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American research group develops worlds first acoustic superlens
Article Type: Mini features From: Sensor Review, Volume 30, Issue 2
Recent years have witnessed a surge in interest in metamaterials – artificial materials which are engineered at the microscopic scale to exhibit properties which do not occur in nature. The most widely researched class are photonic metamaterials which exhibit negative refractive indices through modification of their electric permittivity and magnetic permeability; the properties that determine how a substance interacts with electric and magnetic fields. Perceived applications include so-called “superlenses” for ultra-high resolution optical imaging and, more fancifully, invisibility cloaking devices. The acoustic analogues of permittivity and permeability are mass density and elastic constant which, when suitably modified, can yield phononic metamaterials. A research team at the University of Illinois at Urbana-Champaign have recently developed such a material which has been used to construct the world’s first acoustic superlens.
Funded by the US Defence Advanced Research Projects Agency (DARPA), the group is led by Nicholas Fang (Figure 1), Professor of Mechanical Science and Engineering at Illinois. Writing in Physical Review Letters (Zhang et al., 2009), the team has reported the successful focussing of ultrasound waves in water through a flat metamaterial lens on a spot roughly half the width of a wavelength at 60.5 kHz using a planar network of fluid-filled, sub-wavelength Helmholtz cavity resonators that oscillate at ultrasonic frequencies. The system is analogous to an inductor-capacitor circuit in a photonic metamaterial but here transmission channels act as a series of inductors and the Helmholtz resonators act as capacitors. The device consists of an aluminium plate, 1 cm thick, 15 cm wide and 30 cm long, machined into which are two adjacent 40×40 arrays of Helmholtz resonators, each being less than 3 mm in diameter. The fluid-filled resonators are connected by a network of channels and in the left-hand array the volume of the resonators is around ten times that of one section of the connecting channels, while in the right-hand array the volume of the channels is some ten times greater than that of the resonators. The difference in the way that the pressure gradient builds up in these two differently constructed arrays is such that when an ultrasound wave travels through the fluid in the left-hand array it is positively refracted, whereas sound travelling through the right-hand one is negatively refracted. The group demonstrated this by emitting ultrasound waves at 60.5 KHz from a transducer with a 3 mm tip inserted in a hole in the left-hand array. They then mapped the resulting pressure field in the right-hand array by mounting a hydrophone onto a mechanical stage and then moved the stage around the array. They found that the converging waves from the left-hand array reached the interface with the right-hand array and then reconverged to form an image of the transducer point source with a resolution of half a wavelength of ultrasound in water (about 12 mm at 60 KHz), dimensions that agreed with a computer simulation of the experimental set-up.
Several applications are anticipated for the technology and as the lens displays variable focal lengths at different frequencies, improved 3D ultrasonic medical imaging could emerge as a key use. Fang argues that if the team can achieve sub-wavelength imaging then it may be possible to reduce the minimum spot size to about 0.1 mm, which is about the size of early stage tumours, potentially allowing the ultrasonic diagnosis and therapy of breast and prostate cancers much earlier than is currently possible. However, doing so requires a ratio of refractive indices of the two arrays of −1, so that the angles of incidence and refraction are equal for all rays and the rays can therefore be brought to a single focus. This is not possible in the current arrangement owing to machining errors but Fang believes that these can be eliminated. Other potential uses include high resolution non-destructive testing of concrete bridges, buildings and other structures and, no doubt reflecting the DARPA interest, acoustic cloaking devices, potentially rendering submarines invisible to sonar.
Zhang, S., Yin, L. and Fang, N. (2009), “Focusing ultrasound with an acoustic metamaterial network”, Physical Review Letters, Vol. 102 No. 194301, 4 p