Photonic Crystal Enables Flat-Lens Imaging
Daniel S. Burgess
A team of scientists at Northeastern University in Boston has demonstrated that it is possible to image using a flat photonic crystal lens that displays negative refraction. The work illustrates the potential of "left-handed materials" for applications at microwave and optical frequencies.
More than 30 years ago, Victor G. Veselago posited that materials with both a negative dielectric constant and permeability would display a negative refractive index, bending an incident ray by the same magnitude but in the opposite direction from the normal line as a typical material. He argued that this might enable the design of novel optical elements such as flat-surfaced lenses. Research into these left-handed materials -- and much debate over whether they could in fact exist -- was reinvigorated in 2000 with the suggestion by John B. Pendry of Imperial College of Science, Technology and Medicine in London that they might enable the construction of "superlenses" with a diffraction limit of zero.
Shortly thereafter, David Smith and Sheldon Schultz of the University of California, San Diego, demonstrated that properly constructed composite materials of split rings and wires in a circuit board substrate indeed can refract microwaves through a negative angle. Last year, independent groups at Massachusetts Institute of Technology in Cambridge, Boeing Phantom Works in Seattle and at Northeastern substantiated this report.
The MIT study further identified a focusing effect using a 6-cm-thick flat slab of such a metamaterial, but a high degree of attenuation, attributable to contamination from the fabrication process, prevented imaging. Moreover, it was noted at the time that the complicated design of such materials made it difficult to tailor their refractive index for a particular wavelength.
The Northeastern scientists overcame these problems by replacing the metamaterial slab with a photonic crystal, which offered them a greater degree of control at the design stage over the propagation of electromagnetic radiation through the structure. The photonic crystal comprises a 35 × 17.5-cm periodic array of 1.25-cm-long, 0.63-cm-diameter alumina rods, arranged with a lattice constant of 1.8 cm.
Transmission intensity maps of 9.3-GHz microwaves revealed a high-quality image of the source on the far side of the structure (see figure). Moving the microwave source by 4 cm resulted in a corresponding displacement of the image, indicating that the flat lens has no optical axis.
Researchers at Northeastern University have demonstrated imaging by a flat lens made of a left-handed photonic crystal. Courtesy of Srinivas Sridhar.
Srinivas Sridhar, director of the university's Electronic Materials Research Institute and lead author of the study, said that another advantage of photonic crystals over metamaterials is that the former should be scalable to operate at optical wavelengths. If so, he suggested, one can imagine a myriad of applications, including subwavelength imaging, scanning photon-tunneling microscopy, and devices such as ultrahigh-sensitivity phase shifters and optical switches.
Sridhar is optimistic: "The challenge is to fabricate a similar structure at submicron-length scales using nanofabrication techniques. This is certainly doable."
Currently, the team is investigating the use of left-handed metamaterials for microwave antennas. It also plans further experiments to explore the unique features of imaging with a flat photonic crystal lens and hopes to model new structures for optical wavelengths.
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