Near-infrared emission and upconversion in Er3+-doped TeO2–ZnO–ZnF2 glasses

Journal of Luminescence 140 (2013) 38–44

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Near-infrared emission and upconversion in Er3 þ -doped TeO2–ZnO–ZnF2 glasses A. Miguel a, R. Morea b, J. Gonzalo b, M.A. Arriandiaga c, J. Fernandez a,d, R. Balda a,d,n a

Departamento de Fı´sica Aplicada I, Escuela Superior de Ingenierı´a, Universidad del Paı´s Vasco UPV/EHU, Alda. Urquijo s/n 48013 Bilbao, Spain Instituto de Optica, Consejo Superior de Investigaciones Cientı´ficas CSIC, Serrano 121, 28006 Madrid, Spain c Departamento de Fı´sica Aplicada II, Facultad de Ciencia y Tecnologı´a, Universidad del Paı´s Vasco UPV/EHU, Apartado 644, Bilbao, Spain d Materials Physics Center CSIC-UPV/EHU and Donostia International Physics Center, 20018 San Sebastian, Spain b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 December 2012 Received in revised form 21 February 2013 Accepted 28 February 2013 Available online 13 March 2013

We have investigated the near infrared emission and upconversion of Er3 þ ions in two different compositions of glasses based on TeO2, ZnO, and ZnF2 for different ErF3 concentrations (0.5, 1, 2, and 3 wt%). Judd–Ofelt intensity parameters have been determined and used to calculate the radiative transition probabilities and radiative lifetimes. The infrared emission at around 1532 nm corresponding to the 4I13/2-4I15/2 transition is broader by nearly 30 nm if compared to silica based glasses. The stimulated emission cross section is higher for the glass with the lowest content of ZnF2 which also shows higher values of the figure of merit for bandwidth. On the other hand, the lifetimes of the excited states are longer for the glass with the highest content of ZnF2. Green and red emissions corresponding to transitions (2H11/2,4S3/2)-4I15/2 and 4F9/2-4I15/2 have been observed under excitation at 801 nm and attributed to a two photon process. The temporal evolution of the green emission suggests the presence of excited state absorption and energy transfer upconversion processes to populate the 4S3/2 level. In the case of the red emission, its increase as ErF3 concentration increases together with its temporal behavior indicate that for ErF3 concentrations higher than 0.5 wt%, level 4F9/2 is populated by multiphonon relaxation from level 4S3/2 and energy transfer processes. & 2013 Elsevier B.V. All rights reserved.

Keywords: Laser spectroscopy Erbium Fluorotellurite glasses Upconversion Optical properties

1. Introduction Erbium doped glasses have been a subject of increasing interest in the last years due to their important optical properties which make them suitable for applications in photonics such as lasers, optical amplifiers, and frequency upconverters [1–5]. In particular, they have played an important role in the development of broadband erbium-doped fiber amplifiers (EDFA) due to the 4 I13/2-4I15/2 transition around 1.5 mm where standard optical communications fibers present low losses. In addition to the excellent infrared amplifying and lasing properties of trivalent erbium its rich energy level structure and the possibility of exciting transitions 4I15/2-4I11/2 and 4I15/2-4I9/2 at around 980 and 800 nm respectively, with commercial laser diodes, makes this ion attractive for infrared to visible upconversion applications such as high density optical storage, color displays, optoelectronics, and medical diagnosis. An important factor to be considered for both kinds of applications is the glass host. Though fluoride n Corresponding author at: Departamento de Fı´sica Aplicada I, Escuela Superior de Ingenierı´a, Universidad del Paı´s Vasco UPV/EHU, Alda. Urquijo s/n 48013 Bilbao, Spain. Tel.: þ 34 94 601 4258; fax: þ34 94 601 4178. E-mail addresses: [email protected], [email protected] (R. Balda).

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.02.059

glasses have been extensively studied due to their low phonon energies, oxide glasses are more appropriate for practical applications due to their high chemical durability and thermal stability. As it is also known, oxyfluoride glasses combine the advantages of oxide glasses with the good optical properties of fluorides [6]. On the other hand, the addition of fluorine ions which can release hydrogen species and reduce the OH presence has a great influence in the quenching processes of the radiative emission of excited levels of Er3 þ ions. Tellurite glasses have smaller phonon energies than other oxide glasses such as silicate, phosphate, and borate glasses [7,8] which increases the quantum efficiency from excited states of rare-earth ions in these matrices. 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 mm), which make them promising materials for photonic applications such as optical fiber amplifiers, upconversion lasers, nonlinear optical devices, and others [9–17]. The broad bandwidth of the 1.5 mm emission in tellurite glasses, which is about twice the one of silica-based EDFA, makes these glasses attractive as broadband amplifiers. In fact, broadband Er-doped fiber amplifiers have been achieved by using tellurite-based fibers as the erbium host [13,14]. Concerning upconversion processes, the multiphonon

relaxation rates are critical in determining the upconversion efficiency. Since tellurite glasses have the lowest maximum phonon energy among oxide glasses and a larger refractive index, both beneficial for radiative transition of RE ions, they are suitable for upconversion luminescence [18–20]. In a recent work, we have reported a preliminary characterization of the absorption and emission properties of Er3 þ ions in a fluorotellurite glass of composition 46.6TeO2–18.2ZnO–35.2ZnF2 [21]. In the present study, we analyze the influence of ZnF2 content on the absorption, near infrared emission, as well as upconversion emissions, of Er3 þ ions in two different compositions of TeO2–ZnO–ZnF2 fluorotellurite glasses, with ErF3 concentrations ranging between 0.5 and 3 wt%. The upconverted green and red emissions obtained under near infrared excitation in the 4 I9/2 level are investigated by using steady-state and timeresolved laser spectroscopy and compared with those obtained under one photon excitation. The possible excitation mechanisms responsible for this upconversion luminescence are discussed on the basis of lifetime measurements results.

2. Experimental techniques Fluorotellurite glasses having a nominal composition of 74.6 TeO2–8.8ZnO–16.6ZnF2 (TZF16) and 46.6TeO2–18.2ZnO–35.2ZnF2 (TZF35) mol% were prepared by mixing the corresponding high purity (99.99 %) oxides and fluorides (TeO2, ZnO, ZnF2 and ErF3) in an agate balls mill. The powder mixture was melted in a covered platinum crucible using an electrical vertical furnace at temperatures in the 800–850 1C range. Glass melts were made homogenous by using an electrical platinum stirrer for 20 min, and then poured onto a preheated brass mould. The obtained glass blocks (about 1 cm3 size) were immediately introduced into the annealing furnace, kept for 10 min at temperatures ranging from 300 to 320 1C, and then cooled down to room temperature. The glasses were doped with 0.5, 1, 2, and 3 wt% of ErF3. The optical measurements were carried out on polished planoparallel glass slabs of about 1 mm thickness. Conventional absorption spectra were performed with a Cary 5 spectrophotometer. The steady-state emission measurements were made with an argon laser and a Ti-sapphire ring laser in the 770–920 nm spectral range as exciting light. The fluorescence was analyzed with a 0.25 monochromator, and the signal was detected by a Hamamatsu R928 photomultiplier and finally amplified by a standard lock-in technique. Infrared emissions were detected with an extended IR Hamamatsu R5509-72 photomultiplier. Lifetime measurements were obtained by exciting the samples with a dye laser pumped by a pulsed nitrogen laser and a Ti-sapphire laser pumped by a pulsed frequency doubled Nd:YAG laser (9 ns pulse width), and detecting the emission with Hamamatsu R928 and R5509-72 photomultipliers. Data were processed by a Tektronix oscilloscope.

3. Results and discussion 3.1. Absorption and emission properties The room temperature absorption spectra were obtained for all samples in the 300–1700 nm range with a Cary 5 spectrophotometer. As an example, Fig. 1 shows the absorption cross-section as a function of wavelength for the two glasses doped with 2 wt% ErF3. The spectra are characterized by 10 bands corresponding to the transitions starting from the 4I15/2 ground state to the different higher levels of Er3 þ ions. The integrated absorption coefficient for different absorption bands shows in both glasses a

Absorption cross-section (10-20 cm2)

A. Miguel et al. / Journal of Luminescence 140 (2013) 38–44

8

4

39

G11/2

TZF16 TZF35

6

4

2

2

2

H9/2

4

4

H11/2

4

F7/2 4

F3/2 0 300 500

F9/2

S3/2

4 4

4

I9/2

700

900

I13/2

I11/2 1100

1300

1500

Wavelength (nm) Fig. 1. Room temperature absorption cross-section of Er3 þ in TZF16 and TZF35 glasses.

linear dependence on concentration, which indicates that the relative concentrations of Er3 þ ions are in agreement with the nominal values. The absorption edge in the UV is blue shifted for the glass with the higher content of ZnF2. This behavior is due to the smaller polarizability of the fluorine ligands around the Er3 þ ions (a higher ionicity of the glass matrix ) [22]. In TZF35 glass, the high content of ZnO and ZnF2 modifies the tellurium network increasing the ionic character. Moreover, the structural characterization shows that the TeO4 groups are continuously changed into TeO3 þ 1 and TeO3 which decreases the polarizability [6]. The radiative transition probabilities have been calculated from the absorption spectra by using the Judd–Ofelt (JO) theory [23,24]. The measured absorption bands are all dominated by electric dipole transitions except the 4I15/2-4I13/2 transition, which contains electric-dipole and magnetic-dipole contributions. The magnetic-dipole contribution, fmd, can be obtained from equation fmd ¼nf0 [25], where n is the refractive index of the studied glasses and f0 is a calculated quantity based on the energy-level parameters of lanthanide aquo ions. The electric dipole oscillator strength for this transition is then obtained by subtracting the calculated magnetic-dipole contribution from the experimental oscillator strength. The JO parameters obtained for these glasses together with the density, refractive index, and Er3 þ concentration (calculated from the density) for samples with 2 wt% of ErF3 are displayed in Table 1. As can be seen from this table the density decreases as the ZnO and ZnF2 content increases due to the lower atomic weight of ZnO and ZnF2 compared with TeO2. The refractive index also decreases in the glass with the higher content of ZnF2 due to the presence of ionic bonds. The values for the JO parameters are in agreement with those previously reported by Nazabal [6]. It is well known that O2 is most sensitive to local structure and glass composition and its value is indicative of the amount of covalent bonding between RE ions and ligand anions [26]. As can be seen in Table 1, O2 is lower for the glass with the higher content of ZnF2. The high content of ZnF2, which favors the presence of fluorine ions around Er3 þ , decreases the covalency degree in the rare-earth site and results in a lower value of O2. Moreover, the sum of the JO parameters decreases for TZF35 glass due to the decrease of covalency of the chemical bond between the Er3 þ ion and the ligand anions. The radiative transition probabilities for the excited levels of Er3 þ can be calculated by using the JO parameters. The radiative transition probability is given by [27],   A ðS,LÞJ; ðS0 ,L0 ÞJ 0 ¼

64p4 e2 3

3hl ð2J þ1Þ

" n

ðn2 þ 2Þ2 Sed þ n3 Smd 9

# ð1Þ

40

A. Miguel et al. / Journal of Luminescence 140 (2013) 38–44

Table 1 Density, Er3 þ ions concentration, JO parameters, and r.m.s. deviation for the two glasses doped with 2 wt% ErF3. Glass

Density (g cm  3)

N (cm  3)

n

O2  10  20

O4  10  20

O6  10  20

r.m.s.

TZF16 TZF35

5.53 5.31

2.97  1020 2.85  1020

2.002 1.833

4.71 3.02

1.57 1.28

1.13 1.14

3.24  10  7 2.59  10  7

where nðn2 þ 2Þ2 =9 is the local field correction for electric dipole transitions and n3 for magnetic dipole transitions. The radiative lifetime is related to radiative transition probabilities by

tR ¼

8
0

0

S ,L ,J

91 = 0 0 0 A½ðS,LÞJ; ðS ,L ÞJ  ; 0

ð2Þ

The fluorescence branching ratio can be obtained from the transition probabilities by using   A½ðS,LÞJ; ðS0 ,L0 ÞJ0  b ðS,LÞJ; ðS0 ,L0 ÞJ 0 ¼ P A½ðS,LÞJ; ðS0 ,L0 ÞJ 0 

ð3Þ

S0 ,L0 ,J 0

The radiative transition probabilities, the branching ratios, and the radiative lifetimes of some selected levels of Er3 þ for both glass compositions are shown in Table 2. Since the radiative transition probability is related to the refractive index of the glass host, the glass with the highest content of TeO2, which has a higher refractive index, presents higher radiative transition probabilities.

3.2. Near infrared emission and fluorescence lifetimes The near infrared emission was obtained for all samples at room temperature by exciting at 801 nm in the 4I9/2 level. After excitation of this level, the next lower levels are populated by multiphonon relaxation. As an example, Fig. 2 shows the fluorescence spectra corresponding to the 4I11/2-4I15/2 and 4I13/2-4I15/2 transitions for the samples doped with 0.5, 1, 2, and 3 wt% of ErF3 in the two glasses. As can be seen, the glass with a higher content of ZnF2 (Fig. 2b) shows a higher intensity for the 4I11/2-4I15/2 emission which indicates that in this case the multiphonon relaxation rate between levels 4I11/2 and 4I15/2 is lower than in the TZF16 glass. The fluorescence spectra corresponding to the 4I13/2-4I15/2 transition show in all cases a maximum at around 1532 with an R effective bandwidth (Dleff ¼ IðlÞdl=Imax ) of around 67 nm for the less concentrated samples which increases up to 77 nm and 75 nm for the samples doped with 3 wt% for TZF16 and TZF35 respectively. This broadening of the emission band as concentration increases can be explained by considering radiation trapping effects in which part of the photons emitted by an Er3 þ ion is reabsorbed by a non-excited Er3 þ , inducing the 4I13/2-4I15/2 transition and the subsequent emission. In these samples, this broadening cannot be attributed to changes in the chemical bonding of the ligands surrounding the Er3 þ ions since the effective bandwidth of the 4I15/2-4I13/2 absorption band is around 65 and 64 nm for all concentrations in TZF16 and TZF35 glasses respectively. Another important parameter in an optical amplification process is the stimulated emission cross-section. To obtain a reliable estimation of the stimulated emission cross-section, the fluorescence profile for all concentrations was obtained from the absorption spectra by using the McCumber approach [28], which

Table 2 Predicted radiative transition rates, radiative lifetimes, and branching ratios of some excited levels of Er3 þ in TZF16 and TZF35 glasses. Transition

TZF16 4 I13/2-4I15/2 4 I11/2-4I15/2 4 I11/2-4I13/2 4 I9/2-4I15/2 4 I9/2-4I13/2 4 I9/2-4I11/2 4 F9/2-4I15/2 4 F9/2-4I13/2 4 F9/2-4I11/2 4 F9/2-4I9/2 4 S3/2-4I15/2 4 S3/2-4I13/2 4 S3/2-4I11/2 4 S3/2-4I9/2 4 S3/2-4F9/2 TZF35 4 I13/2-4I15/2 4 I11/2-4I15/2 4 I11/2-4I13/2 4 I9/2-4I15/2 4 I9/2-4I13/2 4 I9/2-4I11/2 4 F9/2-4I15/2 4 F9/2-4I13/2 4 F9/2-4I11/2 4 F9/2-4I9/2 4 S3/2-4I15/2 4 S3/2-4I13/2 4 S3/2-4I11/2 4 S3/2-4I9/2 4 S3/2-4F9/2

Energy (cm  1)

Amd (s  1)

Aed (s  1)

6588 10,211 3623 12,425 5837 2214 15,258 8670 5047 2833 18,345 11,757 8134 5920 3087

81.6

227.8 294.9 33.2 261.7 84.9 1.8 2910.5 147.8 130.7 6.3 2308.3 925.6 74.0 121.3 1.1

6588 10,222 3634 12,431 5843 2209 15,373 8785 5151 2942 18,378 11,790 8156 5947 3005

62.6

16.1

2.5

12.6

1.9

161.9 201.0 23.5 157.0 62.5 1.2 1928.1 94.7 95.5 3.4 1710.7 688.1 54.3 85.7 0.7

Arad (s  1)

srad

b (%)

(ms)

309.4 344.2

3.23 2.91

350.9

2.85

3195.3

0.31

3430.4

0.29

224.6 237.1

4.45 4.22

222.7

4.49

2121.6

0.47

2539.5

0.39

relates the absorption and emission cross- sections by,   ðehvÞ se ðvÞ ¼ sa ðvÞexp KT

100 85.7 14.3 74.6 24.2 1.2 91.1 4.6 4.1 0.2 67.3 27.0 2.2 3.5 0.03 100 84.8 15.2 70.5 28.1 1.4 90.9 4.5 4.5 0.2 67.4 27.1 2.1 3.4 0.03

ð4Þ

where sa and se are the absorption and stimulated emission cross-sections respectively, n is the photon frequency, e is the net free energy required to excite one Er3 þ ion from the 4I15/2 to the 4 I13/2 state at temperature T, h is the Planck constant, and K is the Boltzmann constant. The absorption cross-section has been experimentally obtained and e can be determined by using the simplified procedure provided in Ref. [29]. As an example, Fig. 3 shows the absorption and emission cross-sections obtained for TZF16 glass doped with 2% of ErF3 together with the normalized measured emission spectrum. As can be seen the emission profile is similar being the difference attributed to reabsorption, as mentioned before. The effective linewidth of the 4I13/2-4I15/2 transition, in both glasses, is similar to that of other halotellurite glasses, and larger than those of phosphate ( E46) and silica-based glasses (E37 nm) [30]. A large bandwidth is useful for tunable lasers and also for broadband amplification. The peak positions, effective bandwidths (experimental and calculated) and emission cross-sections for all concentrations in the two glasses are shown in Table 3. As can be seen TZ16 glass

A. Miguel et al. / Journal of Luminescence 140 (2013) 38–44

shows higher values for the stimulated emission cross-section probably due to the higher value of the refractive index which increases the stimulated emission cross-section as ðn2 þ2Þ2 =n2 for electric dipole transitions and linearly for magnetic dipole transitions [31]. These values are close to those found in other tellurite and fluorotellurite glasses [18,32,33]. The gain bandwidth of an amplifier is determined by the width of the emission spectrum

Fig. 2. Near-infrared emission spectra of Er3 þ ions in (a) TZF16 and (b) TZF35 glasses for different ErF3 concentrations.

Cross-section (10-21cm2)

10

σabs(λ)

TZF16

σem(λ)

8

Emission 4I13/2

4

I15/2

6 4

0

1400

and the emission cross-section. The figure of merit (FOM) for bandwidth (the product of the stimulated emission cross-section and bandwidth) shows higher values for TZF16 glass which has higher stimulated emission cross-section and slightly higher values for the bandwidth. The values obtained in both glasses are higher than in phosphate, germanate, and silicate ones [34]. Assuming that the FOM for bandwidth is an indication of the achievable gain band, the values obtained for these glasses suggest that they are promising materials for broadband light sources, in particular TZF16 glass. The experimental decays of luminescence from levels 4I11/2 and 4I13/2 were obtained at room temperature for all samples by exciting at 801 nm in level 4I9/2 and are displayed in Table 4. From this level fast nonradiative relaxation efficiently populates levels 4 I11/2 and 4I13/2. In both glasses, the decays of the excited 4I11/2 level slightly deviate from a perfect exponential behavior for the samples doped with 0.5 and 1% and become single exponential at higher concentrations showing the presence of diffusion processes at high concentrations. Due to the energy gap value between levels 4I11/2 and 4I13/2 (around 3600 cm  1), the lifetime of the former is strongly influenced by the phonon energy. As can be seen these lifetimes are much longer for the glass with the highest content of ZnF2 which weakens the electron–phonon coupling strength of the phonon mode locally coupled to Er3 þ ions. And so, the multiphonon relaxation rate from this level is reduced and the emission efficiency enhanced, which makes the level lifetime much longer if compared to pure tellurite and fluorotellurite glasses with a lower fluorine content [6,33]. The lifetime of level 4I13/2 plays an important role in Er3 þ doped glasses for their applications as optical amplifiers. In these glasses the decays from this level show an initial rise time due to the population from the higher 4I11/2 level and a single exponential behavior at all concentrations. The measured lifetime slightly increases with concentration due to the presence of selfabsorption which causes a delay in the detection of the excited luminescence due to successive re-absorption and re-emission processes inside the sample depending on the absorption crosssection and thickness. In this case, according to Auzels approach, the lifetime corrected for the self-absorption effect is given by t ¼ tPL ð1 þ sa NLÞ, where t is the measured lifetime, tPL is the non affected value by self-absorption, sa is the absorption crosssection, N is the ions concentration, and L is the sample thickness [34]. Table 4 displays the lifetime values obtained by a fit to a single exponential function and the corrected lifetimes for both glasses. The corrected lifetime decreases for the higher concentrated samples. The lifetimes decrease at higher concentrations corrected for self-absorption, indicates the presence of the energy transfer

Table 3 Room temperature emission properties of the 4I13/2 -4I15/2 transition for different concentrations of ErF3 in both glasses. Dl(1) eff corresponds to the effective linewidth of the measured fluorescence spectra whereas Dl(2) eff corresponds to the calculated one from the fluorescence profile by using the McCumber approach. wt% ErF3

2

1450

1500

1550

1600

1650

1700

Wavelength (nm) Fig. 3. Absorption and emission cross-sections of the 4I13/224I15/2 transitions for TZN16 glass doped with 2 wt% ErF3 together with the normalized measured emission spectrum.

41

TZF16 0.5 1 2 3 TZF35 0.5 1 2 3

re(10  21 cm2) FOM

kp

Dk(1) eff

Dk(2) eff

(nm)

(nm)

(nm)

1532 1532 1532 1532

67.7 73.4 74.6 77.0

65.0 65.5 65.4 65.0

1.995 2.001 2.002 2.003

8.2 8.8 8.7 9.2

555 645 649 708

1532 1532 1532 1532

66.7 68.8 74.5 75.1

64.8 64.8 64.4 64.3

1.835 1.823 1.833 1.840

7.1 7.5 7.5 7.8

474 516 559 586

n

(10  21 cm2 nm)

42

A. Miguel et al. / Journal of Luminescence 140 (2013) 38–44

Table 4 Lifetimes of the 4I11/2 and 4I13/2 levels obtained under excitation at 801 nm at room temperature. tPL is the corrected value by self-absorption.

lexc ¼ 801 nm TZF16

kem ¼980 nm 4

kem ¼1532 nm

wt% ErF3

s ( I11/2) (ms)

s (4I13/2) (ms)

sPL (4I13/2) (ms)

0.5 1 2 3 TZF35 0.5 1 2 3

0.49 0.42 0.36 0.32

5.4 5.9 6.1 6.8

5.1 5.3 4.9 4.8

1.9 2.0 1.7 1.5

7.7 8.2 8.5 7.6

7.3 7.4 6.8 5.5

processes. In the case of the 4I13/2 level which is the first excited state, the quenching of luminescence when active ion concentration increases has been mainly considered as due to diffusion toward unidentified impurities (such as OH or others present in the starting materials) or another type of self-generated quenching centers [35]. In these glasses, the decays are single exponential at all concentrations which indicates that we are dealing with a fast diffusion process. This behavior can be associated with a rapid energy diffusion between Er3 þ ions that can lead to a spatial equilibrium of the excitation within the Er3 þ system in a time shorter than the decay time. In the transfer rapid limit the donor transfer takes place so quickly that transfer times are averaged out and the whole system exhibits a simple exponential decay as is experimentally observed.

Fig. 4. Upconversion emission spectra of Er3 þ doped TZF glasses with different ErF3 concentrations, obtained under excitation at 801 nm. The emission intensities are normalized to the 549 nm band. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

3.3. Infrared to visible upconversion Visible upconversion has been observed for all samples at room temperature under continuous wave (cw) and pulsed laser excitation in resonance with the 4I9/2 level. The upconverted emission spectra obtained under cw excitation were measured by using a Ti-sapphire ring laser. Cut-off filters were used to remove the pumping radiation. As an example Fig. 4 shows the room temperature upconverted emission spectra of Er3 þ in both glasses obtained under cw near infrared excitation at 801 nm for all concentrations normalized to the green emission. The observed emissions correspond to transitions 2H11/2-4I15/2, 4S3/24 I15/2, and 4F9/2-4I15/2. The 2H11/2-4I15/2 emission can be observed at room temperature because the 2H11/2 level is populated from 4S3/2 via fast thermal equilibrium between levels. As can be observed the most intense emission corresponds to the green emission from level 4S3/2. At low concentration, the weak red emission from the 4F9/2 level is due to its population from level 4 S3/2 through multiphonon relaxation. This relaxation occurs with a moderate rate due to the energy gap values between both levels ( 3000 cm  1) and the maximum phonon energy ( 750– 775 cm  1) [6]. These upconversion emission spectra are similar to those found under one photon excitation at 488 nm in the 4F7/2 level, exception made of the red emission intensity. Under OP excitation the ratio between the red and green emission intensities varies from 0.03 to 0.07 as ErF3 concentration increases for TZF16 glass, and from 0.03 to 0.13 for TZF35 glass. In the case of the upconversion emission spectra the ratio increases from 0.04 to 0.37 for TZF16 glass, and from 0.06 to 0.86 for TZF35 as ErF3 concentration increases from 0.5 to 3 wt%. This increase of the red emission as concentration increases indicates the presence of additional mechanisms in order to populate level 4F9/2.

To investigate the excitation mechanisms for populating the H11/2, 4S3/2, and 4F9/2 levels after IR excitation, we have obtained the evolution of the upconverted emission intensities for different pumping powers. Upconversion intensities were recorded at 529, 549, and 655 nm for different pump powers. The upconversion emission intensity (Iem) depends on the incident pump power (Ppump) according to the relation Iemp(Ppump)n, where n is the number of photons involved in the pumping mechanism. The dependence of the intensity on the pump power is quadratic which indicates a two photon (TP) upconversion process to populate the 2H11/2, 4S3/2, and 4F9/2levels. The same behavior is observed for the two glasses. This in turn may be associated to excited state absorption (ESA) and/or to energy transfer upconversion (ETU) [36]. Fig. 5 shows the possible upconversion mechanisms which may account for the green and red emissions under 801 nm excitation. The (2H11/2, 4S3/2) levels can be populated by ESA and/or ETU. In the first case, in a first step the absorption of one NIR pump-photon excites the electrons to level 4I9/2, then multiphonon relaxation occurs to level 4I11/2 , subsequent ESA of a second NIR pump photon promotes the electrons to the 4F3/2,5/2 levels, and finally, by nonradiative relaxation, 2H11/2 and 4S3/2 levels are reached. Part of the excitation energy in the 4I11/2 level further relaxes, radiatively and nonradiatively, to level 4I13/2. Under this excitation condition, ESA from level 4I13/2 to 2H11/2 can occur. Another possibility is an energy transfer from the 4I11/2 level, in which two Er3 þ ions in this level interact, and one ion gains energy and reaches the 4F7/2 level, whereas the other one loses energy and goes to the ground state. The upconverted red emission can be the result of multiphonon relaxation from the 4 S3/2 level and ETU processes. Energy transfer can take place via 2

A. Miguel et al. / Journal of Luminescence 140 (2013) 38–44

E (103 cm-1)

4G 11/2

25

2H

20

4F 4 3/2, F5/2 4F 7/2 2H 11/2 4S 3/2

ETU 15

10

ESA

9/2

4F 9/2 4I 9/2 4I 11/2

4I 13/2

5

0

4I 15/2

Fig. 5. Energy level diagram of Er3 þ ion in TZF16 glass and possible upconversion mechanisms.

transitions (4I9/2-4I13/2) and (4I11/2-4F9/2) and/or (4I11/2-4I15/2) and (4I13/2-4F9/2). There exists another possible process to populate the 4F9/2 level in which two Er3 þ ions interact, one of them in level 4I11/2 and the other one in level 4F7/2 , going both to level 4F9/2 (See Fig. 5). In these glasses, the upconverted emission shows an enhancement of the red emission relative to the green one, which is not observed in the spectra obtained by exciting at 488 nm which indicates that level 4F9/2 is populated by energy transfer processes in addition to multiphonon relaxation from the 4 S3/2 level. As it is well known, the time evolution of the upconversion luminescence after an excitation pulse provides a useful tool in discerning which the operative mechanism is. The radiative ESA process occurs during the excitation pulse and leads to an immediate decay of the upconversion luminescence after excitation. Upconversion by energy transfer leads to a time-dependent emission that shows a rise of the upconverted population after the laser pulse, followed by a decay of the population, with a lifetime longer than the one after direct excitation. The rise and decay times are determined by both the intermediate and the upper excited state lifetimes. This distinction is possible when the pulse width is much shorter than the time constant of the relevant energy transfer step. To clarify which are the mechanisms responsible for the upconverted emission in these glasses we have measured the lifetimes of the 4S3/2, and 4F 9/2 levels under pulsed excitation in resonance with the 4 I 9/2 level and also under one photon excitation at 488 nm with a dye laser. The time dependence of the upconverted luminescence from the 4S 3/2 level does not show any rise time even at high concentrations which indicates the presence of an ESA process from level 4 I 13/2 and also a small contribution from level 4I 11/2. However, lifetimes are longer than under OP excitation, which suggests that ETU processes such as (4 I 11/2 -4 I 15/2) and ( 4I 11/2- 4F 7/2 ) can also be responsible for the green emission. In this process two Er3 þ ions in the 4 I 11/2 level interact and one of them goes to the ground state whereas the other one reaches the 4F 7/2 level from where level 4S3/2 is populated by nonradiative relaxation. This ETU process is responsible for the long lifetime of the

43

green emission. The lifetime values obtained at 488 and 801 nm are shown in Table 5. In the case of the red emission from the 4F9/2 level, its population at low concentrations is due to multiphonon relaxation from the 4S3/2 level. However, the concentration dependence of the upconversion red emission suggests the presence of an additional population channel of level 4F9/2. As we have seen in Fig. 4, in the case of the upconverted emission there is an increase in the red emission intensity relative to the green one with increasing Er3 þ concentration. In addition, the experimental decays for the red emission obtained after NIR excitation, show a lengthening of the 4F9/2 lifetime compared to level 4S3/2 as ErF3 concentration increases. As an example, the time evolution of the red emission obtained under excitation at 801 nm for the two glasses doped with 3 wt% is shown in Fig. 6. As can be observed, there is an initial rise time, similar to the one observed under OP excitation and due to the population from level 4S3/2 and a clear double exponential decay. The fraction of the low component increases with ErF3 concentration. Table 5 shows the lifetime values of the 4F9/2 level obtained under OP excitation at 488 nm and at 801 nm, as well as the lifetime values of the 4I11/2 level. The decays for the red emission obtained under excitation at 801 nm in the lowest concentrated samples are too weak to be accurately measured. It can be noticed that the short lifetime of the decays of the upconverted red emission is close to the 4F9/2 lifetime whereas the lifetime of the long decay is close to the one of level 4 I11/2. This behavior indicates that for ErF3 concentrations higher than 0.5 wt%, the 4F9/2 level emission is not only populated by multiphonon relaxation from level 4S3/2 but, rather, additional ETU processes are involved. For example, as we suggest in Fig. 5, energy transfer can take place via transitions (4I9/2-4I13/2) and (4I11/2-4F9/2) (DE¼790 and 692 cm  1 for TZF16 and TZF35 respectively) and/or (4I11/2-4I15/2) and (4I13/2-4F9/2) (DE¼1541 and 1437 cm  1). Another possibility is an energy transfer involving two Er3 þ ions, one in level 4F7/2 and the other one in level 4 I11/2 , both going to level 4F9/2. This process is nearly resonant with an energy mismatch around 49 and 33 cm  1 for TZF16 and TZF35 respectively. By considering that the lifetime of a higherenergy level excited by ETU reflects those of the intermediate levels from which upward excitation occurs, and that in both glasses the long components of the decays of the red emission are very close to the lifetime of level 4I11/2, the (4I9/2-4I13/2);(4I11/24 F9/2) and (4F7/2-4F9/2);(4I11/2-4F9/2) processes seem to be the likeliest ones to explain the population of the 4F9/2 level by ETU.

Table 5 Lifetime values of the 4S3/2 and 4F9/2 levels obtained under excitation at 488 nm and 801 nm together with the lifetime values of the 4I11/2 level. wt% ErF3

kexc ¼ 488 (4S3/2) (ls)

TZF16 0.5 71 1 54

kexc ¼ 801 (4S3/2) (ls)

kexc ¼ 488 (4F9/2) (ls)

108 82

75 61

2

36

58

48

3

25

55

34

TZF35 0.5 178 1 143

776 445

204 196

2

83

252

150

3

48

192

122

kexc ¼801 (4F9/2) (ls)

–

ts ¼61, tl ¼ 377 ts ¼47, tl ¼ 409 ts ¼30, tl ¼ 375 –

ts ¼199, tl ¼ 1770 ts ¼163, tl ¼ 1720 ts ¼121, tl ¼ 1640

kexc ¼ 801 (4I11/2) (ms)

0.49 0.42 0.36 0.32

1.9 2.0 1.7 1.5

44

A. Miguel et al. / Journal of Luminescence 140 (2013) 38–44

T= 295 K

9/2

(a) λexc= 488 nm

3

Intensity (arb. units)

4F

(b) λexc=801 nm

2

TZF16 1

(b) (a)

Intense green emission due to the (2H11/2,4S3/2)-4I15/2 transitions together with red emission corresponding to 4F9/2-4I15/2 have been observed in all samples under excitation at 801 nm, and attributed to two photon processes. The time evolution of the green upconverted emission suggests that ESA and ETU are responsible for the observed emission. In the case of the red emission from level 4F9/2, at low concentrations this level is populated by multiphonon relaxation from level 4S3/2, whereas as ErF3 concentration increases there is an increase of the red emission attributed to the presence of ETU processes involved. The temporal behavior of the red upconverted emission suggests that ETU processes, with level 4I11/2 as intermediate state, are responsible for the increase of this emission with ErF3 concentration.

0 0.0

0.5

1.0

1.5

2.0

2.5

Time (ms)

Acknowledgments

T= 295 K

Intensity (arb. units)

This work was supported by the Spanish Government under projects MAT2009-14282-C02-02 and MAT2009-14282-C02-01, FIS2011-27968, and Consolider CSD2007-00013 (SAUUL), and the Basque Country Government (IT-659-13).

4F

9/2

(a) λexc= 488 nm (b) λexc=801 nm

1

TZF35

References

(b)

[1] [2] [3] [4] [5]

(a)

0 0

2

4

6

8

10

Time (ms)

[6] [7] [8]

Fig. 6. Temporal behavior of the upconverted 4F9/2 luminescence for the samples doped with 3 wt% ErF3 under excitation at (a) 488 nm and (b) 801 nm for the two glasses. Data correspond to room temperature. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

[9] [10]

4. Conclusions

[14] [15]

Absorption and luminescence measurements have been performed in Er3 þ doped 74.6TeO2–8.8ZnO–16.6ZnF2 and 46.6TeO2– 18.2ZnO–35.2ZnF2 fluorotellurite glasses for different ErF3 concentrations up to 3 wt%. The Judd–Ofelt intensity parameters and radiative transition rates have been calculated. The glass with the highest content of ZnF2 shows a lower value for O2 parameter due to the decrease of covalency of the chemical bond between the Er3 þ ion and the ligand anions. The presence of ionic bonds also leads to lower values of the refractive index and radiative transition probabilities. The near infrared emissions at around 980 and 1530 nm have been characterized for different ErF3 concentrations (0.5, 1, 2, and 3 wt%). The glass with the highest content of ZnF2 shows a higher intensity for the 4I11/2-4I15/2 emission which indicates a lower multiphonon relaxation rate. Moreover, the lifetimes of the 4I11/2 level which are highly affected by multiphonon relaxation are much longer in this glass. Fluorescence measurements show that in both glasses the 4I13/2-4I15/2 emission is broader by nearly 30 nm if compared to silica-based glasses and the FOM for bandwidth is higher than in other oxide glasses. In particular, the highest values of the FOM correspond to the TZF16 glass which has higher values for the stimulated emission crosssection. These features make these glasses attractive for broadband amplifiers.

[16]

[11] [12] [13]

[17] [18] [19] [20] [21]

[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]

R.J. Mears, L. Reekie, I.M. Jauncey, D.N. Payne, Electron. Lett. 23 (1987) 1026. E. Desurvire, J. Simpson, P.C. Becker, Opt. Lett. 12 (1987) 888. A. Mori, Y. Oshishi, S. Sudo, Electron. Lett. 33 (1997) 863. J.Y. Allain, M. Monerie, H. Poignant, Electron. Lett. 28 (1992) 111. J.F. Massicott, M.C. Brierley, R. Wyatt, S.T. Davey, D. Szebesta, Electron. Lett. 29 (1993) 2119. V. Nazabal, S. Todoroki, A. Nukui, T. Matsumoto, S. Suehara, T. Hondo, T. Araki, S. Inoue, C. Rivero, T. Cardinal, J. Non-Cryst. Solids 325 (2003) 85. J.S. Wang, E.M. Vogel, E. Snitzer, Opt. Mater. 3 (1994) 187. R.A.H. El-Mallawany, Tellurite Glasses Handbook-Physical Properties and Data, CRC Boca Raton, FL, 2001. S.Q. Man, E.Y.B. Pun, P.S. Chung, Opt. Commun. 168 (1999) 369. M. Yamada, A. Mori, K. Kobayashi, H. Ono, T. Kanamori, K. Oikawa, Y. Nishida, Y. Ohishi, IEEE Photon. Technol. Lett. 10 (1998) 1244. S. Shen, A. Jha, L. Huang, P. Joshi, Opt. Lett. 30 (2005) 1437. S. Tanabe, K. Hirao, N. Soga, J. Non-Cryst. Solids 122 (1990) 79. Y. Ohishi, A. Mori, M. Yamada, H. Ono, Y. Nishida, K. Oikawa, Opt. Lett. 23 (1998) 274. A. Mori, IEEE J. Lightwave Technol. 20 (2002) 822. R. Balda, J. Ferna´ndez, M.A. Arriandiaga, J. Ferna´ndez-Navarro, J. Phys.: Condens. Matter 19 (2007) 086223. I. Iparraguirre, J. Azkargorta, J.M. Ferna´ndez-Navarro, M. Al-Saleh, J. Ferna´ndez, R. Balda, J. Non-Cryst. Solids 353 (2007) 990. B. Richards, Y. Tsang, D. Binks, J. Lousteau, A. Jha, Opt. Lett. 33 (2008) 402. N. Jaba, A. Kanoun, H. Mejri, A. Selmi, S. Alaya, H. Maaref, J. Phys: Condens. Matter 12 (2000) 4523. F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, Appl. Phys. Lett. 80 (2002) 1752. K. Kumar, S.B. Rai, D.K. Rai, J. Non-Cryst. Solids 353 (2007) 1381. A. Miguel, M. Al-Saleh, J. Azkargorta, R. Morea, J. Gonzalo, M.A. Arriandiaga, J. Ferna´ndez, R. Balda, Opt. Mater., http://dx.doi.org/10.1016/j.optmat.2012. 09.022, In press. M.J. Weber, D.C. Ziegler, C.A. Angell, J. Appl. Phys. 53 (1982) 4344. B.R. Judd, Phys. Rev. 127 (1962) 750. G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4412. C.K. Jorgensen, R. Reisfeld, J. Less-Common Met. 93 (1983) 107. M.J. Weber, Phys. Rev 157 (1967) 262. D.E. McCumber, Phys. Rev. 136 (1964) A954. W.J. Miniscalco, R.S. Quimby, Opt. Lett. 16 (1991) 258. Y. Ding, S. Jiang, B.C. Hwang, T. Luo, N. Peyghambarian, Y. Himei, T. Ito, Y. Miura, Opt. Mater. 35 (2000) 123. R. Reisfeld, Struct. Bonding 22 (1975) 123. U.R. Rodrı´guez-Mendoza, E.A. Lalla, J.M. Ca´ceres, F. Rivera-Lo´pez, S.F. Leo´nLuı´s, V. Lavı´n, J. Lumin. 131 (2011) 1239. R. Balda, M. Al-Saleh, A. Miguel, J.M. Fdez-Navarro, J. Ferna´ndez, Opt. Mater. 34 (2011) 481. Q. Qian, Y. Wang, Q.Y. Zhang, G.F. Yang, Z.M. Yang, Z.H. Jiang, J. Non-Cryst. Solids 354 (2008) 1981. F. Auzel, G. Baldacchini, L. Laversenne, G. Boulon, Opt. Mater. 24 (2003) 103. R. Balda, A.J. Garcı´a-Adeva, J. Ferna´ndez, J.M. Fdez-Navarro, J. Opt. Soc. Am. B 21 (2004) 744.