650nm emitting lasers are required for a number of applications including DVD. A combination of requirements, such as narrow far-field divergence, low temperature dependence of threshold current and low absolute operating current make device design difficult. We have adopted the approach of using coupled multiple quantum wells to minimise quasi-Fermi level separation and hence carrier leakage while using a simple low optical confinement structure, with small refractive index step between waveguide core and cladding, to optimise far-field divergence. The samples studied consist of either 3, 4 or 5 coupled quantum wells set in a waveguide core of (AlxGa1-x)0.5In0.5P (0.520.7Ga0.3)0.5In0.5P. The lasing wavelength was between 650 and 655nm with a threshold current density of between 1267Acm-2 and 1417Acm-2 for a 450μm long device with a maximum T0 of 105K (20-70°C). The measured far-field divergence of 27 ±1 degrees (FWHM) was similar for all three structures. We have performed an in-depth analysis using a single pass, multi-section method to deduce modal gain and loss, spontaneous current and efficiency. This demonstrates that the extra wells provide a smaller than expected improvement in gain at room temperature and negligible improvement in gain-current performance at 220K and we attribute this to imperfect carrier distribution amongst the wells. The measured values of the internal optical mode loss of 9-20cm-1 and approximately 50% for the internal efficiency provide room for improvement in laser performance.
The fourth generation light source (4GLS) is a new synchrotron radiation facility proposed for the United Kingdom. It is based on an energy recovery linac, and will consist of a suite of instruments providing radiation from the soft x-ray to the far infrared. In addition to undulator sources, three free electron lasers (FELs) are proposed. Two cavity-based lasers will provide infrared and vacuum-ultraviolet (VUV) radiation, and one FEL will use the self-amplification of spontaneous emission (SASE) phenomenon to produce extreme-ultraviolet (XUV) radiation. The combination of sources will provide unprecedented wavelength coverage, power, and timing structure. The 4GLS facility is expected to have great potential for many areas of research in the biomedical sciences. We discuss some potential biomedical applications of 4GLS FELs in, for example, the areas of macromolecular conformation dynamics, imaging, and radiation damage, and show where the unique properties of this combination of sources will benefit these areas of research.
The Confocal Synchrotron Radiation Microscope at Daresbury was used in a study of the transport and distribution of the steroid Coumestrol in single Leydig cells. The broad spectrum of synchrotron radiation in combination with UV compatible microscope optics affords the extension of confocal microscopy from the visible to the UV region down to about 200 nm. Consequently fluorescent molecules with absorption bands in the UV can be imaged. In addition the pulsed nature of the light source allows us to perform time-resolved fluorescence spectroscopy experiments on microscopic volumes. Coumestrol is a naturally fluorescing plant steroid exhibiting estrogenic activity. In physiological environments it has an absorption peak in the UV at 340 nm and it emits around 440 nm. First results indicate that the Coumestrol transport through the cell membrane is diffusion limited. The weak fluorescence observed in the nuclei of the Leydig cells may be due to fluorescence quenching arising from the interaction of the Coumesterol with nuclear components. However, micro-volume time-resolved fluorescence spectroscopy experiments on cell nuclei have revealed the same decay behavior for Coumesterol in both the cytoplasm and nucleus of the cells.
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