Interplay between amplified spontaneous emission, Forster resonant energy transfer, and self-absorption in hybrid poly(9,9-dioctylfluorene)-CdSe/ZnS nanocrystal thin films.

We investigated the excitation density dependence of the photoluminescence spectra of hybrid poly(9,9-dioctylfluorene)-CdSe/ZnS nanocrystals (PF8-NCs) thin films. We demonstrate that this experiment allows the determination of the efficiency of all the CdSe/ZnS NCs excitation processes and that the presence of amplified spontaneous emission (ASE) from the PF8 leads to a strong dependence of the NC excitation processes from the laser excitation density. Below the PF8 ASE threshold only about 6% of the excitons in the NCs are due to pump laser absorption, while about 94% of the NC excitation is due to the interaction with the PF8, and it is due for about 58% to PF8-->NC Forster resonant energy transfer (FRET) and for about 37% to reabsorption by the NCs of the PF8 luminescence. The presence of PF8 ASE significantly modifies this scenario by strongly decreasing the FRET importance and strongly increasing the reabsorption one. The interplay between reduced FRET and increased reabsorption overall decreases the NC excitation due to PF8 indicating that ASE from the donors should be avoided if efficient NCs excitation under strong pumping is wished.

Isolated homonymous lateral hemianopsia revealing central nervous system toxoplasmosis as the initial manifestation of AIDS.

A 25-year-old-healthy man, with previous history of parenteral drug abuse, developed a left homonymous hemianopsia. Computed tomographic scanning showed a single ring enhancing lesion in the right parieto-occipital area. After the demonstration of seropositivity for human immunodeficiency virus and high serum immunoglobulin G antibody titer against Toxoplasma gondii, a diagnosis of cerebral toxoplasmosis in an AIDS patient was made. An isolated visual field defect revealing cerebral toxoplasmosis may be the first presentation of AIDS. Opportunistic cerebral lesions should be always considered in subjects at risk for AIDS who present focal neurological signs.