P. I. Tamborenea

Electronic transitions in disk-shaped quantum dots induced by twisted light

We theoretically investigate the absorption and emission of light carrying orbital angular momentum (twisted-light) by quasi-two-dimensional (disc-shaped) quantum dots in the presence of a static magnetic field. We calculate the transition matrix element for the light-matter interaction and use it to explore different scenarios, depending on the initial and final state of the electron undergoing the optically-induced transition. We make explicit the selection rule for the conservation of the z-projection of the orbital angular momentum. For a realistic set of parameters (quantum dots size, beam waist, photon energy, etc.) the strength of the transition induced by twisted light is 10% of that induced by plane-waves. Finally, our analysis indicates that it may be possible to select precisely the electronic level one wishes to populate using the appropriate combination of light-beam parameters suggesting technological applications to the quantum control of electronic states in quantum dots.

Localization and entanglement of two interacting electrons in a quantum-dot molecule

The localization of two interacting electrons in a coupled-quantum-dots semiconductor structure is demonstrated through numerical calculations of the time evolution of the two-electron wave function including the Coulomb interaction between the electrons. The transition from the ground state to a localized state is induced by an external, time-dependent, uniform electric field. It is found that while an appropriate constant field can localize both electrons in one of the wells, oscillatory fields can induce roughly equal probabilities for both electrons to be localized in either well, generating an interesting type of localized and entangled state. We also show that shifting the field suddenly to an appropriate constant value can maintain in time both types of localization.

Thermal effectiveness of multipass plate exchangers

Abstract Plate heat exchanger arrangements, with a constant number of streams per pass for each fluid, and uniform fluid distribution, are classified in terms of five basic parameters. A program based on the diagonalization of a system of linear differential equations is constructed for the numerical calculation of the thermal effectiveness of an arbitrary arrangement. An alternative program is developed in the limit of a large number of plates. A comparative analysis of the results obtained is performed. Basic trends are discussed, and the early onset of asymptotic behaviour is pointed out.

Orbital and spin dynamics of intraband electrons in quantum rings driven by twisted light

We theoretically investigate the effect that twisted light has on the orbital and spin dynamics of electrons in quantum rings possessing sizable Rashba spin-orbit interaction. The system Hamiltonian for such a strongly inhomogeneous light field exhibits terms which induce both spin-conserving and spin-flip processes. We analyze the dynamics in terms of the perturbation introduced by a weak light field on the Rasha electronic states, and describe the effects that the orbital angular momentum as well as the inhomogeneous character of the beam have on the orbital and the spin dynamics.

Theory of the optical absorption of light carrying orbital angular momentum by semiconductors

We develop a free-carrier theory of the optical absorption of light carrying orbital angular momentum (twisted light) by bulk semiconductors. We obtain the optical transition matrix elements for Bessel-mode twisted light and use them to calculate the wave function of photo-excited electrons to first-order in the vector potential of the laser. The associated net electric currents of first and second-order on the field are obtained. It is shown that the magnetic field produced at the center of the beam for the l=1 mode is of the order of a millitesla, and could therefore be detected experimentally using, for example, the technique of time-resolved Faraday rotation.

Spin-orbit effects in nanowire-based wurtzite semiconductor quantum dots

We study the effect of the Dresselhaus spin-orbit interaction on the electronic states and spin relaxation rates of cylindrical quantum dots defined on quantum wires having wurtzite lattice structure. The linear and cubic contributions of the bulk Dresselhaus spin-orbit coupling (SOC) are taken into account, along with the influence of a weak external magnetic field. The previously found analytic solution for the electronic states of cylindrical quantum dots with zinc blende lattice structures with Rashba interaction is extended to the case of quantum dots with wurtzite lattices. For the electronic states in InAs dots, we determine the spin texture and the effective

Twisted-light-induced optical transitions in semiconductors: Free-carrier quantum kinetics

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Coherent control of multisubband wavepackets with terahertz (Thz) pulses

factor, which shows a scaling collapse when plotted as a function of an effective renormalized dot-size-dependent spin-orbit coupling strength. The acoustic-phonon-induced spin relaxation rate is calculated and the transverse piezoelectric potential is shown to be the dominant one.

Molecular beam epitaxial growth: simulation and continuum theory☆

We theoretically investigate the interband transitions and quantum kinetics induced by light carrying orbital angular momentum, or twisted light, in bulk semiconductors. We pose the problem in terms of the Heisenberg equations of motion of the electron populations, and interband and intraband coherences. Our theory extends the free-carrier semiconductor Bloch equations to the case of photoexcitation by twisted light. The theory is formulated using cylindrical coordinates, which are better suited to describe the interaction with twisted light than the usual Cartesian coordinates used to study regular optical excitation. We solve the equations of motion in the low excitation regime, and obtain analytical expressions for the coherences and populations; with these, we calculate the orbital angular momentum transferred from the light to the electrons and the paramagnetic and diamagnetic electric current densities.

Photoexcitation of graphene with twisted light

We perform numerical calculations to study coherent control of multisubband wavepackets by means of pairs of subpicosecond terahertz laser pulses in suitably designed quantum well structures. We employ a single-particle, effective-mass model of the semiconductor structures. Our purpose is to explore the applicability of the ideas of coherent control and wavepacket interferometry to a new physical system (multisubband wavepackets in doped semiconductor quantum wells) and in a new frequency range (terahertz radiation). Similar ideas have been successfully demonstrated in recent years in atomic, molecular, and excitonic systems, in the optical or near-infrared spectrum. We analyze in detail four quantum well structures, with emphasis on the wavepacket dynamics and interference, and demonstrate numerically the possibility of measurable coherent control of the population of the excited electrons. These wavepacket-interference effects could be used to study decoherence times in doped semiconductor structures. A...