From what I recall, an optical hard drive is kind of like a multi-layer dvd. Data is stored in small images that are stacked on top of each other.
They are amazingly fast at retrieving data and the amount of data stored per unit volume is huge. The downside is that as of now the writing process is painstakingly slow.
fckm:
http://online.cctt.org/physicslab/co...re/Compton.asp
When the photon collides with the crystal in this example, it re-emerges from the crystal with a longer wavelength and therefore less energy and less momentum. Are you saying that the photon with the longer wavelength is not the same one that collided with the object?