Lasing-threshold condition for oblique TE and TM modes, spectral singularities, and coherent perfect absorption

Abstract

We study spectral singularities and their application in determining the threshold gain coefficient g((E/M)) for oblique transverse electric or magnetic (TE/TM) modes of an infinite planar slab of homogenous optically active material. We show that g((E)) is a monotonically decreasing function of the incidence angle theta (measured with respect to the normal direction to the slab), while g((M)) has a single maximum, theta(c), where it takes an extremely large value. We identify theta(c) with the Brewster's angle and show that g((E)) and g((M)) coincide for theta = 0 (normal incidence), tend to zero as theta -> 90 degrees, and satisfy g((E)) theta(c) is smaller inside the gain region than outside it. The converse is true for the TM waves with theta < theta(c) and all TE waves.