Papel del TENS en el tratamiento de la DP:

Further investigations of the illumination of the +z face through a phase mask were com- pleted in collaboration with Iain Wellington and Collin Sones (University of Southamp- ton). Phase masks with Λ = 726 nm were used with 248-, 266-, and 298-nm wavelengths from a KrF, Nd:YVO4, and dye laser, respectively.

Unpatterned illumination by 20 pulses of λ = 306 nm light with a fluence near the ablation threshold formed a random array of etch pits and short line segments, as shown in Figure 5.13(a). These pits had a triangular shape with vertices oriented along the y axes, and were the inverse of the etch frustration pyramidal structures resulting from UV illumination on the opposite −z face. Illumination through a phase mask using several pulses ofλ= 298 nm light with a fluence near the ablation threshold produced similar etch pits, but instead were strictly aligned along the periodic lines of high-intensity, as shown in Figure 5.13(b). Close inspection of (b) shows that in fact all the etch pits aligned not in the center of the high intensity, but at a very precise distance to the left side toward the low intensity region. Several etch pits also aligned at the precise symmetric point on the right side (circled), indicating that either a specific intensity or intensity-gradient is ideal for their formation.

Increasing the fluence above the ablation threshold produced quasi-periodic UV-induced lines strictly aligned along the phase mask intensity patterns, as shown in Figure 5.14(a). The non-uniformity of the beam prevented alignment throughout the beam, resulting in lines aligning along the other y axes of the crystal. Nevertheless, preferential alignment along a single y axis was achieved over much of the beam.

On closer inspection of the irradiated crystal, shown in Figure 5.14(b), two very im- portant characteristics were revealed. Firstly, each UV-induced line was composed of a chain of etch pits and line segments that was reminiscent of domain lines formed by correlated nucleation [Shur05a]. Secondly, thequasi-periodicity of the structure became clear. An ablation grating corresponding to the intensity profile from the phase mask appeared across the entire beam. However the UV-induced lines did not form in every period, but instead at some multiple of the phase mask period only. The separation between adjacent lines typically ranged from 3–8 multiples of the phase mask period.

This separation between UV-induced lines is further supported by Figure 5.15. Lines forming along adjacent periods or within two periods terminated when they approached within∼1–2 micron. However, a three-period separation was allowed over long distances, as shown in Figure 5.14(b).

Figure 5.13: Nucleated AOP domains formed via low fluence exposures of (a) un-

patterned illumination using 20 pulses ofλ= 306 nm, and (b) illumination through a phase mask (Λ = 726 nm) using several pulses of λ= 298 nm.

UV-induced lines formed strictly along the high-intensity peaks, and when a segment deviated from this path, the line quickly reasserted alignment along the nearest peak. Even under conditions where the UV-induced lines did not alignalongthe high-intensity peaks, each segment of the lines did form precisely at one of these peaks, as shown in Figure 5.16(a). Here, each line is aligned along any one of the three y axes, but the line segments still formed only in the presence of the high intensity. However, this alignment of the intensity grating with the UV-induced lines was strongly dependent upon the accurate alignment of the intensity grating with a y axis of the crystal. When the phase mask was rotated 20◦ away from a y axis, alignment of the UV-induced lines remained primarily along each of the three y axes without preference, as shown in Figure 5.16(b). Some dotted lines also formed along the high-intensity peaks, although the segments were typically very small, indicating that the individual segments would not grow along this arbitrary angle.

All UV-induced lines discussed above have been formed using fluences significantly above the ablation threshold, simultaneously causing the deep ablation grating. However, UV- induced lines were also formed where the fluence was sufficiently low to cause very

Figure 5.14: AOP domain lines aligned byλ= 266 nm illumination through a phase

mask, showing (a) alignment across the majority of the laser spot, and (b) formation of domain chains running along some of the lines of illumination only, typically 3–8 phase

mask periods apart.

Figure 5.15: Interaction between adjacent AOP domain lines formed via illumination

through a phase mask (Λ = 726 nm), limiting the minimum spacing to three phase mask periods. Domain lines were prevented from propagating when they came into

Figure 5.16: SEM micrographs showing (a) AOP domain nucleation only at the peaks of light intensity, even when the domain-chains do not preferentially align along these

peaks, and (b) the effect of misalignment between the phase mask and the y axes.

Figure 5.17: Aligned AOP domain lines formed without observable ablation, using

λ= 266 nm with an average fluence of∼100 mJ/cm2_.

little or even no visible ablation, as shown in Figure 5.17. Some chains of UV-induced segments appeared very straight and with almost regular periodicity, whereas chains in between exhibited more bending and deviation from a straight line. This may be a result of the same interaction between neighboring lines that prevents line formation along adjacent periods of the phase mask illumination.

Figure 5.18: Polarized optical microscope images ofE-field poling of AOP domains formed byλ= 266 nm unpatterned illumination of the +z face of undoped CLN. Spots 1–4 were exposed to 20 pulses, and 5–8 exposed to 10 pulses. The approximate fluence per pulse of spots 1 and 5 was 700 mJ/cm2_{, spots 2 and 6 was 580 mJ/cm}2_{, spots 3 and}

7 was 400 mJ/cm2_{, and spots 4 and 8 was 180 mJ/cm}2_{. The images show the domains}

formed by 20 kV/mm applied in the poling direction for (a) 2 and (b) 20 minutes.

The effect of illumination via a phase mask was observed by Iain Wellington (University of Southampton) to be dependent upon temperature of the substrate [Wellington07]. Between room temperature and 100◦C, no qualitative difference was observed. How- ever, with temperature increasing up to 200◦C, the density of the UV-induced lines was observed to decrease significantly, forming with a greater separation than at room tem- perature. Additionally, the features formed at high temperature were continuous lines, and not composed of individual line segments.