Direct demonstration of sensitization at 980nm optical excitation in erbium-ytterbium silicates

Direct demonstration of sensitization at 980nm

optical excitation in erbium-ytterbium silicates

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Vanhoutte, Michiel, Bing Wang, Zhiping Zhou, Jurgen Michel, and

Lionel C. Kimerling. “Direct demonstration of sensitization at

980nm optical excitation in erbium-ytterbium silicates.” In 7th IEEE

International Conference on Group IV Photonics, 308-310. Institute

of Electrical and Electronics Engineers, 2010. © Copyright 2010

IEEE

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http://dx.doi.org/10.1109/GROUP4.2010.5643345

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Institute of Electrical and Electronics Engineers (IEEE)

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Final published version

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http://hdl.handle.net/1721.1/79771

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Direct demonstration of sensitization at 980nm

optical excitation in erbium-ytterbium silicates

Michiel Vanhoutte*, Student Member, IEEE, Bing Wang**, Zhiping Zhou**,

Jürgen Michel, Member, IEEE and Lionel C. Kimerling, Member, IEEE

Microphotonics Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

Abstract

—

I. INTRODUCTION

Erbium-doped materials have important applications in silicon photonics thanks to their strong light emission at 1.54µm. Nevertheless, the low solubility of erbium (typically

NEr < 1020cm-3) limits the achievable gain in erbium-doped

materials. Erbium compounds such as erbium oxide (Er2O3)

and erbium silicates (Er2SiO5 and Er2Si2O7) have been

proposed as a solution to this problem [1]-[3]. In these material systems, erbium concentrations are of the order NEr =

1022_cm-3_{. At these high erbium concentrations however, the}

small Er-Er distances lead to increased interionic interactions such as upconversion and concentration quenching. These phenomena deplete the population of the excited erbium level at 1.54µm and are hence detrimental for light emission. Dilution of erbium by means of partial substitution with other rare earth ions helps to decrease Er-Er interactions. This idea has already been implemented in ErxY2-xO3 and ErxY2-xSiO5,

where erbium is partially substituted by yttrium [4]-[5]. Another strategy to achieve dilution in erbium compounds is to use ytterbium as a dilutant instead of yttrium. The successful fabrication of erbium-ytterbium monosilicates (ErxYb2-xSiO5) and their light emission at 1.54um have

recently been demonstrated [6]. In addition to dilution of erbium, ytterbium is an excellent candidate for sensitization of erbium at 980nm optical pumping. With an absorption

* Corresponding author (e-mail: mvhoutte@mit.edu) ** Current address: State Key Laboratory of Advanced Optical Communication Systems and Networks, Peking University, Beijing, 100871, China.

section at 980nm of about an order of magnitude higher than that for erbium and energy resonance of the 2F5/2  2F7/2

transition in Yb3+ _{with the} 4_I

11/2  4I15/2 transition in Er3+,

ytterbium can absorb the pump energy and subsequently transfer it to erbium [7].

In the present work, the effect of ytterbium as a sensitizer at 980nm is demonstrated by comparison of the photoluminescence (PL) intensity excited at 488nm and 980nm, causing direct excitation of erbium and excitation through ytterbium respectively. Secondly, through a study of the decay rate of the 1.54µm PL signal, the effect of erbium dilution with ytterbium is investigated.

II. EXPERIMENTAL

Erbium-ytterbium silicate thin films were deposited by means of RF-magnetron co-sputtering of Er2O3, Yb2O3 and

SiO2 targets on crystalline p-type Si. The composition was

controlled by variation of the power applied to the different targets during co-sputtering and was verified by energy-dispersive X-ray spectroscopy (EDS) and Rutherford backscattering (RBS) measurements. The deposited films were annealed in O2 for 1h at 1200ºC.

Room-temperature PL experiments were done with free space excitation with 488nm light from an argon laser and 980nm diode laser light. The pump signal was modulated at 81Hz and detected with a liquid N2-cooled photomultiplier

tube connected to a lock-in amplifier.

III. RESULTS AND DISCUSSION

A. Sensitization of erbium by ytterbium at 980nm pumping

Figures 1a and 1b show the photoluminescence spectra of erbium-ytterbium silicate thin films with different compositions, excited with 488nm and 980nm light respectively. It can be seen that the shape of the PL spectrum for a given composition does not depend on the pump wavelength. However, a very different dependence of PL

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intensity on erbium-ytterbium composition is clearly observed for the two different pump wavelengths.

At 488nm pumping, erbium ions are directly excited since ytterbium has no atomic levels in this energy range. The only role ytterbium can play in this case is to dilute the erbium concentration. Figure 1a shows that the film with intermediate erbium-ytterbium content Er0.5Yb1.5SiO5 emits the strongest

PL, with a an intensity of about 1.25x stronger than the PL for composition Er0.2Yb1.8SiO5 and about 2.8x stronger than the

PL for composition Er1.0Yb1.0SiO5. At 980nm pumping, both

ytterbium and erbium can absorb the pump light and sensitization by ytterbium can occur. Comparing the PL of the same films pumped by 980nm light, Figure 1b shows that the PL increases sharply with decreasing erbium and increasing ytterbium content: the PL of Er0.2Yb1.8SiO5 is about 2.3x

stronger than the PL of Er0.5Yb1.5SiO5 and 28x stronger than

the PL of Er1.0Yb1.0SiO5.

This result can be explained by Equation 1 for the photoluminescence intensity in the low-flux regime. In this expression, σex is the effective erbium excitation cross-section

at the excitation wavelength, Φ is the pump flux, τrad is the

radiative lifetime and τ is the overall signal decay time.

(Eq.1) The overall decay time τ follows from the rate equation of the first excited erbium level given in Equation 2. This equation includes effects such as radiative decay, non-radiative decay by phonon emission, upconversion and parasitic effects such as concentration quenching. In the latter effect, the excitation energy migrates between neighboring erbium ions until a luminescence quenching center (e.g. an OH-impurity) is reached and the excitation energy is lost. The energy migration rate is proportional to the concentration of quenching centers Nq and depends strongly on erbium

concentration NEr (see below) [8].

(Eq. 2) In the studied low-flux regime, the effects of upconversion can be neglected. The decay rate of the other effects may depend on the refractive index and erbium environment (τr),

the phonon energy (τnr), the erbium concentration (CErEr, NEr)

and the fabrication conditions (quenching center concentration

Nq). However, for a given film, the overall decay rate and

hence the factor τ/τrad in Equation 1 should not depend on the

pump wavelength.

(a) 488 optical pumping

(b) 980nm optical pumping

Fig. 1. Room temperature photoluminescence spectra around 1550nm of erbium-ytterbium silicate films with different compositions at 488nm and 980nm optical pumping. All films were annealed for 1h in O2 at 1200ºC.

For a film with a given erbium concentration NEr, it follows

from Equation 1 that the ratio of photoluminescence intensities at 488nm pumping and 980nm pumping is given by

(Eq. 3) Since the fluxes are independent of erbium concentration, the only remaining explanation of the different dependence of

x5

PL intensity on erbium concentration must be due to an increasing effective excitation cross-section at 980nm with increasing ytterbium concentration. This is in other words a demonstration of sensitization by ytterbium at 980nm.

B. Effect of erbium dilution by ytterbium

Figure 2 shows the rise and decay of the PL signal at 1540nm of the different silicate films on a semilog plot. Two features of the lifetime curves draw attention. First of all, the decay is singly exponential. Secondly, the rise and decay rates increase significantly with erbium concentration.

Fig. 2. Semilog plot of rise and decay times of the PL signal at 1540nm (excited with 980nm light) for different erbium-ytterbium concentrations. The rise and decay rates are exponential and increase with erbium concentration.

This behavior can be explained by the rate equation of the first excited erbium level given in Equation 2. In this equation, concentration quenching due to energy migration to luminescence quenching centers is the only effect that has a strong erbium concentration dependence while still remaining singly exponential. The coefficient of energy migration CErEr

describes dipole-dipole interactions between erbium ions and scales with the distance between erbium ions as 1/d6_{. Since the}

interionic distance scales with concentration as 1/NEr3, this

coefficient will increase quadratically with erbium concentration NEr.

Though the upconversion coefficient C11 has a similar

quadratic dependence on the erbium concentration, this effect becomes only important at higher pump fluxes and does not cause an exponential decay.Upconversion can therefore not explain the observed lifetime behavior.

The dependence of rise and decay times on erbium concentration demonstrates how dilution of the erbium concentration with ytterbium can indeed mitigate parasitic effects such as energy migration to luminescence centers and thus increase the internal quantum efficiency of light emission.

IV. CONCLUSION

Based on the study of 1.54 µm photoluminescence intensity and lifetimes of ErxYb2-xSiO5 thin films, we have shown that

the role of ytterbium in erbium-ytterbium silicates is twofold. The difference in dependence on ytterbium concentration of the PL intensity at 980nm and 488nm excitation can only be explained by a strong dependence of the 980nm excitation cross-section on ytterbium concentration. This is thus a demonstration of sensitization of erbium through ytterbium at 980nm excitation. Secondly, the rise and decay rates of the 1.54µm signal are singly exponential and increase strongly with increasing erbium concentration. This behavior is explained by concentration quenching due to migration of the excitation energy to luminescence quenching centers, which is reduced by means of dilution of the erbium concentration by ytterbium.

ACKNOWLEDGMENTS

This work was supported by the Si-based Laser Initiative of the Multidisciplinary University Research Initiative (MURI) sponsored by the Air Force Office of Scientific Research (AFOSR).

Bing Wang acknowledges support from the Chinese Scholarship Council during his stay at MIT.

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