  We present an approach to increase the effective light-receiving area of
superconducting nanowire single-photon detectors (SNSPD) by means of free-form
microlenses that are printed in situ on top of the sensitive detector area
using high-resolution multi-photon lithography. We demonstrate a detector based
on a niobium-nitride (NbN) nanowire with a 4.5 $\mathrm \mu$m $\times$ 4.5
$\mathrm \mu$m sensitive area, supplemented with a lens of 60 $\mathrm \mu$m
diameter. For free-space illumination at a wavelength of 1550 nm, the lensed
sensor has a 100-fold-increased effective collection area, which leads to
strongly enhanced system detection efficiency without the need for long
nanowires. Our approach can be readily applied to a wide range of sensor types
and effectively overcomes the inherent design conflict between high counting
speed due to short sensor reset time, high timing accuracy, and high
fabrication yield on the one hand and high collection efficiency through large
effective detection areas on the other hand.
