Is There a Perfect Lens?
Daniel S. Burgess
According to researchers at the University of Texas at Austin and at Consejo Superior de Investigaciones Cientificas in Madrid, Spain, the reports of the death of the diffraction limit may have been greatly exaggerated. In independent reports, their calculations contradict those of John B. Pendry of Imperial College in London, who theorized that negative refractive index materials would amplify the evanescent components of the wave field and thereby act as perfect, lossless lenses.
Prashant M. Valanju and colleagues in Texas argue that, if such materials displayed negative refraction, fundamental principles such as causality and finite signal speed would not hold. Instead, they postulate, although phase diffraction may be negative through negative refractive index materials, group refraction will remain positive, distorting any signals and leading to dispersion.
Nicolas Garcia and Manuel Nieto-Vesperinas in Spain also note that dispersion prevents negative refraction. They concede that an ideal lens made of a negative refractive index material would amplify the evanescent waves -- if it were infinitely thick. A real optical element, however, has two interfaces, which, they calculate, force the waves to decay just as they do in positive refractive index media.
When Photonics Spectra spoke with Pendry in 2001, the physicist noted that the theories needed to be tested in practical experiments. And in April of that year, a team at the University of California, San Diego, in La Jolla reported an index of refraction of approximately -2.7 for 10.5-GHz microwave radiation in a cellular array of copper split-ring resonators.
Valanju's group dismisses that work, positing that it confused near-field effects for a negative refractive index. Nevertheless, more research -- theoretical and experimental and addressing dispersion -- may be required to put the issue to rest.
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