High-efficiency grating-couplers: demonstration of a new design strategy.

We present a simple and practical strategy that allows to design high-efficiency grating couplers. The technique is based on the simultaneous apodization of two structural parameters: the grating period and the fill-factor, along with the optimization of the grating coupler etching depth. Considering a 260 nm Si-thick Silicon-on-insulator platform, we numerically demonstrated a coupling efficiency of -0.8 dB (83%), well matching the experimental value of -0.9 dB (81%). Thanks to the optimized design, these results represent the best performance ever reported in the literature for SOI structures without the use of any back-reflector.

Group-velocity dispersion in SOI-based channel waveguides with reduced-height.

We report on the experimental characterization, in the telecom C-band, of group-velocity dispersion (D) in 100-nm high rectangular strip waveguides realized by silicon-on-insulator technology. We compare the experimental results with numerical predictions, showing that 100-nm high waveguides exhibit normal dispersion and that the absolute value of the dispersion coefficient D decreases as the waveguide width is increased. D at 1550 nm varies from -8130 to -3900 ps/(nm·km) by increasing the waveguide width from 500 to 800 nm.

Stationary growth and unique invariant harmonic measure of cylindrical diffusion limited aggregation.

We prove that the harmonic measure is stationary, unique, and invariant on the interface of diffusion limited aggregation (DLA) growing on a cylinder surface. We provide a detailed theoretical analysis puzzling together multiscaling, multifractality, and conformal invariance, supported by extensive numerical simulations of clusters built using conformal mappings and on a lattice. The growth properties of the active and frozen zones are clearly elucidated. We show that the unique scaling exponent characterizing the stationary growth is the DLA fractal dimension.

A case-crossover study of sleep and work hours and the risk of road traffic accidents.

STUDY OBJECTIVES: Sleepiness, prolonged wakefulness, and extended work hours have been associated with increased risk of injuries and road accidents. The authors' objective was to study the relation between those factors and road accidents using a case-crossover design, effective in estimating the risk of acute events associated with transient, short effect exposures. DESIGN: Five hundred seventy-four injured drivers presenting for care after road accidents to the Emergency Room of Udine, Italy, were enrolled in the study from March 2007 to March 2008. Sleep, work, and driving patterns in the 48 h before the accident were assessed through an interview. MEASUREMENTS AND RESULTS: The relative risk (RR) of accident associated with each exposure was estimated using the case-crossover matched pair interval approach. Sleeping > or = 11 h daily was associated with a decrease of the RR, as was sleeping less than usual. Being awake > or = 16 h and, possibly, working > 12 h daily were associated with increases in the RR. CONCLUSIONS: Extended work hours and prolonged wakefulness increase the risk of road accidents and suggest that awareness should be raised among drivers. The findings regarding acute sleep amount are less clear, possibly due to an effect of chronic sleep loss.

A case-crossover study of alcohol consumption, meals and the risk of road traffic crashes.

BACKGROUND: The case-crossover (CC) design has proved effective to investigate the association between alcohol use and injuries in general, but has never been applied to study alcohol use and road traffic crashes (RTCs) specifically. This study aims at investigating the association between alcohol and meal consumption and the risk of RTCs using intrapersonal comparisons of subjects while driving. METHODS: Drivers admitted to an Italian emergency room (ER) after RTCs in 2007 were interviewed about personal, vehicle, and crash characteristics as well as hourly patterns of driving, and alcohol and food intake in the 24 hours before the crash. The odds ratio (OR) of a RTC was estimated through a CC, matched pair interval approach. Alcohol and meal consumption 6 and 2 hours before the RTC (case exposure window) were compared with exposures in earlier control windows of analogous length. RESULTS: Of 574 patients enrolled, 326 (56.8%) reported previous driving from 6 to 18 hours before the RTC and were eligible for analysis. The ORs (mutually adjusted) were 2.25 (95%CI 1.11-4.57) for alcohol and 0.94 (0.47-1.88) for meals. OR for alcohol was already increased at low (1-2 units) doses - 2.17 (1.03-4.57) and the trend of increase for each unit was significant - 1.64 (95%CI 1.05-2.57). In drivers at fault the OR for alcohol was 21.22 (2.31-194.79). The OR estimate for meal consumption seemed to increase in case of previous sleep deprivation, 2.06 (0.25-17.00). CONCLUSION: Each single unit of acute alcohol consumption increases the risk of RTCs, in contrast with the 'legal' threshold allowed in some countries. Meal consumption is not associated with RTCs, but its combined effects with sleepiness need further elucidation.