Taking advantage of the piezoelectricity of lithium niobate, we achieve nonreciprocal transmission of 10 decibels for a 200-MHz surface acoustic wave using parity-time- symmetric resonators and demonstrate one-way circulation of acoustic waves

Hu, Yaowen

Lončar, Marko

Maity, Smarak

Mao, Wenbo

Shao, Linbo

Sinclair, Neil

Yang, Lan

English

Caltech Authors - Main

FTh4Q.2.pdf CLEO 2020 © OSA 2020
Nonreciprocal Acoustic Transmission using Lithium Niobate 
Parity-Time-Symmetric Resonators 
Linbo Shao1, Wenbo Mao1,2, Smarak Maity1, Neil Sinclair1,3, Yaowen Hu1, 
Lan Yang2, and Marko Lončar1 
1John A. Paulson School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138, USA 
2Department of Electrical and Systems Engineering, Washington University, St. Louis, MO 63130, USA and 
3Division of Physics, Mathematics and Astronomy, and Alliance for Quantum Technologies (AQT), California Institute of Technology, 1200 E. 
California Boulevard, Pasadena, CA 91125, USA 
shaolb@seas.harvard.edu (L.S.); loncar@seas.harvard.edu (M.L.) 
Abstract: Taking advantage of the piezoelectricity of lithium niobate, we achieve nonreciprocal 
transmission of 10 decibels for a 200-MHz surface acoustic wave using parity-time-
symmetric resonators and demonstrate one-way circulation of acoustic waves. 
1. Introduction
Phonons have emerged as versatile on-chip information carriers with applications ranging from microwave filters to
transducers. Nonreciprocal devices are desired to control and route these high-frequency phonons. Nonreciprocal
phonon transmission can be achieved by breaking the time-reversal symmetry of propagating waves. Previous
demonstrations, which employ bulk media based on circulating fluids [1] and superlattices with nonlinear media [2,
3], were limited to acoustic frequencies below a few MHz. Meanwhile, ferromagnetic materials demonstrate
nonreciprocal phonon device at microwave frequencies but with a weak isolation [4]. Here, we construct a nonlinear
acoustic parity-time(PT)-symmetric system and enable nonreciprocal transmission. We achieve a nonreciprocity of
10 decibels for a 200-MHz surface acoustic wave (SAW) and further demonstrate one-way circulation of SAW by
cascading nonreciprocal devices.
2. PT-symmetric SAW resonators
The PT-symmetric SAW system consists of two coupled SAW resonators defined by the Bragg mirrors (Fig. 1(a)).
The Bragg mirrors provide more than 30 dB reflectivity over a bandwidth of 8 MHz around a frequency of 200 MHz
(Fig. 1(b)). The SAW resonators exhibit three resonant modes with intrinsic quality factor up to 104 within the
reflection band of the Bragg mirror. The interdigital transducers (IDTs) within the resonator are connected to external
electronic circuits to provide the gain or loss for the SAW. Specifically, the variable gain IDT is connected (using
wire bonding) to a negative resistance electric circuit implemented by an operational amplifier with feedback resistors,
and the loss IDT is connected to a tunable electronic resistor. Two more IDTs are situated outside the coupled resonator
system and are used as an emitter and a receiver of SAWs, for measurements of acoustic transmission.
3. Results
We numerically simulate the nonreciprocal SAW transmission in the broken PT-symmetric region (Fig. 1(c)). The PT
symmetry breaking induces a stronger localization in the active resonator for the backward propagating wave than
that of the forward propagating wave. The stronger localization results in a lower gain due to the saturation and leads
to a lower transmission in the backward direction.
 Our PT-symmetric SAW devices operate in the unbroken or broken symmetry regimes by varying the number of 
Bragg mirror grooves (i.e. the coupling strength) between the two resonators. An 80-groove (30-groove) mirror is 
used for measurements in the nonreciprocal broken (reciprocal unbroken) PT-symmetric regime. A nonreciprocity of 
10.9 dB is observed in the broken PT-symmetric regime (Fig. 2(a)), while a reciprocal transmission is observed in the 
unbroken-PT-symmetric regime (Fig. 2(b)). 
 Further, we demonstrate one-way SAW circulation using two nonreciprocal devices. Expected circulating behavior 
is observed at the resonant frequency of the device (Fig. 3). A SAW from Port 1 is preferably transmitted to Port 2 
(S21). At the resonant frequency, a 20 dB higher transmission is observed in the clockwise direction S21 than that in 
the counterclockwise direction S31 (Fig. 3(a)). Similarly, when a SAW is excited from Port 2, a 10 dB higher 
transmission to Port 3 is observed than that to Port 1 (Fig. 3(b)). 
4. Conclusion
We demonstrate a compact piezoelectric platform on lithium niobate for non-Hermitian and nonreciprocal acoustics.
The operating frequency can be adjusted from a hundred megahertz to a few gigahertz by geometrically scaling the
design [5]. Our work would enable exploration of acoustic non-Hermitian physics and extend acoustic signal
processing functionalities for next-generation wireless communication.
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FTh4Q.2.pdf CLEO 2020 © OSA 2020
 
Fig. 1. Nonreciprocal phonon transmission using nonlinear PT-symmetric SAW resonators. (a) Schematic and microscopic image of our 
coupled SAW resonator system for nonreciprocal transmission. Loss and gain are introduced in the passive and active resonators, 
respectively. IDTs are used to create gain, loss and to generate and receive SAWs. (b) Measured transmission spectra of one device with a 
strong coupling in the unbroken PT-symmetric (reciprocal) regime and the other device with a weak coupling in the broken PT-symmetric 
(nonreciprocal) regime. (c) Numerical simulations of the magnitude of elastic displacement due to SAWs propagating through broken PT-
symmetric (nonreciprocal) resonators in the forward and backward directions.  
 
Fig. 2. (a) Nonreciprocal and (b) reciprocal transmission measurements of the SAW resonators in the broken (unbroken) PT-symmetric 
regime. The microwave powers applied to input IDTs are -25 dBm (3 μW) in both plots.  
 
Fig. 3. One-way circulation of SAW. Transmission measurements among different ports.  
Acknowledgment  
This work is supported by the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319, NSF CQIS Grant No. ECCS-1810233, 
ONR MURI Grant No. N00014-15-1-2761, AFOSR MURI Grant No. FA9550-14-1-0389. N.S. acknowledges support by the Natural Sciences and 
Engineering Research Council of Canada (NSERC), the AQT Intelligent Quantum Networks and Technologies (INQNET) research program, and 
by the DOE/HEP QuantISED program grant, QCCFP (Quantum Communication Channels for Fundamental Physics), award number DE-
SC0019219. 
References 
1. R. Fleury, et al., "Sound Isolation and Giant Linear Nonreciprocity in a Compact Acoustic Circulator," Science 343, 516-519 (2014). 
2. B. Liang, et al., "An acoustic rectifier," Nat. Mater. 9, 989-992 (2010). 
3. B. Liang, et al., "Acoustic diode: rectification of acoustic energy flux in one-dimensional systems," Phys. Rev. Lett. 103, 104301 (2009). 
4. T. Nomura, et al., "Phonon Magnetochiral Effect," Phys. Rev. Lett. 122, 145901 (2019). 
5. L. Shao, et al., "Phononic Band Structure Engineering for High-Q Gigahertz Surface Acoustic Wave Resonators on Lithium Niobate," Physical 
Review Applied 12, 014022 (2019). 
 
Authorized licensed use limited to: CALIFORNIA INSTITUTE OF TECHNOLOGY. Downloaded on September 22,2020 at 00:43:19 UTC from IEEE Xplore.  Restrictions apply. 


Nonreciprocal Acoustic Transmission using Lithium Niobate Parity-Time-Symmetric Resonators

Nonreciprocal Acoustic Transmission using Lithium Niobate Parity-Time-Symmetric Resonators

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