Optical spectroscopy and luminescence properties of a Tm3+-doped LiKB4O7 glass

Journal of Non-Crystalline Solids 521 (2019) 119477

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Optical spectroscopy and luminescence properties of a Tm3+-doped LiKB4O7 glass

T

⁎

I.I. Kindrata, , B.V. Padlyaka,b, R. Lisieckic, V.T. Adamivb a

University of Zielona Góra, Institute of Physics, Division of Spectroscopy of Functional Materials, 4a Szafrana Str., 65-516 Zielona Góra, Poland Vlokh Institute of Physical Optics, Department of Optical Materials, 23 Dragomanov Str., 79-005 Lviv, Ukraine c Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Division of Optical Spectroscopy, Group for Spectroscopy of Laser Materials, 2 Okólna Str., 50-422 Wrocław, Poland b

A R T I C LE I N FO

A B S T R A C T

Keywords: Borate glasses Tm3+ ions Luminescence spectra Optical absorption Luminescence kinetics Judd–Ofelt analysis

Spectroscopic properties of the Tm-doped borate glass with LiKB4O7 (0.5Li2O–0.5K2O–2B2O3) basic composition are investigated and analysed using optical absorption and photoluminescence (excitation, emission, decay kinetics) experimental techniques as well as Judd–Ofelt analysis. Optical absorption spectra exhibit absorption bands related with transitions of Tm3+ ions. The theoretical and experimental oscillator strengths for observed absorption transitions as well as Judd–Ofelt intensity parameters (Ω2, Ω4, Ω6) have been calculated. Photoluminescence spectra reveal a lot of emission bands in the visible and infrared spectral ranges related with 4f – 4f transitions of Tm3+ ions. Luminescence decay kinetics of several excited states was detailed investigated and analysed. Radiative properties such as radiative transition probabilities (Arad), luminescence branching ratios (β), radiative lifetimes (τrad), quantum efficiency (η), and stimulated emission cross-section (σem) have been estimated. The obtained results show that the investigated LiKB4O7:Tm glass belongs to perspective luminescent and laser materials in the visible and infrared spectral ranges.

1. Introduction In the last decades, research of borate glasses receives a considerable attention as a result of interesting spectroscopic and physical properties of these disordered materials [1–4]. Borate glasses are more promising materials than corresponding borate crystals owing to uncomplicated and low-cost producing technology and possibility to introduce high content of lanthanide ions in glass networks. At present time, borate glasses doped with lanthanide ions are promising luminescent materials for illumination technology and laser techniques [5–11]. Generally, the thulium impurity introduces into the structure of oxide compounds as Tm3+ (4f12, 3H6) and Tm2+ (4f13, 2F7/2). The luminescent properties of the Tm2+ (4f13, 2F7/2) ions in the SrB4O7 crystal were reported in [12,13], but in oxide glasses the Tm2+ luminescence was studied only in [14]. The Tm2+ ions in Tm2+-Tm3+ co-doped germanosilicate glass, fabricated in strongly reduced helium atmosphere, show a strong very broad absorption band in the range of 350–900 nm corresponding to the 4f13 → 4f125d1 transition and a weak absorption band peaked at 1115 nm that is related with the 2F7/2 → 2F5/ 2+ reveals a broad 2 transition [14]. The luminescence spectrum of Tm

⁎

emission band in the spectral range 600–1050 nm due to the 5d – 4f transition and an emission band from 1050 nm to 1300 nm due to the aforementioned 4f – 4f transition [14]. The study of spectroscopic properties of the disordered glass materials doped with Tm3+ ions represents great interest due to their wide applications as luminescent materials, laser media, optical fibre amplifiers and up-converters [15–19]. In particular, Tm3+-doped borate glasses should be considered as potential luminescent and laser materials in the visible and near infrared (NIR) ranges [20–22]. The spectroscopic properties of Tm3+ (4f12, 3H6) ions, generally, are focused on four excited multiplets, namely, 3F4, 3H4, 1G4, and 1D2. Transitions from these excited states result in several emission bands about 0.36 μm, 0.45 μm, and 0.65 μm in the visible spectral range and around 0.8 μm, 1.2 μm, 1.5 μm, and 1.8 μm in the NIR region. Moreover, the up-conversion also can be observed in the Tm3+-doped glasses as conversion of the infrared light into the visible emission [18,19]. During last decades, the spectroscopic properties of Tm-doped oxide glasses were intensively studied [20–28]. In particular, the luminescent properties of the Tm3+ ions were investigated in silicate [23,24], germanate [25], phosphate [26], lithium borate [20], bismuth borate [22], lead borate [27], and antimony borate [28] glasses. Analysis of the

Corresponding author. E-mail address: [email protected] (I.I. Kindrat).

https://doi.org/10.1016/j.jnoncrysol.2019.119477 Received 24 April 2019; Received in revised form 31 May 2019; Accepted 1 June 2019 Available online 21 June 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.

Journal of Non-Crystalline Solids 521 (2019) 119477

I.I. Kindrat, et al. 100

available references shows that optical spectroscopy of the Tm3+ centres in borate glasses represents an actual problem. Spectroscopic properties of the Tm3+ ions in a number of borate glasses, particularly in the lithium potassium tetraborate glass, were not investigated yet. Therefore, the primary purpose of this paper is to study spectroscopic and luminescent properties of the Tm-doped glass with 0.5Li2O–0.5K2O–2B2O3 composition, which is identical to the wellknown lithium potassium tetraborate (LiKB4O7) crystal. Furthermore, radiative transitions probabilities, luminescence branching ratios, radiative lifetimes and stimulated emission cross-section in the LiKB4O7:Tm glass are calculated basing on the Judd-Ofelt analysis [29,30]. 2. Experimental

cm ]

10

-1

[cm ]

3

60

40

1

5

G4 3

4

H6

3

F3+ F2 3

H4

3

3

H5

3

2

20

400

600

800

1000

1200

Wavelenght,

2.1. Preparation of the glass samples

-1

Wavenumber, [

15

6

80

Absorption coefficient,

Absorption coefficient,

-1

[cm ]

25 20

1400

F4

1600

1800

[nm]

0 300

Glass with LiKB4O7 (0.5Li2O–0.5K2O–2B2O3) chemical composition doped with Tm was obtained in the air atmosphere according to standard technology of glass preparation [31]. High chemical purity carbonates (Li2CO3 and K2CO3) and boric acid (H3BO3) (99.9% Aldrich) taken in stoichiometric proportion have been used for synthesis of the LiKB4O7 compound. Thulium impurity was embedded to the glass composition as Tm2O3 oxide of chemical purity 99.9% in amount of 0.4 mol%. Synthesis of the LiKB4O7 compound was carried out in corundum crucible in the air atmosphere using the following multi-step reactions:

1200

1500

1800

2100

2400

[nm]

Fig. 1. Optical absorption spectrum of the LiKB4O7:Tm glass containing 0.4 mol % Tm2O3, registered at T = 300 K.

3. Results and discussion 3.1. Optical absorption of the LiKB4O7:Tm glass Optical absorption spectrum of the Tm-doped LiKB4O7 glass in the 200–2500 nm range is shown in Fig. 1. Observed absorption bands according to [32] were attributed to the Tm3+ transitions from the ground state 3H6 to the following excited states: 3F4, 3H5, 3H4, 3F3, 3F2, and 1G4. The partial energy levels diagram of Tm3+ ions in the LiKB4O7 glass is given below. Strong absorption below 350 nm is ascribed to the fundamental absorption edge of the LiKB4O7 glass matrix. Other absorption bands corresponding to 4f – 4f and/or 4f – 5d transitions of the Tm2+ (4f13, 2F7/2) ions have been not registered in the absorption spectrum of the investigated glass. The spectrum of optical absorption in the LiKB4O7:Tm glass is given in the units of cross-section in Fig. 2. At first, background of un-doped sample was subtracted from the registered optical absorption spectrum of the LiKB4O7:Tm glass. At the second stage, spectrum was calibrated in the cross-sectional unit (σabs) based on the following formula:

4H2 O ↑

170 C 2H2 O ↑ (α ‐НВО2) →o 250 C

CO2 ↑

720 C

900

Wavelenght,

0.5Li2 CO3 + 0.5K2CO3 + 4H3 BO3 →o 0.5Li2 CO3 + 0.5K2CO3 + 4

0.5Li2 CO3 + 0.5K2CO3 + 2B2 O3 →o LiKB4 O7

600

(1)

The Tm-doped LiKB4O7 glass samples were prepared by rapid cooling of the melt that was overheated about 100 K than melting temperature (Tmelt = 807 °C (1080 K) for LiKB4O7) for excluding of crystallisation [31]. For optical absorption and luminescence measurements the samples of obtained glasses have been cut to the 10 mm × 6 mm × 2 mm size and polished. 2.2. Research methods and equipment Structural investigations were performed with usage commercial Xray diffractometer (model DRON-3). The obtained Tm-doped samples reveal typical XRD pattern that is identical to XRD pattern of the LiKB4O7:Sm glass, presented in our article [6]. The observed XRD pattern confirms disordered glassy structure of the obtained LiKB4O7:Tm samples. The optical absorption spectrum of the LiKB4O7:Tm glass was registered using Cary 5000 (“Agilent Technologies”) spectrophotometer, working in the UV–Vis–NIR spectral range. The luminescence (emission and excitation) spectra in the UV–Vis spectral range were registered using FluoroMax–4 (“Horiba Yvon”) spectrofluorimeter. Photoluminescence emission spectra in the NIR spectral range were recorded with Dongwoo system (model DM711) that consists of excitation and emission monochromators, photomultiplier tube, and InGaAs detector. Luminescence decay kinetics was registered using experimental setup, which includes Continuum Surelite I Optical Parametric Oscillator (OPO) pumped by third harmonic (λ = 355 nm) of YAG:Nd pulsed laser for excitation, grating GDM monochromator with 1000 mm focal length for monochromatisation of emitted light, Hamamatsu R928 photomultiplier tube and EG&G InSb detector for detection of luminescence emission in the visible and NIR spectral regions, respectively, and Tektronix digital oscilloscope for recording of luminescence decay curves.

Wavelenght, 1 8 0 0 1 50 0

H6

2

cm ]

-20

5 00

0.7

3

3

F3+ F2

0.6 3

0.5 0.4

450

H4

3

abs

[ 10

[nm]

900 3

0.8

Absorption cross-section,

12 0 0

H5

3

F4

0.3 0.2 1

G4

0.1 0.0 6000

8000

10000

12000

14000

20000

22000

-1

Wavenumber, [cm ] Fig. 2. Optical absorption bands observed in the LiKB4O7:Tm glass, calibrated in the absorption cross-section (σabs) unit. 2

Journal of Non-Crystalline Solids 521 (2019) 119477

I.I. Kindrat, et al.

α (∼ ν) σabs (∼ ν) = N

Table 2 Comparison of the Judd-Ofelt intensity parameters (Ωλ × 10−20 cm2) for Tm3+ centres in borate glasses with different basic compositions.

(2)

where α is the absorption coefficient given in [cm−1] as a function of wavenumber ν͠ and N = 3.11 × 1019 cm−3 is the number of Tm3+ ions in the LiKB4O7:Tm glass per cm3. 3.2. Judd-Ofelt analysis of Tm3+ centres in the LiKB4O7:Tm glass Experimental oscillator strengths (fexp) of the Tm3+ absorption bands were computed by the following equation [33]:

fexp =

2303me c 2 NA πe 2

∫ ε (∼ν ) dν∼ = 4.32 × 10−9 ∫ ε (∼ν ) dν∼

A (ν͠ ) cl

(4)

8π 2mcν͠ (n2 + 2)2 3h (2J + 1) 9n

∑

(5)

5972 1.78 8259 1.09 12,655 3.61 14,564 2.73 21,367 0.56 −20 4.37 × 10 0.05 × 10−20 1.41 × 10−20 6.75 × 10−7 0.22

F4 H5 3 H4 3 F3 + 3F2 1 G4 Ω2 (cm2) Ω4 (cm2) Ω6 (cm2) σrms δrms 3

4.37

0.05

1.41

Present work

2.19 3.05 4.12 3.65 2.92 4.65

0.03 0.45 0.04 1.32 0.59 1.24

0.50 0.58 0.46 1.37 1.55 1.32

[20] [20] [21] [22] [27] [28]

β (ψ′, ψ) =

∑

Ωλ | 〈ψ′ ‖U λ‖ ψ〉 |2 + n3AMD′ ,

λ = 2,4,6

Arad (ψ′, ψ) ∑ Arad (ψ′, ψ) ψ

τrad (ψ′) =

1 ∑ Arad (ψ′, ψ) ψ

(7)

(8)

where ∑ Arad (ψ′, ψ) represents summation of the probabilities of raψ

diative transitions from an excited state ψ' to all lower states ψ. Obtained probabilities of radiative transitions (Arad), branching ratios of luminescence (β) as well as radiative lifetimes (τrad) for the Tm3+ excited states in the LiKB4O7:Tm glass are given in the Table 3. 3.3. Tm3+ luminescence spectra in the LiKB4O7:Tm glass Luminescence emission spectra of the LiKB4O7:Tm glass registered at different wavelengths of photoexcitation are presented in Fig. 3 (a – d). Let us detailed consider the observed emission spectra. Fig. 3 (a) presents luminescence spectra of the LiKB4O7:Tm glass (solid curve) and undoped LiKB4O7 glass (dashed curve) registered upon excitation 287 nm. Broad band in the range 400–600 nm with maximum about 470 nm registered in both LiKB4O7:Tm and LiKB4O7 glasses is related with intrinsic luminescence of the LiKB4O7 glass [38]. Based on our study [38] of intrinsic luminescence in undoped borate glasses, including LiKB4O7 glass, one can state that observed intrinsic luminescence is related with recombination of electrons with O– hole centres created by UV-excitation. Such intrinsic luminescence in the undoped LiKB4O7 glass reveals second order luminescence kinetics with lifetimes in the microseconds range due to trapping and re-trapping of electrons by shallow electronic traps before their recombination with O– hole centres [38]. The Tm3+ ions in the LiKB4O7:Tm glass under photoexcitation at 287 nm (3H6 → 3P0 excitation transition) reveal intense

Table 1 Band barycentre positions (ν͠ ), experimental (fexp) and calculated (fcal) oscillator strengths of the observed Tm3+ optical absorption transitions, Judd-Ofelt intensity parameters (Ω2, Ω4, and Ω6), absolute (σrms) and relative (δrms) root mean square deviations for investigated LiKB4O7:Tm glass.

3

LiKB4O7:Tm or 16.6Li2O–16.6K2O–66.4B2O3–0.4Tm2O3 49Li2CO3–50H3BO3–1Tm2O3 39Li2CO3–10BaCO3–50H3BO3–1Tm2O3 15Na2O–15K2O–69.5B2O3–0.5Tm2O3 45Bi2O3–55B2O3–1.5Tm2O3 PbO–B2O3–Al2O3–WO3–Tm2O3 20CaO–40Sb2O3–39B2O3–1Tm2O3

where AED and AMD are the electric and magnetic dipole contributions, AMD′ is the rate of magnetic dipole vacuum spontaneous emission reported in [35]. The luminescence branching ratio (β) and radiative lifetime (τrad) of an excited state ψ' were calculated using well-known relations [36]:

where fED and fMD are the electric and magnetic dipole contributions, n is the refractive index, h is the Planck constant, J is the total angular momentum of the ground state, Ωλ (λ = 2, 4, and 6) are the Judd-Ofelt intensity parameters [29,30], |〈ψ‖Uλ‖ψ′〉|2 are squared reduced matrix elements for transition ψ → ψ' [32,34], and fMD′ is the magnetic dipole vacuum oscillator strength [35]. Magnetic dipole contribution for the 3 H6 → 3H5 magnetic dipole transition was taken into account for calculation of the total oscillator strength (fcal). The Judd-Ofelt intensity parameters were calculated with usage of Eqs. (3) and (5) as well as the method of least squares [36]. The JuddOfelt parameters (Ω2, Ω4, and Ω6) for the LiKB4O7:Tm glass (Tm2O3 amount – 0.4 mol%) are given in Table 1. Besides this, the comparison of the Judd-Ofelt parameters for the investigated LiKB4O7:Tm glass and some other Tm3+-doped borate glasses is given in Table 2. The phenomenological Judd-Ofelt intensity parameters (Ω2, Ω4, and Ω6) in the

fexp (×10−6)

References

(6)

Ωλ | 〈ψ ‖U λ‖ ψ′〉 |2 + nfMD′

ν͠ (cm−1)

Ω6

Arad (ψ′, ψ) = AED + AMD 64π 4e 2ν͠ 3 n (n2 + 2)2 = 3h (2J + 1) 9

λ = 2,4,6

Transitions from the ground state 3H6 to

Ω4

LiKB4O7:Tm glass exhibit the Ω2 > Ω6 > Ω4 trend that has been reported also for Tm3+-doped lithium borate [20], mixed alkali borate [21], bismuth borate [22], lead borate [27] and antimony borate [28] glasses. Using obtained Judd-Ofelt intensity parameters the probabilities of radiative transitions, branching ratios of luminescence, and radiative lifetimes can be evaluated [36,37]. The rate of spontaneous emission or probability of radiative transition (Arad) from an excited state ψ' to one of a lower state ψ was calculated according to the following formula:

where A (ν͠ ) represents the absorbance or optical density as a function of wavenumber ν͠ , c [mol/l] is the Tm content, and l [cm] is the thickness of sample or optical pathlength. The experimental oscillator strengths for five well separated absorption bands and one group of overlapped absorption bands (3H6 → 3F3, 3F2 transitions) in the LiKB4O7:Tm glass containing 0.4 mol% of Tm2O3 are given in Table 1. Calculated theoretical oscillator strength (fcal) for an induced transition from a ground state ψ to an excited state ψ' is expressed by relation:

fcal = fED + fMD =

Ω2

(3)

The electron's mass (me), the velocity of light (c), the Avogadro's number (NA), and the electron's charge (e) are taken in the CGS units. The molar absorptivity ε (ν͠ ) as a function of wavenumber ν͠ was obtained using the Beer–Lambert's law:

ε (ν͠ ) =

Glass composition

fcal (×10−6) 1.82 1.28 2.69 2.58 0.57

3

Journal of Non-Crystalline Solids 521 (2019) 119477

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bands in the range 250–300 nm according to [32] refer to transitions from the ground state 3H6 to the excited states 1I6, 3P0, 3P1, and 3P2. The luminescence excitation spectrum of the LiKB4O7:Tm glass, registered by monitoring of intensity of the Tm3+ emission band at 786 nm corresponding to the 3H4 → 3H6 transition, is presented in Fig. 4 (b). This spectrum exhibit strong excitation band peaked at 358 nm (3H6 → 1D2 transition), slightly weaker excitation band with a maximum at 467 nm (3H6 → 1G4 transition), weakly resolved excitation bands in the range 250–300 nm (3H6 → 1I6, 3H6 → 3P0, 3H6 → 3P1, and 3 H6 → 3P2 transitions) as well as asymmetrical excitation band in the range 650–700 nm that corresponds to overlapped 3H6 → 3F2 and 3 H6 → 3F3 transitions. It should be noted that luminescence excitation spectra reveal five bands belonging to transitions from the ground state 3H6 to the excited states 1D2, 1I6, 3P0, 3P1, and 3P2 (see Figs. 4 (a) and 4 (b)), which have been not observed in the optical absorption spectrum (see Fig. 1). Based on detailed analysis of the observed optical absorption, photoluminescence (emission and excitation) spectra in the LiKB4O7:Tm glass was constructed partial energy levels diagram of the Tm3+ ions (see Fig. 5).

Table 3 Radiative transition probabilities (Arad), luminescence branching ratios (β), and radiative lifetimes (τrad) for excited states of the Tm3+ ions in the LiKB4O7:Tm glass. Excited state, ψ'

Final state, ψ

Arad (s−1)

β (%)

τrad (μs)

3

3

H6 H6 F4 3 H6 3 F4 3 H5 3 H6 3 F4 3 H5 3 H4 3 H6 3 F4 3 H5 3 H4 3 F3 3 H6 3 F4 3 H5 3 H4 3 F3 3 F2 3 H6 3 F4 3 H5 3 H4 3 F3 3 F2 1 G4

116 205 8 974 86 22 1215 91 231 1 548 500 128 10 0 577 117 683 202 37 5 1558 14,507 83 1402 835 314 110

100 96.4 3.6 90.0 8.0 2.0 79.0 5.9 15.0 0.1 46.2 42.2 10.8 0.8 0 35.6 7.2 42.1 12.5 2.3 0.3 8.3 77.1 0.4 7.5 4.4 1.7 0.6

8620 4695

F4 H5

3

3

3 3

H4

3

F3

3

F2

1

G4

1

D2

924

650

843

617

3.4. Luminescence kinetics of Tm3+ emission in the LiKB4O7:Tm glass and its analysis 53

The luminescence decay curves of several emission bands of Tm3+ ions in the LiKB4O7 glass are given in Fig. 6. The luminescence decay kinetics was registered for emission transitions originated from the following excited states: 1D2, 1G4, 3H4, and 3F4. The obtained experimental luminescence lifetimes are presented in Fig. 6 and Table 4. Quantum efficiency of the corresponding emitting level was calculated as ratio of the experimental lifetime, evaluated from luminescence decay curves, to the radiative lifetime that was determined using the Judd–Ofelt analysis (η = τexp/τrad). The quantum efficiencies of the 1D2, 1 G4, 3H4, and 3F4 excited states in the LiKB4O7:Tm glass are listed in Table 4. Let us detailed consider the registered luminescence decay curves. Fig. 6 (a) shows luminescence decay curve of emission band that was monitored at 452 nm (1D2 → 3F4 transition). Single exponent with lifetime value 14 μs approximates this decay curve. The obtained experimental lifetime is smaller than the radiative lifetime. This fact indicate significant non-radiative processes in the investigated LiKB4O7:Tm glass. Generally, the non-radiative relaxation is caused by the energy transfer process and multiphonon relaxation [39,40]. In our opinion, the energy transfer between Tm3+ ions via cross-relaxation mechanism dominates in the 1D2 state deactivation. The 1D2 excited state in the LiKB4O7:Tm glass can be depopulated by the following cross-relaxation channels (see Fig. 5): 1 D2 → 1G4 (6597 cm−1) ⇒ 3H6 → 3F4 (5972 cm−1) 1 D2 → 3F3 + 3F2 (13,400 cm−1) ⇒ 3H6 → 3H4 (12,655 cm−1) 1 D2 → 3H4 (15,309 cm−1) ⇒ 3H6 → 3F3 + 3F2 (14,564 cm−1) The luminescence decay kinetics of the 1G4 → 3F4 emission monitored at 650 nm is given in Fig. 6 (b). The registered decay curve is slightly non-exponential. Therefore, the average lifetime (τavg) was estimated according to the following formula [40]:

band peaked at 452 nm (1D2 → 3F4 transition) and weaker bands about 358 nm (1D2 → 3H6 transition), 467 nm (1G4 → 3H6 transition), and 511 nm (1D2 → 3H5 transition). Luminescence emission spectrum of the LiKB4O7:Tm glass registered upon excitation at 358 nm (3H6 → 1D2 excitation transition) is presented in Fig. 3 (b). Spectrum shows intense narrow band with maximum at 452 nm (1D2 → 3F4 transition) and several weak bands about 467 nm (1G4 → 3H6 transition), 511 nm (1D2 → 3H5 transition), 650 nm (1G4 → 3F4 transition), 665 nm (1D2 → 3H4 transition), 755 nm (1D2 → 3 F3 transition), and 785 nm (1G4 → 3H5 and 3H4 → 3H6 overlapped transition). Basing on emission spectra presented in Fig. 3 (a) and (b) one can notice that blue emission band with a maximum at 452 nm is most intense in comparison with other emission bands. This result well correlates with radiative properties of the LiKB4O7:Tm glass (see Table 3) predicted basing on Judd-Ofelt analysis. This intense emission that corresponds to the 1D2 → 3F4 transition reveals highest radiative transition probability Arad = 14,507 s−1 and very high branching ratio that equals 77.1%. In Fig. 3 (c) is presented luminescence emission spectrum of the LiKB4O7:Tm glass registered under excitation at 467 nm (3H6 → 1G4 excitation transition). Spectrum consists of two emission bands. First band at 650 nm is related with 1G4 → 3F4 transition. Second band in the infrared range can be deconvoluted onto two very close bands peaked at 782 nm and 786 nm, which correspond to the 1G4 → 3H5 and 3H4 → 3 H6 transitions, respectively. At last, the LiKB4O7:Tm glass upon excitation at 808 nm (near the 3H6 → 3H4 excitation transition) reveals emission band with a maximum at 1805 nm, which correspond to 3 F4 → 3H6 transition (see Fig. 3 (d)). Excitation spectrum of the Tm3+ luminescence registered in the LiKB4O7:Tm glass by monitoring of intensity of the 1D2 → 3F4 emission band at 452 nm is showed in Fig. 4 (a). Excitation spectrum reveals intense excitation band with a maximum at 358 nm and several weak excitation bands in the range 250–300 nm. The strong excitation band is related with 3H6 → 1D2 transition of Tm3+ ions. The weak excitation

τavg =

∫ t⋅I (t )⋅dt ∫ I (t )⋅dt

(9)

where I(t) is luminescence intensity at time t. The estimated average lifetime equals 98 μs. The non-radiative relaxation of the 1G4 excited state in the LiKB4O7:Tm glass can be carried out by the following crossrelaxation channels (see Fig. 5): 1 G4 → 3F3 + 3F2 (6803 cm−1) ⇒ 3H6 → 3F4 (5972 cm−1), 1 G4 → 3H4 (8712 cm−1) ⇒ 3H6 → 3H5 (8259 cm−1), 1 G4 → 3H5 (13,108 cm−1) ⇒ 3H6 → 3H4 (12,655 cm−1), 1 G4 → 3F4 (15,395 cm−1) ⇒ 3H6 → 3F3 + 3F2 (14,564 cm−1). 4

Journal of Non-Crystalline Solids 521 (2019) 119477

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Fig. 3. Luminescence emission spectra of the LiKB4O7:Tm glass containing 0.4 mol% Tm2O3, registered under excitation at 287 nm (a), 358 nm (b), 467 nm (c), and 808 nm (d). Observed emission bands are identified and denoted in Figure.

relation:

The non-exponentiality of the luminescence decay kinetics of the G4 level was analysed by Inokuti-Hirayama model [41] with the aim to discuss the Tm3+ − Tm3+ interaction. The Inokuti-Hirayama model describes luminescence kinetics in the presence of energy transfer between randomly distributed donors and acceptors when the donor acceptor energy transfer is much faster than energy migration between donors [41]. This model is successfully applied when energy is transferred from an excited rare-earth ion to those non-excited rare-earth ions [22,27,40]. In our case, the Tm3+ ions in the 1G4 excited state are donors, whereas Tm3+ ions in the ground 3H6 state are acceptors. According to the Inokuti-Hirayama model, luminescence decay curve is described by the following formula: 1

α=

⎜

(11)

where Г(1–3/S) is the gamma function to be approximately equals 1.77 for S = 6, 1.43 for S = 8, and 1.30 for S = 10, N0 is the acceptors concentration, R0 is the critical transfer distance between acceptor and donor at which the rate of energy transfer equals to the rate of spontaneous emission [41]. Theoretical decay curves for various S calculated with applying of the Inokuti-Hirayama model are presented in Fig. 6 (b). Best fit of experimental decay curve was obtained for S = 6. This fact indicates that the electric dipole-dipole interaction is responsible for the energy transfer via cross-relaxation mechanism between Tm3+ ions. Fitting of the luminescence decay curve of the 1G4 state results in the following values of the energy transfer parameter (α) and critical transfer distance (R0): α = 0.22 and R0 = 9.84 Å. The luminescence decay kinetics of the 3H4 → 3H6 emission monitored at 786 nm is given in Fig. 6 (c). The average lifetime of registered non-exponential decay curve is estimated to be equals 81 μs. Non-radiative deactivation of the 3H4 excited state is attributed to the

3

⎛ t t s⎞ I (t ) = I0 exp⎜− − α⎛ ⎞ ⎟ τ0 ⎝ τ0 ⎠ ⎠ ⎝

4π 3 Γ ⎛1 − ⎞ N0 R 03 3 ⎝ S⎠

⎟

(10)

where I(t) is the luminescence intensity at time t after action of excitation pulse, I0 is the initial intensity, τ0 is the lifetime of the donors without acceptors. Parameter S indicates dipole-dipole (S = 6), dipolequadrupole (S = 8) or quadrupole-quadrupole (S = 10) interactions between ions. Energy transfer parameter (α) is defined by the following 5

Journal of Non-Crystalline Solids 521 (2019) 119477

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Fig. 4. Luminescence excitation spectra of the LiKB4O7:Tm glass containing 0.4 mol% Tm2O3, registered by monitoring intensity of emission band at 452 nm (1D2 → 3 F4 transition) (a) and 786 nm (3H4 → 3H6 transition) (b). Observed excitation bands are identified and denoted in Figure.

40

3

P 3 2 P 3 1 P 1 0 I6

35 30 -1

cm ]

1

D2

25

358 nm 452 nm 511 nm 665 nm 755 nm

287 nm 358 nm 467 nm

10

G4

3 3F2 F3 3 H4

15 467 nm 650 nm 782 nm

Energy [

1

20

3

H5 F4

3

786 nm 1.8 m

5

3

0

excitation

emission

H6

cross-relaxation

3+

Tm ions 3+

Fig. 5. The partial energy diagram of Tm channels via cross-relaxation mechanism.

ions in the LiKB4O7:Tm glass showing luminescence emission and excitation transitions as well as energy transfer

recorded optical absorption and luminescence bands in the LiKB4O7:Tm glass are characterised by significant inhomogeneous broadening (see Figs. 2–4) caused by multisite character of the Tm3+ luminescence centres as well as glassy-like structural disordering. That is why the obtained values of α and R0 are relatively high even at low Tm2O3 content of 0.4 mol%. Evaluation of the distance between Tm3+ ions is not simple experimental task and needs advanced techniques. In the very general consideration of random distribution of ions, the mean distance between Tm3+ ions can be approximately estimated as the radius of a sphere having per-particle volume 1/N as follow:

following cross-relaxation channel: 3 H4 → 3F4 (6683 cm−1) ⇒ 3H6 → 3F4 (5972 cm−1) Besides this, the 3H4 → 3H6 ⇔ 3H6 → 3H4 energy migration process between Tm3+ ions also can be responsible for non-radiative relaxation of the 3H4 excited state. Fit of luminescence kinetics of the 3H4 → 3H6 emission by the Inokuti-Hirayama model also confirms dipole-dipole interaction between Tm3+ ions. Fitting procedure gives α = 4.16 and R0 = 26.2 Å. Generally, the energy transfer parameter (α) as well as the critical transfer distance (R0) is proportional to spectral overlapping of donor emission and acceptor absorption ∫ σDem(λ)σAabs(λ)dλ [40]. The 6

Journal of Non-Crystalline Solids 521 (2019) 119477

I.I. Kindrat, et al.

Fig. 6. Luminescence decay curves of emission transitions originated from the 1D2 (a), 1G4 (b), 3H4 (c), and 3F4 (d) states in the LiKB4O7:Tm glass. Decay curves for the 1G4 and 3H4 levels were fitted by the Inokuti-Hirayama model with different S = 6, 8, 10.

with usage of Eq. (9) equals about 41 μs. Obtaining of decay curve with strong noises and short lifetime value is a rather unexpected result. The radiative lifetime (τrad) of the 3F4 level according to the Judd–Ofelt analysis is 8620 μs. The small quantum efficiency of the 3F4 → 3H6 emission clearly indicates strong non-radiative processes of the 3F4 level deactivation in the LiKB4O7:Tm glass. The cross-relaxation energy transfer between Tm3+ ions in the first excited 3F4 state and ground 3H6 state is impossible. Of course, the 3F4 → 3H6 ⇔ 3H6 → 3F4 energy migration process may occur. But, in our opinion, strong non-radiative deactivation of the 3F4 excited state is mainly attributed to multiphonon relaxation. The multiphonon relaxation rate to bridge the gap between 3 H6 and 3F4 states in borate glasses with maximal energy of phonons about 1400 cm−1 is slightly less than 104 s−1 [40,42]. Comparison of multiphonon relaxation rate with calculated emission rate Arad = 116 s−1 gives an answer about reason of small quantum efficiency of the 3F4 → 3H6 transition in the LiKB4O7:Tm glass. It should be noted that low quantum efficiency about 1% for the 3F4 state also was observed in the Ca4GdO(BO3)3 crystal [43]. Obtained our results on luminescence kinetics have a scientific interest, because published spectroscopic studies of the Tm-doped borate glasses [22,27,44,45] describe luminescence kinetics only one or two excited states of the Tm3+ ions. Let us compare the obtained results

Table 4 Experimental (τexp) and radiative (τrad) lifetimes as well as quantum efficiency (η) of the 1D2, 1G4, 3H4, and 3F4 excited states of Tm3+ ions in the LiKB4O7:Tm glass. Excited state

τexp (μs)

τrad (μs)

η (%)

1

14 98 81 41

53 617 924 8620

26.4 15.9 8.8 0.5

D2 G4 3 H4 3 F4 1

1

3 ⎞3 RTm − Tm = ⎛ 4 πN Tm ⎠ ⎝ ⎜

⎟

(12) 3+

According to this formula the mean distance between Tm ions in the LiKB4O7:Tm glass approximately equals 19.7 Å. This value is higher than obtained critical transfer distance (R0) for the 1G4 excited state and lower than corresponding R0 value for the 3H4 state. Hence, the radiative emission rate of the 1G4 state is greater than the cross-relaxation rate, whereas the energy transfer rate via cross-relaxation mechanism is much higher than the spontaneous emission rate from the 3H4 state. The luminescence decay kinetics of the 3F4 → 3H6 emission monitored at 1805 nm is given in Fig. 6 (d). The average lifetime estimated 7

Journal of Non-Crystalline Solids 521 (2019) 119477

I.I. Kindrat, et al.

βλ5I (λ ) ∫ λI (λ ) dλ

-1

Wavenumber, [cm ] 5200

em

Cross-section,

[ 10

-20

2

cm ]

0.5

0.4

p = 0.2

-0.3

p=0 1600

1650

1700

1750

1800

1850

1900

[nm]

(14)

4. Conclusions

abs

0.3

p = 0.4

-0.2

where p is the inversion population definited as a fraction of the Tm3+ centres in the excited state. Calculated effective cross-section (σeff) of the 3F4 → 3H6 transition in the LiKB4O7:Tm glass for different p values is presented in Fig. 8. The positive effective cross-section is observed even for p = 0.1 and σeff monotonously grows with the inversion population increasing. Positive effective cross-section at small p values is related with large splitting between absorption and emission band (see Fig. 7). As result, absorption at wavelength of emission maximum is very small. This effect can compensate relatively high non-radiative processes in the LiKB4O7:Tm glass. Weak absorption at wavelength of emission peak and positive effective cross-section at small inversion population can be useful for laser generation. The experimental verification of possibility and efficiency of laser generation about 1800 nm in the LiKB4O7:Tm glass is in progress.

ν͠ min

5400

p = 0.6

-0.1

σeff (λ ) = pσem (λ ) − (1 − p) σabs (λ )

tively. The Stokes shift between barycentre wavenumbers of absorption

5600

p = 0.8

and emission amounts to 401 cm−1. The highest cross-sections of absorption at 1662 nm and stimulated emission at 1805 nm are 0.36 × 10−20 cm2 and 0.45 × 10−20 cm2, respectively (see Fig. 7). Obtained stimulated emission cross-section in the LiKB4O7:Tm glass is higher than its evaluated for cadmium zinc phosphate glass (0.15 × 10−20 cm2) [54] and multi-component silicate glass (0.389 × 10−20 cm2) [24], comparable with silica glass (0.46 × 10−20 cm2) [23], and lower than emission cross-section reported for germanate glass (0.565 × 10−20 cm2) [25] and Y2Te4O11 microcrystalline powder (1.12 × 10−20 cm2) [55]. In order to eliminate the self-absorption the effective emission crosssection (σeff) was evaluated with usage of the following equation:

ν͠

5800

p=1

0.0

Fig. 8. Effective emission cross-section (σeff) of the 3F4 → 3H6 transition in the LiKB4O7:Tm glass, calculated for several values of inversion population.

(13)

6000

0.1

Wavelength,

∫ σ (ν͠ ) dν͠ = 0.5 are located at 5972 cm−1 and 5571 cm−1, respec-

6200

0.2

-0.4 1550

where β is the branching ratio of luminescence, n is the refractive index, and τrad is the radiative lifetime. The absorption and luminescence spectra between the ground 3H6 state and first excited 3F4 state are presented in Fig. 7 in absorption (σabs) and stimulated emission crosssection (σem) units. The Stokes shift between maximum of absorption at 1662 nm and emission at 1805 nm equals 477 cm−1. Barycentre energies of absorption and emission band obtained using relation

6400

2

cm ] [ 10 eff

It should be noted that numerous studies [46–48] of spectroscopic properties of Tm3+ ions in different crystals are mainly focused on infrared emission, peaked about 1.8 μm for diode-pumped laser generation. Many studies [49–53] of Tm3+-doped glasses are focused on visible luminescence of Tm3+ ions for application in solid-state lighting. In particular, combination of blue emission of Tm3+ ions with green and red emission of Tb3+ and Eu3+ ions is widely used to obtain white light [52,53]. Moreover, significant number of works [23–26] also analyse the infrared emission of Tm3+-doped glasses for potential fibre lasers. Let us examine observed infrared emission about 1.8 μm in the LiKB4O7:Tm glass. The stimulated emission cross-section (σem) is important criterion to estimate possible applications of optical materials [1,40]. The intensity of luminescence I(λ) as a function of wavelength λ given in Fig. 3 (d) can be replaced by cross-sectional unit according to the Füchtbauer–Ladenburg formula:

8πn2cτrad

0.3

Effective cross-section,

3.5. Cross-section of stimulated emission of the 3F4 → 3H6 infrared band

σem (λ ) =

0.4

-20

0.5

with those reported for some Tm-doped oxide glasses. Previously were reported the following lifetimes: 14.7 μs for the 1D2 state in sodium borate glass [44], 73 μs for the 1G4 multiplet in bismuth borate glass [22], 74 μs and 34 μs for the 1G4 state in different lead borate glasses [27,45], 50 μs and 190 μs for the 3F4 and 3H4 levels, respectively, in phosphate glass [26], 420 μs and 20 μs for the 3F4 and 3H4 states, respectively, in silicate glass [23]. The obtained experimental lifetimes are comparable with corresponding published data for other glasses, especially for borate glasses.

The Tm-doped borate glass with LiKB4O7 (0.5Li2O–0.5K2O–2B2O3) basic composition has been studied in detail using optical absorption, luminescence (emission, excitation, decay kinetics) spectroscopy, and Judd–Ofelt theory. Based on the obtained results analysis it is possible to summarise the following:

0.2

0.1

• The thulium dopant is incorporated into the structure of the LiKB O 4

0.0 1550

1600

1650

1700

1750

Wavelength,

1800

1850

1900

1950

•

[nm]

Fig. 7. Absorption and emission spectra of the F4 ↔ H6 transition in the LiKB4O7:Tm glass, calibrated in the absorption (σabs) and stimulated emission cross-section (σem) units. 3

3

• 8

7

glass exclusively as Tm3+ (4f12, 3H6) ions, which exhibit characteristic absorption and luminescence spectra. Optical absorption spectrum of the Tm-doped LiKB4O7 glass reveals several 4f – 4f transitions of Tm3+ ions in the visible and infrared spectral ranges. The Tm3+ optical spectra have been analysed within the Judd–Ofelt

Journal of Non-Crystalline Solids 521 (2019) 119477

I.I. Kindrat, et al.

• •

•

theory. The oscillator strengths (fexp and fcal) and the Judd–Ofelt parameters (Ω2, Ω4, and Ω6) were evaluated. Probabilities of radiative transitions (Arad), branching ratios of luminescence (β), and radiative lifetimes (τrad) have been estimated. Photoluminescence spectra of the LiKB4O7:Tm glass reveal intense blue (1D2 → 3F4 transition, λmax = 452 nm), red (1G4 → 3F4 transition, λmax = 650 nm) as well as infrared (3H4 → 3H6 transition, λmax = 786 nm and 3F4 → 3H6 transition, λmax = 1805 nm) emission bands. The luminescence decay kinetics of the 1D2, 1G4, 3H4, and 3F4 excited states of Tm3+ centres in the LiKB4O7:Tm glass were detailed studied and discussed. Based on the Inokuti-Hirayama model it was found that non-radiative processes mainly are related with crossrelaxation energy transfer between Tm3+ ions coupled by dipoledipole interaction. The cross-section of stimulated emission (σem) and effective emission cross-section (σeff) for the 3F4 → 3H6 transition have been calculated. Based on obtained results it was suggested that efficient laser generation around 1.8 μm can be obtained in the LiKB4O7:Tm glass.

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[49]

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10

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