High-Q GaAs Microcavities for Quantum Photonics

• High-Q planar GaAs microcavities are epitaxially grown nanophotonic structures combining DBRs and GaAs active layers to achieve high quality factors and minimal mode volumes.
• They employ advanced digital-alloy DBRs and precise cavity design to optimize both vertical and lateral optical confinement, enhancing light–matter interaction and reducing losses.
• These microcavities underpin applications in cavity QED, low-threshold polariton and quantum-dot lasing, as well as hybrid opto-acoustic systems for quantum photonics.

High-Q planar GaAs-based microcavities are epitaxial and nanophotonic structures that combine distributed Bragg reflectors (DBRs) and GaAs active regions to confine photons with high quality factor ($Q$), small mode volume ($V_m$), and high field overlap with embedded quantum emitters. These microcavities serve as foundational platforms for research in cavity quantum electrodynamics, low-threshold polariton and quantum-dot lasing, quantum acoustics, and strong exciton–photon coupling. Their performance arises from careful optimization of vertical (DBR) and lateral (defect, grating, or acoustic) confinement, materials engineering, and advances in epitaxial growth and device processing.

1. Structural Design and Mirror Technology

State-of-the-art planar GaAs microcavities use DBR stacks as vertical mirrors, typically of the AlAs/GaAs or Al$_x$Ga$_{1-x}$As/GaAs type, to achieve reflectivities exceeding 99.9% (Wang et al., 2017). Recent developments replace traditional AlGaAs DBR high-index layers with short-period GaAs/AlAs superlattices (digital alloys, “SPSLs”), lowering interface roughness (RMS ≲0.1 nm), enhancing $\lambda/4$ thickness control, and enabling miniband engineering to minimize absorption at the photon energy (Stolyarov et al., 2 Dec 2025). Alternative vertical mirrors include dielectric DBRs (e.g., SiO$_2$/SiN$_x$ top stacks), metallic Tamm–plasmon structures, and sub-wavelength gratings (SWGs), each providing a trade-off between fabrication ease, $Q$, and vacuum field strength (Wang et al., 2017).

Cavity layer design centers on one‐$\lambda/n$ or $\lambda/2$ GaAs slabs (with $n\approx3.5$–3.68 near 800–930 nm). Embedded active layers (typ. InGaAs quantum dots, GaAs quantum wells) are positioned at cavity antinodes to maximize light–matter coupling (Gaur et al., 2023, Wang et al., 2017). Lateral confinement is achieved via buried photonic defects (smooth parabolic profile), focusing interdigital surface-acoustic-wave transducers (FIDTs), or by etching micropillars or microdisks, with the planar defect/SAW methods offering superior thermal and spectral properties (Msall et al., 2019, Gaur et al., 2023).

2. Quality Factor ($Q$), Mode Volume, and Loss Channels

The performance-defining metric is the cavity Q-factor, $Q = \omega_0/\Delta\omega$ (resonance frequency over FWHM linewidth). Planar GaAs-based microcavities routinely achieve $Q = 3\,000$–$10\,000$ using conventional AlGaAs/GaAs Bragg reflectors (Wang et al., 2017). Digital-alloy-based DBRs (SPSLs) have produced experimental $Q$ as high as $5.4\times10^4$, surpassing conventional alloy predictions and highlighting the benefits of interface control and quantum-confinement engineering (Stolyarov et al., 2 Dec 2025). Hybrid semiconductor-dielectric DBRs (e.g., SiO$_2$/SiN$_x$) yield $Q$ up to $17\,000$ with reduced mode volume ($V_m \sim 0.28\,\mu\text{m}^3$) by utilizing buried photonic defects for lateral confinement (Gaur et al., 2023).

Principal loss channels include finite DBR reflectivity, intrinsic absorption, interface roughness, deviations from ideal $\lambda/4$ layer periodicity, and scattering at structural defects. In digital-alloy structures, quantum-confinement and miniband engineering shift excitonic absorption away from optical resonance, minimizing $Q_\text{abs}$ losses (Stolyarov et al., 2 Dec 2025). In hybrid semiconductor-dielectric DBRs, loss is set by air/dielectric contrast and the number of mirror pairs, with $Q$ saturating beyond $\sim$15 dielectric pairs due to reflectivity limits (Gaur et al., 2023). For phononic cavities, disorder-induced scattering from MBE-grown mirror thickness fluctuations dominates at low temperatures (Lagoin et al., 2018).

3. Cavity Architectures: Comparative Analysis

The following table summarizes representative mirror and cavity configurations, benchmarking $Q$, mode volume, and relevant figures of merit (Wang et al., 2017, Gaur et al., 2023, Stolyarov et al., 2 Dec 2025):

Mirror Type	$Q$ Range	Mode Volume ($\mu$m$^3$)	Key Features
Digital-alloy DBR	$5\times10^4$	$\sim0.3$	Minimized roughness, tunable bandgap
AlGaAs/GaAs DBR	$3\times10^3$–$1\times10^4$	$0.3$–$0.5$	Standard platform, ease of MBE
Hybrid dielectric DBR	$1\times10^4$–$1.7\times10^4$	$0.28$	Ex-situ top mirror, parabolic defect
SWG–DBR hybrid	$3\times10^3$–$8\times10^3$	$0.16$–$0.26$	CMOS process, strong coupling
Metal (Tamm–plasmon)	$50$–$100$	$0.21$	High field, absorption loss

Conventional DBR–DBR structures yield $Q$ up to $10^4$ with careful epitaxy. SWG and air–DBR configurations allow increased vacuum field strength and reduced mode volume, at the cost of increased scattering and fabrication complexity. Hybrid defect–dielectric microcavities enable simple planar fabrication and post-growth $Q$ tuning, as their top dielectric DBR can be modified ex situ (Gaur et al., 2023).

4. Acoustic and Optomechanical Microcavities

Planar GaAs/AlAs superlattice structures are also engineered as phononic cavities, with DBRs for acoustic rather than optical confinement (Lagoin et al., 2018). Fabry–Pérot structures with $\lambda/2$ GaAs defects and 15–25 GaAs/AlAs bilayer mirrors demonstrate mechanical Q-factors up to $2.7\times10^4$ for longitudinal acoustic modes near 20–180 GHz, with $Q\cdot f \sim 5\times10^{14}$ Hz and energy-decay lifetimes exceeding 400 ns. Loss is predominantly due to layer thickness disorder. These platforms are promising for quantum acoustics, phonon storage, cavity optomechanics, and integration with optical/quantum-dot modes for hybrid quantum systems.

Surface-acoustic-wave (SAW) microcavities employ focusing IDTs lithographically shaped using the GaAs group-velocity wavefront and precise phase/gouy corrections for diffraction-limited spot sizes. Acoustic mode volumes $V_m \sim 3$–$5\,\mu\text{m}^3$ with $Q\sim2\,000$ have been demonstrated, and downscaling to $\lambda_\text{SAW}\,\sim 0.5\,\mu$m is feasible for strong strain–electron coupling applications (Msall et al., 2019).

5. Quantum and Lasing Performance Metrics

Best-in-class planar GaAs microcavities achieve lasing thresholds as low as $200$ W/cm$^2$ for polariton lasing (Stolyarov et al., 2 Dec 2025), and sub-milliwatt thresholds for quantum-dot microlasers in defect–dielectric microcavities with $\beta$ factors up to 0.015 (Gaur et al., 2023). Enhancement of light–matter interaction is quantified via mode volume and vacuum Rabi splitting, with SWG and air–DBR structures maximizing $E_\text{max}$ and anticrossing splittings of $\hbar\Omega_R > 10$ meV using multi-QW schemes (Wang et al., 2017).

Phononic and hybrid acoustic–photonic microcavities exploit high $Q\cdot f$ products for resolved-sideband optomechanics and quantum storage, with mechanical lifetimes exceeding 0.4 $\mu$s at cryogenic temperatures (Lagoin et al., 2018). The thermal robustness and optimized anchoring provided by planar designs prevent spectral diffusion and heating effects even at high optical pump powers (Gaur et al., 2023).

6. Advanced Growth, Material Engineering, and Future Prospects

Recent advances demonstrate that digital-alloy DBRs grown via MBE with GaAs/AlAs SPSLs enable precise $\lambda/4$ control, atomic smoothness, and structural defect suppression by using growth interruptions and real-time flux stabilization (Stolyarov et al., 2 Dec 2025). Quantum-confinement and excitonic miniband engineering in the SPSL suppress sub-bandgap absorption near the cavity mode, resulting in measured $Q$ nearly double the best theoretical value for a comparable ternary alloy. Growth protocols include substrate rotation, in situ RHEED monitoring, and spatial compositional control to impart intended detuning and field profile gradients (Stolyarov et al., 2 Dec 2025).

Hybrid architectures—combining semiconductor and dielectric or SWG reflectors, buried parabolic photonic defects, and planar geometry—extend the operational wavelength range from the UV (300 nm) to mid-IR (3 μm), and allow flexible integration of diverse emitters, such as perovskite nanocrystals, color centers, or 2D material excitons (Gaur et al., 2023). Electrically driven devices and cQED embodiments can be realized by substituting transparent contacts or incorporating electrodes within the parabolic defect structure.

A plausible implication is that interface engineering at the atomic scale, combined with customizable field and thermal properties through planar and hybrid architectures, will drive further reduction of mode volumes, increase $\beta$ factors, and open routes for ultralow-threshold quantum light sources, high-coherence polariton condensates, and strongly-coupled hybrid quantum–opto–acoustic systems across a broad spectral range (Stolyarov et al., 2 Dec 2025, Gaur et al., 2023, Wang et al., 2017).

7. Applications and Integration

High-Q planar GaAs-based microcavities are fundamental components for:

• Low-threshold polariton and quantum-dot lasers, where the ability to tune $Q$, $\beta$, and $V_m$ enables ultra-efficient coherent light generation (Gaur et al., 2023, Stolyarov et al., 2 Dec 2025).
• Single-photon and entangled photon sources for quantum communication, due to small mode volume and strong emitter–field overlap (Gaur et al., 2023, Wang et al., 2017).
• Cavity QED experiments across visible to IR regions, exploiting flexible field profiles and strong coupling (Stolyarov et al., 2 Dec 2025).
• Cavity optomechanics and quantum phononics, by embedding phononic DBRs or SAW cavities for hybrid photon–phonon platform development (Lagoin et al., 2018, Msall et al., 2019).
• Monolithic or hybrid integration on-chip, benefitting from ex situ mirror processing, field engineering, and defect-free growth methods (Stolyarov et al., 2 Dec 2025, Gaur et al., 2023).

Such versatility ensures that high-Q planar GaAs-based microcavities remain central to advancements in semiconductor quantum photonics, coherent optomechanics, and polaritonic circuits.