This year, the Center for Metamaterials and Integrated Plasmonics was represented at Phononics by Yaroslav Urzhumov, who is sharing with us some of the exciting things he learned there.
Sharm El-Sheikh is an exotic, tourist area distributed along the southeast coast of Sinai Peninsula, and it is easily accessible from Cairo or Europe by air. The conference venue was chosen wisely in a quiet neighborhood of Nabq, at a four-star resort of Laguna Vista Beach.
The five-day, all-serial conference program was packed with talks and events. The big ones were the Bloch Medal award, received by Prof. Bahram Djafari-Rouhani (Lille, France), and the Brillouin Medal award shared by Profs. Ping Sheng, Che Ting Chan and Zhiyu Yang, all three of Hong Kong, China. The associated Brillouin lecture was delivered by C.T. Chan, in which he talked about analogues of graphene in acoustics and the implications of Dirac cones in phononic metacrystals. The Bloch lecture and associated paper of Djafari-Rouhani discussed the exciting topic of the so-called phoXonic crystals – periodic structures that modulate dispersion properties of both phoTons and phoNons, thus the name. Simultaneous confinement of photons and phonons and their phase matching over long distances are just two effects achievable with phoxonic crystals, which are thought of as a promising medium for enhanced optomechanical effects.
The topics of talks were organized into five clusters: Phononic crystals, Phononic metamaterials, Phonon transport, Optomechanics, and Wave propagation in periodic structures. Below, I highlight several hand-picked topics, which is by no means a reflection of lesser importance of all other papers, which are uniformly excellent.
The session on Phononic crystals was opened with a plenary talk of Prof. Economou, who talked about the importance of phonon resonance scattering effects, destructive interference and their applications such as acoustic stop bands and vibration absorbers. Prof. Sigalas (Greece) presented his work on atomically engineered graphene-like structures containing a mixture of group-IV atoms such as carbon, silicon and germanium. His ab initio calculations of the band structure and phonon density of states (PDOS) showed that germanene, a germanium analog of graphene can be stable and possess wide band gaps.
The talk from Prof. Norris group (Rutgers Univ., USA) delivered by Dr. Hladky-Hennion discussed the unusual properties of foam-like metallic metacrystals suitable for underwater acoustics. Prof. Pennec (Lille, France) discussed applications of phoxonic crystals for sensing of liquids. Dr. Zwinteck (U-Arizona) described their development of ultrasoft polymer-based elastic metamaterials and their shear vibrational modes. The applications were not revealed, but with this work being sponsored by Toyota, we can hope it will give rise to ultra-soft cushions in the high-end Toyota cars next year, or something fancy like that. Prof. Park (Seoul, Korea) discussed his new approaches to phononic bandgap maximization using topology optimization. Gradient-based optimization of eigenvalue problems is a daunting task due to issues with band crossings, periodicity reduction and spurious eigenmodes, but Dr. Park found a way around all these hurdles and engineered a substantially enlarged band gap.
The day closed with two excellent talks about dynamic tunability in phononic crystals. Ms. Sklan (MIT) presented a magneto-acoustic scheme for phonon manipulation based on magnetostrictive materials, and Dr. Degraeve presented a new way to tune elastic crystals with electroded piezoelectric layers.
The Phononic Metamaterials sessions opened with a plenary talk by Prof. Ping Sheng, who talked about perfect acoustic absorbers (dampers) based on sub-wavelength resonances of low-frequency sound. Another plenary talk by Nick Fang, delivered by his assistant, was loaded with the most recent findings from his group, including acoustic cloaking and beam shaping devices. Prof. Gengkai Hu presented theoretical and experimental results for an acoustic superlens based on negative-mass metamaterials. Negative mass density and its relationship to surface “plasmons” was also a subject of Prof. Christensen’s (Denmark) invited talk.
The culmination of the day was the excitement-filled colloquium delivered by Prof. Sanchez-Dehesa (Spain). He showed lots of experimental results for unidirectional aeroacoustic cloaks both in two and three dimensions. Some of his cloaking structures are as ephemeral as heated air of Sinai desert, but others are as rigid as hard plastic used in 3D printers.
The Metamaterials session would not be fun without at least one hilarious blunder. When Dr. Lai started speaking about noise control applications of acoustic absorbers, the sound speakers formed a positive-feedback loop with his microphone, and noise went out of control, in a room filled with acoustics specialists. For the next Phononics (to be held in Paris, we hear), the speakers are advised to bring more absorbing metamaterials to prevent such mishaps.
The breadth of the expertise in this session was really impressive – from an application-oriented of Dr. Urzhumov and Dr. Zubtsov on poroelastic metamaterials for underwater acoustics to an exotic scientific proposal of Dr. Garcia-Raffi to use fractal dimensions in sonic crystals.
The third major thrust, phononic transport, was covered by a great selection of talks about phonon contributions to thermal conductivity, and methods to manipulate the latter. Dr. Reinke (Sandia National Labs) presented an approach for reducing thermal conductivity based on phononic superlattices, combinations of natural atomic and engineered mesoscopic lattices. Reduction of thermal conductivity is particularly important for thermoelectric devices, whose figure of merit is inversely proportional to the thermal conductance. It is equally important for thermoacoustics, where low thermal conductivity leads to more efficient mechanical wave generation from a heat flux. Prof. Hussein, one of the organizers, has demonstrated another way to utilize mesoscopic resonances in phononic crystals for the reduction of thermal conductivity.
Perhaps the most complicated and exciting thrust covered at the conference is Optomechanics, to which most of Day 4 was devoted. The plenary talks of Prof. Psarobas (Greece) and Prof. Jusserand (France) were concerned with phoxonic crystals and the enormous enhancements of optomechanical couplings available in such systems. Prof. Farhat (Saudi Arabia) presented a novel device that exploits extraordinary properties of graphene to efficiently couple phonons to light, with potentially enormous implications in biosensing. The Optomechanics session transitioned smoothly into Quantum Phononics, with outstanding invited talks delivered by Dr. Camacho (Sandia), Prof. Ludwig (Germany), Prof. Santos (Germany) and the organizer’s colloquium of Ihab El-Kady (Sandia). The latter talk introduced a novel quasiparticle dubbed “phoniton”, potentially useful in quantum computing, and discussed the group’s efforts to detect it.
The schedule of Day 4 was densely packed with other events: the poster session and the Bedouin Night banquet in the desert. At the latter, guests had to sit on the ground, Bedouin-style, while watching the mesmerizing motion of Arab dancers, an unforgettable first-time experience for many of the guests.
The closing Day 5 was slightly less attended, partly due to the temptations of nearby coral reefs and the renowned diving sites. Yes, the talks of the Periodic Structures session were just as interesting as all others. There were bio-inspired tunable metafluid cloaks (Prof. Baz, Maryland), topology-optimized band structures (Prof. Jensen, Denmark), chirped sonic crystals (Dr. Sanchez-Morcillo) and nonlinear waves in elastic crystals. All in all, it was one of the most interesting conferences in the world of metamaterials that I have attended (and I have been to quite a few), and the venue was not only exotic and entertaining, but also truly relaxing, as a world-class resort is supposed to be.