The Other Left Hand Cavity Transducer Laser beam Listen carefully: In order to test for structural defects in an important material (an aircraft wing in the example shown), several transducers generate ultrasound waves that bounce around inside a cavity, transforming a brief, simple tone into a complex set of echo-like pulses. The wave pulses eventually emerge from a small opening and cause a vibration on the surface of the wing, and a laser records that vibration (upper waveform). Then the transducers are made to amplify and play the time reverse (lower waveform) of the previous laser-recorded sound. This time, the bounces within the cavity merge the complex pulses into a single burst powerful enough to reveal any hidden defects. as much as possible. But while the first time they get a lower-amplitude response from the cavity, the second time they get a higher-amplitude response. That latter vibration is once again recorded by a laser vibrometer pointed at the material’s surface. This time it contains the detectable harmon- ics that reveal internal defects. The noncontact acoustic source capable of producing evidence of internal defects is revolutionary, yet in one important respect, it almost designed itself: The cavity does not need to be exquisitely designed or constructed with exacting tolerances. Rather, a complex and misshapen cavity is ideal because it generates a convoluted and stretched-out ringing signal—one that becomes a concentrated burst with time reversal. As project leader le Bas explains it, “In a sense, the more imperfect our cavity design, the better the results.” — Craig Tyler It’s a case of technology imitating nature, only doing it one better. A team led by los Alamos researchers Antoinette Taylor and Hou-Tong Chen, collaborating with a group from UC Berkeley, took the concept of molecular chirality, or molecular handed- ness, and created a novel polarizer that can be dynamically switched to transmit either left- or right-circularly polarized radiation. There’s nothing quite like it, either in nature or in industry. Chirality refers to a lack of symmetry be- tween an object and its mirror image. Your hands, for example, are chiral; while mirror images of each other, to make your left hand resemble your right (not recommended) you have to remove your left thumb and reattach it to the base of the palm below the pinky, then remove and reattach your fingers in reverse order. Similarly, chiral molecules— those lacking any mirror symmetry planes— would require some rearrangement of atoms to be identical to their mirror images. They are of particular interest because they are optically active. A chiral molecule will rotate linearly polarized light in one direc- tion, while its mirror image will rotate the light the opposite way. Chiral molecules may also absorb one type of circularly polarized light more than the other. Taylor and Chen were looking to mimic this effect in metamaterials. Typically fab- ricated on a silicon-on-sapphire wafer, a metamaterial consists of tiny, custom- designed metal and silicon structures arranged in a pattern on the wafer’s surface. It is the interaction of those structures with electromagnetic waves that determines the metamaterial’s electromagnetic properties. The materials work particularly well for terahertz-frequency waves (THz), which occupy the portion of the electromagnetic spectrum between the microwave and far infrared. THz waves have been earmarked for several types of imaging applications, but many of the components needed to fully exploit the radiations—lenses, polarizers, amplifiers, switches, etc.—have yet to be developed. Chen and colleagues designed a meta- material structure that was inherently chiral and created a surface pattern such that the metamaterial transmitted left-circularly po- larized THz waves. Each structure contained two silicon pads that when illuminated by an infrared laser switched from being insula- tors to conductors. This effectively switched the structure to its opposite chirality, so the metamaterial transmitted right-circularly po- larized radiation. By pulsing the laser on and off, the transmitted THz waves would switch between polarizations or, if the incoming radiation was elliptically polarized, rotate its polarization axis. One possible application would be to probe biological systems and measure the relative abundance of right- versus left- chiral molecules. But says Chen, “We’ve never had this capability before. It opens up entirely new avenues of research.” And that deserves a hand. — Jay Schecker 25 µm 10 µm (Top) Scanning electron microscope image of the chiral structure of the fabricated metamaterial. (Bottom) The purple, blue, and yellow colors represent gold structures; the silicon pads (see text) are shown in green. CREDIT: MACMIllAn pUBlISHERS lTD: nATURE COMMUnICATIOnS 3:942, ZHAng, S. ET Al., COpYRIgHT 2012 1663 los alamos science and technology magazine october 2012 25