The second innovation is to emulate a charged-particle beam by means of a waveguide with a periodic array of slots, instead of using real charged particles [Fig. 1, right]. By doing so, Xi et al. solved the problem of extremely weak Čerenkov radiation in the microwave frequencies associated with moving charged particles. As an electromagnetic wave travels inside the waveguide, it emerges at each slot with a fixed phase retardation relative to the neighboring slots. This leaking radiation from the waveguide is equivalent to the radiation from a phased antenna array. The Fourier transform of the electric current carried by a moving charged particle results in a broad spectrum in the frequency domain. But as long as a single frequency is concerned, the current of the charged particle is equivalent to that of a phased dipole array, as mathematically proved by the authors. In this analog, the phase velocity of the electromagnetic wave propagating inside the waveguide corresponds to the moving speed of the charged particles in a regular Čerenkov radiation configuration.
In the experiment, the authors designed a waveguide with comparatively low refractive index of n ~0.5, which is equivalent to a charged particle moving with a speed twice as great as that of light in a vacuum. With this new experimental configuration, the radiation signal can be many orders of magnitude stronger than the traditional Čerenkov radiation induced by a fast charged-particle beam, and thus the Čerenkov radiation was directly observed along the backward direction within the left-handed frequency range of the metamaterials. It is worth noting that Grbic and Eleftheriades carried out an experimental attempt earlier at the University of Toronto, in which they observed the backward radiation from a low-index left-handed microwave transmission line into free space . However, considering the fact that it is the phase velocity rather than group velocity of the electromagnetic wave propagating in the transmission line that corresponds to the speed of an equivalent moving charge, the Čerenkov radiation observed by Grbic et al. was indeed in the forward direction with respect to the direction of the equivalent moving charge.
I guess this is fine, but I'd say that one still needs to show this with actual particle beams. If this can't be done, then the claim that such phenomenon can be used as beam diagnostics doesn't quite hold.
Ironically, in the same issue of PRL, another paper gave a theoretical analysis of the detection of this reverse Cherenkov radiation of an electron beam passing into a left-handed metamaterial. So it would be nice if one can actually detect this direction from electron beams, rather than simulated ones.
 S.N. Galyamin et al. Phys. Rev. Lett. v.103, p.194802 (2009).