Richard Shek-kwan Chang

Learn More
'Mah-jong epilepsy' is a rare reflex epilepsy syndrome, manifesting as recurrent epileptic seizures triggered by either playing or just watching mah-jong. We present three patients with this condition and review all the reported cases. Mah-jong-induced seizures can be considered a subtype of cognition-induced epilepsy. Nonetheless, these patients have(More)
Recent experiments on similarly shaped polymer microcavity lasers show a dramatic difference in the far-field emission patterns. We show, for different deformations of the ellipse, quadrupole and hexadecapole, that the large differences in the far-field emission patterns are explained by the differing ray dynamics corresponding to each shape. Analyzing the(More)
We study lasing emission from asymmetric resonant cavity GaN microlasers. By comparing far-field intensity patterns with images of the microlaser we find that the lasing modes are concentrated on three-bounce unstable periodic ray orbits; i.e., the modes are scarred. The high-intensity emission directions of these scarred modes are completely different from(More)
It is proposed that the spoiling of high Q whispering gallery modes in deformed dielectric spheres can be understood as a transition to chaotic ray dynamics which can no longer be confined by total internal reflection. This is a KAM/Lazutkin transition for light. The theory is applied to explain the lasing properties of droplets. The highest Q optical(More)
By combining detailed imaging measurements at different tilt angles with simulations of ray emission from prolate deformed lasing micro-droplets, we conclude that the probability density for the lasing modes in a three-dimensional dielectric microcavity must reside in the chaotic region of the ray phase space. In particular, maximum emission from such(More)
A ray-optics model is developed to describe the spoiling of the high-Q (whispering gallery) modes of ring-shaped cavities as they are deformed from perfect circularity. A sharp threshold is found for the onset of Q spoiling as predicted by the Kolmogorov-Arnol'd-Moser (KAM) theorem of nonlinear dynamics. Beyond the critical deformation b(c), Q ~ (b -(More)
BACKGROUND Clinical outcome of Chinese relapsing remitting multiple sclerosis (RRMS) patients is uncertain. AIM To study the long-term clinical outcome of Chinese RRMS patients. METHOD RRMS patients with duration of 10 years or longer followed up in our hospital is retrospectively studied. RESULTS 61 RRMS patients (75% female) were studied. Their mean(More)
BACKGROUND AND PURPOSE Cancer patients are at increased risk of cardiovascular and cerebrovascular events. It is unclear whether cancer confers any additional risk for recurrent stroke or cardiovascular mortality after stroke. METHODS This was a single center, observational study of 1,105 consecutive Chinese ischemic stroke patients recruited from a large(More)