Browsing by Author "Popa, Bogdan-Ioan"
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Item Open Access Phase conjugation and negative refraction using nonlinear active metamaterials.(Phys Rev Lett, 2010-09-17) Katko, Alexander R; Gu, Shi; Barrett, John P; Popa, Bogdan-Ioan; Shvets, Gennady; Cummer, Steven AWe present an experimental demonstration of phase conjugation using nonlinear metamaterial elements. Active split-ring resonators loaded with varactor diodes are demonstrated theoretically to act as phase-conjugating or time-reversing discrete elements when parametrically pumped and illuminated with appropriate frequencies. The metamaterial elements were fabricated and shown experimentally to produce a time-reversed signal. Measurements confirm that a discrete array of phase-conjugating elements act as a negatively refracting time-reversal rf lens only 0.12λ thick.Item Open Access Three-dimensional broadband omnidirectional acoustic ground cloak.(Nat Mater, 2014-04) Zigoneanu, Lucian; Popa, Bogdan-Ioan; Cummer, Steven AThe control of sound propagation and reflection has always been the goal of engineers involved in the design of acoustic systems. A recent design approach based on coordinate transformations, which is applicable to many physical systems, together with the development of a new class of engineered materials called metamaterials, has opened the road to the unconstrained control of sound. However, the ideal material parameters prescribed by this methodology are complex and challenging to obtain experimentally, even using metamaterial design approaches. Not surprisingly, experimental demonstration of devices obtained using transformation acoustics is difficult, and has been implemented only in two-dimensional configurations. Here, we demonstrate the design and experimental characterization of an almost perfect three-dimensional, broadband, and, most importantly, omnidirectional acoustic device that renders a region of space three wavelengths in diameter invisible to sound.