$\"Fig.$ <\/a>

Fig. 1. (a) SEM of a SiNx membrane (~ 350-nm thick) with SNSPD on top. (b) Optical micrograph of a membrane-SNSPD that was transferred and aligned to a Si waveguide on a PIC chip. (c) Instrument response function (IRF) of a membrane-SNSPD transferred onto a secondary substrate. The IRF was measured using a mode-locked fiber-coupled laser with sub-ps-pulse-width and 1550-nm wavelength. We measured sub-35-ps timing jitter for detectors with IC > 13 \u03bcA.<\/p><\/div>\n

Superconducting nanowire single-photon detectors (SNSPDs)^[1<\/a>]<\/sup> based on niobium nitride (NbN) nanowires have shown high speed (< 3ns dead time^[2<\/a>]<\/sup> ) and timing resolution (sub-30-ps timing jitter^[3<\/a>]<\/sup> ), making SNSPDs a leading single-photon detection technology in the near-infrared range.<\/p>\n

Recently SNSPDs have attracted interest as components in photonic integrated circuits (PICs), which enable the generation, manipulation, and detection of non-classical photons on a single chip. Previous approaches to integrating SNSPDs with photonic structures^[4<\/a>]<\/sup>^[5<\/a>]<\/sup>^[6<\/a>]<\/sup> were compatible with only a handful of substrate materials and required additional fabrication steps on the sample. These steps can be incompatible with complex and delicate PICs. Based on a micron-scale flip-chip concept^[7<\/a>]<\/sup>, we have developed a technology that enables integration of SNSPDs on PICs without exposing the PIC to chemicals or high temperatures. We used this method to integrate SNSPDs with silicon waveguides designed for 1550-nm center wavelength.<\/p>\n

NbN-SNSPDs based on sub-100-nm-wide nanowires were fabricated on top of a SiN_{x<\/sub>-on-Si substrate. The underlying silicon was removed via isotropic dry etch in XeF_{2<\/sub>, resulting in SNSPDs on suspended ~350-nm-thick SiN_{x<\/sub> membranes. The scanning electron micrograph (SEM) of a membrane-SNSPD is shown in Figure 1(a). Micro-manipulated tungsten probes were used to pick up the membrane and visually align it to the waveguide on the PIC chip. Figure 1(b) shows a membrane-SNSPD after transfer onto a waveguide.<\/p>\n}}}

Preliminary measurements on membrane-SNSPDs that were transferred on a secondary substrate showed that the contact between the membrane and the secondary substrate was sufficient to cool the detector in a continuous-flow cryostat, where we measured sub-35-ps timing jitter, as shown in Figure 1(c). While the photonic structures demonstrated here are waveguides, this technology could be applied to other PICs that require on-chip integration and near-field single-photon detection.<\/p>\n