Superlattic-like phase-change films are considered a promising phase-change material because it provides more controllable degree of freedoms for the simultaneous optimization of multiple parameters of phase-change films. However, the mechanism on the effect of superlattice-like structure on parameters of phase-change films is still controversial. At present there are two opinions: interfacial effect and the reduction of thermal conductivity. Here four superlattice-like phase-change films, [Ge8Sb92(15 nm)/Ge (x nm)]3, are fabricated. Their behaviors of crystallization are investigated using the measurements of sheet resistance and coherent phonon spectroscopy. Two measurements show the crystallization temperature of the four superlattice-like films increases with the thickness of Ge layers. However, this increase cannot be explained by both the interfacial effect and the reduction of thermal conductivity. It is proposed that true superlattice effect should be considered to explain the effect of superlattice-like structure. Electron diffusion between two different constituent layers should be considered, as done in semiconductor superlattice structures. Electron diffusion can lead to the establishment of built-in electric field inside the superlattice-like films, which causes the change of band structures of two constituent materials and long-range coupling of superlattice-like films, further change of physical parameters. Based on this long-range coupling, the effect of cycle number in superlattice-like films on crystallization temperature can be explained. Some primary evidence on electric field effect on crystallization temperature of phase-change films is provided.
The crystallization properties of as-deposited amorphous AgInSbTe thin films irradiated by single-shot picosecond laser pulses were studied using in-situ transient optical reflectance and electrical resistance measurements with nanosecond resolution. It was found that the real-time optical and electrical signal responses were different under the same pumping conditions. The optical signals showed a multistage crystallization process with a total time of approximately 150 ns, while the electrical signals showed a negative exponential trend decreasing to the final stable state within about several microseconds. A resistor–capacitor model was constructed to explain this delayed electrical response. The fluencedependent evolution dynamics maybe implied a non-fully crystallization process under ultra-short pulse stimulation.
Ultrafast dynamics of spin waves and coercivity of a FePt/FeNi exchange coupling film is studied by femtosecond laser Kerr spectroscopy. Magnetization-precession spin waves are observed. Time-delayed magneto-optical Kerr hysteresis loop (TDMOKHL) technique is used to study the dynamics of coercivity of the magnetic film under laser excitations as done usually. However, we find that the dynamics of coercivity revealed by TDMOKHL technique does not reflect the evolution of initial coercivity given by the initial hysteresis loop with no pump excitation, but does that of final coercivity under laser excitation. The final coercivity is given by the hysteresis loop measured at a negative pump-probe delay time under pump excitation. The evolution of initial coercivity can be observed only under a weak pump excitation in which magnetic system behaves reversible magnetically. In general, TDMOKHL technique can not track the evolution of coercivity of a magnetic system because TDMOKHLs do not reflect the time evolution of initial hysteresis loop with no laser excitation. The correctness of all previous reports on the dynamics of coercivity and magnetic-order transition of a laser-excited magnetic system needs reexamining if those reports were dependent on TDMOKHL technique. The oscillatory recovery behavior of final coercivity is observed under pump excitation and ascribed to metastability of zero magnetization state.
A general recording model of in-line far-field holography is first proposed, which includes three illumination modes: divergent, convergent and collimated beam illuminations. The general irradiance distribution on a hologram is then derived from the general recording model. Based on the general irradiance distribution of in-line far-field holography, the analytical solutions of recordable depth of view (RDV) and allowable farthest far-field distance (AFFD) of in-line far-field holography have been given for different illumination modes. The analytical solutions of RDV and AFFD show that AFFD can be not limited if micro- objects are positioned in one special space and illuminated by convergent beams, but RDV isn't improved. When micro- objects are placed in another special space and illuminated by divergent beam, the RDV and AFFD can be improved simultaneously, but recording object space is split into two sub-spaces. These results are very important for the design of holographic recording system.
A general recording model of off-axis holography is first proposed, which includes three illumination modes: divergent,
convergent and collimated beam illuminations. The interference pattern is then derived based on the general
recording model. The general solutions on the recordable depth of view (RDV) and allowable farthest far-field distance
(AFFD) are given analytically, which show that the RDV and AFFD may be improved by using non-collimated beam
illumination and placing particle fields into a special spatial zone.
The effects of bandfilling and bandgap shrinkage in the femtosecond absorption saturation measurements of GaAs have been studied using femtosecond pulses generated from CPM dye laser and self-mode-locked Ti:sapphire laser. For exciting photon energy of 2 eV and carriers density of 1 by 1018 cm-3, an optical induced absorption increase is observed and is attributed to the bandgap shrinkage. The dependence of the change of absorption coefficient on photon energy, temperature and excited carrier densities is discussed.
The some of problems related to automatic analysis of strongly noisy particle images are discussed in this paper. The new double thresholding and correlating focussing recognition methods are developed and used to automatically analyze a particle image reconstructed from the hologram of solid rocket propellant combustion. The tested results are given and show that the new methods are effective in automatic analysis of strongly noisy particle images.
A double-exposure phase-shifting holography applied to particle velocimetry is proposed and analyzed theoretically in this paper. The background noise of the reconstructed particle image can be eliminated by means of the proposed holography so that the SNR of the image is increased significantly, which makes it easy for a computer to automatically analyze the reconstructed particle image. The experimental results of a simulated velocity field are given.
The influence of illuminating mode on the recordable depth of field in in-line holography of particle fields is discussed theoretically. The results show that the recordable depth of field can be increased using the divergent illumination.
An offaxis farfield holography with a filter is studied. The intensity distribution in the reconstructed i*age plane is calculated. The influences of the radius and absorptive index of the filter on the form and contrast of the reconstructed image are discussed.
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