A monolithic optical absorber and methods of making same. The monolithic optical absorber uses an array of mutually aligned carbon nanotubes that are grown using a PECVD growth process and a structure that includes a conductive substrate, a refractory template layer and a nucleation layer. Monolithic optical absorbers made according to the described structure and method exhibit high absorptivity, high site densities (greater than 109 nanotubes/cm2), very low reflectivity (below 1%), and high thermal stability in air (up to at least 400° C.). The PECVD process allows the application of such absorbers in a wide variety of end uses.