Yb3+-codoped tellurite glasses

Yb3+-codoped tellurite glasses

Journal of Alloys and Compounds 344 (2002) 304–307 L www.elsevier.com / locate / jallcom Infrared-to-visible frequency upconversion in Pr 31 / Yb 3...

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Journal of Alloys and Compounds 344 (2002) 304–307

L

www.elsevier.com / locate / jallcom

Infrared-to-visible frequency upconversion in Pr 31 / Yb 31 - and Er 31 / Yb 31 -codoped tellurite glasses a a a a a, P.V. dos Santos , M.V.D. Vermelho , E.A. Gouveia , M.T. de Araujo , A.S. Gouveia-Neto *, F.C. Cassanjes b , S.J.L. Ribeiro b , Y. Messaddeq b a

´ , Universidade Federal de Alagoas, Maceio´ 57072 /970, AL, Brazil Departamento de Fısica b ´ , UNESP, Araraquara 14800 /900, SP, Brazil Instituto de Quımica

Abstract Upconversion luminescence and thermal effects in Pr 31 / Yb 31 - and Er 31 / Yb 31 -codoped 60TeO 2 –10GeO 2 –10K 2 O–10Li 2 O– 10Nb 2 O 5 tellurite glasses excited by CW infrared radiation at 1.064 mm is reported. Generation of intense green and red fluorescence emission in Er 31 / Yb 31 -codoped samples and appreciable upconversion luminescence in the wavelength region of 450–680 nm in Pr 31 / Yb 31 -codoped samples is observed. Temperature-induced enhancement of 312 in the upconversion efficiency in Er 31 / Yb 31 - and 310 in the Pr 31 / Yb 31 -doped samples is demonstrated.  2002 Elsevier Science B.V. All rights reserved. Keywords: Luminescence; Rare-earth; Glass; Thermal-effect; Upconversion

1. Introduction There has been a great deal of interest in the search for novel glass materials to be used as host in the conversion of infrared radiation into visible light through frequency upconversion in rare-earth doped (RED) systems [1]. The RED solid-state upconverter can find applications in photonic devices such as color displays, high density optical data reading and storage, biomedical diagnostics, infrared laser viewers and indicators, amongst many. To pursue that goal, it is important to study the frequency upconversion mechanism in novel alternative host materials and identify the major relaxation and interaction mechanisms of rare-earth ions implanted into the novel glass. Among the novel materials available, TeO 2 -based glasses emerge as serious contenders for photonic devices applications. The tellurite glass present good optical quality, is stable against atmospheric moisture, and it exhibits low optical attenuation from 0.4 to 5 mm. Due to the relatively low maximum phonon-energy (|800 cm 21 ) as compared to silicate glasses, and the high refractive index (|2.0), it is expected to provide significantly high radiative decay rates and low nonradiative relaxation rates of rareearths excited-state levels. The material also exhibits high *Corresponding author. Fax: 155-82-214-1645. E-mail address: [email protected] (A.S. Gouveia-Neto).

solubility allowing the incorporation of high lanthanide concentrations apart from being nonhygroscopic and to possess high thermal stability against crystallization. For the majority of rare-earth single-doped systems the upconversion process has proven inefficient for pumping in the wavelength region of 1.0–1.1 mm. However, when RED materials are codoped with trivalent ytterbium [2–4], the high absorption cross-section of this sensitizer and the efficient energy-transfer mechanism between rare-earth ions, yield a considerable enhancement in the upconversion efficiency as demonstrated for Tm 31 / Yb 31 [2], Pr 31 / Yb 31 [3], and recently in Er 31 / Yb 31 [4] doped chalcogenide glasses. In Ref. [4], an enhancement by a factor of 320 was observed in the infrared-to-visible upconversion efficiency as compared to an Er 31 single-doped sample. In this work, we report on upconversion emission and temperature-induced upconversion efficiency enhancement in Pr 31 / Yb 31 - and Er 31 / Yb 31 -codoped TeO 2 -based glasses pumped at 1.064 mm.

2. Experimental The glass samples had a composition of 60TeO 2 – 10GeO 2 –10K 2 O–10Li 2 O–10Nb 2 O 5 doped with 30 000 ppm / wt of Yb 31 and 1000 ppm / wt of Er 31 or Pr 31 . The fluorescence signal was collected by a fiber-bundle, and

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00374-2

P.V. dos Santos et al. / Journal of Alloys and Compounds 344 (2002) 304–307

was dispersed by a 0.34 m scanning spectrograph with operating resolution of 0.5 nm and detected by a S-20 uncooled photomultiplier tube. A lock-in amplifier in conjunction with a storage-scope coupled to a microcomputer was used for data acquisition and storage. The temperature of the samples was increased from 20 to 230 8C by placing it into an aluminum oven heated by resistive wire elements. A copper–constantan thermocouple (reference at 0 8C) attached to one of the sample’s faces was used to monitor the temperature within |2 8C accuracy.

3. Results and discussion

3.1. Upconversion fluorescence spectroscopy A typical room-temperature emission spectrum of the Pr 31 / Yb 31 -codoped sample is presented in Fig. 1. It exhibits emission bands centered around 490, 540, 625 and 655 nm corresponding to the 3 P0 → 3 H 4 , 3 P0 → 3 H 5 , 3 3 1 3 3 3 31 P0 → H 6 1 D 2 → H 4 , and P0 → F 2 transitions of Pr ions, respectively. Pumping of the Pr 31 excited-state emitting levels is accomplished through a combination of multiphonon-assisted absorption of the Yb 31 -sensitizer, energy-transfer and multiphonon-assisted excited-state absorption of the Pr 31 -acceptor, as portrayed in the inset of

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Fig. 1. In a first step, a pump photon at 1.064 mm provokes a multiphonon-assisted anti-Stokes excitation of the Yb 31 sensitizer from the 2 F 7 / 2 ground-state to the 2 F 5 / 2 excitedstate level. The excited Yb 31 transfers its energy to a neighbor Pr 31 ion in the 3 H 4 ground-state, exciting it to the 1 G 4 level. This excited Pr 31 ion undergoes a multiphonon-assisted anti-Stokes excited-state absorption of a second pump photon which promotes it to the 3 P0 upper emitting level. Finally, the excited Pr 31 ion decays from 3 P0 either radiatively to generate the main visible fluorescence emission bands or nonradiatively to populate lowerlying luminescent levels. The dependence of the blue emission intensity upon the excitation power at room temperature was examined and the results exhibited a quadratic power law behavior as expected for a process involving two pump photons. Fig. 2 shows typical roomtemperature visible emission for the Er 31 / Yb 31 -codoped sample. The spectrum shows emission bands centered around 530, 555, and 670 nm corresponding to the 2 H 11 / 2 , 4 S 3 / 2 , and 4 F 9 / 2 transitions to the 4 I 15 / 2 ground state of erbium ions. The upconversion excitation mechanism responsible for the population of the visible emitting levels 31 31 in Er / Yb -codoped samples excited at 1.064 mm, already reported for bulk glasses [4], is shown in the inset of Fig. 2. So, it suffices to mention here that the upconversion pumping process of the excited-state levels of the Er 31 -acceptor is obtained by means of the phonon-assisted anti-Stokes excitation of the Yb 31 -sensitizer from the 2 F 7 / 2

Fig. 1. Frequency upconversion emission spectrum for the Pr 31 / Yb 31 -codoped sample at room temperature.

P.V. dos Santos et al. / Journal of Alloys and Compounds 344 (2002) 304–307

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Fig. 2. Frequency upconversion emission spectrum for the Er 31 / Yb 31 -codoped sample at room temperature.

ground state to the 2 F 5 / 2 excited-state. The excited Yb 31 transfers its energy to an Er 31 ion, exciting it to the 4 I 11 / 2 level, and a subsequent energy-transfer process promotes 31 4 4 the Er ions from the I 11 / 2 to the upper F 7 / 2 excitedstate. Multiphonon-assisted nonradiative decays from the 4 F 7 / 2 excited-state populate the 2 H 11 / 2 , 4 S 3 / 2 , and 4 F 9 / 2 emitting levels. The dependence of the visible signals upon excitation intensity was inspected and results presented an approximately quadratic power law behavior with pump intensity, owing to the onset of saturation on the emission intensity for pump powers above 400 mW.

mands at least one optical phonons in order to match the 21 energy difference of approximately 930 cm between the 1 3 pump-photon energy and that of the G 4 → P0 transition of 31 31 Pr . Accordingly, the population of the Pr excited-state 3 P0 level relies strongly upon the phonon occupation number in the host matrix. The multiphonon-assisted absorption leads to temperature dependent effective absorption cross-sections for both sensitizer and acceptor,

3.2. Thermal effects The dependence of the upconversion fluorescence for the Pr 31 / Yb 31 -codoped sample upon temperature was investigated for a fixed excitation power and the results are presented in Fig. 3. As one observes, the upconversion visible fluorescence has enhanced by a factor of 310 in the temperature range of 20 to 190 8C. The excitation of the Yb 31 -sensitizer from the 2 F 7 / 2 ground-state to the 2 F 5 / 2 excited-state requires the participation of one optical phonon in order to compensate for the energy mismatch of |800 cm 21 between the incident photon at 1.064 mm and the ytterbium transition energy [5] which corresponds approximately to the value of the peak of the phonon spectrum for tellurium-oxide glasses. Furthermore, the praseodymium 1 G 4 → 3 P0 excited-state absorption also de-

Fig. 3. Temperature dependence of the visible upconversion emission signals for the Pr 31 / Yb 31 - and Er 31 / Yb 31 -codoped samples at a fixed excitation power. The solid lines stand for the theoretical fit for the experimental data.

P.V. dos Santos et al. / Journal of Alloys and Compounds 344 (2002) 304–307

which are increasing functions of the sample temperature yielding the enhancement of the populations of the emitting levels. The temperature dependence of the population of the Pr 31 excited-state 3 P0 level can be described by the following set of rate-equations [6]: ne n~ e 5 n g sge (T )F 2 n e CS2 n 0 2 ], tS

(1.a)

n2 n~ 2 5 n e CS2 n 0 2 n 2 s23 (T )F 2 ], t2

(1.b)

n3 n~ 3 5 n 2 s23 (T )F 2 ], t3

(1.c)

(2)

0

where s is the absorption cross-section at resonance, hnphonon is the phonon energy, k B is the Boltzmann constant and T the absolute temperature. The exponent p accounts for the number of phonons taking part in the anti-Stokes absorption processes. Combining the above equations, one obtains the steady-state population, already reported [7], of the 3 P0 emitting level as

t2t3tS s23 (T )NA NS CS2 sge (T )F 2 n 3 ( ]]]]]]]]], s1 1 tS CS2 NAd

In conclusion, upconversion luminescence emission and 31 31 31 31 thermal effects in Pr / Yb - and Er / Yb -codoped tellurite-based glasses pumped by CW infrared radiation at 1.064 mm was investigated for the first time. Generation of intense green and red fluorescence emission in Er 31 / Yb 31 -codoped samples and appreciable upconversion fluorescence in the region of 450–680 nm in Pr 31 / Yb 31 codoped samples was observed. Our results revealed a temperature-induced enhancement in the upconversion efficiency of 312 in Er 31 / Yb 31 - and 310 in Pr 31 / Yb 31 doped samples in the temperature range of 20 to 190 8C.

Acknowledgements The financial support for this research by FINEP, CNPq, CAPES, PADCT, FAPESP and PRONEX-NEON is gratefully acknowledged.

(3)

where NA and NS 5n e 1n g , are the Pr 31 and Yb 31 concentrations, respectively. The light intensity emitted of the 3 P0 level is then given by I( 3 P0 → i) 5 hn3i A 3i n 3

as a function of the temperature was also investigated and the results are depicted in Fig. 3. As can be seen, the visible emission efficiency increased by a factor of 312 when the sample was heated from room temperature up to 230 8C. By using a theoretical approach similar to the one demonstrated for the Er 31 / Yb 31 system in Ref. [7], we have obtained the effective populations for the pertinent coupled emitting levels of the Er 31 / Yb 31 system and with the proper parameters, the visible emission intensity is obtained and generates the theoretical fit indicated by the solid line in Fig. 3.

4. Conclusions

where n e C S2 is the sensitizer-acceptor energy-transfer rate, tS , t2 , and t3 are the lifetimes of the levels 2 F 5 / 2 (level e), 1 3 G 4 (level 2), and P0 (level 3), respectively, and F is the power flux. In Eq. (1), sge (T ) and s23 (T ) represent the temperature-dependent effective absorption cross-sections for the Yb 31 excitation and Pr 31 excited-state absorption, respectively, owing to the so-called multiphonon-assisted anti-Stokes excitation process [5]. The absorption crosssections can be written in a general form as

s (T ) 5 s 0 f exp(hnphonon /k B T ) 2 1 g 2p ,

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(4)

where A is the radiative transition rate from level 3 to the i level and n3i its frequency. Introducing Eq. (3) into Eq. (4) we have obtained the temperature dependence of the visible emission intensities of the 3 P0 level and the result is illustrated by the solid lines in plot of Fig. 3. As can be observed, indeed the theoretical model matches very well the experimental results. The dependence of the visible upconversion emission for the Er 31 / Yb 31 -codoped sample

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