Spectroscopic investigations of 1.06 µm emission and time resolved Z-scan studies in Nd3+-doped zinc tellurite based glasses

Journal of Luminescence 192 (2017) 1047–1055

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Spectroscopic investigations of 1.06 µm emission and time resolved Z-scan studies in Nd3+-doped zinc tellurite based glasses C.R. Kesavulua,c, K. Sureshb, J.F.M. dos Santosc, T. Catundac, H.J. Kima, C.K. Jayasankarb, a b c

MARK

⁎

Department of Physics, Kyungpook National University, Daegu 41566, Republic of Korea Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Instituto de Fisica de São Carlos, Universidade de São Paulo, Av. Trabalhador Sãocarlense 400, São Carlos, SP, Brazil

A R T I C L E I N F O

A B S T R A C T

Keywords: Nd3+ glasses Thermo-optical properties Thermal lens technique Z-scan Nonlinear refractive index NIR luminescence Decay time

Zinc tellurite based glasses (TZNbTiNd: TeO2 + ZnO + Nb2O5 + TiO2) doped with various Nd3+ concentrations (0.01, 0.05, 0.1, 0.5, 1.0, 1.5 and 2.0 mol% Nd2O3) were prepared and characterized through absorption, emission and decay rate measurements. Judd-Ofelt intensity parameters were derived from the absorption spectrum and used to calculate the radiative properties for the 4F3/2 → 4IJ/2 (where J = 9, 11 and 13) transitions of Nd3+ ions. Strong near infrared emission at 1.062 µm (4F3/2 → 4I11/2) has been obtained for all the glasses upon 808 nm diode laser excitation. The decay times from 4F3/2 level is found to be quite single exponential for different concentrations of Nd3+ ions with a shortening of lifetime with increasing concentration. In these measurements, the electronic refractive index variation is associated with the difference in the polarizabilities (Δαp) of the Nd3+ ion in its ground and excited states. The results indicate that in tellurite glasses, Δαp is relatively very high (4 × 10−25 cm3) compared to other Nd3+-doped glasses. We also measured the imaginary part of the non-linear refractive index (n2′′) due to absorption saturation. Hence, the spectroscopic results indicate that the investigated glasses are potentially applicable as a 1062 nm laser host as well as optical devices.

1. Introduction Rare-earth (RE) doped tellurite glasses have been a subject of increasing interest in the last few years. Among the oxide glasses, tellurite glasses exhibit the lowest phonon energy (∼ 780 cm−1), which increases the quantum efficiency of the excited states of Nd3+ ions in these matrices and provides the possibility of developing more efficient lasers and optical fiber amplifiers [1,2]. Moreover, these glasses combine good mechanical stability, chemical durability, and high linear and nonlinear refractive indices, with a wide transmission window (typically 0.4–6.0 μm), which make them promising materials for photonic applications such as upconversion lasers, optical fiber amplifiers, nonlinear optical devices, and so on [3–9]. Consequently, tellurite glasses are attracting hosts in order to obtain an efficient NIR fluorescence and NIR-to-VIS energy transfer up-conversion through electronic transition of RE3+ ions [10–12]. The addition of optically transparent transition metal oxides, such as TiO2 and Nb2O5 to TeO2, the non-linear optical characteristic values of TeO2 based glasses can be enhanced further [13,14], due to the high polarizability of Ti4+ ions (with a solitary electron pair 5s2) can be even more enhanced by means of the incorporation of other heavy metal

⁎

Corresponding author. E-mail address: [email protected] (C.K. Jayasankar).

http://dx.doi.org/10.1016/j.jlumin.2017.08.037 Received 24 May 2017; Received in revised form 17 August 2017; Accepted 18 August 2017 Available online 19 August 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.

oxides that can be easily polarized. For example, addition of Bi3+, Pb2+, Ti+ and Zn2+ or with empty d orbital of Ti4+ and Nb5+ yields the enhancement of polarization [15]. Also TiO2 is one of the best modifiers in the ternary TeO2 based glassy systems that can provide high non-linear refractive index with low wavelength dispersion and a wide transmittance window in UV–visible and IR regions [16]. Moreover, TiO2 can increase the thermal stability of TeO2 based glasses by replacing Te-O-Te linkages with more rigid Te-O-Ti ones [17]. Apart from these special optical properties, other advantages of such glasses are their good thermal and chemical stability, low tendency to crystallization and their ability to host RE ions. Therefore, the presence of Ti in tellurite glasses may exhibit interesting optical and electro-optical properties with potential applications [14]. Population Lens (PL) is a well know effect in ion photonic materials, where a refractive index profile appears due to the difference of polarizability of the active ions in metastable versus the ground state [18]. Due to the transverse and longitudinal modulation of the excited state population in a laser cavity, PL affects critically the laser behavior. The longitudinal modulation of a standing wave causes the effect of spatial hole-burning. Since the gain is directly related to the excited state population, PL is relevant to the laser stability, line width transverse mode

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C.R. Kesavulu et al.

quality, and etc. In rare earth doped glasses (bulk, fibers and waveguides), the PL effect has been applied to transient Bragg gratings, optical switches, coherent beam combining [19–21], etc. Although the nonlinearity of undoped tellurite glasses has been extensively studied [22], to the best of our knowledge, this paper reports the first investigation of PL in a rare earth doped tellurite glass. In this paper, zinc tellurite glasses with the nominal composition of (75-x) TeO2 + 15ZnO + 5Nb2O5 + 5TiO2 (mol%) doped with Nd2O3 (0.01, 0.05, 0.1, 0.5, 1.0, 1.5 and 2.0 mol%) were studied to investigate the spectroscopic properties and Z-scan studies. Judd-Ofelt theory [23,24] was applied to evaluate the radiative properties based on the phenomenological intensity parameters (Ωλ=2,4,6), which yield spontaneous transition probabilities (AR), luminescence branching ratios (βR) and the radiative lifetime (τrad). The luminescence decay curves of 4 F3/2 excited level and the energy transfer among the excited Nd3+ ions are analyzed and discussed in detail. Thermo-optical properties, such as thermal diffusivity and the temperature coefficient of optical path, were studied by thermal lens in the mode-mismatched configuration [25]. From the Z-scan studies, we have obtained the nonlinear refractive index (n2) in the cw regime and the polarizability difference (Δαp) between excited and ground states of the Nd3+ ion. It is well known that Δαp exhibits a strong host dependence, varying in the range of 10−26–10−25 cm3 for Nd3+ -doped glasses [26–28]. The obtained value is found to be one order of magnitude larger than observed in Nd3+doped oxide crystals and glasses, making Nd3+-doped tellurite glasses as an interesting gain material with high pump induced refractive index changes. This property is important for many applications such as optical switching, coherent beam combining, phase conjugation and demonstrations of fundamental phenomena such as slow and fast light propagation [19].

n2 =

(5)

3.1. Preparation of samples and spectral analysis The zinc tellurite based glasses with a chemical composition of (75x) TeO2 + 15ZnO + 5 Nb2O5 + 5TiO2 + xNd2O3 (here after referred as TZNbTiNd001, TZNbTiNd005, TZNbTiNd01, TZNbTiNd05, TZNbTiNd10, TZNbTiNd15 and TZNbTiNd20, for x = 0.01, 0.05, 0.1, 0.5, 1.0, 1.5 and 2.0 mol%, respectively) were prepared by the conventional melt quenching technique. The stochiometric compositions of the batch materials (∼25 g) were melted in a platinum crucible at 780 °C for 30 min. The melt was stirred with a platinum rod and then poured onto a preheated brass plate, annealed at 350 °C for 10 h to relieve thermal stresses and strains and then slowly cooled to room temperature. Finally, the glass samples were cut and polished for optical measurements. The refractive index (2.04) was determined by the Brewster’s angle method using the 650 nm diode laser. Density measurements were carried out by Archimedes’s principle with xylene as immersion liquid. Absorption spectrum was measured on a Perkin-Elmer Spectrophotometer (Lambda-950) in the wavelength range of 400–950 nm with a spectral resolution of 1 nm. The NIR emission spectra were measured with Edinburgh FLS-980 spectrometer by exciting at 808 nm laser diode with a spectral resolution of 1 nm and incident power of 600 mW. NIR decay times were measured by exciting the samples with 808 nm laser diode (P = 600 mW) with frequency of 100 Hz, detected with an InGaAs detector and a digital oscilloscope. All measurements were carried out at room temperature.

(1)

3.2. Thermal lens technique

where,

fL = (no2+2)/3

(2)

The thermal lens (TL) effect takes place when a laser beam passes through an absorbing medium. The absorbed energy causes thermal expansion and a refractive index gradient in the medium, changing the wavefront curvature, like a lens. This effect produces a transversal phase shift in the beam, whose amplitude is [25]:

is the Lorenz local field correction factor, Nt is the total ion concentration and Nex is the excited state population. Δn is a complex quantity where the real part is proportional to

Δα p = Re

⎟

3. Experimental methods

In ion doped materials, such as rare-earth and transition metal ions, there is one kind of refractive index change which originates from the population of metastable state of the dopant ion, which has a complex susceptibility different from that of the ground state. The complex refractive index change, Δn, is proportional to the susceptibility change due to the excited state population:

2π 2 Nex f ( χ − χg ), n 0 L Nt ex

⎜

If the pump beam has a Gaussian intensity profile, the excited state population will follow the same profile, resulting in a refractive spatial variation profile similar to a lens-like profile. Consequently, this phenomenon is known as the PL effect. The experimental technique described and implemented in the case of Nd3+-doped glass in the following section is a very sensitive technique which allows the measurement of the real and imaginary parts of the nonlinear index of refraction, n2′ and n2′′. The polarizability and the difference between the ground- and excited-state absorption cross-sections of the Nd3+ ions can be evaluated by using the Eq. (5).

2. Theoretical background

∆n =

Nt ⎛ 2π 2 λ f ∆α p − i ∆σ ⎞ Is ⎝ n 0 L 4π ⎠

(χex − χg ) Nt

θ = −φ

, (3)

(6)

where φ, Pabs, K, λ, and ds/dT are the fraction of absorbed energy converted into heat, absorbed pump power, thermal conductivity, probe beam wavelength and temperature coefficient of the optical path length, respectively. The φ parameter is related to fluorescence quantum efficiency (η) by [29]:

the polarizability difference between excited and ground states experienced by the dopant ion, and the imaginary part of Δn is proportional to the difference between the ground- and excited-state absorption cross-sections of the dopant ions:

∆σ = (σex − σg ),

Pabs ds Kλ dT

(4)

λ φ = 1−η ⎛⎜ ex ⎞⎟ λ ⎝ em ⎠

we shall first consider the unsaturated pump regime, for which the excited state population is given by Nex ∼ Nt·I/Is, where Is = hν/στ is the pump saturation intensity, hν is the pump photon energy and ‘τ’ is the lifetime of the metastable excited state. Therefore, Δn is proportional to the pump intensity and can be written, similar to the Kerr nonlinear effect, n = n0 +n2I, with n2 = n2′ − in2′′ as [20]:

(7)

where 〈λem〉 and λex are the average emission and excitation wavelength, respectively. The TL experiments were performed using the time-resolved modemismatched configuration. The Ar+ laser at 514 nm was used as the excitation source and a He-Ne laser at 632.8 nm as a probe beam of the 1048

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TL effect. The pump signal is modulated at 8 Hz to get enough time to induce a TL effect in the gain medium and to observe the system to reach the stationary state. The experimental curves were fitted using the theoretical model discussed in [25]. The experimental geometrical parameters were: probe beam radius, wp = 275 µm, the excitation beam radius, woe = 41 µm, at the sample and Z= 5.5 cm, where Z is the position of the sample with regard to the probe beam waist. Further details of the experimental arrangement can be found in the reference [25]. 3.3. Z-scan technique Among the techniques used to characterize the complex susceptibility of a material, the Z-scan technique is the most popular one. Since its introduction in 1989 [30], this technique has gained importance due to its sensitivity and also due to its simplicity compared to the other techniques. Similar to TL, Z-scan technique relies on the basic idea to relate the beam center intensity variation to the refractive index variation. This can be done by monitoring the transmittance variation of the pumped sample as a function of the sample position along the incident beam. This transmittance presents a peak and a valley and the variation between these peak and valley is proportional to the induced phase shift through the normalized transmittance (ΔTpv) as given by:

∆Tpv ~0.406. ∆Φ0

Fig. 1. Optical absorption spectrum of TZNbTiNd10 glass.

Table 1 The absorption band positions (λ, nm), experimental (fexp) and calculated (fcal) oscillator strengths (10−6) of TZNbTiNd10 glass along with JO intensity parameters (Ωλ, × 10−20 cm2).

(8)

Transition 4I9/2 →

λ

fexp

4

875 805 748 682 628 585 527 514 474 431

2.36 2.44 9.48 8.79 8.98 9.46 0.66 0.74 0.15 0.20 31.80 31.75 3.40 4.60 1.01 1.75 1.50 1.18 0.23 0.64 σ (N)a = ± 0.55 (10)

F3/2 F5/2 + 2H9/2 F7/2 + 4S3/2 4 F9/2 2 H11/2 4 G5/2 + 2G7/2 4 G7/2 4 G9/2 2 G9/2 + 2D3/2 + 2K15/2 2 P1/2 + 2D5/2 4

where ΔΦ0 = kLeffn2′I0 with k is the pump wavenumber, Leff = [1-exp (-αL)/α] is the effective sample length, α is the linear absorption coefficient, n2′ is the real part of n2, I0 is the on-axis intensity, and S is the transmittance through some aperture. Moreover, the separation between the peak and valley positions is related to the Rayleigh range through ΔZpv∼1.7zo. The Z-scan technique can also be used to determine the absorption changes by focusing all beam on to the detector. In this case, the maximum transmittance variation is related to the imaginary part of n2 as Tp−1∼kLeffn2′′I0. Therefore, using the shape of the Z-scan curve and the above relations, it is possible to determine the sign and the magnitude of both the real and imaginary parts of the nonlinear refractive index in the material. In slow absorbers, where Δn(t) ∝ Ne(t) ∝Nt(1-e-t/τ) and τ ≥ 100 μs, it is possible to apply the time resolved Z-scan procedure [18]. In this case, a chopper modulates the beam and the transmittance is defined as the ratio between the intensity at a time ti ≪ τ, where there is only linear effects, and at a time tf ≫ τ, where both linear and nonlinear effects are present. The main advantage of this technique is to eliminate linear parasitic effects due to unparallel faces of the sample, polish imperfections, etc.; enhancing the sensitivity. In this way, the signal to noise ratio achieved is much better than the one obtained in the standard Z-scan set up which is similar to the classical Z-scan one [30], with the introduction of a chopper in order to allow time-resolved detection and eliminate parasitic linear effects [20]. In this work, this method was applied to study Nd3+-doped tellurite glass with τ = 131 μs. The Z-scan measurements were performed by chopping the cw Ti-sapphire laser (748 nm) at the frequency f = 410 Hz.

4

fcal

Ω2 = 6.32, Ω4 = 3.06 and Ω6 = 4.56 a σ refers rms deviation between experimental and calculated values and N refers the number of levels used in the fit.

transitions, respectively, and are shown in Table 1. The band at 805 nm (4F5/2+2H9/2) is most commonly used for the optical pumping of neodymium-based lasers, either by flash lamps or by semiconductor GaAs laser diodes [34]. From the absorption spectrum, the experimental oscillator strengths (fexp) of absorption bands of TZNbTiNd10 glass are determined using the relation [35]

fexp =

2.303mc 2 NA πe 2

∫ ∈(ν) dν = 4.318 × 10−9 ∫ ∈(ν) dν

(9)

where ‘NA’ is the number of Nd absorbing ions per unit volume, ‘m’ is the mass of the electron, ‘c’ is the velocity of light in vacuum, ‘e’ is the electron charge, ‘ε’ is the molar absorptivity at a wavenumber υ (cm−1). The theoretical oscillator strength of an f-f transition can be evaluated using the Judd-Ofelt (JO) theory [23,24]. In this theory, calculated oscillator strength (fcal) for an induced electric-dipole transition from the ground state to an excited state is given by 3+

4. Results and discussions

8π 2mcν (n2+2)2 3h (2J +1) 9n

∑

Ωλ (ΨJ U λ Ψ′J ′)

2

4.1. Absorption spectrum – Judd-Ofelt intensity parameters

fcal =

Fig. 1 shows the optical absorption spectrum of 1.0 mol% of Nd3+doped tellurite (TZNbTiNd10) glass recorded in the 400–950 nm spectral region along with the assignments of absorption bands. The assignment of the observed absorption bands has been made according to the earlier studies on Nd3+-doped glasses [31–33]. The absorption bands at 431, 474, 514, 527, 585, 628, 682, 748, 805 and 875 nm correspond to 4I9/2→2P1/2+2D5/2, 2G9/2+2D3/2+2K15/2, 4G9/2, 4G7/2, 4 G5/2+2G7/2, 2H11/2, 4F9/2, 4F7/2+4S3/2, 4F5/2+2H9/2 and 4F3/2

where ‘h′ is the Planck’s constant, ‘n’ is the refractive index, ‘υ’ is the energy of the transition in cm−1, ‘J’ is the total angular momentum of the ground state, (n2 + 2)2/9 n is the Lorentz local field correction for the absorption band, Ωλ (λ = 2, 4 and 6), are the JO intensity parameters and ||Uλ||2 are the doubly squared reduced matrix elements of the unit tensor operator of the rank λ = 2, 4 and 6 which are calculated from the intermediate coupling approximation for the transition, ΨJ → Ψ'J', which are considered to be the independent of the host. 1049

λ = 2,4,6

(10)

Journal of Luminescence 192 (2017) 1047–1055

C.R. Kesavulu et al.

Table 2 The Judd-Ofelt parameters (Ωλ, × 10−20 cm2), trend and spectroscopic quality factor ( χ = Ω4/Ω6) for the TZNbTiNd10 glass along with reported Nd3+:glasses. Glass

Ω2

Ω4

Ω6

Trend

χ

TZNbTiNd10 [Present work] TTNW [36] TNd25 [37] TZ [38] TAKLNP10 [39] LTTNd10 [40] Lead fluorosilicate [41] BPG [42] Borate [43] Silica borate [44] BIGaZLuTMn [45]

6.32 5.37 4.21 3.86 5.93 4.54 3.66 0.95 5.74 2.14 1.26

3.06 4.29 5.97 4.02 3.23 5.79 1.81 2.01 9.37 2.57 2.58

4.56 5.17 5.45 3.79 4.69 5.69 2.10 4.30 9.99 1.93 4.08

Ω2 > Ω6 > Ω4 Ω2 > Ω6 > Ω4 Ω4 > Ω6 > Ω2 Ω4 > Ω2 > Ω6 Ω2 > Ω6 > Ω4 Ω4 > Ω6 > Ω2 Ω2 > Ω6 > Ω4 Ω6 > Ω4 > Ω2 Ω6 > Ω4 > Ω2 Ω4 > Ω2 > Ω6 Ω6 > Ω4 > Ω2

0.67 0.82 1.09 1.06 0.69 1.01 0.86 0.47 0.94 1.33 0.63

The experimentally measured oscillator strengths (fexp) along with the calculated oscillator strengths (fcal) are presented in Table 1. The small root-mean-square deviation (σrms) of ± 0.55 × 10−6, indicates the good fit between the fexp and fcal oscillator strengths. The evaluated JO intensity parameters (Ω2,4,6), spectroscopic quality factors (χ) and their trend in the TZNbTiNd10 glass are compared in Table 2 with those of reported Nd3+-doped glasses that include 79TeO2 + 10TiO2 + 10WO3 + 1Nd2O3 (TTNW) [36], 85TeO2 + 10PbF2 + 2.5AlF3 + 2.5 Nd2O3 (TNd25) [37], 85TeO2 + 15ZnO + 0.5Nd2O3 (TZ) [38], 65TeO2 + 10P2O5 + 6Al2O3 + 18K2O + 0.5La2O3 + 0.5Nd2O3 (TAKLNP10) [39], 59TeO2 + 25WO3 + 15PbF2 + 1Nd2O3 (LTTNd10) [40], 67SiO2 + 10PbO + 2PbF2 + 12K2O+ 8Na2O + 0.24AS2O3 + 0.76Nd2O3 (Lead fluorosilicate) [41], 17.6Ga2O3 + 24.9Bi2O3 + 56.7PbO + 1Nd2O3 (BPG) [42], 0.75Nd2O3 + 4.25Y2O3 + 40CaO + 55B2O3 (Borate) [43], 54.5B2O3 + 10SiO2 + 25Gd2O3 + 10CaO + 0.5Nd2O3 (Silica borate) [44] and 30BaF2 + 18InF3 + 12GaF3 + 20ZnF + 9LuF3 + 6ThF4 + 4MnF2 + 1NdF3 (BIGaZLuTMn) [45]. The trend of the intensity parameters in the present Nd3+-doped glass has been found to be in the order of Ω2 > Ω6 > Ω4. Generally, the magnitude of Ω2 parameter depends on the asymmetry and the covalent nature between rare-earth ions and ligand anions, whereas the values of Ω4 and Ω6 are related to the bulk properties such as viscosity and rigidity of the host medium [46]. From Table 2, for the present TZNbTiNd10 glass, the Ω2 value (6.32 × 10−20 cm2) is found to be larger than those of other Nd3+-doped glasses [36–45]. This indicates that the present TZNbTiNd10 glass exhibited higher covalency nature between Nd-O bond and asymmetry in the vicinity of Nd3+ ions. The intensities of certain f–f transitions are larger compared to other transitions and are characterized by the higher values of reduced matrix element ||U2||2 [47]. These transitions are known as ‘hypersensitive transitions’ obeying the selection rules, ΔJ ≤ 2; ΔL ≤ 2; ΔS = 0. The most intense absorption band at 585 nm corresponds to the 4I9/2 → 4G5/ 2 3+ ions. As can be seen from 2 + G7/2 hypersensitive transition of Nd Table 1, these transitions possess the highest experimental oscillator strength (fexp) than those of the other transitions. The emission intensities of 4F3/2 → 4IJ (J = 9/2, 11/2, 13/2 and 15/2) transitions are uniquely characterized by the ratio of (Ω4/Ω6) intensity parameters. The ratio of Ω4/Ω6 is known as spectroscopic quality factor (χ) [47], which decides the fluorescence efficiency between 4F3/2 → 4I11/2 and 4 F3/2 → 4I9/2 transition. The increase in the value of χ factor increases fluorescence efficiency of the 4F3/2 → 4I11/2 transition in the host. The χ value of present TZNbTiNd10 glass is found to be 0.67 which is comparable to the other Nd3+-doped glasses [36–45] (see Table 2). Hence, the obtained χ value of present TZNbTiNd10 glass indicating that the 4 F3/2 → 4I11/2 transition possesses the possibility of obtaining lasing action at 1.062 µm.

Fig. 2. Partial energy level diagram showing the possible near-infrared emission transitions of TZNbTiNd glasses along with non-radiative (NR) and cross-relaxation (CR) channels.

Fig. 3. Near-infrared emission spectra (λex = 808 nm) of the TZNbTiNd glasses for different Nd3+ ion concentrations.

transitions and cross-relaxations channels in TZNbTiNd glasses. The emission spectra in the wavelength range from 825 to 1500 nm have been measured as a function of Nd3+ ion concentration by exciting at 808 nm and are shown in Fig. 3. The emission spectra exhibited three emission bands centered at 902, 1062 and 1336 nm that corresponds to 4 F3/2 → 4I9/2, 4I11/2 and 4I13/2 transitions, respectively. Among these three transitions, the transition at 1062 nm attributed to 4F3/2 → 4I11/2 transition is a potential transition with high intensity than the rest of the transitions. The intensity of 4F3/2 → 4I11/2 transition increases with increase of Nd3+ ion concentration up to 0.5 mol% and then quenches with increase of Nd3+ ion concentration due to the enhanced interaction between excited state of Nd3+ ions, and/or ground state of Nd3+ ions and leads to energy transfer process through cross-relaxation channels that could be attributed to (4F3/2: 4I9/2 → 4I15/2 : 4I15/2) or (4F3/2 : 4I9/2 → 4I13/2 : 4I15/2) resonant energy transfer between the active ions [35,48], as shown in Fig. 2. The JO parameters and refractive index have been used to predict the radiative properties, such as radiative transition probabilities (AR, s−1), branching ratios (βR) and radiative lifetimes (τrad, μs) for the

4.2. NIR emission and radiative properties Fig. 2 shows the partial energy level diagram along with emission 1050

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Table 3 Emission transitions (SLJ → S′L′J′), predicted radiative transition probabilities (A, s−1), branching ratios (βR) and radiative lifetimes (τrad, μs) for some important emission levels of TZNbTiNd10 glass. SLJ 4

F9/2

S′L′J′

A

βR

τrad

4

∼0 13 13 19 31 2369 3400 3143 608 1 302 1568 829 5067 32 656 3122 2228

0 0.001 0.001 0.002 0.003 0.247 0.354 0.327 0.063 ∼0 0.039 0.202 0.107 0.652 0.005 0.109 0.517 0.369

104

S3/2 F7/2 2 H9/2 4 F3/2 4 F5/2 4 I15/2 4 I13/2 4 I11/2 4 I9/2 4 F3/2 4 I15/2 4 I13/2 4 I11/2 4 I9/2 4 I15/2 4 I13/2 4 I11/2 4 I9/2 4

4

F5/2

4

F3/2

Table 5 Emission band positions (λp, nm), effective linewidth (Δλeff, nm), experimental branching ratio (βR), stimulated emission cross-section (σ(λp), × 10−20 cm2), experimental lifetime (τexp, μs), radiative lifetime (τrad, μs) and figure of merit (σ(λp) × τexp, × 10−25 cm2 s) for 4F3/2 →4I11/2 level of Nd3+ ion in TZNbTiNd10 glass along with other Nd3+:glasses.

128

165

4

I9/2 I11/2 I13/2

4 4

→

λp

902 1062 1336

Δλeff

45.51 26.10 50.06

AR

2228 3122 656

βR

σ (λp)

Exp.

Cal.

0.33 0.59 0.08

0.37 0.52 0.11

λp

Δλeff

βR

σ (λp)

τexp

τrad

σ (λp) × τexp

TZNbTiNd10[Present work] BZBNd1 [33] TTNW [36] TNd25 [37] TZ [38] TAKLNP10 [39] LTTNd10 [40] Lead fluorosilicate [41] BPG [42] Silicate [50] Fluorophosphate [51] TZN10 [52] ED-2 [53]

1062

26.1

0.59

4.83

131

165

63.27

1063 1064 1065 1062 1058 1062 1056

29.0 29.17 37.6 24.0 35.0 4.16 44.2

0.86 0.42 – 0.47 0.51 0.71 0.48

4.33 4.53 1.67 4.20 3.00 4.75 0.87

62 124 85 158 221 68 586

145 162 258 198 256 83 845

26.85 56.17 14.19 66.36 66.30 32.30 50.98

1066 1061 1054 1061 1062

30.0 48.4 28.0 30.9 30.0

0.50 – – 0.60 0.51

1.10 2.23 3.67 4.27 2.90

110 355 286 104 310

369 748 348 153 372

12.10 79.16 104.96 44.41 89.90

0.8Al2O3 + 0.1(Na2O + CaO) + Nd2O3 (silicate) [50], 55.5 P2O5 + 14K2O + 6KF + 14.5BaO + 9Al2O3 + 1Nd2O3 (fluorophosphate) [51], 60 TeO2 + 39 ZnO + 1.0 Nd2O3 (TZN10) [52] and commercial ED-2 [53] glasses. For efficient lasers, the product of the emission cross-section and the lifetime of a laser transition are recognized as a figure of merit (σ(λp) × τexp) of the laser transition, since σ(λp) × τexp is proportional to the slope efficiency and inversely proportional to the threshold pump power of a laser. The σ(λp) × τexp for the 4F3/2 → 4I11/2 transition was calculated and presented in Table 5 along with the other reported glasses [33,36–42,50–53]. As can be seen from Table 5, the figure of merit was found to be 63.27 × 10−25 cm2 s for the TZNbTiNd10 glass, and it is higher than those of BZBNd1 [33], TTNW [36], TNd25 [37], LTTNd10 [40], lead fluorosilicate [41], BPG [42] and TZN10 [52] glasses, whereas lower than those of TZ [38], TAKLNP10 [39], silicate [50], fluorophosphate [51] and commercial ED-2 [53] glasses. Other important properties for the development of fiber laser gain media are non-linear refractive index and high solubility of rare earths. High nonlinear refractive index reduces the energy extraction efficiency of the laser and promotes the risk of laser induced damage [54]. Since the present tellurite glasses, shows high non-linear refractive index and very high solubility of rare earths, it is expected that tellurite glasses exhibit good optical properties for fiber laser gain media. Hence, the present study reveals that the TZNbTiNd10 glass can be recommended as a suitable host for lasing emission at 1062 nm and also for optical amplification.

Table 4 Emission peak wavelengths (λp, nm), effective bandwidths (Δλeff, nm), radiative transition probabilities (AR, s−1), experimental and calculated branching ratios (βR), and stimulated emission cross-sections (σ(λp), × 10−20 cm2) for selected transitions of TZNbTiNd10 glass. Transition 4F3/2

Glass

1.03 4.83 1.33

TZNbTiNd10 glass as described elsewhere [31,32,35,46,47]. Table 3 presents radiative properties for some of the important luminescent levels of TZNbTiNd10 glass. From the emission spectra, the experimental emission peak position (λp), effective linewidths (Δλeff), branching ratio (βR) and stimulated emission cross-section (σ(λp)) are obtained for TZNbTiNd10 glass and are presented in Table 4. The predicted (βR) and radiative transition probability (AR) values using the JO theory are also presented in Table 4. In general, the stimulated emission cross-section (σ(λp)) is dependent on the JO intensity parameters and effective bandwidths (Δλeff) of the emission bands, which are affected by host glass matrices. The emission intensity of 4F3/2 → 4I11/2 laser transition at 1062 nm depends only on the Ω4 and Ω6 parameters due to triangle rule |J′ − J| ≤ λ ≤ |J′ + J| [47,49]. Whereas, the Ω2 parameter does not have any effect on the stimulated emission cross-section (σ(λp)) of 4F3/2 → 4I11/2 emission transition of Nd3+ ion. The radiative branching ratios (βR) used to evaluate the relative intensities of all the emission transitions originating from an excited state are given in Table 4 along with experimental branching ratios. The experimental branching ratios (βR) can be determined from the relative areas of the emission transitions. In general, βR is a critical parameter to the laser designer, since it characterizes the possibility of attaining stimulated emission from any specific transition. In the present study, the values of βR for the 4F3/2 → 4 I11/2 lasing transition was found to be higher compared to other transitions. Moreover, the experimental branching ratios (βR) for the 4 F3/2 → 4I11/2 transition is greater than 0.50 suggesting that this TZNbTiNd10 glass can be used as an efficient laser transition. As can be seen from Table 5, it is noticed that the stimulated emission crosssection (σ(λp)) for the 4F3/2 → 4I11/2 transition of TZNbTiNd10 glass found to be the highest than the other reported 33Bi2O3 + 33ZnO + 33B2O3 + 1Nd2O3 (BZBNd1) [33], [36–42], 97SiO2 +2.1B2O3 +

4.3. Decay time analysis The decay profile of the luminescence from 4F3/2 level of Nd3+ ion in TZNbTiNd glasses has been measured by monitoring the 4F3/2 → 4I11/ 3+ 2 transition and is shown in Fig. 4 for all the concentrations of Nd ions. It is interesting to note that the decay curves are quite single exponential for all the concentrations of Nd3+ ions. From the decay curves, fluorescence lifetime (τexp) of the 4F3/2 level has been determined by finding the first e-folding times of the decay intensity. The τexp values for the 4F3/2 level of Nd3+ ions are found to be 174, 169, 166, 158, 131, 98 and 66 μs for TZNbTiNd001, TZNbTiNd005, TZNbTiNd01, TZNbTiNd05, TZNbTiNd10, TZNbTiNd15 and TZNbTiNd20 glasses, respectively. It is observed that with the increase of Nd3+ ion concentration, the lifetime of the 4F3/2 level is decreased. The quenching of lifetime with increasing the concentration of Nd3+ ions could be due to the presence of cross-relaxation processes (4F3/2 : 4 I9/2 → 4I15/2 : 4I15/2) or (4F3/2 : 4I9/2 → 4I13/2 : 4I15/2) (resonant energy 1051

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Fig. 6. TL transient signal for Pex = 5.0 mW and λex = 514 nm. The solid line corresponds to adjustment using Ref. [27]. The curve fitting provide θ = − (1.99 ± 0.01) × 10−2 rad and tc = (1.43 ± 0.01) × 10−3 s for TZNbTiNd05. For the TZNbTiNd10 sample, θ = − (6.98 ± 0.01) × 10−2 rad and tc = (1.50 ± 0.01) × 10−3 s was determined.

Fig. 4. Decay curves for the 4F3/2 level of TZNbTiNd glasses for different Nd3+ ion concentrations under 808 nm excitation.

transfer) between the Nd3+ ions in the host [48] as shown in the Fig. 2. This process is usually analyzed with the following empirical formula [55].

τobs =

τ0 1 + (N / Q) P

agreement indicates the absence of multiphonon relaxation, so that the fluorescence quenching is only due to the increase of concentration. Moreover, it assures the absence of eventual impurities, like OH- ions, that could degrade the quantum efficiency. For the TZNbTiNd10 glass, η ~ 0.79 has been estimated.

(11)

where τobs is the observed luminescence lifetime, τo is the ideal luminescence lifetime in the limit of zero concentration, N is the Nd3+ ion concentration, Q is the concentration quenching and P is the phenomenological parameter characterizing the steepness of the corresponding quenching curve. Fig. 5 shows the measured luminescence lifetime of the 4F3/2 level for the studied glasses as a function of Nd3+ ion concentration. As can be seen, the observed luminescence lifetime decreased with increasing dopant concentration. The curve is well fitted by Eq. (11) resulting in τo = (169 ± 2) µs, Q = (1.68 ± 0.03) mol% and p = 2.4 ± 0.1. This p ~ 2.4 value is close to p = 2, as theoretically expected for a two-ion cross-relaxation mechanism. The optimum quenching concentration (Q) is 1.68 mol%, which corresponds to 6.05 × 1020 ions/cm3 in the present Nd3+-doped tellurite glasses. This is comparable to Nd3+-doped silicate glasses [56]. Most of the oxide and fluoride based Nd3+-doped glasses, the Q value varies in the range of 3.0–9.0 × 1020 ions/cm3 [56]. At the low concentration limit (NNd → 0)τexp → τ (Eq. (11)), the obtained value of τo = 169 µs is almost equal to τrad = 165 µs, closer to the estimated uncertainty of JO analysis (~ 15%) [56]. This good

4.4. Thermal lens result Fig. 6 shows TL transient signals for the TZNbTiNd05 and TZNbTiNd10 glasses, both with the same excitation power (Pex = 5 mW). For this study, a focused He-Ne laser beam was used as probe with Zc ~ 1.0 cm (the beam diameter measured with a Thorlabs WM100 Omega meter) and thickness of the sample used is 0.2 cm which should be thinner than the confocal parameter probe beam. In the present case, the increase of signal reveals a convergent character of the induced thermal lens indicating ds/dT > 0. The TL signal is larger for the TZNbTiNd10 glass due to the larger absorption and smaller quantum efficiency (η). The solid line represents the theoretical fit [25] which provides two parameters: θ and the characteristic TL response time tc , given by:

tc =

woe2 4D

(12)

where woe is the excitation beam radius at the sample position and D is the sample thermal diffusivity. The parameter θ is approximately proportional to the amplitude of the TL signal and θ is ~3.5 times larger for the 1.0 mol% sample compared to the 0.5 mol%. The tc parameter is nearly the same (within ± 3%) indicating that both samples have similar thermal diffusivities. In order to improve the accuracy of the data, several transient signals were taken as a function of Pex up to ~100 mW. In all samples, a linear increase of 'θ' versus Pex was observed, as expected by Eq. (6). The tc values remained approximately constant, resulting in values with ~ 1% accuracy. The thermal diffusivity values determined using Eq. (12) are shown in Table 6. These values are similar to those obtained for TeO2 + (20-x)Li2O + xTiO2, with x = 0.5 and 1.0 (mol%) [55,57]. The φ value was estimated using Eq. (7), considering that the heat is generated due to a green photon absorption (λex = 514 nm) by Nd3+ ion. The excited ion rapidly decays to the metastable 4F3/2 level, which can decay radiatively to states: 4I15/2, 4I13/2, 4I11/2 and 4I9/2 with emission at 1880, 1350, 1060 and 880 nm, respectively. Using the calculated branching ratios (Table 3), the average emission wavelength, 〈λem〉 = 1018 nm, was obtained. The quantum efficiency of 4F3/2 level is obtained by η = τexp/τrad. Using these data in Eq. (7), φ =0.53 and

Fig. 5. Variation of lifetimes of 4F3/2 level for different Nd3+ ion concentrations in TZNbTiNd glasses. The red line represents theoretical fit (using Eq. (11)) to experimental data.

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from TL measurements and was found to be negligible for these samples, in the present Z-scan set-up. Similarly, an electronic effect could also affect the TL signal. However, this contribution is also negligible in our TL setup because the probe beam area is ~ 45 times larger than the excitation. Consequently, it takes a long time for the heat to diffuse, so the TL response time is 2 orders of magnitude longer than the electronic one, as shown in Fig. 7. The open and closed aperture normalized Z-scan transmittance curves could be almost perfectly fitted by the proper expression [30,59], as it is clearly shown in Fig. 7(a) and (b). From this fit, z0 = (0.47 ± 0.006) cm has been obtained and the real and imaginary parts of the nonlinear refraction index are n2′ = 2.1 × 10−8 and n2′′ = 2.4 × 10−10 cm2/W, respectively. Therefore, the condition of z0 < L (sample thickness of 0.2 cm) has been satisfied. The values of Δαp = 4.2 × 10−25 cm3 and Δσ = −1.1 × 10−20 cm2 were then calculated through Eqs. (3) and (4). Reminding that Δσ = (σex-σg) and using σg = 2.3 × 10−20 cm2, σex = 1.2 × 10−20 cm2 has been determined corresponding to an absorption at 748 nm from 4F3/2 metastable state reaching an energy ~ 24,700 cm−1, close to the band gap energy, Eg ~ 24,000 cm−1 (~ 413 nm). Using the four-wave-mixing method, Powell et al. [26] determined the value of Δαp for 22 different Nd3+ doped glasses and crystals. The average values of Δαp ~ 1.4 for fluorides, 2.6 for phosphates, 4.3 for silicates and 5.1 for oxides (in units of 10−26 cm3) have been observed. Andrade et al. [27] obtained Δαp between 1.1 and 2.7 × 10−26 cm3 in fluoride glasses [27]. Nd:YAG was the most extensively studied Nd3+doped material and its Δαp value is ~ 4 × 10−26 cm3 which is in good agreement with the values obtained by several techniques, such as fourwave-mixing [26], transient-grating, pump-probe interferometry and Zscan [19,28,60]. Moreover, the line shape of n2(ν) or Δαp(ν) in resonance with Nd3+: 4f → 4f lines indicates the presence of a nonresonant contribution, which is usually attributed to the 4f → 5d transitions in the UV. Using this assumption, Powell et al. [26] estimated Δαp ~ 8 × 10−26 cm3 for the free Nd3+ ion and the host dependence of Δαp is attributed to the high sensitivity of the 4f → 5d transitions to the crystal field. According to Soulard et al. [28], the charge transfer bands may also contribute to Δαp. It is interesting to remark that Δαp of Nd3+ in YVO4 is highest value observed in Nd3+ doped crystals, about twice of the value found for YAG [26,28]. Powell et al. [26] attribute this high value to the strong molecular orbital transitions in the near UV, since Nd3+ should substitute the Y3+. Moreover, Δαp of Yb3+ in YVO4 is ~ 4 times larger than in YAG [58]. In fact, YVO4 matrix has a low Eg value (3.6 eV) compared to YAG (~ 6 eV). In the present work, a very large Δαp value (4 × 10−25 cm3), one order of magnitude larger than in Nd3+ YAG, was observed for Nd3+ in a glass matrix with low Eg (~ 3.0 eV). Although it is clear that Δαp is strongly affected by states in UV, it´s physical origin is not well understood since it involves the estimation of energy and oscillator strength of states in the UV range, which are difficult to determine. In a laser cavity, the standing waves produce light-induced gratings of two kinds: phase gratings (due to refractive index modulation) or gain/absorption grating. The dominant character of this grating is governed by the parameter β = (8πfL2/nλp).Δαp)/σp [19], where σp represents the peak stimulated emission cross-section at the wavelength, λp. For the 1.06 µm transition in Nd: YAG, using Δαp = 5 × 10−25 cm3 and σp= 6.5 × 10−19 cm2, β ~ 0.1 has been estimated [61], indicating predominance of the gain over the phase grating. For a typical oxide glass (ED-2), with σp= 2.5 × 10−20 and assuming Δαp ~ 5 × 10−25 cm3, β ~ 2 has been estimated. For Nd3+-doped tellurite glasses, β ~ 13 was calculated, a very high value due to large Δαp and fL. In our study, the analyses have been made within the limits of the TL and Z-scan models.

Table 6 Thermo-optical parameters of the Nd3+-doped tellurite glasses. Glass

Nd2O3 (mol%)

D (cm2/ s)

θ/Pabs (rad/W)

ds/dQ (cm3/J)

K W/ K cm

ds/dT (K−1)

TZNbTiNd05

0.5

− 12.11

TZNbTiNd10

1.0

2.9 × 10−3 2.8 × 10−3

4.3 × 10−6 4.8 × 10−6

6.8 × 10−3 6.4 × 10−3

1.0 × 10−5 1.1 × 10−5

− 19.91

0.70 were obtained for the 0.5 and 1.0 mol% of Nd3+ samples, respectively. Reminding that K = ρCD, where ρ is the density (g/cm3) and C is the specific heat (J/gK), Eq. (6) can be rewritten as θ/Pabs = φ·(λ· D)−1× ds/dQ, where ds/dQ = (ρC)−1 × (ds/dT) represents the change of optical path due to a unit of heat deposit per volume. The obtained values of all thermo-optical parameters are shown in Table 6. The thermal conductivity (K) was calculated from the experimental D values, the factor ρC = 2.3 J/K cm3 has been estimated using the data of glasses of similar compositions [58]. Then ds/dT can be estimated using Eq. (6) and all the obtained thermo-optical parameters are shown in Table 6. Within the experimental uncertainty, the same ds/dQ and ds/dT values were calculated for both samples. This agreement means that the thermo optical parameters are determined mainly by the glass matrix composition, with minor contribution of Nd3+ ions, as expected for low doping level. For thin disk geometry, the parameter ds/dT is related to temperature coefficient of the refractive index (dn/dT), the thermal expansion coefficient (αexp) and the Poisson´s ratio (ν) by:

ds dn = + α exp. (n−1)(1+ν ) dT dT

(13) −5

So, using the values of n = 2.0, ν = 0.6 and αexp= 2.4 × 10 , dn/ dT~ − 2.7 × 10−5 has been estimated. This negative value indicates that ds/dT results from two counteracting contributions, where thermal expansion gives the positive term. Therefore, a negative dn/d/T is an interesting property because it allows the minimization of ds/dT by minor changes in the matrix composition, as done in phosphate and fluoride glasses [29]. 4.5. Z-scan analysis The open and closed Z-scan measurements have been carried out in Nd3+-doped TZNbTiNd10 glass by chopping the cw Ti-sapphire laser with the frequency, f = 410 Hz, and by collecting the data at times ti = 10 μs and tf = 750 μs after the chopper opening. The laser was tuned at 748 nm, to the line center of 4I9/2 → 4F7/2, 4S3/2 absorption line. Measurements were recorded at a low laser pump power (~10 mW), resulted in intensity smaller than 300 W/cm2, in order to avoid saturation effects. After its passage through the sample, the laser beam was detected behind a full aperture (S = 100%), then behind a partially closed one (S = 30%), as it is usually done in Z-scan measurements [21,30]. Since the magnitude of the nonlinear absorption was significant, as shown in Fig. 7(a), the open aperture (S = 100%) normalized Z-scan transmittance signal presents a maximum in the central position of the Z-scan curve. This indicates that absorption decreases with pump intensity, denoting that the ground state absorption crosssection is larger than the excited-state absorption one at that particular wavelength (748 nm). From Fig. 7(b), the closed aperture signal (S = 30%) has to be normalized as it is typically done in the Z-scan technique [21,59]. In a general case, the refractive index change detected by the Z-scan method may have electronic and thermal contributions. However, in the particular case of these tellurite glasses the response time of the TL effect is much longer than the electronic one, which is given by fluorescence lifetime (τ ~ 100 μs). The contribution of the thermal lens signal was simulated using the thermo-optical parameters obtained

5. Conclusions The zinc tellurite (TZNbTiNd) glasses doped with Nd3+ ions have 1053

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been prepared and characterized their optical and luminescence properties and time resolved Z-scan and thermal lens studies. The Judd-Ofelt parameters have been used to predict the radiative properties of the important luminescent levels of TZNbTiNd10 glass. The emission spectra of the present glasses showed strong near infrared emission at 1062 nm corresponding to 4F3/2 → 4I11/2 transition with higher stimulated emission cross-section of 4.8 × 10−20 cm2, experimental branching ratio of 59%, figure of merit of 63.2× 10−25 cm2 s and quantum efficiency of 79%. The decay time of the 4F3/2 level decreases with increasing of Nd3+ ion concentration in the title glasses. From the thermal lens studies, the thermal properties such as: thermal diffusivity, thermal conductivity, temperature coefficient of the optical path length change and the change in the optical path with the heat deposited per unit volume have been determined and discussed and found to be similar to other glass systems. From the Z-scan analysis, Δαp= 4.2 × 10−25 cm3 and Δσ = −1.1 × 10−20 cm2 has been obtained in the present TZNbTiNd10 glass. This Δαp value is one order of magnitude larger than the values found in others Nd3+-doped materials. This property is very interesting for nonlinear applications. Hence, the obtained results indicate that the present TZNbTiNd glasses could be aptly suitable for optical amplification as well as for efficient NIR emission at 1.062 µm. Acknowledgments These investigations have been funded by the Ministry of Science and Technology, Korea (MEST) (No. 2015R1A2A1A13001843). One of the authors (CKJ) is grateful to DAE-BRNS, Government of India for the sanction of Mega Research Project (No. 2009/34/36/BRNS/3174) under MoU between Sri Venkateswara University, Tirupati and RRCAT, Indore and BARC, Mumbai. References [1] J.S. Wang, E.M. Vogel, E. Snitzer, Opt. Mater. 3 (1994) 187–203. [2] A. Jha, B. Richards, G. Jose, T. Teddy-Fernandez, P. Joshi, X. Jiang, J. Lousteau, Prog. Mater. Sci. 57 (2012) 1426–1491. [3] S. Tanabe, K. Hirao, N. Soga, J. Non-Cryst. Solids 122 (1990) 79–82. [4] S.Q. Man, E.Y.B. Pun, P.S. Chung, Opt. Commun. 168 (1999) 369–373. [5] A. Narazaki, K. Tanaka, K. Hirao, N. Soga, J. Appl. Phys. 85 (1999) 2046–2051. [6] N. Jaba, A. Kanoun, H. Mejri, A. Selmi, S. Alaya, H. Maaref, J. Phys.: Condens. Matter 12 (2000) 4523–4534. [7] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, Appl. Phys. Lett. 80 (2002) 1752–1754. [8] S. Shen, A. Jha, L. Huang, P. Joshi, Opt. Lett. 30 (2005) 1437–1439. [9] P. Babu, H.J. Seo, C.R. Kesavulu, K.H. Jang, C.K. Jayasankar, J. Lumin. 129 (2009) 444–448. [10] S.F. Leon-Lusi, U.R. Rodriguez-Mendoza, E. Lalla, V. Lavin, Sens. Actuators B 158

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