Out of the blue : semiconductor laser pumped visible rare-earth doped lasers

The rise of semiconductor-based pump sources such as In$_x$Ga$_{1-x}$N-laser diodes or frequency-doubled optically pumped semiconductor lasers with emission wavelengths in the blue encourages a revisitation of the rare-earth ions Pr$^{3+}$, Sm$^{3+}$, Tb$^{3+}$, Dy$^{3+}$, Ho$^{3+}$ and Er$^{3+}$ with respect to their properties as active ions in crystalline solid-state laser materials with direct emission in the visible spectral range. Nowadays, some of these blue-pumped visible lasers compete with Nd$^{3+}$-lasers in terms of efficiency and direct lasing at various colors from the cyan-blue to the deep red can be addressed in very simple and compact laser setups. This paper highlights the spectroscopic properties of suitable rare-earth ions for visible lasing and reviews the latest progress in the field of blue-pumped visible rare-earth doped solid-state lasers