Abstract: Strong terahertz (1THz=1012Hz) radiation can be generated by the electron oscillation in fs-laser-induced wake fields. The interaction of a fs-laser pulse with a low-density plasma layer is studied in detail using numerical simulations. The spatial distribution and temporal evolution of terahertz electron current developed in a low-density plasma layer are presented, which enables us to calculate the intensity distribution of THz radiation. It is shown that laser and plasma parameters, such as laser intensity, pulse width, and background plasma density, are of key importance to the process. The optimum condition for wake-field excitation and terahertz emission is discussed upon the simulation results. Radiation peaked at 6.4 THz, with 900 fs duration and 9% bandwidth, can be generated in a plasma of density 5×1017cm−3. It turns out that the maximum radiation intensity scales as n03a04 when wake field is resonantly excited, where n0 and a0 are, respectively, the plasma density and the normalized field amplitude of the laser pulse.

Abstract: Quantum mechanical coupling and strain in two vertically arranged InP/InGaP quantum dots is studied as a function of the size of the dots and the spacer thickness. The strain distribution is determined by the continuum mechanical model, while the single-band effective-mass equation and the multiband k (.) p theory are employed to compute the conduction and valence band energy levels, respectively. The exciton states are obtained from an exact diagonalization approach, and we also compute the oscillator strength for recombination. We found that the light holes are confined by strain to the spacer, which is the reason that the hole states exhibit coupling at much larger distances as compared with the electrons. At small d, the doublet structure of the hole energy levels arises as a consequence of the relocation of the light hole from the matrix to the regions located-outside the stack, close to the dot-matrix interface. When d varies, the exciton ground state exhibits numerous anticrossings with other states, which are related to the changing spatial localization of the hole as a function of d. The oscillator strength of the exciton recombination is strongly reduced in a certain range of spacer thicknesses, which effectively turns a bright exciton state into a dark one. This effect is associated with anticrossings between exciton energy levels.

Abstract: Ballistic electron transport through a finite chain of quantum circular rings is studied in the presence of the Rashba coupling, of strength a, and of a perpendicular magnetic field B. The transmission and reflection coefficients for a single ring, obtained analytically, help obtain the conductance through a chain of rings as a function of the strength a, the field B, and of the wave vector k of the incident electron. Due to destructive spin interferences caused by the Rashba coupling the chain can be totally opaque for certain ranges of k the width of which depends on values of a and B. Outside these ranges the conductance oscillates with high values between e(2)/h and 2e(2)/h. The effect of a periodic modulation of a or B on the conductance gaps is investigated. A periodic, square-wave conductance pattern, pertinent to the development of the spin transistor, results within wide stripes in the parameter space spanned by k, a, and B. Finite temperatures smoothen the square-wave profile of the conductance but do not alter its periodic character.