Exchange bias in Fe0.6Zn0.4F2/Fe heterostructures

The exchange bias field, H_e, is measured in Fe_0.6Zn_0.4F₂/Fe heterosystems prepared from Fe layers of 14 and 5 nm thickness which are deposited on top of the compensated (1 1 0) surface of the antiferromagnet. Deviations from a linear dependence of H_e on the magnetization of the Fe layer are attributed to ferromagnetic domains. Moreover, piezomagnetism and its influence on H_e are evidenced.     

Hypericin phototoxicity induces different modes of cell death in melanoma and human skin cells

Hypericin, the major component of St. John's Wort, absorbs light in the UV and visible ranges whereupon it becomes phototoxic through the production of reactive oxygen species. Although photodynamic mechanisms (i.e. through endogenous photosensitizers) play a role in UVA phototherapy for the treatment of skin disorders such as eczema and psoriasis, photodynamic therapy employing exogenous photosensitizers are currently being used only for the treatment of certain forms of non-melanoma skin cancers and actinic keratoses. There are few reports however on its use in treating melanomas. This in vitro study analyses the phototoxic effect of UVA (400-315 nm) - activated hypericin in human pigmented and unpigmented melanomas and immortalised keratinocytes and melanocytes. We show that neither hypericin exposure nor UV irradiation alone reduces cell viability. We show that an exposure to 1 microM UVA-activated hypericin does not bring about cell death, while 3 microM activated hypericin induces a necrotic mode of cell death in pigmented melanoma cells and melanocytes and an apoptotic mode of cell death in non-pigmented melanoma cells and keratinocytes. We hypothesis that the necrotic mode of cell death in the pigmented cells is possibly related to the presence of melanin-containing melanosomes in these cells and that the hypericin-induced increase in reactive oxygen species leads to an increase in permeability of melanosomes. This would result in toxic melanin precursors (of an indolic and phenolic nature) leaking into the cytoplasm which in turn leads to cell death. Hypericin localisation in the endoplasmic reticulum in these cells shown by fluorescent microscopy, further support a disruption in cellular processing and induction of cell death. In contrast, this study shows that cells that do not contain melanosomes (non-pigmented melanoma cells and keratinocytes) die by apoptosis. Further, using a mitochondrial-specific fluorescent dye, we show that intracellular accumulation of hypericin induces a mitochondrial-associated caspase-dependent apoptotic mode of cell death. This work suggests that UVA is effective in activating hypericin and that this phototoxicity may be considered as treatment option in some cases of lentigo maligna or lentigo maligna melanoma that are too large for surgical resection.     

Perfect blazing with echelle gratings in TE and TM polarization

Perfect blazing with echelle gratings in a high-order Littrow mount exists not only with TM polarization, as can perhaps be assumed, but also with TE and TM polarizations simultaneously. This finding contradicts with a heuristic explanation as to why perfect blazing is not possible simultaneously in TE and TM polarizations given by Loewen and Popov [Diffraction Gratings and Applications (Marcel Dekker, 1997)]. As a matter of fact, in the ideal case of infinite conductivity, light is diffracted in the Littrow order with 100% efficiency for both polarization states. For metal gratings, a small loss occurs. Three conditions are necessary for perfect blazing: (A) the apex angle must be very close to 90°, (B) a specific Littrow order has to be used, and (C) the blaze angle has to be 4° to 6° larger than the Littrow angle. The last property is particularly intriguing because typically the difference between the two angles is smaller than 1.0°.     

High-transparency films for nonlinear integrated optics based on P(VDF-TrFE) and poly(α-methyl styrene) host materials

In search of waveguiding polymer films with both highly efficient NonLinear Optical (NLO) and optimum transmission properties the second harmonic generation of light is measured on poled films of P(VDF-TrFE) blended with 10% PMMA or covered by the polarizable guest-host system PMS, both pure and doped with 2% NLO dye The PMS films turn out to be stable electrets after annealing at about 50°C and poling at room temperature. They are thus particularly easily processable candidates for NLO and waveguiding applications.     

Creep and relaxation dynamics of domain walls in periodically poled KTiOPO4

Creep and relaxation of domain walls under ac electric fields are observed in an ideal model system, periodically poled KTiOPO4, to occur in different regimes, which are separated by dynamic phase transitions at frequencies f(m)(T)=f(m0)exp((-DeltaE/k(B)T), with f(m0)=3 x 10(9) Hz and DeltaE=0.6 eV. Power law dispersion of the creep susceptibility, chi proportional to 1+(iomegatau)(-beta), with beta approximately equal to 0.4, and large nonlinearity encountered at f < f(m), is contrasted with Cole-Cole-type relaxational dispersion, chi proportional to (1+[iomegatau](1-alpha))(-1), with alpha approximately 0.3, at f > f(m).