There is an old Science Fiction concept - slow glass(SG). 1/8 inch of SG could be formulated so it delayed light by 12 hours. So pieces were left in the sunlight to light houses at night. They even conceived of glass with a 1 year delay - that they made into bombs and artillery shells. Formulaic changes could give you years even. A fully charged piece had the energy of a year stored in it before it came out the other side, To make a bomb, a small charge would shatter it, releasing the energy stored in in a light beam of 1 kilowatt per square meter x one year is stored in a square meter piece. Quite an energy release on shattering. There were many other technical inventions that used this SG - if only. Many materials pass light at less than light speed, but is a small effect in the real world.