Global Cooling to Become Increasing Concern 1

Full spectrum incident and reflective (readmitted) electromagnetic (EM) radiation originating
from the sun provides a constant energy source to the earth. Approximately 30% of this
energy is reflected back to space from the atmosphere, 19% is absorbed by atmospheric
gases and reradiated to the earth’s surface in the mid-IR range (7-14 um), and 51% is
absorbed by the surface or organic life and reradiated at around 10 um [1]. The energy
reaching the earth in both the visible and IR regions and the reradiated IR energy are
under-utilized by current technology.

Several approaches have been pursued to harvest energy from the sun.  Conversion of solar
energy to electricity using photovoltaic cells is the most  common.  An alternative to
photovoltaics is the rectenna, which  is a combination of a receiving antenna and a rectifier.  
The initial rectenna concept was demonstrated for microwave power transmission by
Raytheon Company in 1964 [2]. This illustrated the ability to capture electromagnetic energy
and convert it to DC power at efficiencies approaching 84% [3].  Since then much research
has been performed to extend the concept of rectennas to the infrared and visible regime for
solar power conversion.  

Progress has been made in fabrication and characterization of metal-insulator-metal diodes
for use in an infrared rectenna [4-5].  It has been demonstrated that optical antennas can
couple electromagnetic radiation in the visible in the same way as radio antennas do at their
corresponding wavelengths [6].

The major technical challenges continue to be in developing economical manufacturing
methods  for large-scale fabrication of antenna-based solar collectors.  Further research is
required to improve the efficiency of rectification of antenna induced terahertz currents to a
usable DC signal.  The material properties and behavior of  antennas/circuits in the THz solar
regions need to be further characterized.