How to Build a Laser Death Ray
Heat Ray is the terminology I will be using for a laser that shines a beam of near constant power on its target for a prolonged period of time (from a few hundredths of a second or more). It causes damage primarily by heating the illuminated part of the target hot enough to cause softening, warping, blistering, cracking, charring, cooking, ignition, melting, or vaporization. Usually the heat ray heats up the surface of the target, but highly penetrating radiation (such as visible light on a transparent material or hard x-rays to most matter) can raise the temperature of a considerable volume along the beam path.
While heat rays can instantly scorch or ignite a surface, they generally require time to burn through the surface to vulnerable parts underneath. If the target can sense damage (such as our feeling of pain) and can writhe or turn away, the heat ray may be ineffective at reaching vital components or organs since it constantly has to burn through new parts of the surface. This drastically limits their use as antipersonnel weapons unless the beam is used as a sort of long range flamethrower, to ignite clothes and char the skin over a wide area. Against soft skinned, numb targets, the heat ray is more useful. In tests of modern heat rays against rockets in flight, the heat ray does not even have to burn through the skin, simply heating and softening the thin shell is sufficient to cause the rocket to blow itself to pieces.
A dramatic description of a heat ray is found in H.G. Wells "The War of the Worlds."
Beyond the pit stood the little wedge of people with the white flag at its apex, arrested by these phenomena, a little knot of small vertical black shapes upon the black ground. As the green smoke arose, their faces flashed out pallid green, and faded again as it vanished. Then slowly the hissing passed into a humming, into a long, loud, droning noise. Slowly a humped shape rose out of the pit, and the ghost of a beam of light seemed to flicker out from it.
Technical Note - A heat ray is not a ray of heat. It is a ray that heats what it illuminates. In thermodynamics, heat is defined as energy flow that has an associated entropy. The entropy of a laser beam is vanishingly small, so any laser beam carries essentially no heat at all even though it may be carrying huge amounts of energy. On the other hand, once incident on a surface, absorption of that energy by the surface creates a lot of entropy and the surface gains heat. This sort of nit-picking is completely unimportant for our purpose, but useful for snobbish pedants who want to think they are impressing people with how smart they are.