94 research outputs found
Optical properties of nitrogen doped gallium arsenide under pressure
We present new insights into the problem of the isoelectronic nitrogen (N) impurity in gallium arsenide (GaAs). By performing photoluminescence (PL), photoluminescence excitation (PLE), transmission, and time decay measurements through a broad range of pressures, we were able to positively identify and track previously unseen states arising from the N impurity.;The first observation we make is of a dramatic increase in luminescence as N forms an isolated state within the gap around 22kbar. We attribute this effect to N acting as a nonradiative trapping center while still resonant with the conduction band states. We also note the appearance, above 22kbar, of a broad band-to-acceptor like emission below the band edge, which we associate with a band to N acceptor transition.;The next major observation we report is of the excited state of the N x exciton. We find this state to be clearly identifiable for the pressure range ~25kbar to ~30kbar, and is weak in PL, but shows enormous absorption signals in PLE and transmission. The spacing of this level from Nx-B is in good agreement with the established theory of shallow acceptor levels.;We then move on to the identification of a second bound state associated with N. This state, designated NGamma, was previously reported for the GaP system and the GaAs1-xPx alloy, but has not, until now, been positively identified in GaAs. We find the state to be degenerate with the shallow donor level for pressures up to ~30kbar. The emission from this state is found to possess unique N characteristics and is seen to move in pressure with both shallow and deep like properties.;We conclude with a discussion of the process of lasing in N doped GaAs. We present data consistent with previous studies of the lasing process in GaAs. However, we find an intriguing trend with pressure as we approach the Gamma-X crossover. The lasing gain begins to shift in a manner similar to NGamma . In fact, NGamma appears to act as an upper bound on the laser emission. We believe this to be preliminary evidence of the influence of N on the lasing mechanism
Stark effect on the exciton spectra of vertically coupled quantum dots: horizontal field orientation and non-aligned dots
We study the effect of an electric-field on an electron-hole pair in an
asymmetric system of vertically coupled self-assembled quantum dots taking into
account their non-perfect alignment. We show that the non-perfect alignment
does not qualitatively influence the exciton Stark effect for the electric
field applied in the growth direction, but can be detected by application of a
perpendicular electric field. We demonstrate that the direction of the shift
between the axes of non-aligned dots can be detected by rotation of a weak
electric field within the plane of confinement. Already for a nearly perfect
alignment the two-lowest energy bright exciton states possess antilocked
extrema as function of the orientation angle of the horizontal field which
appear when the field is parallel to the direction of the shift between the dot
centers
Forster signatures and qubits in optically driven quantum dot molecules
An interesting approach to achieve quantum gate operations in a solid state
device is to implement an optically driven quantum gate using two vertically
coupled self-assembled quantum dots, a quantum dot molecule (QDM). We present a
realistic model for exciton dynamics in InGaAs/GaAs QDMs under intense laser
excitation and applied electric fields. The dynamics is obtained by solutions
of the Lindblad master equation. A map of the dressed ground state as function
of laser energy and applied electric field exhibits rich structure that
includes excitonic anticrossings that permit the identification of the relevant
couplings. The optical signatures of the dipole-dipole Forster energy transfer
mechanism show as splittings of several (spatially) indirect excitonic lines.
Moreover, we construct a model for exciton qubit rotations by adiabatic
electric field cyclic sweeps into a Forster-tunneling regime which induces
level anticrossings. The proposed qubit exhibits Rabi oscillations among two
well defined exciton pairs as function of the residence time at the
anticrossing.Comment: Paper presented in the International Conference on Electronic
Properties of Two-dimensional Systems and Modulated Semiconductor Structures
Genova Magazzini del Cotone, July 15-20 200
Photoluminescence Spectroscopy of the Molecular Biexciton in Vertically Stacked Quantum Dot Pairs
We present photoluminescence studies of the molecular neutral
biexciton-exciton spectra of individual vertically stacked InAs/GaAs quantum
dot pairs. We tune either the hole or the electron levels of the two dots into
tunneling resonances. The spectra are described well within a few-level,
few-particle molecular model. Their properties can be modified broadly by an
electric field and by structural design, which makes them highly attractive for
controlling nonlinear optical properties.Comment: 4 pages, 5 figures, (v2, revision based on reviewers comments,
published
Electrically tunable g-factors in quantum dot molecular spin states
We present a magneto-photoluminescence study of individual vertically stacked
InAs/GaAs quantum dot pairs separated by thin tunnel barriers. As an applied
electric field tunes the relative energies of the two dots, we observe a strong
resonant increase or decrease in the g-factors of different spin states that
have molecular wavefunctions distributed over both quantum dots. We propose a
phenomenological model for the change in g-factor based on resonant changes in
the amplitude of the wavefunction in the barrier due to the formation of
bonding and antibonding orbitals.Comment: 5 pages, 5 figures, Accepted by Phys. Rev. Lett. New version reflects
response to referee comment
Spin Fine Structure in Optically Excited Quantum Dot Molecules
The interaction between spins in coupled quantum dots is revealed in distinct
fine structure patterns in the measured optical spectra of InAs/GaAs double
quantum dot molecules containing zero, one, or two excess holes. The fine
structure is explained well in terms of a uniquely molecular interplay of spin
exchange interactions, Pauli exclusion and orbital tunneling. This knowledge is
critical for converting quantum dot molecule tunneling into a means of
optically coupling not just orbitals, but spins.Comment: 10 pages, 7 figures, added material, (published
Two level anti-crossings high up in the single-particle energy spectrum of a quantum dot
We study the evolution with magnetic field of the single-particle energy
levels high up in the energy spectrum of one dot as probed by the ground state
of the adjacent dot in a weakly coupled vertical quantum dot molecule. We find
that the observed spectrum is generally well accounted for by the calculated
spectrum for a two-dimensional elliptical parabolic confining potential, except
in several regions where two or more single-particle levels approach each
other. We focus on two two-level crossing regions which show unexpected
anti-crossing behavior and contrasting current dependences. Within a simple
coherent level mixing picture, we can model the current carried through the
coupled states of the probed dot provided the intrinsic variation with magnetic
field of the current through the states (as if they were uncoupled) is
accounted for by an appropriate interpolation scheme.Comment: 4 pages, 4 figures, accepted for publication in Physica E in MSS 13
conference proceeding
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