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An omnidirectional antireflection (AR) coating for a deep-ultraviolet (UV) AR band is designed and fabricated on the sapphire substrate of a deep-UV flip-chip light-emitting diode (LED) device. The two-layer AR coating uses the tailored- and low-refractive index nanoporous alumina fabricated by glancing-angle deposition methodology. The AR coating effectively matches the refractive indices between the air and sapphire substrate. At close-to-normal angles of incidence, this AR coating almost completely eliminates the Fresnel reflection at the sapphire/air interface of the deep-UV LED device. The resulting improvement of the light-extraction efficiency by 8% is in good agreement with the simulation results. For a total thickness of 172 nm for the two-layer AR coating, extinction was negligible (<2%). The results show that nanoporous alumina thin films are excellent tailored- and low-refractive index thin film materials for high-performance deep-UV AR coating applications.