The composition of the P840-reaction center complex (RC), energy and electron transfer within the RC, as well as its topographical organization and interaction with other components in the membrane of green sulfur bacteria are presented, and compared to the FeS-type reaction centers of Photosystem I and of Heliobacteria. The core of the RC is homodimeric, since pscA is the only gene found in the genome of Chlorobium tepidum which resembles the genes psaA and -B for the heterodimeric core of Photosystem I. Functionally intact RC can be isolated from several species of green sulfur bacteria. It is generally composed of five subunits, PscA-D plus the BChl a-protein FMO. Functional cores, with PscA and PscB only, can be isolated from Prostecochloris aestuarii. The PscA-dimer binds P840, a special pair of BChl a-molecules, the primary electron acceptor A(0), which is a Chl a-derivative and FeS-center F(X). An equivalent to the electron acceptor A(1) in Photosystem I, which is tightly bound phylloquinone acting between A(0) and F(X), is not required for forward electron transfer in the RC of green sulfur bacteria. This difference is reflected by different rates of electron transfer between A(0) and F(X) in the two systems. The subunit PscB contains the two FeS-centers F(A) and F(B). STEM particle analysis suggests that the core of the RC with PscA and PscB resembles the PsaAB/PsaC-core of the P700-reaction center in Photosystem I. PscB may form a protrusion into the cytoplasmic space where reduction of ferredoxin occurs, with FMO trimers bound on both sides of this protrusion. Thus the subunit composition of the RC in vivo should be 2(FMO)(3)(PscA)(2)PscB(PscC)(2)PscD. Only 16 BChl a-, four Chl a-molecules and two carotenoids are bound to the RC-core, which is substantially less than its counterpart of Photosystem I, with 85 Chl a-molecules and 22 carotenoids. A total of 58 BChl a/RC are present in the membranes of green sulfur bacteria outside the chlorosomes, corresponding to two trimers of FMO (42 Bchl a) per RC (16 BChl a). The question whether the homodimeric RC is totally symmetric is still open. Furthermore, it is still unclear which cytochrome c is the physiological electron donor to P840(+). Also the way of NAD(+)-reduction is unknown, since a gene equivalent to ferredoxin-NADP(+) reductase is not present in the genome.
We investigated the degradation of cleaved facets of (Al,In)GaN laser diodes in different atmospheres. We found that operation in water-free atmospheres with sufficient oxygen shows a slow degradation. Operation in atmospheres with water vapor causes a fast degradation and an oxidation on the facet. This deposition is a permanent damage to the laser diode. If the laser diode is operated in pure nitrogen, we find a thick deposition on the facet, which shows high absorption. This deposition can be removed by either high optical output powers or by operation in atmospheres with sufficient oxygen. We also explain the influence of these coatings to the degradation behavior and see these coatings as the reason for unstable kinks in the L–I characteristics during operation.
In our study, III-nitride laser diodes with uncoated facets obtained by cleavage show a much faster degradation than coated ones. An increase in threshold current and drop of slope efficiency suggest increased absorption losses. Degradation experiments in different atmospheres prove the influence of the respective atmosphere and indicate the growth of an oxide film leading to increased absorption. Because the observed degradation is insensitive to the photon density we suggest nonradiative centers, which are saturated at low photon densities, to be at the origin of degradation. No evidence for photon enhanced degradation of coated laser diodes was found. A dielectric coating efficiently protects the facets.
We study the facet degradation behavior of (Al,In)GaN multiple quantum well laser diodes. Water vapor causes a fast degradation due to facet oxidation of the uncoated facet. Degradation in an inert nitrogen atmosphere is slow and comparable to degradation of GaN laser diodes with coated facets. We also observe a reversible increase in the threshold current density due to a change in absorption caused by surface charges. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
We study the degradation behaviour of GaN gain guided laser diodes (LDs) on SiC substrates with cleaved facets and reflective coatings on none, one, or both facets. This allows us to demonstrate that in addition to volume effects there is a contribution of the laser facets to laser degradation. We observe that for the uncoated LDs the threshold current density is increasing considerably faster compared to LDs with mirror coatings. Degradation is observed during operation but not during storage at ambient conditions and thus expected to be photon or current induced. Operation of the uncoated laser in a nitrogen atmosphere reduces the degradation rate with respect to operation in air.
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