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designed a device structure using a dielectric multilayer membrane shown in Fig.5 which enables exible design to be sensitive to light of wavelengths shorter than 1μm [13][14]. e wavelength of light absorbed by the supercon-ducting nanowire can be freely designed by varying the thicknesses of two kinds of dielectric membrane of dier-ent refractive indexes (we used SiO2 and TiO2).In order to save time spent on optimization of structure, we designed an optimization method by combining a matrix method and a nite element method. First of all, the thickness and period of SiO2 and TiO2 is optimized so that high optical absorptance is achieved at the desired wavelength for the structure with NbN thin lm that is not processed as a mono layer nanowire on the dielectric multilayer membrane, using the matrix method (for ex-ample, soware to optimize optical thin lm, Essential MacLeod, etc.). en, light wavelength dependency of optical absorptance covering polarization dependency of the nanowire made of NbN thin lm which is a part of the real SSPD structure is calculated using the nite element method (using soware such as COMSOL, etc.). A larger amount of time for calculation is reduced using both the matrix method and nite element method rather than using only the nite element method. e light wavelength dependency of optical absorptance is obtained by optimiza-tion of the structure of a dielectric multilayer, targeting wavelength of 650 ～ 900 nm by this method. e light wavelength dependency of optical absorptance obtained by optimizing structure of dielectric multilayer targeting wavelength of 650 ～ 900 nm by this method is shown in Fig. 5. It is seen that high optical absorptance is achieved in the wavelength range of 650-900 nm and low at other wavelengths. e optical absorptance obtained for the SSPD we manufactured and evaluated is presented in Fig. 5. e optical absorptance obtained from the experiment coincides well with the result obtained from calculation, which means that our optimization method is eective [13][14].ere are many theories for the origin of dark count rate of SSPD. e dark count rate of SSPD in a low bias region is mostly due to black body radiation at room temperature incident from optical ber [15]. As photon absorption at wavelengths other than that to be detected can be suppressed by using a dielectric multilayer, it is expected to be useful to decrease the dark count rate by black body radiation. We will apply a device structure using a dielectric multilayer to various light wavelengths in the future and verify the eectiveness from the viewpoint of decreasing the dark count rate.3.2Multi-pixel detectore detection eciency of Si APD for light wavelength of less than 1μm achieves 70%. So, in order for SSPD to extend its application elds, it is necessary to verify its advantages in not only high detection eciency but total performance such as maximum count rate, dark count rate, and jitter over other existing techniques.One of the merits of SSPD is the high maximum count rate. In principle, it is determined by the relaxation time of a quasi-particle of a hot spot generated by photon ab-sorption and it is supposed to be possible to operate at 1 GHz. However, in order to couple it with single-mode ber with a core diameter of about 10 μm, a photosensitive area of 15 μm × 15 μm is necessary. If the sensitive area is covered with superconducting nanowire of 100 nm width in meander, the kinetic inductance of nanowire LK reaches 1 μH. Hence, the dead time (time needed to recover to the state of next photon detection aer detecting a photon) of the SSPD is constrained by the ratio of Lk to load resistance R (Lk/R time constant) and the maximum count rate of the FiF5　SSPD with dielectric multilayer and its optical wavelength dependence of optical absorptance (a)(b)50060070080090010001100020406080100Optical absorptance and SDEmax (%)Wavelength (nm)  Simulation (TE)  ExperimentIncident photonSuperconducting nanowire (NbN)Dielectric multilayer(SiO2, TiO2)FiF4　Application field of single-photon detectorMedical fieldBiological fieldIndustrial fieldTelecommunicationMeasurement field‣Clinical examinationBlood test, biochemical test‣Nuclear medicine imagingFluorescence measurement‣Confocal microscopy‣Fluorescence correlation spectroscopyDiagnostics of semiconductor circuitand wafer‣Quantum Cryptography‣Space communication‣Light-detection and ranging (LIDAR)Academic field‣Quantum optics‣High-energy physicsSingle-photon detector4 Quantum Node Technology60　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)