  In this paper several designs to maximize the absorption efficiency of
superconducting-nanowire single-photon detectors are investigated. Using a
simple optical cavity consisting of a gold mirror and a SiO2 layer, the
absorption efficiency can be boosted to over 97%: this result is confirmed
experimentally by the realization of an NbTiN-based detector having an overall
system detection efficiency of 85% at 1.31 micrometers. Calculations show that
by sandwiching the nanowire between two dielectric Bragg reflectors, unity
absorption (> 99.9%) could be reached at the peak wavelength for optimized
structures. To achieve broadband high efficiency, a different approach is
considered: a waveguide-coupled detector. The calculations performed in this
work show that, by correctly dimensioning the waveguide and the nanowire,
polarization-insensitive detectors absorbing more than 95% of the injected
photons over a wavelength range of several hundred nm can be designed. We
propose a detector design making use of GaN/AlN waveguides, since these
materials allow lattice-matched epitaxial deposition of Nb(Ti)N films and are
transparent on a very wide wavelength range.
