  Unidirectional invisibility of a PT-symmetric optical system is of great
interest, but challenging as well since it is infeasible to fulfill it through
wide optical frequency ranges in all angular directions. Accordingly we study
reflectionless and invisible patterns in the TE and TM modes of an optical slab
system consisting of adjacent or separated pair of balanced gain and loss
layers with a gap. We provide a comprehensive study of one of the simplest
experimentally accessible examples of a unidirectionally reflectionless and
invisible PT-symmetric optical slab system. We obtain the physically optimal
conditions for the realization of these phenomena. We derive analytic
expressions, and show that only certain gain amounts restricted to take values
between certain minimum and maximum values give rise to uni/bi-directionally
invisible configurations. The size of gap decides the measure of
reflectionlessness and invisibility parameters, especially on gain value and
incident angle.
