  Realizing an NbN superconducting nanowire single-photon detector (SNSPD) with
a 100% intrinsic detection efficiency (IDE) at the near-infrared wavelengths is
still challenging. Herein, we developed a post-processing method to increase
the IDE of NbN SNSPDs to near unity using a 20 keV helium ion irradiation. The
IDE enhancement was achieved owing to the ion-induced reduction of the
superconducting energy gap and the electron density of states at the Fermi
level, determined with the electrical and magnetic transport measurements. The
change in optical absorptance of the irradiated SNSPD was negligible as
confirmed by the measured optical reflectance and system detection efficiency
(SDE). Benefited with the IDE enhancement, the SDE of an irradiated device was
significantly increased from 49% to 92% at 2.2 K for a 1550 nm wavelength.
