Non-reciprocal and highly nonlinear active acoustic metamaterials

Abstract

Unidirectional devices that pass acoustic energy in only one direction have numerous applications and, consequently, have recently received significant attention. However, for most practical applications that require unidirectionality at audio and low frequencies, subwavelength implementations capable of the necessary time-reversal symmetry breaking remain elusive. Here we describe a design approach based on metamaterial techniques that provides highly subwavelength and strongly non-reciprocal devices. We demonstrate this approach by designing and experimentally characterizing a non-reciprocal active acoustic metamaterial unit cell composed of a single piezoelectric membrane augmented by a nonlinear electronic circuit, and sandwiched between Helmholtz cavities tuned to different frequencies. The design is thinner than a tenth of a wavelength, yet it has an isolation factor of 410 dB. The design method generates relatively broadband unidirectional devices and is a good candidate for numerous acoustic applications.

PT-Symmetric Acoustics

“…Other structures of one-way acoustic devices, such as asymmetric nonlinear bead chains, have been demonstrated thereafter [23]. By introducing nonlinear active electric circuits, with incoherent amplification, into the resonant unit cells in acoustic metamaterials, one can realize a nonreciprocal acoustic metamaterial with a large contrast ratio [24]. It is also noted that the acoustic one-way propagation in linear timevarying media is exploited [25].…”

“…The acoustic gain material with a negative imaginary part of the modulus has not yet been observed in nature, which, however, can be effectively realized by delicate feedback systems using the active sound-controlling apparatus [24]. Since the setup respects PT symmetry, the mass density and the real part of the bulk modulus are an even function of position z ¼ 0, while the imaginary part of the bulk modulus is an odd one.…”

“…Recent advancements in man-made materials ("metamaterials") have resulted in intriguing achievements in acoustic and phononic transport manipulation [1]. These discoveries include dynamic negative density and a bulk modulus [2][3][4][5][6][7][8], subwavelength imaging [9][10][11], acoustic and surface wave cloaking [12][13][14][15], a phononic band gap [16,17], extraordinary acoustic transmission [18,19], Anderson localization [20], and asymmetric transmission [21][22][23][24][25]. These works, so far, are based on the modulation of the real part of the acoustic parameters.…”

Subwavelength ultrasonic circulator based on spatiotemporal modulation

“…Plugging this expansion into (10), and considering the fact that the eigenmodes of 0 M have been normalized with respect to the scalar product…”

“…Large acoustic isolation has been obtained using option (i) in a non-linear medium paired with a frequency selective mirror [8,9], or with the help of nonlinear acoustic inclusions [10]; however, all these nonlinear solutions typically introduce severe signal distortions and only work for large acoustic intensities. According to the Casimir-Onsager principle of microscopic reversibility [11], linear isolation is possible if the system is biased with an odd-vector upon time reversal, just like the static magnetic field in the case of the Faraday isolator [option (ii)].…”