Suspension-Free Chip Integration

• The suspension-free chip platform is defined as a substrate-integrated system that supports optical, phononic, and other functional domains without released or undercut structures, thereby enhancing mechanical stability.
• It leverages engineered material contrasts, total internal reflection, and encapsulation to confine waves and enable efficient photon–phonon and microwave coupling in devices such as LNOS racetracks and planar quantum stacks.
• The architecture enables wafer-scale manufacturability with improved thermal management and integration density, advancing hybrid quantum, sensing, and bio-analytical applications.

A suspension-free chip platform is defined as an integrated microscale or nanoscale system in which all essential elements—optical, phononic, or other functional domains—are supported directly on a substrate without requiring any released, undercut, or mechanically suspended structures. Such platforms exploit total internal reflection, velocity and refractive-index contrast, or encapsulation to confine waves or maintain sample environments, while maximizing mechanical stability, reproducibility, and large-scale manufacturability. Suspension-free architectures have become foundational across Brillouin optomechanics, quantum acoustodynamics, quantum photonics, chip-based atomic sensors, and hydrated spectromicroscopy, leveraging bulk-supported, lithographically patterned films to co-integrate multiple physical modalities and eliminate the fabrication and reliability constraints imposed by conventional "suspended" devices.

1. Foundational Platform Architectures and Materials

Suspension-free chip platforms are rapidly proliferating across device physics, based on a range of substrates and waveguide materials:

• Lithium-niobate-on-sapphire (LNOS) racetrack or microring circuits: X-cut LN (index ≈2.2 at 1550 nm, thickness 100–600 nm) is grown or bonded atop a c-plane sapphire wafer (index ≈1.7), then patterned into ridge waveguides (1.2 μm wide, 400 nm thick) by electron-beam lithography and dry etching. No undercut or air gap is used; both optical and GHz-frequency acoustic waves are confined by total internal reflection at the LN-sapphire interfaces (Yang et al., 23 Oct 2025, Duan et al., 20 Nov 2025, Xu et al., 18 Sep 2025).
• Planar quantum photonic stacks: Thin (≈200 nm) hexagonal boron nitride flakes can be stamped onto aluminum nitride ridge waveguides (1 μm wide, 200 nm thick) on sapphire, forming a fully planar structure without any suspended elements. Quantum emitters in hBN are thus coupled directly to guided photonic modes (Kim et al., 2019).
• Hydrated sample environments: Graphene-encapsulated sample holders utilize silicon nitride membranes and polymer lids patterned with observation ports, sealed by atomically thin graphene. Capillary and van der Waals forces ensure conformal contact and water retention, without any liquid suspension or free-standing films (Arble et al., 2021).
• MEMS atomic vapor cells: Vacuum cells for chip-based laser cooling are fabricated by anodic bonding silicon and aluminosilicate glass windows, with all optical/MEMS elements lithographically defined, yielding completely planar cells for cold-atom trapping and interrogation (McGilligan et al., 2020).

The elimination of mechanical suspension leads to improved thermal anchoring, ease of back-end integration, robustness to packaging and environmental cycling, and high structural yield in wafer-scale fabrication.

2. Physical Principles Underlying Confinement and Coupling

Suspension-free platforms rely on engineered material contrast and careful waveguide or cavity design to support low-loss confinement of the relevant physical excitations:

• Photon and phonon guiding in LNOS: A high index and low acoustic velocity in LN guide both telecom-band TE optical modes and 5–10 GHz phononic (acoustic) modes. For a racetrack perimeter $L=13.43$ mm, the free spectral range matches the Brillouin phonon frequency ($\mathrm{FSR} = c / n_g L \approx 9.79$ GHz), supporting triply resonant cavity Brillouin optomechanics (Yang et al., 23 Oct 2025).
• Waveguide cross-section and phase-matching: The ridge geometry ensures that TE-like photonic and quasi-Love phononic modes are co-localized. In ring or racetrack geometries, momentum matching for Brillouin scattering is achieved by integer FSR matching, automatically ensuring phase matching for photon–phonon coupling throughout the cavity (Duan et al., 20 Nov 2025, Yang et al., 23 Oct 2025).
• Piezoelectric coupling to qubits: Interdigital transducers (IDTs) patterned atop unsuspended LN waveguides enable microwave–phonon coupling. The coupling Hamiltonian $H_I = \hbar g(a^\dagger b + ab^\dagger)$ quantifies the interaction, with $g/2\pi$ predicted in the 1–20 MHz range for realistic device dimensions and IDT geometries (Xu et al., 18 Sep 2025).
• Encapsulation and environmental confinement: For hydrated spectromicroscopy, atomic-layer graphene provides an electron-, X-ray-, and IR-transparent yet molecularly impermeable seal, preserving water around bio-specimens for $\gtrsim 10$ h in vacuum, enhanced by integrated hydrogel pads (Arble et al., 2021).

3. Key Performance Metrics and Theoretical Modeling

Metrics for suspension-free device performance follow from detailed modeling of the coupled photon–phonon or photonic–electronic dynamics:

Quantity	Typical Value/Formula	Relevant Platform
Optical $Q$	$Q_{\mathrm{int}} \approx 2.3 \times 10^5$	LNOS Brillouin
Phonon $Q$	$Q_m \approx 10^3$	LNOS Brillouin, QAD
Vacuum optomechanical $g_0$	$g_0/2\pi \approx 2.7$ kHz	LNOS Brillouin
Cooperativity $C$	$C=\frac{4g_0^2 n_p}{\kappa\gamma_m}$ up to $0.41$	LNOS Brillouin
$Q_m f_m$ product	$\approx 10^{13}$ Hz	LNOS Brillouin
Piezoelectric $g/2\pi$	$1.3$–$20$ MHz (scalable with $N_\mathrm{IDT}$)	LNOS QAD
Coupling efficiency $\eta$	$1.2\%$ measured, $15.5\%$ theoretical	hBN–AlN on sapphire
Hydration lifetime	$>10$ h in vacuum, $>200$ h in air	Graphene-encapsulated

• Multi-channel operation: The Brillouin process in LNOS racetracks supports a continuous mapping between optical wavelength and phonon frequency, with $d\Omega/d\lambda\approx-7.6$ MHz/nm, allowing $>40$ nm optical tuning to access $>300$ MHz of phonon bandwidth for parallel photonic–phononic operations (Yang et al., 23 Oct 2025).
• Gyroscope noise and sensitivity: Brillouin saser gyroscopes in LNOS exhibit angle random walk $\sim 0.1$ deg/$\sqrt{\mathrm{h}}$ at Qac $=5\times10^3$, exceeding the performance of purely optical gyroscopes (which would require Qopt $>10^{10}$ or high pump powers to match) (Duan et al., 20 Nov 2025).
• Photon–phonon–qubit strong coupling: Unsuspended phononic circuits in LNOS achieve $g/2\pi$ exceeding 10 MHz, with phonon lifetimes of order 100 ns and transmon coherence times of 20–50 μs, readily realizing the $g\gg(\gamma_{\rm ph}+\gamma_q)/2$ regime critical for hybrid quantum information processing (Xu et al., 18 Sep 2025).
• Planar quantum photonics: The absence of suspended structures improves mechanical yield (robustness to stiction, collapse) and enables high packing density and deterministic placement of quantum emitters (Kim et al., 2019).

4. Fabrication, Scalability, and Integration Capabilities

Suspension-free chips exploit standard thin-film, lithographic, and transfer techniques adapted from semiconductor processing, yielding several advantages:

• Wafer-scale compatibility: Platforms such as LNOS and AlN-on-sapphire are fully compatible with back-end photonics foundry processes. There are no fragile release steps, and all structures are directly anchored on crystalline or glassy substrates (Yang et al., 23 Oct 2025, Kim et al., 2019).
• Piezoelectric and photonic co-integration: Patterned electrodes, qubit wiring, and both photonic and phononic elements can be formed by lift-off and aligned atop the same unsuspended waveguide layer; transverse fields enable electrical gating or detection of confined phonons (Xu et al., 18 Sep 2025).
• Hydrated sample platforms: Silicon nitride windows and patterned SU-8 polymer lids, sealed by graphene transfer, allow fabrication of multi-compartment, reusable chips for wet-cell spectromicroscopy, with dimensions precisely controlled by standard MEMS processes (Arble et al., 2021).
• MEMS cold-atom cells: Direct Si/glass anodic bonding, laser-cut window apertures, and resistive alkali sources allow the monolithic assembly of UHV-compatible cells for atom trapping and laser cooling, eliminating the need for external suspensions (McGilligan et al., 2020).

The Editor’s term “suspension-free architecture” thus denotes a class of chip-based devices fabricated without underetch or air-gap sacrificial layers, enabling dense integration, high yields, and robust thermal management.

5. Impact on Applications: Classical and Quantum Information, Sensing, and Microscopy

Suspension-free chip platforms have a broad and growing application span:

• Hybrid quantum circuits: Direct photon–phonon–qubit interfaces in LNOS enable microwave-to-optical quantum transduction, multi-qubit coupling via GHz phonons, and on-chip integration with superconducting electronics (Yang et al., 23 Oct 2025, Xu et al., 18 Sep 2025).
• Multi-channel processing: The one-to-one mapping of optical resonances to phonon frequency channels allows simultaneous, parallel photonic–phononic operations across $>40$ nm wavelength / $>300$ MHz phononic span, useful for multiplexed RF signal processing and quantum networking (Yang et al., 23 Oct 2025).
• Inertial sensing: Integrated Brillouin saser gyroscopes in planar LNOS chips demonstrate sub–deg/$\sqrt{\mathrm{h}}$ random walk at mW pump power, exceeding prior performance by leveraging direct acoustic out-coupling and suppressed thermal/frequency noise (Duan et al., 20 Nov 2025).
• Quantum photonics: Planar coupling of single-photon emitters (hBN) to AlN waveguides on sapphire allows deterministic, scalable realization of quantum photonic circuits for applications in secure communication, quantum computing, and metrology (Kim et al., 2019).
• Correlative spectromicroscopy of hydrated matter: Suspension-free graphene-encapsulated platforms enable label-free examination of live cells, bacteria, and gels by SEM, FTIR, and X-ray fluorescence in vacuum and in situ, preserving sample hydration and providing electron and photon transparency unmatched by other architectures (Arble et al., 2021).
• Laser cooling and atom interferometry: Microfabricated Si/glass UHV cells, rigidly bonded and aligned with micro-optics, set the basis for compact cold-atom sensors, portable clocks, and quantum sensors without any isolated/suspended elements (McGilligan et al., 2020).

6. Advantages and Limitations Versus Suspended Architectures

Suspension-free platforms offer distinct benefits over conventional suspended chips:

• Mechanical robustness: Substrate-supported waveguides resist stiction, collapse, and damage during fabrication, packaging, and operational cycling (Yang et al., 23 Oct 2025, Xu et al., 18 Sep 2025).
• Thermal management: Bulk-substrate anchoring dissipates heat efficiently, mitigating "hot spots" and supporting higher pump powers or densities without degrading performance.
• Wafer-scale scalability: Thin-film and back-end-of-line compatibility allow massively parallel integration of hundreds of photonic, phononic, and superconducting circuit elements per wafer (Xu et al., 18 Sep 2025).
• Integration density: Planar architectures facilitate the co-fabrication of photonic, phononic, and microwave electronic elements without the spacing or design rules imposed by suspension steps.
• Device performance: While ultrahigh Q (10⁵–10⁷) is possible in suspended structures, Q ≈ 10³–10⁴ in unsuspended chips suffices for strong photon–phonon or phonon–qubit coupling if coupling $g$ is engineered in the MHz regime (Duan et al., 20 Nov 2025, Xu et al., 18 Sep 2025).

Limitations include:

• Upper bound to Q: Highest attainable mechanical Q is generally lower than in air-clad, suspended membranes, potentially limiting coherence times for some applications.
• Acoustic leakage paths: Nonzero substrate coupling may limit lifetime of high-frequency phonons, and requires precise mode engineering to minimize loss (Xu et al., 18 Sep 2025).

7. Outlook and Prospects

Suspension-free chip platforms are enabling a new era of on-chip hybrid systems that combine photonics, phononics, microwave electronics, and quantum functionalities by leveraging mature thin-film and MEMS processes. Anticipated developments include:

• Hybrid quantum transducers: On-chip interfaces between optical, phononic, and superconducting domains for scalable quantum networking (Yang et al., 23 Oct 2025, Xu et al., 18 Sep 2025).
• Active phononic integrated circuits: Brillouin gain enables on-chip amplifiers, delay lines, and oscillators, supporting RF communication, nonreciprocal devices, and ultra-low-power signal processing (Duan et al., 20 Nov 2025).
• Multiplexed quantum photonic platforms: Direct placement and integration of quantum emitters with optimized waveguides and detectors in large-scale circuits (Kim et al., 2019).
• Bio-analytical technologies: Correlative, label-free spectromicroscopy with preserved hydration, structural fidelity, and multi-modal readout for fundamental biology and nanomedicine (Arble et al., 2021).
• Field-deployable atomic sensors: Monolithic, suspension-free MEMS vapor cells forming the nucleus of portable, robust cold-atom platforms (McGilligan et al., 2020).

The suspension-free paradigm is therefore propelling quantum, classical, sensing, and analytical chip technologies toward robust, scalable, and multifunctional integration.