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.