Polaron induced double electron in a quantum dot is investigated using the exact diagonalization techniques and the compact density-matrix approach. The dependence of nonlinear optical processes on the incident photon energies and the polaronic effect are brought out. The linear, third order non-linear optical absorption coefficients and the refractive index changes of singlet and triplet states as a function of photon energy are obtained with and without the inclusion of polaronic effect. It is found that the geometrical confinement and the effect of polaron have great influence on the optical properties of dots.
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.
WANG YanFeiWANG DongYingZHANG TongGangHONG YeLongZHANG ShengNanTAO ZhiMingXIE XiaoPengCHEN JingBiao
When femtosecond （fs） timeresolved experiments are used to study ultrafast processes, quantum beat phenomena are often observed. In this paper, to analyze the fs timeresolved spectra, we will present the density matrix method, a powerful theoretical technique, which describes the dynamics of population and coherence of the system. How to employ it to study the pumpprobe experiments and fs ultrafast processes is described. The transition of pyrazine is used as an example to demonstrate the application of the density matrix method. Recently, Suzuki＇s group have employed the 22 fs time resolution laser to study the dynamics of the state of pyrazine. In this case, conical intersection is commonly believed to play an important role in this nonadiabatic process. How to treat the effect of conical intersection on nonadiabatic processes and fs timeresolved spectra is presented. Another important ultrafast process, vibrational relaxation, which takes place in subps and ps range and has never been carefully studied, is treated in this paper. The vibrational relaxation in water dimer is chosen to demonstrate the calculation. It should be noted that the vibrational relaxation of （H20）2 has not been experimentally studied but it can be accomplished by the pump-probe experiments.
