Compact discs (CDs) literally scratch the surface of optical data storage. About 99.99 percent of a CD is wasted because only one side is used for recording the bits of computer code. Physicist Min Gu at Victoria University of Technology hopes to change this by adding depth to data storage. In the July 1999 issue of Optics Letters, he described how he used a new version of a three-dimensional recording technique and an inexpensive plastic to achieve effects previously attained only with rare and costly elements. Known as bit storage by two-photon excitation, the technique records data by causing changes in a substance's refractive index. When a photon strikes the electrons in a photorefractive material, it permanently redistributes the electrons and, hence, their pattern of electric charges. By changing the pattern, any light passing through is slowed and bent. If these refractive changes are localized enough, they can stand for the zeros and ones of digital information. Two-photon excitation enables writing, erasing and writing over the same spot on a photorefractive polymer. From left to right: A laser writes the letter "A"; a two-second burst of ultraviolet light erases it; the letter "B" is written over the site. The numerals marked "1" and "2" establish the positions of the letters. Unfortunately, whenever a beam of light passes through a photorefractive material, it affects everything in its path, not just the target molecules. Other researchers solved this problem by using two laser beams positioned at different angles and aimed at the target. Each beam was composed of photons that were not energetic enough by themselves to change the refractive index but that could do so if they hit the same molecule simultaneously. Instead of this two-beam method, Gu fired one Spectra-Physics Tsunami 800-nm Ti:sapphire ultra-short pulsed laser beam. Only the molecules at the focused beam waist received enough energy to cause two-photon excitation. Other molecules were not excited because the intensity was too low. He made another important change as well, replacing the standard lithium niobate crystal used as a photorefractive medium with a simple plastic, poly(n-vinylcarbazole). At $1000 for a 1-mm-thick wafer that is half the size of a postage stamp, the lithium crystal's prospects for commercial use were limited. In contrast, the plastic costs pennies for the same size wafer. Gu discovered that poly(n-vinylcarbazole) can store information just as well as the expensive crystal; 8 cu cm of this polymer can store 5 GB, or the same amount of information as a modest home computer's hard drive. Some obstacles remain. Recording each change, or bit, in the refractive index takes about 1 ms -- a slow pace when recording data on a CD. And after writing, erasing and rewriting at the same location 1000 times, the plastic's refractive index reverts to normal, causing the individual bits to degrade. But Gu said he is optimistic that this technology can break the two-dimensional barrier. He stated that while its current recording speed is slow, the plastic can be read in real time.