How to Build a Laser Death Ray
Everyone is probably familiar with how mirrors work - put a mirror in the path of the beam, and the beam will be reflected. The general principle behind reflection is that the angle at which the beam strikes the mirror is equal to the angle at which the beam leaves the mirror. This is called specular reflection.
If you have a bent or warped mirror, the light will hit different parts of the surface at different angles, so the light will be reflected at different angles. You can use this to design mirrors that intentionally focus or defocus the beam. Mirrors which can bend on the fly exactly how you want them to are used to make adaptive optics for correcting twinkle.
Mirrors have many advantages for directing the death ray beam. They reflect from their surface, so they can be supported by their entire back side rather than just the edges. Likewise, they can be cooled along the back side if they absorb too much heat from the beam. If you make the mirror out of something flexible, actuators behind the mirror can deform it on the fly for adaptive optics and minor very rapid adjustments to focus or aim.
If you know the wavelength of the beam your death ray will be emitting, the best mirrors are made of dielectric layers. The simplest of these use thin layers of transparent material with a thickness close to the wavelength of the light being reflected; The layers altnernate between two different kinds of transparent substance. At each interface, the light is partially reflected. The reflected waves add together to amplify each other, and soon the entire beam has been reflected. Mirrors of this design can reflect 99.999% or more of the incident light of the correct wavelength when incident at the optimum angle.
A dielectric mirror is a simple, one dimensional example of a general class of nanostructured materials called photonic crystals. More complicated two dimensional photonic crystals can be made by drilling lots of tiny holes in a transparent surface with a spacing between holes close to that of the light's wavelength. Engineers today are trying hard to figure out how to build three dimensional photonic crystals which would be made of a regular, periodic array of transparent components extending in all three directions; each component would have a size not larger than the wavelength of the light. Photonic crystals can be made to reflect light over a wider range of angles and over a finite band of wavelengths with the same sort of efficiency as simple dielectric mirrors. However, the band of reflected wavelengths will not be arbitrarily wide - you might be able to reflect all visible light with a photonic crystal mirror, but not mid infrared or vacuum ultraviolet, for example.
Photonic crystals and dielectric mirrors can be made for all wavelengths from far infrared through the extreme ultraviolet. Modern dielectric mirrors can be made to withstand intensities from heat rays as high as 5 MW/cm2 and blaster pulses in the range of 20 J/cm2 over a few nanoseconds at 1.06 micron wavelengths in the near infrared. The reflectivity of modern dielectric mirrors falls off in the vacuum and extreme ultraviolet - the mirrors used for EUV photolithography (which is how we make computer chips) reflect only about 70% of the light that hits them. Presumably other photonic crystals would have similarly poor performance in the VUV and EUV, but no one has made them yet.
If you need to reflect a wide range of wavelengths, it is usually best to turn to metal mirrors. Gold coatings on the mirror surface can reflect around 98 to 99% of incident mid and far infrared light. Silver is preferred for visible and near infrared, with 90 to 95% reflectivity to about 0.4 microns and special coatings can boost that to >98%. Finally, aluminum is the metal of choice for reflecting a broad spectrum of near infrared, visible, and near ultraviolet light.