Three researchers at the Istituto di Ricerca sulle Onde Eletromagnetiche Nello Carrara (Firenze, Italy) have used methods already developed for optics to enable propagation of localized microwave packets over distances of tens of wavelengths at velocities in excess of the speed of light in a vacuum. Superluminal effects for evanescent waves have been previously demonstrated in both optical and microwave tunneling experiments over distances on the order of one wavelength. The optical distance has been further extended to the centimeter range by using X-shaped Bessel beams; Daniela Mugnai, Anedio Ranfagni, and Rocco Ruggeri report in that they have achieved a similar extension for microwaves [see Physical Review Letters 84, 4830 (2000)].
In scaling the optical experiment into the microwave wavelength range of about 3.5 cm, the researchers modulated an 8.6-GHz microwave carrier to provide rectangular pulses with rise- and falltimes of a few nanoseconds. The input signal was fed via a horn-antenna launcher through a circular slit with a mean diameter of 7-10 cm and into the focal plane of a spherical mirror with a 50-cm diameter and 12-cm focal length. A second horn antenna, placed 30-130 cm from the mirror focal plane, received both launched and reflected signals for detection by a two-channel digital real-time oscilloscope. For receiver-mirror distances on the order of tens of centimeters, transmission velocities were achieved that exceeded the speed of light in a vacuum by more than 5%. Contact Rocco Ruggeri at [email protected].