3,243 research outputs found
Replica-molded electro-optic polymer Mach–Zehnder modulator
A Mach-Zehnder electro-optic polymer amplitude modulator is fabricated by a simple and high-throughput soft-stamp replica-molding technique. The modulator structure incorporates the highly nonlinear and stable chromophore, AJL8, doped in amorphous polycarbonate. Single-arm phase-retardation results in a halfwave voltage (V-pi) of 8.4 V at 1600 nm. The on/off extinction ratio is better than 19 dB, resulting from precise Y-branch power splitters and good waveguide uniformity. These results indicate that the simple fabrication process allows for good optical performance from high-fidelity replicas of the original master devices
Large-Amplitude, Pair-Creating Oscillations in Pulsar and Black Hole Magnetospheres
A time-dependent model for pair creation in a pulsar magnetosphere is
developed. It is argued that the parallel electric field that develops in a
charge-starved region (a gap) of a pulsar magnetosphere oscillates with large
amplitude. Electrons and positrons are accelerated periodically and the
amplitude of the oscillations is assumed large enough to cause creation of
upgoing and downgoing pairs at different phases of the oscillation. With a
charge-starved initial condition, we find that the oscillations result in
bursts of pair creation in which the pair density rises exponentially with
time. The pair density saturates at , where is the parallel electric field in the
charge-starved initial state, and is the Lorentz factor for
effec tive pair creation. The frequency of oscillations following the pair
creation burst is given roughly by . A positive feedback keeps the system stable, such that the average pair
creation rate balances the loss rate due to pairs escaping the magnetosphere.Comment: 21 pages, 6 figures, ApJ submitte
Statistical Properties of Exciton Fine Structure Splittings and Polarization Angles in Quantum Dot Ensembles
We propose an effective model to describe the statistical properties of
exciton fine structure splitting (FSS) and polarization angle of quantum dot
ensembles (QDEs). We derive the distributions of FSS and polarization angle for
QDEs and show that their statistical features can be fully characterized using
at most three independent measurable parameters. The effective model is
confirmed using atomistic pseudopotential calculations as well as experimental
measurements for several rather different QDEs. The model naturally addresses
three fundamental questions that are frequently encountered in theories and
experiments: (I) Why the probability of finding QDs with vanishing FSS is
generally very small? (II) Why FSS and polarization angle differ dramatically
from QD to QD? and (III) Is there any direct connection between FSS, optical
polarization and the morphology of QDs? The answers to these fundamental
questions yield a completely new physical picture for understanding optical
properties of QDEs.Comment: 6 pages, 3 figures, 1 tabl
THE IMPACT OF TEAM RANKING ON TEAM LENDING PERFORMANCE: AN EMPIRICAL STUDY ON KIVA
Prosocial crowdfunding, such as Kiva, puzzles researchers regarding what motivates online peers to lend for free, and how voluntary online participation could be organized to create great social goods. A common practice of prosocial lending websites is to enable self-organizing teams. In this paper, we are interested in the impact of team ranking, and thus team reputation on its lending performance. Contradicting predictions could be derived depending on the theoretical lenses. While social identity theory suggests that better ranking strengthens individual identification and promotes lending participation; economic theory on public goods indicates that good ranking may trigger a crowd-out effect. To empirically explore the relationship between team ranking and team performance, we collected data from Kiva, the largest prosocial crowdfunding platform. Kiva enables lenders to form teams, and teams are ranked monthly on both lending performance and member recruitment. Our data analysis suggests that appearance on the top ranking list leads to a reduction in future team lending indicating that good team-rank triggers the crowd-out effect. Meanwhile, salience on the member recruitment list does not show any significant impact on lending performance. Our finding suggests that team reputation may not promote identification in this context
Supercoupling between heavy-hole and light-hole states in self-assembled quantum dots
Spintronics, quantum computing and quantum communication science utilizing
cubic semiconductors rely largely on the properties of the hole states,
composed of light and heavy hole wavefunction components. The admixture of
light-hole (LH) into ground hole state predominately by the heavy hole (HH)
would induce unique features of LH in optical transitions, spin relaxation, and
spin polarization. We point to an unexpected source of HH-LH mixing in quantum
dots, arguing that in contrast with current models the mixing does not reflect
the strain between the dot and its matrix and does not scale inversely with the
energy splitting between the bulk HH and LH states. Instead, we show via
atomistic pseudopotential calculations on a range of strained and unstrained
dots of different symmetries that the HH-LH mixing is enabled by the presence
in the QD of a dense ladder of intermediate states between the HH and LH states
which amplifies and propagates this interaction and leads to "supercoupling"
(analogous to super-exchange in magnetism). This explains a number of
outstanding puzzles regarding the surprising large coupling seen in unstrained
QD (GaAs/AlAs) of ideal shapes and the surprising fact that in strained QD
(InAs/GaAs) the coupling is very strong despite the fact that the 12-fold
increase in bulk HH-LH splitting overrides the ~4 fold enhancement of the
coupling matrix element by strain in comparison with unstrained GaAs QDs.Comment: 20 pages, 4 figure
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