Integrated Phononic Waveguides

• Integrated phononic waveguides are chip-scale structures that confine and guide coherent GHz mechanical waves using mechanisms like index guiding and phononic-crystal defects.
• They enable practical applications such as compact RF delay lines, filters, and resonators while interfacing with electronic, photonic, and quantum systems.
• Advances in platforms like GaN-on-SiC and LN-on-diamond support low-loss propagation and dynamic reconfigurability, essential for scalable, integrated acoustic circuits.

Searching arXiv for recent and foundational work on integrated phononic waveguides to ground the article. Integrated phononic waveguides are on-chip structures that guide coherent mechanical waves in close analogy to integrated photonic waveguides, but at radio-frequency and microwave acoustic wavelengths. Across current implementations, the term encompasses wavelength-scale ridge or strip waveguides that confine guided Lamb-, Rayleigh-, Love-, or Sezawa-like modes; phononic-crystal defect waveguides; and waveguides formed from periodically coupled micromechanical resonators. The field spans compact RF delay lines, microring and Fabry–Pérot resonators, directional couplers, dispersive filters, programmable signal-processing circuits, and hybrid interfaces to electrons, photons, superconducting qubits, and solid-state spins (Bicer et al., 2023, Bicer et al., 2021, Modica et al., 2020, Li et al., 25 Feb 2025, Wang et al., 4 Dec 2025).

1. Definitions and physical scope

Integrated phononic waveguides are most directly defined as the acoustic analog of integrated photonic waveguides: micron-scale structures that guide coherent GHz phonons with low loss on a chip (Bicer et al., 2023). In practice, the category includes several distinct physical realizations.

One class uses continuous solid-state waveguides with acoustic index guiding. In these systems, confinement is achieved because the guiding layer has lower acoustic phase velocity than the surrounding medium, so the acoustic field remains localized in the patterned core. GaN on SiC, GaN on sapphire, SiN on LN, AlScN on SiC, LN on diamond, and AlN/diamond all instantiate this principle in different geometries and frequency ranges (Bicer et al., 2021, Wang et al., 2020, Ji et al., 29 Mar 2026, Deng et al., 23 Mar 2025, Malik et al., 29 May 2025, Ding et al., 2023).

A second class uses phononic-crystal defect waveguides. In suspended GaAs-on-insulator, a 2D Kagome phononic crystal with a five-line defect supports a single guided mode around $2.061\,\text{GHz}$, with group velocity below $1000\,\text{m/s}$ and intrinsic filtering set by the band structure (Modica et al., 2020). In suspended LiNbO$_3$ thin films, boundary-induced chiral anomalous bulk states create bulk waveguides with unidirectional, low-loss transport and slow-wave delay functionality (Li et al., 25 Feb 2025).

A third class uses periodic arrays of mechanically coupled resonators. In MEMS drumhead waveguides, the waveguide is a 1D chain of drumhead resonators whose Bloch passbands, localization behavior, and thermoelastic-buckling-induced transmission switching are captured by a reduced-order model (Kanj et al., 2023). Earlier GaAs/AlGaAs phononic crystal waveguides formed from periodic suspended membranes already demonstrated guided mechanical bands, controllable group velocity, and coupling to a localized phonon cavity (Hatanaka et al., 2014). h-BN phononic crystal waveguides likewise implement guided RF phonons in an array of coupled nanomechanical resonators with pass and stop bands in the 15–40 MHz range (Wang et al., 2020).

This breadth implies that “integrated phononic waveguide” is not restricted to one mode family, one confinement mechanism, or one fabrication style. A plausible implication is that the unifying criterion is not topology of implementation but the presence of lithographically defined, chip-scale pathways for controlled acoustic propagation, routing, and interaction.

2. Guiding mechanisms and modal physics

The fundamental guiding mechanisms are vertical confinement, lateral confinement, and, in resonant geometries, whispering-gallery or cavity confinement. In GaN-on-SiC, vertical confinement comes from the velocity contrast between slow GaN and fast SiC, while lateral confinement arises from etched ridge sidewalls. At $f \approx 3.4\,\text{GHz}$ with $\lambda_a = 1.6\,\mu\text{m}$, the implemented waveguides use $t_{wvg} = 1.5~\mu\text{m}$ and $w_{wvg} = 1.8~\mu\text{m}$, so the dimensions are on the order of the acoustic wavelength, enabling tight confinement and low-loss bending (Bicer et al., 2023). The same slow-on-fast logic underlies earlier GaN-on-SiC strip waveguides for $>3\,\text{GHz}$ sound, where a GaN Lamb mode at $v \approx 5414\,\text{m/s}$ is slower than the nearest SiC mode by about $1300\,\text{m/s}$ over the full in-plane direction space (Bicer et al., 2021).

Mode content is highly platform-dependent. GaN-on-SiC and GaN-on-sapphire support guided Lamb-like, quasi-Rayleigh, and quasi-Love families; AlScN-on-SiC supports Rayleigh-like and Sezawa-like waveguide families; SiN-on-LN supports guided Rayleigh surface acoustic waves around $1000\,\text{m/s}$0; diamond waveguides support shearing and Rayleigh SAWs at $1000\,\text{m/s}$1–$1000\,\text{m/s}$2; and suspended membrane platforms support flexural or Lamb-like modes with strong geometric dispersion (Wang et al., 2020, Deng et al., 23 Mar 2025, Ji et al., 29 Mar 2026, Ding et al., 2023, Hirsch et al., 2023, Hirsch et al., 24 Mar 2026).

For dispersive waveguides and delay lines, the central kinematic relation is

$1000\,\text{m/s}$3

with $1000\,\text{m/s}$4 inferred from measured delay and physical length. In GaN spiral delay lines, $1000\,\text{m/s}$5 and $1000\,\text{m/s}$6 imply $1000\,\text{m/s}$7 (Bicer et al., 2023). In suspended GaAs-on-insulator, the defect mode in the Kagome waveguide gives measured $1000\,\text{m/s}$8 and $1000\,\text{m/s}$9, consistent with a simulated $_3$0 near the band edge (Modica et al., 2020). In BI-CABS LiNbO$_3$1, extracted group velocities remain below $_3$2 and reach as low as $_3$3 (Li et al., 25 Feb 2025).

Microring and ring-like resonators are the canonical resonant extension of an integrated waveguide. In GaN-on-SiC, the microring uses whispering-gallery-mode guidance by total internal reflection of sound along the circular interface, which minimizes excess dissipation relative to metal boundaries or phononic Bragg reflectors at multi-GHz frequencies (Bicer et al., 2023). In SiN-on-LN, ring resonators are formed from guided Rayleigh SAW waveguides; the measured free spectral range of $_3$4 at $_3$5 implies $_3$6 (Ji et al., 29 Mar 2026). In waveguide-cavity systems, the free spectral range obeys

$_3$7

for rings and

$_3$8

for Fabry–Pérot cavities (Bicer et al., 2021, Wang et al., 4 Dec 2025).

3. Materials platforms and device archetypes

The present landscape is best understood as a set of material-platform archetypes, each emphasizing a different balance among confinement, electromechanical coupling, loss, integration, and compatibility with active or quantum subsystems.

Platform	Guiding structure	Representative reported metrics
GaN on SiC	Ridge waveguides, microrings, spirals	$_3$9 at $f \approx 3.4\,\text{GHz}$0; delays $f \approx 3.4\,\text{GHz}$1 (Bicer et al., 2023)
Suspended GaAs-on-insulator	Kagome phononic crystal defect waveguide	$f \approx 3.4\,\text{GHz}$2; $f \approx 3.4\,\text{GHz}$3 delay (Modica et al., 2020)
Suspended LiNbO$f \approx 3.4\,\text{GHz}$4 thin film	BI-CABS bulk waveguide	nearly flat $f \approx 3.4\,\text{GHz}$5 in $f \approx 3.4\,\text{GHz}$6–$f \approx 3.4\,\text{GHz}$7; delays $f \approx 3.4\,\text{GHz}$8 (Li et al., 25 Feb 2025)
SiN on LN	Slot-defined guided SAW waveguides, couplers, rings	$f \approx 3.4\,\text{GHz}$9 single-mode loss; $\lambda_a = 1.6\,\mu\text{m}$0 ring (Ji et al., 29 Mar 2026)
AlScN on SiC	2D-confined strip waveguides	Sezawa-like $\lambda_a = 1.6\,\mu\text{m}$1; $\lambda_a = 1.6\,\mu\text{m}$2 (Deng et al., 23 Mar 2025)
LN on diamond	Thin-film LN rib on bulk diamond	$\lambda_a = 1.6\,\mu\text{m}$3 total insertion loss at $\lambda_a = 1.6\,\mu\text{m}$4; $\lambda_a = 1.6\,\mu\text{m}$5 transducer efficiency (Malik et al., 29 May 2025)
AlN/diamond	Ridge and suspended SAW waveguides	SAW transmission at $\lambda_a = 1.6\,\mu\text{m}$6–$\lambda_a = 1.6\,\mu\text{m}$7; cross section $\lambda_a = 1.6\,\mu\text{m}$8 (Ding et al., 2023)

GaN-on-SiC occupies a prominent position because it combines low-loss GHz acoustics with a semiconductor suitable for co-integrated electronics. Implemented structures include straight buses, $\lambda_a = 1.6\,\mu\text{m}$9 microrings, and spiral delay lines up to $t_{wvg} = 1.5~\mu\text{m}$0 in length with footprint $t_{wvg} = 1.5~\mu\text{m}$1 (Bicer et al., 2023). Earlier work already framed GaN-on-SiC as a route to monolithic RF front-ends, emphasizing $t_{wvg} = 1.5~\mu\text{m}$2 Lamb-wave strip guides, bends, and microrings in a platform already used for GaN HEMTs (Bicer et al., 2021).

Thin-film piezoelectrics on fast substrates supply another major branch. LN-on-diamond combines strong LN piezoelectricity with the high acoustic velocity and color-center compatibility of diamond, enabling a $t_{wvg} = 1.5~\mu\text{m}$3 delay line at $t_{wvg} = 1.5~\mu\text{m}$4 with $t_{wvg} = 1.5~\mu\text{m}$5 total insertion loss at $t_{wvg} = 1.5~\mu\text{m}$6 (Malik et al., 29 May 2025). SiN-on-LN achieves low-loss guided SAW components without etching LN itself, using a SiN slot geometry on bulk X-cut LN to realize waveguides, directional couplers, and rings around $t_{wvg} = 1.5~\mu\text{m}$7 (Ji et al., 29 Mar 2026). Suspended LiNbO$t_{wvg} = 1.5~\mu\text{m}$8 thin films host BI-CABS waveguides, where boundary engineering rather than a conventional edge channel creates a bulk transport state compatible with wide-aperture SPUDTs (Li et al., 25 Feb 2025).

Membrane and periodic-resonator platforms emphasize different strengths. High-stress SiN membranes provide ultra-long propagation and single-mode guidance at MHz frequencies, enabling directional emission with over $t_{wvg} = 1.5~\mu\text{m}$9 directional suppression in a $w_{wvg} = 1.8~\mu\text{m}$0-wide, $w_{wvg} = 1.8~\mu\text{m}$1-long spiral waveguide (Hirsch et al., 2023). The same membrane architecture can be driven deeply nonlinear to support dark solitons over metre-scale effective propagation, with direct imaging of hundreds of collisions (Hirsch et al., 24 Mar 2026). MEMS drumhead chains and h-BN coupled-resonator lattices extend the concept to on-chip mechanical band structures, passbands, stopbands, and tunable localization (Kanj et al., 2023, Wang et al., 2020).

This diversity suggests that integrated phononic waveguides have become a platform concept rather than a single device class. A plausible implication is that platform choice is increasingly determined by the desired co-integrated subsystem—RF electronics, photonics, nonlinear acoustics, or quantum defects—rather than by acoustic guidance alone.

4. Loss, dispersion, and performance metrics

Low dissipation is a central requirement because many of the most valuable waveguide functions—delay, resonant enhancement, routing through many stages, and coherent transduction—accumulate loss over distance or storage time. In GaN-on-SiC, this requirement is addressed with microrings whose best device reaches $w_{wvg} = 1.8~\mu\text{m}$2 at $w_{wvg} = 1.8~\mu\text{m}$3, corresponding to

$w_{wvg} = 1.8~\mu\text{m}$4

the highest reported in GaN to date according to the paper (Bicer et al., 2023). The same work extracts intrinsic and coupling quality factors from temporal coupled-mode theory, using the drop-port relation

$w_{wvg} = 1.8~\mu\text{m}$5

For a representative doublet with $w_{wvg} = 1.8~\mu\text{m}$6 and loaded $w_{wvg} = 1.8~\mu\text{m}$7, the authors obtain $w_{wvg} = 1.8~\mu\text{m}$8 and $w_{wvg} = 1.8~\mu\text{m}$9, indicating strong under-coupling (Bicer et al., 2023).

The same GaN platform also directly links waveguide and resonator metrics. Ring-based extraction gives on-chip propagation loss $>3\,\text{GHz}$0, while spiral delay lines yield $>3\,\text{GHz}$1 after removing IDT contributions (Bicer et al., 2023). In the earlier GaN-on-SiC strip-guide work, the straight waveguide link showed $>3\,\text{GHz}$2 through a $>3\,\text{GHz}$3 guide, with the loss budget decomposed into bidirectional IDT loss, impedance mismatch, mode mismatch, propagation loss, and residual scattering (Bicer et al., 2021). By contrast, SiN-on-LN reports direct propagation loss of $>3\,\text{GHz}$4 for a single-mode $>3\,\text{GHz}$5 SAW guide and $>3\,\text{GHz}$6 for multimode guides near $>3\,\text{GHz}$7, with ring-based extraction giving $>3\,\text{GHz}$8 from $>3\,\text{GHz}$9 (Ji et al., 29 Mar 2026).

Dispersion is often a feature rather than a defect. In suspended GaAs-on-insulator, the defect mode operates near a flat band edge and therefore provides slow sound and filtering in the same element (Modica et al., 2020). In BI-CABS LiNbO$v \approx 5414\,\text{m/s}$0, slow-wave propagation produces group delays up to about $v \approx 5414\,\text{m/s}$1 in sub-mm structures (Li et al., 25 Feb 2025). In large-scale GaN-on-sapphire PnICs, the group velocity of a $v \approx 5414\,\text{m/s}$2-wide waveguide is measured as $v \approx 5414\,\text{m/s}$3, and this dispersion is explicitly exploited to design an acoustic arrayed waveguide grating with $v \approx 5414\,\text{m/s}$4 and channel spacing $v \approx 5414\,\text{m/s}$5 (Xu et al., 30 Oct 2025).

Temporal diagnostics have become increasingly important because the combination of low propagation loss, slow sound, and modern VNA bandwidth makes it possible to reconstruct pulse motion, ringdown, and multipath interference directly from frequency-domain measurements. In GaN-on-SiC PnICs, temporal dynamics reveal pulse circulation and ringdown in acoustic microrings as well as parasitic multipath effects in resonator geometries, providing a time-domain reflectometry method for mapping interface reflection and loss (Malik et al., 9 Apr 2025).

The loss question also has a materials-physics dimension. In GaN-on-SiC, the measured $v \approx 5414\,\text{m/s}$6 exceeds the simplified isotropic Akhiezer estimate for GaN,

$v \approx 5414\,\text{m/s}$7

although the comparison is explicitly described as a rough estimate because GaN and SiC are anisotropic and phonon-phonon scattering is mode- and direction-dependent (Bicer et al., 2023). This suggests that integrated geometries need not be fundamentally loss-limited by confinement alone.

5. Control, nonlinearity, and reconfigurability

Integrated phononic waveguides increasingly serve as active and programmable media rather than passive transmission lines. One route uses geometry- and disorder-sensitive mechanics. In MEMS drumhead chains, thermoelastic buckling amplifies weak fabrication disorder, breaking periodicity and localizing first-passband modes in an Anderson-like manner. Near critical buckling, the first passband becomes effectively “off,” while the second passband remains transmitting, producing a thermally controlled transmission switch in a finite disordered waveguide (Kanj et al., 2023). Earlier membrane-array phononic crystal waveguides also demonstrated cavity-mediated dynamic switching and transfer of vibrational energy between a waveguide mode and a localized cavity (Hatanaka et al., 2014).

A second route uses directed excitation. In a single-mode high-stress SiN membrane waveguide, two localized electrostatic actuators separated along the guide act as a phased array for the guided mode. By tuning relative phase and amplitude, the device achieves $v \approx 5414\,\text{m/s}$8 and $v \approx 5414\,\text{m/s}$9 power differences between left and right, corresponding to $1300\,\text{m/s}$0 and $1300\,\text{m/s}$1 directional emission, respectively (Hirsch et al., 2023). This avoids back-propagation and crosstalk in dense phononic circuits.

A third route uses thermo-acoustic or other integrated phase shifters within larger guided-wave circuits. Large-scale programmable PnICs built from GaN waveguides on sapphire demonstrate Y-splitters, directional couplers, MMIs, polarization converters, microrings, gratings, and thermo-acoustic Mach–Zehnder interferometers. Combined into larger systems, these elements realize an ultra-compact $1300\,\text{m/s}$2 acoustic power splitter with integration density of $1300\,\text{m/s}$3, a 21-port acoustic frequency demultiplexer with $1300\,\text{m/s}$4 resolution, and a four-channel reconfigurable frequency synthesizer (Xu et al., 30 Oct 2025).

Nonlinearity provides a fourth route. In high-stress Si$1300\,\text{m/s}$5N$1300\,\text{m/s}$6 membrane waveguides, anomalous group-velocity dispersion and mechanical Kerr nonlinearity produce dark solitons governed by the lossy NLSE

$1300\,\text{m/s}$7

This platform supports dark soliton compression, fission, collisions, and a melting soliton Wigner crystal over metre-scale effective propagation (Hirsch et al., 24 Mar 2026). The earlier GaN spiral work likewise identifies strong dispersion and spectral selectivity as promising for integrated RF filtering, pulse shaping, and temporal signal manipulation (Bicer et al., 2023).

These demonstrations indicate that reconfigurability in phononic waveguides can be implemented through thermal, electromechanical, interference-based, and nonlinear mechanisms. A plausible implication is that future PnICs will resemble photonic integrated circuits not only in topology but also in the diversity of available control knobs.

6. Hybrid integration, applications, and field trajectory

The strongest strategic motivation for integrated phononic waveguides is hybrid integration. GaN-on-SiC is simultaneously piezoelectric and a high-electron-mobility semiconductor, so the same stack can host waveguides, resonators, and active RF electronics such as HEMTs. This opens the prospect of traveling-wave acoustoelectric interactions in micron-scale guides for amplification, modulation, mixing, and compact RF front-ends (Bicer et al., 2023, Bicer et al., 2021). AlScN-on-SiC extends this idea with strongly electromechanical Sezawa-like modes, where the concentrated strain and piezoelectric fields can substantially reduce DC power in acoustoelectric amplifiers and enhance three-wave mixing relative to slab devices (Deng et al., 23 Mar 2025).

Quantum integration is now a major branch of the field. LN-on-diamond is explicitly targeted at phonon-mediated hybrid quantum systems involving strain-sensitive color centers in diamond, with estimated single-phonon spin–phonon coupling $1300\,\text{m/s}$8 for SiV centers and a path to $1300\,\text{m/s}$9 in ring resonators (Malik et al., 29 May 2025). AlN/diamond SAW waveguides support 4–5 GHz transmission in ridge and suspended geometries with wavelength-scale cross sections, with estimated single-phonon Rabi rates of $1000\,\text{m/s}$00–$1000\,\text{m/s}$01 for SiV centers and projected driven Rabi frequencies $1000\,\text{m/s}$02 at $1000\,\text{m/s}$03 input (Ding et al., 2023). In suspension-free LNOS PnICs, superconducting transmons coupled to Fabry–Pérot and microring phononic cavities realize circuit quantum acoustodynamics with Purcell factors up to $1000\,\text{m/s}$04, demonstrating elementary building blocks for scalable phononic circuits (Wang et al., 4 Dec 2025).

At the systems level, integrated phononic waveguides already support several application classes that recur across platforms: compact RF delay lines, filters, multiplexers, oscillators, acoustic power splitters, signal synthesizers, and sensing architectures (Modica et al., 2020, Ji et al., 29 Mar 2026, Xu et al., 30 Oct 2025). In SiN-on-LN, a $1000\,\text{m/s}$05 phononic oscillator based on a ring resonator reaches a phase noise of $1000\,\text{m/s}$06 at $1000\,\text{m/s}$07 offset (Ji et al., 29 Mar 2026). In GaN-on-SiC, on-chip RF delays exceeding $1000\,\text{m/s}$08 correspond to an equivalent electromagnetic delay of $1000\,\text{m/s}$09 (Bicer et al., 2023). In BI-CABS LiNbO$1000\,\text{m/s}$10, low-loss, nearly flat $1000\,\text{m/s}$11 in the passband and slow-wave delay lines point toward dense signal processing and sensing (Li et al., 25 Feb 2025).

A recurring misconception is that guided acoustic circuits must either be suspended for low loss or topological to be robust. The literature summarized here does not support either restriction. Low-loss guidance has been demonstrated in fully supported GaN-on-SiC, SiN-on-LN, AlScN-on-SiC, and LNOS architectures (Bicer et al., 2023, Ji et al., 29 Mar 2026, Deng et al., 23 Mar 2025, Wang et al., 4 Dec 2025). Topological or boundary-induced transport provides one route to robustness, but strong confinement, good material choice, and careful coupler and taper design also yield practical low-loss integrated circuits (Li et al., 25 Feb 2025, Xu et al., 30 Oct 2025).

Taken together, current results define integrated phononic waveguides as a mature device concept with multiple viable material stacks and a rapidly broadening function set. The dominant trajectory is toward architectures that combine low-loss routing, high-$1000\,\text{m/s}$12 resonators, efficient transducers, and direct interfaces to electrons, photons, superconducting qubits, and spin defects on the same chip (Ji et al., 29 Mar 2026, Wang et al., 4 Dec 2025).