Ultrabroadband electromagnetically indefinite medium formed by aligned carbon nanotubes

Anisotropic materials with different signs of components of the permittivity tensor are called indefinite materials. The known realizations of indefinite media at ir and optical frequencies suffer from high absorption losses and narrow bandwidth. Here we show that periodic arrays of parallel carbon nanotubes (CNTs) can behave as low-loss indefinite media in the infrared range. We demonstrate that a finite-thickness slab of CNTs supports propagation of backward waves with small attenuation in an ultrabroad frequency band.