Light converter: The emission of visible light (Vis) upon excitation in the near‐infrared (NIR) has been observed for the first time in a transparent solution. Excitation at 975 nm (marked red in the picture) of optically clear colloidal solutions of LuPO4:Yb3+, Tm3+, and YbPO4:Er3+ nanocrystals in chloroform leads to visible upconversion luminescence in the blue, green, or red spectral regions.

A variety of redispersible Eu(3+)-doped LaPO(4) nanoparticles have been prepared in a high-boiling coordinating solvent mixture, and the Eu(3+) lattice sites of these materials have been investigated by luminescence line-narrowing measurements. In this spectroscopic method, Eu(3+) ions occupying different lattice sites are selectively excited with a tunable narrow-bandwidth laser system and distinguished by their luminescence spectra ("site-selective spectroscopy"). Depending on the concentration of the dopant, up to three different lattice sites could be identified in the interior of the LaPO(4) nanoparticles. These sites correspond to those known from bulk LaPO(4). In addition, a variety of surface sites is observed, which could be converted completely into bulk sites by overgrowing the nanoparticles with a shell of pure LaPO(4). The surface sites are identical to those obtained by reacting Eu(3+) with the surface of pure LaPO(4) nanoparticles. The spectroscopic properties of Eu(3+)-doped LaPO(4) nanoparticles differ from those of pure EuPO(4) nanoparticles, which were also investigated. Remarkably, the core/shell synthesis investigated in this paper allows one to prepare doped nanoparticles that contain no other dopant sites than those known from the corresponding bulk material.

Three-dimensional periodic microstructures of aluminum oxide, which are important for creating photonic band-gap structures (PBGs), were fabricated by laser rapid prototyping by means of laser-induced direct-write deposition from the gas phase. The structures consisted of layers of parallel rods forming a face-centered tetragonal lattice with lattice constants of 66 and 133 micrometers. These structures showed transmission minima centered around 4 terahertz (75 micrometers) and 2 terahertz (150 micrometers), respectively. PBGs will allow precise control of the optical properties of materials, including lasers without threshold.

The complete series of all nonradioactive lanthanide phosphates has been prepared in organic solution as ligand-capped nanoparticles. In all cases, well-dispersed particles with mean sizes below 10 nm are obtained. Despite the similar chemical properties of the lanthanides, the growth, the crystal structure, and the mean size of the nanocrystals are found to be strongly affected by the lanthanide ion employed. Very small nanoparticles are obtained for lanthanides for which the lattice energies of the bulk tetragonal xenotime phase and the bulk monoclinic monazite phase are similar. These smaller nanoparticles do not show the expected phase transition from monoclinic to tetragonal and seem to have their own unique crystal structure. Possible explanations for these observations are discussed. Finally, we present the first results on the IR emission of Er 3+ -doped YbPO 4 and (Lu, Yb)PO 4 nanoparticles in solution.

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