{\it In-situ} Laser Microprocessing at the Quantum Level

One of the biggest challenges of nanotechnology is the fabrication of nano-objects with perfectly controlled properties. Here we employ a focused laser beam both to characterize and to {\it in-situ} modify single semiconductor structures by heating them from cryogenic to high temperatures. The heat treatment allows us to blue-shift, in a broad range and with resolution-limited accuracy, the quantized energy levels of light and charge carriers confined in optical microcavities and self-assembled quantum dots (QDs). We demonstrate the approach by tuning an optical mode into resonance with the emission of a single QD and by bringing different QDs in mutual resonance. This processing method may open the way to a full control of nanostructures at the quantum level.Comment: 3 figure

Lifetime of sub-THz coherent acoustic phonons in a GaAs-AlAs superlattice

We measure the lifetime of the zone-center 340 GHz longitudinal phonon mode in a GaAs-AlAs superlattice excited and probed with femtosecond laser pulses. By comparing measurements conducted at room temperature and liquid nitrogen temperature, we separate the intrinsic (phonon-phonon scattering) and extrinsic contributions to phonon relaxation. The estimated room temperature intrinsic lifetime of 0.95 ns is compared to available calculations and experimental data for bulk GaAs. We conclude that ∼0.3 THz phonons are in the transition zone between Akhiezer and Landau-Rumer regimes of phonon-phonon relaxation at room temperature.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0001299)United States. Dept. of Energy (Grant DE-FG02-00ER15087

Intrinsic to extrinsic phonon lifetime transition in a GaAs–AlAs superlattice

We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon–phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the 'interfacial atomic disorder' model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-FG02-00ER15087)United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0001299/DE-FG02-09ER46577

Electrical techniques for the measurement of deep states

AbstractWhen point defects, either intrinsic (such as vacancies or interstitials) or extrinsic (most impurities with the exception of shallow donors or acceptors) are introduced into a semiconductor, they can result in the occurrence of ‘deep states’. These are electronic levels that are not normally ionized at room temperature, but can affect both carrier concentrations and minority carrier lifetime. The purpose of this review is to provide an outline of the techniques that are commonly used to characterize deep defect states in terms of their electrical properties

Evaluation of the Cape Town Protocol for the isolation of Campylobacter spp. from environmental waters: short communication

Campylobacter is recognised as one of the major causes of bacterial gastro-enteritis world-wide. In addition to poultry products, milk and water have also been implicated as possible sources of infection. Methods for the detection and isolation of this organism have been developed specifically for the medical field and select for Campylobacter jejuni and C. coli, excluding all other species of Campylobacter, whereas the filter-based Cape Town Protocol reportedly yields most Campylobacter spp. The Cape Town Protocol was evaluated for possible use in analysis of environmental water samples. It yielded only 0.1% of the total number of actively growing C. jejuni and C. coli cells, whereas the selective medium mCCDA yielded 10%. Analysis of 60 water samples yielded 221 putative Campylobacter isolates, but only four could be confirmed as Arcobacter butzleri and none as Campylobacter. Our results indicated that neither the Cape Town Protocol nor mCCDA can be used for the direct enumeration or isolation of Campylobacter spp. from environmental water samples. Keywords: Campylobacter, Arcobacter, Water-borne pathogens (WaterSA: 2003 29(2): 225-229