It was May 1996 when we were walking in Alcobendas, Madrid, to find a place
to enjoy a coffee. Once we got updated on personal matters, we were ready to
jump into the scientific and technical issues that always light a spark with us.
The topic that day was the capability of very tiny pieces of metal, deposited on a
dielectric substrate, to resonate at optical frequency in the manner of the old-fashioned
antenna. A research stay at the ETHZ in Switzerland showed that,
besides the exotic application of optical frequency multiplication for time
standard measurements, antennas could move upward in the frequency domain
and get into the optical regime with awesome capabilities yet to be explored.
This was probably the first conversation on optical antennas that we had.
Since that walk, significant advances have been made in this field, and in some
of them, the efforts made at the Infrared System Lab of CREOL (University
of Central Florida), at the Applied Optics Complutense Group (Universidad
Complutense de Madrid), and at the Physics and Optical Science Department
(University of North Carolina at Charlotte) have contributed to a better
understanding and application of this area.
Optical antennas, infrared antennas, antenna-coupled detectors, plasmonic
antennas, nano-antennas, resonant optics, meta-surfaces. . . are all
different names representing the same idea: how the electromagnetic interaction
of light and a wise combination of material and geometry may provide
new elements and devices that did not previously exist. However, naming the
devices we were studying was not our main concern, nor was the visibility of
our findings. The Infrared System lab has been primarily devoted to
complying with the sponsors’ requirements and supporting the students
graduating from the lab. These 20 years of activity in this field have been
mainly focused on the practical application of antenna-coupled devices and
resonant structures.
At times we have been tempted to present this topic as a mere translation
of the radiofrequency and microwave antenna principles to optical frequencies.
Although optical antennas inherited some basic ideas from their lowerfrequency
counterparts, there are couple of very substantial differences. First
of all, materials, especially metals, that behave as ideal conductors in the
radiofrequency regime become dispersive at optical frequency. Charge carriers are not able to follow the incident electric field, and new phenomena,
sometimes degrading device performance, are at play. Secondly, the size of the
elements able to resonate at optical bands is in the range of tens and hundreds
of nanometers. We are then in the nanoworld with all that goes with it. Mesooptics
and nano-photonics are the two areas that aid in understanding this
technology, where the systems are moving closer and closer to the quantum
realm. Besides, fabrication technologies in the nanoscale are complex, hard to
master, and extremely expensive. Therefore, computational electromagnetism
plays a decisive role in sorting out the best possible design that finally goes to
the nanofabrication lab. These labs mainly involve electron-beam lithography,
focused ion beam, and all of the auxiliary techniques for the precise
deposition of materials. These techniques, although well established and
capable, produce geometries departing from the specification to some extent,
with the result of a loss of performance. The fabricated devices are tested
using dedicated characterization setups, and the measured discrepancies
require further refinement of the models and simulations.
The purpose of this book is to document the advances in technology
achieved by our research groups over the period of approximately 1995–2016 in
the area of infrared antennas and frequency-selective surfaces. Our approach
was primarily experimental and consisted of three main thrusts: design,
fabrication, and testing. The goal was always to have operational devices to
measure, and the comparison between the designed-for and measured
performance “closed the loop,” giving additional insight and guidance for
device optimization. This volume summarizes a large part of our activity in
antenna-coupled detectors, optical antennas, and resonant structures, including
frequency-selective surfaces, resonant polarization elements, and reflectarrays.
Our main goal is to show the reader the practical issues related to the design,
modeling, fabrication, and testing of these devices.
Our over-riding principle was to implement and demonstrate radiofrequency
technology at infrared frequencies, using electron-beam lithography.
Translation of longer-wavelength designs to infrared by wavelength-ratio
scaling of dimensions provides a starting point, but modifications are needed
because of the non-ideal properties of the metals in the infrared, and because
of the effect of the substrate refractive index.
The two main types of devices were antenna-coupled sensors that had an
electrical connection for signal readout, and frequency-selective surfaces that
were purely passive devices without electrical connection. Owing to the
practical difficulties of establishing connections between the antenna structure
and the sensor, the passive devices were always easier to work with. We did
not pursue antenna-coupled emitters, so antenna-coupled devices were always
seen from a sensor perspective. We were not typically concerned with field
enhancement for its own sake, but rather for the response of the sensor.
The authors want to recognize those persons who over the years have
contributed to making possible the type of applied research that is described
here: Mohamed Abdel-Rahman, Jeffrey Bean, Christos Christodoulou, Iulian
Codreanu, Kevin Coffey, Alexander Cuadrado, Jeffrey D’Archangel, Aristide
Dogariu, Mahmoud Elshorbagy, José Miguel Ezquerro, Louis Florence,
William Folks, Bill Franklin, Chi Fu, Christophe Fumeaux, James Ginn, José
Antonio Gómez-Pedrero, Francisco Javier González, Michael Gritz, Robert
Hudgins, Edward Kinzel, Fritz Kneubühl, Dale Kotter, Peter Krenz,
Brian Lail, Nuria Llombart, José Manuel López-Alonso, Donald Malocha,
Tasneem Mandviwala, Juan Carlos Martínez-Antón, Christopher
Middlebrook, Charles Middleton, Brian Monacelli, Camilo Moreno, Daniel
Mullaly, Ben Munk, Robert Olmon, Robert Peale, Wolfgang Porod, James
Potter, Menelaos Poutous, Irina Puscasu, Markus Raschke, José María
Rico-García, Jason Schaefer, William Schaich, David Shelton, Manuel
Silva-López, Brian Slovick, Jeff Tharp, Eric Tucker, Sam Walsworth, Arthur
Weeks, Scott Williams, and Guy Zummo.
Finally, it is joy to also recognize the unconditional support and
encouragement that we have received at home from Paz, Irene, Maggie,
and Eddie. Their contributions to the work presented here were indispensable.
Javier Alda
Glenn Boreman
July 2017
