The slow light propagation in a line waveguide in the two-dimensional triangular photonic crystal has been numerically studied, based on which a wideband photonic crystal waveguide with low group-velocity and low disp...The slow light propagation in a line waveguide in the two-dimensional triangular photonic crystal has been numerically studied, based on which a wideband photonic crystal waveguide with low group-velocity and low dispersion is proposed. The numerical simulation analysis shows that it is possible to maximize the group index and minimize the group-velocity dispersion in wide bandwidth by increasing the radius of the basic air hole and changing the position of the first two rows of air holes in photonic crystal waveguides. Such a photonic crystal waveguide exhibits low group velocity and low group-velocity dispersion over a broad wavelength range. A larger group index-bandwidth product is achieved in this type of waveguide structure. The numerically computed results present the normalized bandwidth as 0.32%, 0.48% and 0.642% corresponding to the group index of 85, 58 and 45, respectively.展开更多
Based on Kukhtarev＇s equations, we derive the formulae of intensity-coupling coefficient, the phase-coupling coefficient and the group velocity at large modulation depth. It is theoretically shown that the signal bea...Based on Kukhtarev＇s equations, we derive the formulae of intensity-coupling coefficient, the phase-coupling coefficient and the group velocity at large modulation depth. It is theoretically shown that the signal beam can be amplified after passing through the photorefractive crystal and the group velocity reduced to m/s, even cm/s. Meanwhile, we also analyse the influence of the thermal excitation rate and the large signal effects on optical amplification and reduction of light propagation in photorefractive two-wave mixing.展开更多
Novel highly birefringent photonic bandgap fibres （PBGFs） are obtained by filling of a high index material in the air holes of total internal reflection birefringent photonic crystal fibres. The effect of the fillin...Novel highly birefringent photonic bandgap fibres （PBGFs） are obtained by filling of a high index material in the air holes of total internal reflection birefringent photonic crystal fibres. The effect of the filling high index material on the transmission characteristics has been theoretically investigated. The photonic bandgap has been achieved by using plane-wave method. Moreover, the phase and group modal birefringence have been studied by a full-vector finite-element method. Numerical results show that very high group and phase modal birefringence with magnitude of order of 10^-2 and 10^-3 has been respectively acquired, which is much higher than those of the non-filled fibres. Furthermore, strong coupling between surface modes and the fundamental modes has been found in the bandgap of the birefringent PBGFs, whose effect on the birefringence and confinement loss has also been discussed.展开更多
The photosensitivity of silicon is inherently very low in the visible electromagnetic spectrum,and it drops rapidly beyond 800 nm in near-infrared wavelengths.We have experimentally demonstrated a technique utilizing ...The photosensitivity of silicon is inherently very low in the visible electromagnetic spectrum,and it drops rapidly beyond 800 nm in near-infrared wavelengths.We have experimentally demonstrated a technique utilizing photon-trapping surface structures to show a prodigious improvement of photoabsorption in 1-μm-thin silicon,surpassing the inherent absorption efficiency of gallium arsenide for a broad spectrum.The photon-trapping structures allow the bending of normally incident light by almost 90 deg to transform into laterally propagating modes along the silicon plane.Consequently,the propagation length of light increases,contributing to more than one order of magnitude improvement in absorption efficiency in photodetectors.This high-absorption phenomenon is explained by finitedifference time-domain analysis,where we show an enhanced photon density of states while substantially reducing the optical group velocity of light compared to silicon without photon-trapping structures,leading to significantly enhanced light–matter interactions.Our simulations also predict an enhanced absorption efficiency of photodetectors designed using 30-and 100-nm silicon thin films that are compatible with CMOS electronics.Despite a very thin absorption layer,such photon-trapping structures can enable high-efficiency and high-speed photodetectors needed in ultrafast computer networks,data communication,and imaging systems,with the potential to revolutionize on-chip logic and optoelectronic integration.展开更多
A large number of Rayleigh wave dispersion curves recorded at twenty three seismic stations was used to investigate the 3-D shear wave velocity structure of the northeastern Brazilian lithosphere. A simple procedure t...A large number of Rayleigh wave dispersion curves recorded at twenty three seismic stations was used to investigate the 3-D shear wave velocity structure of the northeastern Brazilian lithosphere. A simple procedure to generate a three-dimensional image of Mohorovicic;discontinuity was applied in northeastern Brazil and the Moho 3-D image was in agreement with several isolated crustal thicknesses obtained with different geophysical methods. A detailed 3-D S wave velocity model is proposed for the region. In the crust, our model is more realist than CRUST2.0 global model, because it shows more details either laterally or in depth than global model, i.e., clear lateral variation and gradual increase of S wave velocity in depth. Down to 100 km depth, the 3-D S wave velocity model in northeastern Brazil is dominated by low velocities and this is consistent either with heat flow measurements or with measurements of the flexural strength of the lithosphere developed in the South American continent. Our 3-D S wave velocity model was also used to obtain the lithosphere thickness in each cell of the northeastern Brazil and the results were consistent with global studies about the Lithosphere-Asthenosphere Boundary worldwide.展开更多
Under the condition of combined effects of group--velocitydispersion and self- phase modulation, the step Fourier method isused to simulate the propagation of initial chirped super-Gaussianpulses inside fiber. The ini...Under the condition of combined effects of group--velocitydispersion and self- phase modulation, the step Fourier method isused to simulate the propagation of initial chirped super-Gaussianpulses inside fiber. The initial chirp influences the shapes of superGaussian pulses in propagation process, and positive and negativechirps have different effects. For the existing of initial chirp, thesplits of pulses and the spreading speed move ahead and increase.When the amplitude of super-Gaussian pulses increases by 1.4 times,in the range of │C│<1.5, pulses can keep good shapes along theirpropagation distance.展开更多
We show analytically that in phase-mismatched second-harmonic generation, an effective diffraction is induced at the second-harmonic （SH） frequency. Numerical simulation results agree with the analytical predictions...We show analytically that in phase-mismatched second-harmonic generation, an effective diffraction is induced at the second-harmonic （SH） frequency. Numerical simulation results agree with the analytical predictions. Compared to the case of linear propagation, the effect of the overall diffraction at the SH frequency becomes doubled due to the induced diffraction, which causes an interesting result that the SH beam width will be larger than that of the fundamental field.展开更多
Supercontinuum generation in dispersion-flattened fibers is studied theoretically. It is found that the flat spectral width of the supereontinuum generation in normal dispersion-flattened fiber can be increased from 6...Supercontinuum generation in dispersion-flattened fibers is studied theoretically. It is found that the flat spectral width of the supereontinuum generation in normal dispersion-flattened fiber can be increased from 66 nm to over 100 nm when the absolute value of the initial frequency chirps is increased from zero to 10. It is further found that initial frequency chirps are adverse to flat and wideband supercontinuum generation in anomalous dispersion-flattened fiber, and when the absolute value of the frequency chirps is increased to a certain degree, supercontinuum spectrum even can not be achieved.展开更多
文摘The slow light propagation in a line waveguide in the two-dimensional triangular photonic crystal has been numerically studied, based on which a wideband photonic crystal waveguide with low group-velocity and low dispersion is proposed. The numerical simulation analysis shows that it is possible to maximize the group index and minimize the group-velocity dispersion in wide bandwidth by increasing the radius of the basic air hole and changing the position of the first two rows of air holes in photonic crystal waveguides. Such a photonic crystal waveguide exhibits low group velocity and low group-velocity dispersion over a broad wavelength range. A larger group index-bandwidth product is achieved in this type of waveguide structure. The numerically computed results present the normalized bandwidth as 0.32%, 0.48% and 0.642% corresponding to the group index of 85, 58 and 45, respectively.
基金Supported by the National Natural Science Foundation of China under Grant Nos 60478014 and 60272075, and the Programme of Excellent Team of Harbin Institute of Technology.
文摘Based on Kukhtarev＇s equations, we derive the formulae of intensity-coupling coefficient, the phase-coupling coefficient and the group velocity at large modulation depth. It is theoretically shown that the signal beam can be amplified after passing through the photorefractive crystal and the group velocity reduced to m/s, even cm/s. Meanwhile, we also analyse the influence of the thermal excitation rate and the large signal effects on optical amplification and reduction of light propagation in photorefractive two-wave mixing.
基金Supported by the National Basic Research Programme of China under Grant No 2003CB314906, the National High Technology Project of China under Grant No 2002AA313110, the National Natural Science Foundation of China under Grant Nos 60407005 and 60137010, and the Science and Technology Innovation Fund of Nankai University.
文摘Novel highly birefringent photonic bandgap fibres （PBGFs） are obtained by filling of a high index material in the air holes of total internal reflection birefringent photonic crystal fibres. The effect of the filling high index material on the transmission characteristics has been theoretically investigated. The photonic bandgap has been achieved by using plane-wave method. Moreover, the phase and group modal birefringence have been studied by a full-vector finite-element method. Numerical results show that very high group and phase modal birefringence with magnitude of order of 10^-2 and 10^-3 has been respectively acquired, which is much higher than those of the non-filled fibres. Furthermore, strong coupling between surface modes and the fundamental modes has been found in the bandgap of the birefringent PBGFs, whose effect on the birefringence and confinement loss has also been discussed.
基金supported in part by the US Army’s Night Vision and Electronic Sensors Directorate(Grant No.W909MY-12-D-0008)NSF ECCS(Grant No.1428392)。
文摘The photosensitivity of silicon is inherently very low in the visible electromagnetic spectrum,and it drops rapidly beyond 800 nm in near-infrared wavelengths.We have experimentally demonstrated a technique utilizing photon-trapping surface structures to show a prodigious improvement of photoabsorption in 1-μm-thin silicon,surpassing the inherent absorption efficiency of gallium arsenide for a broad spectrum.The photon-trapping structures allow the bending of normally incident light by almost 90 deg to transform into laterally propagating modes along the silicon plane.Consequently,the propagation length of light increases,contributing to more than one order of magnitude improvement in absorption efficiency in photodetectors.This high-absorption phenomenon is explained by finitedifference time-domain analysis,where we show an enhanced photon density of states while substantially reducing the optical group velocity of light compared to silicon without photon-trapping structures,leading to significantly enhanced light–matter interactions.Our simulations also predict an enhanced absorption efficiency of photodetectors designed using 30-and 100-nm silicon thin films that are compatible with CMOS electronics.Despite a very thin absorption layer,such photon-trapping structures can enable high-efficiency and high-speed photodetectors needed in ultrafast computer networks,data communication,and imaging systems,with the potential to revolutionize on-chip logic and optoelectronic integration.
文摘A large number of Rayleigh wave dispersion curves recorded at twenty three seismic stations was used to investigate the 3-D shear wave velocity structure of the northeastern Brazilian lithosphere. A simple procedure to generate a three-dimensional image of Mohorovicic;discontinuity was applied in northeastern Brazil and the Moho 3-D image was in agreement with several isolated crustal thicknesses obtained with different geophysical methods. A detailed 3-D S wave velocity model is proposed for the region. In the crust, our model is more realist than CRUST2.0 global model, because it shows more details either laterally or in depth than global model, i.e., clear lateral variation and gradual increase of S wave velocity in depth. Down to 100 km depth, the 3-D S wave velocity model in northeastern Brazil is dominated by low velocities and this is consistent either with heat flow measurements or with measurements of the flexural strength of the lithosphere developed in the South American continent. Our 3-D S wave velocity model was also used to obtain the lithosphere thickness in each cell of the northeastern Brazil and the results were consistent with global studies about the Lithosphere-Asthenosphere Boundary worldwide.
文摘Under the condition of combined effects of group--velocitydispersion and self- phase modulation, the step Fourier method isused to simulate the propagation of initial chirped super-Gaussianpulses inside fiber. The initial chirp influences the shapes of superGaussian pulses in propagation process, and positive and negativechirps have different effects. For the existing of initial chirp, thesplits of pulses and the spreading speed move ahead and increase.When the amplitude of super-Gaussian pulses increases by 1.4 times,in the range of │C│<1.5, pulses can keep good shapes along theirpropagation distance.
基金Supported by the National Natural Science Foundation of China under Grant Nos 60538010, 10335030 and 10576009, and the Science and Technology Commission of Shanghai under Grant Nos 05JC14005 and 05SG02.
文摘We show analytically that in phase-mismatched second-harmonic generation, an effective diffraction is induced at the second-harmonic （SH） frequency. Numerical simulation results agree with the analytical predictions. Compared to the case of linear propagation, the effect of the overall diffraction at the SH frequency becomes doubled due to the induced diffraction, which causes an interesting result that the SH beam width will be larger than that of the fundamental field.
基金National Natural Science Foundation of China(60677051)Guangdong Natural Science Foundation of China(8151063301000004)
文摘Supercontinuum generation in dispersion-flattened fibers is studied theoretically. It is found that the flat spectral width of the supereontinuum generation in normal dispersion-flattened fiber can be increased from 66 nm to over 100 nm when the absolute value of the initial frequency chirps is increased from zero to 10. It is further found that initial frequency chirps are adverse to flat and wideband supercontinuum generation in anomalous dispersion-flattened fiber, and when the absolute value of the frequency chirps is increased to a certain degree, supercontinuum spectrum even can not be achieved.
