Author(s): Christensen Johan
Classical systems containing cleverly devised combinations of loss and gain elements constitute extremely rich building units that can mimic non-Hermitian properties, which conventionally are attainable in quantum mechanics only. Parity-time (PT) symmetric media, also referred to as synthetic media, have been devised in many optical systems with the ground breaking potential to create nonreciprocal structures and one-way cloaks of invisibility. Here we demonstrate a feasible approach for the case of elasticity where the most important ingredients within synthetic materials, loss and gain, are achieved through electrically biased piezoelectric semiconductors . We study first how wave attenuation and amplification can be tuned, and when combined, can give rise to a mechanical PT synthetic media with unidirectional suppressed reflectance, a feature directly applicable to evading sonar detection . In the same context we also focus on the possibility to create a truly non-reciprocal device permitting ultrasonic signals to propagate along a one-way only .\n\nThe second part of my talk focused on topological insulators in a non-Hermitian environment. Acoustic analogues of electronic topological insulators have recently lead to a wealth of new opportunities in manipulating sound propagation with strikingly unconventional acoustic edge modes immune to backscattering. Earlier fabrications of topological insulators are characterized by a fixed geometry and a very narrow frequency response, which severely hinders the exploration and design of useful devices. Here we establish topologically protected sound in reconfigurable phononic crystals that can be switched on and off simply by rotating its meta-atoms without altering the lattice structure. Furthermore, we introduce non-Hermitian ingredients in terms of loss and gain in order to explore topologically protected acoustic valley pseudospin transport properties with amplifying or attenuating states [4,5].\n\n J. Christensen, M. Willatzen, V. R. Velasco, and M.-H. Lu, Phys. Rev. Lett. 116, 207601 (2016)\n\n S. A. Cummer, J. Christensen, and A. Alu, Nature Reviews Materials 1, Article number: 16001 (2016)\n\n A. Merkel, M. Willatzen, and J. Christensen, submitted (2017).\n\n Zhiwang Zhang, Ye Tian, Ying Cheng, Qi Wei, Xiaojun Liu, and Johan Christensen, submitted (2017).\n Mudi Wang, Johan Christensen, Zhengyou Liu, submitted (2017).
Name: Dr Johan Christensen