When a wavelength source and an observer are moving in relation to each other, a fundamental frequency shift is observed called the Doppler effect due to wave compression or extension in a positive refractive-index media. In astronomy, for example, light is shifted to shorter wavelengths (blue shift) for objects moving toward Earth, while light is shifted to longer wavelengths (red shift) for objects moving away; in fact, Doppler shifts in spectral lines are used to study ejections from our own Sun. Soviet physicist Victor Veselago theorized in 1968 that in negative-refractive-index metamaterials, the opposite would occur: A red-shifted signal would be detected for a wavelength source approaching a detector, and a blue-shifted signal would be seen for a source moving away from a detector. Until now, the inverse Doppler effect had only been observed at radio frequencies. But a group of researchers from the University of Shanghai for Science and Technology (USST; Shanghai, China), Jiangxi Normal University (JNU; Nanchang, China), and Swinburne University of Technology (SUT; Melbourne, Victoria, Australia) have now observed the inverse Doppler effect at optical frequencies in a photonic-crystal metamaterial.
展开▼
