Visible-Spectrum Photonic Integrated Circuits

• Visible-spectrum PICs are chip-scale optical circuits that operate from 400–700 nm, integrating both passive and active elements using advanced materials and microfabrication.
• They enable high-performance applications in quantum photonics, biosensing, imaging, and astronomical interferometry by achieving ultralow losses and precise alignment.
• Innovative designs incorporate hybrid laser bonding, edge couplers, and integrated detectors, delivering sub-dB losses, high modulation speeds, and single-photon sensitivity.

Visible-spectrum photonic integrated circuits (PICs) are monolithically or heterogeneously integrated optical circuits that confine and manipulate visible-wavelength (approximately 400–700 nm) electromagnetic waves on a chip-scale platform. These circuits incorporate passive and active elements, including waveguides, resonators, beam splitters, phase shifters, modulators, sources, and detectors, in high-density arrangements. Advances in materials engineering, microfabrication, and hybrid integration have enabled visible-spectrum PICs to support key functionalities in quantum information science, biosensing, imaging, displays, and more. The visible regime is uniquely challenging due to shorter wavelengths that amplify scattering and loss, but it offers access to atomic transitions, efficient biological imaging windows, and superior color rendering for consumer and scientific technologies.

1. Materials Systems and Waveguide Platforms

Visible-spectrum PICs utilize a variety of materials systems optimized for low loss, index contrast, and compatibility with visible light. Silicon nitride (Si₃N₄) on silicon dioxide (SiO₂) is the prevailing low-loss passive platform, capable of propagation losses below 0.1 dB/cm at 461 nm, thanks to precise control over LPCVD/PECVD deposition, tripleX waveguide geometry, and photonic Damascene processing (Buzaverov et al., 16 May 2024). Ion-exchanged glass waveguides (K⁺:Na⁺) provide single-mode guiding from ~530–820 nm, enabling robust beam combiners for astronomical interferometry (Lallement et al., 5 Sep 2024, Lallement et al., 5 Sep 2024). Thin-film lithium niobate (TFLN), offering high second-order nonlinearity, supports integrated frequency doubling and generation of visible light when combined with III–V laser gain sections via direct bonding and adiabatic tapers (Morin et al., 12 Dec 2024). Silicon photonics platforms have been extended to the visible by co-integration with SiN, enabling foundry-scale hybrid integration of InGaN laser sources at 450 nm (Mu et al., 19 Oct 2025).

Material choice also dictates compatibility with phase-actuation mechanisms: Si₃N₄ and SiO₂ can be integrated with AlN piezo-optomechanical actuators for high-speed, low-power, large-scale programmability (Dong et al., 2021), while glass diffusion processes inherently yield low propagation loss but lower index contrast than semiconductor platforms.

2. Light Sources and Laser Integration

Scalable integration of visible-wavelength laser sources into PICs is a core challenge due to tight modal confinement and strict alignment tolerances at short wavelengths. Hybrid integration strategies using passive-alignment flip-chip bonding have demonstrated sub-micron-precision placement of 450-nm InGaN laser diodes onto SiN Si photonic platforms. This approach leverages co-designed alignment marks, mechanical stoppers, nanoscale mode size edge couplers, and carefully optimized solder bump interconnects, yielding minimum on-chip coupling loss of 1.1 dB and wall-plug efficiency of 7.8%. Multiple independent lasers can be passively bonded onto the same chip, supporting complex circuits with active routing and chip-integrated photodetectors (Mu et al., 19 Oct 2025).

Heterogeneous integration—in which GaAs-based quantum well or quantum dot lasers emitting in the near-IR are directly bonded to thin-film lithium niobate, followed by on-chip frequency doubling via periodically poled TFLN—provides access to the “green gap” (515–595 nm) through cavity-coupled separated gain architectures and efficient nonlinear wavelength separation (Morin et al., 12 Dec 2024). Such architectures allow high optical powers to circulate in the fundamental mode without thermal crosstalk, enabling conversion efficiencies limited primarily by waveguide quality and modal overlap.

Visible-spectrum PICs thereby integrate both direct and frequency-converted laser sources, enabled by advances in edge coupling, adiabatic tapers, and wafer-scale alignment methodologies.

3. Modulators, Phase Shifters, and Programmability

Phase and amplitude manipulation at visible wavelengths is addressed using both passive and active devices. Limitations of thermo-optic phase shifters—mainly slow (μs) response and high (10–100 mW) power dissipation—have directed research towards aluminum nitride (AlN) piezo-optomechanical actuators monolithically integrated with SiN waveguides (Dong et al., 2021). These actuators achieve >100 MHz phase modulation bandwidth, 5 ns rise/fall times, and ~2 nW holding power, supporting operation from room to cryogenic temperatures with minimal cross-talk.

Micromechanical cantilever phase shifters, placed in Mach–Zehnder interferometer meshes, can be synchronously driven at mechanical eigenfrequencies to leverage resonant enhancement. The resulting modulation factor,

$$G_m = \frac{A_c(\omega_s)}{A_c(\omega_s \approx 0)} = \left[ (1-(\omega_s/\omega_c)^2)^2 + (\omega_s/\omega_c)^2 (1/Q_m)^2 \right]^{-1/2},$$

scales with the mechanical quality factor $Q_m$, reducing the required drive voltage and overall power with bandwidths limited by $Q_m$ (Dong et al., 2023). This architecture, demonstrated for 1x8 programmable switches with ~11 ns channel cycling, is compatible with high-density optical phased arrays and neural networks.

Reconfigurable networks using meshes of Mach–Zehnder interferometers, often programmed via integrated heaters or ultrafast electromechanical actuators, can implement arbitrary linear transformations on N-mode input/output vectors, supporting many-mode interferometry, quantum state processing, and high-throughput measurement (Dong et al., 2021, Bütow et al., 2022).

4. Integrated Detectors and Quantum Photonics

Detection at the single-photon level, critical for quantum information and sensing, has been addressed by integrating superconducting nanowire single-photon detectors (SNSPDs) with visible-to-mid-IR sensitivity directly onto waveguides via a micrometer-scale flip-chip process. This method decouples SNSPD fabrication from PIC processing, achieves 100% device yield, and allows pre-selection of high-efficiency, low-jitter detectors. On-chip detection efficiencies reach up to 45% per detector and >10% system efficiency for multi-detector circuits—substantially higher than the <0.2% threshold of prior systems (Najafi et al., 2014). Detection latencies (<50 ps timing jitter) and nanosecond reset times enable high-fidelity on-chip photon correlation measurements, a requirement for scalable quantum photonic processors.

Hybrid architectures that couple tree-mesh PICs of piezo-actuated MZIs with inverse-taper matched diamond nanostructures realize fast, high-fidelity optical pulse carving, individualized qubit addressing, and g^2(0) < 0.14 autocorrelation for efficient single-photon emission and quantum memory readout (Palm et al., 2023).

5. Circuit Architectures for Sensing, Imaging, and Interferometry

High-density PICs enable applications in imaging, spectroscopy, and astronomical interferometry. Reconfigurable meshes of Mach–Zehnder interferometers, coupled to spatially distributed grating couplers (“pixels”), provide spatially resolved amplitude and phase detection of free-space beams. Calibrated operation, including robust off-design usage, supports quantitative mode mapping, polarization sensitive detection, and adaptable field sampling (Bütow et al., 2022).

In astronomical beam-combining, K⁺:Na⁺ ion-exchanged glass waveguides support robust, achromatic beam combining from 530–820 nm with low loss (~0.3 dB/cm at 780 nm). Advanced building blocks, such as symmetric/asymmetric directional couplers and multi-segment achromatic phase shifters, meet stringent requirements for fringe phase sampling and pupil remapping at the Hα transition (656.3 nm), vital for high-dynamic-range, high-stability telescopic imaging (Lallement et al., 5 Sep 2024, Lallement et al., 5 Sep 2024).

Flat-panel laser displays leverage large-scale visible PICs combining thousands of elements (Y-splitters, wavelength-multiplexers, impedance-matched grating emitters) on SiN/SiO₂/AlOx multilayers for ultra-thin, high-uniformity, wide-gamut AR/VR display modules (Shi et al., 26 Dec 2024). Integration with LCoS panels and targeted photonic emission maximizes color volume and miniaturization.

6. Circuit Design, Fabrication, and Characterization

Simulation and design of visible-spectrum PICs require rigorous modeling due to enhanced sensitivity to fabrication variation and loss. Core simulation tools include the beam propagation method (BPM), finite difference time domain (FDTD), and eigenmode expansion (EME) methods. For instance,

$$\frac{\partial E(x,y,z)}{\partial z} = \frac{i}{2k_0}\nabla^2_\perp E(x,y,z) + ik_0\, n_1(x,y) E(x,y,z)$$

employs the slowly-varying envelope approximation of BPM, critical for swift propagation calculations (Oquendo et al., 23 Jun 2025). Machine learning techniques (genetic algorithms, direct binary search, deep neural networks) accelerate inverse design, especially for devices requiring pixel-level control at the sub-micron scale. Quantum and quantum-inspired optimization methods (VQE, QAOA, tensor networks) are emerging as scalable solutions for non-convex, high-dimensional photonic layout challenges.

Nondestructive circuit characterization uses embedded nonlinear elements—such as Kerr resonators—as optical power discriminators, enabling extraction of individual component loss/gain (accuracy better than 0.1 dB) via threshold power difference measurements:

$$\alpha_L = \frac{\alpha + \Delta}{2},\quad \alpha_R = \frac{\alpha - \Delta}{2}$$

where $\Delta$ is the measured difference in threshold in dB (Chen et al., 21 Oct 2025). Such techniques support precise optical balancing, loss library validation, and accurate quantum efficiency assessment.

Fabrication employs high-resolution electron beam or DUV lithography, advanced dry etching, chemical-mechanical planarization, low-hydrogen deposition, and thermal annealing to minimize both scattering and absorption losses (Buzaverov et al., 16 May 2024). Heterogeneous integration of III–V gain elements, active flip-chip laser bonding, and local optical/electrical testing is standard for platforms targeting wide spectral coverage and advanced quantum/optical computation.

7. Applications and Outlook

Visible-spectrum PICs are applied in quantum technologies (entanglement distribution, single-photon sources/detectors, quantum memory control), AR/VR displays, LiDAR, biosensing, optical neural networks, and astronomical instrumentation. Modular architectures—comprising independently actuated trees of photonic phase shifters and hybridized atomic or solid-state emitters—improve scalability, channel isolation, and functional extension (Palm et al., 2023). Rewritable PCM-based PIC platforms, leveraging wide-bandgap non-volatile materials, offer iterative rapid prototyping and educational utility via laser-assisted local reconfiguration (Miller et al., 2023).

Ongoing research targets tighter process control (sub-0.1 nm RMS sidewall), novel active integration strategies, and robust large-scale circuit design for progressively more complex, reliable, and scalable PICs. Future advances in component-level diagnostics, programmable electronics, and active light source integration are expected to further solidify visible-spectrum PICs as foundational platforms for next-generation quantum, sensing, and display technologies.