In 2007, superconducting nanowire single photon detectors (SSPD or SNSPD)[1] made an outstanding impact in the field of quantum information technology by demonstrating quantum key distribution (QKD) over a 200-km optical fiber with a 42-dB optical loss using a practical SNSPD system [2]. This successful demonstration was realized thanks to its extremely low dark count rate (DCR) of a few Hz and short timing jitter of 60 ps, while the system detection efficiency (SDE) showed a poor value of 0.7% at a wavelength of 1550 nm. Afterwards, significant efforts have been devoted to the improvement of SDE in practical systems, and remarkable advancements have been continually achieved in the past decade. The SDE of SNSPD is primarily determined by the product of three factors: optical coupling efficiency (ηoc), absorption efficiency (ηabs), and intrinsic detection efficiency (ηint). Therefore, the advancements to increase each factor cumulatively resulted in the improvement of SDE. In the initial stage of the SNSPD research, the nanowire device consists only of an NbN superconducting nanowire on the substrate. In this case, the nanowire cannot absorb the incident photons efficiently, because the thickness of the superconducting nanowire is only few nanometers. To achieve high ηabs, Rosfjord et al.[3] demonstrated an integrated λ/4 optical-cavity structure on the superconducting nanowire, and this approach could be successfully adapted in practically available devices with relatively large sensitive area [4, 5]. The optical cavity design has evolved through various ap-