We demonstrate that very few (2-4) quantum dots as a gain medium are sufficient to realize a photonic-crystal laser based on a high-quality nanocavity. Photon correlation measurements show a transition from a thermal to a coherent light state proving that lasing action occurs at ultralow thresholds. Observation of lasing is unexpected since the cavity mode is in general not resonant with the discrete quantum dot states and emission at those frequencies is suppressed. In this situation, the quasicontinuous quantum dot states become crucial since they provide an energy-transfer channel into the lasing mode, effectively leading to a self-tuned resonance for the gain medium.
Nanocrystalline diamond microdisks have been fabricated and characterized. The process conditions were chosen to ensure smooth and vertical sidewalls. Focused ion beam milling was used to create ultrasmooth sidewalls. Whispering gallery modes were observed near the nitrogen-vacancy center emission wavelength (637nm) by photoluminescence and near ∼1550nm by evanescent fiber coupling. The cavity quality factors (Q) are about 100 in both experiments. The Q’s for these disks were calculated to be as high as 105 by three-dimensional finite-difference time-domain simulations. The authors believe the Q’s to be limited by absorption and scattering within the nanocrystalline cavity material.
Recently, small hydropower attracts attention because of its clean, renewable and abundant energy resources to develop. Therefore, a cross-flow hydraulic turbine is proposed for small hydropower in this study because the turbine has relatively simple structure and high possibility of applying to small hydropower. The purpose of this study is to investigate the effect of the turbine's structural configuration on the performance and internal flow characteristics of the cross-flow turbine model using CFD analysis. The results show that nozzle shape, runner blade angle and runner blade number are closely related to the performance and internal flow of the turbine. Moreover, air layer in the turbine runner plays very important roles of improving the turbine performance.
GaN-based photonic-crystal membrane nanocavities with Q factors up to 800 have been realized at the wavelength of ∼480nm. The tuning behavior agrees well with numerical calculations using the finite-difference time-domain method. Theoretically, the lowest energy mode of a cavity that consists of seven missing holes in the Γ-K direction promises a Q factor as high as 4×104 with a mode volume of about 1.3×(λ∕n)3.
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