20 research outputs found
Fine structure of the exciton absorption in semiconductor superlattices in crossed electric and magnetic fields
The exciton absorption coefficient is determined analytically for a
semiconductor superlattice in crossed electric and magnetic fields, for the
magnetic field being parallel and the electric field being perpendicular to the
superlattice axis. Our investigation applies to the case where the magnetic
length, while being much smaller than the exiton Bohr radius, considerably
exceeds the superlattice period. The optical absorption in superlattices
displays a spectral fine structure related to the sequences of exciton states
bound whose energies are adjacent to the Landau energies of the charge carriers
in the magnetic field. We study effects of external fields and of the
centre-of-mass exciton motion on the fine structure peak positions and
oscillator strengths. In particular, we find that the inversion of the
orientation of the external fields and of the in-plane total exciton momentum
notably affects the absorption spectrum. Conditions for the experimental
observation of the exciton absorption are discussed
Impurity center in a semiconductor quantum ring in the presence of a radial electric field
The problem of an impurity electron in a quantum ring (QR) in the presence of
a radially directed strong external electric field is investigated in detail.
Both an analytical and a numerical approach to the problem are developed. The
analytical investigation focuses on the regime of a strong wire-electric field
compared to the electric field due to the impurity. An adiabatic and
quasiclassical approximation is employed. The explicit dependencies of the
binding energy of the impurity electron on the electric field strength,
parameters of the QR and position of the impurity within the QR are obtained.
Numerical calculations of the binding energy based on a finite-difference
method in two and three dimensions are performed for arbitrary strengths of the
electric field. It is shown that the binding energy of the impurity electron
exhibits a maximum as a function of the radial position of the impurity that
can be shifted arbitrarily by applying a corresponding wire-electric field. The
maximal binding energy monotonically increases with increasing electric field
strength. The inversion effect of the electric field is found to occur. An
increase of the longitudinal displacement of the impurity typically leads to a
decrease of the binding energy. Results for both low- and high-quantum rings
are derived and discussed. Suggestions for an experimentally accessible set-up
associated with the GaAs/GaAlAs QR are provided.Comment: 16 pages, 8 figure
Role of dynamical screening in excitation kinetics of biased quantum wells: Nonlinear absorption and ultrabroadband terahertz emission
Turchinovich D, Monozon BS, Jepsen PU. Role of dynamical screening in excitation kinetics of biased quantum wells: Nonlinear absorption and ultrabroadband terahertz emission. Journal of Applied Physics. 2006;99(1).In this work we describe the ultrafast excitation kinetics of a biased quantum well, arising from the optically induced dynamical screening of a bias electric field. The initial bias electric field inside the quantum well is screened by the optically excited polarized electron-hole pairs. This leads to a dynamical modification of the properties of the system within an excitation pulse duration. We calculate the excitation kinetics of a biased quantum well and the dependency of resulting electronic and optical properties on the excitation pulse fluence, quantum well width, and initial bias field strength. Our calculations, in particular, predict the strongly nonlinear dependency of the effective optical absorption coefficient on the excitation pulse fluence, and ultrabroadband terahertz emission. Our theoretical model is free of fitting parameters. Calculations performed for internally biased InGaN∕GaN quantum wells are in good agreement with our experimental observations [Turchinovich et al., Phys. Rev. B 68, 241307(R) (2003)], as well as in perfect compliance with qualitative considerations
Bound and resonant impurity states in a narrow gaped armchair graphene nanoribbon
An analytical study of discrete and resonant impurity quasi-Coulomb states in
a narrow gaped armchair graphene nanoribbon (GNR) is performed. We employ the
adiabatic approximation assuming that the motions parallel ("slow") and
perpendicular ("fast") to the boundaries of the ribbon are separated
adiabatically. The energy spectrum comprises a sequence of series of
quasi-Rydberg levels relevant to the "slow" motion adjacent from the low
energies to the size-quantized levels associated with the "fast" motion. Only
the series attributed to the ground size-quantized sub-band is really discrete,
while others corresponding to the excited sub-bands consist of quasi-discrete
(Fano resonant) levels of non-zero energetic widths, caused by the coupling
with the states of the continuous spectrum branching from the low lying
sub-bands. In the two- and three-subband approximation the spectrum of the
complex energies of the impurity electron is derived in an explicit form.
Narrowing the GNR leads to an increase of the binding energy and the resonant
width both induced by the finite width of the ribbon. Displacing the impurity
centre from the mid-point of the GNR causes the binding energy to decrease
while the resonant width of the first excited Rydberg series increases. As for
the second excited series their widths become narrower with the shift of the
impurity. A successful comparison of our analytical results with those obtained
by other theoretical and experimental methods is presented. Estimates of the
binding energies and the resonant widths taken for the parameters of typical
GNRs show that not only the strictly discrete but also the some resonant states
are quite stable and could be studied experimentally in doped GNRs
Persistent Currents in Small, Imperfect Hubbard Rings
We have done a study with small, imperfect Hubbard rings with exact
diagonalization. The results for few-electron rings show, that the
imperfection, whether localized or not, nearly always decrease, but can also
\emph{increase} the persistent current, depending on the character of the
imperfection and the on-site interaction. The calculations are generally in
agreement with more specialized studies. In most cases the electron spin plays
an important role.Comment: 6 pages, 4 figure
Multi-photon absorption spectra involving the magneto-optical and Wannier - Stark effects in superlattices
Terahertz electro-absorption effect enabling femtosecond all-optical switching in semiconductor quantum dots
We demonstrate an instantaneous all-optical manipulation of optical absorption in InGaAs/GaAs quantum dots(QDs) via an electro-absorption effect induced by the electric field of an incident free-space terahertz signal. A terahertz signal with the full bandwidth of 3 THz was directly encoded onto an optical signal probing the absorption in QDs, resulting in the encoded temporal features as fast as 460 fs. The instantaneous nature of this effect enables femtosecond all-optical switching at very high repetition rates, suggesting applications in terahertz-range wireless communication systems with data rates of at least 0.5 Tbit/s