Mode coupling by scattering in chiral nematic liquid crystal ring lasing.

Lasing in dye-doped chiral nematic liquid crystal can be realized with low pump energy and relatively high efficiency, thanks to the high reflectivity of the periodic structure. When the helical axis is oriented perpendicular to the substrates, the main lasing peak is normal to the substrates. In some cases, ring lasing of a particular wavelength is observed into an emission cone with axial symmetry. In this paper we explain how scattering of light in the liquid crystal layer leads to optical coupling between normal modes and inclined modes. Based on a numerical model that takes into account spontaneous emission, gain and scattering we show that scattering leads to emission characteristics that are similar to experimental results.

Generation of multiple solitons using competing nonlocal nonlinearities.

We discuss the dynamics of fundamental Gaussian beams launched in saturable and nonlocal nonlinear media. Solely in the presence of a self-focusing saturable nonlinearity, the breathing solitons undergo strong deformation. The addition of a defocusing nonlinearity leads to the generation of couples of solitons. Experimentally, we demonstrate in nematic liquid crystals the formation of multiple spatial solitons starting from a bell-shaped input, with both direction and the number of filaments depending on the input power, confirming the theoretical predictions.

Active commuting to school, cognitive performance, and academic achievement: an observational study in Dutch adolescents using accelerometers.

BACKGROUND: The current study examined the associations between active commuting to school, cognitive performance, and academic achievement in Dutch adolescents. In addition, it was explored whether these associations were moderated by sex and mediated by depressive symptoms. METHODS: Students in grades 7 and 9 (N = 270; mean age 13.4 years; 53% boys) were included. Active commuting to school was measured objectively by an ActivPAL3™ accelerometer. Cognitive performance was measured by the d2 Test of attention (key components of executive functioning) and the Symbol Digit Modalities Test (information-processing speed). Academic achievement was determined by the mean of the school grades obtained in Dutch, mathematics and English. Depressive symptoms were self-reported. RESULTS: Active commuting to school constituted 28% of the total amount of time spent moving per week. Active commuting to school was not significantly associated with cognitive performance and academic achievement, overall. However, active commuting to school was positively associated with performance on the d2 Test of attention in girls (ß = .17, p = .037), but not in boys (ß = -.03, p = .660). The associations were not mediated by depressive symptoms. CONCLUSIONS: The associations between active commuting to school and cognitive performance and academic achievement are weak and might be moderated by sex, while the greatest benefits on cognition due to active commuting to school might be with regard to executive functioning. Future studies might make use of experimental designs, because causal relations between active commuting to school and cognitive performance or academic achievement would provide important implications for both education and public health.

The association between objectively measured physical activity and academic achievement in Dutch adolescents: findings from the GOALS study.

The main goal of this study was to investigate the association between objectively measured physical activity and academic achievement in adolescents. Students in Grades 7 and 9 (N = 255) were included. Overall, we found no significant dose-response association between physical activity and academic achievement. However, in Grade 7 total physical activity volume (Total PA) was negatively associated with academic achievement, while moderate-to-vigorous intensity physical activity (MVPA) was negatively associated with both academic achievement and mathematics performance. In contrast, in Grade 9 both Total PA and MVPA were positively associated with mathematics performance. In addition, the overall association between MVPA and academic achievement followed an inverted U-shaped curve. Finally, Total PA was positively associated with executive functioning, while executive functioning in turn mediated the associations between Total PA and both academic achievement and mathematics performance. These results indicate that the association between physical activity and academic achievement in adolescents is complex and might be affected by academic year, physical activity volume and intensity, and school grade.

Quantum dot lasing from a waterproof and stretchable polymer film.

Colloidal quantum dots (QDs) are excellent optical gain materials that combine high material gain, a strong absorption of pump light, stability under strong light exposure and a suitability for solution-based processing. The integration of QDs in laser cavities that fully exploit the potential of these emerging optical materials remains, however, a challenge. In this work, we report on a vertical cavity surface emitting laser, which consists of a thin film of QDs embedded between two layers of polymerized chiral liquid crystal. Forward directed, circularly polarized defect mode lasing under nanosecond-pulsed excitation is demonstrated within the photonic band gap of the chiral liquid crystal. Stable and long-term narrow-linewidth lasing of an exfoliated free-standing, flexible film under water is obtained at room temperature. Moreover, we show that the lasing wavelength of this flexible cavity shifts under influence of pressure, strain or temperature. As such, the combination of solution processable and stable inorganic QDs with high chiral liquid crystal reflectivity and effective polymer encapsulation leads to a flexible device with long operational lifetime, that can be immersed in different protic solvents to act as a sensor.

Cross-national comparability of the WHOQOL-BREF: a measurement invariance approach.

PURPOSE: To evaluate whether the WHOQOL-BREF measures the QOL construct in the same way across nations. METHODS: Students from Flanders, Belgium and Iran completed the WHOQOL-BREF as part of a larger Quality of Life questionnaire. Their responses were compared using a multi-group confirmatory factor analysis. RESULTS: In general, the QOL construct appears rather similar in both cultures; however, participants from both countries seem to respond differently to particular items of the WHOQOL-BREF. Especially for the physical and psychological domain, this is problematic, because none of their indicators works in the same way across samples. CONCLUSIONS: Notwithstanding some limitations of this study, it must be concluded that the WHOQOL-BREF should only be used with great caution in cross-national comparisons.

Physical Activity and School Absenteeism Due to Illness in Adolescents.

BACKGROUND: Knowledge about the beneficial role of physical activity (PA) for health and school performance is growing. Studies investigating the link between PA and school absenteeism due to illness are lacking. Therefore, we investigated associations between habitual PA and school absenteeism due to illness in adolescents and explored whether mental health and cardiovascular fitness mediated this association. METHODS: We studied 328 students in grades 7 and 9 (mean age 13.8 years; 49% boys). The PA was measured objectively by an ActivPAL3™ accelerometer attached on the thigh during 1 full week (24 hours/day). Depressive symptoms and self-esteem were self-reported by the Center for Epidemiologic Studies Depression Scale (CES-D) and Rosenberg Self-Esteem Scale, respectively, and included as a proxy for mental health in the analyses. Cardiovascular fitness was measured by the 20-m shuttle-run test. School absenteeism due to illness data was provided by the school administration. RESULTS: The PA was not significantly associated with school absenteeism, though there was an indirect association between PA and school absenteeism by cardiovascular fitness. CONCLUSIONS: Cardiovascular fitness mediates the association between PA and school absenteeism due to illness. Thus, cardiovascular fitness of students should be improved to reduce school absenteeism due to illness.

Impaired facial emotion recognition in patients with ventromedial prefrontal hypoperfusion.

Empathy refers to our ability to recognize and share emotions by another human being. Impairment may underlie many of the emotional deficits commonly associated with a range of neuropsychiatric and neurological conditions. The prefrontal cortex (PFC) has long been implicated in these processes, but the specific contribution of subregions of the PFC remain unclear. Studies regarding the role of subregions of the prefrontal cortex such as the ventromedial prefrontal cortex (vmPFC)-in facial emotion recognition have yielded inconsistent results. The present study aimed to investigate the capacity to recognize nonverbal emotional facial expressions in a group of patients with the following: (a) perfusion deficits in the vmPFC (vmPFC group; N = 13), (b) hypoperfusions sparing the vmPFC (nonvmPFC group; N = 12), and in (c) a control group of healthy volunteers (control group; N = 17). Regions of hypoperfusion were identified by means of Single Photon Emission Computed Tomography (SPECT). Participants were asked to recognize facial expressions of the 7 basic emotions (happiness, fear, surprise, anger, disgust, sadness, or neutral). Detection of facial expressions of fear, disgust, and surprise was affected after functional disruption of the vmPFC. The present study confirms the role of the vmPFC in recognizing emotional facial expressions.

Comparative sensitivity of outcome variables of a software-based Behavioral Sleep Resistance Task.

Unintentional sleep and performance impairment due to extended wakefulness are often the cause of traffic and work accidents. Therefore, large-scale screening instruments assessing the ability to resist falling asleep during monotonous tasks are needed. The current widespread computer use in industrial and home settings offers the possibility to use software-based instruments as an alternative for expensive hardware interfaces to assess sleep resistance. Therefore, we propose a software-based sleep resistance task based on the procedure of the Oxford SLEep Resistance test (OSLER): the Behavioral Sleep Resistance Task (BSRT). In order to validate this instrument we submitted 36 healthy individuals to four 40-min sleep resistance challenges during a night of extended wakefulness (14, 16, 18 and 20 h). Subjective sleepiness was assessed by means of the Karolinska Sleepiness Scale and a Visual Analogue Scale for sleepiness/alertness and objective sleepiness by means of a 4-min EEG-monitored fixation task. Our results show significant correlations of BSRT hit ratio, error profiles and sleep onset latency variables with subjective sleepiness and of BSRT hit ratio and 3-6 error profiles with High-Beta Central EEG activity. Additionally, using a distribution- and scale-free sensitivity index, we found that subjective sleepiness measures are the most sensitive to the effects of sustained wakefulness, closely followed by the BSRT hit ratio and BSRT error profile 3-6.