 102-103 (2003) 106. 12. Q. Zhang, J. Ding, B. Tang, Y. Qiao, Q. Zhou, J. Qiu, Q. Chen and D. Chen, J. Phys. D: Appl. Phys. 42 (2009) 235405. 13. L.R.P. Kassab, S.H. Tatumi, A.S. Morais, L.C. Courrol, N.U.Wetter and V.L.R. Salvador, Opt. Exp. 8 (2001) 585. 14. X. Zou and H. Toratani, Phy. Rev. B 52 (1995) 15889. 15. P. Nandi and G. Jose, IEEE J. Quantum Electron. 42 (2006) 1115. 16. S. Taccheo, P. Laporta and C. Svelto, Appl. Phys. Lett 68 (1996) 2621.
Physics Procedia 29 (2012) 109 – 113
16th International Conference on Luminescence- (ICL`11)
Fluorescence and spectroscopic properties of Yb3+-doped phosphate glasses K. Venkata Krishnaiah1, K. Upendra Kumar2, V. Agarwal3, C.G. Murali4, S. Chaurasia4, L.J. Dhareshwar4, C.K. Jayasankar1*, Victor Lavin5 1
Department of Physics, Sri Venkateswara University, Tirupati - 517 502, Andhra Pradesh, India División de Ciencias e Ingenierías, Campus León, Universidad de Guanajuato, Lomas del Bosque 103, Col. Lomas del Campestre, C.P. 37150, León Guanajuato, Mexico 3 Centro de Investigacion en Ingenieria y Ciencias Aplicadas, Universidad Autonoma del Estado de Morelos, Avenida Universidad 1001,Colonia Chamilpa, C.P. 62210, Cuernavaca, Morelos, Mexico 4 Laser and Nutron Physics Division, Bhabha Atomic Research Centre, Mumbai - 400 085, India 5 Departamento de Física Fundamental y Experimental, Electrónica y Sistemas, Universidad de La Laguna, E-38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain 2
Received 25 July 2011; accepted 25 August 2011
Abstract The concentration dependent Yb3+: phosphate (P2O5 + K2O + MgO + Al2O3 + Yb2O3) glasses have been prepared and characterized their fluorescence and laser properties. The stimulated emission cross-section and laser performance parameters were determined from the measured absorption spectra using the method of reciprocity. The refractive index, absorption and emission cross-sections and fluorescence lifetimes varied with Yb3+ ion concentration. The higher emission cross-section was found to be 1.01 × 10-20 cm2 at the extraction wavelength of 1005 nm. The fluorescence lifetime of 2F5/2 level decreases from 1.04 ms to 0.28 ms with increase of Yb2O3 concentration from 0.05 to 6.0 mol %. The gain cross-section spectra can be obtained from the measured absorption and emission cross-sections with different population levels. The values of emission cross-section, fluorescence lifetime, minimum pulse duration, pump power, extraction efficiency and gain coefficients suggest that these glasses can be used as a laser gain media for the generation of ultrashort pulse and high power laser applications. © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the organizing committee © 2011 Published by Elsevier represented by Stephen C. RandB.V. and the guest editors.
Keywords: Ytterbium, phosphate glasses, fluorescence, laser parameters
1. Introduction In recent years, the advances of solid state lasers have witnessed tremendous progress in the development of both laser materials and the laser system engineering. The properties of laser host materials determine the performance of a laser system that should have a high gain, high laser induced material damage threshold, extremely good thermal properties and scalability of growth to large volumes. Among several laser glass materials, ytterbium doped glass materials are very important and interesting since they possess long fluorescence lifetime (~2 ms) and broad fluorescence bandwidth (~50 nm) [1]. The investigation on spectroscopic and laser properties of Yb3+-doped glasses are widely fascinating due to its special features: (i) a simple energy level scheme with the 2F7/2 ground and the 2F5/2 excited states, for which excited state absorption and up-conversion can be ignored, (ii) the absence of cross-relaxation mechanisms that can decrease the effective lifetime, (iii) a higher trivalent lanthanide (Ln3+) ion solubility is possible because of a weak concentration quenching, (iv) an excellent sensitizer for other Ln3+ (Er3+, Tm3+, Ho3+, etc.) ions through energy transfer and back-transfer efficient processes [2]. Among various glass hosts, Tel : + 91 877 2248033, fax: +91 877 2289472 * Corresponding author: [email protected] (C.K. Jayasankar) 1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the organizing committee represented by Stephen C. Rand and the guest editors. doi:10.1016/j.phpro.2012.03.700
110
K. Venkata Krishnaiah et al. / Physics Procedia 29 (2012) 109 – 113
phosphate glasses received much attention because they can store optical energy at greater densities and this energy can be efficiently extracted [3]. The phosphate glasses offer good laser and optical properties such as low nonlinear refractive index, high cross-section for stimulated emission, long fluorescence lifetime and thermo-mechanical properties. Moreover, these properties can be improved by chemical strengthening [4,5]. The selection of Al(PO3)3, Ba(PO3)2 and KPO3 as a raw materials for phosphate glass has its own advantages compared to other metaphosphates [6]. The drawback of phosphate glasses are that they have a lower mechanical strength than silicate glasses which do not allow one to use high average pump powers and a low resistance to atmospheric moisture. These draw backs are caused by low chemical bonds among the main structural elements of phosphate glasses (PO4 tetrahedrons) [7]. In the present work, the detailed study focuses on Yb3+: phosphate glasses which show very good fluorescence and lasing properties. 2. Experimental details Phosphate glasses of composition PKMAYb: (60-x/2) P2O5 - 15 K2O - (15-x/2) MgO - 10 Al2O3 - x Yb2O3, where x = 0.05, 0.5, 1.0, 2.0, 4.0 and 6.0 mol % referred as PKMAYb005, PKMAYb05, PKMAYb10, PKMAYb20, PKMAYb40 and PKMAYb60, respectively, have been prepared by melt quenching technique. After fabrication glasses were characterized through absorption, emission and decay rates. The high purity metaphosphates of KPO3, Al(PO3)3, Mg(PO3)2 and Yb2O3 are used as starting materials. The procedure of glass preparation, measurements and analysis of physical and spectral properties are similar to our earlier work [8]. All the glasses were prepared at ambient condition where no attempt has been made to control the OH groups in the glass matrix. 3. Results and discussion Figure 1 shows the absorption and emission cross-section spectra of PKMAYb glasses for different concentrations. Because of the simple energy level scheme and the effect of radiation trapping, the emission crosssection of Yb3+ is generally obtained by the reciprocity method (RM) [9]. The absorption and emission crosssections decrease with increase of Yb2O3 concentration. According to the McCumber theory [9], the emission crosssection is inversely proportional to the concentration. Higher the Yb3+ ion concentration, occupy different sites in the glass host, that can change the local field and also reabsorption takes place for Yb3+ ion concentration. The fluorescence trapping is also increases with increase of concentration. Hence, the radiation trapping depends on the concentration and occurs even for small doping levels. Therefore, decrease in emission cross-section with increase in Yb3+ ion concentration has been noticed. Table 1 presents the spectroscopic as well as laser properties of
1.5
Vem 1.0
Vab
0.5
0.0
2.0
1.5
Vem
1.0
Vab
0.5
0.0
860
910
960
1010
(c) PKMAYb10
2.5
2
PKMAYb05
-20
2.0
2.5
3.0
Cross-section ( x 10
2
cm )
-20
PKMAYb005
Cross-section ( x 10
2
cm )
-20
Cross-section (x 10
2.5
(b)
3.0
cm )
(a)
3.0
2.0
1.5
Vem
1.0
Vab 0.5
0.0
1060
860
Wavelength (nm)
910
960
1010
1060
860
910
960
Wavelength (nm)
1.5
Vemi
1.0
Vabs 0.5
1.5
Vem
1.0
Vab
0.0
860
910
960
1010
Wavelength (nm)
1060
PKMAYb60
2 -20
2.0
0.5
0.0
(f)
2.5
cm )
2.5
1060
3.0
PKMAYb40
Cross-section (x10
2
cm )
-20
PKMAYb20
2.0
Cross-section (x10
2
cm )
-20
Cross-section ( x 10
2.5
(e)
3.0
(d)
3.0
1010
Wavelength (nm)
860
2.0
1.5
Vem
1.0
Vab
0.5
0.0
910
960
1010
1060
860
Wavelength (nm)
Fig.1 Absorption and emission cross-section spectra of PKMAYb glasses.
910
960
1010
Wavelength (nm)
1060
111
K. Venkata Krishnaiah et al. / Physics Procedia 29 (2012) 109 – 113
PKMAYb glasses. They are compared with the Yb3+-doped fluorophosphate [10], heavy metal oxide (HMO) [11], oxy-fluoride silicate (SACF0.4) [12], lead fluoroborate (PbFB) [13], PNK25 (P2O5 + Nb2O5 + K2O) [14] and phospho-tellurite (35PT1Y) [15] glasses. The larger emission cross-section is obtained for the PKMAYb005, which is higher than the value found in HMO [11] and SACF0.4 [12] glasses as listed in the Table 1. No systematic trend has been observed in the effective bandwidth with increase of Yb2O3 concentration. The highest effective bandwidth of 111 nm is obtained for PKMAYb20 glass than those glasses shown in Table 1. Table 1. Glass lable, absorption cross-section (ıab, x 10-20 cm2), emission cross-section (ıem, x 10-20 cm2 at peak absorption (Ȝp) and extraction (Ȝ0) wavelengths), effective bandwidth (ǻȜeff, nm), fluorescence lifetime (IJf, ms), saturation pumping intensity (Isat, kWcm-2), minimum pumping intensity (Imin, kWcm-2) pump power (Usat, Jcm-2), minimum pulse duration (IJmin, fs) and gain coefficient (G (ıab(Ȝp) x IJf x ıem(Ȝo)), cm4 ms) of Yb3+:glasses. Parameters o
ıab
ıem
ıem
Glass lable p
(Ȝp)
(Ȝp)
(Ȝo)
ǻȜeff
IJf
Isat
Imin
Usat
IJmin
G
PKMAYb005
2.35
1.84
1.01
89
1.04
8.42
5.88
6.07
38
0.31
PKMAYb05
1.57
1.17
0.43
81
0.96
9.08
7.13
10.20
47
0.79
PKMAYb10
1.51
1.13
0.41
74
0.56
13.50
10.6
10.60
45
1.52
PKMAYb20
1.48
1.11
0.38
111
0.42
21.80
18.1
11.42
30
1.51
PKMAYb40
1.44
1.08
0.31
73
0.32
44.70
34.7
11.02
46
1.73
PKMAYb60
1.24
0.93
0.28
80
0.28
60.00
46.5
12.75
42
1.77
FP[10]
1.77
-
1.39
-
0.66
17.3
3.70
-
-
1.65
HMO [11]
2.20
-
0.75
86
0.40
-
3.40
22.60
-
-
SACF0.4[12]
-
1.17
0.61
42
1.43
-
1.51
11.20
-
-
PbFB [13]
-
-
1.07
60
0.81
-
1.69
-
-
-
PNK25 [14]
-
-
-
-
0.96
15.55
1.45
-
-
-
35PT1Y[15]
-
-
1.79
-
1.17
-
1.84
-
-
-
The decay rates for 2F5/2 level of Yb3+ ion in PKMAYb glasses exhibit single exponential (Figure 2) for all the concentrations. The lifetime decreases from 1.04 ms to 0.28 ms with increase of Yb2O3 concentation from 2
F5/2o
(a) PKMAYb005, W 1.04 ms (b) PKMAYb05, W 0.96 ms
Oex= 950 nm Oem= 975 nm
1.8
1.8
1.6
1.6
1.4
1.4
1.2
1.2
1.0
1.0
(c) PKMAYb10, W 0.56 ms (d) PKMAYb20, W 0.42 ms (f) PKMAYb60, W 0.28 ms
0.1 (f)
(e) (d)
(b)
(c)
(a)
0.01
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0
1
2
3
4
5
Time (ms)
Fig. 2. Decay rates of the 2F5/2 level of PKMAYb glasses
-2
0
2
4
6
8
10
12
14
0.0 16
Concentration (ions/cc)
Fig. 3. Variation of emission cross-section and the gain coefficient with Yb2O3 concentration
G
(e) PKMAYb40, W 0.32 ms
Vem(O0)
Normalised intensity (arb. units)
1
112
K. Venkata Krishnaiah et al. / Physics Procedia 29 (2012) 109 – 113
0.05 to 6.0 mol %. It is noticed that the value of IJf (1.04 ms) found to be longer for PKMAYb005 than that of other concentrations and higher than fluorophosphate [10], HMO [11], PbFB [13] and PNK25 [14] glasses. The product of ıab(Ȝp) x IJf is proportional to the stored energy[10].
2 Gain cross-section (x 10-20 cm )
The high stored energy and emission cross-section indicates good potential for laser host materials. Therefore, it has been suggested that the laser glass should have high gain coefficient, G, for laser applications. The emission cross-section decreases whereas the gain coefficient increases with increase of Yb3+ ion concentration as shown in Figure 3. The low pump fluence value of 6.07 Jcm-2 obtained for 0.05 mol % Yb2O3 doped glass is far better than HMO [11] and SACF0.4 [12] glasses. The IJmin and Usat were found to be 38 fs and 6.07 J cm-2, respectively, obtained for 0.05 mol % Yb2O3 doped glass suggested that it can be used for ultrashort pulse E and high power laser applications. The wavelength 1.2 PKMAYb10 dependent gain cross-section can be expressed as 1.0 ıg(Ȝ) = ȕıem(Ȝ)-(1-ȕ) ıab(Ȝ), where ȕ (0 to 1 with the 0.8 increments of 0.1) denotes that the ratio of inverted 0.6 ions to the total Yb3+ ion density [16]. The obtained 0.4 gain cross-section (ıg(Ȝ)) spectra of PKMAYb10 glass 0.2 is shown in Figure 4. The ıg(Ȝ) was calculated from 0.0 the measured emission cross-section (ıem) and absorption cross-section (ıab) determined after the -0.2 McCumber method and are given for the values ȕ=1 -0.4 and ȕ=0, respectively. The present glass shows the -0.6 largest gain at 975 nm and also broad emission is -0.8 promising for the generation of tunable and ultrashort E -1.0 pulse lasers. It is observed from the gain spectra, a 890 910 930 950 970 990 1010 1030 1050 wide tenable wavelength range from 980 to 1040 nm is Wavelength (nm) expected whenever the value of ȕ is larger than 0.4. From the point of view of laser operation, it is Fig. 4 Gain spectra of PKMAYb10 glass for generally desirable that the emission cross-section to different population levels be as large as possible to provide greater gain, the fluorescence lifetime to be long in order to permit high inversion densities and the absorption cross-section at the pump wavelength to be as large as possible to allow for efficient diode pumping. These considerations lead the laser parameters ((saturation pumping intensity (Isat) and minimum pumping intensity (Imin)) with low values. An efficient host for laser operation should also incorporate a high concentration of the trivalent rare-earth ion as the laser gain is proportional to the doping concentration. Conclusions Absorption and emission spectra and fluorescence decay rates of Yb3+-doped phosphate glasses with different concentrations were measured and analyzed. The systematic studies reveal that better spectroscopic and laser properties with large absorption (>1.50 x 10-20 cm2) and emission (>1.00 x 10-20 cm2 at 1005 nm) crosssections, effective band width (>70 nm), longer fluorescence lifetime (>1.00 ms) and higher gain coefficient (>1.5 x 10-20 cm2 ms) has been obtained for Yb3+-doped phosphate glasses, which can be used as gain media. The positive gain can be obtained when the population inversion is more than 40 %. The minimum pulse duration and pump fluence values, which are found to be relatively better for the Yb3+-doped phosphate glasses are useful to generate ultrashort pulse and high power lasers. Acknowledgements One of the authors (CKJ) is grateful to DAE-BRNS, Mumbai, Govt. of India for the sanction of major research project (No. 2007/34/25-BRNS/2415, dt. 18-01-2008). Dr. K. Upendra Kumar is gratefully acknowledged the partial financial support by FONCICYT PROJECT 94142.
K. Venkata Krishnaiah et al. / Physics Procedia 29 (2012) 109 – 113
113
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