2. METODOLOGÍA
2.2. Recogida de datos
2.2.3. Realización de la prueba
Themechanismof zinc indiffusion into the lithium niobate substrate is similar to that of titanium indiffusion which is the most common and most well known method of creating waveguides in lithium niobate [26,27,28].
The previous work done by T. S. Chernaya et al. [28] suggested that during the indiffusion processes the Zn atoms diffuse through Li sites if the zinc concentration is below 7.6mol% but change their locations in thelattice and partially occupy the Nb positions if the concentration is in excess of this level. Zinc indiffusion into the Li position was accompanied by a decrease in the concentration of intrinsic Nb-Li defects. This clarifies thestructural nature of the “threshold” of Zn concentration (7.6mol%), which manifests itselfas singularities in the concentration dependences of various optical properties.The structural origin of the threshold concentration is likely a common feature of all non-photorefractive impurities (Mg, Zn, In, and Sc) in LiNbO3 [28].
Chapter 6: Fabrication of Zinc-Indiffused PPLN Channel Waveguides
While some physical aspects of the mechanism of zinc indiffusion into lithium niobate are still not fully understood, it seems clear that the congruent composition of as-grown lithium niobate crystal is important, as it has a lithium atom concentration ratio (Li/(Li+Nb)) of 48.6% [29,30], which represents a considerable deficiency of lithium ions, and this kind of crystal contains specific structural vacancies (i.e., empty oxygen octahedra) [28,29] and other intrinsic defects (Nb-Li) in its crystal structure [28,29,31,32]. Therefore, various metal atoms, such as Ti, Mg,Zn, In, Fe, Er, Nd and Sc, can be readily introduced into the crystallographic frame by occupying Li sites through thermal indiffusion or doping during the crystal growth processes. This wide ranges of dopants accounts for the versatility of LiNbO3 for many important
applications [33].
Nominally pure LiNbO3 devices suffer serious optical damage problems due to the
photorefractive effect when exposed to high intensity illumination. This problem can be greatly reduced by doping LiNbO3 single crystals with MgO at a concentration of
about 5mol% [34,35]. Moreover, it has been shown that the photorefractive damage in LiNbO3 single crystals can be prevented by doping or indiffusing with ZnO even
more efficiently than with MgO [36,37].
One paper [37] reported that photoresistance of zinc indiffused or doped lithium niobate (6mol%) increases two orders of magnitude higher than pure LiNbO3 crystal,
the measurement of resistance to optical damage was carried with Ar laser (λ=488 nm) by means of the direct observation of the facula distortion. Although the reason for the optical damage resistance of zinc doped or indiffused lithium niobate is not clear, Zn doped or indiffused LiNbO3 crystal with 6mol% concentration ZnO has a slightly
higher photorefractive resistance that that of magnesium doped Lithium niobate (5mol%). Table 6.01 lists data on the resistance to optical damage of different Zn doped LiNbO3 crystals and MgO doped LiNbO3 [37].
In choosing the process parameters it should be noted that rapid temperature changes have a major effect on LiNbO3. Experimentally, spontaneous poling dots can be
found on the surface of 500µm thick LiNbO3 samples when the rate of temperature
increase or decrease is larger than 10oC per minute. Therefore during the diffusion process the samples must be raised to and decreased from the diffusion temperature at a rate lower than this in order to avoid spontaneous poling dots.
Table 6.01 The resistance to optical damage of Zn:LiNbO3 crystals (λ=488 nm) [37].
No. ZnO (mol%) MgO (mol%) Resistance (W⋅cm-2)
0 0 0 3.1 2 10 × 1 2 0 6.6 2 10 × 2 5 0 7.1 2 10 × 3 6 0 9.8 4 10 × 4 8 0 9.8 4 10 × 5 0 5 9.7 4 10 ×
The Curie temperature of LiNbO3 is 1210oC. Above this temperature, the properties
of LiNbO3 change from a ferroelectric to a paraelectric phase which is nonpolar, and
therefore any PPLN domain structure in the sample will be lost. Thus the diffusion temperature must be below the Curie temperature to preserve the polarisation of the switched domain grating, otherwise they will be flipped back into their original polarisation direction.
In addition, when z-cut LiNbO3 samples were subjected to high temperatures, of over
1000oC, significant lithium outdiffusion on the positive z face occurs [38,39]. When using zinc, Li2O is lost from the LiNbO3 to form Li-Zn-O compounds on the +z
crystal surface, which act as a source for zinc indiffusion and as a barrier for lithium outdiffusion, this problem can be avoided by diffusing zinc atoms on the –z face of sample. But such outdiffusion will cause particular problems during the indiffusion of zinc waveguides on PPLN, because the PPLN has a periodically switched domain structure, so the outdiffusion layer will be formed periodically, resulting in a periodic change in the zinc concentration, and thus higher losses and a less well confined waveguide. It has also been found that Li-outdiffusion can be effectively reduced through decreasing the diffusion temperature [20]. Therefore a diffusion temperature of lower than 1000oC was adopted to reduce the effect of Li-outdiffusion on the surface of LiNbO3 samples in previous work [20].