The need for higher performance fiber optic telecommunications receivers has provided the impetus for substantial progress during the last decade in the understanding and performance of InP-based linear mode avalanche photodiodes (APDs) for the wavelength range from 1.0 to 1.7 um. However, these advances have not been paralleled in the performance and availability of single photon avalanche diodes (SPADs) based on similar design and materials platforms. Moreover, the limited work that has been done to date has been focused on optimizing devices for telecommunications wavelengths in the vicinity of 1550 nm, and there has been even less effort towards devices for use at 1064 nm. For this SBIR program, we propose to apply innovative design concepts for the development of high performance SPADs optimized for 1064 nm applications. In particular, we will implement novel bandgap and electric-field engineering approaches to tailor the SPAD avalanche gain properties to realize higher single photon detection efficiency while maintaining the very low dark count rates that are made possible by optimizing the absorption region design for the detection of 1064 nm photons. We will apply design concepts that we have innovated during Phase I of this program (as well as in the course of developing state-of-the-art 1550 nm SPADs) that involve optimization of the device electric profile for photon counting as well as epitaxial layer compositions. These efforts will culminate in 1064 nm large area detectors (with active area diameters up to 500 um) that demonstrate feasibility in meeting NASA performance targets including 50% detection efficiency, bandwidth of 500 MHz, saturation levels of 50 Mcounts/s, and non-gated operation. We will also use the detectors developed to deliver linear SPAD arrays for use as photon-counting line-scan imagers.