Experimental Demonstration of a Coherent Perfect Absorber with PT Phase Transition

School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom(Received 30 June 2013; revised manuscript received 14 December 2013; published 10 April 2014)We report the realization of a coherent perfect absorber, using a pair of passive resonators coupled to amicrowave transmission line in the background, which can completely absorb light in its parity-time (PT-)symmetric phase but not in its broken phase. Instead of balancing material gain and loss, we exploit theincident waves in the open system as an effective gain so that ideal PT symmetry can be established byusing only passive materials. Such a route will be effective to construct PT-symmetric metamaterials andalso tunable PT-symmetric optical elements in general. It also provides a flexible platform for studyingexceptional-point physics with both electric and magnetic responses.