  We report on a direct generation of efficient and wavelength-tunable
third-harmonic generation (THG) from ultrapure electronic-grade (EG) diamond
crystals. Under an ultrafast infrared excitation at 1280 nm, the considerably
high optical conversion efficiency of ~ 0.7% at a THG wavelength of 427 nm is
obtained, and the THG signal can be tuned over ultra-broadband range from 420
to 730 nm. We argue that the THG efficiency is originating from minimum
absorption loss and phase-matching conditions in EG diamond. Enhanced THG from
EG diamond crystal represents a new paradigm for establishing efficient
diamond-based frequency converters, quantum sensing, and quantum communications
platforms.
