A new conception of nano-laser is proposed in which depending on the size of nano-clusters (silicon quantum dots (QD)), the pumping level of laser can be tuned by the quantum confinement (QC) effect, and the populatio...A new conception of nano-laser is proposed in which depending on the size of nano-clusters (silicon quantum dots (QD)), the pumping level of laser can be tuned by the quantum confinement (QC) effect, and the population inversion can be formed between the valence band and the localized states produced from the surface bonds. The nano-laser belongs to the emission of type Ⅱ. The peaks of stimulated emission are observed at 605 nm and 693 nm. Through the micro-cavity of nano-laser, a full width at half maximum of the peak at 693 nm can reach to 0.5 nm. The theoretical model and the experimental results indicate that it is a necessary condition for setting up nano-laser that the smaller size of nano-clusters (d<3 nm) can make the localized states into band gap below the conduction band opened and the states of conduction band become the pumping level of nano-laser. The emission energy of nano-laser will be limited in the range of 1.7～2.3 eV generally due to the position of the localized states in gap, which is good in agreement between the experiments and the theory.展开更多
Fine structures of supersonic flow over a 5 mm high backward facing step(BFS),including expansion wave fan,reattachment shock,supersonic boundary layer were measured in a Ma=3.0 low-noise indraft wind tunnel.By varyin...Fine structures of supersonic flow over a 5 mm high backward facing step(BFS),including expansion wave fan,reattachment shock,supersonic boundary layer were measured in a Ma=3.0 low-noise indraft wind tunnel.By varying the superficial roughness of the wall upstream from the step,supersonic laminar flow and supersonic turbulent flow could be formed over a BFS.Measurements on the spatiotemporal features of the holistic flow field and the fine structures in four typical regions were carried out using NPLS(nano-based planar laser scattering).Flow structures,including expansion wave fan,reattachment shock,supersonic boundary layer and its separation,reattachment and redevelopment are revealed by measuring the holistic structure of the transient flow field.Comparing the two time-averaged flow fields with each other,it is apparent that supersonic turbulent flow over a BFS(STF-BFS) has a larger expansion angle and a shorter recirculation region,and its redeveloped boundary layer increases at a smaller obliquity while the angle of reattachment shock is the same for the supersonic laminar flow over a BFS(SLF-BFS).With regard to time-evolution features,the K-H vortices in the SLF-BFS suffers from shearing,expansion,reattachment and three-dimensional effects while in the STF-BFS large-scale structures are affected by the incline and distortion at the reattachment point due to expansion,viscosity and reverse-pressure.Studies on local regions indicate that in the SLF-BFS,the emergence of compression waves which distinctly converge into a reattachment shock is due to the local convective Mach number and the inducement of K-H vortices in the free shear layer.Nevertheless,in the STF-BFS,compression waves and K-H vortices are barely evident,and the formation of a reattachment shock is related to the wall compressive effect.展开更多
We numerically investigate the population dynamics in a single photon resonant three-level cascade and non-cascade energy level molecules at 532-nm wavelength. The time-dependent population in the energy levels in the...We numerically investigate the population dynamics in a single photon resonant three-level cascade and non-cascade energy level molecules at 532-nm wavelength. The time-dependent population in the energy levels in the presence of 100 ps(pico-second) and 100 ns(nano-second) laser pulses is described in the form of rate equations. We provide a brief idea of how the optical energy transfer takes place in the light-matter interaction and we also discuss the absorption as a function of pulse width and repetition rate. We also plot the z-scan transmittance curve as a function of number of excitation pulses participating in the absorption.展开更多
基金Support from the National Natural Science Foundation of China ( Grant No.10764002,60966002,11264007)the National Key Laboratory of Surface Physics in Fudan University
文摘A new conception of nano-laser is proposed in which depending on the size of nano-clusters (silicon quantum dots (QD)), the pumping level of laser can be tuned by the quantum confinement (QC) effect, and the population inversion can be formed between the valence band and the localized states produced from the surface bonds. The nano-laser belongs to the emission of type Ⅱ. The peaks of stimulated emission are observed at 605 nm and 693 nm. Through the micro-cavity of nano-laser, a full width at half maximum of the peak at 693 nm can reach to 0.5 nm. The theoretical model and the experimental results indicate that it is a necessary condition for setting up nano-laser that the smaller size of nano-clusters (d<3 nm) can make the localized states into band gap below the conduction band opened and the states of conduction band become the pumping level of nano-laser. The emission energy of nano-laser will be limited in the range of 1.7～2.3 eV generally due to the position of the localized states in gap, which is good in agreement between the experiments and the theory.
文摘Fine structures of supersonic flow over a 5 mm high backward facing step(BFS),including expansion wave fan,reattachment shock,supersonic boundary layer were measured in a Ma=3.0 low-noise indraft wind tunnel.By varying the superficial roughness of the wall upstream from the step,supersonic laminar flow and supersonic turbulent flow could be formed over a BFS.Measurements on the spatiotemporal features of the holistic flow field and the fine structures in four typical regions were carried out using NPLS(nano-based planar laser scattering).Flow structures,including expansion wave fan,reattachment shock,supersonic boundary layer and its separation,reattachment and redevelopment are revealed by measuring the holistic structure of the transient flow field.Comparing the two time-averaged flow fields with each other,it is apparent that supersonic turbulent flow over a BFS(STF-BFS) has a larger expansion angle and a shorter recirculation region,and its redeveloped boundary layer increases at a smaller obliquity while the angle of reattachment shock is the same for the supersonic laminar flow over a BFS(SLF-BFS).With regard to time-evolution features,the K-H vortices in the SLF-BFS suffers from shearing,expansion,reattachment and three-dimensional effects while in the STF-BFS large-scale structures are affected by the incline and distortion at the reattachment point due to expansion,viscosity and reverse-pressure.Studies on local regions indicate that in the SLF-BFS,the emergence of compression waves which distinctly converge into a reattachment shock is due to the local convective Mach number and the inducement of K-H vortices in the free shear layer.Nevertheless,in the STF-BFS,compression waves and K-H vortices are barely evident,and the formation of a reattachment shock is related to the wall compressive effect.
文摘We numerically investigate the population dynamics in a single photon resonant three-level cascade and non-cascade energy level molecules at 532-nm wavelength. The time-dependent population in the energy levels in the presence of 100 ps(pico-second) and 100 ns(nano-second) laser pulses is described in the form of rate equations. We provide a brief idea of how the optical energy transfer takes place in the light-matter interaction and we also discuss the absorption as a function of pulse width and repetition rate. We also plot the z-scan transmittance curve as a function of number of excitation pulses participating in the absorption.
