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Magnetic-free silicon nitride integrated optical isolator

Nature Photonics volume 15, pages 828–836 (2021)Cite this article

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Abstract

Integrated photonics enables signal synthesis, modulation and conversion using photonic integrated circuits (PICs). Many materials have been developed, among which silicon nitride (Si3N4) has emerged as a leading platform particularly for nonlinear photonics. Low-loss Si3N4 PICs have been widely used for frequency comb generation, narrow-linewidth lasers, microwave photonics and photonic computing networks. Yet, among all demonstrated functionalities for Si3N4 integrated photonics, optical non-reciprocal devices such as isolators and circulators have not been achieved. Conventionally, they are realized based on the Faraday effect of magneto-optic materials under an external magnetic field; however, it has been challenging to integrate magneto-optic materials that are not compatible with complementary metal–oxide–semiconductors and that require bulky external magnet. Here we demonstrate a magnetic-free optical isolator based on aluminium nitride (AlN) piezoelectric modulators monolithically integrated on low-loss Si3N4 PICs. The transmission reciprocity is broken by spatio-temporal modulation of a Si3N4 microring resonator with three AlN bulk acoustic wave resonators that are driven with a rotational phase. This design creates an effective rotating acoustic wave that allows indirect interband transition in only one direction among a pair of strongly coupled optical modes. A maximum of 10 dB isolation is achieved under 300 mW total radiofrequency power applied to three actuators, with minimum insertion loss of 0.1 dB. An isolation bandwidth of 700 MHz is obtained, determined by the optical resonance linewidth. The isolation remains constant over nearly 30 dB dynamic range of optical input power, showing excellent optical linearity. Our integrated, linear, magnetic-free, electrically driven optical isolator could be a key building block for integrated lasers and optical interfaces for superconducting circuits.

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Fig. 1: Principle of the nitride optical isolator.
Fig. 2: Characterization of optical and mechanical properties of the isolator device.
Fig. 3: Optical isolation and radiofrequency phases dependency.
Fig. 4: RF power dependency and anti-Stokes TM sideband generation.
Fig. 5: Influence of optical mode spacing, time-domain response and optical power linearity.

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Data availability

The code and data used to produce the plots within this work are available on Zenodo (https://doi.org/10.5281/zenodo.5120854). All other data used in this study are available from the corresponding authors on reasonable request.

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Acknowledgements

This work was supported by US National Science Foundation’s RAISE TAQS program under grant no. PHY 18-39164, by NSF QISE-Net under grant no. DMR 17-47426, by the Air Force Office of Scientific Research under award no. FA8655-20-1-7009, by funding from the EU H2020 research and innovation programme under grant agreement no. 732894 (HOT), and by Swiss National Science Foundation under grant agreement no. 176563 (BRIDGE). Samples were fabricated in the EPFL center of MicroNanoTechnology (CMi), and Birck Nanotechnology Center at Purdue University. AlN deposition was performed at Plasma-Therm LLC. We thank Y. Shi for valuable discussions.

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  1. These authors contributed equally to the work. Hao Tian and Junqiu Liu.

Authors and Affiliations

  1. OxideMEMS Lab, Purdue University, West Lafayette, IN, USA

    Hao Tian & Sunil A. Bhave

  2. Institute of Physics, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland

    Junqiu Liu, Anat Siddharth, Rui Ning Wang, Terence Blésin, Jijun He & Tobias J. Kippenberg

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Contributions

H.T. and J.L. designed the devices. J.L., H.T. and R.N.W. developed the process and fabricated the samples, with the assistance from J.H.. H.T. performed the experiment and simulations, and analysed the data. A.S. performed the experiment on the overcoupled device with the assistance from T.B.. H.T. and J.L. wrote the manuscript, with input from others. S.A.B and T.J.K supervised the collaboration.

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Correspondence to Tobias J. Kippenberg or Sunil A. Bhave.

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Peer review information Nature Photonics thanks Chun-Hua Dong and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–10, Tables 1 and 2, Notes 1–11.

Supplementary Video 1

Experimental demonstration of optical transmission spectrum of TE and TM light in forwards and backwards directions.

Supplementary Video 2

Unidirectional transmission of optical pulses with 10 ns pulse width.

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Tian, H., Liu, J., Siddharth, A. et al. Magnetic-free silicon nitride integrated optical isolator. Nat. Photon. 15, 828–836 (2021). https://doi.org/10.1038/s41566-021-00882-z

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