Abstract
Negative-index materials (NIMs) support optical anti-parity-time (anti-) symmetry even when they are lossless. Here we prove the feasibility in achieving higher-order exceptional points (EPs) in lossfree waveguide arrays by utilizing the anti- symmetry induced by NIM. Numerical simulation about a third-order EP fits well with the coupled-mode theory. A scheme of achieving fourth-order EPs is also discussed. This work highlights the great potential of NIM in overcoming the obstacles of ordinary non-Hermitian optics, and the possibilities of combining anti-, , and Hermitian couplings for various purposes.
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