Non-diffraction Bloch modes in low-symmetric photonic crystals

Summary form only given. In this study, all-angle self-collimation capability of two-dimensional (2D) low-symmetric PCs is investigated. Breaking the rotational symmetry in PC primitive cell provides additional dispersive characteristics, i.e. anomalous equi-frequency contours (EFCs), especially at higher bands [4]. Low symmetric PCs were previously studied in exploring extraordinary self-collimation property [5]. The schematic of proposed PC structure with C2 rotational symmetry is shown in Fig 1(a) and the calculated EFCs for the fourth transverse magnetic (TM) mode is presented in Fig. 1(b). As can be seen from Fig. 1(a) unit cell consists of three dielectric PC rods with same refractive index equal to nr=3.13. Frequency domain calculations show that the almost flat regions of EFCs have a frequency range of a/λ= 0.570-0.660, which corresponds to a broad bandwidth of ~15%. The variation of transmittance for different incident angles is shown in Fig. 1(c). The electric field intensity distributions for the incident angles {0°,30°,60°} are shown in Figs. 1(d)-1(f), respectively. The calculated field intensities show the propagating beam is still well collimated without significant diffraction along the longitudinal direction. As can be seen from the results, the dielectric distribution profile and rotational symmetry of the PC unit cell play a crucial role on the dispersion characteristics. The unique dispersion property of designed PC occurs due to the anomalous nature of the EFCs that yield broadband and wide-angle (up to the incidence angle of 60°) non-diffracted wave guiding inside the structure where wide-angle excitement phenomenon is the inherent characteristic of the investigated PC structure. The crucial contribution of the proposed PC design is that one can engineer single-mode based optical devices with considerable low insertion loss such as sharp bends, interferometers and optical interconnects using square lattice low symmetry periodic structure without introducing any external or internal defects. Furthermore, the approach also removes the requirement for critical alignment of source and waveguide to route incident light inside the photonic structures.