  In recent years, particular physical phenomena enabled by non-Hermitian
metamaterial systems have attracted significant research interests. In this
paper, a non-Hermitian three-dimensional metamaterial near the exceptional
point (EP) is proposed to demonstrate extremely asymmetric absorption and
reflection. Unlike its conventional counterparts, this proposed metamaterial is
constructed with a loss-assisted design. Localized losses are introduced into
the structure by combining our technique of graphene-based resistive inks with
conventional printed circuit board (PCB) process. Extremely asymmetric
absorption and reflection near the EP are experimentally observed by tuning the
loss between split ring resonators (SRRs) in the meta-atoms. Simultaneously, by
linking the equivalent circuit model (ECM) with the Hamiltonian quantum
physical model, the equivalent non-Hermitian Hamiltonian is obtained and a
non-Hermitian transmission matrix is constructed. We show that tuning the
structure and circuit parameters of the ECM produces a metamaterial system with
EP response. Our system can be used in the design of asymmetric metamaterial
absorbers. Our work lays down the way for the manipulation of EP to develop
perfect absorption, sensing and other applications in the 3D metamaterial
platform.
