Nondegenerate Fermions in the Background of the Sphaleron Barrier

We consider level crossing in the background of the sphaleron barrier for nondegenerate fermions. The mass splitting within the fermion doublets allows only for an axially symmetric ansatz for the fermion fields. In the background of the sphaleron we solve the partial differential equations for the fermion functions. We find little angular dependence for our choice of ansatz. We therefore propose a good approximate ansatz with radial functions only. We generalize this approximate ansatz with radial functions only to fermions in the background of the sphaleron barrier and argue, that it is a good approximation there, too.Comment: LATEX, 20 pages, 11 figure

Point-by-point inscription of apodized fiber Bragg gratings

We demonstrate apodized fiber Bragg gratings inscribed with a point-by-point technique. We tailor the grating phase and coupling amplitude through precise control over the longitudinal and transverse position of each laser-inscribed modification. This method of apodization is facilitated by the highly-localized, high-contrast modifications generated by focussed IR femtosecond laser inscription. Our technique provides a simple method for the design and implementation of point-by-point fiber Bragg gratings with complex apodization profiles.Comment: 6 pages, 4 figures, article in revie

Eight-band calculations of strained InAs/GaAs quantum dots compared with one, four, and six-band approximations

The electronic structure of pyramidal shaped InAs/GaAs quantum dots is calculated using an eight-band strain dependent $\bf k\cdot p$ Hamiltonian. The influence of strain on band energies and the conduction-band effective mass are examined. Single particle bound-state energies and exciton binding energies are computed as functions of island size. The eight-band results are compared with those for one, four and six bands, and with results from a one-band approximation in which m(r) is determined by the local value of the strain. The eight-band model predicts a lower ground state energy and a larger number of excited states than the other approximations.Comment: 8 pages, 7 figures, revtex, eps