83 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
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
Coherent optical control of spin-spin interaction in doped semiconductors
We provide a theory of laser-induced interaction between spins localized by
impurity centers in a semiconductor host. By solving exactly the problem of two
localized spins interacting with one itinerant exciton, an analytical
expression for the induced spin-spin interaction is given as a function of the
spin separation, laser energy, and intensity. We apply the theory to shallow
neutral donors (Si) and deep rare-earth magnetic impurities (Yb) in III-V
semiconductors. When the photon energy approaches a resonance related to
excitons bound to the impurities, the coupling between the localized spins
increases, and may change from ferromagnetic to anti-ferromagnetic. This
light-controlled spin interaction provides a mechanism for the quantum control
of spins in semiconductors for quantum information processing; it suggests the
realization of spin systems whose magnetic properties can be controlled by
changing the strength and the sign of the spin-spin interaction.Comment: 10 pages, 5 figure
Efficient spin control in high-quality-factor planar micro-cavities
A semiconductor microcavity embedding donor impurities and excited by a laser
field is modelled. By including general decay and dephasing processes, and in
particular cavity photon leakage, detailed simulations show that control over
the spin dynamics is significally enhanced in high-quality-factor cavities, in
which case picosecond laser pulses may produce spin-flip with high-fidelity
final states.Comment: 6 pages, 4 figure
Donor-donor interaction mediated by cavity-photons and its relation to interactions mediated by excitons and polaritons
I report theoretical predictions of two models of donor-donor indirect
interaction mediated by photons in zero- and two-dimensional cavities. These
results are compared to previously studied cases of indirect interactions
mediated by excitons and/or polaritons in bulk semiconductor and
two-dimensional cavities. I find that photons mediate an Ising-like interaction
between donors in the same manner polaritons do, in contrast to the
Heisenberg-like interaction mediated by exciton. For the particular case of a
two-dimensional cavity, the model shows that the dependence on distance of the
donor-donor coupling constant is the same for photons and polaritons when the
donor-donor distance is large. Then, it becomes clear that photons are
responsible for the long range behavior of the polariton indirect interaction
Long-range spin-qubit interaction mediated by microcavity polaritons
We study the optically-induced coupling between spins mediated by polaritons
in a planar micro-cavity. In the strong coupling regime, the vacuum Rabi
splitting introduces anisotropies in the spin coupling. Moreover, due to their
photon-like mass, polaritons provide an extremely long spin coupling range.
This suggests the realization of two-qubit all-optical quantum operations
within tens of picoseconds with spins localized as far as hundreds of
nanometers apart.Comment: 5 pages, 3 figure
Twisted-light-induced optical transitions in semiconductors: Free-carrier quantum kinetics
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 inter- and intra-band coherences. Our
theory extends the free-carrier Semiconductor Bloch Equations to the case of
photo-excitation 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.Comment: 11 pages, 3 figure
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