InGaP-on-Insulator Photonics

• InGaP-on-Insulator photonic platforms are integrated circuits using lattice-matched InGaP films on low-index insulators to achieve high χ(2) nonlinearity and strong optical confinement.
• They employ both transfer-free and wafer-scale bonding techniques to fabricate waveguides and resonators with low propagation loss and high Q-factors.
• The platforms enable ultrafast second-harmonic generation and efficient photon-pair production, supporting scalable quantum photonic integrated circuits.

Indium Gallium Phosphide on Insulator (InGaP-on-Insulator, or InGaP-OI) photonic platforms comprise integrated photonic circuits leveraging thin films of In$_{x}$Ga$_{1-x}$P, typically lattice-matched and epitaxially grown, with high refractive index contrast on a low-index insulator such as SiO$_2$ or Al$_2$O$_3$. These platforms enable ultrafast and ultra-efficient nonlinear photonic processes—most notably second-order (χ$^{(2)}$) phenomena—spanning from the visible into the telecommunication bands. The core features are high χ$^{(2)}$ (∼220 pm/V), large optical bandgap (∼1.9 eV), low propagation loss, and the ability to be monolithically integrated with III–V quantum light sources and detectors. InGaP-OI supports both passive waveguides and high confinement resonators, exhibiting figures-of-merit and quantum nonlinear performance that exceed those of established thin-film χ$^{(2)}$ materials, positioning it as a leading candidate for scalable quantum photonic integrated circuits (QPICs) (Zhao et al., 2021, Akin et al., 2024, Akin et al., 2024, Thiel et al., 2024).

1. Material Stack and Fabrication Methods

InGaP-OI platforms utilize thin (100–115 nm) epitaxial In$_{x}$Ga$_{1-x}$P films (x ≈ 0.48–0.5) providing a direct, wide bandgap (≈1.9–1.92 eV). The high refractive index (n$_{1-x}$0(1550 nm) ≈ 3.10–3.15) ensures strong vertical and lateral optical confinement. The insulator, typically thermal SiO$_{1-x}$1 (n ≈ 1.44) or ALD Al$_{1-x}$2O$_{1-x}$3 (n ≈ 1.70), provides a refractive index contrast ∆n ≥ 1.4, supporting tight optical modes and efficient nonlinear overlap.

There are two principal fabrication flows:

• Transfer-free approach: InGaP epitaxial layers are grown on GaAs, patterned via e-beam lithography and etched (ICP–RIE), followed by selective undercut (e.g., citric-acid etch) to create a suspended or oxide-supported device. Top oxide (35–50 nm ALD SiO$_{1-x}$4 or Al$_{1-x}$5O$_{1-x}$6) provides surface passivation and mechanical stability (Zhao et al., 2021, Akin et al., 2024).
• Wafer-scale bonding: MOCVD-grown InGaP/AlGaAs/GaAs stacks are plasma-activated and bonded to oxide-on-Si handle wafers (100 mm). The GaAs substrate is removed using wet etches (NH$_{1-x}$7OH:H$_{1-x}$8O$_{1-x}$9), and AlGaAs etch stop is removed by dilute HF. Deep-UV stepper lithography and ICP etching define devices with 102–115 nm InGaP thickness and 200 nm minimum feature sizes. Devices are conformally clad with ALD and PECVD SiO$_2$0, and Ti/Pt microheaters are optionally integrated for thermal tuning (Thiel et al., 2024, Akin et al., 2024).

A summary of principal material stacks is shown below:

Core	Thickness (nm)	Substrate/Underclad	Top Cladding
In$_2$1Ga$_2$2P	102–115	SiO$_2$3/Si or air	SiO$_2$4, Al$_2$5O$_2$6
In$_2$7Ga$_2$8P	110	GaAs	Al$_2$9O$_2$0 (35 nm)

Surface roughness RMS < 1 nm and interface defect density ≪ 1000 cm$_2$1 are routinely achieved (Thiel et al., 2024).

2. Waveguide and Resonator Engineering

Single- and multimode ridge or strip-loaded waveguides are defined with core widths ranging from 400 nm (single-mode) to 1.5 μm (multimode, dispersion engineered), with typical heights of 102–115 nm. High-index contrast supports deep sub-micron bending radii (≥5 μm for microrings; ≥50 μm for spirals).

Resonator geometries include microring and spiral designs:

• Microring resonators: Radii from 5 μm (high FSR, 560 GHz) to 40 μm (low FSR, ∼400 GHz), with azimuthal mode indices carefully engineered for phase matching in χ$_2$2 interactions (|2m$_2$3 – m$_2$4|=2 for 775 nm↔1550 nm). Coupling is achieved via straight or pulley-style bus waveguides with gaps of 250–400 nm (Zhao et al., 2021, Thiel et al., 2024).
• Spiral waveguides: Enable extended interaction lengths (up to 12.5 cm per spiral) for high conversion efficiency in waveguide-based χ$_2$5 processes (Thiel et al., 2024).

Inverse-taper edge couplers provide chip-to-fiber coupling with losses as low as 3.5 dB/facet for 1.5 μm mode field diameter PM fiber. Sidewall angle is ∼88–90°, and etched profiles preserve verticality critical for high-Q and phase-matched operation (Zhao et al., 2021, Thiel et al., 2024).

3. Linear and Nonlinear Optical Properties

Propagation loss and Q-factor: Optimized InGaP-OI waveguides demonstrate intrinsic Q$_2$6 up to 440,000 (corresponding to propagation losses as low as 1.22 dB/cm) at 1550 nm for split-resonance modes. Loss increases at smaller radii (e.g., R=20 μm yields α ≈ 5.4 dB/cm), reflecting enhanced scattering. Sidewall roughness of 0.6–1 nm RMS is a dominant extrinsic loss source (Thiel et al., 2024, Akin et al., 2024).

Nonlinear susceptibilities: InGaP exhibits χ$_2$7 ≈ 220 pm/V and a third-order nonlinearity n$_2$8 ∼ 1×10$_2$9 m$_3$0/W (Zhao et al., 2021, Thiel et al., 2024). The symmetry class is $_3$1, with the principal χ$_3$2 tensor elements being χ$_3$3, χ$_3$4, and χ$_3$5.

Modal phase matching is enabled by exploiting the large index contrast and dimensional control to realize n$_3$6(TE$_3$7, ω) ≈ n$_3$8(TM$_3$9, 2ω). This provides Δk = β$^{(2)}$0 – 2β$^{(2)}$1 ≈ 0 for second-harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC), supporting ultra-efficient nonlinear conversion in both microring and straight waveguide geometries (Zhao et al., 2021, Akin et al., 2024, Thiel et al., 2024, Akin et al., 2024).

Normalized SHG conversion efficiencies η$^{(2)}$2 = P$^{(2)}$3/P$^{(2)}$4L$^{(2)}$5 of up to 128,000%/W/cm$^{(2)}$6 have been demonstrated at 1.55 μm pump, nearly two orders of magnitude higher than previous C-band platforms (Akin et al., 2024, Akin et al., 2024).

4. Quantum Nonlinear Photonic Functions

The InGaP-OI platform is engineered for quantum photonic functionality, especially for photon-pair generation and nonlinear frequency conversion in the C-band.

• SPDC performance: Pair-generation rates reach 97 GHz/mW in 1.6 mm-long waveguides, with a bandwidth Δλ=115 nm (Δν≈14.4 THz) and per-THz brightness of 6.7 GHz/mW/THz. Coincidence-to-accidentals ratios (CAR) >10$^{(2)}$7 and two-photon interference (Franson) visibility V$^{(2)}$8 > 98% have been achieved (Akin et al., 2024, Zhao et al., 2021).
• Microring-based schemes provide enhanced photon indistinguishability, spectrally narrow pair sources, and potential frequency-bin entanglement through dispersion-engineered χ$^{(2)}$9 or χ$^{(2)}$0 wave mixing (Thiel et al., 2024).
• Single-photon-level χ$^{(2)}$1: The single-photon nonlinear coupling rate g/2π = 11.2 MHz and nonlinearity-to-loss ratio g/κ$^{(2)}$2 exceed all prior thin-film χ$^{(2)}$3 systems, enabling paradigms such as quantum nondemolition measurement, continuous-variable squeezed states, and coherent up/downconversion (Zhao et al., 2021).

5. Benchmarking and Comparison to Other χ$^{(2)}$4 Platforms

Quantitative benchmarks of InGaP-OI versus state-of-the-art thin-film χ$^{(2)}$5 platforms are summarized below:

Platform	χ$^{(2)}$6 (pm/V)	Max g/κ (%)	η$^{(2)}$7 (%/W/cm$^{(2)}$8)	SPDC Rate (MHz/μW)
InGaP (OI)	~220	1.5	128,000 (guide), 71,200 (ring)	27.5–97 (device-dependent)
GaAs	∼238	≤0.5	lower	≤5
Al$^{(2)}$9Ga$^{(2)}$0As	~100	≤0.3	lower	≤5
PPLN (LiNbO$^{(2)}$1)	~54	≤1.0	~20,000 (ultra-high-Q)	2.8
AlN	1–6	≤0.1	∼2,500	≤5

InGaP surpasses other platforms in both absolute and normalized SHG/SPDC efficiency and nonlinearity-to-loss ratio. The wide bandgap (∼1.9 eV) prevents two-photon absorption at telecom wavelengths, supporting high-power operation (Zhao et al., 2021, Akin et al., 2024).

6. Limitations and Optimization Strategies

Key technical challenges for InGaP-OI include:

• Thickness nonuniformity: Variations σ$^{(2)}$2>2 nm over wafer-scale lead to spatial variations in phase matching, especially limiting for >2 mm device lengths (Akin et al., 2024).
• Sidewall roughness: σ$^{(2)}$3 ≈ 0.6–1 nm yields propagation losses of 1–2 dB/cm; improved etch recipes and resist reflow are under investigation for further reduction (Thiel et al., 2024, Akin et al., 2024).
• Bonding/interface defects: Absorption or Qi degradation (vs. suspended) results from voids at the InGaP–SiO$^{(2)}$4 interface; addressed via low-temperature, plasma-activated bonding, advanced surface cleaning, and ALD passivation/anneal (Thiel et al., 2024, Akin et al., 2024).
• Limited thermal tuning: The low thermo-optic coefficient of SiO$^{(2)}$5 underclad limits tuning range; microheaters and potentially AlN piezoelectric actuators are implemented for wavelength fine control (Thiel et al., 2024).

Future optimizations include thicker oxide cladding (power handling), deuterated SiO$^{(2)}$6 for reduced absorption, and monolithic integration of III–V sources and detectors for fully functional, scalable quantum PICs (Akin et al., 2024, Thiel et al., 2024).

7. Functional Applications and Integration Roadmap

InGaP-OI platforms enable a spectrum of quantum and nonlinear optical technologies, including:

• On-chip entangled photon-pair and heralded single-photon sources for quantum communication;
• Wavelength-multiplexed entanglement distribution over 115 nm bandwidth in the C-band;
• Coherent wavelength conversion and frequency conversion for connecting disparate quantum systems;
• Squeezed-light sources and parametric amplifiers for continuous-variable quantum optics;
• Integrated quantum repeaters and multiplexed photonic circuits.

Scalability is supported by 100 mm wafer-scale fabrication, CMOS-compatible passivation, and deep-UV lithography (200 nm resolution), yielding >1000 components per wafer. The platform is compatible with monolithic integration of III–V pump lasers and photodetectors on the same substrate owing to the lattice-matched growth (InGaP→GaAs) (Akin et al., 2024, Akin et al., 2024, Thiel et al., 2024).

A plausible implication is that further advances in InGaP-OI PICs will establish a foundation for large-scale, monolithically integrated quantum photonic processors, with deployment in quantum information, spectroscopy, and high-speed classical and quantum networking.