  We investigate the detection efficiency of a spiral layout of a
Superconducting Nanowire Single-Photon Detector (SNSPD). The design is less
susceptible to the critical current reduction in sharp turns of the nanowire
than the conventional meander design. Detector samples with different nanowire
width from 300 to 100 nm are patterned from a 4 nm thick NbN film deposited on
sapphire substrates. The critical current IC at 4.2 K for spiral, meander, and
simple bridge structures is measured and compared. On the 100 nm wide samples,
the detection efficiency is measured in the wavelength range 400-1700 nm and
the cut-off wavelength of the hot-spot plateau is determined. In the optical
range, the spiral detector reaches a detection efficiency of 27.6%, which is
~1.5 times the value of the meander. In the infrared range the detection
efficiency is more than doubled.
