The dynamic population processes of infrared radiation in dysprosium-doped different host materials (LaF<sub>3</sub>, Y<sub>2</sub>O<sub>3</sub>, YAlO<sub>3</sub> and silicate glass) are theoretically investigated. The radiation and non-radiation transition rates of each energy level are calculated using Judd-Ofelt (J-O) theory and according to “energy-gap law”. It is demonstrated that the non-radiative transition rate increases significantly as the phonon energy increases, indicating that the choice of host materials has a great influence on the infrared transition processes. By solving the rate equations we establish, it is found that the population profiles of the same energy levels are almost the same, but the time to reach equilibrium population varies greatly among different materials. The population probability of <sup>6</sup>H<sub>9/2</sub> and <sup>6</sup>H<sub>11/2</sub> energy levels increases first and then decreases, whereas that of 6H13/2 and <sup>6</sup>H<sub>15/2</sub> monotonically increases or decreases with time. The excited state <sup>6</sup>H<sub>13/2</sub> has a quite long decay lifetime of 38.97 ms in dysprosium-doped LaF<sub>3</sub>, which is a good metastable state for mid-infrared emission.. These results are helpful to the material selection and application of infrared lasers.
Impact ionization in charge layer and multiplication layer of InAlAs/InGaAs avalanche photodiodes (APDs) with separated absorption, grading, charge and multiplication structures has been studied by two-dimensional simulations using Silvaco TCAD. Special attention has been paid to the charge layer and multiplication layer with different thicknesses and doping concentrations in order to optimize the structure for low band discontinuities and an appropriate electric field distribution. Band-edge profile calculations as well as current–voltage characteristic and electric field results of the APDs will be discussed in this article.
Transmission properties of transverse magnetic light through periodic sub-wavelength slit apertures on a metallic film, behind which is another planar metallic film, are studied by finite-difference-time-domain method with constant periodicity and slit width. The result shows that the transmitted energy is strongly correlated to both the thickness of the metallic grating and the distance between such two films at a specific wavelength. The thickness of the grating acts as a filter that allows specific wavelengths to go through the slits, while the distance of dual metallic film dominantly determines a constructive or destructive interference between the transmitted light through the slits and the reflected wave from the back film. Besides, a strong vibration in the transmission spectrum as a function of the grating thickness is interestingly observed, which can be interpreted by the resonance of the surface plasmons of the front and the back metallic films.