Presence of fast quenching mechanisms in Al
2O
3:Er
3+L. Agazzi*, J.D.B. Bradley, F. Ay, K. Wörhoff, and M. Pollnau
Integrated Optical Microsystems Group, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
* Corresponding author: l.agazzi@ewi.utwente.nl
Abstract
The measurement of luminescence decay curves and non-saturable absorption in erbium-doped aluminum oxide waveguides reveals the presence of fast quenching effects, leading to a revised value of the microscopic and macroscopic parameters of energy-transfer upconversion.
1. Introduction
Decay mechanisms such as energy-transfer upconversion (ETU) have a detrimental influence on many rare-earth-ion-doped infrared amplifiers and lasers [1], including Er3+ -doped waveguide amplifiers [2]. In this paper we investigate such mechanisms by lifetime and nonsaturable absorption experiments. The latter reveal a fast quenching of a fraction of ions, which is not detectable in the luminescence decay curves.
2. Experimental details
Al2O3:Er3+ layers with a thickness of 1.0 μm were deposited on thermally oxidized silicon
substrates by reactive co-sputtering [3] and ridge waveguides with a width of 4.0 μm were defined using reactive ion etching [4] to a depth of 50 nm. Recently, in such channel waveguides optical net gain was demonstrated over a bandwidth of 80 nm (1500-1580 nm) with a peak value of 2.0 dB/cm at 1533 nm [5].
For the present investigation, the Er concentration was varied from 0.27 to 3.66 × 1020 cm-3. Luminescence decay measurements were performed by exciting the channel waveguides with a diode laser emitting at 1480 nm and modulated by a square-pulse generator. The pulse had a duration of 40 ms, allowing the Er3+ population to reach a steady state before the pump was switched off. The luminescent decay at 1530 nm was collected using a high-N.A. liquid fiber mounted normal to the sample surface and the resulting signal was acquired with a digital oscilloscope.
Non-saturable absorption experiments were carried out by launching 1480-nm pump light into the waveguides and recording the transmission as a function of launched power.
3. Results and discussion
The measured luminescence decay curves, see Fig. 1(a), were analyzed within the frame of the microscopic model developed by Zubenko et al. [6], which takes into account both ETU and energy migration. The microparameter CDA of ETU from the 4I13/2 first excited level
of Er3+ and the mean time of a migration hop τ0 were extracted. CDA yields a value of
(6.10.6) × 10-41 cm6/s, while τ0 exhibits a decrease from more than 100 ms down to 1 ms
with increasing Er3+ concentration, due to the decreasing distance among Er3+ ions which enhances the probability of energy migration. The macroscopic upconversion parameter WETU was obtained from the equation
0 2 3 DA ETU C
W . It increases linearly with Er3+ concentration from 0.069 to 0.664 × 10-18 cm3/s.
The amount of absorbed pump power propagating through the waveguide versus launched pump power for an Er3+ concentration of 2.12 × 1020 cm-3 is shown in Fig. 1(b). The experimental data were simulated with the rate-equation model presented in [3] and the ETU
parameters reported here, but no agreement was found (dashed line). A non-saturable component, indicated by the arrow in Fig. 1(b), is present. This is due to additional fast quenching effects induced by, e.g., active ion pairs and clusters, undesired impurities, or host material defects. Such fast quenching mechanisms, which according to another investigation [7] may occur at a time scale on the order of 1-10 μs, do not reveal their existence in the luminescence decay curves due partly to the limited time resolution (~10 µs) of our setup. We have modified the model to account for this quenching effect (solid line) and extracted the fraction of quenched ions fq, which increases from 10% to 33% when the Er3+ concentration
increases from 1.17 to 3.66 × 1020 cm-3. This result suggests a higher probability of fast quenching with increasing concentration. Investigations to understand the exact nature of the fast quenching mechanism are in progress.
(a) 0 5 10 15 20 25 30 -5 -4 -3 -2 -1 0 Time (ms) ln (I nten sity) (arb. un its) 5 1,2 3 4 (b) 0 10 20 30 40 50 0 1 2 3 4 fq=0% f q=25% Abso rbe d Pump Powe r (dB)
Launched Pump Power (mW)
Figure 1. (a) Normalized luminescence decay curves at 1530 nm of Al2O3 doped with Er3+ concentrations of
1) 0.27, 2) 1.17, 3) 2.12, 4) 2.91, 5) 3.66 × 1020 cm-3; (b) absorbed versus launched pump power for a waveguide with 2.12 1020 cm-3 Er3+ concentration and 1 cm length (blue squares). The solid and dashed lines represent the calculations for 25% and 0 % quenched ions, respectively. The nonsaturable absorption (NSA) contribution is indicated by the arrow.
Acknowledgement
Funding was provided by the Smartmix Memphis programme of the Dutch Ministry of Economic Affairs.
References
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