70 research outputs found
Analysis of optical properties of strained semiconductor quantum dots for electromagnetically induced transparency
Using multiband k*p theory we study the size and geometry dependence on the
slow light properties of conical semiconductor quantum dots. We find the V-type
scheme for electromagnetically induced transparency (EIT) to be most favorable,
and identify an optimal height and size for efficient EIT operation. In case of
the ladder scheme, the existence of additional dipole allowed intraband
transitions along with an almost equidistant energy level spacing adds
additional decay pathways, which significantly impairs the EIT effect. We
further study the influence of strain and band mixing comparing four different
k*p band structure models. In addition to the separation of the heavy and light
holes due to the biaxial strain component, we observe a general reduction in
the transition strengths due to energy crossings in the valence bands caused by
strain and band mixing effects. We furthermore find a non-trivial quantum dot
size dependence of the dipole moments directly related to the biaxial strain
component. Due to the separation of the heavy and light holes the optical
transition strengths between the lower conduction and upper most valence-band
states computed using one-band model and eight-band model show general
qualitative agreement, with exceptions relevant for EIT operation.Comment: 9 pages, 12 figure
Comparison of EIT schemes in semiconductor quantum dot structures: Impact of many-body interactions
We investigate the impact of many-body interactions on group-velocity
slowdown achieved via electromagnetically induced transparency in quantum dots
using three different coupling-probe schemes (ladder, V, and Lambda,
respectively). We find that for all schemes many-body interactions have an
important impact on the slow light properties. In the case of the Lambda and V
schemes, the minimum required coupling power to achieve slow light is
significantly reduced by many-body interactions. V type schemes are found to be
generally preferable due to a favorable redistribution of carriers in energy
space
Mid-Devensian climate and landscape in England : new data from Finningley, South Yorkshire
While there is extensive evidence for the Late Devensian, less is known about Early and Middle Devensian (approx. 110–30 ka) climates and environments in the UK. The Greenland ice-core record suggests the UK should have endured multiple changes, but the terrestrial palaeo-record lacks sufficient detail for confirmation from sites in the British Isles. Data from deposits at Finningley, South Yorkshire, can help redress this. A channel with organic silts, dated 40 314–39 552 cal a BP, contained plant macrofossil and insect remains showing tundra with dwarf-shrub heath and bare ground. Soil moisture conditions varied from free draining to riparian, with ponds and wetter vegetated areas. The climate was probably low arctic with snow cover during the winter. Mutual climatic range (MCR), based on Coleoptera, shows the mean monthly winter temperatures of −22 to −2°C and summer ones of 8–14°C. Periglacial structures within the basal gravel deposits and beyond the glacial limits indicate cold-climate conditions, including permafrost. A compilation of MCR reconstructions for other Middle Devensian English sites shows that marine isotope stage 3—between 59 and 28 ka—experienced substantial variation in climate consistent with the Greenland ice-core record. The exact correlation is hampered by temporal resolution, but the Finningley site stadial at approximately 40 ka may correlate with the one of the Greenland stadials 7–11
- …