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Investigation of optical and spectroscopic properties of neodymium doped oxyfluoro-titania-phosphate glasses for laser applications
Scripta Materialia 162 (2019) 246–250
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Investigation of optical and spectroscopic properties of neodymium doped oxyfluoro-titania-phosphate glasses for laser applications G. Neelima a,b, K. Venkata Krishnaiah b,⁎, N. Ravi b,⁎, K. Suresh c, K. Tyagarajan d, T. Jayachandra Prasad e a
Department of Physics, JNT University, Anantapuramu 515002, India Department of Physics, RGM College of Engineering and Technology, Nandyal 518501, India Department of Physics, Sri Venkateswara University, Tirupati 517502, India d Department of Physics, JNT University College, Pulivendhula 516390, India e Department of ECE, RGM College of Engineering & Technology, Nandyal 518501, India b c
a r t i c l e
i n f o
Article history: Received 11 September 2018 Received in revised form 8 November 2018 Accepted 8 November 2018 Available online xxxx Keywords: Neodymium ions Titania-fluorophosphate glasses Non-linear refractive index High-energy high-power lasers Judd-Ofelt parameters
a b s t r a c t Nd3+-doped oxyfluoro-titania-phosphate glasses with a chemical combination, P2O5–CaF2–BaF2–TiO2–Nd2O3 (PCfBfTiNd), were synthesized by the regular melt-quenching method. These glass samples were investigated through optical absorption, emission, decay curves and spectroscopic ellipsometry analysis. Non-linear refractive index (n2) is evaluated using the Boling method and it was found to be as low as 3.68 × 10−16 cm2/W (1.36 × 10−13 esu). Emission spectra of Nd3+-doped glasses were obtained by laser excitation at a wavelength of 808 nm. Results were endorsed that the 0.5 mol% Nd3+-doped PCfBfTiNd05 glass could be a suitable gain media for the development of high-energy high-peak power lasers. © 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Rare earth (RE3+) activated glasses have been opted for the advance of solid-state lasers, optical fiber communication, sensing, plasma displays and white light emitting diodes (W-LEDs) [1–3]. Generally, RE3 + -doped phosphate glasses are the active host media since they possess long fluorescence lifetime, high quantum efficiency, low non-linear refractive index and high stimulated emission cross-section for highenergy high-power solid-state lasers [4,5]. Particularly, neodymium (Nd3+)-doped phosphate glasses are predominantly impressive for low-power diode laser pumping as a consequence of their high emission cross-section and low fluorescence quenching [6]. One of the significant applications of Nd3+: glass lasers in the plasma generation [7,8]. However, there is a big task for the researchers to develop laser glasses for high-power laser applications with much improved thermomechanical properties while sustaining the optical and laser properties. To encounter these specifications, titania (TiO2) is one of the modifiers in our glass system (P2O5–CaF2–BaF2–TiO2) which can improves the thermo-mechanical properties of the glasses. Broadband laser emission of Nd3+ ion at 1055 nm from the Nd3+doped CaF2 and SrF2 fluoride crystals has been studied [9]. The Nd3+doped phosphate glasses co-doped with other RE3+ ions enact as a diluting agent [10,11]. Nd3+ ions in the laser amplifier system can establish a prospective activator for enhancing the magneto-optical (MO) ⁎ Corresponding authors. E-mail addresses: [email protected] (K. Venkata Krishnaiah), [email protected] (N. Ravi).
https://doi.org/10.1016/j.scriptamat.2018.11.018 1359-6462/© 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
properties of the glasses [12]. Generally, the paramagnetic nature of RE3+ ions is quite interesting for optoelectronic applications. Fluoride based glasses possess lower phonon energy than oxides, zirconates, phosphates, and so on, this consequence leads to decrease of non-radiative relaxation and simultaneously increase of photoluminescence (PL) quantum efficiency. With these advantages, fluoride based glasses were predominated over oxide glasses. Nd3+doped fluorides, such as CaF2, BaF2, SrF2 and LaF3, can reduce the concentration quenching effects which mainly replicates the emission intensity and the fluorescence lifetime [13–16]. It is evinced that the Nd3+-doped oxyfluoro-phosphate glasses unveil a high grade of inhomogeneous spectral line widening and smooth line shapes that leads to the shorter laser pulses [17–19]. With these virtues, new glassy systems are essential to enhance the emission cross-section and bandwidth. Moreover, fluoride based glass fibers were testified as low transmission loss of optical energy in the near infrared region [20,21]. Fluorescence properties of Nd3+-doped glasses can be improved significantly with the use of alkali or alkaline earth metal modifiers with less field strength in these phosphate glasses [22,23]. Present work examines the physical, optical and PL properties of Nd3 + -doped PCfBfTiNd glasses. Judd-Ofelt (JO) phenomenological intensity parameters (Ω2, Ω4, Ω6) were estimated by means of the absorption spectrum. The PL spectra of these glasses were obtained upon laser excitation at a wavelength of 808 nm. Decay curves were analysed and evaluated for the lifetime of excited level, 4F3/2 of Nd3+ ion. The results are compared with those of other reported Nd3+:systems.
G. Neelima et al. / Scripta Materialia 162 (2019) 246–250
Glasses were fabricated by regular melt-quenching method [24] with a molar composition of (60-x) P2O5–20CaF2–15BaF2–5TiO2– xNd2O3 where x = 0.05, 0.1, 0.5, 1.0, 1.5, 2.0 and 2.5 (mol%), referred as PCfBfTiNd0.05, PCfBfTiNd0.1, PCfBfTNd0.5, PCfBfTNd1.0, PCfBfTNd1.5, PCfBfTNd2.0, and PCfBfTNd2.5, respectively. Absorption spectrum of the Nd3+-doped oxyfluoro-titania-phosphate (PCfBfTiNd2.0) powder glass was recorded in the diffuse reflectance (DRS) mode (VARIAN Cary 5000 in the region 175–3300 nm) by UV– visible–NIR double beam spectrophotometer. The PL spectra were recorded using Edinburg Spectrofluorometer (FLS — 980, λ = 200–1700 nm) with excitation of 808 nm CW laser. Decay curves of the glass samples were recorded with the same spectrometer by exciting with a 808 nm laser in the pulsed mode. Refractive index (n) of Nd3+-doped glass was measured by J.A. Woollam Spectroscopic Ellipsometer (Model: M-2000VI) in the spectral range of 370–1690 nm with an error of ±0.0023. Error of the parameters was evaluated by regular standard deviation method. Non-linear behaviour of laser materials is crucial for high-power laser applications as the absorption and emission properties depending on their refractive index [25]. Abbe numbers are used to categorize glass and other optically transparent materials. Two modifiers, BaF2 and CaF2 offer low non-linearity, wide transmission and low hydroxyl content of the glass. Addition of transition metal ions, such as Ti4+ improves linear refractive index and dispersion of the fluorophosphate glasses [26]. A similar trend was perceived in our glass matrix. The non-linear refractive index of the PCfBfTiNd glasses can be evaluated using a formula suggested by Boling [27] and it is given by 2 −1=2 n2 ¼ KðnD −1Þ n2D þ 2 ðvd Þ−1 1:517 þ n2D þ 2 ðnD þ 1ÞvD =6nD ð1Þ where K = 68 × 10−13 esu for the fluorophosphates glass. Abbe number measures the dispersion of a material in the visible region and is given by the following equation vd ¼
ðnD −1Þ ðn F −nE Þ
ð2Þ
where nD, nF and nE are the linear refractive indices at λD = 587.6 nm, λF = 486.1 nm and λE = 656.3 nm, respectively. It is well identified that the index of refraction decreases with an increase in the incident laser wavelength. Refractive indices at three different wavelengths and Abbe number of Nd3+-doped glass are compared to other reported Nd3+-doped laser glasses in Table 1 [28–31]. It is noteworthy that the
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Abbe number was estimated to be 61.63 which is higher than the reported Nd3+-doped glasses that include N21 [28], FCD600 [29] and STIM28 [31]. These PCfBfTiNd glasses show a low dispersion due to a high value of Abbe number compared to the newly released commercial glass FCD600 (HOYA) [30]. Non-linear refractive index of PCfBfTiNd10 glass is found to be 3.68 × 10−16 cm2/W (1.36 × 10−13 esu) which demonstrations a higher value compared to other reported glasses [28,29] and lower than the majority of the commercial glasses (SCHOTT, HOYA etc.) reported in the literature [29–31]. Absorption spectrum of 2.0 mol% Nd3+-doped PCfBfTiNd2.0 powder glass at room temperature in the spectral region of 300–1100 nm is shown in Fig. 1. Spectrum unveils ten characteristic bands witnessed at around 430 nm (23,255 cm−1), 476 nm (21,008 cm−1), 512 (19,531 cm−1), 525 (19,047 cm−1), 583 (17,152 cm−1), 626 (15,974 cm−1), 683 (14,641 cm−1), 746 (13,404 cm−1), 799 (12,515 cm−1) and 873 (11,454 cm−1) equivalent to the transitions from the ground state, 4I9/2 to higher excited states, 2D5/2 + 2P1/2, 2 K15/2 + 4G11/2 + 2D3/2, 4G9/2, 2K13/2 + 4G7/2, 2G7/2 + 4G5/2, 2H11/2, 4F9/ 4 4 4 2 4 2, S3/2 + F7/2, F5/2 + H9/2 and F3/2, respectively. All the absorption peaks of Nd3+ ions were resolved in our glass system. An intense absorption peak is noted at 583 nm (17,152 cm−1) which was a result of hypersensitive nature of the transition, 4I9/2 → 4G5/2 + 2G7/2 and it follows the selection rules |ΔL| ≤ 2, |ΔS| ≤ 0 and |ΔJ| ≤ 2 [32,33]. Based on absorption bands and transition energies, relative intensity parameters of Nd3+ were determined by Judd-Ofelt (JO) theory [34,35]. The JO intensity parameters Ωλ (where λ = 2, 4, 6), and calculated oscillator strength (fcal) were obtained from the values of experimental oscillator strength (fexp) by the use of least-square fitting method. The reduced matrix elements were adopted from the literature [36]. The oscillator strength can be evaluated from the area under the absorption curve. The fexp and fcal of PCfBfTiNd glasses were listed in the Table 2. The root mean square (δrms) is obtained as 0.110 which states that reliable to evaluate the radiative parameters. The JO intensity parameters (Ω2, Ω4 and Ω6) are presented in Table 3. It clearly indicates that the intensity parameter, Ω2 illustrates the asymmetry and the covalancy of Ln3+ sites, whereas the Ω4 and Ω6 are associated with the dynamics of the host glass matrix. High value of the Ω2 (0.57 × 10−20 cm2) is compared to Ω4 which indicates less centrosymmetrical ion site and chemically more covalent with the ligands. However, the emission intensity of Nd3+-doped laser glasses could be characterized by the intensity parameters Ω4 and Ω6 which can be used for evaluating the spectroscopic quality factor χ (=Ω4/Ω6). The quality factor found to be as high as 0.75 which reports less than unity. The value of χ found to be higher than the reported glasses that include 0.54 for Glass-B [22], 0.70 for PKFBAN10
Table 1 Refractive indices at different wavelengths (nD, nF and nE) and Abbe numbers (vd) of Nd3 : systems along with proposed PCfBfTiNd1.0 glass.
+
Glasses
Ref.
PCfBfTiNd1.0 (present work) N21 [28] N31 LG-770 APG-1 [29] N-FK5 NBFD30 FCD600 (new) [30] FCD1 TAFD65 S-BSL 7 S-TIH 1 [31] S-TIM28 S-FPL51
nD
nF
nE
1.5485 1.5550 1.5461
n (@1054 nm)
vd
n2 (×10−13) esu
1.5480 ± 0.0023
61.63 ± 0.6717
1.36 ± 0.0215
1.5758 1.5280 1.5086 1.5370 1.4874 1.8588 1.5941
1.5731 1.5413 – 1.5350 1.4922 1.8791 1.6009
1.5652 1.5332 1.5070 – 1.4891 1.8656 1.5964
1.5652 – 1.4996 1.5260 1.4785 1.8307 1.5835
65.3 66.2 – – 70.41 30.00 60.47
1.30 1.10 – – 1.47 1.83 1.58
1.4970 2.0509 1.5163 1.7173 1.6889 1.4970
1.5012 2.0786 1.5219 1.7346 1.7046 1.5012
1.4984 2.0601 1.5182 1.7231 1.6941 1.4984
1.4901 2.0134 1.5069 1.6935 1.6669 1.4901
81.61 26.94 64.14 29.52 31.07 81.54
1.49 2.01 1.51 1.71 1.68 1.49
Fig. 1. Absorption spectrum of 2.0 mol% Nd3+-doped PCfBfTiNd2.0 glass in the visible and NIR regions.
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Table 2 Oscillator strengths fexp and fcal of Nd3+-doped PCfBfTiNd2.0 glass. Wavelength (nm)
4
I9/2→
fexp
23,255 21,008
0.090 0.360
0.024 0.183
0.065 0.177
19,531 19,047 17,152 15,974 14,641 13,404 12,515 11,454
0.064 0.254 2.220 0.022 0.210 0.890 0.630 0.360
0.137 0.395 2.210 0.009 0.063 0.810 0.753 0.229
0.073 0.141 0.009 0.013 0.146 0.079 0.123 0.130
D5/2 + 2P1/2 K15/2 + 4G11/2 + 2 D3/2 4 G9/2 512 2 525 K13/2 + 4G7/2 2 583 G7/2 + 4G5/2 2 626 H11/2 4 683 F9/2 4 746 S3/2 + 4F7/2 4 799 F5/2 + 2H9/2 4 873 F3/2 δrms = ±0.110 (10)
430 476
Δf = |fexp ~ fcal|
Energy (cm−1)
2 2
fcal
[37], 0.62 for NaPZN05 [38], 0.74 for BaABP05 [39], 0.32 for LBZN05 [40] and lower than 0.82 for LCBALPN04 [46] and 0.87 for BiPN05 [55] glasses. The JO intensity parameters of PCfBfTiNd2.0 glasses are found to be Ω2 = 0.57 × 10−20 cm2, Ω4 = 0.43 × 10−20 cm2 and Ω6 = 0.58 × 10−20 cm2. It is noticeable that the value of Ω2 is greater than the value of Ω4, further, Ω6 was found to be greater than Ω4 (Ω6 N Ω2 N Ω4). Glasses with fluoride content possess a high ionic bond and more brittle, that exhibit a low value of Ωλ compared to phosphate hosts. However, a large variation of Ωλ values for fluorophosphate glasses can be observed with no specific trend. The JO intensity parameters, quality factor and nature of the intensity parameters were compared in Table 3. The PCfBfTiNd glasses show a similar behaviour compared to other reported glasses. JO intensity parameters were utilized to assess the radiative parameters. The radiative properties of Nd3+-doped PCfBfTiNd2.0 glass can be obtained by using JO intensity parameters. Consequently, the spectroscopic quality factor χ can be used to describe the emission transitions of Nd3+:systems. In general, the most probable transition, 4F3/2 → 4I11/2 is obtained at 1.05–1.06 μm for the Ω6 ≫ Ω4 and the 4F3/2 → 4I9/2 transition is dominant at around 890 nm for Ω6 ≪ Ω4. The spectroscopic quality factor of PCfBfTiNd2.0 glass is found to be 0.75 and satisfies the condition Ω6 ≫ Ω4 that results the laser emission from 4F3/2 → 4I11/2 at 1.05 μm in the near infrared wavelength region. Stimulated emission cross-section (σem) is an important parameter to evaluate the rate of laser emission from the optical materials. The emission cross-sections are determined using the Fuchtbauer-Ladenburg formula. The value of σem of PCfBfTiNd glass is reported as 13.70 × 10−20 cm2, which is found to be higher than CaB4O7N10 (2.34) [41], PKSFAN10 (3.29) [43], TCZNBN10 (3.86) [45], LCBALP10 (6.97) [46], PKBaAN05 (7.0) [47] and lower than BNPZLiN05 (15.89) [50] glasses and are displayed in the Table 4. A comparison of radiative transition probabilities, branching ratio and fluorescence lifetime for various Nd3 + -doped systems are listed in Table 5. The PL spectra of Nd3+-doped PCfBfTiNd glasses excited by laser at a wavelength of 808 nm are shown in Fig. 2. The emission spectra disclose three peaks at around 874, 1054 and 1325 nm corresponds to the f-f Table 3 Comparison of JO parameters (Ω2, Ω4, Ω6), their trend and spectroscopic quality factor (χ) of Nd3+:systems.
Table 4 Comparison of stimulated emission cross-sections (σem) of various Nd3+:systems compositions. σem (×10−20) cm2
S·no.
Composition
1 2 3 4 5 6 7 8 9 10 11 12 13
PCfBfTiNd2.0 [present work] APG-1 [29] LG-770 [29] CaB4O7N10 [41] TAKLNP20 [42] PKSFAN10 [43] PKCFAN10 [44] TCZNBN10 [45] LCBALP10 [46] PKBaAN05 [47] TZN05 [48] PCdN02 [49] BNPZLiN05 [50]
13.70 ± 0.0536 3.40 3.90 2.34 2.97 3.29 3.42 3.86 6.97 7.00 2.18 3.79 15.89
transitions of 4F3/2 → 4IJ (J = 9/2, 11/2 and 13/2), respectively [14,51–54]. Under 808 nm excitation, Nd3+ ions rapidly excite to the higher energy state, 4F7/2 from the ground state, 4I9/2. Then, these excited Nd3+ ions of higher energy state are spontaneously de-excite to the lower energy state, 4F5/2 through non-radiative transition that acts as a metastable state. It holds a longer lifetime compared to other excited states. Therefore, Nd3+ ions relax much time in the metastable state leads to population inversion and then lasing. The 4F3/2 → 4I9/2 transition separates into two peaks owed to the Stark splitting of Nd3 + ionic states in the host [55]. This nature of splitting reveals the asymmetric environment around Nd3+ about titania-phosphate network, which rises with increasing Nd3+ concentration up to 0.5 mol%. Moreover, it is noticed that the splitting was evidenced significantly for 0.5 mol% Nd3+-doped glass. On the other hand, the overlap of these bands is observed with further increase of Nd3+ ion concentration. Emission intensity increased as the concentration of Nd3+ increases up to 0.5 mol% thereafter it decreases abruptly at 1.0 mol% Nd3+ ion concentration. Again an increase in the intensity was noticed till 2.0 mol% and then decreased to 2.5 mol%. The maximum intensity of emission at 1054 nm was perceived for 0.5 mol% Nd3+-doped PCfBfTiNd0.5 glass corresponds to the transition of 4F3/2 → 4I11/2. Excitation, emission and cross-relaxation channels were presented in the partial energy level diagram of Nd3+ ions in Nd3+-doped PCfBfTiNd glasses shown in the Fig. 3. The cross relaxation process was observed among the levels 4F3/2 → 4I15/2 and 4I9/2 → 4I15/2. Under 808 nm laser excitation, a bandwidth of around 17 nm with high emission cross-section is obtained for the PCfBfTiNd0.5 glass, which is higher than those of other reported Nd3+-doped glasses [56]. Fluorescence decay curves for the 4F3/2 → 4I11/2 transition of Nd3+ ions in PCfBfTiNd glasses were recorded under 808 nm laser excitation, as shown in the Fig. 4. Decay curves of PCfBfTiNd glasses display exponential behaviour at low Nd3+ ions concentration and deviated to non-exponential at high concentration. Lifetime of 4F3/2 level is found to be 140, 143, 137, 92, 89, 64 and 62 μs for 0.05, 0.1, 0.5, 1.0, 1.5, 2.0 and 2.5 mol% Nd3+-doped PCfBfTiNd glasses, respectively. Variation of lifetime with the effect of Nd3+ ion concentration is shown in the inset of Fig. 4. It is observed that the lifetime of 4F3/2 level increases Table 5 A comparison of radiative properties of Nd3+-doped glasses.
Glass matrix
References
Ω2
Ω4
Ω6
χ = Ω4/Ω6
Trend
Matrix
β
PCfBfTiNd2.0 Glass-B10 PKFBAN10 NaPZN05 BaABP05 LBZN05 LCBALPN01 BiPN05
Present work [22] [37] [38] [39] [40] [46] [55]
0.57 17.79 4.92 3.76 3.73 2.69 4.35 9.61
0.43 8.39 3.67 3.27 2.48 1.09 2.87 10.71
0.58 15.52 5.26 5.23 3.33 3.40 3.49 12.34
0.75 0.54 0.70 0.62 0.74 0.32 0.82 0.87
Ω6 N Ω2 N Ω6 Ω2 N Ω6 N Ω4 Ω6 N Ω2 N Ω4 Ω6 N Ω2 N Ω4 Ω2 N Ω6 N Ω4 Ω6 N Ω2 N Ω4 Ω2 N Ω6 N Ω4 Ω6 N Ω4 N Ω2
τexp μs
AR s−1
PCfBfTiNd0.1 CaB4O7N10 [41] TAKLNP20 [42] PKSFAN10 [43] TCZNBN10 [45] PKBaAN05 [47] BNPZLiN05 [50]
50 45 50 64 53 – 48.12
143 ± 0.2 55 249 211 – 170 48.4
6449 – 1844 – 1871 – 4041
σem × τexp 10−23
(Δλ, nm)
σem × Δλ 10−25 cm3
1.95 – 7.39 – – – 7.64
17.22 36 35 32.6 35.37 22.5 16.18
2.35 0.84 1.03 1.07 1.20 1.57 2.57
G. Neelima et al. / Scripta Materialia 162 (2019) 246–250
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Fig. 4. Decay profiles of Nd3+-doped PCfBfTiNd glasses with various content of Nd3+ ions. Fig. 2. Emission spectra of Nd3+-doped PCfBfTiNd glasses upon excitation of 808 nm laser.
with the increase of Nd3+ ion concentration up to 0.1 mol%. Then, it decreases with further increase of Nd3+ ion concentration till 2.5 mol%. The decrease in lifetime may be the result of concentration quenching and increase in lifetime may be because of reabsorption. Lifetime found to be as high as 143 μs for PCfBfTiNd0.1 glasses. The PCfBfTiNd0.1 glass possesses longer lifetime than those of other reported Nd3+: glasses that include 95 μs for PbFPbOBN01 [57], 131 μs for PKFBAN10 [37], and lower than 281 μs for LCBALPN01 [46], 348.72 μs for YCaF2N08 [58], and 489 μs for BSAYPN05 [59] systems. In summary, we report, the optical and photoluminescence (PL) properties of Nd3+-doped oxyfluoro-titania-phosphate glasses as a function of Nd3+ concentration through optical absorption, emission spectra, spectroscopic ellipsometry and decay curve analysis. Linear and non-linear refractive indices of the glasses were found to be 1.5485 (at 587.6 nm) and 3.68 × 10−16 cm2/W (1.36 × 10−13 esu), respectively. Spectroscopic quality factor, χ (=Ω4/Ω6) is determined as 0.754 for 0.5 mol% Nd3+-doped PCfBfTiNd0.5 glass. The PCfBfTiNd glasses revealed lower non-linearity compared to other commercial glasses that include N21, N31 (FOCTEK) and LG 770 (SCHOTT). Upon 808 nm laser excitation, PL intensity at a wavelength of 1054 nm corresponds to the 4F3/2 → 4I11/2 transition was increased up to 0.5 mol% and decreased with further increase of Nd3+ ions concentration. Figure of
Fig. 3. Energy level diagram of Nd3+ ion along with excitation, de-excitation and crossrelaxation channels.
merit (FOM) (σnem ) of 0.5 mol% Nd3+-doped glass is found to be 3.72 2
× 10−4 W which was comparable to the reported Nd3+:glasses [60]. Decay curves of Nd3+-doped glasses were exhibited exponential behaviour for low concentrations of Nd3+, and non-exponential for higher concentrations. The results are noteworthy and endorsing that the Nd3+-doped oxyfluoro-titania-phosphate glasses could be a suitable candidate for the development of high-energy high-peak power solidstate lasers. Acknowledgement One of the authors Dr. KVK is thankful to SERB-DST, Govt. of India, New Delhi for the award of a major research project (File Number: EMR/2017/000009). The authors acknowledge MoU-DAE-BRNS Project (No. 2009/34/36/BRNS/3174), Department of Physics, Sri Venkateswara University, Tirupati, India for extending the PL experimental facility. We also thank the Central Electro Chemical Research Institute (CECRI-CSIR), Karaikudi and Indian Institute of Technology Madras (IIT-M) for providing UV-VIS-NIR absorption and Ellipsometry studies, respectively. References [1] B. Afef, H.H. Hegazy, H. Algarni, Y. Yang, K. Damak, E. Yousef, R. Maâlej, J. Rare Earths 35 (4) (2017) 361–367. [2] Kummara Venkata Krishnaiah, Kagola Upendra Kumar, Chalicheemalapalli Kulala Jayasankar, Mater. Express 3 (1) (2013) 61–70. [3] Hansol Lee, Seonghyeon Kim, Jong Heo, Woon Jin Chung, Opt. Lett. 43 (4) (2018) 627–630. [4] K. Venkata Krishnaiah, C.K. Jayasankar, S. Chaurasia, C.G. Murali, L.J. Dhareshwar, Sci. Adv. Mater. 5 (3) (2013) 276–284. [5] Francisco Muñoz, Akira Saitoh, Rafael J. Jiménez-Riobóo, Rolindes Balda, J. NonCryst. Solids 473 (2017) 125–131. [6] D. Kopf, F.X. Kärtner, U. Keller, K.J. Weingarten, Opt. Lett. 20 (1995) 1169–1171. [7] O. Deutschbein, M. Faulstich, W. Jahn, G. Krolla, N. Neuroth, Appl. Opt. 17 (1978) 2228–2232. [8] Yongchun Xu, Shunguang Li, Lili Hu, Wei Chen, J. Rare Earths 29 (6) (2011) 614–617. [9] A. Halperin, Handbook on the Physics and Chemistry of Rare Earths, 28, 2000, pp. 187–309. [10] S. Normani, A. Braud, R. Soulard, J.L. Doualan, A. Benayad, V. Menard, G. Brasse, R. Moncorgé, J.P. Goossens, P. Camy, Cryst. Eng. Commun. 18 (2016) 9016–9025. [11] Vadim O. Kozhanov, Alexander N. Zaderko, Olga Yu. Boldyrieva, Vladyslav V. Lisnyak, Mol. Cryst. Liq. Cryst. 639 (1) (2016) 78–86. [12] E. Golis, RSC Adv. 6 (2016) 22370–22373. [13] Roy C. Gunter Jr., Joseph V. Closs, Appl. Opt. 14 (1) (1975) 174–176. [14] Weikuan Duan, Yanyan Zhang, Zhongyue Wang, Jingyi Jiang, Chen Liang, Wei Wei, Nanoscale 6 (2014) 5634–5638. [15] Christopher M. Bender, James M. Burlitch, Duane Barber, Clifford Pollock, Chem. Mater. 12 (2000) 1969–1976.
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Investigation of optical and spectroscopic properties of neodymium doped oxyfluoro-titania-phosphate glasses for laser applications G. Neelima a,b, K. Venkata Krishnaiah b,⁎, N. Ravi b,⁎, K. Suresh c, K. Tyagarajan d, T. Jayachandra Prasad e a
Department of Physics, JNT University, Anantapuramu 515002, India Department of Physics, RGM College of Engineering and Technology, Nandyal 518501, India Department of Physics, Sri Venkateswara University, Tirupati 517502, India d Department of Physics, JNT University College, Pulivendhula 516390, India e Department of ECE, RGM College of Engineering & Technology, Nandyal 518501, India b c
a r t i c l e
i n f o
Article history: Received 11 September 2018 Received in revised form 8 November 2018 Accepted 8 November 2018 Available online xxxx Keywords: Neodymium ions Titania-fluorophosphate glasses Non-linear refractive index High-energy high-power lasers Judd-Ofelt parameters
a b s t r a c t Nd3+-doped oxyfluoro-titania-phosphate glasses with a chemical combination, P2O5–CaF2–BaF2–TiO2–Nd2O3 (PCfBfTiNd), were synthesized by the regular melt-quenching method. These glass samples were investigated through optical absorption, emission, decay curves and spectroscopic ellipsometry analysis. Non-linear refractive index (n2) is evaluated using the Boling method and it was found to be as low as 3.68 × 10−16 cm2/W (1.36 × 10−13 esu). Emission spectra of Nd3+-doped glasses were obtained by laser excitation at a wavelength of 808 nm. Results were endorsed that the 0.5 mol% Nd3+-doped PCfBfTiNd05 glass could be a suitable gain media for the development of high-energy high-peak power lasers. © 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Rare earth (RE3+) activated glasses have been opted for the advance of solid-state lasers, optical fiber communication, sensing, plasma displays and white light emitting diodes (W-LEDs) [1–3]. Generally, RE3 + -doped phosphate glasses are the active host media since they possess long fluorescence lifetime, high quantum efficiency, low non-linear refractive index and high stimulated emission cross-section for highenergy high-power solid-state lasers [4,5]. Particularly, neodymium (Nd3+)-doped phosphate glasses are predominantly impressive for low-power diode laser pumping as a consequence of their high emission cross-section and low fluorescence quenching [6]. One of the significant applications of Nd3+: glass lasers in the plasma generation [7,8]. However, there is a big task for the researchers to develop laser glasses for high-power laser applications with much improved thermomechanical properties while sustaining the optical and laser properties. To encounter these specifications, titania (TiO2) is one of the modifiers in our glass system (P2O5–CaF2–BaF2–TiO2) which can improves the thermo-mechanical properties of the glasses. Broadband laser emission of Nd3+ ion at 1055 nm from the Nd3+doped CaF2 and SrF2 fluoride crystals has been studied [9]. The Nd3+doped phosphate glasses co-doped with other RE3+ ions enact as a diluting agent [10,11]. Nd3+ ions in the laser amplifier system can establish a prospective activator for enhancing the magneto-optical (MO) ⁎ Corresponding authors. E-mail addresses: [email protected] (K. Venkata Krishnaiah), [email protected] (N. Ravi).
https://doi.org/10.1016/j.scriptamat.2018.11.018 1359-6462/© 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
properties of the glasses [12]. Generally, the paramagnetic nature of RE3+ ions is quite interesting for optoelectronic applications. Fluoride based glasses possess lower phonon energy than oxides, zirconates, phosphates, and so on, this consequence leads to decrease of non-radiative relaxation and simultaneously increase of photoluminescence (PL) quantum efficiency. With these advantages, fluoride based glasses were predominated over oxide glasses. Nd3+doped fluorides, such as CaF2, BaF2, SrF2 and LaF3, can reduce the concentration quenching effects which mainly replicates the emission intensity and the fluorescence lifetime [13–16]. It is evinced that the Nd3+-doped oxyfluoro-phosphate glasses unveil a high grade of inhomogeneous spectral line widening and smooth line shapes that leads to the shorter laser pulses [17–19]. With these virtues, new glassy systems are essential to enhance the emission cross-section and bandwidth. Moreover, fluoride based glass fibers were testified as low transmission loss of optical energy in the near infrared region [20,21]. Fluorescence properties of Nd3+-doped glasses can be improved significantly with the use of alkali or alkaline earth metal modifiers with less field strength in these phosphate glasses [22,23]. Present work examines the physical, optical and PL properties of Nd3 + -doped PCfBfTiNd glasses. Judd-Ofelt (JO) phenomenological intensity parameters (Ω2, Ω4, Ω6) were estimated by means of the absorption spectrum. The PL spectra of these glasses were obtained upon laser excitation at a wavelength of 808 nm. Decay curves were analysed and evaluated for the lifetime of excited level, 4F3/2 of Nd3+ ion. The results are compared with those of other reported Nd3+:systems.
G. Neelima et al. / Scripta Materialia 162 (2019) 246–250
Glasses were fabricated by regular melt-quenching method [24] with a molar composition of (60-x) P2O5–20CaF2–15BaF2–5TiO2– xNd2O3 where x = 0.05, 0.1, 0.5, 1.0, 1.5, 2.0 and 2.5 (mol%), referred as PCfBfTiNd0.05, PCfBfTiNd0.1, PCfBfTNd0.5, PCfBfTNd1.0, PCfBfTNd1.5, PCfBfTNd2.0, and PCfBfTNd2.5, respectively. Absorption spectrum of the Nd3+-doped oxyfluoro-titania-phosphate (PCfBfTiNd2.0) powder glass was recorded in the diffuse reflectance (DRS) mode (VARIAN Cary 5000 in the region 175–3300 nm) by UV– visible–NIR double beam spectrophotometer. The PL spectra were recorded using Edinburg Spectrofluorometer (FLS — 980, λ = 200–1700 nm) with excitation of 808 nm CW laser. Decay curves of the glass samples were recorded with the same spectrometer by exciting with a 808 nm laser in the pulsed mode. Refractive index (n) of Nd3+-doped glass was measured by J.A. Woollam Spectroscopic Ellipsometer (Model: M-2000VI) in the spectral range of 370–1690 nm with an error of ±0.0023. Error of the parameters was evaluated by regular standard deviation method. Non-linear behaviour of laser materials is crucial for high-power laser applications as the absorption and emission properties depending on their refractive index [25]. Abbe numbers are used to categorize glass and other optically transparent materials. Two modifiers, BaF2 and CaF2 offer low non-linearity, wide transmission and low hydroxyl content of the glass. Addition of transition metal ions, such as Ti4+ improves linear refractive index and dispersion of the fluorophosphate glasses [26]. A similar trend was perceived in our glass matrix. The non-linear refractive index of the PCfBfTiNd glasses can be evaluated using a formula suggested by Boling [27] and it is given by 2 −1=2 n2 ¼ KðnD −1Þ n2D þ 2 ðvd Þ−1 1:517 þ n2D þ 2 ðnD þ 1ÞvD =6nD ð1Þ where K = 68 × 10−13 esu for the fluorophosphates glass. Abbe number measures the dispersion of a material in the visible region and is given by the following equation vd ¼
ðnD −1Þ ðn F −nE Þ
ð2Þ
where nD, nF and nE are the linear refractive indices at λD = 587.6 nm, λF = 486.1 nm and λE = 656.3 nm, respectively. It is well identified that the index of refraction decreases with an increase in the incident laser wavelength. Refractive indices at three different wavelengths and Abbe number of Nd3+-doped glass are compared to other reported Nd3+-doped laser glasses in Table 1 [28–31]. It is noteworthy that the
247
Abbe number was estimated to be 61.63 which is higher than the reported Nd3+-doped glasses that include N21 [28], FCD600 [29] and STIM28 [31]. These PCfBfTiNd glasses show a low dispersion due to a high value of Abbe number compared to the newly released commercial glass FCD600 (HOYA) [30]. Non-linear refractive index of PCfBfTiNd10 glass is found to be 3.68 × 10−16 cm2/W (1.36 × 10−13 esu) which demonstrations a higher value compared to other reported glasses [28,29] and lower than the majority of the commercial glasses (SCHOTT, HOYA etc.) reported in the literature [29–31]. Absorption spectrum of 2.0 mol% Nd3+-doped PCfBfTiNd2.0 powder glass at room temperature in the spectral region of 300–1100 nm is shown in Fig. 1. Spectrum unveils ten characteristic bands witnessed at around 430 nm (23,255 cm−1), 476 nm (21,008 cm−1), 512 (19,531 cm−1), 525 (19,047 cm−1), 583 (17,152 cm−1), 626 (15,974 cm−1), 683 (14,641 cm−1), 746 (13,404 cm−1), 799 (12,515 cm−1) and 873 (11,454 cm−1) equivalent to the transitions from the ground state, 4I9/2 to higher excited states, 2D5/2 + 2P1/2, 2 K15/2 + 4G11/2 + 2D3/2, 4G9/2, 2K13/2 + 4G7/2, 2G7/2 + 4G5/2, 2H11/2, 4F9/ 4 4 4 2 4 2, S3/2 + F7/2, F5/2 + H9/2 and F3/2, respectively. All the absorption peaks of Nd3+ ions were resolved in our glass system. An intense absorption peak is noted at 583 nm (17,152 cm−1) which was a result of hypersensitive nature of the transition, 4I9/2 → 4G5/2 + 2G7/2 and it follows the selection rules |ΔL| ≤ 2, |ΔS| ≤ 0 and |ΔJ| ≤ 2 [32,33]. Based on absorption bands and transition energies, relative intensity parameters of Nd3+ were determined by Judd-Ofelt (JO) theory [34,35]. The JO intensity parameters Ωλ (where λ = 2, 4, 6), and calculated oscillator strength (fcal) were obtained from the values of experimental oscillator strength (fexp) by the use of least-square fitting method. The reduced matrix elements were adopted from the literature [36]. The oscillator strength can be evaluated from the area under the absorption curve. The fexp and fcal of PCfBfTiNd glasses were listed in the Table 2. The root mean square (δrms) is obtained as 0.110 which states that reliable to evaluate the radiative parameters. The JO intensity parameters (Ω2, Ω4 and Ω6) are presented in Table 3. It clearly indicates that the intensity parameter, Ω2 illustrates the asymmetry and the covalancy of Ln3+ sites, whereas the Ω4 and Ω6 are associated with the dynamics of the host glass matrix. High value of the Ω2 (0.57 × 10−20 cm2) is compared to Ω4 which indicates less centrosymmetrical ion site and chemically more covalent with the ligands. However, the emission intensity of Nd3+-doped laser glasses could be characterized by the intensity parameters Ω4 and Ω6 which can be used for evaluating the spectroscopic quality factor χ (=Ω4/Ω6). The quality factor found to be as high as 0.75 which reports less than unity. The value of χ found to be higher than the reported glasses that include 0.54 for Glass-B [22], 0.70 for PKFBAN10
Table 1 Refractive indices at different wavelengths (nD, nF and nE) and Abbe numbers (vd) of Nd3 : systems along with proposed PCfBfTiNd1.0 glass.
+
Glasses
Ref.
PCfBfTiNd1.0 (present work) N21 [28] N31 LG-770 APG-1 [29] N-FK5 NBFD30 FCD600 (new) [30] FCD1 TAFD65 S-BSL 7 S-TIH 1 [31] S-TIM28 S-FPL51
nD
nF
nE
1.5485 1.5550 1.5461
n (@1054 nm)
vd
n2 (×10−13) esu
1.5480 ± 0.0023
61.63 ± 0.6717
1.36 ± 0.0215
1.5758 1.5280 1.5086 1.5370 1.4874 1.8588 1.5941
1.5731 1.5413 – 1.5350 1.4922 1.8791 1.6009
1.5652 1.5332 1.5070 – 1.4891 1.8656 1.5964
1.5652 – 1.4996 1.5260 1.4785 1.8307 1.5835
65.3 66.2 – – 70.41 30.00 60.47
1.30 1.10 – – 1.47 1.83 1.58
1.4970 2.0509 1.5163 1.7173 1.6889 1.4970
1.5012 2.0786 1.5219 1.7346 1.7046 1.5012
1.4984 2.0601 1.5182 1.7231 1.6941 1.4984
1.4901 2.0134 1.5069 1.6935 1.6669 1.4901
81.61 26.94 64.14 29.52 31.07 81.54
1.49 2.01 1.51 1.71 1.68 1.49
Fig. 1. Absorption spectrum of 2.0 mol% Nd3+-doped PCfBfTiNd2.0 glass in the visible and NIR regions.
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G. Neelima et al. / Scripta Materialia 162 (2019) 246–250
Table 2 Oscillator strengths fexp and fcal of Nd3+-doped PCfBfTiNd2.0 glass. Wavelength (nm)
4
I9/2→
fexp
23,255 21,008
0.090 0.360
0.024 0.183
0.065 0.177
19,531 19,047 17,152 15,974 14,641 13,404 12,515 11,454
0.064 0.254 2.220 0.022 0.210 0.890 0.630 0.360
0.137 0.395 2.210 0.009 0.063 0.810 0.753 0.229
0.073 0.141 0.009 0.013 0.146 0.079 0.123 0.130
D5/2 + 2P1/2 K15/2 + 4G11/2 + 2 D3/2 4 G9/2 512 2 525 K13/2 + 4G7/2 2 583 G7/2 + 4G5/2 2 626 H11/2 4 683 F9/2 4 746 S3/2 + 4F7/2 4 799 F5/2 + 2H9/2 4 873 F3/2 δrms = ±0.110 (10)
430 476
Δf = |fexp ~ fcal|
Energy (cm−1)
2 2
fcal
[37], 0.62 for NaPZN05 [38], 0.74 for BaABP05 [39], 0.32 for LBZN05 [40] and lower than 0.82 for LCBALPN04 [46] and 0.87 for BiPN05 [55] glasses. The JO intensity parameters of PCfBfTiNd2.0 glasses are found to be Ω2 = 0.57 × 10−20 cm2, Ω4 = 0.43 × 10−20 cm2 and Ω6 = 0.58 × 10−20 cm2. It is noticeable that the value of Ω2 is greater than the value of Ω4, further, Ω6 was found to be greater than Ω4 (Ω6 N Ω2 N Ω4). Glasses with fluoride content possess a high ionic bond and more brittle, that exhibit a low value of Ωλ compared to phosphate hosts. However, a large variation of Ωλ values for fluorophosphate glasses can be observed with no specific trend. The JO intensity parameters, quality factor and nature of the intensity parameters were compared in Table 3. The PCfBfTiNd glasses show a similar behaviour compared to other reported glasses. JO intensity parameters were utilized to assess the radiative parameters. The radiative properties of Nd3+-doped PCfBfTiNd2.0 glass can be obtained by using JO intensity parameters. Consequently, the spectroscopic quality factor χ can be used to describe the emission transitions of Nd3+:systems. In general, the most probable transition, 4F3/2 → 4I11/2 is obtained at 1.05–1.06 μm for the Ω6 ≫ Ω4 and the 4F3/2 → 4I9/2 transition is dominant at around 890 nm for Ω6 ≪ Ω4. The spectroscopic quality factor of PCfBfTiNd2.0 glass is found to be 0.75 and satisfies the condition Ω6 ≫ Ω4 that results the laser emission from 4F3/2 → 4I11/2 at 1.05 μm in the near infrared wavelength region. Stimulated emission cross-section (σem) is an important parameter to evaluate the rate of laser emission from the optical materials. The emission cross-sections are determined using the Fuchtbauer-Ladenburg formula. The value of σem of PCfBfTiNd glass is reported as 13.70 × 10−20 cm2, which is found to be higher than CaB4O7N10 (2.34) [41], PKSFAN10 (3.29) [43], TCZNBN10 (3.86) [45], LCBALP10 (6.97) [46], PKBaAN05 (7.0) [47] and lower than BNPZLiN05 (15.89) [50] glasses and are displayed in the Table 4. A comparison of radiative transition probabilities, branching ratio and fluorescence lifetime for various Nd3 + -doped systems are listed in Table 5. The PL spectra of Nd3+-doped PCfBfTiNd glasses excited by laser at a wavelength of 808 nm are shown in Fig. 2. The emission spectra disclose three peaks at around 874, 1054 and 1325 nm corresponds to the f-f Table 3 Comparison of JO parameters (Ω2, Ω4, Ω6), their trend and spectroscopic quality factor (χ) of Nd3+:systems.
Table 4 Comparison of stimulated emission cross-sections (σem) of various Nd3+:systems compositions. σem (×10−20) cm2
S·no.
Composition
1 2 3 4 5 6 7 8 9 10 11 12 13
PCfBfTiNd2.0 [present work] APG-1 [29] LG-770 [29] CaB4O7N10 [41] TAKLNP20 [42] PKSFAN10 [43] PKCFAN10 [44] TCZNBN10 [45] LCBALP10 [46] PKBaAN05 [47] TZN05 [48] PCdN02 [49] BNPZLiN05 [50]
13.70 ± 0.0536 3.40 3.90 2.34 2.97 3.29 3.42 3.86 6.97 7.00 2.18 3.79 15.89
transitions of 4F3/2 → 4IJ (J = 9/2, 11/2 and 13/2), respectively [14,51–54]. Under 808 nm excitation, Nd3+ ions rapidly excite to the higher energy state, 4F7/2 from the ground state, 4I9/2. Then, these excited Nd3+ ions of higher energy state are spontaneously de-excite to the lower energy state, 4F5/2 through non-radiative transition that acts as a metastable state. It holds a longer lifetime compared to other excited states. Therefore, Nd3+ ions relax much time in the metastable state leads to population inversion and then lasing. The 4F3/2 → 4I9/2 transition separates into two peaks owed to the Stark splitting of Nd3 + ionic states in the host [55]. This nature of splitting reveals the asymmetric environment around Nd3+ about titania-phosphate network, which rises with increasing Nd3+ concentration up to 0.5 mol%. Moreover, it is noticed that the splitting was evidenced significantly for 0.5 mol% Nd3+-doped glass. On the other hand, the overlap of these bands is observed with further increase of Nd3+ ion concentration. Emission intensity increased as the concentration of Nd3+ increases up to 0.5 mol% thereafter it decreases abruptly at 1.0 mol% Nd3+ ion concentration. Again an increase in the intensity was noticed till 2.0 mol% and then decreased to 2.5 mol%. The maximum intensity of emission at 1054 nm was perceived for 0.5 mol% Nd3+-doped PCfBfTiNd0.5 glass corresponds to the transition of 4F3/2 → 4I11/2. Excitation, emission and cross-relaxation channels were presented in the partial energy level diagram of Nd3+ ions in Nd3+-doped PCfBfTiNd glasses shown in the Fig. 3. The cross relaxation process was observed among the levels 4F3/2 → 4I15/2 and 4I9/2 → 4I15/2. Under 808 nm laser excitation, a bandwidth of around 17 nm with high emission cross-section is obtained for the PCfBfTiNd0.5 glass, which is higher than those of other reported Nd3+-doped glasses [56]. Fluorescence decay curves for the 4F3/2 → 4I11/2 transition of Nd3+ ions in PCfBfTiNd glasses were recorded under 808 nm laser excitation, as shown in the Fig. 4. Decay curves of PCfBfTiNd glasses display exponential behaviour at low Nd3+ ions concentration and deviated to non-exponential at high concentration. Lifetime of 4F3/2 level is found to be 140, 143, 137, 92, 89, 64 and 62 μs for 0.05, 0.1, 0.5, 1.0, 1.5, 2.0 and 2.5 mol% Nd3+-doped PCfBfTiNd glasses, respectively. Variation of lifetime with the effect of Nd3+ ion concentration is shown in the inset of Fig. 4. It is observed that the lifetime of 4F3/2 level increases Table 5 A comparison of radiative properties of Nd3+-doped glasses.
Glass matrix
References
Ω2
Ω4
Ω6
χ = Ω4/Ω6
Trend
Matrix
β
PCfBfTiNd2.0 Glass-B10 PKFBAN10 NaPZN05 BaABP05 LBZN05 LCBALPN01 BiPN05
Present work [22] [37] [38] [39] [40] [46] [55]
0.57 17.79 4.92 3.76 3.73 2.69 4.35 9.61
0.43 8.39 3.67 3.27 2.48 1.09 2.87 10.71
0.58 15.52 5.26 5.23 3.33 3.40 3.49 12.34
0.75 0.54 0.70 0.62 0.74 0.32 0.82 0.87
Ω6 N Ω2 N Ω6 Ω2 N Ω6 N Ω4 Ω6 N Ω2 N Ω4 Ω6 N Ω2 N Ω4 Ω2 N Ω6 N Ω4 Ω6 N Ω2 N Ω4 Ω2 N Ω6 N Ω4 Ω6 N Ω4 N Ω2
τexp μs
AR s−1
PCfBfTiNd0.1 CaB4O7N10 [41] TAKLNP20 [42] PKSFAN10 [43] TCZNBN10 [45] PKBaAN05 [47] BNPZLiN05 [50]
50 45 50 64 53 – 48.12
143 ± 0.2 55 249 211 – 170 48.4
6449 – 1844 – 1871 – 4041
σem × τexp 10−23
(Δλ, nm)
σem × Δλ 10−25 cm3
1.95 – 7.39 – – – 7.64
17.22 36 35 32.6 35.37 22.5 16.18
2.35 0.84 1.03 1.07 1.20 1.57 2.57
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Fig. 4. Decay profiles of Nd3+-doped PCfBfTiNd glasses with various content of Nd3+ ions. Fig. 2. Emission spectra of Nd3+-doped PCfBfTiNd glasses upon excitation of 808 nm laser.
with the increase of Nd3+ ion concentration up to 0.1 mol%. Then, it decreases with further increase of Nd3+ ion concentration till 2.5 mol%. The decrease in lifetime may be the result of concentration quenching and increase in lifetime may be because of reabsorption. Lifetime found to be as high as 143 μs for PCfBfTiNd0.1 glasses. The PCfBfTiNd0.1 glass possesses longer lifetime than those of other reported Nd3+: glasses that include 95 μs for PbFPbOBN01 [57], 131 μs for PKFBAN10 [37], and lower than 281 μs for LCBALPN01 [46], 348.72 μs for YCaF2N08 [58], and 489 μs for BSAYPN05 [59] systems. In summary, we report, the optical and photoluminescence (PL) properties of Nd3+-doped oxyfluoro-titania-phosphate glasses as a function of Nd3+ concentration through optical absorption, emission spectra, spectroscopic ellipsometry and decay curve analysis. Linear and non-linear refractive indices of the glasses were found to be 1.5485 (at 587.6 nm) and 3.68 × 10−16 cm2/W (1.36 × 10−13 esu), respectively. Spectroscopic quality factor, χ (=Ω4/Ω6) is determined as 0.754 for 0.5 mol% Nd3+-doped PCfBfTiNd0.5 glass. The PCfBfTiNd glasses revealed lower non-linearity compared to other commercial glasses that include N21, N31 (FOCTEK) and LG 770 (SCHOTT). Upon 808 nm laser excitation, PL intensity at a wavelength of 1054 nm corresponds to the 4F3/2 → 4I11/2 transition was increased up to 0.5 mol% and decreased with further increase of Nd3+ ions concentration. Figure of
Fig. 3. Energy level diagram of Nd3+ ion along with excitation, de-excitation and crossrelaxation channels.
merit (FOM) (σnem ) of 0.5 mol% Nd3+-doped glass is found to be 3.72 2
× 10−4 W which was comparable to the reported Nd3+:glasses [60]. Decay curves of Nd3+-doped glasses were exhibited exponential behaviour for low concentrations of Nd3+, and non-exponential for higher concentrations. The results are noteworthy and endorsing that the Nd3+-doped oxyfluoro-titania-phosphate glasses could be a suitable candidate for the development of high-energy high-peak power solidstate lasers. Acknowledgement One of the authors Dr. KVK is thankful to SERB-DST, Govt. of India, New Delhi for the award of a major research project (File Number: EMR/2017/000009). The authors acknowledge MoU-DAE-BRNS Project (No. 2009/34/36/BRNS/3174), Department of Physics, Sri Venkateswara University, Tirupati, India for extending the PL experimental facility. We also thank the Central Electro Chemical Research Institute (CECRI-CSIR), Karaikudi and Indian Institute of Technology Madras (IIT-M) for providing UV-VIS-NIR absorption and Ellipsometry studies, respectively. References [1] B. Afef, H.H. Hegazy, H. Algarni, Y. Yang, K. Damak, E. Yousef, R. Maâlej, J. Rare Earths 35 (4) (2017) 361–367. [2] Kummara Venkata Krishnaiah, Kagola Upendra Kumar, Chalicheemalapalli Kulala Jayasankar, Mater. Express 3 (1) (2013) 61–70. [3] Hansol Lee, Seonghyeon Kim, Jong Heo, Woon Jin Chung, Opt. Lett. 43 (4) (2018) 627–630. [4] K. Venkata Krishnaiah, C.K. Jayasankar, S. Chaurasia, C.G. Murali, L.J. Dhareshwar, Sci. Adv. Mater. 5 (3) (2013) 276–284. [5] Francisco Muñoz, Akira Saitoh, Rafael J. Jiménez-Riobóo, Rolindes Balda, J. NonCryst. Solids 473 (2017) 125–131. [6] D. Kopf, F.X. Kärtner, U. Keller, K.J. Weingarten, Opt. Lett. 20 (1995) 1169–1171. [7] O. Deutschbein, M. Faulstich, W. Jahn, G. Krolla, N. Neuroth, Appl. Opt. 17 (1978) 2228–2232. [8] Yongchun Xu, Shunguang Li, Lili Hu, Wei Chen, J. Rare Earths 29 (6) (2011) 614–617. [9] A. Halperin, Handbook on the Physics and Chemistry of Rare Earths, 28, 2000, pp. 187–309. [10] S. Normani, A. Braud, R. Soulard, J.L. Doualan, A. Benayad, V. Menard, G. Brasse, R. Moncorgé, J.P. Goossens, P. Camy, Cryst. Eng. Commun. 18 (2016) 9016–9025. [11] Vadim O. Kozhanov, Alexander N. Zaderko, Olga Yu. Boldyrieva, Vladyslav V. Lisnyak, Mol. Cryst. Liq. Cryst. 639 (1) (2016) 78–86. [12] E. Golis, RSC Adv. 6 (2016) 22370–22373. [13] Roy C. Gunter Jr., Joseph V. Closs, Appl. Opt. 14 (1) (1975) 174–176. [14] Weikuan Duan, Yanyan Zhang, Zhongyue Wang, Jingyi Jiang, Chen Liang, Wei Wei, Nanoscale 6 (2014) 5634–5638. [15] Christopher M. Bender, James M. Burlitch, Duane Barber, Clifford Pollock, Chem. Mater. 12 (2000) 1969–1976.
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