Unbounded spatial spectrum of propagating waves in a polaritonic wire medium

In this paper, we study a topological phase transition in a wire medium operating at infrared frequencies. This transition occurs in the reciprocal space between the indefinite (open-surface) regime of the metamaterial and its dielectric (closed-surface) regime. Due to the spatial dispersion inherent to a wire medium, a hybrid regime turns out to be possible at the transition frequency. Both such surfaces exist at the same frequency and touch one another. At this frequency, all values of the parallel wave vector correspond to propagating spatial harmonics. The implication of this regime is the overwhelming radiation enhancement. We numerically investigate the gain in radiated power for a subwavelength dipole source submerged into such medium. In contrast to previous works, this gain (called the Purcell factor) turns out to be higher for a parallel dipole than for a perpendicular one.