Preparation and characterization of stable aqueous suspensions of up-converting Er3+/Yb3+-doped LiNbO3 nanocrystals

The preparation of LiNbO₃:Er³⁺/Yb³⁺ nanocrystals and their up-conversion properties have been studied. It is demonstrated that polyethyleneimine- (PEI) assisted dispersion procedures allow obtaining stable aqueous LiNbO₃:Er³⁺/Yb³⁺ powder suspensions, with average size particles well below the micron range (100–200 nm) and the isoelectric point of the suspension reaching values well above pH 7. After excitation of Yb³⁺ ions at a wavelength of 980 nm, the suspensions exhibit efficient, and stable, IR-to-visible (green and red) up-conversion properties, easily observed by the naked eye, very similar to those of the starting crystalline bulk material.     

Photoacoustic detection of heat generation in proton-implanted Nd:YAG waveguides with and without laser operation

In this work the heat generation in proton-implanted planar Nd:YAG waveguides with and without laser action is determined by using photoacoustic techniques. From the analysis of the photoacoustic signal in the two emission regimes (stimulated and spontaneous) as a function of the absorbed power, it was found that the heat generated is substantially reduced during laser action. The quotient of the heat generated in both emission channels (stimulated and spontaneous) was found to be S_stimulated/S_spontaneous=0.65±0.03. This value is in agreement with the theoretical estimation considering a Nd³⁺ quantum efficiency _waveguide=0.78, extracted from the lifetime measured in the waveguide, slightly lower than the value in the bulk. This work demonstrates that photoacoustic measurements provide a sensitive method to characterize the performance of waveguide lasers. PACS 42.79.Gn; 42.82.Et; 42.60.Lh; 43.60.+d     

Comparative Studies of Er3☎ Ions in LiNbO3 Waveguides Produced by Different Methods

We compare the optical properties of Er^3? in LiNbO₃ waveguides produced by different techniques and find by confocal luminescence microscopy characteristic differences in the excitation and emission transitions. Besides a small redistribution among the regular defect sites, essentially no direct Er^3?—Ti^4? interaction can be observed in Ti^4? diffused waveguides. However a significant shift in transition energies is found, which increases with Ti^4?concentration. Based on earlier results we associate this shift with a decrease in the intrinsic electric field. In addition to a similar shift, we find in Zn^2? diffused waveguides new defect types which suggest a change in direct environment. Finally, in annealed proton exchange waveguides we find no interaction effects at all.     

Dynamics of the Yb3+ to Er3+ energy transfer in LiNbO3

The energy transfer dynamics between Yb³⁺ and Er³⁺ ions in lithium niobate is investigated after ytterbium-pulsed excitation at 920 nm. The sensitisation of the LiNbO₃:Er³⁺ system with Yb³⁺ ions does not modify the lifetime of the ⁴I_13/2 erbium level (1.5-μm emission), whereas it induces a marked, concentration-dependent change in the lifetime of the ²F_5/2 (Yb³⁺) and ⁴S_3/2 (Er³⁺) multiplets (1060-nm and 550-nm emissions, respectively). The results are analysed by using the rate-equation formalism and cross-relaxation model for the energy transfer. Received: 15 October 1998 / Revised version: 24 November 1998 / Published online: 24 February 1999     

Compositional characterisation of Zn-diffused lithium niobate waveguides

Rutherford backscattering (RBS) and secondary-ion mass spectrometry techniques have been used to investigate the two-step process involved during waveguide fabrication in LiNbO₃ using Zn-vapour diffusion. Compositional analysis (O, Nb, Li and Zn) in the two steps has been characterised. RBS analysis reveals that the first step, involving a heating of the substrate under a metallic Zn atmosphere, gives rise to a partial exchange between the Nb and Li ions from the crystals and the Zn from the vapour source. The second treatment at higher temperature in an open atmosphere diffuses the Zn deeper into the substrate, thus forming an optical waveguide, while the Nb and Li ions recover their bulk values. Received: 16 May 2001 / Revised version: 7 September 2001 / Published online: 30 October 2001     

Infrared energy transfer in Tm3+:LiNbO3

The infrared optical properties of LiNbO₃ crystals doped with variable thulium concentrations have been studied. In particular, the infrared emissions that appear after direct pumping to the ³H₄ multiplet (795 nm) have been analyzed. The emission spectra, as well as the lifetimes, show that an efficient energy transfer between ³F₄ and ³H₄ levels takes place. The concentration dependence of the lifetimes has been studied and the transfer probability has been characterized.     

Polarized emission and absorption cross-section calculation in LiNbO3:Tm3+

Absorption and emission cross-sections are essential spectroscopic parameters that determine the performance of rare-earth solid state lasers and amplifiers. A precise characterization of such parameters is of great interest in order to proceed with the necessary optimization stage which will assure the development of photonic devices with high efficiency. In particular, in order to model Tm³⁺ infrared lasers under 800 nm pumping, the absorption and emission cross-sections associated with optical transitions involving the ³H₄ and ³F₄ manifolds are needed. In this work, the classical McCumber theory is applied to evaluate the polarized absorption and emission cross-sections of Tm³⁺ ions in LiNbO₃.     

Modelling of Tm3+-doped LiNbO3 Waveguide Lasers

In this work, CW laser operation of Tm³⁺-doped LiNbO₃ channel waveguides has been modelled. The model is based on time dependent laser rate equations coupled with the laser signal and pump photon flux equations. Steady state solutions for the population densities, pump and signal powers are obtained by using finite difference discretization of the active volume. The effects of spectroscopic parameters such as concentration dependent cross-relaxation and excess waveguide loss have been analyzed. We demonstrate good agreement with experimental data previously reported in Zn-diffused LiNbO₃:Tm³⁺ channel waveguide lasers. It is shown also that laser performance can be substantially improved by optimizing the cavity length.     

Analysis of the absorption coefficient in carbon implanted Nd:YVO<Subscript>4</Subscript> by an annealing process

This work reports the behaviour of the absorption coefficient in a C²⁺-implanted Nd:YVO₄ crystal by successive annealing steps. The absorption coefficient was monitored with respect to time and temperature after an annealing process. Firstly, the annealing duration increased in a constant form keeping a constant temperature and secondly, the annealing temperature was increased with equal time intervals. The results indicate that the variation of the absorption coefficient has an exponential behaviour.     

Cr<Superscript>3+</Superscript> spectroscopy study in ZnO-codoped and Zn-in-diffused congruent LiNbO<Subscript>3</Subscript>:Cr crystals

This paper reports on the spectroscopy properties, absorption and luminescence, of Cr³⁺ ions in singly doped, ZnO-codoped, and Zn in-diffused LiNbO₃:Cr crystals. In addition to the broad absorption, inter-ionic transitions ascribed to Cr³⁺ ions located in Li⁺ and Nb⁵⁺ sites; [Cr]_Li and [Cr]_Nb centres two absorption bands at higher energy are reported and ascribed to the charge transfer transitions of the Cr³⁺ ions of the two defect centres. The charge transfer transitions are used as optical probe to study the role of the Zn ions in the Zn in-diffused LiNbO₃:Cr samples. It has been observed that the Zn-in-diffused processes created [Cr]_Nb centres in the diffusion zone. The location of the diffused Zn²⁺ ions is considered to be in Li⁺ site, displacing the Cr³⁺ ions from the Li⁺ sites, [Cr]_Li, to the Nb⁵⁺ positions, [Cr]_Nb.