Efficient bidirectional piezo-optomechanical transduction between microwave and optical frequency

ARTICLE https://doi.org/10.1038/s41467-020-14863-3 OPEN Efficient bidirectional piezo-optomechanical transduction between microwave and optical frequency 1234567890():,; Wentao Jiang 1 ✉, Christopher J. Sarabalis1, Yanni D. Dahmani1, Rishi N. Patel1, Felix M. Mayor Timothy P. McKenna1, Raphaël Van Laer1 & Amir H. Safavi-Naeini1 ✉ 1, Efficient interconversion of both classical and quantum information between microwave and optical frequency is an important engineering challenge. The optomechanical approach with gigahertz-frequency mechanical devices has the potential to be extremely efficient due to the large optomechanical response of common materials, and the ability to localize mechanical energy into a micron-scale volume. However, existing demonstrations suffer from some combination of low optical quality factor, low electrical-to-mechanical transduction efficiency, and low optomechanical interaction rate. Here we demonstrate an on-chip piezo-optomechanical transducer that systematically addresses all these challenges to achieve nearly three orders of magnitude improvement in conversion efficiency over previous work. Our modulator demonstrates acousto-optic modulation with V π = 0.02 V. We show bidirectional conversion efficiency of 10
5 with 3.3 μW red-detuned optical pump, and 5:5% with 323 μW blue-detuned pump. Further study of quantum transduction at millikelvin temperatures is required to understand how the efficiency and added noise are affected by reduced mechanical dissipation, thermal conductivity, and thermal capacity. 1 Department of Applied Physics and Ginzton Laboratory, Stanford University, 348 Via Pueblo Mall, Stanford, CA 94305, USA. ✉email: ; NATURE COMMUNICATIONS | (2020)11:1166 | https://doi.org/10.1038/s41467-020-14863-3 | www.nature.com/naturecommunications 1 ARTICLE I NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-14863-3 t takes energy to sufficiently change the optical properties of a device or medium to impart information onto an optical field1,2. Electro-optic devices are engineered to minimize this energy by using low-loss optical waveguides and resonators that localize the optical field in a small volume, and reduce the amount of energy it takes to set up the electric fields needed for modulation3,4. Mechanical vibrations change the local optical properties with less energy than is typically possible via the electro-optic effect in common materials. Whereas voltages corresponding to  1010 microwave photons are typically needed in the most highly optimized electro-optic systems, only  104 microwave phonons of the same energy are needed in the best optomechanical systems5. However, this efficient modulation requires localization of both optical and mechanical energy into a wavelength-scale volume5. This complicates electrical driving of this localized mechanical motion and must be addressed by careful co-engineering of a piezo-optomechanical system. An energy-efficient modulator, whether electro-optic or piezooptomechanical, also operates as a quantum transducer between microwaves and light where a large optical pump coherently and reversibly couples an optical sideband to microwave-frequency excitations6–8. Such transducers may one day enable quantum networks that perform distributed quantum sensing and information processing9,10. There are many physical mechanisms that mediate the exchange of microwave and optical photons11. Among these, as in the classical case, mechanically mediated conversion offers strong coupling rates and low energy consumption5,12,13. Electro-optomechanical conversion using MHz-frequency mechanical membranes14,15 to mediate interactions between a Fabry–Pérot cavity and a superconducting microwave resonator has been demonstrated with 47% efficiency, and 38 added noise photons16. Desire for larger conversion rates, lower added noise, and lower heating have motivated investigations of integrated gigahertz-frequency devices which require less optical pump power to operate. Several approaches using aluminum nitride (AlN)17–20, silicon21,22, gallium arsenide (GaAs)23–25, and lithium niobate (LN)26–28 have been pursued, but the best end-to-end conversion efficiencies have remained on the order of 10
8 19. Efficient piezo-optomechanical modulation is challenging. The mechanical modes must be highly co-localized with the optical resonances to achieve high optomechanical interaction rates, while maintaining electrical access to the mechanical motion. Optomechanical crystals (OMC) provide a natural way to achieve the former by implementing a simultaneous photonic–phononic crystal to confine both optical and mechanical waves29. However, efficient electrical coupling to the micron-scale mechanical resonances of a phononic crystal has only recently been achieved30,31. These demonstrations leverage the high piezoelectric coefficient of lithium niobate32 and electrodes on or near the resonator to efficiently couple motion to high-impedance superconducting microwave circuits. Such an approach is difficult to integrate with photonic devices due the large optical absorption of metals, which ruins the optical quality factor and destroys the superconductivity. We overcome the low microwave-to-mechanical efficiency of previously demonstrated lithium niobate piezo-optomechanical crystals27 by integrating an interdigitated transducer (IDT) that excites a wavelength-scale mechanical waveguide33 to efficiently drive the localized mechanical mode of the OMC. Moreover, the phononic waveguide spatially separates the optical and microwave circuits, an important feature in a cryogenic setting where absorption in metals needs to be minimized. We characterize the device as a classical modulator where an incoming microwave signal at the mechanical mode frequency modulates the optical cavity resonance. In a separate experiment, we characterize the 2 potential of the device as a quantum transducer, by imparting a laser tone red-(blue-)detuned by a mechanical frequency from the optical resonance to introduce an interaction between photons resonant with the cavity and the mechanical motion of the device, which is coupled to the external microwave channel. We demonstrate bidirectional conversion between microwave and optical photons with quantum efficiency up to 10
5 (5:5%) using the red-(blue-)detuned optical pump. The integrated piezooptomechanical transducer is fabricated with X-cut thin-film lithium niobate on silicon (LNOS), a material platform demonstrated to be compatible with superconducting circuits and qubits30,31—opening a path for integration with quantum sensors and processors. Results Design. An incident microwave signal on the IDT is converted to a propagating mechanical wave in the second-order horizontal shear mode (SH2) in the transducer region. The mechanical wave is then scattered by the linear horn into the first-order longitudinal mode (L1) of a 1.3-µm-wide waveguide (Fig. 1b). From finite-element simulation and separate measurem (...truncated)