Daniel Zaksas

Directional Signals in the Prefrontal Cortex and in Area MT during a Working Memory for Visual Motion Task

Neurons in the middle temporal visual area (MT) have been implicated in the perception of visual motion, whereas prefrontal cortex (PFC) neurons have been linked to temporary storage of sensory signals, attentional and executive control of behavior. Using a task that placed demands on both sets of neurons, we investigated their contribution to working memory for visual motion. Monkeys compared the direction of two moving random-dot stimuli, sample and test, separated by a brief memory delay. Neurons in both areas showed robust direction-selective activity during all phases of the task. During the sample, ∼60% of task-related PFC neurons were direction selective, and this selectivity emerged 40 ms later than in MT. Unlike MT, the PFC responses to sample did not correlate with behavioral choices, but their selectivity was modulated by task demands and diminished on error trials. Reliable directional signals were found in both areas during the memory delay, but these signals were transient rather than sustained by neurons of either area. Responses to the test in both areas were modulated by the remembered sample direction, decreasing when the test direction matched the sample. This decrease arose in the PFC 100 ms later than in MT and was predictive of the forthcoming decision. Our data suggest that neurons in the two regions are functionally connected and make unique contributions to different task components. PFC neurons reflect task-related information about visual motion and represent decisions that may be based, in part, on the comparison in MT between the remembered sample and test.

One step closer to the intrinsic laser damage threshold of HfO2 and SiO2 monolayer thin films

Results from monolayer-film laser-damage studies by various authors have remained difficult to compare, owing to many extrinsic factors having impact on measured damage thresholds and observed damage morphology. Prominent among these factors are the deposition method and conditions during deposition, the choice of starting materials, and the condition of supporting substrates. Here special attention is paid to the film-supporting surface with the goal of eliminating interfacial absorption effects. Fused-silica and float-glass substrates are prepared by various techniques: cleaved, conventionally polished, conventionally polished with added magnetorheological finish, and ion milled after conventional polish. Atomic-force microscopy is employed in determining microroughness and mapping laser-damage morphology features after irradiation at 1054 nm and 351 nm. HfO2 and SiO2 monolayers deposited on these surfaces showed large variations in damage threshold and morphology, depending on substrate-finish conditions. In spite of highest microroughness, cleaved-float-glass surfaces yielded the highest damage thresholds in both bare and coated forms. A comparison between HfO2 and SiO2 monolayer damage thresholds proved SiO2 to be generally far superior to HfO2.

Perfluorinated polymer films with extraordinary UV-laser damage resistance

UV damage thresholds are reported together with other optical characterization results for perfluorinated, frame-supported, micron-thickness polymer membranes. These thresholds are among the highest for any material ever measured in the UV. Transmitted wavefront uniformity for pellicles up to 30 cm in clear aperture approaches the wavefront-error-detection limits of state-of-the-art interferometers. Owing to benign fabrication procedures, these foils are highly defect free and lack the birefringence characteristics typical of extruded polymers.

Characterization of freestanding polymer films for application in 351-nm, high-peak-power laser systems

Perfluorinated pellicles are screened for thickness uniformity, absence of birefringence, photolytic stability under prolonged UV irradia- tion, and survival strength under pulsed (0.5-ns) high-peak-power laser irradiation. Results prove them to be extraordinarily robust against laser damage, photolytically stable, and better in transmitted wavefront quality than can be measured by state-of-the-art interferometry.

Nonlinear UV damage mechanism in polymer thin films observed from below to above damage threshold

In thin cellulose acetate films, 351-nm laser damage at subnanosecond pulse length can be demonstrated to be driven solely by a slow-buildup-time, transverse stimulated scattering process in which the transverse-scattered wave exhibits neither Raman nor Brillouin spectral shifts. Above threshold, the damage morphology exhibits an orientational selectivity that coincides with the scattered-wave orientation observable down to fluences 20 percent below the damage threshold. The physics of the scattering process remains to be identified.