  We experimentally demonstrate imaging in the longwave-infrared (LWIR)
spectral band (8um to 12um) using a single polymer flat lens based upon
multi-level diffractive optics. The device thickness is only 10{\mu}m, and
chromatic aberrations are corrected over the entire LWIR band with one surface.
Due to the drastic reduction in device thickness, we are able to utilize
polymers with absorption in the LWIR, allowing for inexpensive manufacturing
via imprint lithography. The weight of our lens is less than 100 times those of
comparable refractive lenses. We fabricated and characterized two different
flat lenses. Even with about 25% absorption losses, experiments show that our
flat polymer lenses obtain good imaging with field of view of ~35degrees and
angular resolution less than 0.013 degrees. The flat lenses were characterized
with two different commercial LWIR image sensors. Finally, we show that by
using lossless, higher-refractive-index materials like silicon, focusing
efficiencies in excess of 70% can be achieved over the entire LWIR band. Our
results firmly establish the potential for lightweight, ultra-thin, broadband
lenses for high-quality imaging in the LWIR band.
