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. 2016 Mar 30;7:11110. doi: 10.1038/ncomms11110

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(a) The measured complex-valued refractive indices of the loss material formed by the leaky waveguide at 5, 5.3, 5.7 and 6 kHz (open marks). The active gain material is tuned to have the refractive indices that are complex conjugate of those of the loss material at the corresponding frequencies (filled marks). (b) The measured amplitudes of the scattering matrices for the controlled exceptional points at these four frequencies with the appropriately tuned spacing between the loss and gain materials. The reflections from the loss side almost vanish and total transmissions are observed for all of the frequencies. The reflections from the gain side are nonzero.