Asymmetrical Reflection in Passive Non-Hermitian Structure

Alternating a lossy with a near-lossless material within a structure can lead to remarkable asymmetry in wave propagation. The field of passive (without control of gain and loss by external stimuli) non-Hermitian acoustics is quickly emerging. Here, we propose a simple system comprised of a single pair of lossless (metallic) and lossy (polymeric) resonators in the form of circular ~0.1 mm thin sheets – that demonstrates close to 100 % asymmetry in the acoustic reflection coefficient, while at the same time the transmission coefficients are identical. Experimental measurements and theoretical modeling over a wide range of audible frequencies (reflection and transmission coefficients from 1 kHz up to 6.3 kHz) reveal a nearly ideal sonic unidirectional reflector. Such asymmetric acoustic properties are valuable, for example, for concealing objects from sonars, while allowing communication.