Optical properties of Yb3+ ions in fluorophosphate glasses for 1.0 μm solidstate infrared lasers K. Venkata Krishnaiah, C. K. Jayasankar, V. Venkatramu, S. F. LeόnLuis, V. Lavín, S. Chaurasia & L. J. Dhareshwar Applied Physics B Lasers and Optics ISSN 0946-2171 Appl. Phys. B DOI 10.1007/s00340-013-5502-6
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Author's personal copy Appl. Phys. B DOI 10.1007/s00340-013-5502-6
Optical properties of Yb3+ ions in fluorophosphate glasses for 1.0 lm solid-state infrared lasers K. Venkata Krishnaiah • C. K. Jayasankar • V. Venkatramu • S. F. Leo´n-Luis • V. Lavı´n • S. Chaurasia • L. J. Dhareshwar
Received: 2 November 2012 / Accepted: 2 May 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Yb3?-doped fluorophosphate glasses were prepared by melt-quenching technique and characterized their spectroscopic properties to assess the laser performance parameters. The magnitude of absorption (emission) crosssections at 975 nm for all the studied Yb3?-doped glasses is found to be in the range of 0.29–1.50 9 10-20 (0.59–1.99 9 10-20 cm2) which is much higher than those of commercial Kigre QX/Yb: 1.06 9 10-20 (0.5 9 10-20 cm2) laser glass. The luminescence lifetimes of 2F5/2 level decrease (1.15–0.45 ms) with increase in Yb2O3 concentration (0.1–4.0 mol%). Effect of OH- content on luminescence properties of Yb3? ions has also been investigated. The effect of radiative trapping has been discussed by using McCumber (McC) and Fuchtbauer– Ladenburge (F–L) methods. The product of experimental K. Venkata Krishnaiah C. K. Jayasankar (&) Department of Physics, Sri Venkateswara University, Tirupati 517 502, India e-mail:
[email protected] V. Venkatramu Department of Physics, Yogi Vemana University, Kadapa 516 003, India S. F. Leo´n-Luis V. Lavı´n Departamento de Fı´sica Fundamental y Experimental, Electro´nica y Sistemas, and MALTA Consider Team, Universidad de La Laguna, E-38200 San Cristo´bal de La Laguna, Santa Cruz de Tenerife, Spain S. Chaurasia High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Center Trombay, Mumbai 400 085, India L. J. Dhareshwar Raja Ramanna Fellow, Physics Group, Bhabha Atomic Research Centre, Mumbai 400 085, India
lifetimes and emission cross-sections for 0.1 mol% Yb2O3doped glass is found to be 2.28 9 10-20 cm2 ms which indicates that the higher energy storage and extraction capability could be possible. The detailed spectroscopic results suggest that the studied glasses can be considered for high-power and ultrashort pulse laser applications.
1 Introduction In recent years, trivalent ytterbium (Yb3?) ion has attracted great attention as a dopant for the development of tunable and ultrafast lasers [1–6]. The developments in high-field lasers for nuclear fusion have indicated that Yb3?-doped materials, particularly glasses, are the best host materials for efficient energy storage in the excited state [7]. Yb3? (4f13) ion has a very simple energy level scheme, which consists of only two manifolds, the ground 2F7/2 and excited 2F5/2 states which are well separated by about 10,000 cm-1. The Yb3?-doped laser materials can be efficiently pumped by high-power diode lasers in the range of 0.9–1.1 lm, and the laser operation takes place in the 1.0 lm region, close to 1.06 lm laser line of Nd3? ion. An efficient lasing is possible in Yb3?-doped materials because of the small quantum defect (the energy difference between pump and laser photons) which is the primary source of heating. Thus, the laser materials with smaller quantum defect may be possible to have lower heating and, in general, it is about 3–4 times less for Yb3? ion compared to Nd3? ion on the 1.06 lm transitions [8]. In addition, as there are no intermediate levels, the efficiency is not degraded by the undesirable processes such as concentration quenching through cross-relaxation and excited-state absorption. In contrast to many other rare earths (RE3?)doped solid-state laser materials, high solubility of Yb3?
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Author's personal copy K. Venkata Krishnaiah et al.
ions is possible without much reduction in the upper-state lifetime. Moreover, the problems originating from the thermal load, in which high-power lasers can be severed at high doping levels, are minimized by the high intrinsic efficiency. The Yb3? ions are also of great interest as a sensitizer of energy transfer for infrared (IR) to visible (Vis) up-conversion [9, 10] and IR lasers [11]. Among the host materials, the fluorophosphate (FP) glasses are of immense interest as they possess fascinating features such as high transparency from near ultraviolet (UV) to mid-IR, low melting temperature, low thermal expansion co-efficient, high gain, good thermo-optical performance, large absorption cross-section for the Yb3? ions, low-refractive index and resistance to radiation damage, which can be tuned by the chemical composition [12, 13]. FP glasses exhibit a wide transmission range, lowlinear and nonlinear refractive index, athermal behavior, low OH- content, high solubility of RE3? ions with broad absorption and emission bands, longer fluorescence lifetime and tailorable properties by varying the fluoride to phosphate ratio [14]. The performance of the laser devices depends on the radiative and nonradiative rates. In Yb3?-doped materials, the main nonradiative processes are the energy migration among the Yb3? ions due to superposition between the absorption and emission bands and also the energy transfer to impurities leading to heat generation. Migration alone does not result in loss of excitation, but it may increase the energy transfer to impurities. Particularly, in phosphate glasses, the presence of OH- radicals which are the most prominent channels for fluorescence quenching leads to decrease in luminescence lifetimes. The vibrational frequency of OH- groups falls in the range from 2,700 to 3,500 cm-1 [15], much larger than the phonon energies of the glass host, usually varying from 300 to 1,500 cm-1. The 2F5/2 ? 2F7/2 transition of Yb3? ion may be bridged by three or four phonons that lead to nonradiative deexcitation of Yb3? IR transition. The main sources of OHimpurities may be from the starting materials and atmospheric moisture during the melting [16]. Energy transfer processes between Yb3?–OH- and Yb3?-other impurities have also been reported in the phosphate glasses [17]. Moreover, to achieve optimum laser performance in commercial laser glasses, like LHG-8 and LG700 of Nd3?doped phosphate laser glasses, the hydroxyl group absorption should be less than 2 cm-1 at 3,000 cm-1 [18]. The OH- content is effectively minimized by using nitrogen (N2) atmosphere [16] and oxygen bubbling [19] during the melting process. Hence, it is great challenge to optimize superior spectroscopic properties that can yield better laser performance in Yb3?-doped fluorophosphate glasses. In this direction, it is interesting to investigate the effect of concentration on
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spectroscopic properties of Yb3?-doped fluorophosphate glasses of base composition:P2O5–K2O–KF–BaO–Al2O3– Yb2O3. The absorption and emission cross-sections, fluorescence bandwidth, lifetimes, gain co-efficient, minimum pulse duration, and the minimum pumping intensities have been evaluated as a function of Yb3? concentration in order to assess their capabilities of being used as a laser gain media. The obtained results are also compared with those of other reported Yb3?-doped systems.
2 Experimental details The fluorophosphate glasses of composition (in mol%) (54x/2) P2O5–14 K2O–10 KF–(13-x/2) BaO–9 Al2O3– x Yb2O3, where x = 0.1, 1.0, 2.0 and 4.0 mol% and hereafter referred as PKFBAYb, were prepared by the melt-quenching technique [20]. The densities of the samples were determined by the Archimedes method with water as the immersion liquid. The refractive indices of the samples were examined with an Abbe refractometer at sodium wavelength using 1-bromo naphthalene as an adhesive coating with an accuracy of ±0.01. Optical absorption spectra were measured using a Perkin-Elmer Lambda-950 spectrophotometer in the range of 870–1,070 nm. The emission spectra were measured by exciting glass samples with the 860 nm radiation of Ti:sapphire pumped by a multiline 10 W Ar? laser (206010 Beam Lock Spectra Physics). Infrared transmittance spectrum of the sample (thickness of 1 mm) was measured by using TENSOR-27 Fourier transform infrared spectrometer (FTIR) in the range of 4,000–1,500 cm-1. The emissions were focused with a convergent lens onto a 0.18 m single-grating monochromator (Jobin–Yvon Triax180) and then detected with the NIR-extended photomultiplier tube (Hamamatsu R406). The decay curves were measured by exciting the samples with the 950 nm radiation of optical parametric oscillator (EKSPLA/NT342/ 3/UVE) by monitoring the 980 nm emission. The signal was acquired by a digital oscilloscope (LeCroy 200 MHz Oscilloscope). The spectra were corrected from the instrument response, and all the measurements were performed at room temperature (RT).
3 Results and discussion 3.1 Spectroscopic properties The refractive indices of the PKFBAYb glasses increase with the increase in the Yb3? ion concentration (see Table 1). Normally, the refractive index of the glass varies with the individual polarizabilities of cation, size of the
Author's personal copy Optical properties of Yb3? ions Table 1 The glass label, compositions and their physical properties [density (d, g/cm3), optical path length (l, mm) concentration (C, 91020 ions/cm3) and refractive index (n)] of Yb3?-doped fluorophosphate glasses Label
Glass composition
d
l
PKFBAYb01
53.95 P2O5 ? 14 K2O ? 10 KF ? 12.95 BaO ? 9 Al2O3 ? 0.1 Yb2O3
2.80
2.57
0.27
1.521
PKFBAYb10
53.50 P2O5 ? 14 K2O ? 10 KF ? 12.50 BaO ? 9 Al2O3 ? 1.0 Yb2O3
2.94
2.58
3.82
1.523
PKFBAYb20
53.00 P2O5 ? 14 K2O ? 10 KF ? 12.00 BaO ? 9 Al2O3 ? 2.0 Yb2O3
3.96
2.94
5.46
1.533
PKFBAYb40
52.00 P2O5 ? 14 K2O ? 10 KF ? 11.00 BaO ? 9 Al2O3 ? 4.0 Yb2O3
4.08
4.05
14.60
1.536
cation and the concentration of cation per unit volume [21]. As all the cations are fixed in the present glasses, the increase in refractive index of the glass may be due to the increase of Yb3? ions concentration per unit volume. This can be due to the high ionic polarizability of Yb3? ions and increase in high atomic weight which leads to increase in glass density [22]. The absorption spectra of PKFBAYb glasses and a linear increase in integrated absorbance with increase in Yb2O3 concentration are shown in Fig. 1. It is observed that the line shapes of the absorption spectra are similar to those found in other Yb3?-doped glasses [21, 23] indicating similar environments for Yb3? ions in all the studied glasses. It is interesting to observe from Fig. 1, and the well-resolved Stark levels have been observed at higher Yb3? concentration along with slight changes in the intensities with increase in Yb3? ion concentration. The observed broad absorption line shapes are due to the inhomogeneous broadening as well as strong influence of electron–phonon interactions [24] that characterizes a glassy host. The inhomogeneously broadened absorption spectra have been deconvoluted into five Gaussian peaks centered at around 915 nm (10,928 cm-1), 925 nm (10,811 cm-1), 960 nm (10,416 cm-1), 975 nm (10,256 cm-1) and 1,000 nm (10,000 cm-1) corresponding to different transitions between Stark levels, as shown in the inset of Fig. 2. It is observed that the band around 915 nm is not resolved in the other reported systems and it is indicative of a higher crystal-field around Yb3? ions in these glasses [25]. The emission spectra of Yb3? ions in all the studied glasses have been measured under 860 nm excitation and are shown in Fig. 3. The line shape of the spectra is due to inhomogeneous broadening characteristic of glassy host. Large bandwidth is attributed to the 2 F5/2 ? 2F7/2 transition from Stark sublevels of Yb3? levels. The variation of integrated emission intensity for different Yb3? ion concentrations is shown in the inset of Fig. 3. It is observed that the intensity of 2F5/2 ? 2F7/2 transition increases with increase in Yb2O3 concentration which is also identified in Yb3?-doped phosphate [26] and heavy-metal oxide [27] glasses. The performance of a Yb3?-glass laser can be accessed from the spectroscopic properties such as effective absorption and emission cross-sections of 4f13-4f13
C
n
Fig. 1 Normalized absorption spectra and variation of the integrated absorbance (inset) with Yb2O3 concentration in PKFBAYb glasses
Fig. 2 Deconvolution of the absorption spectrum of PKFBAYb10 glass. Inset shows partial energy level structure of Yb3? ions in fluorophosphate glasses along with absorption and emission channels
transitions and lifetimes of 2F5/2 fluorescent level [21, 23] which can be obtained from absorption and emission spectra. The absorption and emission cross-section spectra
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Fig. 3 Normalized emission spectrum of PKFBAYb glasses and inset shows the variation of integrated intensity for different Yb2O3 concentrations (Lines are used as a guide for the eyes)
of PKFBAYb glasses for different concentrations of Yb2O3 are shown in Fig. 4 in the range of 860–1,070 nm. There are considerable superpositions between absorption and emission in the range from 950 to 980 nm, which allows energy migration among Yb3? ions and self-trapping of radiation. As can be seen from Fig. 4, the emission crosssection decreases with increase in Yb2O3 concentration where as the absorption cross-section decreases monotonically (about 5 times) for 4.0 mol% doped glass with respect to 0.1 mol% Yb2O3-doped glass. The change in absorption cross-section may be due to increase in thickness which leads to re-absorption or energy migration between Yb3?–OH- and Yb3?-other impurities. The emission cross-section of Yb3? ions can also be obtained from F–L equation [28] in order to confirm the validity of the evaluated results which are obtained from the McC method [29]. The stimulated emission cross-sections obtained by both (McC and F–L) methods are in good agreement which may be due to the negligible effect of radiation trapping in the titled glasses (see Table 2). From the point of view of laser operation, it is necessary to have higher emission cross-sections for greater gain as well as higher absorption cross-sections at the pump wavelength for an efficient diode laser pumping. It is well known that the stimulated emission cross-section depends strongly on the local environment of the Yb3? ion which is characterized by the Yb3? site symmetry and its bonding characteristics [30, 31]. The more asymmetric site of the [YbO6] octahedron, the higher the stimulated emission cross-section will be. The emission cross-section decreases with increase in the Yb3? ion concentration at kp (975 nm) and k0 (1.0 lm). This may be due to increase in energy transfer or energy migration among Yb3? ions as well as
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Yb3?–OH- and other impurities. Therefore, decrease in emission cross-section with increase in Yb3? ion concentration has been noticed. The spectroscopic properties such as absorption [rab(k)] and emission [rem(k)] cross-sections, effective line-widths (Dkeff), spontaneous transition probabilities (A), luminescence lifetimes (sf) and quantum efficiencies (g) for the PKFBAYb glasses are listed in Table 3. It is clearly observed that there is a systematic decrease in absorption and emission cross-sections and radiative transition probabilities with the increase in Yb2O3 concentration. It is noticed from Table 3, and the absorption cross-section at kp (975 nm) and k0 (1.0 lm) decreases from 1.5 to 0.29 9 10-20 cm2 (0.19–0.08 9 10-20 cm2) when Yb2O3 concentration is increased from 0.1 to 4.0 mol%. It is also identified from Table 2, the emission cross-sections at the pump wavelength, kp (975 nm), decrease from 1.68 to 0.4 9 10-20 cm2 (F–L method) and from 1.99 to 0.59 9 10-20 cm2 (McC method) while at the laser wavelength, k0 (1.0 lm), it decreases from 0.73 to 0.25 9 10-20 cm2 (F–L method), and from 0.77 to 0.28 9 10-20 cm2 (McC method) for PKFBAYb glasses when Yb2O3 concentration is increased from 0.1 to 4.0 mol%. Similar trend has also been noticed in the other reported systems [24, 32]. It is interesting to note that there is a good agreement between experimental (F–L) and theoretical (McC) emission cross-sections. Figure 5 shows the variation of the Dkeff and emission cross-section values with respect to Yb2O3 concentration. It is found that PKFBAYb01 glass has absorption (emission) cross-section of 1.50 9 10-20 (1.99 9 10-20 cm2) which are much higher than that of the other reported commercial Kigre QX/Yb: 1.06 9 10-20 (0.5 9 10-20 cm2) laser glass. The magnitudes of spectroscopic parameters for different Yb3?-doped systems are compared in Table 4 that includes Y3Al5O12 (YAG) [32], 25Bi2O3–57PbO–18Ga2O3 (BPG) [25], 43.5B2O3–22.5PbO–34PbF2 (BPF) [25], 80TeO2–10Nb2O5–5K2O–5Li2O (TNKL) [25], 25P2O5– 20Nb2O5–24CaO–10SrO–20BaO (PN-20) [31], 18P2O5– 18Nb2O5–13B2O3–20ZnO–15SrO–15BaO (PNB-18) [31], 20Bi(PO3)3–10Sr(PO3)3–35BaF2–35MgF2 (BSBM) [33], (60–65)P2O5–(4–8)B2O3–(5–10)Al2O3–(5–10)BaO–(0–2) La2O3–(0–2)Nb2O3 (New/Yb) [34], Kigre QX/Yb [34], 0.4MgF2–0.4BaF2–0.1Al(PO3)3–0.1Ba(PO3)2 (MBABP) [35], 60TeO2–30P2O5–10Na2O (30PT1Y) [36] and 55TeO2– 35P2O5–10Na2O (35PT1Y) [36]. It is represented that the magnitude of absorption cross-sections of Yb3? ions at kp in all the studied glasses is lower than YAG crystal [32], BPG [25], BPF [25], TNKL [25], BSBM [33], and MBABP [35] glasses, while the magnitude of emission cross-sections of Yb3? ions at kp is higher than those of the commercial Kigre QX/Yb [34], New/Yb [34] laser glasses and YAG crystal [32]. The emission cross-section at 1.0 lm is
Author's personal copy Optical properties of Yb3? ions
Fig. 4 Absorption and emission cross-section (using McCumber method) spectra for different concentrations of Yb2O3-doped PKFBAYb glasses Table 2 Comparison between the Fu¨chtbauer–Landenburg (F–L) and McCumber (McC) methods of emission cross-sections at primary (remp) and secondary (rems) wavelengths, their ratios and radiative trapping co-efficient (rtc) for PKFBAYb glasses remp(kp) (910-20 cm2)
rems(k0) (910-20 cm2)
rems/remp
F–L
McC
F–L
McC
F–L
McC
PKFBAYb01
1.68
1.99
0.73
0.77
0.39
0.34
0.87
PKFBAYb10
1.40
1.71
0.58
0.61
0.41
0.35
0.85
PKFBAYb20
1.13
1.35
0.53
0.49
0.46
0.37
0.80
PKFBAYb40
0.40
0.59
0.25
0.28
0.63
0.47
0.75
Glasses
comparable with BPG [25] and MBABP [35]. On the other hand, the effective bandwidth is also comparable to the other reported systems except BPG [25] glass, which is strongly affected by re-absorption. The measured luminescence decay rates for the 2 F5/2 ? 2F7/2 transition of Yb3? ions in the PKFBAYb
rtc
glasses are found to be single exponential for all the concentrations of Yb3? ions (see Fig. 6). For laser applications, it is necessary to have longer luminescence lifetime (sf) in order to allow a high inversion density. The fluorescence lifetimes of the 2F5/2 level in PKFBAYb glasses have been evaluated from the single exponential fit and are
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Author's personal copy K. Venkata Krishnaiah et al. Table 3 Spectroscopic and laser properties of Yb3?-doped PKFBAYb glasses Glasses
rab (kp) (910-20 cm2)
rab (k0) (910-20 cm2)
k0 (nm)
Dkeff (nm)
rem (k0) (910-20 cm2) F–L
McC
A (s-1)
sf (ms)
g (%)
PKFBAYb01
1.50
0.19
1008
84
0.73
0.77
668
1.15
93
PKFBAYb10
1.28
0.12
1002
61
0.58
0.61
511
0.73
35
PKFBAYb20
0.97
0.09
1002
66
0.53
0.49
387
0.60
27
PKFBAYb40
0.29
1004
69
0.25
0.28
263
0.45
12
Glasses
bmin
0.08 2
Isat (kW/cm )
2
Imin (kW/cm )
smin (fs)
2
G (cm ms)
2
rem (k0) 9 sf (cm ms) F–L
McC
Usat (J/cm2)
PKFBAYb01
0.63
1.05
0.66
40
0.18
0.83
0.80
7.09
PKFBAYb10
0.55
2.36
0.89
50
1.83
0.42
0.45
8.51
PKFBAYb20
0.38
3.18
1.74
54
1.34
0.32
0.29
8.72
PKFBAYb40
0.62
11.89
7.35
50
0.56
0.11
0.11
32.38
Fig. 5 Variation of a the effective bandwidth (Dkeff) and emission cross-section [rem(k0)] b the product of the emission cross-section at kp and fluorescence lifetime [rem(kp) 9 sf] and gain co-efficient (G) as a function of Yb2O3 concentration (Lines are used as a guide for the eyes)
shown in Table 3. It is observed that the sf of the 2F5/2 level of Yb3? shortens from 1.15 to 0.45 ms in PKFBAYb glasses when Yb2O3 concentration is increased from 0.1 to 4.0 mol%. Therefore, the quenching of lifetime may be due to the interaction between Yb3? and OH- radicals [17] that are dominant at lower Yb3? ion concentration (NYb * 391020 ions/cc). At higher Yb3? ion concentration, Yb3?-other impurities and also the energy transfer among the Yb3? ions (diffusion limited) [20] are predominant. The magnitudes of sf for Yb3?-doped systems are compared in Table 4. It is observed that the sf values are higher for present glasses than those of the BPG [25], TNKL [25], BSBM [33] and MBABP [35] glasses and comparable with those of YAG [32], PNB-18 [31] and 30PT1Y [36] systems and lower to those of other
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commercial Kigre QX/Yb [34] and New/Yb [34] laser glasses. As observed from Table 4, the decrease in quantum efficiency (g) with increase in Yb2O3 concentration has been noticed. This may be due to the reduction in Yb3? interionic distances which in turn leads to increase the interactions among Yb3? ions and OH- radicals [17]. It is well known that OH- impurities are effective quenchers for IR emission in glasses. The IR transmittance spectrum of PKFBAYb10 glass is shown in Fig. 7. As can be seen from Fig. 7, the residual OH- frequencies are situated in the range of 2,500–3,700 cm-1 which is higher than the vibrational frequencies of the host glass. As a result, three or four phonons are required for nonradiative de-excitation of Yb3? ion emission. The quenching of luminescence through OH- impurities can be schematically illustrated in the inset of Fig. 7 which is similar to the energy transfer from Nd3? to OH- observed in phosphate glasses [18]. According to the studies concern, the Nd3? decay rate is approximately proportional to the product of Nd3? and OH- concentrations. The OH- absorption coefficient (aOH) of PKFBAYb10 glass is estimated as 10.12 cm-1 at 3,000 cm-1, and the OH- content is 303 ppm which are lower than those of reported (12.7 cm-1 and 387 ppm) samples prepared in air atmosphere [37]. The total transition rate is given by radiative (Wrad = 1/srad) and nonradiative (Wnr) rates. The linear behavior of Wnr is presented in Table 4 with increase in Yb2O3 concentration where the excited Yb3? ions correspond to the donor and acceptors to the impurities, mainly OH-. Free OH- contents in the present sample are estimated as 1.23 9 1020 cm-3 which is lower than those of reported 1.6 9 1020 cm-3 [17] for phosphate glass. In order to know the phonon sideband (PSB) to determine the multi-phonon relaxation, the excitation spectrum of the 1.0 mol% Eu2O3-doped fluorophosphate glass of
Author's personal copy Optical properties of Yb3? ions Table 4 Spectroscopic and laser performance parameters of Yb3?-doped systems Parameters ? Systems ;
k0 (nm)
rab (kp) (910-20 cm2)
rem (kp) (910-20 cm2)
rem (k0) (910-20 cm2)
Dkeff (nm)
sf (ms)
Wnr (s-1)
Imin (kW/ cm2)
rem (kp) 9 sf (910-20 cm2 ms)
rem (k0) 9 sf (910-20 cm2 ms)
Usat (J/cm2)
PKFBAYb01
1,008
1.50
1.99
0.77
84
1.15
0.19
0.66
2.28
0.80
7.09
PKFBAYb10
1,002
1.28
1.71
0.61
61
0.73
0.84
0.89
1.03
0.45
8.52
PKFBAYb20
1,002
0.97
1.30
0.49
66
0.60
1.27
1.74
0.95
0.29
8.72
PKFBAYb40
1,004
0.29
0.59
0.28
69
0.45
1.96
7.35
0.26
0.11
32.38
YAG [32]
1,031
2.00
0.80
2.00
18
1.08
–
1.53
0.86
2.16
–
BPG [25]
1,012
2.20
–
0.75
86
0.40
–
3.40
–
0.30
22.60
BPF [25]
1,022
2.56
–
1.07
61
0.81
–
1.69
–
0.87
16.20
TNKL [25]
1,028
4.09
–
1.10
66
0.59
–
1.62
–
0.65
14.02 –
PN-20 [31]
1,019
–
–
1.36
–
1.09
–
0.51
–
1.48
PNB-18 [31]
1,020
–
1.31
–
–
1.15
–
0.45
1.51
–
–
BSBM [33]
977
1.77
–
1.39
–
0.71
–
3.70
–
0.92
–
New/Yb [34]
975
–
0.53
–
–
2.20
–
–
1.00
–
–
Kigre QX/Yb [34]
976
1.06
0.50
–
–
2.00
–
–
1.12
–
–
MBABP [35]
976
1.64
–
0.87
84
0.65
–
–
–
0.57
–
30PT1Y [36]
1,004.7
1.26
1.70
–
–
1.26
–
1.79
2.14
–
–
35PT1Y [36]
1,002.4
1.33
1.79
–
–
1.17
–
1.84
2.09
–
–
Fig. 6 Luminescence decay curves for the 2F5/2 level of Yb3? ions in PKFBAYb glasses. Inset shows the exponential decrease in lifetime with the Yb2O3 concentration (Lines are used as a guide for the eyes)
similar base composition (53.5 P2O5–14 K2O–10 KF–12.5 BaO–9 Al2O3–1.0 Eu2O3, referred as PKFBAEu glass) is measured by monitoring at 612 nm of 5D0 ? 7F2 transition, shown in Fig. 8. The PSB is associated with the 7 F0 ? 5D2 transition (the pure electronic transition, PET) of Eu3? ions can be clearly observed in the wavelength range of 420–480 nm. The difference between the positions of PET and PSB corresponds to the energy of the phonons coupled to the electronic levels of RE3? ions. The PKFBAEu glass exhibits one weak PSB around 640 cm-1
Fig. 7 IR transmittance spectra of 1.0 mol% Yb2O3-doped PKFBAYb10 glass and inset show the energy transfer of Yb3? to OHimpurities
due to coupling of P–O–P asymmetric stretching vibrations of the PO2 group [38], and an intense band around 1,175 cm-1 caused by the coupling of symmetric vibrations of the PO2 groups to the electronic levels of Eu3? ions. The electron–phonon coupling strength (g) calculated as the intensity ratio of the PSB to the PET is found to be 0.047, which is nearly two times larger than the value obtained for the Sr(PO3)–Eu(PO3)3 metaphosphate glass [14]. The multiphonon relaxation processes can be
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Author's personal copy K. Venkata Krishnaiah et al.
Fig. 9 Gain cross-section spectra of 1.0 mol% Yb2O3-doped PKFBAYb10 glass for different populations
Fig. 8 Phonon sideband (PSB) spectrum of 1.0 mol% Eu2O3-doped PKFBAEu10 glass
neglected as it needs 8–9 phonons to bridge the large energy gap of 10,256 cm-1 between 2F5/2 and 2F7/2 levels in phosphate glasses. In order to assess the potential of Yb3?-doped PKFBAYb glasses for laser devices, the laser performance parameters [32] like the minimum fraction of Yb3? ions (bmin), the pump saturation (Isat) and the minimum pump intensities (Imin) are evaluated using spectroscopic parameters of absorption and emission cross-sections and lifetimes and are listed in Table 3. As can be seen from Table 3, bmin, Isat and Imin values are in the range of 0.62–0.38, 1.05–11.89 and 0.66–7.35 kW/cm2, respectively. These values should be as low as possible to minimize the pump losses. The parameter Imin should be less of the order of 4.5 kW/cm2 for diode pumping [32]. The value of Imin is found to be lower for PKFBAYb01 glass which is lower than YAG [32], BPF [25], BSBM [33] systems. The Isat and Imin increase with increasing the concentration of Yb3? ions. For good laser glasses, the rem (kp) 9 sf product is desirable to be as large as possible to provide high gain [21]. The product is found to decrease from 2.28 to 0.26 9 10-20 cm2ms when the Yb2O3 concentration is increased from 0.1 to 4.0 mol% and is comparable to commercial Yb3?-doped laser glasses of Kigre QX/Yb (1.12 9 10-20 cm2ms) [34] and New/Yb (1.0 9 10-20 cm2ms) [34]. It is noticed that the product is higher for the PKFBAYb01 glass of the order of 2.28 9 10-20 cm2 ms which indicates that the studied glass possess better energy storage and extraction capability. The rem (k0) 9 sf product is higher for BPG [25], BPF [25],
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PN-20 [31], YAG [32] systems than studied glasses. The variation of rem (k0) 9 sf product and gain co-efficient (G) [21] with respect to Yb3? ion concentration is shown in Fig. 5. The minimum pulse duration (smin) and pump power (Usat) [39] are the key parameters for the development of ultrashort and high-power lasers. From Table 2, the values of smin and Usat are found to be in the range of 40–50 fs and 7.09–32.4 J/cm2, respectively, for PKFBAYb glasses. It is found that the smin is lower (40 fs) for PKFBAYb01 glass, indicating that this can be considered for ultrashort laser applications. From Fig. 5, it is noticed that the value of G is higher for PKFBAYb10 glass. Hence, gain cross-section [rg(k)] [40] spectra (see Fig. 9) of PKFBAYb10 glass were evaluated from the measured absorption cross-section (rab) and emission cross-section (rem). It is evidence that a wide tunable wavelength range from 940 to 1030 nm is expected whenever the value of b is larger than 0.4 and exhibits an intense peak at 975 nm. Zhang and Hu [19] proposed that radiation trapping can be evaluated by radiation trapping coefficient (rtc) which can be evaluated from the emission cross-sections at kp (975 nm) and k0 (1.0 lm) derived from both McC and F–L methods. The absorption spectrum is used to evaluate emission cross-section from the McC method, which avoids the effect of fluorescence trapping at highly Yb3?doped glasses. From Table 2, it is observed that there is no marginal change in the ratio of the (rems)/(remp)McC and (rems)/(remp)F–L with respect to Yb2O3 concentration. Here, the remp and rems are the emission cross-sections at primary (975 nm) and secondary (1.0 lm) wavelengths, respectively. The systematic spectroscopic studies reveal that the present glasses could be considered as a laser gain media for 1.0 lm solid-state infrared laser applications.
Author's personal copy Optical properties of Yb3? ions
4 Conclusions Ytterbium-doped fluorophosphate glasses have been sintered by melt-quenching technique and characterized their spectroscopic properties by means of McCumber method to evaluate emission cross-sections for 2F5/2 ? 2F7/2 transition. The effect of concentration on the laser performance parameters which include pump saturation intensity, minimum pump intensity, pulse duration, and energy storage parameters has been explained. The magnitude of absorption and emission cross-sections of present Yb3?-doped glasses are higher than those of the commercial phosphate laser glasses. The emission cross-sections are obtained by McC and F–L methods and compared in order to study the radiation trapping co-efficient. The fluorescence lifetime of 2 F5/2 level shortened from 1.15 to 0.45 ms when Yb2O3 concentration is increased from 0.1 to 4.0 mol%. It is found that the product of emission cross-section and lifetime for these Yb3?-doped glasses is higher than commercial laser glasses, which indicates better energy storage and extraction capability. The smoother gain profile suggests that a larger laser tuning range of 940–1,030 nm with an intense peak at 975 nm. The superior values of gain coefficient, minimum pulse duration and bandwidth suggest that these glasses have potential applications in ultrashort and highpower lasers. Acknowledgments Prof. C.K. Jayasankar and Dr. L.J. Dhareshwar are grateful to the DAE-BRNS, Govt. of India, for the award of the Major Research Project (No. 2007/34/25-BRNS/2415). Dr. V. Venkatramu is grateful to DAE-BRNS, Govt. of India, for the award of DAE Research Award for Young Scientists (No. 2010/20/34/5/ BRNS/2223). Dr. Lavı´n is grateful for the financial support to MICINN within The National Program of Materials (MAT2010-21270C04-02) and to the EU-FEDER. The Consolider-Ingenio 2010 Program (MALTA CSD2007-0045) and the authors are also grateful to MINECO (PRI-PIBIN-2011-1153) from Spain and Department of Science and Technology (DST/INT/Spain/P-38/11), Govt. of India for financial support with in the Indo-Spanish Joint Programme of Cooperation in Science and Technology and S. F. Leo´n-Luis wishes to thank MICINN for the FPI grant (BES-2008-003353).
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