Tailoring of electric dipoles for highly directional propagation in parity-time-symmetric waveguides

Photonic structures offer a flexible platform for studying and demonstrating parity-time (PT ) symmetry phenomena. In these platforms, electric dipoles are often used as accurate models for electromagnetic sources, and elliptical dipoles were shown to provide for directional mode excitation. Here we introduce tailored dipole sources for directional excitation in a PT -symmetric structure made of two coupled waveguides. By eliminating one mode in the device, a qualitatively different wave propagation on the two sides of the dipole is achieved. Interestingly, before the exceptional point, a linear dipole suffices to have mode beatings on only one side. Furthermore, beyond the exceptional point, gain can be created on one side only. Finally, at the exceptional point, a near-complete directionality can be achieved due to the mode merging. We explain these effects via a detailed analysis of the modes, and the subsequent mode excitation of the dipole is analytically described. In the end, these various types of contrasting phenomena offer possibilities for integrated photonics applications, routing setups, and lasing behavior.