Métodos de detección de mutaciones

The mechanism behind waveguide formation from Ȝ = 244 nm direct UV writing

in congruent lithium niobate is not completely understood. Nevertheless, there are strong indicators that point to a plausible explanation. It is believed that the mechanism behind the UV writing process is thermally driven, indeed the temperature increase and profile from an incident cw Gaussian beam is well known [30] and will be examined in detail in chapter 7. Lithium ion movement is the key to this process – from chapter 3 we know that in the early days of optical waveguide fabrication in lithium niobate, lithium out-diffusion was used to fabricate guides but in some methods such as Ti-indiffusion it seen as a hindrance to low loss waveguides. For optical waveguides it was found that those which had the lowest propagation loss and the longest lifetime required surface melting during the writing process and these waveguides had surfaces considerably above

the melting temperature, Tm ~1526 K. Lithium will have a thermal diffusivity

increase of several magnitudes when the crystal becomes amorphous and hence there will be a greater movement/out-diffusion of lithium in this region.

During early experiments to investigate properties of LiNbO3 crystals with

different stoichiometries, an increase in the extraordinary refractive index only

with a decrease in Li2O concentration was observed [31], this was also seen in

LiTaO3 [32]. It was also observed that by heating samples to 1100ºC for several

hours, an out-diffusion of Li2O occurs at the surface which causes an increase in

the extraordinary refractive index, forming a waveguide [33]. The waveguides formed had unaffected electro-optic properties and modulators were fabricated [34]. This out-diffusion effect has been fully characterized by Carruthers [35]. A separate cause of lower index refractive index layers has been shown to be due to hydroxyl ions [36].

A suggestion for the main cause of guiding in UV written samples is an out- diffusion and concurrent sideway diffusion of lithium ions, induced by the incident optical power heating the surface, forming the guiding region. The waveguides fabricated also show a similar increase in extraordinary refractive index only and the crystal is still electro-optic. Samples were sent to

Loughborough Surface Analysis3 for secondary ion mass spectroscopy (SIMS)

analysis to identify if any lithium ion concentration change occurred in irradiated samples. This technique should be able to quantify the concentration of Li ions on the surface due to the writing process.

Figs. 4.34 and 4.35 show the same single waveguide (written at 30 mW, velocity

50 mm/min and spot radius 4 ȝm) that has undergone SIMS analysis. Fig. 4.34

shows the contrast of lithium ions and fig 4.35 the concentration of niobium ions; the measurement of actual ion concentration was too difficult with the combination of such a small area, a small change in ion concentration and the

white of higher concentration indicating a lithium ion deficiency in the waveguide. A similar deficiency is seen with niobium but as niobium is a much larger ion, the contrast is smaller.

Other studies of UV machining on lithium niobate with far higher incident intensities have been able to confirm that incident UV exposure causes an out- diffusion of lithium ions [37, 38] and a reduction of niobium ions due to ablation.

Figure 4.34: Li ion contrast with black area along waveguide denoting a lithium deficient region.

Figure 4.35: Nb ion contrast with black area along waveguide denoting niobium deficient region.

Aside from this material picture, a change in lithium concentration has a subsequent effect on the refractive index of lithium niobate as reported by Schlarb [26]. Fig. 4.36 illustrates how the change in lithium concentration will affect the

extraordinary refractive index. From this analysis, the concentration of Li2O for

congruent LiNbO3 is ~ 48.4 mol% and for an increase in ne of ~9 x 10-4 at Ȝ =

1549 nm from section 4.4.5, a concentration change of 0.3 mol% Li2O is

predicted. Numerical aperture measurements, taken at Ȝ = 633 nm on a +z face

sample show an index change of 6 x 10-4 from results in section 4.3 estimate an

index change corresponding to a possible difference of 0.1 mol% in lithium concentration. The graph shows that even small changes in lithium concentration are enough to produce an index difference large enough to form a waveguide. These concentration changes are challenging to measure but nevertheless, lithium

WG

Waveguide

out-diffusion and possible sideways diffusion presents a viable mechanism for waveguide formation due to UV writing.

Li Concentration (mol%) 47.2 47.4 47.6 47.8 48.0 48.2 48.4 Δ Q H 10-5 10-4 10-3 10-2 10-1 1549 nm 633 nm

Figure 4.36: Lithium ion concentration change vs. change in ne for congruent lithium niobate as predicted by Schlarb [26].

There are several points to make that suggest lithium out-diffusion may not be the sole effect occurring at the crystal surface and assisting waveguide formation. The decay of the output intensity over several days coupled with the fact that low temperature annealing dissipates the waveguide suggests a related mechanism is responsible for this decay. Further to the lithium out-diffusion, there is lithium ion diffusion in the crystal from the high temperature due to the incident writing beam at the surface to regions of lower temperature. This temperature gradient drives lithium ions from their sites in the irradiated region to lithium vacancy sites in the cooler areas forming an index difference. After writing, there is a charge

Ȝ = 633nm