156 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
Non-perturbative Correlation Effects in Diluted Magnetic Semiconductors
The effects of carrier-impurity correlations due to a Kondo-like spin-spin
interaction in diluted magnetic semiconductors are investigated. These
correlations are not only responsible for a transfer of spins between the
carriers and the impurities, but also produce non-perturbative effects in the
spin dynamics such as renormalization of the precession frequency of the
carrier spins, which can reach values of several percent in CdMnTe quantum
wells. In two-dimensional systems, the precession frequency renormalization for
a single electron spin with defined wave vector shows logarithmic divergences
similar to those also known from the Kondo problem in metals. For smooth
electron distributions, however, the divergences disappear due to the
integrability of the logarithm. A possible dephasing mechanism caused by the
wave-vector dependence of the electron spin precession frequencies is found to
be of minor importance compared to the spin transfer from the carrier to the
impurity system. In the Markov limit of the theory, a quasi-equilibrium
expression for the carrier-impurity correlation energy can be deduced
indicating the formation of strongly correlated carrier-impurity states for
temperatures in the mK range
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 and
. The differences between cases with different values of 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 -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
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