Luminescence properties of Dy3+ doped lithium zinc borosilicate glasses for photonic applications.

Different concentrations of Dy3+ ions doped lithium zinc borosilicate glasses of chemical composition (30-x) B2O3 - 25 SiO2 -10 Al2O3 -30 LiF - 5 ZnO - x Dy2O3 (x = 0, 0.1, 0.5, 1.0 and 2.0 mol%) were prepared by the melt quenching technique. The prepared glasses were investigated through X-ray diffraction, optical absorption, photoluminescence and decay measurements. Intensities of absorption bands expressed in terms of oscillator strengths (f) were used to determine the Judd-Ofelt (J-O) intensity parameters Ωλ (λ = 2, 4 and 6). The evaluated J-O parameters were used to determine the radiative parameters such as transition probabilities (AR), total transition probability rate (AT), radiative lifetime (τR) and branching ratios (βR) for the excited 4F9/2 level of Dy3+ ions. The chromaticity coordinates determined from the emission spectra were found to be located in the white light region of CIE chromaticity diagram.

Introduction

The development of rare earth (RE3+) ions doped glasses with good thermal, mechanical and chemical durability are highly essential for the design of high performance lasers, optical amplifiers and white light emitting diodes (LEDs) [1, 2, 3, 4, 5, 6]. Different multi-component borate glasses have been considered for technological applications in many areas and among them, the lithium zinc borosilicate (LZBS) glasses have attracted the researchers for the development of visible lasers and amplifiers due to their good optical transmission, low phonon energy, low glass transition temperature, high refractive index, high dielectric constant and high thermal expansion coefficient [7, 8, 9, 10].

Among various RE3+ ions, the dysprosium (Dy3+) ion exhibits intense yellow (574 nm) and blue (481 nm) emissions corresponding to the 4F9/2 → 6H13/2 and 4F9/2 → 6H15/2 transitions, respectively together with a feeble red emission (665 nm) due to 4F9/2 → 6H11/2 transition. Thus, the Dy3+ - doped glasses are more suitable to produce two primary colors as well as white light emission. The 4F9/2 → 6H13/2 (yellow) transition is a host sensitive and the 4F9/2 → 6H15/2 (blue) transition is host insensitive. Thus, the yellow to blue (Y/B) intensity ratios can be modulated by varying the coordination environment of the host material and/or by changing the concentration of Dy3+ ions.

The aim of this work is to investigate the optical absorption, photoluminescence (PL) and decay properties of 4F9/2 level of Dy3+ ions in lithium zinc borosilicate (LZBS) glasses. Optimization of concentration of Dy3+ ions for efficient luminescent properties and the quenching in PL and decay time has been explained. The aptness of the studied glasses for solid state lasers as well as white LEDs has also been discussed.

Experimental

Materials and method of preparation

Different concentrations of Dy3+ ions doped LZBS glasses with molar compositions of (30 – x) B2O3 + 25 SiO2 + 10 Al2O3 +30 LiF + 5 ZnO + x Dy2O3 (where x = 0, 0.1, 0.5, 1.0 and 2.0 mol %) were prepared by the conventional melt quenching method. About 10 g batch compositions of homogeneous mixtures of high purity B2O3, SiO2, Al2O3, LiF, ZnO and Dy2O3 (99.99%) chemicals were taken into clean and dry alumina crucible and then heated at 1200 °C continuously for 1 h until to get molten state. The melts were poured on a preheated brass mould and annealed at 350 °C for 7 h and then cooled to room temperature in order to strengthen the glasses and also to remove the thermal strains during the preparation of glasses. Based on Dy3+ ions concentration (x = 0, 0.1, 0.5, 1.0 and 2.0 mol %), the glasses were labelled as LZBSDy0.0, LZBSDy0.1, LZBSDy0.5, LZBSDy1.0 and LZBSDy2.0, respectively as shown in the Table 1. The prepared LZBSDyx glasses are shown in the Fig. 1.

Table 1

Glass composition and labelling of LZBSDyx glasses (x = 0.0, 0.1, 0.5, 1.0 and 2.0 mol %).

Glass code	B2O3 (mol %)	SiO2 (mol %)	LiF (mol%)	Al2O3 (mol%)	ZnO (mol%)	xDy2O3 (mol%)
LZBSDy0.0	30	25	30	10	5	0.0
LZBSDy0.1	29.9	25	30	10	5	0.1
LZBSDy0.5	29.5	25	30	10	5	0.5
LZBSDy1.0	29.0	25	30	10	5	1.0
LZBSDy2.0	28.8	25	30	10	5	2.0

Fig. 1

Prepared LZBSDyx glasses (x = 0.1, 0.5, 1.0 and 2.0 mol %).

Physical and optical measurements

The thickness and refractive indices of the prepared glasses play an important role in the estimation of luminescence parameters. So, the refractive indices (n) were determined by the Abbe's refractometer using sodium vapour lamp as a source. The optical absorption spectra were recorded using Jasco V-770 UV-VIS-NIR spectrophotometer. The luminescence and decay measurements were performed using FLS-980 fluorescence spectrometer with xenon flash lamp as a source. All the spectral measurements were carried out at room temperature.

Results and discussion

Absorption spectrum and spectral analysis

The optical absorption spectrum of LZBSDy0.5 glass recorded in the region 500–2000 nm is shown in Fig. 2. The spectrum exhibited twelve absorption bands and are assigned based on the earlier work done by Carnall et al. [11]. The absorption bands located at 347, 364, 386, 426, 451, 471, 752, 797, 895, 1084, 1264 and 1670 nm are assigned to the 6H15/2 → 6P7/2, 6P5/2, 4I13/2, 4G11/2, 4I15/2, 4F9/2, 6F3/2, 6F5/2, 6F7/2, 6F9/2, 6F11/2 and 6H11/2 transitions, respectively. To find the nature of the covalence between the rare earth ion and its ligands as well as the radiative properties of the Dy3+ ions in the prepared glasses, the experimental oscillator strength (fexp) of the absorption bands were determined by using the Eq. (1) [12].where is the molar absorptivity of a band at a wavenumber (cm−1) which can be calculated from the Beer-Lambert's law. The calculated oscillator strengths (fcal) and the Judd - Ofelt (J-O) intensity parameters (Ωλ = 2, 4 and 6) are obtained from the experimental oscillator strength (fexp) by the least square fitting method using the Eq. (2) [13, 14].where c is the speed of light, is the wavenumber, h is the Planck's constant, n is refractive index of the glass, is the Lorentz local field correction factor and accounts for dipole-dipole correction and (2J + 1) is the degeneracy of the ground state. The electric () and magnetic () dipole strengths can be calculated using the Eqs. (3) and (4) [11].where e is the charge of electron, Ωλ (λ = 2, 4 and 6) are the J–O intensity parameters and ||U|| are the doubly reduced matrix elements of rank ‘λ’. While calculating the intensity parameters, the oscillator strengths of 6H15/2 → 6P7/2, 5/2 transitions were not taken into consideration due to their weak intensities. In order to know the quality of fit between the fexp and fcal values, it is necessary to find the root mean square deviation (δrms). In the present case, the obtained δrms value is ± 0.35×10−6, which indicates the best fit between the experimental and calculated oscillator strengths. For LZBSDy0.5 glass, the evaluated experimental (fexp) and calculated (fcal) oscillator strengths are shown in Table 2 and from which it is observed that, the intensity of 6H15/2 → 6F11/2 hypersensitive transition which is very sensitive to the host environment is higher when compared to the other transitions obeying the selection rules , and .

Fig. 2

Optical absorption spectrum of LZBSDy0.5 glass in the UV-VIS-NIR regions.

Table 2

Experimental and calculated oscillator strengths (fexp & fcal) for the observed absorption bands of LZBSDy0.5 glass.

Transition from 6H15/2 →	Oscillator strengths (×10−6)
	fexp	fcal
4I13/2	0.50	0.90
4G11/2	0.27	0.15
4I15/2	0.73	0.82
4F9/2	0.22	0.29
6F3/2	0.47	0.32
6F5/2	2.64	1.72
6F7/2	3.66	3.72
6F9/2	4.58	4.59
6F11/2	12.00	11.97
6H11/2	2.07	2.27
δrms	±0.39×10−6

The evaluated J-O intensity parameters for the LZBSDy0.5 glass are = 11.75×10−20 cm2, = 3.90×10−20 cm2, = 3.61×10−20 cm2 and follows the trend . Similar trend has been observed for other Dy3+ doped host matrices such as L6BD [15], Ge-Ga-Se [16], PKBFAD [17] and Dy:KLTB [18] as compared in Table 3. The higher magnitude quantity of parameter indicates the lower degree of symmetry around the active ion and stronger covalence of active ion-oxygen ligand bond. Whereas, the Ω4 and Ω6 parameters are related to the bulk properties such as viscosity, stability and rigidity of the host medium in which the ions are doped [19, 20, 21]. In the present investigation, considerably higher magnitude of indicates higher degree of asymmetry around the Dy3+ ions and relatively weaker covalence of active ion-oxygen bond.

Table 3

Comparison of J-O intensity parameters of LZBSDy0.5 glass with other Dy3+ doped glasses.

Glass system	J–O intensity parameters (×10−20 cm2)
	Ω_2	Ω_4	Ω_6
LZBSDy0.5 [Present work]	11.75	3.90	3.61
L6BD [15]	12.83	3.47	3.43
Ge-Ga-Se [16]	11.61	2.79	1.11
PKBFAD [17]	10.41	2.29	2.07
Dy:KLTB [18]	9.86	3.39	2.41

Photoluminescence

The luminescence spectra of LZBSDyx (x = 0.1, 0.5, 1.0 and 2.0 mol %) glasses recorded by exciting with 385 nm wavelength are shown in Fig. 3. The emission spectra revealed three characteristic emission bands of Dy3+ ions in the blue (481 nm), yellow (574 nm) and red (665 nm) regions which are assigned to the 4F9/2 → 6H15/2, 4F9/2 → 6H13/2 and 4F9/2 → 6H11/2 transitions, respectively. The intensity of emission bands increased from 0.1 to 0.5 mol% then decreased with increase of Dy3+ ions concentrations. From this, it is concluded that, the optimum concentration as well as efficient luminescence of Dy3+ ions is obtained for LZBSDy0.5 glass. The concentration quenching exhibited by Dy3+ ions at higher concentrations could be due to the energy transfer among the excited Dy3+ ions. The variation of intensities of 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 emission transitions as a function of Dy3+ ions concentration is illustrated in Fig. 4.

Fig. 3

Photoluminescence spectra for different concentrations of LZBSDyx glasses (x = 0.1, 0.5, 1.0 and 2.0 mol %).

Fig. 4

Variation of emission intensities of 4F9/2 → 6H13/2 and 4F9/2 → 6H15/2 transitions with the increase of Dy3+ ions concentration in LZBS glasses.

From the luminescence spectra, it is noticed that, the emission intensity of 4F9/2 → 6H15/2 (481 nm) transition is slightly higher than that of 4F9/2 → 6H13/2 (574 nm) transition. Similar results were reported for Dy3+ ion doped alkali lead tellurofluoroborate glasses [18]. Additionally, the optical properties of rare earth doped materials are influenced by the coordination environment around the active ions. The 4F9/2 → 6H13/2 transition is electric dipole (ΔJ = ±2), which is host sensitive whereas the transition 4F9/2 → 6H15/2 is magnetic dipole (ΔJ = 0, ±1 but 0↔0 forbidden) and is host insensitive. Thus, the yellow to blue (Y/B) intensity ratio has been used to determine the Dy3+-O2− bond covalence. In the present study for all the prepared glasses, the values of Y/B ratios are calculated and presented in the Table 4.

Table 4

CIE chromacity color coordinates (x, y), correlated color temperature (CCT) values and Y/B ratios for the LZBSDyx glasses (x = 0.1, 0.5, 1.0 and 2.0 mol %).

Glass code	Chromacity coordinates		CCT	Y/B ratio
	x	Y
LZBSDy0.1	0.321	0.347	6002	2.35
LZBSDy0.5	0.319	0.363	6040	2.45
LZBSDy1.0	0.318	0.357	6102	2.65
LZBSDy2.0	0.327	0.358	5722	2.39

Color perception

In order to know the emission color perception of LZBSDyx (x = 0.1, 0.5, 1.0 and 2.0 mol %) glasses, the Commission International de I'Eclairagein 1931 (CIE) color coordinates (x, y) have been calculated from the intensities of emission spectral profiles [22, 23]. The evaluated CIE chromaticity coordinates (x, y) for LZBSDy0.1, LZBSDy0.5, LZBSDy1.0 and LZBSDy2.0 glasses are (0.321, 0.347), (0.319, 0.363), (0.318, 0.357) and (0.327, 0.358), respectively and are found to be located in the white region of CIE diagram as shown in Fig. 5. From these results, one can conclude that the Dy3+ doped LZBS glasses are more suitable for the white light generation.

Fig. 5

CIE chromaticity diagram showing the color coordinates (x, y) for LZBSDyx glasses (x = 0.1, 0.5, 1.0 and 2.0 mol %).

To know the spectral behaviour of near white light produced by the broadband light sources can be characterised by the correlated color temperature (CCT) values which can be calculated by using the Eq. (5) and is familiar as Mc Camy's formula [24].where .

As per the CCT ratings for a light source if CCT values below 3200 K are referred as “warm sources” and above 4000 K are considered as “cool in appearance” [24].

The CCT values obtained for LZBSDy0.1, LZBSDy0.5, LZBSDy1.0 and LZBSDy2.0 glasses are 6002, 6040, 6102 and 5722 K and also presented in Table 4. It is observed that, all the CCT values are above 4000 K and can be concluded that the prepared glasses are usually considered as “cool” in appearance [24].

Radiative properties

From the emission spectra of LZBSDyx (x = 0.1, 0.5, 1.0 and 2.0 mol%) glasses, various radiative properties such as transition probabilities (AR), total radiative transition rate (AT), radiative lifetimes (τR), luminescence branching ratios (βR) and emission cross-sections (σe) are determined for the emission transitions using the Eqs. (6), (7), (8), (9) and (10) [25].

The experimental branching ratios (βexp) are determined from the relative areas under the emission peaks and stimulated emission cross-sections (σe) between and levels are given by [25].where is the emission peak wavelength and is the effective linewidth of the emission band. Various radiative parameters of 4F9/2 → 6H15/2, 13/2 & 11/2 transitions in LZBSDy0.5 glass are presented in Table 5. The values of σe and (σe×τR) for 4F9/2 → 6H13/2 transition are found to be 20.15×10−22 cm2 and 9.87 ×10−25 cm2, respectively and are in good agreement (Table 6) with other Dy3+ doped glass hosts [26, 27, 28, 29]. Relatively higher values of the gain bandwidth (σe×ΔλP) and optical gain (σe×τR) parameters for the 4F9/2→4H13/2 transition are critical to predict the amplification of the medium and so it is confirmed that the LZBSDy0.5 glass is a suitable candidate for photonic applications.

Table 5

Radiative parameters of emission transitions of LZBSDy0.5 glass.

Radiative parameters	4F9/2 → 6H15/2	4F9/2 → 6H13/2	4F9/2 → 6H11/2
λp (nm)	483	574	665
Δλeff (nm)	15.93	14.38	54.33
AR (s−1)	386	1428	154
βR	0.18	0.65	0.69
βexp	0.55	0.45	0.40
σe (×10−22 cm2)	6.53	52.50	2.67
τR ((×10−3 s)	0.45	0.45	0.45
(σe×τR) (×10−25 cm2)	2.94	23.65	1.20
(σe×Δλeff) (×10−28 cm3)	10.23	76.49	14.50

*AT = Total radiative transition rate = 1968 s−1; Whereas τR = Total radiative lifetime = 0.508 ms.

Table 6

Comparison of peak positions λp (nm), stimulated emission cross-sections σe (×10−22 cm2), experimental lifetime τexp (ms) and optical gain (σe×τexp) (×10−25 cm2) parameters for the 4F9/2 → 6H13/2 transition level of Dy+3 doped LZBS glasses.

Glass	λp (nm)	σe	τexp (ms)	σe×τexp
LZBSDy0.5 [Present work]	574	52.50	0.33	17.38
Fluoroborate [26]	573	2.74	0.53	1.45
PbPKANDy10[27]	576	54.5	0.47	25.67
Lead borosilicate Dy0.5[28]	577	44.23	0.53	23.70
PKAlCaFDy10 [29]	575	4.77	0.59	2.84

Luminescence decay analysis

The decay profiles of 4F9/2 excited level in LZBSDyx (x = 0.1, 0.5, 1.0 and 2.0 mol %) glasses are obtained by exciting with 385 nm wavelength and monitoring the emission at 574 nm. The experimental (τexp) lifetime values are found to be 0.58, 0.39, 0.36 and 0.32 ms for the LZBSDy0.1, LZBSDy0.5, LZBSDy1.0 and LZBSDy2.0 glasses, respectively. The decrease of τexp values of 4F9/2 emission level with the increase of Dy3+ ions concentrations is shown in the inset of Fig. 6. This could be due to the energy transfer through non-radiative decay rates (WNR) at higher concentrations. One way of evaluating the WNR is using the Eq. (11) [25]:where and are the radiative and experimental lifetimes, respectively. The evaluated values of τR, τexp and WNR values for LZBSDy0.1, LZBSDy0.5, LZBSDy1.0 and LZBSDy2.0 glasses are presented in Table 7. Moreover, the fluorescence quantum efficiency (η) of an emission level 4F9/2 can be obtained using the Eq. (12) [25] and are presented in the Table 7.

Fig. 6

Decay profiles of 4F9/2 excited level for different concentrations of LZBSDyx glasses. (x = 0.1, 0.5, 1.0 and 2.0 mol %).

Table 7

Experimental (τexp), radiative lifetime (τR), quantum efficiency (η) and non-radiative relaxation rate (WNR) for the 4F9/2 excited level in Dy+3 doped LZBS glasses.

Glass	τexp (ms)	τR (ms)	η (%)	WNR (s−1)
LZBSDy0.1	0.334	0.508	66	1026
LZBSDy0.5	0.331	0.508	65	1052
LZBSDy1.0	0.180	0.508	35	3587
LZBSDy2.0	0.160	0.508	31	4281

Conclusions

In the present study, different concentrations Dy3+ ions doped LZBS glasses of high optical quality were prepared and characterized through XRD, optical absorption, emission and decay techniques. The intensity analysis of absorption levels has been done using the Judd-Ofelt theory. The evaluated intensity parameters follow the trend as Ω2 > Ω4> Ω6 and are further used to evaluate the radiative parameters such as transition probabilities (AR), total transition probability rate (AT), branching ratios (βR), radiative lifetimes (τR) and stimulated emission cross sections (σe) for all the emission transitions of Dy3+ doped LZBS glasses. The PL spectra recorded for different concentration of Dy3+ ions in LZBS glasses exhibited two strong intense emissions in the blue and yellow regions. Reasonably, large stimulated emission cross-section of the 4F9/2→ 6H13/2 transition confirmed that the present glasses are good hosts for laser active materials. From the evaluated chromaticity coordinates, it is concluded that the present glasses are also useful for the generation white light.

Declarations

Author contribution statement

N. Jaidass: Conceived and designed the experiments; Performed the experiments; Wrote the paper.

C. Krishna Moorthi and A. Mohan Babu: Contributed reagents, materials, analysis tools or data.

M. Reddi Babu: Analyzed and interpreted the data.

Funding statement

This work was supported by the grant of funds by DST-SERB, New Delhi (Lr. No. SR/FTP/PS-109/2012) and DAE – BRNS Mumbai (No.2012/34/72).

Competing interest statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.