Subwavelength ultrasonic circulator based on spatiotemporal modulation

Enabling efficient nonreciprocal acoustic devices is challenging, yet very desirable for a variety of applications, including acoustic imaging, underwater communications, energy concentration and harvesting, signal processing,and noise control. We discuss the theory and design of a fully linear compact acoustic circulator based on spatiotemporal modulation of the effective acoustic index, providing a compact and practical way to realize large sound circulation at any desired frequency. Our proposal enables tunable isolation levels of over 40 dB, with insertion losses as low as 0.3 dB, in a noise-free, integrable, frequency scalable device whose total size does not exceed λ/6.