Contents I. Introduction
Square optical resonators have attracted great attention for the potential applications of optical add–drop filters, microlasers and plasmon lasers [1]–[16]. Square resonators with flat resonator sidewalls can provide a long interaction length with the bus waveguides for the optical add-drop filters [1], [2]. Multimode resonances were observed in square fused silica microresonators [3], single spatial mode selection was realized in a layered square microresonator laser [4], and directional lasing was observed from a deformed square organic–inorganic hybrid glass microring cavity [5]. Mode wavelengths and field distributions were derived for confined modes in a square resonator and were compared with the numerical results of two dimensional (2D) finite difference time domain (FDTD) technique [7]. In addition, mode characteristics were numerically simulated for imperfect and deformed square resonators [8], and vertical loss was investigated by three dimensional (3D) FDTD technique [9]. Metallically coated square microlasers with single and multiple output waveguides were used to obtain unidirectional emission and multiport output from square resonator microlasers [12]–[15]. However, square resonator microlasers laterally confined by the p-electrode have complicated mode spectra [12], due to the additional mode confinement provided by the p-electrode metallic layer.
