Sub Surface Laser Engraving (SSLE) is the process of engraving an image below the surface of a solid material, usually glass with an optical clarity to minimize distortion of the laser.
Sub-Surface Laser Engraving or SSLE as it is being termed nowadays, is a technique "discovered" in Russia in the early 1980's. Originally the creation of these "dots" in optical glass was a problem known as "Laser Induced Damage" and was exhaustingly studied.
At the time, the idea was to avoid these problems by selecting specific material compositions and laser optics to improve laboratory use. In the process, a list of material compositions was created that provided good transparency with minimal heat absorption. There was also another list, which included materials that didn’t, and it was somewhere there that laser technicians found they could write their name in these materials.
Someone decided there might be some commercial applications for this and began writing dissertations about the controlled placement of these dots inside of the crystal. Over the years, the science of this process from a purist view has become more of an art form that balances the technical aspects of high-powered lasers and delicate balance of image design within them.

A technical overview:
The creation of 3D laser crystals utilize high-energy laser beams to produce a phenomenon known as “Multi-photon Absorption” within optically perfect crystal. This phenomenon, which uses the electromagnetic wave of the laser beam known as coherent light creates an electric field greater than 10 million volts per centimeter. When the laser beam is focused within the interior of the subject crystal the energy creates unattached electrons also known as “free” electrons. These “free” electrons, accelerated by the electric field created by the laser beam causes the high energy electrons to collide with atoms and ions in the focus area. As the process continues it causes a chain reaction and produces about 1 million trillion free electrons per cubic centimeter in about 1 trillionth of a second. The laser then emits a short pulse beam of a few billionths per second and produces a tiny micro crack. The laser head then align and position tens of thousands of additional micro cracks to create 2 or 3 dimensional images. Although, the laser generates power densities of 10 billion watts per square centimeter, the surface of the crystal is not damaged due to the highly transparent nature of optically perfect crystal. The resulting images appear to be suspended within the crystal