Strong transmittance above the light line in mid-infrared two-dimensional photonic crystals

The mid-infrared region of the electromagnetic spectrum between 3 and 8μm hosts absorption lines of gases relevant for chemical and biological sensing. 2D photonic crystal structures capable of guiding light in this region of the spectrum have been widely studied, and their implementation into miniaturized sensors has been proposed. However, light guiding in conventional 2D photonic crystals is usually restricted to a frequency range below the light line, which is the dispersion relation of light in the media surrounding the structures. These structures rely on total internal reflection for confinement of the light in z-direction normal to the lattice plane. In this work, 2D mid-infrared photonic crystals consisting of microtube arrays that mitigate these limitations have been developed. Due to their high aspect ratios of ∼1:30, they are perceived as semi-infinite in the z-direction. Light transmission experiments in the 5-8μm range reveal attenuations as low as 0.27dB/100μm, surpassing the limitations for light guiding above the light line in conventional 2D photonic crystals. Fair agreement is obtained between these experiments, 2D band structure and transmission simulations.