Spectroscopy and Near-Infrared to Visible Upconversion of Er³⁺ Ions in Aluminosilicate Glasses Manufactured with Controlled Optical Transmission

Abstract

In this work we report on the spectroscopic properties and the near-infrared to visible upconversion of Er³⁺ ions in aluminosilicate glasses manufactured by directionally solidification with the laser floating zone technique. Glasses were manufactured in a controlled oxidizing atmosphere to provide them with high optical transmission in the visible spectral range. Absorption and emission spectra, and lifetimes were assessed in both the visible and the near infrared spectral range. Green upconversion emissions of the ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions at 525 nm and 550 nm attributed to a two-photon process were observed under excitation at 800 nm. Mechanisms responsible for the upconversion luminescence were discussed in terms of excited state absorption and energy transfer upconversion processes. Excitation spectra of the upconverted emission suggest that energy transfer upconversion processes are responsible for the green upconversion luminescence.

1. Introduction

In recent years rare-earth-doped glasses have been subject of intense research as host materials because of their significant optical properties, which make them adequate as infrared and upconversion lasers, optical amplifiers, and active photonic devices [1,2,3,4,5,6,7,8,9]. In particular, silicate and aluminosilicate glasses present excellent thermal and mechanical properties, and corrosion resistance to be used in practical applications [10,11,12,13,14,15,16,17]. In addition, their maximum phonon energy (~1050 cm⁻¹) is much lower than that of phosphate and borate glasses, ~1300 cm⁻¹ and ~1350 cm⁻¹, respectively, so that quantum efficiency is less influenced by multiphonon relaxation processes [16,17,18,19].

Er³⁺ ions are among the most interesting active centers to be studied because of its potential applications in the field of infrared optical amplification related to the radiative efficiency of the ⁴I_13/2→⁴I_15/2 emission at around 1.55 µm [1,2,20]. Furthermore, the rich energy level structure of this rare-earth allows the excitation of the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2 upconversion emission bands centered at around 530, 550, and 665 nm using wavelengths in the near infrared spectral regions [1,2,21,22]. NIR-to-visible energy conversion mechanism involves the conversion of low-excitation-energy photons into high-energy emitted light in the visible range through non-linear anti-Stokes processes. In addition, these ions can also be used as local ordering probe because of the close relation between their spectroscopic properties and the local structure and bonding at the ion site [23,24,25,26,27].

In a previous work, we reported on how to control the optical transmission of aluminosilicate glasses manufactured departing from commercial glass-ceramics by means of the Laser Floating Zone (LFZ) technique [28]. This technique utilizes an infrared laser source to create a molten zone in the material from which, by controlling the solidification rates, a new material with controlled microstructure is produced. Surrounding medium during the fabrication process, in terms of oxidizing or reducing atmosphere, can also be controlled. We reported that when the fabrication of these aluminosilicate glasses took place in an oxidizing atmosphere, Ti³⁺ centers contained in the glass turned into Ti⁴⁺ ions, giving rise to colorless glasses, the transmittance of which ranged 80% in the visible spectral range. Therefore, it was possible to tailor the resulting optical transmission. In this work we have taken advantage of this feature to fabricate erbium-doped glasses in oxidizing atmosphere to give rise to optical active glasses of high optical transmission. Spectroscopic and NIR-to-visible upconversion properties have been studied and the possible excitation mechanisms responsible for this upconversion luminescence were discussed.

2. Materials and Methods

2.1. Sample Fabrication

Glass-ceramic powder was obtained from a commercial glass-ceramic, Ceran Suprema^®, manufactured by Schott. Next, 1 wt% and 4 wt% of Er₂O₃, ultra-pure 99.99% (Sigma-Aldrich, St. Louis, MO, USA) were mixed and isostatically pressed at 200 MPa for 3 min and sintered at 1200 °C for 12 h to obtain the precursor rods. Glass samples were obtained departing from these erbium-doped glass-ceramic precursors by means of the laser floating zone (LFZ) technique. This manufacturing technique has been described elsewhere [29,30,31]. Glass samples were obtained at a growth rate of 300 mm/h, which provided both high axial and radial cooling gradients to manufacture glass samples. In addition, the manufacturing process was carried out in a semi-sealed chamber, which allowed working in different atmospheres such as oxygen, nitrogen, and air. Specifically, Er-doped glasses were fabricated in oxygen atmosphere to obtain samples with high optical transmission. Taking into account the theoretical weight percentage the samples were doped with, from now on they will be named as Er1 and Er4.

2.2. Characterization Techniques

Composition of glasses was determined by means of field emission scanning electron microscopy microscope (FESEM) with energy dispersive X-ray detector (EDX) (Carl Zeiss, Jena, Germany). EDX technique allows the determination of most elements present in concentration above 0.1% with an estimated accuracy of ±5%.

Absorption spectra were recorded with a Cary 5 spectrophotometer. Steady-state emission were obtained by exciting the sample with an argon laser and a Ti-sapphire ring laser (0.4 cm⁻¹ linewidth) in the 770–920 nm spectral range. The fluorescence was analyzed with a 0.25 m Jobin-Ybon monochromator (Horiba, Kyoto, Japan), and the signal was detected by a Hamamatsu R928 photomultiplier and finally amplified by a standard lock-in technique. Infrared emission at 1.5 µm was detected with an extended IR Hamamatsu R5509-72 photomultiplier (Hamamatsu, Hamamatsu-city, Japan).

Lifetime measurements were performed 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 pulsewidth) (Coherent, Santa Clara, USA), and detecting the emission with Hamamatsu R928 and R5509-72 photomultipliers (Hamamatsu, Hamamatsu-city, Japan). Data were processed by a Tektronix MDO3104 oscilloscope (Tektronix-Inc, Beaverton, OR, USA).

3. Results

3.1. Compositional Characterization

The composition of the Er-doped glass samples after the fabrication process in an oxygen atmosphere was carried out by EDX microanalysis.

Table 1. Compositional analysis in at% of the Er-doped samples manufactured in an oxygen atmosphere.

	Na	Mg	Al	Si	Ti	Zr	Er
Er1	0.70	1.81	28.11	65.99	2.00	0.98	0.41
Er4	0.86	1.75	28.12	65.11	1.84	0.89	1.43

shows the composition for both the samples. It can be observed that SiO₂ and Al₂O₃ were the majority components of the samples, which also included low percentages of NaO, MgO, TiO₂, ZrO₂, and Er₂O₃. The content of Er³⁺ ions in both glasses was calculated accounting the measured content of Er₂O₃ and the density of both glasses, 2.35 g/cm³ and 2.36 g/cm³ for Er1 and Er4, respectively, resulting in 7.92 × 10¹⁹ at/cm³ for Er1 and 2.71 × 10²⁰ at/cm³ for Er4.

3.2. Absorption and Emission Properties

The room temperature absorption spectra were obtained for both samples in the 300–1700 nm range. As an example, Figure 1 shows the absorption spectra as a function of the wavelength for the sample doped with a 4 wt% of Er₂O₃. The spectrum consists of 10 absorption bands corresponding to the transition from the ⁴I_15/2 ground state to the ⁴G_11/2, ²H_9/2, ⁴F_3/2,5/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2 of Er³⁺ excited states ions [1].

Visible emission spectra were obtained at room temperature under excitation of the ⁴F_7/2 level at 488 nm. Multiphonon relaxation processes populated the lower levels resulting in the emission bands observed at around 530, 548, and 660 nm which corresponded to transitions from the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 levels to the ground state. Figure 2 shows the emission spectra of both glasses. The main emission corresponded to the (²H_11/2, ⁴S_3/2)→⁴I_15/2 transition. A weak red emission was also observed from the ⁴F_9/2 level. This level was populated through multiphonon relaxation processes from the ⁴S_3/2 level.

The experimental decays of the luminescence from ⁴S_3/2 and ⁴F_9/2 levels were obtained at room temperature for both glasses under excitation at 488 nm.

Table 2. Lifetimes at room temperature of the ⁴S_3/2 and ⁴F_9/2 levels obtained under excitation at 488 nm and ⁴I_13/2 levels obtained under excitation at 800 nm.

	548 nm (4S3/2) (λexc = 488 nm)	660 nm (4F9/2) (λexc = 488 nm)	1528 nm (4I13/2) (λexc = 800 nm)
Er1	3.60 ± 0.11 μs	3.06 ± 0.09 μs	4.91 ± 0.12 ms
Er4	3.20 ± 0.07 μs	3.17 ± 0.03 μs	3.01 ± 0.06 ms

shows the values obtained by a fit to a single exponential function. It can be observed that lifetimes of these levels were found to be similar for both glasses, with values slightly higher for the glass doped with a 1 wt% of Er₂O₃. As an example, Figure 3 shows the experimental decays from the ⁴S_3/2 and ⁴F_9/2 levels under excitation at 488 nm for the sample doped with 1 wt%. Lifetime was also measured at room temperature for the ⁴I_13/2 level under excitation at 800 nm corresponding to the level ⁴I_9/2. It was also found that lifetime was slightly higher for the glass doped with a 1 wt% of Er₂O₃. The decay from the ⁴I_13/2 level was found to behave like a perfect single exponential, whereas decays from ⁴S_3/2 and ⁴F_9/2 excited levels slightly deviated from a perfect exponential behavior. The observed lifetimes were in the same order of magnitude than those reported for other silicate and aluminosilicate glasses [10,11,32].

The fluorescence spectra at room temperature corresponding to the ⁴I_13/2→⁴I_15/2 transition were measured by exciting the samples at 802 nm. As shown in Figure 4 both samples presented a maximum at around 1528 nm. Effective bandwidth (Δλ_eff) was measured according to

$$\Delta {\lambda }_{eff}=\int \raisebox{1ex}{$I\left(\lambda \right)d\lambda $}\!\left/ \!\raisebox{-1ex}{$I_{max}$}\right.,$$

(1)

where I(λ) is the intensity of the emission spectrum as a function of the wavelength and I_max is the peak intensity. It was found that the effective bandwidth increased from 62.5 nm for the sample doped with 1 wt% to 66.3 nm for the sample doped with 4 wt% of Er₂O₃. These values are larger than those obtained for other silicate and phosphate glasses, the bandwidth of which ranges from 30–40 nm for silicate and 46 nm for phosphate glasses, respectively [33]. This feature is highly significant since broadband amplifiers and tunable lasers require large bandwidth.

In addition to the bandwidth, the stimulated emission cross-section, σ_em, is another important parameter that provides information about the optical amplification. This parameter was estimated from the absorption spectra by using the McCumber approach [34], in which the absorption and emission cross-section are related according to

$${\sigma }_{em}\left(\upsilon \right)={\sigma }_{abs}\left(\upsilon \right)exp\left[\frac{\epsilon -h\upsilon }{KT}\right],$$

(2)

where σ_em and σ_abs are the stimulated emission and absorption cross-section, respectively, υ is the photon frequency, h is the Planck constant, K is the Boltzmann constant, and ε is the net free energy required to excite one Er³⁺ ion from states ⁴I_15/2 to ⁴I_13/2 at temperature T. The absorption cross-section was experimentally obtained and ε was determined by using the simplified procedure provided by Miniscalco [35]. The maximum emission cross-sections were found to be 3.51 × 10⁻²¹ cm² and 4.19 × 10⁻²¹ cm² at 1530 nm for the samples doped with 1 wt% and 4 wt% of Er₂O₃, respectively. These values were similar to those found in other aluminosilicate glasses [32]. As an example, Figure 5 shows the absorption and emission cross-section for the sample doped with 4 wt% of Er₂O₃.

3.3. Infrared to Visible Upconversion

Visible upconversion at room temperature was observed in both samples under continuous laser excitation in resonance with the ⁴I_9/2 level, as shown in Figure 6. The observed green emissions correspond to ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions of Er³⁺ ions, which were located approximately at 525 nm and 550 nm, respectively. Nevertheless, red emission corresponding to the ⁴F_9/2→⁴I_15/2 transition was not observed in these glasses. This is due to the fact that the energy gap between levels ⁴F_9/2 and ⁴I_9/2 is 2264 cm⁻¹ and the maximum phonon energy is 1000 cm⁻¹ approximately [32].

Excited state absorption (ESA) and energy transfer upconversion (ETU) are the main processes associated to the upconversion emission of rare-earth ions [36]. Excitation mechanisms for populating the ²H_11/2 and ⁴S_3/2 levels under NIR excitation were investigated by studying the upconversion emission intensity I_up as a function of the infrared excitation power I_IR. It is well-known that upconversion emission intensity increases proportionally to the nth power of the infrared excitation power according to the relation I_up k ∝ (I_IR)ⁿ, where n is the number of photons involved in the pumping mechanism. Figure 7 shows the logarithmic plot of the green upconversion intensity I_up compared to the excitation power I_IR under excitation at 800 nm for both glass samples. Linear fit allowed determining the n-values, resulting in 1.47 and 1.52 for Er1 and Er4, respectively. Slope values below two indicates a saturation of the intermediate levels [37]. These results confirm that a two-photon step was involved in the upconversion process to populate the emitting levels.

Possible mechanisms accounting for green upconversion emission under 800 nm excitation is presented in Figure 8. Green upconverted emission requires the population of levels with at least the energy of the ²H_11/2 level or higher. The ⁴I_9/2 level is resonantly excited by the pumping wavelength at 800 nm. Next, the thermalized levels ²H_11/2 and ⁴S_3/2 can be populated by means of two ESA processes or two ETU. On the first ESA, labelled as ESA1, a non-radiative relaxation from the ⁴I_9/2 level to the lower ⁴I_11/2 level is produced, from which the absorption of one photon populates the ⁴F_3/2,5/2 level, followed by a non-radiative de-excitation to the ²H_11/2 and ⁴S_3/2 levels. On the second ESA, labelled as ESA2, an additional non-radiative relaxation from the ⁴I_11/2 level to the ⁴I_13/2 level is produced. Then, the absorption of one photon promotes the Er³⁺ ions to the ²H_11/2 and ⁴S_3/2 levels. These two processes involve only one Er³⁺ ion. Nevertheless, two upconversion mechanisms involving the interaction of two nearby Er³⁺ ions in the ⁴I_11/2 level and entailing an energy transfer are possible. In the first ETU, denoted as I, the mechanism can be described as Er³⁺(⁴I_11/2) + Er³⁺(⁴I_11/2) → Er³⁺(⁴I_15/2) + Er³⁺(⁴F_7/2), followed by a non-radiative de-excitation to the ²H_11/2 and ⁴S_3/2 levels. The second ETU, denoted as II, can be described as Er³⁺(⁴I_11/2) + Er³⁺(⁴I_13/2) → Er³⁺(⁴I_15/2) + Er³⁺(²H_11/2, ⁴S_3/2).

A method to distinguish between ESA and ETU mechanisms is provided by the excitation spectra of the upconverted luminescence [38]. In ESA processes upconversion excitation spectra are the result of excited state absorption and the one photon absorption. In ETU, excitation spectra are proportional to the square of the ground state absorption. Hence, excitation spectra of the upconversion green emission were performed in the ⁴S_3/2→⁴I_15/2 transition at 550 nm in both Er-doped aluminosilicate glasses. Figure 9 shows these spectra for the sample doped with a 4 wt%. It can be observed that both excitation and absorption spectra are similar. This behavior was also observed for the sample doped with a 1 wt%. Consequently, upconversion mechanisms were due to energy transfer upconversion processes.

4. Conclusions

Directionally solidified Er-doped aluminosilicate glasses were manufactured by the laser floating zone in a controlled oxidizing atmosphere to provide them with a high optical transmission in the visible spectral range. The infrared and visible emissions were assessed at room temperature. Infrared emission corresponding to the ⁴I_13/2→⁴I_15/2 transition at 1528 nm presented an effective bandwidth nearly 30 nm broader than other silicate glasses, which makes them suitable for broadband amplifiers. The visible emission was dominated by the green emission corresponding to the ²H_11/2 and ⁴S_3/2 levels. Lifetimes from levels ⁴S_3/2 and ⁴F_9/2 at room temperature under excitation at 488 nm were found to be similar for both glasses, whereas lifetime from the ⁴I_13/2 level was found to be shorter for the glass with the higher content of Er³⁺ ions.

NIR-to-visible upconversion of Er³⁺ ions in these glasses under excitation at 800 nm presented intense green emissions corresponding to ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions placed at 525 nm and 550 nm which were attributed to a two-photon process. Nevertheless, red upconversion emission for the ⁴F_9/2→⁴I_15/2 transition was not observed due to the high maximum phonon energy. Excitation spectra of the upconverted luminescence from the ⁴S_3/2 level suggests the energy transfer upconversion as the mechanism responsible for the upconversion process in these glasses.