Because of the increasing incidence of skin cancer, interest in using the autofluorescence of skin tissue as a noninvasive tool for early diagnosis is enforced. Focus is especially on malignant melanotic melanoma. On the basis of a newly developed method to selectively excite melanin fluorescence of skin tissue by stepwise two-photon excitation with nanosecond laser pulses at 810 nm, we have investigated information from this melanin fluorescence with respect to the differentiation of pigmented lesions. A distinct difference in the melanin fluorescence spectrum of malignant melanoma (including melanoma in situ) when compared to that of benign melanocytic lesions (i.e., common nevi) has been found for freshly excised samples as well as for histopathological samples. There is also specific fluorescence from dysplastic nevi. In this way, early detection of malignant melanoma is possible.
Due to its extremely low fluorescence quantum yield, in the conventionally (one-photon) excited autofluorescence of
skin tissue, melanin fluorescence is masked by several other endogenous and possibly also exogenous fluorophores (e.g.
NADH, FAD, Porphyrins). A first step to enhance the melanin contribution had been realized by two-photon fs-pulse
excitation in the red/near IR, based on the fact that melanin can be excited by <i>stepwise</i> two-photon absorption, whereas
all other fluorophores in this spectral region allow only <i>simultaneous</i> two-photon excitation.
Now, the next and decisive step has been realized: Using an extremely sensitive detection system, for the first time twophoton
fluorescence of skin tissue excited with pulses in the ns-range could be measured. The motivation for this step
was based on the fact that the population density of the fluorescent level resulting from a stepwise excitation has a
different dependence of the pulse duration than that from a simultaneous excitation (&Dgr;t<sup>2</sup> vs. &Dgr;t). Due to this strong discrimination between the fluorophores, practically pure melanin fluorescence can be obtained. Examples for in-vivo,
ex-vivo as well as paraffin embedded skin tissue will be shown. The content of information with respect to early
diagnosis of skin deseases will be discussed.
We report on a miniature solid state emitter structure, which allows electrical pumping of only one single InAs quantum
dot (<i>QD</i>) grown in the Stranski-Krastanow mode. The emitter is based on a single layer of low density (~10<sup>8</sup> cm<sup>-2</sup>)<i> QDs </i>
grown by Molecular Beam Epitaxy and a submicron AlOX current aperture defined by selective oxidation of high
aluminium content AlGaAs layers. The device demonstrates strongly monochromatic polarized emission of the single
<i>QD</i> exciton at subnanoampere current pumping. No other emission is observed across a spectral range of 500 nm, proving that indeed just one single<i> QD </i>is contributing. Correlation measurements of the emitted photons show a clear
High-efficient single-photon sources are important for fundamental experiments as well as for modern applications in the field of quantum information processing. Therefore, both the overall collection efficiency as well as the photon generation rate are important parameters. In this article, we use cascaded two-photon emission from a single quantum dot in order to double the efficient transmission rate in a quantum key distribution protocol by multiplexing on a single photon level. The energetically non-degenerate photons are separated with a single photon add/drop filter based on a Michelson interferometer. For optimizing the collection efficiency, coupling of quantum emitters to microcavities is advantageous. We also describe preliminary results towards coupling of a single quantum dot grown on a micrometer-sized tip to the whispering gallery modes of a microsphere cavity.