Backward propagation of surface slow light in photonic crystals through morphological diversity

We propose and demonstrate photonic crystals (PCs) providing backward-directional propagation of surface slow waves, which is significant for potential PC-based photonic applications. An effective pathway for backward directing of surface slow light along with the modification of other important characteristics is presented via implementing surface morphological diversity in PCs. With the surface orientation angle varying from 900 to 300, the newly appearing bands inside the band gap shift to higher frequencies, and negative group indices up to -100 are observed as the strong indication of backward propagation. Furthermore, dependence of the propagation direction on the surface corrugation angle has been verified via detailed time-domain analyses and microwave experiments using dipole source. As obtained from both the numerical and experimental results, for instance, the structure with 600 provides a well-defined backward propagation. In addition, normalized-delay-bandwidth-product can easily be modified by varying the surface orientation angle in the proposed structures according to the necessities of the application. Furthermore, the group velocity dispersion spectra extracted for each periodic structure exhibit considerably high-range near-zero values as 0.139 ps(2)/m at 900 for the range of 495.54-501.25 nm and 0.176 ps(2)/m at 850 for the range of 495.66-501.25 nm. Third order dispersion spectra also obtained for the proposed PCs show near-zero values as 0.098 ps(3)/m at 900 and 0.113 ps(3)/m at 850 in the corresponding frequency regimes. Facile control of the key characteristics such as backward-directed surface wave propagation in the periodic dielectric structures having morphological diversity serves a great potential for next-generation photonic applications.