  High-power lasers have numerous scientific and industrial applications. Some
key areas include laser cutting and welding in manufacturing, directed energy
in fusion reactors or defense applications, laser surgery in medicine, and
advanced photolithography in the semiconductor industry. These applications
require optical components, in particular mirrors, that withstand high optical
powers for directing light from the laser to the target. Ordinarily, mirrors
are comprised of multilayer coatings of different refractive index and
thickness. At high powers, imperfections in these layers lead to absorption of
light, resulting in thermal stress and permanent damage to the mirror. Here we
design, simulate, fabricate, and demonstrate monolithic and highly reflective
dielectric mirrors which operate under high laser powers without damage. The
mirrors are realized by etching nanostructures into the surface of
single-crystal diamond, a material with exceptional optical and thermal
properties. We measure reflectivities of greater than 98% and demonstrate
damage-free operation using 10 kW of continuous-wave laser light at 1070 nm,
with intensities up to 4.6 MW/cm2. In contrast, at these laser powers, we
observe damage to a standard dielectric mirror based on optical coatings. Our
results initiate a new category of broadband optics that operate in extreme
conditions.
