SPIE Proceedings [SPIE Integrated Optoelectronic Devices 2006 - San Jose, CA (Saturday 21 January 2006)] Optical Components and Materials III - Optical properties and energy-transfer

Optical properties and energy-transfer frequency upconversion of Yb3+-sensitized Ho3+- and Tb3+-doped lead-cadmium-germanate glass

and glass ceramic

Artur S. Gouveia-Neto*a, Antônio C. M. Afonsoa, Jehan F. Nascimentoa, Ernande B. Costaa, Luciano A. Buenob, Younes Messaddeqc and Sidney J. L. Ribeiroc

aLaboratório de Fotônica, DFM/UFRPE, Recife, PE, 52171/900, Brasil bUNOESC – ACET – Videira – SC – 89560/000 – Brasil

cInstituto de Química, UNESP, Araraquara, 14800/900, SP, Brasil

ABSTRACT In this report we investigate the optical properties and energy-transfer upconversion luminescence of Ho3+- and Tb3+/Yb3+-codoped PbGeO3-PbF2-CdF2 glass-ceramic under infrared excitation. In Ho3+/Yb3+-codoped sample, green(545 nm), red(652 nm), and near-infrared(754 nm) upconversion luminescence corresponding to the 4S2(5F4) → 5I8, 5F5 → 5I8, and 4S2(5F4) → 5I7, respectively, was readly observed. Blue(490 nm) signals assigned to the 5F2,3 → 5I8 transition was also detected. In the Tb3+/Yb3+ system, bright UV-visible emission around 384, 415, 438, 473-490, 545, 587, and 623 nm, identified as due to the 5D3(5G6) → 7FJ(J=6,5,4) and 5D4→7FJ(J=6,5,4,3) transitions, was measured. The comparison of the upconversion process in glass ceramic and its glassy precursor revealed that the former samples present much higher upconversion efficiencies. The dependence of the upconversion emission upon pump power, and doping contents was also examined. The results indicate that successive energy-transfer between ytterbium and holmium ions and cooperative energy-transfer between ytterbium and terbium ions followed by excited-state absorption are the dominant upconversion excitation mechanisms herein involved. The viability of using the samples for three-dimensional solid-state color displays is also discussed. Keywords: glass-ceramics, upconversion, rare-earth, luminescence, optical properties, glass

1. INTRODUCTION

There has recently been a widespread interest in using lanthanide-doped glassy materials as near-infrared(NIR) pumped luminescence sources of visible radiation. The infrared-to-visible light conversion is accomplished through the so called frequency upconversion process in rare-earth doped materials[1-7], which involves the generation of photons with energies higher than the energy of the excitation source. The practical interest of the rare-earth doped solid-state upconverter resides in the possibility of producing photonic devices including solid-state three-dimensional color displays, infrared pumped visible lasers for high density optical data storage, high resolution printing, biomedical diagnostics, sensors, and undersea optical communications. [8-13]. Thus, there exists a demand for novel hosts that produce high upconversion efficiencies and allow suitable combinations of rare-earth luminescence centers which can diversify the visible upconversion emission wavelengths. Glass-ceramics have recently emerged as auspicious contenders for such photonic devices applications. The advantage of using glass ceramic resides in the fact that the rare-earth doping ions are confined in crystalline environments of low phonon energies, yielding large excited-state lifetimes and optical absorption cross sections when compared to vitreous surroundings. Furthermore, the glass host matrix in which the crystals are immersed, possess the durability and mechanical properties of an oxide glass[14-21]. Among rare-earth ions that efficiently generate visible upconversion and infrared fluorescence, holmium and terbium have emerged as promising candidates as the active ions for the conversion of infrared semiconductor laser radiation into the blue-green-red spectral region. Infrared-to-visible upconversion fluorescence emission has recently been investigated in holmium-doped crystals under excitation of different infrared pump wavelengths [22-27]. Frequency upconversion

Optical Components and Materials III, edited by Michel J.F. Digonnet, Shibin Jiang, Proc. of SPIE Vol. 6116, 61160R, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.639820

Proc. of SPIE Vol. 6116 61160R-1

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luminescence in has also been investigated in ytterbium-sensitized terbium-doped glasses[28-34]. Thus, it is imperative the development of novel solid-state hosts capable of producing efficient frequency upconversion processes. Moreover, the glass host matrices possess the durability and mechanical properties of an oxide glass. In this work upconversion luminescence spectroscopy in Ho3+- and Tb3+/Yb3+-codoped PbGeO3-PbF2-CdF2 transparent glass ceramic pumped by a diode laser source around 975 nm, is experimentally investigated.

2. EXPERIMENTAL

Transparent glass ceramic samples were obtained after an appropriate heat treatment of glasses with a composition of 60PbGeO3-10PbF2-30CdF2 in mol%. The glasses were prepared with reagent grade PbF2 and CdF2 and glassy PbGeO3. Starting reagents were mixed in an agate mortar using n-heptane as homogenizing medium. After melting in an open Pt-Au crucible at 800°C for 30 minutes in air, liquids were quenched at room temperature in graphite molds. Some 30 minutes annealing treatments at temperatures around the glass transition were performed. Rare-earth ions were introduced in the form of oxides. Starting PbGeO3 glass was obtained using PbO and GeO2 mixed and melted at 800°C for 30 minutes and quenched to room temperature between two copper plaques. Heat treatment was carried out at the temperatures defined by the near Tg exothermic effects given by DSC(Differential Scanning Calorimetry) curves [29,30]. For treatments with time intervals of up to 48 h samples remained fully transparent and side with halo pattern, and the X-ray diffractograms, have shown the reflections peaks expected from crystalline particles. Typical samples with concentrations of 0.2, 0.3, 0.4 and 0.5 mol% of Ho3+ doping and 0.1 mol% of Yb3+, presenting crystals immersed in the amorphous material, with no observable clustering and a size distribution of ~25 nm, were obtained. The same procedure was applied to obtain terbium samples with concentrations of 0.1 to 0.5 mol% and 0.4 mol% of ytterbium ions. Figure 1 shows a TEM(transmission electronic microscopy) micrograph obtained for an Ho3+/Yb3+-doped sample treated for 48h at 335oC. The samples were approximately 1.0 mm thick and the excitation source was a cw diode laser operated around 975 nm. The pump beam was focused down onto the samples by a 5 cm focal length lens and the beam waist at the samples location was ∼60 µm. Thus, excitation densities in the samples of up to 25x106 W/m2 were achieved for the maximum laser output power of 70 mW. Furthermore, the luminescence signal was collected perpendicularly to the pump beam direction in order to avoid pump scattered light into the detection system, and 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. Phase detection was used for data acquisition and storage. A typical absorption spectrum of our Ho3+/Yb3+-codoped samples is depicted in figure 2. All measurements throughout our experiment were performed at room temperature. All rare-earth-doped glass and glass ceramic samples present very good optical quality, they are stable against atmospheric moisture, and exhibit low optical attenuation in the spectral region of interest as indicated by the absorption spectrum depicted in figure 2. The material also exhibits good solubility allowing the incorporation of suitable lanthanide concentrations apart from being nonhygroscopic and possess high mechanical and chemical stability.

3. RESULTS AND DISCUSSION 3.1 Upconversion in Ho3+/Yb3+-doped glass and glass ceramic

Figure 3 shows a typical room-temperature visible emission spectrum of the Ho3+/Yb3+-doped glass sample for 70 mW of excitation power at 975 nm. The spectra exhibit emission bands centered around 490, 545 nm corresponding to the 5F2,3 - 5I8 and 4S2(5F4) – 5I8 transitions, respectively. Red and NIR emission around 652 and 754 nm which was respectively assigned to the 5F5 – 5I8 and 4S2,5F4 – 5I7 transitions of holmium ions, was also observed. Its is worthwhile to mention that the fluorescence emanating from the holmium-ytterbium codoped samples could easely be seen by the protected naked-eye. The luminescence emission intensities of the green, red, and NIR signals against the diode laser power were examined and the results revealed a quadratic behavior as depicted in the log-log plot of Figure 4. The upconversion emission intensity Iupconv depends upon the pump intensity according to Iupconvα(Ipump)n, where n accounts for the number of pump photons involved in the upconversion excitation mechanism. Our results then indicate the participation of two(slope ∼ 2) pump photons in the upconversion excitation mechanism for the green, red and NIR signals, respectively. Based upon this results one would conclude that the processes for green, red and NIR emissions under infrared laser excitation was accomplished through two successive transfers from Yb3+ ions. In the first transfer, a holmium ion is excited from its ground-state 5I8 level to the 5I6 level and the exceeding energy (~1600 cm-1) is transferred to the host matrix. Finally, a second transfer from a nearby ytterbium ion excites the same holmium ion from the 5I6 excited-state to the 4S2(5F4) thermalized emitting levels and the exceeding energy is again transferred to the



host matrix as optical phonons. The excited Ho3+ ion at the 4S2(5F4) then is demoted radiatively to the ground-state and 5I7 state producing green around 545 nm and NIR around 754 nm signals, respecitvely. There exist two possible energy upconversion mechanisms accounting for the resulting red emission band recorded in our measurements. The red emission can be obtained by populating the 5F5 level by nonradiative phonon-assisted relaxation from the 4S2(5F4) excited-state. The holmium ion is excited from its ground-state 5I8 level to the 5I6 level and the exceeding energy (~1600 cm-1) is transferred to the host matrix. From the 5I6 level, it relaxes nonradiatively to the 5I7 level and another pump photon is absorbed populating the 5F5 excited-state emitting level producing the recorded red emission signal as indicated in the energy-level diagram of Figure 5. The upconversion emission intensity of the glass and glass ceramic samples were compared by fixing the pump intensity and all the experimental parameters (i.e., collection and detection system) and utilizing both samples with the same thickness. The samples were placed side-by-side in a translation stage which moved perpendicularly related to the pump beam direction. It can be seen in figure 6, that the transparent glass ceramic sample exhibits a well resolved spectrum with narrower emission lines. In addition, the emission bands present signal intensities approximately two times larger than the ones obtained with the precursor glass sample. This enhancement is attributed to the fact that in the transparent glass ceramic sample, the fluorescent ions are confined in a crystalline environment of low phonon-energy, which are in general ≤ 500 cm-1 [18-21], providing large excited-state lifetimes and narrower spectral lines, as compared to the vitreous surrounding in the glassy sample. We have also investigated the dependence of the upconversion signals upon the holmium content in the sample. For a fixed Yb3+ concentration and varying Ho3+ content, the upconversion intensity presented the results depicted in the series of spectra of figure 7. As one observes, the green emission signal rapidly grows with Ho concentration by an order of magnitude while the red signal changes by a factor of two. The important feature of this result is that one may generate two of the three primary colors with controlled relative intensities. Furthermore, as we have shown recently[27] by pumping a holmium single-doped sample with 975 nm radiation just a very intense red signal is generated. The results indicate that by using a proper combination of rare-earth fluorescent ions one can generate the three primary colors in one single sample using a single NIR pump source, for application in three-dimensional solid-state color displays[8].

3.2 Upconversion in Tb3+/Yb3+-doped glass and glass ceramic

Figure 8 shows typical room-temperature UV-visible emission spectra of the bright light emanating from the Tb3+/Yb3+-codoped glass and glass ceramic samples for 70 mW of excitation power at 975 nm. The spectra exhibits upconversion emission around 384, 415, 438, 487, 545, 587, and 623 nm, identified as due to the5D3(5G6) → 7FJ(J=6,5,4) and 5D4→7FJ(J=6,5,4,3) transitions of the Tb3+ ions, respectively. The population of the terbium-ions excited-states emitting levels was accomplished through cooperative energy-transfer from pairs of Yb3+ ions to a neighbor Tb3+ ion for the 5D4→7FJ(J=6,5,4,3) transitions[28-31], and cooperative energy-transfer from pairs of Yb3+ ions to a neighbor Tb3+ ion followed by excited-state absorption of the Tb3+ ions in the 5D4 level[29,30,32]. Firstly, incident pump photons at 975 nm are absorbed by Yb3+ ions at the 2F7/2 ground-state, promoting them to the 2F5/2 excited-state level. A pair of excited Yb3+ ions relax simultaneously and transfer their energy to a nearby Tb3+ ion at the 7F6 ground-state, exciting it to the upper excited-state 5D4 level. From the 5D4 level the Tb3+ ions radiatively relax to the 7FJ lower laying levels generating the recorded 487, 545, 587, and 623 nm emission signals shown in Figure 8. For the 384, 415, and 438 nm signals, the excited Tb3+ ions at the 5D4 absorb another pump photon at 975 nm and are promoted to the 5D1 energy level. The population of the 5D3(5G6) emitting level is sustained by nonradiative decay from the 5D1 level. The intensity of the upconversion emission lines as a function of the laser power was investigated and the results obtained is portrayed in the log-log plot of Figure 9. The results corroborate with the proposed upconversion excitation mechanism for the emitting levels, with the participation of two pump photons in the upconversion excitation mechanism for the 490, 545, 587, and 623 nm emissions(slope ~2) as expected for a cooperative energy-transfer process involving a pair of Yb3+ ions and a Tb3+ ion, and a process involving the participation of three pump photons for the 384, 415, and 438 nm signals(slope ~3).There exist three possible routes for the population of the emitting level responsible for the UV-blue rcorded emissions. However, it was experimentally demonstrated in reports of Ref. 28 that, pump excited state-absorption is the mechanism that supresses the green emission while the pump laser is on and gives rise to the UV-blue emission. The possibility of simultaneous energy-transfer from three Yb3+ ions to one Tb3+ is ruled out, as no increase in the blue fluorescence was observed on turning the pump laser off, and the blue lifetime remains constant in the presence of energy-transfer. Multiphoton absorption is most unlikely to occur due to the low intensity of the laser used throughout our measurements[28,29]. The upconversion and emission processes can be better envisioned with the help of the simplified energy level diagram for the terbium-ytterbium ion pair shown in Figure 10, where it is indicated the energy-transfer and the excited-state absorption excitation steps and the energy levels involved in the seven transitions responsible for the recorded UV-visible emission signals.



4. CONCLUSIONS

In conclusion, we have experimentally investigated the optical properties and energy-transfer upconversion luminescence of Ho3+- and Tb3+/Yb3+-codoped lead-cadmium-fluorogermanate glass and glass-ceramic under infrared excitation around 975 nm. In Ho3+/Yb3+-codoped samples, green(545 nm), red(652 nm), and near-infrared(754 nm) upconversion luminescence corresponding to the 4S2(5F4) → 5I8, 5F5 → 5I8, and 4S2(5F4) → 5I7, respectively, was readly observed. Blue(490 nm) signals assigned to the 5F2,3 → 5I8 transition was also detected. The upconversion excitation processes for the green, red and NIR emissions under infrared laser excitation was accomplished through two successive energy-transfer from Yb3+ ions to Ho3+ ions. In the Tb3+/Yb3+-codoped system, bright UV-visible emission around 384, 415, 438, 473-490, 545, 587, and 623 nm, identified as due to the 5D3(5G6) → 7FJ(J=6,5,4) and 5D4→7FJ(J=6,5,4,3) transitions, was measured. The population of the terbium-ions excited-states emitting levels was accomplished through cooperative energy-transfer from pairs of Yb3+ ions to a neighbor Tb3+ ion for the 5D4→7FJ(J=6,5,4,3) transitions, and cooperative energy-transfer from pairs of Yb3+ ions to a neighbor Tb3+ ion followed by excited-state absorption of the Tb3+ ions in the 5D4 level. The comparison of the upconversion process in glass ceramic and its glassy precursor revealed that the ceramic samples present much higher upconversion efficiencies. The results indicate that by using a proper combination of rare-earth fluorescent ions(holmium, tulium and ytterbium for instance), one can generate the three primary colors in one single sample using a single NIR pump source, which may find application in three-dimensional solid-state color displays.

ACKNOWLEDGEMENTS The financial support for this work by FINEP (Financiadora de Estudos e Projetos), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) grant ref: 470273/2003-08, PADCT (Programa de Apoio ao Desenvolvimento Científico e Tecnológico), and PRONEX-NEON (UFPE/UFAL/UFPB) is gratefully acknowledged. The research of Sidney J. L. Ribeiro and Younes Messadeq has the financial support from FAPESP(Fundação de Amparo à Pesquisa do Estado de São Paulo – Brasil). Luciano A. Bueno is supported by UNOESC. Antônio C. M. Afonso, and Jehan F. Nascimento are supported by undergraduate studentship from PIBIC/CNPq/UFRPE

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Fig. 1.TEM micrography obtained for the transparent Ho3+/Yb3+-codoped sample submitted to a 48h treatment at 335oC.

Fig. 2. Absorption spectrum for the holmium-ytterbium doped glass ceramic sample

400 500 600 700 8000.1

0.2

0.3

0.4

5G4,4K7

5F5

4S2+5F4

3K8,5F2

5F3

5G5

5G6

Abs

orpt

ion(

a.u.

)

Wavelength (nm)



500 600 700 8000

1

2

3

4

5

6

7

8

9

λP= 975 nm

Pp = 70 mW

450 475 500 525

gain x10

(5S2,5F

4) - 5I

7

5F2,3

-5I8

(5S2,5F4) -

5I8

5F5-5I8

Upc

onve

rsio

n in

tens

ity(a

.u.)

Wavelength(nm)

Fig. 3. Typical upconversion spectrum for the Ho3+/Yb3+-doped sample under 70 mW excitation.

35 40 50 60 70 80 909010-1

100

101

Upc

onve

rsio

n in

tens

ity(a

.u.)

Pump power(mW)

545 nm - slope 2.0 754 nm - slope 1.8 652 nm - slope 1.9

Fig. 4. Log-log plot of the green, red and NIR emission intensities as a function of the excitation power at 975 nm



Fig. 5. Energy-level diagram of the holmium-ytterbium ions system

Fig. 6.Upconversion spectra for the Ho/Yb-doped glass ceramic(outer), and precursor glass(inner) sample for a fixed pump power

5F5

5S2, 5F4

5I8Ho3+

5F1, 5G6

5I55I6

5I7

5I4

5F3

5F2, 3K8

5G5

490

nm

652

nm

545

nm

Yb3+

2F7/2

2F5/2

754

nm

975 nm

450 500 550 600 650 700 750 8000

1

2

3

4

5

glass ceramic sample

glass sample

0.2Yb3+/0.1Ho3+(mol%)λp= 975 nm

Upc

onve

rsio

n in

tens

ity(a

.u.)

Wavelength(nm)



450 500 550 600 650 7000

1

2

3

4

5

6

7

8

Blue

Green

Red

λp= 975 nm

0.1Yb/0.2Ho

0.1Yb/0.3Ho

0.1Yb/0.4Ho

0.1Yb/0.5Ho

Upc

onve

rsio

n in

tens

ity(a

.u.)

Wavelength(nm)

Fig. 7. Upconversion spectra for glass ceramic samples with fixed Yb3+ concentration and varying Ho3+ content

350 400 450 500 550 600 650 7000

1

2

3

glass ceramic sample

glass sample

0.4Yb3+/0.2Tb3+(mol%)λp= 975 nm

5D4 - 7F3

5D4 - 7F4

5D4 - 7F

5

5D4 - 7F6

5 D3,

5 G6 -

7 F 4

5 D3,

5 G6 -

7 F 5

5 D3,

5 G6 -

7 F 6

Upc

onve

rsio

n in

tens

ity(a

.u.)

Wavelength(nm)

Fig. 8.Upconversion spectra for the Tb/Yb-doped glass ceramic(outer), and precursor glass(inner) sample for a fixed pump power



Fig. 9. Log-log plot of the green, blue and UV emission intensities as a function of the pump power at 975 nm

Fig. 10. Energy-level diagram of the terbium-ytterbium ions system

Yb3+ Tb3+

2F7/2

2F5/2

975 nm 7F0

7F6

5D4

7F5

7F4

7F3

7F2

7F1

490

nm

545

nm

587

nm

623

nm

5D1

5D3, 5G6

384

nm

415

nm

438

nm

3030 40 50 6060

103

104

Upc

onve

rsio

n in

tens

ity(a

.u.)

Pump power(mW)

545 nm - slope 1.85415 nm - slope 2.83384 nm - slope 2.70



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SPIE Proceedings [SPIE Integrated Optoelectronic Devices 2006 - San Jose, CA (Saturday 21 January 2006)] Optical Components and Materials III - Optical properties and energy-transfer