26 research outputs found
Broadband lightweight flat lenses for longwave-infrared imaging
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
Unique prospects of graphene-based THz modulators
The modulation depth of 2-D electron gas (2DEG) based THz modulators using
AlGaAs/GaAs heterostructures with metal gates is inherently limited to < 30%.
The metal gate not only attenuates the THz signal (> 90%) but also severely
degrades the modulation depth. The metal losses can be significantly reduced
with an alternative material with tunable conductivity. Graphene presents a
unique solution to this problem due to its symmetric band structure and
extraordinarily high mobility of holes that is comparable to electron mobility
in conventional semiconductors. The hole conductivity in graphene can be
electrostatically tuned in the graphene-2DEG parallel capacitor configuration,
thus more efficiently tuning the THz transmission. In this work, we show that
it is possible to achieve a modulation depth of > 90% while simultaneously
minimizing signal attenuation to < 5% by tuning the Fermi level at the Dirac
point in graphene.Comment: 15 pages, 3 figures, 1 tabl