We have studied terahertz ͑THz͒ emission from InAs and GaAs in a magnetic field, and find that the emitted radiation is produced by coupled cyclotron-plasma charge oscillations. Ultrashort pulses of THz radiation were produced at semiconductor surfaces by photoexcitation with a femtosecond Ti-sapphire laser. We recorded the integrated THz power and the THz emission spectrum as a function of magnetic field at fields up to 5.5 T, and as function of temperature for Tϭ10-280 K. The maximum observed THz power is ϳ1.6ϫ10 Ϫ13 J/pulse ͑12 W average power͒ from n-InAs (1.8ϫ10 16 cm Ϫ3 ͒ at Bϭ3.2 T. We compare our results to semiclassical models of magnetoplasma oscillations of bulk free carriers and damped motion of free carriers in a twodimensional electron gas. The bulk model describes THz emission from n-GaAs at all magnetic fields, and InAs at Bϭ0. It fails to describe THz emission from InAs at nonzero magnetic fields. We show that a model including both bulk plasma oscillations and THz emission from a surface accumulation layer describes THz emission from InAs in a moderate magnetic field, but this model does not completely describe emission at fields ͉B͉Ͼ1.0 T.
Bioregenerative life-support systems (BLSS) involving plants will be required to realize self-sustaining human settlements beyond Earth. To improve plant productivity in BLSS, the quality of the solar spectrum can be modified by lightweight, luminescent films. CuInS2/ZnS quantum dot (QD) films were used to down-convert ultraviolet/blue photons to red emissions centered at 600 and 660 nm, resulting in increased biomass accumulation in red romaine lettuce. All plant growth parameters, except for spectral quality, were uniform across three production environments. Lettuce grown under the 600 and 660 nm-emitting QD films respectively increased edible dry mass (13 and 9%), edible fresh mass (11% each), and total leaf area (8 and 13%) compared with under a control film containing no QDs. Spectral modifications by the luminescent QD films improved photosynthetic efficiency in lettuce and could enhance productivity in greenhouses on Earth, or in space where, further conversion is expected from greater availability of ultraviolet photons.
While luminescent concentrators (LCs) are mainly designed to harvest sunlight and convert its energy into electricity, the same concept can be advantageous in alternative applications. Examples of such applications are demonstrated here by coupling the edge-guided light of high-performance LCs based on CuInSe x S2 - x /ZnS quantum dots into optical fibers with emission covering visible-to-NIR spectral regions. In particular, a cost-efficient, miniature broadband light source for medical diagnostics, a spectral-conversion and light-guiding device for agriculture, and a large-area broadband tunable detector for telecommunications are demonstrated. Various design considerations and performance optimization approaches are discussed and summarized. Prototypes of the devices are manufactured and tested. Individual elements of the broadband light source show coupling efficiencies up to 1%, which is sufficient to saturate typical fiber-coupled spectrometers at a minimal integration time of 1 ms using 100 mW blue excitation. Agricultural devices are capable of delivering ∼10% of photosynthetically active radiation (per device) converted from absorbed sunlight to the lower canopy of plants, which boosted the tomato yield in a commercial greenhouse by 7% (fresh weight). Finally, large-scale prototype detectors can be used to discern time-modulated unfocused signals with an average power as low as 1 μW, which would be useful for free-space telecommunication systems. Fully optimized devices are expected to make significant impacts on speed and bandwidth of free-space telecommunication systems, medical diagnostics, and greenhouse crop yields.
Cadmium germanium diarsenide glasses were synthesized in bulk form (B2.4 cm 3 ) using procedures adapted from the literature. Several issues involved in the fabrication and quenching of amorphous CdGe x As 2 (x 5 0.45, 0.65, 0.85, and 1.00, where x is the molar ratio of Ge to 1 mol of Cd) are described. An innovative processing route is presented to enable fabrication of high-purity, vitreous, crack-free ingots with sizes up to 10 mm diameter, and 30-40 mm long. Specimens from selected ingots were characterized using thermal analysis, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, particle-induced X-ray emission, Rutherford backscattering, secondary ion mass spectrometry, X-ray diffraction, density, and optical spectroscopy. Variations in properties as a function of processing conditions and composition are described. Results show that the density of defect states in the middle of the band gap and near the band edges can be decreased three ways: through suitable control of the processing conditions, by doping the material with hydrogen, and by increasing the concentration of Ge in the glass.
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