How to Build a Laser Death Ray

Diffractive Resonant Cavities

Sometimes pure reflective cavities are not desired. Sometimes they are very difficult - vacuum ultraviolet and soft x-rays can only be reflected at grazing angles, meaning you would need a complicated array of mirrors to close the loop and have the optical path meet back up with itself. Sometimes they are impossible - hard x-rays and gamma rays cannot be reflected by normal matter. Sometimes, we can do better by using diffraction gratings. These scatter light by an angle that depends on the wavelength of light, so that a beam of white light will have scattered side-lobes split up into rainbow fans.

So here is a trick - if you have a material that exhibits optical gain on many different wavelengths, putting it in a reflective cavity will give you a wide spread of unstable wavelengths. But you can use a diffraction grating as part of the resonant cavity to select only that wavelength that you want, by choosing the angle at which the diffracted beam is intercepted and reflected by the mirrors that make up the rest of the cavity. An example of using a transmission grating in this way is shown below.

For death rays, however, the main use of diffractive cavities occurs when we want to stabilize the emission of x-rays from our gain medium into a high quality beam whose divergence is limited by the diffraction limit. We are helped in this because the periodic arrays of atoms in a perfect crystal act like a diffraction grating to x-rays. A beam of x-rays incident on a crystal at a given orientation can have its beam partially diffracted at a specific angle to the beam and the crystal orientation. The image below shows how a crystal that difracts the x-ray beam at 120° can form a path that leads back to the gain medium (in this case an incandescent plasma), stabilizing the beam and improving its ability to focus.

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