RESUMO
We study THz-driven condensate dynamics in epitaxial thin films of MgB_{2}, a prototype two-band superconductor (SC) with weak interband coupling. The temperature and excitation density dependent dynamics follow the behavior predicted by the phenomenological bottleneck model for the single-gap SC, implying adiabatic coupling between the two condensates on the ps timescale. The amplitude of the THz-driven suppression of condensate density reveals an unexpected decrease in pair-breaking efficiency with increasing temperature-unlike in the case of optical excitation. The reduced pair-breaking efficiency of narrow-band THz pulses, displaying minimum near ≈0.7 T_{c}, is attributed to THz-driven, long-lived, nonthermal quasiparticle distribution, resulting in Eliashberg-type enhancement of superconductivity, competing with pair breaking.
RESUMO
The light extraction efficiency of light-emitting diodes (LEDs) patterned with photonic crystals is investigated based on an incoherent multi-reflection-transmission model. The formulas of the extraction efficiency are obtained in an analytic form similar to the unpatterned case. Besides, we generalize the Rouard formulas commonly used in thin-film optics to the case of multilayered photonic crystal slabs. The influence of various types of backside reflectors and the absorption of the semiconductor on the efficiency are demonstrated for GaN-based LEDs. The optimization of the photonic crystal slab based on the LED model is also presented and briefly discussed.
