Light-hole transitions in quantum dots: realizing full control by highly focused optical-vortex beams

An optical-vortex is an inhomogeneous light beam having a phase singularity at its axis, where the intensity of the electric and/or magnetic field may vanish. Already well studied are the paraxial beams, which are known to carry well defined values of spin (polarization $sigma$) and orbital angular momenta; the orbital angular momentum per photon is given by the topological charge $ell$ times the Planck constant. Here we study the light-hole--to--conduction band transitions in a semiconductor quantum dot induced by a highly-focused beam originating from a $ell=1$ paraxial optical vortex. We find that at normal incidence the pulse will produce two distinct types of electron--hole pairs, depending on the relative signs of $sigma$ and $ell$. When sign($sigma$)$=$sign($ell$), the pulse will create electron--hole pairs with band+spin and envelope angular momenta both equal to one. In contrast, for sign($sigma$)$ eq$sign($ell$), the electron-hole pairs will have neither band+spin nor envelope angular momenta. A tightly-focused optical-vortex beam thus makes possible the creation of pairs that cannot be produced with plane waves at normal incidence. With the addition of co-propagating plane waves or switching techniques to change the charge $ell$ both the band+spin and the envelope angular momenta of the pair wave-function can be precisely controlled. We discuss possible applications in the field of spintronics that open up.

Method for finding the critical temperature of the island in a SET structure

We present a method to measure the critical temperature of the island of a superconducting single electron transistor. The method is based on a sharp change in the slope of the zero-bias conductance as a function of temperature. We have used this method to determine the superconducting phase transition temperature of the Nb island of an superconducting single electron transistor with Al leads. We obtain $T_mathrm{c}^mathrm{Nb}$ as high as 8.5 K and gap energies up to $Delta_mathrm{Nb}simeq 1.45$ meV. By looking at the zero bias conductance as a function of magnetic field instead of temperature, also the critical field of the island can be determined. Using the orthodox theory, we have performed extensive numerical simulations of charge transport properties in the SET at temperatures comparable to the gap, which match very well the data, therefore providing a solid theoretical basis for our method.