High quality factor nitride-based optical cavities: microdisks with embedded GaN/Al(Ga)N quantum dots.

We compare the quality factor values of the whispering gallery modes of microdisks (µ-disks) incorporating GaN quantum dots (QDs) grown on AlN and AlGaN barriers by performing room temperature photoluminescence (PL) spectroscopy. The PL measurements show a large number of high Q factor resonant modes on the whole spectrum, which allows us to identify the different radial mode families and to compare them with simulations. We report a considerable improvement of the Q factor, which reflects the etching quality and the relatively low cavity loss by inserting QDs into the cavity. GaN/AlN QDs-based µ-disks show very high Q values (Q>7000) whereas the Q factor is only up to 2000 in µ-disks embedding QDs grown on the AlGaN barrier layer. We attribute this difference to the lower absorption below bandgap for AlN barrier layers at the energies of our experimental investigation.

Does leukoaraiosis predict morbidity and mortality?

OBJECTIVE: To determine whether leukoaraiosis predicts morbidity and mortality. BACKGROUND: Gait disturbance and leukoaraiosis both are common in the elderly. Gait disturbance predicts mortality. Leukoaraiosis may be a unifying factor to both gait disturbance and mortality. METHODS: We followed 221 patients prospectively evaluated for severity of neurologic deficits by the National Institutes of Health (NIH) stroke scale and for leukoaraiosis in seven brain regions by CT, graded as absent (n = 119, 54%), mild (in at least one of seven brain regions; n = 54, 24%), or severe (present in all seven brain regions; n = 48, 22%). Pneumonia (n = 27, 12%), falls resulting in fracture requiring hospitalization (n = 7, 3%), and death (n = 38, 17%) were end points. RESULTS: Severe leukoaraiosis predicted death (Cox hazard ratio [HR] = 2.91; 95% CI = 1.5 - 5.6), pneumonia (HR = 5.1; 95% CI = 2.4 - 10.9), death from pneumonia (HR = 8.3; 95% CI = 1.5 - 46), and falls (HR = 6.8; 95% CI = 1.5 - 30). Severe leukoaraiosis predicted a combined end point of death, pneumonia, and falls (HR = 3.5; 95% CI = 2 - 6). Other predictors were NIH stroke scale score, age, smoking, diabetes, gait score, and referral diagnosis of either dementia or Parkinsonism. Severe leukoaraiosis remained a predictor after adjustment for these other factors (HR = 2.2; 95% CI = 1.2 - 3.9), but was borderline after adjusting for gait (HR = 1.96; 95% CI = 0.97 - 3.94; p = 0.061). The combination of severe leukoaraiosis and gait disturbance had the highest risk (HR = 4.4; 95% CI = 2.4 - 7.9). CONCLUSION: Severe leukoaraiosis predicts morbidity and mortality independently of preexisting neurologic deficits. The combination of leukoaraiosis and gait disturbance carries a poor prognosis.

Cerebral white matter changes (leukoaraiosis), stroke, and gait disturbance.

OBJECTIVES: Leukoaraiosis, a radiological change of cerebral white matter thought to be caused by ischemia, is associated with gait disturbance. However, because of concomitant stroke and cerebral atrophy, the clinical relevance of leukoaraiosis is uncertain. We, therefore, sought to determine if leukoaraiosis is a predictor of gait disturbance after accounting for cerebral atrophy and stroke in patients with a high prevalence of cerebrovascular disease. DESIGN: Cross-sectional observational study. SETTING: Neurology service (inpatient and outpatient) of a Department of Veterans Affairs Hospital. PARTICIPANTS: Consecutive sample of 130 patients, 127 men and three women. MEASUREMENTS: The findings of a gait scale were correlated to vascular risk factors, neurological examination as quantified by the NIH stroke scale and supplemental motor scale, and to brain CT findings. Brain CT scans were rated for leukoaraiosis, cerebral infarction, and cerebral atrophy. RESULTS: Gait disturbance was more frequent and more severe in subjects with leukoaraiosis, of whom 31% had mild and 49% moderate/severe gait disturbance compared with 27% with mild and 12% with moderate/severe gait disturbance in subjects without leukoaraiosis (P < .001). Leukoaraiosis, cerebral atrophy, a history of multiple strokes, and weakness and ataxia of the legs were independent predictors of gait disturbance. The proportion and severity of leukoaraiosis increased with increasing gait disturbance in subgroups without leg deficit (P < .001) and without multiple strokes (P < .001), but no association with leukoaraiosis was shown in patients with leg deficit or a history of multiple strokes (P = .037 and P = .186, respectively). Gait disturbance was more severe when both leukoaraiosis and cerebral atrophy were present (P = .019). CONCLUSION: In our Veteran population, leukoaraiosis is an independent predictor of gait disturbance after accounting for stroke and cerebral atrophy. Although leukoaraiosis is a form of cerebrovascular disease, it appears to be most closely associated to gait disturbance in the absence of symptomatic stroke or leg deficit.