Transmission enhancement in loss-gain multilayers by resonant suppression of reflection

Abstract

Using the transfer-matrix approach and solving time-domain differential equations, we analyze the loss compensation mechanism in multilayer systems composed of an absorbing transparent conductive oxide and dielectric doped with an active material. We reveal also another regime with the possibility of enhanced transmission with suppressed reflection originating from the resonant properties of the multilayers. For obliquely incident and evanescent waves, such enhanced transmission under suppressed reflection turns into the reflectionless regime, which is similar to that observed in the PT -symmetric structures, but does not require PT symmetry. We infer that the reflectionless transmission is due to the full loss compensation at the resonant wavelengths of the multilayers.

PT symmetry breaking in multilayers with resonant loss and gain locks light propagation direction

“…In concordance with Ref. [21], this regime can be called a lasinglike (or quasilasing) mode. It can be treated as a dynamical feature of the phase state of broken PT symmetry.…”

“…S4 [42]). At the exceptional point, we have a very long transient period and the stationary levels of reflection and transmission are again expected to occur in the long time limit [21]. These levels strongly differ from those calculated with the transfermatrix method due to saturation development, discussed further.…”

“…In the steady-state approximation, when the amplitudes of population difference, polarization, and field are time independent, one can use the effective permittivity of a two-level medium [21] …”

“…Equations (1)- (3) are solved numerically using the finitedifference time-domain (FDTD) approach developed in our previous publication [43] and recently adapted to study lossgain structures [21]. As an initial value of the population difference, we employ the pumping parameter, i.e., w(t = 0) = w eq .…”

“…A typical example is the reflectionless transmission and unidirectional "invisibility" for both normal [19,20] and oblique [21] incidence. However, the symmetry breaking at the exceptional points belongs exclusively to the PT symmetry domain.…”