46 research outputs found

### Twisted-light-induced intersubband transitions in quantum wells at normal incidence

We examine theoretically the intersubband transitions induced by laser beams
of light with orbital angular momentum (twisted light) in semiconductor quantum
wells at normal incidence. These transitions become possible in the absence of
gratings thanks to the fact that collimated laser beams present a component of
the light's electric field in the propagation direction. We derive the matrix
elements of the light-matter interaction for a Bessel-type twisted-light beam
represented by its vector potential in the paraxial approximation. Then, we
consider the dynamics of photo-excited electrons making intersubband
transitions between the first and second subbands of a standard semiconductor
quantum well. Finally, we analyze the light-matter matrix elements in order to
evaluate which transitions are more favorable for given orbital angular
momentum of the light beam in the case of small semiconductor structures.Comment: 9 pages, 2 figure

### Insensitivity of spin dynamics to the orbital angular momentum transferred from twisted light to extended semiconductors

We study the spin dynamics of carriers due to the Rashba interaction in
semiconductor quantum disks and wells after excitation with light with orbital
angular momentum. We find that although twisted light transfers orbital angular
momentum to the excited carriers and the Rashba interaction conserves their
total angular momentum, the resulting electronic spin dynamics is essentially
the same for excitation with light with orbital angular momentum $l=+|l|$ and
$l=-|l|$. The differences between cases with different values of $|l|$ are due
to the excitation of states with slightly different energies and not to the
different angular momenta per se, and vanish for samples with large radii where
a $k$-space quasi-continuum limit can be established. These findings apply not
only to the Rashba interaction but also to all other envelope-function
approximation spin-orbit Hamiltonians like the Dresselhaus coupling.Comment: 5 pages, 2 figure

### Photoexcitation of graphene with twisted light

We study theoretically the interaction of twisted light with graphene. The
light-matter interaction matrix elements between the tight-binding states of
electrons in graphene are determined near the Dirac points. We examine the
dynamics of the photoexcitation process by posing the equations of motion of
the density matrix and working up to second order in the field. The time
evolution of the angular momentum of the photoexcited electrons and their
associated photocurrents are examined in order to elucidate the mechanisms of
angular momentum transfer. We find that the transfer of spin and orbital
angular momentum from light to the electrons is more akin here to the case of
intraband than of interband transitions in semiconductors, due to the fact that
the two relevant energy bands of graphene originate from the same atomic
orbitals.Comment: 18 pages, 4 figure

### Case study of the validity of truncation schemes of kinetic equations of motion: few magnetic impurities in a semiconductor quantum ring

We carry out a study on the validity and limitations of truncation schemes
customarily employed to treat the quantum kinetic equations of motion of
complex interacting systems. Our system of choice is a semiconductor quantum
ring with one electron interacting with few magnetic impurities via a
Kondo-like Hamiltonian. This system is an interesting prototype which displays
the necessary complexity when suitably scaled (large number of magnetic
impurities) but can also be solved exactly when few impurities are present. The
complexity in this system comes from the indirect electron-mediated
impurity-impurity interaction and is reflected in the Heisenberg equations of
motion, which form an infinite hierarchy. For the cases of two and three
magnetic impurities, we solve for the quantum dynamics of our system both
exactly and following a truncation scheme developed for diluted magnetic
semiconductors in the bulk. We find an excellent agreement between the two
approaches when physical observables like the impurities' spin angular momentum
are computed for times that well exceed the time window of validity of
perturbation theory. On the other hand, we find that within time ranges of
physical interest, the truncation scheme introduces negative populations which
represents a serious methodological drawback.Comment: 15 pages, 3 figure

### Coherent control of interacting electrons in quantum dots via navigation in the energy spectrum

Quantum control of the wave function of two interacting electrons confined in
quasi-one-dimensional double-well semiconductor structures is demonstrated. The
control strategies are based on the knowledge of the energy spectrum as a
function of an external uniform electric field. When two low-lying levels have
avoided crossings our system behaves dynamically to a large extent as a
two-level system. This characteristic is exploited to implement coherent
control strategies based on slow (adiabatic passage) and rapid (diabatic
Landau-Zener transition) changes of the external field. We apply this method to
reach desired target states that lie far in the spectrum from the initial
state.Comment: Published version. 4 pages, 3 figure