Gigahertz Phononic Integrated Circuits Based on Overlay Slot Waveguides

Ziyao Feng, Yang Liu, Xiang Xi, Lai Wang, and Xiankai Sun

Phys. Rev. Applied 19, 064076 – Published 27 June 2023

Abstract

Harnessing the properties of phonons on a chip, phononic integrated circuits are playing an increasingly important role in optoelectronic integration. To date, most of such phononic waveguides are realized by etching a piezoelectric layer and forming a wire, which not only involves complicated fabrication processes but also limits integration with electronic and photonic elements on the same chip. To overcome these difficulties, here we propose and demonstrate a type of overlay slot phononic waveguide obtained by patterning a thin silicon cladding layer on an unetched gallium-nitride-on-sapphire substrate. We experimentally demonstrate the guiding and power splitting of gigahertz surface acoustic waves in such waveguides, with a measured propagation loss of about 1.80 dB/mm for the Rayleigh mode. We also theoretically predict that the Love mode in such waveguides can turn into a phononic bound state in the continuum under special conditions. Such waveguides introduce an alternative paradigm for phononic integrated circuitry and will enable applications in information processing, environmental monitoring, and noninvasive manipulation of biomolecules on a chip.

Received 31 October 2022
Revised 6 March 2023
Accepted 17 April 2023

DOI:https://doi.org/10.1103/PhysRevApplied.19.064076

Physics Subject Headings (PhySH)

Acoustic phonons Bound states in the continuum Piezoelectric devices

Device fabrication Phononic crystals Semiconductors Waveguides

Surface acoustic wave

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Ziyao Feng ^1,†, Yang Liu ^2,†, Xiang Xi¹, Lai Wang ², and Xiankai Sun ^1,*

¹Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China
²Beijing National Research Center for Information Science and Technology (BNRist), Department of Electronic Engineering, Tsinghua University, Beijing 100084, China

^*xksun@cuhk.edu.hk
^†Z. Feng and Y. Liu contributed equally to this work.

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Vol. 19, Iss. 6 — June 2023

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Images

Figure 1
Cross-sectional illustrations of suspended (a) and nonsuspended (b)–(d) phononic waveguides. Nonsuspended scheme includes overlay strip (b), topographic rib (c), and overlay slot (d) waveguides.
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Figure 2
(a) Conceptual illustration of the proposed overlay slot phononic waveguides for gigahertz phononic integrated circuits. (b) Cross section of the phononic waveguide structure and distribution of effective velocity of the supported acoustic waves. (c),(d) Displacement profiles of the Rayleigh mode (c) and Love mode (d) of the phononic waveguide. (e) Propagation loss as a function of the waveguide width w for the Rayleigh mode and Love mode. (f) Frequency of the propagating acoustic wave as a function of the $Ga N$ layer thickness for the Rayleigh mode and Love mode. Insets plot the modal profiles of the Rayleigh mode and Love mode with h_GaN= 5.5 µm. (g) Propagation loss as a function of the $Ga N$ layer thickness for the Rayleigh mode and Love mode. In (b),(f),(g), the waveguide width is w = 9.2 µm.
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Figure 3
(a) Optical microscope image of a fabricated 90°-bend device. (b) Scanning electron microscope image of a fabricated S-bend device. (c) Magnified scanning electron microscope images showing details of an interdigital transducer. (d) Originally measured and processed ∣S₂₁∣ transmission spectra for a 90°-bend device with a bend radius of 500 µm. Inset plots the measured response in the time domain. (e) Measured ∣S₂₁∣ transmission spectra for S-bend devices with bend radii of 150, 175, and 300 µm. Inset plots the measured ∣S₂₁∣ transmission at an acoustic frequency of 1.47 GHz. (f) Measured ∣S₂₁∣ transmission spectra for straight-waveguide devices with waveguide lengths L = 0.2, 0.4, 0.6, and 1.0 mm. (g) Measured and fitted ∣S₂₁∣ transmission as a function of the waveguide length L at an acoustic frequency of 1.47 GHz.
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Figure 4
(a) Cross-sectional illustration of the topographic rib waveguide structure. (b) Rayleigh mode’s propagation loss as a function of the $Ga N$ layer thickness h_GaN for the slot waveguide and rib waveguide with a bend radius of 300 µm. Insets plot the modal profiles of the Rayleigh mode in the two types of phononic waveguides. (c) Measured ∣S₂₁∣ transmission spectra for slot-waveguide devices where two IDTs are connected with an S bend or a straight waveguide. (d) Measured ∣S₂₁∣ transmission spectra for rib-waveguide devices where two IDTs are connected with an S bend or a straight waveguide.
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Figure 5
(a) Illustration of a power splitter based on a multimode directional coupler. (b) Modal profiles of the fundamental and first-order Rayleigh modes for the multimode phononic waveguide. (c)–(e) Measured T₁₂ and T₁₃ transmission spectra for the multimode waveguide structure with lengths L = 100 µm (c), 300 µm (d), and 500 µm (e). (f) Numerically simulated and experimentally measured ratio of acoustic intensities between that at port IDT 3 and the sum of those at ports IDT 2 and IDT 3.
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Figure 6
Numerical results of the Love mode of the overlay slot waveguides. (a) Illustration of a straight waveguide. (b) Propagation loss of the Love mode in the straight waveguide as a function of the waveguide width w and wavelength λ. The propagation loss approaches zero (black regions) for certain combinations of parameters, where the BIC condition is satisfied. (c) Propagation loss as a function of the waveguide width w at wavelengths λ = 2.45, 2.50, and 2.55 µm. (d) Illustration of a bent waveguide. (e) Propagation loss as a function of the waveguide width w and bend radius at λ = 2.50 µm. Similar to the straight waveguide, the propagation loss approaches zero (black regions) for certain combinations of parameters, where the BIC condition is satisfied. (f) Propagation loss as a function of the bend radius at λ = 2.50 µm with w = 8.5, 9.0, and 9.5 µm.
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