---
title: InGaP-on-Insulator Photonics
url: https://www.emergentmind.com/topics/ingap-on-insulator-photonic-platforms
type: topic
---

# InGaP-on-Insulator Photonics

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) [2105.12705][2406.02434][2410.20507][2406.18788].

## 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$_\mathrm{InGaP}$(1550 nm) ≈ 3.10–3.15) ensures strong vertical and lateral optical confinement. The insulator, typically thermal SiO$_2$ (n ≈ 1.44) or ALD Al$_2$O$_3$ (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$_2$ or Al$_2$O$_3$) provides surface passivation and mechanical stability [2105.12705][2406.02434].

- **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$_4$OH:H$_2$O$_2$), 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$_2$, and Ti/Pt microheaters are optionally integrated for thermal tuning [2406.18788][2410.20507].

A summary of principal material stacks is shown below:

| Core           | Thickness (nm) | Substrate/Underclad   | Top Cladding   |
|----------------|---------------|-----------------------|----------------|
| In$_{0.49}$Ga$_{0.51}$P | 102–115       | SiO$_2$/Si or air      | SiO$_2$, Al$_2$O$_3$   |
| In$_{0.5}$Ga$_{0.5}$P   | 110           | GaAs                   | Al$_2$O$_3$ (35 nm)    |

Surface roughness RMS < 1 nm and interface defect density ≪ 1000 cm$^{-2}$ are routinely achieved [2406.18788].

## 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$^{(2)}$ interactions (|2m$_\mathrm{TE}$ – m$_\mathrm{TM}$|=2 for 775 nm↔1550 nm). Coupling is achieved via straight or pulley-style bus waveguides with gaps of 250–400 nm [2105.12705][2406.18788].

- **Spiral waveguides:** Enable extended interaction lengths (up to 12.5 cm per spiral) for high conversion efficiency in waveguide-based χ$^{(2)}$ processes [2406.18788].

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 [2105.12705][2406.18788].

## 3. Linear and Nonlinear Optical Properties

**Propagation loss and Q-factor:** Optimized InGaP-OI waveguides demonstrate intrinsic Q$_i$ 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 [2406.18788][2410.20507].

**Nonlinear susceptibilities:** InGaP exhibits χ$^{(2)}_{xyz}$ ≈ 220 pm/V and a third-order nonlinearity n$_2$ ∼ 1×10$^{-17}$ m$^2$/W [2105.12705][2406.18788]. The symmetry class is $\overline{4}3m$, with the principal χ$^{(2)}$ tensor elements being χ$^{(2)}_{xyz}$, χ$^{(2)}_{yzx}$, and χ$^{(2)}_{zxy}$.

**Modal phase matching** is enabled by exploiting the large index contrast and dimensional control to realize n$_\mathrm{eff}$(TE$_{00}$, ω) ≈ n$_\mathrm{eff}$(TM$_{00}$, 2ω). This provides Δk = β$_{\mathrm{sh}}$ – 2β$_{\mathrm{p}}$ ≈ 0 for second-harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC), supporting ultra-efficient nonlinear conversion in both microring and straight waveguide geometries [2105.12705][2406.02434][2406.18788][2410.20507].

Normalized SHG conversion efficiencies η$_\mathrm{norm}$ = P$_{2ω}$/P$_{ω}^2$L$^2$ of up to 128,000%/W/cm$^2$ have been demonstrated at 1.55 μm pump, nearly two orders of magnitude higher than previous C-band platforms [2406.02434][2410.20507].

## 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$^4$ and two-photon interference (Franson) visibility V$_\text{corr}$ > 98% have been achieved [2406.02434][2105.12705].

- **Microring-based schemes** provide enhanced photon indistinguishability, spectrally narrow pair sources, and potential frequency-bin entanglement through dispersion-engineered χ$^{(2)}$ or χ$^{(3)}$ wave mixing [2406.18788].

- **Single-photon-level χ$^{(2)}$:** The single-photon nonlinear coupling rate g/2π = 11.2 MHz and nonlinearity-to-loss ratio g/κ$_{a,i} \simeq 1.5\%$ exceed all prior thin-film χ$^{(2)}$ systems, enabling paradigms such as quantum nondemolition measurement, continuous-variable squeezed states, and coherent up/downconversion [2105.12705].

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

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

| Platform                | χ$^{(2)}$ (pm/V) | Max g/κ (%) | η$_\mathrm{SHG}$ (%/W/cm$^2$) | 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$_x$Ga$_{1-x}$As      | ~100             | ≤0.3        | lower                  | ≤5                  |
| PPLN (LiNbO$_3$)        | ~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 [2105.12705][2406.02434].

## 6. Limitations and Optimization Strategies

Key technical challenges for InGaP-OI include:

- **Thickness nonuniformity:** Variations σ$_t$>2 nm over wafer-scale lead to spatial variations in phase matching, especially limiting for >2 mm device lengths [2410.20507].
- **Sidewall roughness:** σ$_\mathrm{rms}$ ≈ 0.6–1 nm yields propagation losses of 1–2 dB/cm; improved etch recipes and resist reflow are under investigation for further reduction [2406.18788][2410.20507].
- **Bonding/interface defects:** Absorption or Qi degradation (vs. suspended) results from voids at the InGaP–SiO$_2$ interface; addressed via low-temperature, plasma-activated bonding, advanced surface cleaning, and ALD passivation/anneal [2406.18788][2410.20507].
- **Limited thermal tuning:** The low thermo-optic coefficient of SiO$_2$ underclad limits tuning range; microheaters and potentially AlN piezoelectric actuators are implemented for wavelength fine control [2406.18788].

Future optimizations include thicker oxide cladding (power handling), deuterated SiO$_2$ for reduced absorption, and monolithic integration of III–V sources and detectors for fully functional, scalable quantum PICs [2410.20507][2406.18788].

## 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) [2406.02434][2410.20507][2406.18788].

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.

Source: https://www.emergentmind.com/topics/ingap-on-insulator-photonic-platforms