Invisibility or “cloaking” technology has been a popular plot device with science fiction writers dating back at least to H. G. Wells’s 1897 novella The Invisible Man, about a scientist who invents a way to change the human body’s refractive index to mimic that of air, absorbing and reflecting no light. The notion was re-popularized by Gene Roddenberry’s Star Trek T.V. series in 1966 as a technology used by the alien Romulans to make objects, such as spaceships or individuals, partially or wholly invisible to parts of the electromagnetic (EM) spectrum, thus rendering the Romulan Bird of Prey starships invisible. And again in 1997 in a different context in J.K. Rowling’s children’s novel Harry Potter and the Philosopher’s Stone, in which the eponymous protagonist receives an invisibility cloak as a Christmas present.
However, invisibility technology is on the verge of making the transition from science fiction to reality, with research by scientists at the University of Texas at Austin in the vanguard. Back in March, BioNews Texas’ Joni Mitchell reported that a team of UT Austin researchers had published a study in the New Journal of Physics entitled “Demonstration of an Ultra-Low Profile Cloak for Scattering Suppression of a Finite-Length Rod in Free Space,” (J C Soric et al 2013 New J. Phys. 15 033037, doi:10.1088/1367-2630/15/3/033037 ) showing that invisibility is actually possible in the real world.
The article’s abstract explains: “We present the first experimental realization and verification of a three-dimensional stand-alone mantle cloak designed to suppress the total scattering of a finite-length dielectric rod of moderate cross-section. Mantle cloaking has been proposed to realize ultralow-profile conformal covers that may achieve substantial camouflage, transparency and high-performance non-invasive near-field sensing. Here, we realize and verify a mantle cloak for radio-waves.”
Ms. Mitchell noted that although mass use for the invisibility cloak is still a distant possibility, the results of this experiment make it seem more likely than ever before, with the most obvious application of such a technology being in military research and development, where the ability to cloak soldiers and vehicles would have a devastating impact on the battlefield.
Indeed, in 2007, the British Army tested an “invisible tank,” using cameras and projectors to beam camouflage rendering of surrounding landscape onto the vehicle’s hull. However, in his timeline outlining the history of invisibility cloaking devices — fictional and real The Week science writer Chris Gayomali observes that the biggest problem with real-world attempts at making cloaking devices has been that they’ve been big, bulky, and cumbersome, needing lab support in order to work — but perhaps not for much longer, with the UT, Austin researchers having created an ultra-thin material just 0.15 mm thick, and citing Sebastian Anthony at ExtremeTech predicting that “it’s really only a matter of time until an actual invisibility cloak is realized.”
In their article “Do Cloaked Objects Really Scatter Less?” published in the journal Physical Review last July (DOI: 10.1103/PhysRevX.3.041005), PhD Student and Graduate Research Assistant Francesco Monticone and Professor Andrea Alù of The University of Texas at Austin Department of Electrical and Computer Engineering note that “from ancient times, humanity has been fascinated by the concept of invisibility, and recently, scientists have moved a step closer to bringing this idea to reality by exploiting engineered artificial materials, or metamaterials. Montecone and Alu observe that several recent studies have indeed shown that a properly tailored metamaterial cover can, in principle, render an object invisible when illuminated by an electromagnetic wave oscillating at the specific frequency of interest. Yet, experimental realizations and theoretical investigations have consistently shown that reducing the visibility of an object with a passive cloak in a specific window of the electromagnetic spectrum is generally accompanied by a drastic increase of its visibility in other frequency ranges.
In their paper, Mr. Monticone and Dr. Alù quantitatively assess the potentials and limitations of passive cloaks in terms of overall visibility, integrated over the entire frequency spectrum, finding that quite surprisingly, their results show that any linear, causal, and passive invisibility cloak, without special superconducting features, is deemed to increase the scattering and visibility of the original uncloaked object, when integrated over all frequencies, a result confirming that the most popular cloaking devices actually scatter more, not less, when considered over a sufficiently broad frequency range, allowing easy detection using, e.g., pulsed excitation, and more generally provide a quantitative measure to compare overall performance of different cloaking devices and generally assess their detectability. They conclude that these findings may open important research directions in the quest for invisibility, not only in the electromagnetic domain but also for acoustic, mechanical, and matter waves.
They go on to discuss the global scattering response of invisibility cloaks over the entire electromagnetic spectrum, from static to very high frequencies. Based on linearity, causality, and energy conservation, we show that the total extinction and scattering, integrated over all wavelengths, of any linear, passive, causal, and nondiamagnetic cloak, necessarily increase compared to the uncloaked case, and in light of that general principle provide a quantitative measure to compare the global performance of different cloaking techniques as well as discussing solutions that minimize the global scattering signature of an object using thin, superconducting shells. Montecone and Alu conclude that their results provide important physical insights on how invisibility cloaks operate and affect the global scattering of an object, suggesting ways to defeat countermeasures aimed at detecting cloaked objects using short impinging pulses. The PDF can be downloaded here: http://prx.aps.org/pdf/PRX/v3/i4/e041005
In a research letter entitled “Broadening the cloaking bandwidth non-Foster metasurfaces,” published Oct. 29 in the journal Physical Review, co-authored by Dr. Alu with colleagues Pai-Yen Chen and Christos Argyropoulos — also of UT Austin — introduces the concept and practical design of broadband, ultra thin cloaks based on non-Foster, negatively capacitive metasurfaces. The researchers note that using properly tailored, active frequency-selective screens that conform to an object — a technique within the realm of a practical realization — shows that it is possible to drastically reduce the scattering over a wide frequency range in the microwave regime in orders of magnitude broader than any available passive cloaking technology, and that the proposed active cloak “may impact not only invisibility and camouflaging, but also practical antenna and sensing applications.”
Dr. Andrea Alù is an Associate Professor and the David & Doris Lybarger Endowed Faculty Fellow in Engineering in the Department of Electrical and Computer Engineering at The University of Texas at Austin. He is affiliated with the Wireless Networking and Communications Group, an interdisciplinary center for research and education at The University of Texas at Austin with an emphasis on industrial relevance, and with the Applied Research Laboratories at the University of Texas at Austin.
Dr. Alù is also an Associate Editor of IEEE Antennas and Wireless Propagation Letters and Optics Express, Editor of Metamaterials and Advanced Electromagnetics and guest editor of several special issues on metamaterials and plasmonics. His
research specializes in metamaterials and plasmonics, with applications spanning microwaves, infrared, optical frequencies and acoustic waves, including nanooptics and nanophotonics, cloaking and transparency, nanocircuits and nanostructures modeling, miniaturized antennas and nanoantennas. His research has significantly contributed to the fields of plasmonic cloaking, optical nanoantennas and nanocircuits and to the theory and modeling of metamaterials. He is currently teaching graduate and undergraduate courses on electromagnetics and metamaterials, is currently seeking brilliant and highly motivated graduate students and postdocs to join his group at UT Austin.
UT ECE graduate student Francesco Monticone received an IEEE Antennas and Propagation Society Doctoral Research Award for his project proposal entitled “Molding the Scattering Response with Metamaterials and Plasmonics. ” The project aims to investigate how plasmonics and metamaterials can be exploited to engineer the scattering response with unprecedented efficiency and flexibility, leveraging on the analogies between scattering and circuit problems. Transplanting relevant concepts from circuit and filter design into the field of photonics may provide new physical insights into scattering phenomena and revolutionize the way we design a variety of devices and systems, for applications ranging from photovoltaics and energy harvesting, to sensing and invisibility.