Dr. Nils Diaz was a scientist working on different warp field techniques at the Tanaline Propulsion Laboratory in the 24th century.

In 2370, Ensign Taurik ran a computer simulation aboard the USS Enterprise-D wherein he tested a new warp field configuration. In the simulation, Taurik deliberately altered the plasma flow to the warp nacelles, so that it was out of sync, and increased the warp field integrity by seven percent. He was simulating a technique first experimented on by Dr. Diaz. Chief Engineer Geordi La Forge was familiar with his work, but not with this new technique on warp field, which had not yet been released. Taurik was able to read a preliminary review of his work while at Starfleet Academy. (TNG: "Lower Decks")

This character was only mentioned in writing.
Nils Diaz was named after "Lower Decks" writer René Echevarria's godfather. (Star Trek: The Next Generation Companion)