  Achieving high quantum efficiency (QE) with low dark count is essential for
highly sensitive photodetectors (PDs), including single photon avalanche
detectors (SPADs). However, high QE requires a thicker absorber region, which
leads to high dark current and noise, which in turn affects the detectivity of
PDs and the photodetection efficiency and dark count of SPADs.The holy grail of
photodetector and avalanche photodiode designs is to achieve highest QE with
thinnest absorber and still enable large avalanche to gain as needed. We have
developed a new design paradigm which exploits the coupling between dielectric
Mie resonance and transverse propagating waves in thin layers. The Mie
resonance launches the incident light at an angle in an ultrathin absorber, and
when coupled to transverse waves, the light propagates laterally and is fully
absorbed owing to the longer optical path. Consequently, with appropriate
choice of materials for a chosen wavelength, a high absorption(~90%) within
typically <100 nm absorber thickness is possible. For illustration, we apply
our approach to design Si-based detector operating at 810 nm and InGaAs-based
detector operating at 1550 nm and predict that the dark current at room
temperature is reduced at least by two orders of magnitude. In addition, the
lateral distances are often in a few microns and hence these designs can
potentially enable avalanching for a large optical gain.
