The relationship between gain and carrier density is analysed. In the quantum well (QW) lasers, initially, the gain increases rapidly with the carrier density and then starts to saturate. It can be seen that QW lasers...The relationship between gain and carrier density is analysed. In the quantum well (QW) lasers, initially, the gain increases rapidly with the carrier density and then starts to saturate. It can be seen that QW lasers have a higher differential gain because of the step-like state density, and that the gain saturates at higher carrier densities because of the constant state density of the lowest subband. It is shown that simple logarithmic gain-carrier density is more accurate than the traditional linearized form for a QW laser.展开更多
Photocatalytic CO2 reduction holds promise as a future technology for the manufacture of fuels and commodity chemicals.However,factors controlling product selectivity remain poorly understood.Herein,we compared the pe...Photocatalytic CO2 reduction holds promise as a future technology for the manufacture of fuels and commodity chemicals.However,factors controlling product selectivity remain poorly understood.Herein,we compared the performance of a homologous series of Zn-based layered double hydroxide(ZnM-LDH)photocatalysts for CO2 reduction.By varying the trivalent or tetravalent metal cations in the ZnM-LDH photocatalysts(M=Ti4+,Fe3+,Co3+,Ga3+,Al3+),the product selectivity of the reaction could be precisely controlled.ZnTi-LDH afforded CH4 as the main reduction product;ZnFe-LDH and ZnCo-LDH yielded H2 exclusively from water splitting;whilst ZnGa-LDH and ZnAl-LDH generated CO.In-situ diffuse reflectance infrared measurements,valence band XPS and density function theory calculations were applied to rationalize the CO2 reduction selectivities of the different ZnM-LDH photocatalysts.The analyses revealed that the d-band center(ed)position of the M3+or M4+cations controlled the adsorption strength of CO2 and thus the selectivity to carbon-containing products or H2.Cations with d-band centers relatively close to the Fermi level(Ti4+,Ga3+and Al3+)adsorbed CO2 strongly yielding CH4 or CO,whereas metal cations with d-band centers further from the Fermi level(Fe3+and Co3+)adsorbed CO2 poorly,thereby yielding H2 only(from water splitting).Our findings clarify the role of trivalent and tetravalent metal cations in LDH photocatalysts for the selective CO2 reduction,paving new ways for the development of improved LDH photocatalyst with high selectivities to specific products.展开更多
We demonstrate experimentally the population inversion between 7S1/2 and 6P3/2 levels of cesium in thermal cesium cell with a 455.5 nm pumping laser.We calculate the relative population probabilities at each level the...We demonstrate experimentally the population inversion between 7S1/2 and 6P3/2 levels of cesium in thermal cesium cell with a 455.5 nm pumping laser.We calculate the relative population probabilities at each level theoretically with the density matrix method.In a steady state,5.8% atoms are at 7S1/2 level and 2.9% at 6P3/2 level,which builds up the population inversion between the two levels.We obtain the fluorescence spectra produced in thermal cesium cell in our experiment.The measured relative intensity of each available fluorescence spectral line in the experiment agrees very well with the theoretical result.The demonstrated population inversion between 7S1/2 and 6P3/2 levels can be used to construct an active optical clock of four-level system with a wavelength of 1469.9 nm.展开更多
文摘The relationship between gain and carrier density is analysed. In the quantum well (QW) lasers, initially, the gain increases rapidly with the carrier density and then starts to saturate. It can be seen that QW lasers have a higher differential gain because of the step-like state density, and that the gain saturates at higher carrier densities because of the constant state density of the lowest subband. It is shown that simple logarithmic gain-carrier density is more accurate than the traditional linearized form for a QW laser.
基金financial support from the National Key Projects for Fundamental Research and Development of China (2016YFB0600901, 2017YFA0206904, 2017YFA0206900, 2018YFB1502002)the National Natural Science Foundation of China (51825205, 51772305, 51572270, U1662118, 21871279, 21802154, 21902168)+10 种基金the Beijing Natural Science Foundation (2191002, 2182078, 2194089)the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17000000)the Royal Society-Newton Advanced Fellowship (NA170422)the International Partnership Program of Chinese Academy of Sciences (GJHZ1819, GJHZ201974)the Beijing Municipal Science and Technology Project (Z181100005118007)the K. C. Wong Education Foundationthe Young Elite Scientist Sponsorship Program by CAST (YESS)the Youth Innovation Promotion Association of the CASthe Energy Education Trust of New Zealandthe Mac Diarmid Institute for Advanced Materials and Nanotechnologythe Dodd Walls Centre for Photonic and Quantum Technologies。
文摘Photocatalytic CO2 reduction holds promise as a future technology for the manufacture of fuels and commodity chemicals.However,factors controlling product selectivity remain poorly understood.Herein,we compared the performance of a homologous series of Zn-based layered double hydroxide(ZnM-LDH)photocatalysts for CO2 reduction.By varying the trivalent or tetravalent metal cations in the ZnM-LDH photocatalysts(M=Ti4+,Fe3+,Co3+,Ga3+,Al3+),the product selectivity of the reaction could be precisely controlled.ZnTi-LDH afforded CH4 as the main reduction product;ZnFe-LDH and ZnCo-LDH yielded H2 exclusively from water splitting;whilst ZnGa-LDH and ZnAl-LDH generated CO.In-situ diffuse reflectance infrared measurements,valence band XPS and density function theory calculations were applied to rationalize the CO2 reduction selectivities of the different ZnM-LDH photocatalysts.The analyses revealed that the d-band center(ed)position of the M3+or M4+cations controlled the adsorption strength of CO2 and thus the selectivity to carbon-containing products or H2.Cations with d-band centers relatively close to the Fermi level(Ti4+,Ga3+and Al3+)adsorbed CO2 strongly yielding CH4 or CO,whereas metal cations with d-band centers further from the Fermi level(Fe3+and Co3+)adsorbed CO2 poorly,thereby yielding H2 only(from water splitting).Our findings clarify the role of trivalent and tetravalent metal cations in LDH photocatalysts for the selective CO2 reduction,paving new ways for the development of improved LDH photocatalyst with high selectivities to specific products.
基金supported by the National Natural Science Foundation of China (Grant Nos. 10874009 and 11074011)
文摘We demonstrate experimentally the population inversion between 7S1/2 and 6P3/2 levels of cesium in thermal cesium cell with a 455.5 nm pumping laser.We calculate the relative population probabilities at each level theoretically with the density matrix method.In a steady state,5.8% atoms are at 7S1/2 level and 2.9% at 6P3/2 level,which builds up the population inversion between the two levels.We obtain the fluorescence spectra produced in thermal cesium cell in our experiment.The measured relative intensity of each available fluorescence spectral line in the experiment agrees very well with the theoretical result.The demonstrated population inversion between 7S1/2 and 6P3/2 levels can be used to construct an active optical clock of four-level system with a wavelength of 1469.9 nm.
