The International Congress on Advanced Electromagnetic Materials in Microwaves and Optics is a yearly event that provides a unique forum to share the latest results of the metamaterials research in Europe and worldwide and bring together the engineering, physics, and material science communities working on artificial electromagnetic materials and their applications at microwaves, millimetre waves, terahertz, and optical frequencies.
This year's edition (Metamaterials 2014) was held in Copenhagen, Denmark on the campus of the Technical University of Denmark. The inherent multidisciplinary nature of metamaterials was reflected in the broad range of research topics covered, which included artificial metamaterials and metamaterial surfaces for radio, microwave, terahertz, optics applications as well as acoustics, superconductor and quantum metamaterials. Special attention was given to the current and future impact of metamaterials on industry and commercial technology.
The congress program contained a healthy mix of keynote, invited, and regular oral and poster presentations that provided a good perspective on the current state of the field of metamaterials. The presentations were divided into three parallel regular sessions that followed the morning plenary session. In addition, six special sessions were dedicated to the most recent and hot trends in metamaterials research.
The growing importance of the acoustic, mechanical and elastic metamaterials field was reflected in the organizer's decision to devote an entire special session to it. I was happy to see presentations outlining new ways to design and fabricate the type of material structures needed to control mechanical waves. One of the main difficulties of current designs is that every acoustic or elastic metamaterial structure demonstrated so far has a significant weakness that makes their employment in real applications problematic. It was good to see how far our group has come to addressing this fundamental limitation.
During my 15 minute time slot, I presented a talk entitled "Unified approach to design linear and nonlinear acoustic metamaterials" that proposed a new way to design acoustic metamaterials. The method involves the use of active electronic circuitry connected to acousto-electric transducers such as piezoelectric materials to control the acoustic response of the metamaterial. I showed that this approach greatly expands the range of acoustic properties that can be engineered in these structures.
The technical content of the congress was complemented by quite a few social activities during which participants could interact in a more relaxed setting. I found particularly interesting the visit to the DTU-ESA Spherical Near-Field Antenna Test Facility situated on the campus of the Technical University of Denmark, a laboratory where the European Space Agency characterizes very accurately the antennas employed by various spacecrafts. The highlight of the social activities was a boat trip through the channels of Copenhagen during the last evening of the conference, which outlined the rich history and architecture of this old and beautiful northern European city.
Bogdan Popa, a Research Scientist with Steve Cummer's Lab in Electrical and Computer Engineering presented the following at this year's conference;
Title: Unified Approach to Design Linear and Nonlinear Acoustic Metamaterials
Short abstract: We review a new approach to design active acoustic metematerials that results in artificial media covering a large range of linear acoustic responses. We show how this method can be extended to obtain metamaterials having highly non-linear responses. We illustrate this approach experimentally by presenting the implementation of a highly sub-wavelength unidirectional media capable of breaking the time-reversal symmetry.
Summary: Recent research on acoustic metamaterials has provided convenient ways to engineer media having acoustic properties not found in common natural materials. Their development started by porting electromagnetics concepts to acoustics in fluids. More specifically, it started with the design of metamaterial structures that mimicked fluids having negative effective material parameters and anisotropy. The subsequent emergence of transformation acoustics, an advanced technique that promises to enable exotic devices such as the scattering reducing “acoustic cloak”, lead to significant improvements to the field of acoustic metamaterials.
However, current acoustic metamaterial designs have limited applicability emphasized by the scarcity of experimental demonstrations of full metamaterial based devices, and especially those designed using transformation acoustics. The inadequacy of current metamaterial approaches was further highlighted when the concept of unidirectional media capable of breaking the time-reversal symmetry was ported from electromagnetics to acoustics. It was shown that these media can in principle be implemented using materials having strong acoustic non-linear behavior. Despite that most fluids are naturally non-linear, obtaining such behavior at low wave levels and low frequencies in the audible range needed in numerous applications is a difficult task, because the fluid non-linear regime is accessed when the wave amplitude is comparable to the wavelength.
In this presentation we outline a metamaterial design technique that leads to both linear and non-linear metamaterial structures composed of arrays of highly subwavelength unit cells working at any frequency and sound level. Due to the number of potential applications, we focus here on metamaterials designed to mimic the behavior of anisotropic fluids, i.e. they are characterized by an isotropic bulk modulus and anisotropic mass density.