Structural consequences of ferroelectric nanolithography.

Domains of remnant polarization can be written into ferroelectrics with nanoscale precision using scanning probe nanolithography techniques such as piezoresponse force microscopy (PFM). Understanding the structural effects accompanying this process has been challenging due to the lack of appropriate structural characterization tools. Synchrotron X-ray nanodiffraction provides images of the domain structure written by PFM into an epitaxial Pb(Zr,Ti)O(3) thin film and simultaneously reveals structural effects arising from the writing process. A coherent scattering simulation including the superposition of the beams simultaneously diffracted by multiple mosaic blocks provides an excellent fit to the observed diffraction patterns. Domains in which the polarization is reversed from the as-grown state have a strain of up to 0.1% representing the piezoelectric response to unscreened surface charges. An additional X-ray microdiffraction study of the photon-energy dependence of the difference in diffracted intensity between opposite polarization states shows that this contrast has a crystallographic origin. The sign and magnitude of the intensity contrast between domains of opposite polarization are consistent with the polarization expected from PFM images and with the writing of domains through the entire thickness of the ferroelectric layer. The strain induced by writing provides a significant additional contribution to the increased free energy of the written domain state with respect to a uniformly polarized state.

Comparing conventional and wavefront-optimized LASIK for the treatment of hyperopia.

PURPOSE: To compare differences in visual outcomes and induced spherical aberration after conventional and wavefront-optimized LASIK for the treatment of hyperopia. METHODS: In a prospective, randomized, single-center clinical trial, 51 consecutive eyes underwent LASIK for the treatment of hyperopia. Eyes were divided between groups treated with conventional LASIK with the Alcon LADAR4000 excimer laser (n=25) and wavefront-optimized LASIK with the WaveLight ALLEGRETTO excimer laser (n=26). Refractive and visual outcomes, induced spherical aberrations, and contrast sensitivity were analyzed. RESULTS: On postoperative day 1, 20% of eyes treated with a conventional profile had uncorrected visual acuity (UCVA) of 20/20 or better compared to 65% of eyes receiving wavefront-optimized treatment (P=.0011). By 6 months, UCVA was 20/20 or better in 72% and 84% of the conventional and wavefront-optimized treatment groups, respectively (P=.3254). At 6 months, the manifest refraction spherical equivalent was -0.21+/-0.47 diopters (D) and 0.16+/-0.27 D (P=.6469) whereas the cylinder was -0.41+/-0.47 D and -0.17+/-0.27 D (P=.0332) for the conventional and wavefront-optimized treatment groups, respectively. Induced spherical aberration was -0.54+/-0.32 microm and -0.42+/-0.21 microm for the conventional and wavefront-optimized treatment groups, respectively (P=.1195). The respective change in mesopic and photopic area under the log contrast sensitivity function was -0.05+/-0.29 and -0.05+/-0.23 for the conventional treatment group and 0.08+/-0.39 and 0.08+/-0.41 for the wavefront-optimized treatment group (P=.1970). CONCLUSIONS: Wavefront-optimized (ALLEGRETTO) and conventional (LADAR4000) ablation predictably and safely correct low to moderate hyperopia. Wavefront-optimized ablation showed superior results with regards to rapid visual recovery and residual cylinder. Although not statistically significant, a trend towards less induced negative spherical aberrations and improved mesopic and photopic contrast sensitivity was noted with wavefront-optimized treatment.

Corneal endothelial cell density after femtosecond thin-flap LASIK and PRK for myopia: a contralateral eye study.

PURPOSE: To compare the effect of femtosecond thinflap LASIK and photorefractive keratectomy (PRK) on postoperative endothelial cell density. METHODS: In a prospective, randomized, contralateral, single-center clinical trial, 25 patients (mean age: 30+/-5 years [range: 21 to 38 years]) underwent PRK in one eye and thin-flap LASIK in the fellow eye for the correction of myopia using a wavefront-guided platform. The central corneal endothelial cell density was measured using the NIDEK Confoscan 4 preoperatively, and at 1 and 3 months postoperatively. Changes in endothelial cell density were analyzed over time between the two refractive techniques. RESULTS: In PRK, the average preoperative endothelial cell density was 3011+/-329 cells/mm(2), which decreased to 2951+/-327 cells/mm(2) at 1 month (P=.5736) and 2982+/-365 cells/mm(2) at 3 months (P=.6513). In thinflap LASIK, the average preoperative endothelial cell density was 2995+/-325 cells/mm(2), which decreased to 2977+/-358 cells/mm(2) at 1 month (P=.5756) and 2931+/-369 cells/mm(2) at 3 months (P=.4106). No statistically significant difference was found between the two groups at 1 (P=.7404) or 3 (P=.3208) months postoperatively. CONCLUSIONS: No statistically significant change was noted in endothelial cell density following either PRK or thin-flap LASIK for the treatment of myopia. Furthermore, no statistically significant difference was found between the two groups out to 3 months postoperatively, indicating that thin-flap LASIK is as safe as PRK with regards to endothelial health.