Electromagnetic materials lacking local time-reversal symmetry, such as gyrotropic materials, are of keen interest and importance technologically. However, most natural materials that lack local time-reversal symmetry require strong external fields, and function only in a limited range of the electromagnetic spectrum. Ferromagnetic resonance requires lower intensity external fields, but is limited to the GHz frequency range. Magnetized plasma and Zeeman splitting of optical diode transitions provide gyrotropy at optical frequencies -albeit, at a weak level with high biasing fields. These constraints, together with associated large absorption peaks, have so far prevented large-scale application of nonreciprocal photonic systems. However, the advent of photonic crystals and metamaterials enabled synthesis of artificial composite materials, possessing previously nonexistent electromagnetic properties, such as negative indices of refraction.