Infrared radiative properties of two-dimensional square optical black holes

Abstract

Optical black hole (OBH) is a special optical structure, in which the dielectric function or refractive index of media becomes gradually larger from the outside to the core. The circular optical black holes (COBH) have been proved remarkably useful for broadband omnidirectional light absorption. The main goal of this paper is to propose an alternative square structure of OBH due to fabrication consideration. The infrared radiative properties of two-dimensional SOBHs are numerically studied with geometric optics approximation (GOA) and the finite-difference time-domain (FDTD) method. A critical wavelength is found in this paper: when the wavelength is smaller than half of the inner core side length, the Poynting vectors obtained by the FDTD method agree well with the ray trajectories calculated by the GOA, and the absorptance is nearly 100% with a proper refractive index gradient. While the wavelength is increasing, the net energy flows do not agree with the ray trajectories and the absorptance will be decreased. The reason is attributed to the diffraction effect, which is discussed in detail in this paper.