Figure 4. Reversing the unidirectional transparency at 5.3 kHz.

(a) Calculated transmission (blue) and reflections from the gain side (orange) and the loss side (red) with varying spacing between the loss and gain materials from zero to half wavelength. Two exceptional points are observed when the spacing is 1.24 and 1.71 cm, respectively. (b,c) Transmission and reflections from the gain and loss sides associated with 1.24-cm spacing. Black, blue, red and green colours denote the incidence, transmission, reflection from the gain side and reflection from the loss side, respectively. Solid curves are calculated values, and marked dots are from measurements. The same as in Fig. 2, unidirectional transparency is observed from the loss side. (d,e) Similar representation to b and c, but with 1.71-cm spacing. The direction of unidirectional transparency is reversed. Nearly zero reflection from gain side and 70% reflection from the loss side are observed.