Demonstration of Er 3+ diffusivity and solubility increases in off-congruent, Li-deficient LiNbO 3 crystal
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win Yue-Bun Pun Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, People’s Republic of China (Received 17 February 2011; accepted 29 April 2011)
We demonstrate Er3+ diffusivity and solubility increases in off-congruent, Li-deficient LiNbO3 crystal. Li-poor vapor transport equilibration was used to reduce Li2O content in initial congruent crystals. Local Er3+ in-diffusion was then performed in a wet O2 atmosphere. Before and after the Er3+ diffusion procedure, surface Li2O content was evaluated from measured birefringence. The results show that the Er3+ diffusion procedure resulted in 0.3–0.5 mol% Li2O content loss at crystal surface. Secondary ion mass spectrometry was used to measure the Er3+ depth profiles, from which the diffusivity and solubility are determined. It is shown that the Er3+ diffusivity is nearly doubled and the solubility increases at least 0.6 mol% as the Li2O content decreases by 1.0 mol%. From the known Li2O content reduction, the solubility increase is also predicted and the results show that the predicted data are considerably smaller than the experimental results, suggesting that the Er3+ ions occupy also the Nb5+ site, besides the Li+ site.
I. INTRODUCTION
Er3+-doped LiNbO3 (Er:LN) is a potential substrate material for active optical waveguide devices as it combines the excellent electro-optic, acousto-optic, and nonlinear optical properties of LN with the laser property of Er3+. In particular, it allows broadband amplification and lasing in the telecommunication wavelength region around 1.5 lm. Since the first continuous-wave waveguide laser was reported in 1991,1 a family of Ti (vapor Zn)diffused Er:LN waveguide lasers (amplifiers) and integrated devices operated in the infrared regime have been demonstrated over the past years.2–10 For any practical Ti: Er:LN waveguide device, selective Er3+ doping is a prerequisite for the monolithic integration of active (optically pumped, Er3+ doped) and passive (unpumped) devices on a same substrate to avoid reabsorption in unpumped Er-doped waveguide. This is the reason why the active ions in almost all the Ti:Er:LN devices demonstrated so far were incorporated by the local Er3+ doping instead of the homogeneously bulk doping method.1–6,8–10 Local Er3+ doping in an LN crystal is usually realized via thermal diffusion of Er metal at the temperature close to the Curie point of the crystal (1142.3 6 0.7 °C at the congruent a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.157 1524
J. Mater. Res., Vol. 26, No. 12, Jun 28, 2011
http://journals.cambridge.org
Downloaded: 17 Mar 2015
point). The diffusion of Er3+ into a congruent (Li2O content ;48.4 mol%) LN (CLN) crystal in an argon or O2 atmosphere has been studied in detail by Baumann et al.11 and Caccavale et al.12 The results show that the solubility of Er3+ ion in the crystal is limited. In a CLN crystal, the solubility is only ;2.65 1020 ions/cm3 (;1.43 mol%) at the Curie point of 1142 °C.11 Since the optical gain o
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