We present the design, performance, and tests of a new generation of cooled large-area (5 cm2) optical composite bolometers with a pure germanium absorber, permitting a good efficiency from near-IR to x-rays. With a sensitivity often better than photomultipliers or semiconductor diodes, they allow fluorescence measurements of cold targets with no window, no infrared background, good optical couplings, and a flat response on a large spectral band. Performance obtained at 25 mK is very promising: noise equivalent power as low as 4×10–17 W/Hz½ in the continuous mode, energy threshold about 50 eV in the pulse detection mode, and time constant τ~3 ms. These detectors of low mass (0.25 g) have been successfully used for detecting the fluorescence emitted by much more massive bolometers, having, for example, a BGO (92 g) or a CaWO4 (54 g) target. The simultaneous detection of heat and light in these double bolometers permits the identification of each event in the massive target (α decay, γ, or cosmic ray interaction, neutron recoil...). Thanks to the consecutive excellent subtraction of the radioactive and cosmic ray background, it is a powerful tool developed by several groups for fundamental research: study of very rare decays of atoms, measurement of internal very low radioactivity content in single crystals, direct detection of dark matter recoils in massive fluorescence targets, and detection of solar neutrino fluorescence events in liquid 4He. Recently obtained results are reviewed: the first detection of the rare alpha decay of 209Bi, and new scintillation data on Al2O3 (sapphire), LiF, or TeO2 at 20 mK. By cooling at 10 mK, sensitivity can yet be increased by more than one order of magnitude.