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

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