The magnitude and sign of the effective magnetic splitting factor ${\mathit{g}}^{\mathrm{*}}$ for conduction electrons in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells have been determined as a function of well width down to 5 nm. The experimental method is based on combined measurements of the decay time of photoluminescence and of the suppression of its circular polarization under polarized optical pumping in a magnetic field perpendicular to the growth axis (Hanle effect). Measurements as a function of hole sheet density in the wells reveal a transition from excitonic behavior with very small apparent g value for low density, to larger absolute values characteristic of free electrons at higher densities. For 20-nm wells ${\mathit{g}}^{\mathrm{*}}$ for electrons is close to the bulk value (-0.44), and increases for narrower wells passing through zero for well width close to 5.5 nm. A theoretical analysis based on three-band k\ensuremath{\cdot}p theory, including allowance for conduction-band nonparabolicity and for wave-function penetration into the barriers, gives a reasonable representation of the data, leading to the conclusion that ${\mathit{g}}^{\mathrm{*}}$ in quantum wells has a value close to that of electrons in the bulk at the confinement energy above the band minimum.
Standard residual pesticides applied to US military materials such as camouflage netting can reduce mosquito biting pressure in the field but may contribute to the evolution of resistance. However, residual applications of a spatial repellent such as transfluthrin could allow mosquitoes the opportunity to escape, only inducing mortality if insects linger, for example after becoming trapped in a treated tent. In this study we investigated the capability of transfluthrin on 2 types of US military material to reduce natural populations of disease vector mosquitoes in a cool-arid desert field environment in southern California. We found that transfluthrin could reduce Culex tarsalis incursion into protected areas by up to 100% upon initial treatment and up to 45% for at least 16 days posttreatment, showing that this compound could be an effective element in the US Department of Defense integrated vector management system appropriate for further study.
The circular polarization of cw photoluminescence as a function of applied magnetic field has been measured at 1.8 K for excitons in a series of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As and GaAs/AlAs type-I quantum wells having well widths between 2.5 and 8.0 nm. The results show evidence of level crossings, which have been analyzed to give the short-range, spin-dependent exchange interaction. The associated experimental exchange splittings between optically allowed and nonallowed exciton states are of the order of 0.15 meV for the narrowest wells and fall monotonically with increasing width. Simulations of the data using solutions of rate equations for the exciton-level populations show that the optically nonallowed exciton states lie below the allowed states and give insight into the field dependence of the spin-relaxation rates of excitons and of holes and electrons in an exciton. Calculations of the enhancement of exchange interaction due to carrier confinement relative to the bulk value give agreement between the quantum-well and bulk-exchange values, and confirm that exchange is particularly sensitive to barrier height and other details of the structure. The data also show evidence for eigenstate-polarization changes, which indicate a much smaller zero-field splitting due to departure of the quantum-well symmetry from the ideal ${\mathit{D}}_{2\mathit{d}}$. The simulation indicates that only a fraction (up to 20%) of excitons in these samples actually experiences a nonideal, distorted environment.
In recent years, much attention has been focused on carrier emission rates and cross-well motion in semiconductor quantum well structures because the efficiency and response speed of many MQW optoelectronic devices depend on the dynamics of vertical carrier transport processes. Thermionic emission is believed to be the principal mechanism for the escape of the carriers from each individual quantum well. In most cases, measurements of the transport dynamics have been carried out in structures that contain multiple quantum wells. However in multiple well structures, effects such as resonant tunneling and carrier retrapping make it difficult to unambiguously separate the effects from one another. In recent measurements on single quantum wells with asymmetric barriers, we monitored electron and hole sweep-out rates simultaneously as a function of electric field (1) , and showed that the carrier emission from the wells is thermionic in nature, but this preliminary analysis did not give very good quantitative agreement with the conventional thermionic emission model (2) . In this paper, we will present a study of the emission for both carrier types as a function of temperature and electric field in order to fully evaluate the applicability of the thermionic emission model to the process of carrier escape in a single quantum well.
An electrooptic modulator that is sensitive to the polarization state of transmitted light is demonstrated by using the intrinsic optical anisotropy of biaxially strained [110]-oriented GaAs-In/sub y/Ga/sub 1-x/As multiple quantum wells. The ellipticity and the direction of polarization of a linearly polarized input pulse are modulated by a change in the in-plane dichroism and birefringence produced by a change in the voltage applied across the p-i-n region containing the quantum wells. Sensitive ellipsometric measurements are used to directly measure the anisotropy in the complex index of refraction between the two principal in-plane axes of the sample as a function of wavelength for selected voltages. The latter information is then used to determine the operating wavelength, the contrast ratio, the optical bandwidth, and the tunability of the modulator. This structure requires only standard elementary post-growth processing.
Treating perimeters with residual insecticides for protection from mosquito vectors has shown promise. These barrier treatments are typically evaluated in temperate or tropical areas using abundant vegetation as a substrate. However, there is an emerging interest to develop this technology to protect deployed US troops in extreme desert environments with sparse vegetation. We used a remote desert area in the Coachella Valley, California, to 1) evaluate bifenthrin barrier treatments on native xeric vegetation and 2) compare treatments applied with electrostatic and conventional spray technologies. Through a combination of laboratory bioassays on treated and control vegetation sampled at specific intervals over 63 days, synchronized with field surveillance of mosquitoes, we measured the temporal pattern of bioactivity of bifenthrin barriers under natural hot, dry, and dusty desert conditions. Regardless of spray technology, mosquito catch in treated plots was about 80% lower than the catch in control plots 1 day after treatment. This reduction in mosquito numbers in treated plots declined each week after treatment but remained at about 40% lower than control plots after 28 days. Field data were corroborated by results from bioassays that showed significantly higher mosquito mortality on treated vegetation over controls out to 28 days postspray. We concluded that barrier treatments in desert environments, when implemented as part of a suite of integrated control measures, may offer a significant level of protection from mosquitoes for deployed troops. Given the comparable performance of the tested spray technologies, we discuss considerations for choosing a barrier treatment sprayer for military scenarios.
The invasive Aedes aegypti is an important disease vector increasing in frequency in hot-arid regions of the USA such as the Southwest. Within hot-arid surroundings this mosquito may be confined to peridomestic locations that tend to be cooler and humid, such as in lush, irrigated ornamental vegetation surrounding homes. However, to reach these habitat refugia, ultra-low volume (ULV) applications of insecticides targeting this mosquito must retain efficacy after being sprayed from the air or street where hot-arid conditions are prevalent. We investigated the efficacy of a biologically based larvicide, spinosad (Natular 2EC), applied as a ULV in a hot-arid environment targeting Aedes aegypti. We found that this pesticide is able to penetrate this environment and has the potential to act as a residual.
Ultra-low-volume (ULV) and thermal fog aerosol dispersals of pesticides have been used against mosquitoes and other insects for half a century. Although each spray technology has advantages and disadvantages, only 7 studies have been identified that directly compare their performance in the field. US military personnel currently operating in hot-arid environments are impacted by perpetual nuisance and disease vector insect problems, despite adulticide operations using modern pesticide-delivery equipment such as ULV. None of the identified comparative studies has looked at the relative feasibility and efficacy of ULV and thermal fog equipment against mosquitoes in hot-arid environments. In this study we examine the impact of ULV and thermal fog applications of malathion against caged sentinel mosquitoes in the field in a warm temperate area of Florida, followed by a similar test in a hot-dry desert area of southern California. Patterns of mortality throughout 150 m x 150 m grids of sentinel mosquitoes indicate greater efficacy from the thermal fog application in both environments under suboptimal ambient weather conditions. We discuss the implications of these findings for future military preventive medicine activities and encourage further investigations into the relative merits of the 2 technologies for force health protection.
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