Rare-earth doped transparent nano-glass-ceramics

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Rare-earth doped transparent nano-glass-ceramics: a new generation of photonic integrated devices Vicente Daniel Rodríguez-Armas1, Victor K. Tikhomirov2, Jorge Méndez-Ramos1, Angel C. Yanes3, Javier Del-Castillo3, David Furniss2 and Angela B. Seddon2. 1

Dpto. Física Fundamental y Experimental, Electrónica y Sistemas, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain. 2 Novel Photonic Glasses Group, Wolfson Centre for Materials Research, Nottingham University, Nottingham, NG7 2RD, UK. 3 Dpto. Física Básica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain.

ABSTRACT

We report on optical properties and prospect applications on rare-earth doped oxyfluoride precursor glass and ensuing nano-glass-ceramics. We find out the spectral optical gain of the nano-glassceramics and show that its flatness and breadth are advantageous as compared to contemporary used erbium doped optical amplifiers. We present the possibility of flat gain cross-section erbium doped waveguide amplifiers as short ‘chip’, all-optical, devices capable of dense wavelength division multiplexing, including the potential for direct writing of these devices inside bulk glasses for threedimensional photonic integration. We carried out a comparative study of the up-conversion luminescence in Er3+-doped and Yb3+-Er3+-Tm3+ co-doped samples, which indicates that these materials can be used as green/red tuneable up-conversion phosphors and white light simulation respectively. Observed changes in the spectra of the up-conversion luminescence provide a tool for tuning the colour opening the way for producing 3-dimensional optical recording. Keywords: luminescence, nanostructured materials, vitroceramics

INTRODUCTION Wavelength division multiplexing (WDM) increases telecommunication capacity, thus up to 100 different ‘channels’ i.e. wavelengths of light around 1.55 µm may be simultaneously transmitted [1]. Achieving more channels for DWDM (dense WDM) would require that the luminescent band of Er3+ be broader and flatter than in present Er3+ doped fibre amplifier (EDFA). On the other hand, infrared-tovisible up-conversion luminescence of rare-earth dopants has attracted recently a new interest due to a substantial increase of the up-conversion efficiency in some novel materials, see e.g. [2] for a review, and due to the possibility of tuning the colour of the up-conversion luminescence by changing the pump power/duration and/or laser-induced ceramming of the host material [3-4]. The applications of the efficient tuneable infrared-to-visible up-conversion emission include three-dimensional optical recording, tuneable infrared phosphors or biological labels [3-4]. The class of materials we have termed nano-glass-ceramics offers broader, flatter gain (fairly flat emission in the range 1530 - 1560 nm, which falls within the most frequently employed C-band of WDM, and also covers the S band) [5], than current EDFA and can accept doping levels of erbium more

6593-37 V. 5 (p.1 of 8) / Color: No / Format: A4 / Date: 3/30/2007 6:22:41 AM SPIE USE: ____ DB Check, ____ Prod Check, Notes:


than two orders of magnitude higher than silica fiber (of ~ 45000 ppm), therefore with promising applications as new, short ‘chip’, all-optical amplifiers with active regions only a few millimetres in length. These nano-glass-ceramics are structurally interesting. Using high resolution transmission electron microscopy we have imaged [6] exceedingly small, 0.5 nm diameter, pre-nucleation ordered regions (embryos) of the nano-crystalline phase and irrefutable evidence of the selective partitioning of rare earth ions into the nano-crystallites; the nano-crystalline phase is rare earth doped PbF2. Specific doped compositions exhibit self-limited growth of spherical nano-crystallites during deliberately prolonged heat treatments above Tg, therefore exhibiting a highly reproducible temperature/time “window of opportunity" for shaping, if required [6]. Moreover, in this work we have studied up-conversion phenomena in singly Er3+-doped and 3+ 3+ Yb -Er -Tm3+ co-doped samples, showing a potential use as infrared tuneable phosphor emitting either green or red visible luminescence and presenting white light generation by additive synthesis of red, green and blue (RGB) simultaneous up-conversion emissions [7,8]. Observed changes in the spectra of the up-conversion luminescence from the precursor glasses to nano-glass-ceramics provide a tool for tuning the colour by ceramming of the precursor glasses, opening the way for producing 3-dimensional optical recording. Recently, we have also demonstrated direct laser writing of Er3+ doped, nanocrystalline buried channels within the bulk of parent glasses using either femtosecond or continuous wave focused laser radiation [9]. This development opens the way for producing 3-dimensional integrated erbium doped waveguides amplifiers (EDWA) and other photonic integrated devices.

EXPERIMENTAL The samples used in this study were prepared by conventional melting technique with the following batch composition in mol%: 32(SiO2) 9(AlO1.5) 31.5(CdF2) 18.5(PbF2) 5.5(ZnF2): 3.5(ErF3) mol% for the Er3+-doped samples and 32(SiO2) 9(AlO1.5) 31.5(CdF2) 18.5(PbF2) 5.5(ZnF2) 2.5(YbF3) 0.5 (ErF3) 0.5 (TmF3) for the Yb3+-Er3+-Tm3+ co-doped samples. We used the same procedures for preparation of the parent precursor glasses and the resulting glass-ceramics as those described in [6]. The transparent oxyfluoride nano-glass-ceramics were obtained by thermal treatment above Tg of precursor glasses to precipitate nanocrystals. In these nano-glass-ceramics, the doping level was two orders of magnitude higher than in typical commercial silica EDFA [1]. X-ray diffraction measurements were carried out. The precursor glasses show a broad diffraction curve characteristic of the amorphous state, while x-ray diffraction curves of the heat-treated samples show reflections peaks from crystalline particles, confirming that cubic lead fluoride nanocrystallites were successfully precipitated during thermal treatments. The nano-particle diameters were about 7 to 8 nm, as detected by means of transmission electron microscopy [6]. Each dopant partitions efficiently to the fluoride PbF2-based nanocrystals. Our procedures for measurements of the emission and absorption spectra are described in [10]. Spectra were performed at room temperature and also at 13 K. The latter was to improve the spectral resolution and to distinguish the contributions from thermally populated Stark sublevels. Up-conversion emission spectra were recorded exciting the samples by means of either a CW tuneable Ti:sapphire laser at 800 nm or a laser diode at 980 nm (up to 200 mW power).

RESULTS AND DISCUSSION We carried out a detailed spectroscopic investigation of the 1.5 micron 4I13/2 ↔ 4I15/2 transition in the Er -doped nano-glass-ceramics, both at room temperature and at 13K. We have investigated the reasons for the broad, flat luminescence of Er3+ as it might be expected that the luminescent bands would tend to sharpen on formation of the nano-glass-ceramics, due to lowered non-homogeneous broadening for Er3+ in the nano-crystalline environment compared to the glass. 3+



We have resolved the Stark sublevels for the ground 4I15/2 and excited 4I13/2 states and proposed an energy level sequence diagram for these states [5]. Comparison of low and room temperature spectra has demonstrated that the unusual spectral broadening may be accounted for the distribution of the total oscillator strength of the 4I13/2→4I15/2 emission band by the extra transitions available at room temperature due to the thermally populated Stark levels [5], see Fig.1.

Er

GC

Intensity (arb. units) 1400

3+

4

I13/2

1450

4

1500 1550 1600 Wavelength (nm)

I15/2

1650

Fig. 1. Emission spectrum 4I13/2→4I15/2 of Er3+-doped nano-glass-ceramics at room temperature (solid lines) and at 13 K (dotted line). The rectangle indicates the part of the room temperature spectrum due to thermal population of the Stark sub-levels of the excited state. Excitation was at 800 nm and 50 mW of Ti: sapphire laser.

0.8

1

Gain cross section σg (*10

-20

2

cm )

Next, we model the gain of the Er3+ doped nano-glass-ceramics, showing the potential for broader flatter gain than current EDFA [11].

3+

Er

0.9 0.4

GC

0.8 0.6

0.0 0.5 0.4

-0.4

0.2 0

-0.8

1400

1450

1500 1550 Wavelength (nm)

1600

1650

Fig. 2. Gain cross-section spectra of the 4I13/2→4I15/2 transition of the Er3+ in the nano-glass-ceramic at room temperature. The respective population inversions P are indicated (0 to 1).



The gain cross-section spectrum σg(λ) is computed by using the equation:

σ g = P ⋅ σ sem − (1 − P) ⋅ σ abs

(1)

where σsem(λ) is the stimulated emission cross-section calculated from the experimental emission spectrum using the method of Füchtbauer-Ladenburg [12], σabs(λ) is the absorption cross-section calculated from experimental absorption spectrum, and P is the population inversion defined as the ratio of Er3+ ions concentration in the lasing level 4I13/2 to the total concentration N. Figure 2 shows the family of gain cross-section spectra in singly Er3+-doped nano-glass-ceramics derived for several values of P, ranging from 0 to 1 [11]. It is seen that the gain spectrum of the nano-glass-ceramics is relatively flat in the range from 1.53 to 1.56 µm covering both C-and S-bands of telecommunications window, which is advantageous as compared to commercial EDFA. Moreover, the flat gain region in nano-glass-ceramic substantially exceeds flat gain region in tellurite glasses and fluoride glasses reported to date, which is advantageous for the overall amplification gain of the Er3+-doped optical amplifier [11]. We report now on the comparative study of the up-conversion luminescence in single Er3+ doped nano-glass-ceramics and their precursor glasses, which indicates that these materials can be used as green/red tuneable up-conversion phosphors [7].

3+

4

2

2

4

2

Er

4

S3/2( H11/2)

I15/2

4

( G F H)9/2 I15/2

Intensity (arb. units)

G

500

4

4

F9/2 I15/2

4

4

4

F5/2 F3/2 I13/2

GC

550 600 650 Wavelength (nm)

700

Fig. 3. Up-conversion luminescence spectra excited at 800 nm at room temperature (thick lines) and 13K (thin lines) of the Er3+-doped precursor glasses (G) and Er3+-doped nano-glass-ceramics (GC).



Figure 3 shows up-conversion emission spectra excited at 800 nm at room and low temperature for the Er3+-doped precursor glasses and ensuing nano-glass-ceramics. Apart from the green (545 nm) and red (660 nm) emission, corresponding to transitions indicated in Figure 3, violet and blue up-conversion emission have also been observed, however their intensity was two orders of magnitude smaller and therefore we do not show them. The up-conversion luminescence mechanism excited at 800 nm may be a result of several processes of excited state absorption and energy transfer. The relative weight of these upconversion processes changes with temperature and average distance between the Er3+ ions. The resulting emission routes/mechanisms and therefore up-conversion emitting colour will change respectively. Additional experiments were carried out by using direct excitation of the Er3+ to determine which of these emission routes/mechanisms are dominant [7]. The dominance of the red emission in the up-conversion luminescence at room temperature in nano-glass-ceramics may be explained as due to decrease of the vibration energy of phonons coupled to the Er3+ ions after nano-ceramming. Previously, we had shown the coupled phonon energy in nano-glass-ceramics is 250 cm-1 while in the precursor glasses it is substantially higher, about 900 cm-1 [10, 13]. Due to this decrease in phonon energies, the red emission from the 4F5/2, 4F3/2 levels is not quenched in nano-glass-ceramics by multi-phonon relaxation as in the precursor glasses, where the green emission dominates the up-conversion luminescence. We also see that the ratio of red-to-green emission does change substantially with ceramming of precursor glasses, opening the way for applications as tuneable up-conversion phosphors materials. Next, we present the up-conversion luminescence in Yb3+-Er3+-Tm3+ co-doped precursor glasses and nano-glass-ceramics. Corresponding spectra under 980 nm pumping is presented in Figure 4, focusing the attention on the red, green and blue part of the spectrum, interesting from the viewpoint of RGB generation devices. Assignation of emissions to transitions of Er3+ and Tm3+ ions are indicated.

3+

3+

3+

3+

Intensity (arb. units)

Yb -Er -Tm

4

Er

Tm

3+

3

3

F3 H6

4

F9/2 I15/2 3+

Tm

1

3

G4 F4

GC 3+

Tm 1

3

G4 H6

4

Er

3+

2

4

S3/2( H11/2) I15/2

G

400

500

600

Wavelength (nm)

700

Fig. 4. Up-conversion luminescence spectra excited at 980 nm at room temperature (thick lines) and 13K (thin lines) of the Yb3+-Er3+-Tm3+ co-doped precursor glasses (G) and Yb3+-Er3+-Tm3+ co-doped nanoglass-ceramics (GC).

In Yb3+-Er3+-Tm3+ co-doped precursor glasses, the low temperature spectrum recorded at 13 shows blue green and red emissions located at 480, 550 and 650 nm, respectively. When temperature is



increased up to room temperature, we can still observe the same three main emission bands, with additional shoulders due to thermal population. On the other hand, in Yb3+-Er3+-Tm3+ co-doped nanoglass-ceramics, up-conversion emission spectrum at 13 K consists of two main emission peaks: the green one, centred at about 550, and the red at 650-700 nm. The emission at around 480 nm is dramatically quenched. At the same time, the red emission peak at about 700 nm, corresponding to Tm3+ transition indicated in Fig. 4, is radically enhanced. The same features can be found at room temperature, with additional shoulders due to thermal population. It is worth noting that up-conversion emission peaks show sharper structure with better-resolved Stark components in the glass-ceramics, indicative of the partition of the dopant ions into fluoride nanocrystals precipitated during the heat treatment. Therefore, when the environment of the ions is modified during nano-ceramming of the precursor glasses, a significant change in the up-conversion peaks and therefore in the colour emitted from the sample is observed. We study the mechanism involved in the quenching of the blue luminescence at 480 nm and the enhancement of the red one at 700 nm corresponding to Tm3+ transitions in nano-glass-ceramics [8]. The quenching of blue luminescence at 480 nm from 1G4 level and also the increase of the population of the 3 F3,2 level, which enhances the red emission at 700 nm, can be due to the low multiphonon relaxation rate of ions in fluoride crystalline environment and to the shortening of distance between them during nanoceramming, which favours cross relaxation processes. Finally, we focus the attention on the application of these up-conversion phenomena in the Yb3+3+ 3+ Er -Tm co-doped glasses and nano-glass-ceramics as a generation of red, green and blue colours. In this sense we study the CIE colour coordinates (standard chromaticity diagram) corresponding to these co-doped sample emissions in the colour space diagram, obtained from the human eye response for short, middle, and long wavelengths, also known as blue, green, and red receptors. The standard CIE diagram represents all the chromaticities visible to the human eye plotted in the tongue-shaped curve presented in Fig. 5. The edge corresponds to monochromatic light, while less saturated colours appear in the interior of the figure with white at the center. So, we present in Fig. 5, the CIE colour coordinates corresponding to Yb3+-Er3+-Tm3+ co-doped precursor glasses at room temperature. As it can be seen the point is very close to the white center of the diagram giving rise to a white light simulation by simultaneous generation of red, green and blue emissions. However, in Yb3+-Er3+-Tm3+ co-doped nano-glass-ceramic, and due to the quenching of the blue emission, the resultant emission gives rise to green light represented in Fig. 5.

0.8

CIE colour coordinates 3+

3+

3+

Yb -Er -Tm

y

0.6

GC

0.4

G

0.2 0.0 0.0

0.2

0.4 x

0.6

0.8

Fig. 5. Colour coordinates in the CIE standard chromaticity diagram corresponding to the up-conversion luminescence excited at 980 nm at room temperature of the Yb3+-Er3+-Tm3+ co-doped precursor glasses (G) and Yb3+-Er3+-Tm3+ co-doped nano-glass-ceramics (GC).



Observed changes in the spectra of the up-conversion luminescence and therefore in the emitting colour from glasses to nano-glass-ceramics provide a tool for tuning the resultant colour by ceramming of the precursor glasses. Nano-glass-ceramics can be developed either by heat-treatment of precursor glasses and excitingly we have demonstrated recently direct laser writing of Er3+ doped, nano-crystalline buried channels within the bulk of the parent glasses using either femtosecond or continuous wave focused laser radiation of appropriate photon energy [9]. This development opens the way for producing 3-dimensional optical recording based on the tuneability of up-conversion emitting colour from glasses to glassceramics.

CONCLUSIONS In summary, the nano-glass-ceramics presented in this work accept high doping levels of rare earth ions, without concentration quenching, and have broader, flatter gain than current EDFA, showing promise therefore as new, short ‘chip’, all-optical amplifiers. The room temperature broadest and flattest 1.55 µm telecommunications emission band 4I13/2→4I15/2 of Er3+ for such glass-ceramics, substantially exceeding that in tellurite and fluoride glasses reported to date, is due to thermal population of high Stark sub-levels in the excited 4I13/2 state and to a fortuitous distribution of the total oscillator strength of the 4 I13/2→4I15/2 transition between its thermally populated Stark components. The gain spectrum of this transition of the Er3+-doped nano-glass-ceramics becomes relatively flat in the range from 1.50 to 1.56 µm, which covers both C- and S-bands of telecommunication window. Here we also report that Er3+doped oxyfluoride precursor glasses and ensuing nano-glass-ceramics have potential use as infrared tuneable phosphor emitting either green or red visible luminescence. The ratio of red-to-green upconversion emission increases by more than order of magnitude with nano-ceramming of the precursor glasses. An especial role of phonon energy coupled to Er3+ ions, changing with nano-ceramming of the precursor glasses, has been pointed out. Finally, we report on the up-conversion luminescence in the Yb3+-Er3+-Tm3+ co-doped precursor glasses and nano-glass-ceramics, giving rise to a white light simulation by simultaneous generation of red, green and blue emissions. Moreover, the tuneability of the up-conversion emitting colour, from glasses to nano-glass-ceramics, opens the way for 3-dimensional optical recording applications based on laser writing techniques.



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