Traditionally accepted design paradigms dictate that only optically isotropic (cubic) crystal structures with high equilibrium solubilities of optically active ions are suitable for polycrystalline laser gain media.The restriction of symmetry is due to light scattering caused by randomly oriented anisotropic crystals,whereas the solubility problem arises from the need for sufficient active dopants in the media.These criteria limit material choices and exclude materials that have superior thermo-mechanical properties than state-of-the-art laser materials.Alumina (Al2O3) is an ideal example;it has a higher fracture strength and thermal conductivity than todays gain materials,which could lead to revolutionary laser performance.However,alumina has uniaxial optical proprieties,and the solubility of rare earths (REs) is two-to-three orders of magnitude lower than the dopant concentrations in typical RE-based gain media.We present new strategies to overcome these obstacles and demonstrate gain in a RE-doped alumina (Nd:Al2O3) for the first time.The key insight relies on tailoring the crystallite size to other important length scales-the wavelength of light and interatomic dopant distances,which minimize optical losses and allow successful Nd doping.The result is a laser gain medium with a thermo-mechanical figure of merit of Rs～19,500 Wm-1 a 24-fold and 19,500-fold improvements over the high-energy-laser leaders Nd:YAG (Rs～800 Wm-1) and Nd:Glass (Rs～1 Wm-1),respectively.Moreover,the emission bandwidth of Nd:Al2O3 is broad:～13 THz.The successful demonstration of gain and high bandwidth in a medium with superior Rs can lead to the development of lasers with previously unobtainable high-peak powers,short pulses,tunability,and high-duty cycles.