The National Institute of Information and Communications Technology (NICT, President: Dr. Masao Sakauchi) has succeeded in the development of flexible optical design method for superconducting nanowire single-photon detectors (SSPDs or SNSPDs).

This technique enables SSPDs with a broadband high detection efficiency reject a specific wavelength, and is effective for multidisciplinary applications in fields such as the quantum cryptography, fluorescence spectroscopy, and remote sensing that require high efficiency over a precise spectral range and strong signal rejection at other wavelengths.

This achievement appeared in the Scientific Reports (Nature Publishing Group) on October 24, 2016. The reported results have been partially obtained as a part of JST-SENTAN program and AMED-SENTAN program from April 2015.

We have developed SSPDs on dielectric multilayers and their optical design method, which enable us to design a variety of wavelength dependences of optical absorptance by optimizing the dielectric multilayer.

In order to achieve the high detection efficiency in SSPDs, it is crucial to optimize the optical absorptance for a target wavelength. In the conventional SSPDs, a simple cavity structure consisting of dielectric resonant layers with a mirror layer has been used.

This structure is relatively simple and can effectively achieve high absorptance at the target wavelength, and the wavelength dependencies of absorptance show a single peak structure. However, in this structure, it is difficult to realize the SSPDs with high efficiency over a carefully controlled spectral range, with rejection at other wavelengths to reduce the noises.

By adopting a new SSPD structure with dielectric multilayers, it became possible to design desired wavelength dependences of the optical absorptance. As materials of dielectric multilayer, silicon dioxide (SiO2) and titanium oxide (TiO2) were used, and the niobium nitride (NbN) superconducting nanowire was put on the dielectric multilayer.

The wavelength dependences of the optical absorptance in the nanowire could be designed by optimizing the layer number and thicknesses of each layer in the dielectric multilayer. We developed the SSPDs based on the optimized design, and experimentally demonstrated that the wavelength dependences of the detection efficiency follow the calculated results well.

Regarding the optical design method, in order to optimize the wavelength dependence of the absorptance effectively, we perform two-step simulation of the optical multilayer calculation and the finite element analysis.

In support of the SSPD measurements, the NICT team collaborated with Osaka University, Japan and the University of Glasgow, Scotland through the NICT internship scheme.

Research paper