Dr. Nils Diaz was a scientist working on different warp drive concepts at the Tanaline Propulsion Laboratory in the 24th century.
In 2370, Ensign Taurik used some of Diaz's theories on warp drive on the USS Enterprise-D by deliberately misaligning the plasma flow and found that it improved the ship's warp drive efficiency by seven percent. Chief engineer Geordi La Forge was familiar with his work, but not with this new theory on warp drive, which had not yet been released. Taurik was able to read a preliminary review of his work while at Starfleet Academy. (TNG: "Lower Decks")
Nils Diaz was named after "Lower Decks" writer René Echevarria's godfather. (Star Trek: The Next Generation Companion)