  We designed, fabricated, and measured superconducting nanowire single-photon
detectors (SSPDs) with low filling factor which achieve high system detection
efficiency (SDE) and counting rate simultaneously. Numerical simulation reveals
that high optical absorptance is possible in SSPDs even for low filing factor
by tuning the device design. The SDEs of fabricated 18-50% filling factor SSPDs
were measured systematically, and all SSPDs showed high SDEs of 61-80% and the
lowest 18% filling factor SSPD achieved a high SDE of 69%.
