Freestanding Photonic Devices Overview

• Freestanding photonic devices are substrate-free optical components that minimize dielectric screening and substrate-induced perturbations to enhance performance.
• They employ diverse materials like 2D semiconductors, single-crystal metals, diamond, and III–V semiconductors using precise fabrication techniques to achieve high uniformity and quantum-scale control.
• These devices enable advanced applications such as quantum light sources, nonlinear modulators, and optomechanical platforms through innovative photonic mode engineering and scalable integration.

Freestanding photonic devices are engineered optical components whose active materials are suspended or otherwise decoupled from supporting substrates. This architecture eliminates or minimizes substrate-induced perturbations, enabling enhanced optical performance, novel functionalities, and integration flexibility. Freestanding designs span atomic membranes (transition metal dichalcogenides, noble metals), subwavelength dielectric and plasmonic metasurfaces, semiconductor nanophotonic circuits, and quantum photonic cavities incorporating atom-like color centers. Device performance and design rules in freestanding systems are shaped by reduced dielectric screening, quantum size effects, mechanical boundary conditions, and the dominance of surface/interface phenomena.

1. Materials Platforms and Fabrication Strategies

Freestanding photonic devices have been realized in diverse material classes using platform-specific fabrication methods optimized to preserve crystalline quality, surface cleanliness, and structural integrity.

• 2D Semiconductors: Suspended single- and few-layer MoS₂ are obtained by mechanical exfoliation or wafer-scale transfer onto pre-patterned substrates with etched holes. The absence of a substrate suppresses parasitic doping and dielectric screening, revealing the intrinsic neutral exciton response and enabling bright, spectrally pure photoluminescence with high quantum yield (Scheuschner et al., 2013).
• Single-Crystal Metals: Atomically thin, large-area freestanding gold films are fabricated by atomic-level-precision chemical etching (ALPE) of wet-grown Au flakes on mica, followed by substrate removal. Thickness is tunable from ~30 nm down to 1 nm (~5 atomic layers) at rates of ~0.2 nm/min, producing continuous, flexible, and highly conductive metallic membranes (Pan et al., 2023).
• Diamond and Wide-Bandgap Semiconductors: Freestanding (111)-oriented single-crystal diamond membranes are generated by ion-beam-induced buried damage and electrochemical etch ("liftoff"), supporting subsequent high-quality overgrowth and integration of color centers. Similarly, wafer-scale triangular photonic devices are patterned in bulk 4H-SiC by angled reactive ion beam etching, achieving sub-3% cross-wafer dimensional uniformity and maintaining emitter properties (Regan et al., 2019, Majety et al., 2024).
• III–V Suspended Cavities: Monolithic freestanding circular Bragg grating (CBG) resonators incorporating deterministically positioned GaAs quantum dots are realized by a single-step aspect-ratio-dependent etch into a sacrificially layered heterostructure, followed by selective undercut and supercritical CO₂ drying to avoid stiction (Dhurjati et al., 4 May 2026).
• Dielectric/Plasmonic Metasurfaces: Large-area freestanding metasurfaces are patterned by subwavelength lithography on thin dielectric or metallic films and released by selective etching, enabling arbitrary phase/amplitude control with mechanical isolation (Kumar et al., 2021).

2. Optical, Quantum, and Nonlinear Properties

Substrate removal fundamentally alters optical response by modifying charge transfer, local dielectric environments, and quantum confinement.

• Transition Metal Dichalcogenides (TMDCs): The neutral exciton transition ("A" peak) in single-layer MoS₂ is only observed in freestanding configurations at $E_{A} = 1.886 \pm 0.008\,\mathrm{eV}$ ($\lambda_{A} \approx 658\,\mathrm{nm}$, FWHM $\approx 47\,\mathrm{meV}$). Supported samples exhibit complete quenching of the neutral exciton and are dominated by trion ($A^{-}$) emission due to substrate-induced $n$-type doping ($\Delta E_F \gtrsim 40\,\mathrm{meV}$; $$n_{2D} \gtrsim 1.6 \times 10^{13}\,\mathrm{cm}^{-2}$$), shifting the PL to $E_{A^-} \approx 1.824\,\mathrm{eV}$ ($\lambda_{A^-} \approx 680\,\mathrm{nm}$) with FWHM $\approx 100\,\mathrm{meV}$. Total PL intensity in freestanding is increased by an order of magnitude (Scheuschner et al., 2013).
• Freestanding Gold Films: 2D gold membranes below 10 nm exhibit quantum-confinement-enhanced nonlinear optical response: SHG and THG intensities are enhanced up to $\lambda_{A} \approx 658\,\mathrm{nm}$0 and $\lambda_{A} \approx 658\,\mathrm{nm}$1, respectively, relative to bulk Au. Optical transmittance exceeds 90% at 600 nm for $\lambda_{A} \approx 658\,\mathrm{nm}$2, and thickness-dependent plasmonic resonances in patterned nanoribbons yield Q-factors up to 5 and near-field enhancements $\lambda_{A} \approx 658\,\mathrm{nm}$3 (Pan et al., 2023).
• Quantum Emitters in Membranes: (111)-diamond microring resonators containing SiV centers exhibit photoluminescence ZPL at 738 nm, with integrated resonance enhancement (Q $\lambda_{A} \approx 658\,\mathrm{nm}$4) and 25$\lambda_{A} \approx 658\,\mathrm{nm}$5 brightness compared to non-resonant areas. Similar enhancement and spectral uniformity are seen for N-V centers and divacancies in 4H-SiC waveguides, with linewidths ($\lambda_{A} \approx 658\,\mathrm{nm}$6–70 pm, $\lambda_{A} \approx 658\,\mathrm{nm}$7–15 GHz) unaffected by the etch process (Regan et al., 2019, Majety et al., 2024).
• Suspended Circular Bragg Gratings (CBGs): GaAs QDs embedded in freestanding CBGs reach free-space extraction up to 68% and fiber coupling of 40% by FDTD simulation; devices achieve $\lambda_{A} \approx 658\,\mathrm{nm}$8 integrated photon count rates, $\lambda_{A} \approx 658\,\mathrm{nm}$9 brightness enhancement, and sub-2 $\approx 47\,\mathrm{meV}$0 fine-structure splitting (FSS) attributed to strain relaxation inherent in suspended architectures (Dhurjati et al., 4 May 2026).

3. Device Geometries and Photonic Mode Engineering

The optical functionality of freestanding devices is governed by their geometry, which determines mode confinement, Purcell regimes, and far-field emission.

• Membrane Thickness and Surface Morphology: Purely freestanding membranes achieve sub-nanometer roughness and deterministic thicknesses, with thickness directly tuning spectral and nonlinearity properties (Au, MoS₂). For diamond and SiC, controlled slab thickness ($\approx 47\,\mathrm{meV}$1 in diamond, $\approx 47\,\mathrm{meV}$2 in SiC) is crucial for single-mode operation and maximal emitter-mode coupling (Pan et al., 2023, Majety et al., 2024, Regan et al., 2019).
• Suspended Ring Resonators and Photonic Crystals: (111)-diamond microrings ($\approx 47\,\mathrm{meV}$3, $\approx 47\,\mathrm{meV}$4, $\approx 47\,\mathrm{meV}$5) display FSR $\approx 47\,\mathrm{meV}$6, Finesse $\approx 47\,\mathrm{meV}$7, and strong TE polarization selectivity. In SiC, triangular cross-section waveguides and microrings with apex angle $\approx 47\,\mathrm{meV}$8 support high-confinement, high-$\approx 47\,\mathrm{meV}$9 ($A^{-}$0) guided modes at $A^{-}$1 (Regan et al., 2019, Majety et al., 2024).
• Freestanding Bragg/Metastructures: CBGs with partially etched concentric trenches exploit vertical index asymmetry and complete undercut for unidirectional emission without the need for reflective back mirrors; ARDE adapts trench depth to local width in a single mask step. Freestanding metasurfaces, patterned by phase-gradient or radially varying cells, achieve mechanical and optical stability for macroscopic manipulation (Kumar et al., 2021, Dhurjati et al., 4 May 2026).

4. Optomechanical and Interference Phenomena

Freestanding architectures present unique optomechanical and wave-interference signatures due to their unconstrained boundaries.

• Optomechanical Self-Stabilization: Analytical frameworks reduce stabilization of freestanding metasurfaces in optical beams to radial integrals over 2D unit-cell scattering parameters, yielding closed-form stiffness coefficients ($A^{-}$2). Proper metasurface design ensures passive trapping in all six degrees of freedom, critical for macroscale optical manipulation (e.g., light sails) (Kumar et al., 2021).
• Interference in Suspended 2D Materials: In freestanding MoS₂ flake regions over $A^{-}$3 holes, PL spectra exhibit energy-periodic interference fringes ($A^{-}$4), modeled by a cosine modulation function. The fringe period is set by membrane-substrate distance, enabling in-situ PL spectral shaping or filtering (Scheuschner et al., 2013).

5. Large-Scale Integration and Wafer-Scale Uniformity

Scaling freestanding photonic devices to large areas and complex circuits hinges on lithography, etch-process uniformity, and mechanical robustness.

• Wafer-Scale SiC PICs: Using angled RIBE and Ni hard masks, uniform triangular waveguides, microrings (R = $A^{-}$5m), PCs, and microdisks are fabricated reproducibly across 5-inch 4H-SiC chips, with etch rate and angle variabilities of 5.4% and 2.9%, respectively. Device yields exceed 95% and process variability is minimized by substrate rotation, mask compensation, and CMOS-compatible process flows (Majety et al., 2024).
• Monolithic III–V Integration: Single-step ARDE etch processes for CBGs enable the simultaneous patterning and undercut of resonator geometry, obviating the need for multi-layer stacking, bonding, or mirror deposition. This minimal processing preserves emitter properties and supports high yield for quantum light sources (Dhurjati et al., 4 May 2026).
• Diamond Membrane Circuits: The liftoff and overgrowth methodology for (111) diamond membranes enables fabrication of multiple devices per chip (area up to 100 µm), with Q improvement tractable via sidewall engineering and undercut optimization (Regan et al., 2019).

6. Performance Metrics and Analytical Frameworks

Quantitative performance descriptors and analytic models underpin device benchmarking and design.

Device Type	Key Metrics	Notable Techniques
Freestanding MoS₂	$A^{-}$6 eV, FWHM $A^{-}$7 meV, $A^{-}$8 PL	2D hydrogenic/trion binding, doping-controlled PL
2D Au Nanoribbons	Q up to 5, $A^{-}$9 T($n$0 nm), $n$1	ALPE, field confinement, nonlinear optics
(111) Diamond Rings	Q $n$2 3000, FSR $n$3 nm, $n$4 PL boost	Liftoff/overgrowth, TE-mode dipole alignment
SiC Triangular PICs	$n$5, $n$6, $n$7 yield	Angled RIBE, mode engineering, mask scaling
GaAs CBG–QD	$n$8, $n$9, $\Delta E_F \gtrsim 40\,\mathrm{meV}$0 brightness	ARDE, FDTD, single-step release/strain control
Freest. Metasurface	Analytical $\Delta E_F \gtrsim 40\,\mathrm{meV}$1, stable trapping criteria	Unit-cell scattering, phase-gradient engineering

Analytical formulations include Wannier–Mott and trion binding for 2D excitons (Scheuschner et al., 2013), Fuchs–Sondheimer corrections for Au conductivity (Pan et al., 2023), ARDE transport models for etch depth (Dhurjati et al., 4 May 2026), effective mode area/overlap definitions for waveguide-emitter coupling (Majety et al., 2024), and universally applicable optomechanical stiffness integrals for optical manipulation (Kumar et al., 2021).

7. Applications and Future Directions

Freestanding photonic devices underpin several advanced optical, quantum, and optomechanical technologies:

• Quantum Networks: Wafer-scale SiC and diamond photonics with color centers enable on-chip quantum light sources, integrated entanglement generation, and robust interfacing with photonic circuits (Regan et al., 2019, Majety et al., 2024).
• Nonlinear and Plasmonic Devices: Quantum-confinement in ultrathin Au enhances nonlinear susceptibility for ultra-compact modulators, nanoscale Kerr switches, and metasurface-based field enhancement sensors (Pan et al., 2023).
• Electrically/Electronically Tunable Emitters: Freestanding TMDCs support switching between exciton/trion emission via gate or chemical doping, enabling visible-to-near-IR active modulators (Scheuschner et al., 2013).
• High-Efficiency Light Extraction: Freestanding CBGs achieve record collection efficiency and minimal FSS for scalable sources of entangled photons (Dhurjati et al., 4 May 2026).
• Macroscopic Optical Manipulation: Freestanding metasurfaces designed for optomechanical self-stability enable passive trapping and alignment in vacuum, relevant for macro-photonic systems and light-driven propulsion (Kumar et al., 2021).

A plausible implication is that continued advances in scalable fabrication/etching and integration of freestanding nanostructures with deterministic emitter placement and low mechanical losses will accelerate the deployment of hybrid quantum, nonlinear, and optomechanical photonics, as well as open avenues for large-scale optical manipulation and space-based applications.