An Introduction to Integrated Photonic Material Platforms
Integrated photonics is revolutionising the way we process information, sense our environment, and enable technologies for quantum computing, telecommunications, and more. At the core of this field are the material platforms that define the capabilities of photonic integrated circuits (PICs). In this post, we’ll explore the key material platforms shaping integrated photonics: silicon-on-insulator, indium phosphide, silicon nitride, thin-film lithium niobate, femtosecond laser-written glass-based, germanium-on-silicon, and aluminium oxide. We'll delve into their histories, driving applications, and considerations for choosing the right material for a given application.
The figure below illustrates the wavelength ranges and key applications of these materials, serving as a roadmap for understanding how material properties influence integrated photonics device performance.
Figure 1 - A diagram of key integrated photonic material platforms, their windows of transparency, with key applications for each (list non-exhaustive).
Silicon-On-Insulator - SOI: The Integrated Photonics Backbone
Brief History: SOI has long been the material foundation of the micro-electronics industry, forming the basis of transistors and microchips. Its entry into photonics came as engineers sought to leverage its mature manufacturing infrastructure for optical applications. SOI integrated photonics gained traction due to its compatibility with complementary metal-oxide-semiconductor (CMOS) processes.
Key Applications: SOI is ideal for applications such as data centre transceivers, telecommunications networks, and photonic computing, operating effectively in the near-infrared wavelengths around 1.3 to 1.6 µm, where optical fibres also perform well and significant laser/detector infrastructure can be found.
Advantages and Challenges:
Advantages:
High refractive index contrast with native oxide cladding allows for compact devices.
Mature fabrication ecosystem with access to high-volume (300 mm) CMOS-compatible processes.
High-speed modulation and detection made possible via the incorporation of dopants and germanium epitaxial layers respectively.
Challenges:
Silicon is opaque beyond 8 µm, limiting its use in the mid-infrared.
Its lack of a second-order nonlinear optical response hinders applications requiring frequency conversion, though third-order optical nonlinearities are available.
Lasers must be integrated heterogeneously, complicating packaging.
Indium Phosphide - InP: The C-band Specialist
Brief History: One of the first processes developed purely motivated by optical considerations, InP has it all. This III-V material facilitates the integration of lasers, high-speed modulators, passive components, and detectors, all on one platform.
Key Applications: InP has been pioneered for usage in data centre transceivers, telecommunications networks, automotive LiDAR, and quantum key distribution. The InP epitaxial layers can be engineered for excellent operation for wavelengths between 1 to 2 µm, and the platform represents the only commercially available platform where all the crucial elements of photonics technology can be monolithically integrated into one chip.
Advantages and Challenges:
Advantages:
A monolithic solution. Lasers, detectors, modulators, and passives on a single chip.
Excellent electro-optic properties.
Engineered specifically for performance at 1.5 µm.
Challenges:
Complex fabrication processes can result in lower yield when compared to other platforms.
Due to the monolithic integration of lasers, careful considerations must be made with regards to thermal management.
Silicon Nitride - SiN: The Versatile Workhorse
Brief History: SiN has emerged as an integrated photonics material in recent years in an attempt to overcome some of the limitations of SOI. Its low-loss properties (both linear and nonlinear) have made it a promising choice for applications requiring ultra-low losses, both on-chip and when coupling into optical fibre.
Key Applications: SiN is extensively used for quantum photonic computing and frequency comb generation due to its ultra-low propagation losses across visible to near-infrared wavelengths.
Advantages and Challenges:
Advantages:
Broad transparency ranges from visible to mid-infrared (0.4 to 9 µm).
Ultra-low propagation losses (< 5 dB/m possible), enabling ultra-high-Q resonators.
Thermal and mechanical stability, crucial for demanding environments.
Challenges:
Difficult to integrate with active materials required for on-chip lasers and detectors.
Low thermo-refractive coefficient (about 1/10th that of SOI), leading to large thermo-optic phase shifting components.
Fabrication processes require precise stress management to avoid SiN film cracking.
Thin-Film Lithium Niobate - TFLN: The Nonlinear Powerhouse
Brief History: Lithium niobate has been a cornerstone material for electro-optic modulators in telecommunications for decades. Its use in integrated photonics has accelerated recently with the development of the TFLN platform.
Key Applications: TFLN excels in telecom, co-packaged optics, and quantum computing and sensing owing to its ultra-high second-order nonlinear optical properties and low-propagation losses.
Advantages and Challenges:
Advantages:
Exceptional electro-optic and nonlinear optical properties.
High transparency across a wide wavelength range (0.4 to 5 µm).
Low optical losses.
Challenges:
Integration with other materials like silicon or indium phosphide requires complex bonding techniques.
Fabrication costs remain relatively high compared to SOI platform.
Femtosecond laser-written glass-based waveguides - FLWGB: The Higher-dimensional Platform
Brief History: Glass is one of the oldest optical materials, forming the basis of optical fibres that transformed global communications. In integrated photonics, glass remains essential for its low-loss properties. Recently, significant advances in FLWGB waveguides have enabled an integrated photonics technology platform with ultra-low propagation and coupling losses to optical fibre, along with the possibility of 3D architectures, facilitating dense integration.
Key Applications: FLWGB is critical for biological sensing and quantum computing and sensing, due to its extremely low optical losses and dense integration.
Advantages and Challenges:
Advantages:
Extremely low propagation losses in the visible to near-infrared.
High thermal and chemical stability.
Excellent compatibility with existing optical fibres.
Extremely high component densities facilitated via 3D waveguide patterning.
Challenges:
Large mode size due to low refractive index contrast, resulting in less compact devices.
Limited nonlinear and electro-optic capabilities.
Germanium-on-Silicon - GeOSi: The Mid-IR Adventurer
Brief History: GeOSi is a newer integrated photonics platform, developed to address the growing demand for mid-infrared photonic devices. GeOSi’s broader transparency range makes it a strong candidate for sensing applications, with good transparency between 2 and 14 µm.
Key Applications: Germanium is particularly suited for gas sensing and mid-infrared applications in environmental monitoring, defence, and healthcare.
Advantages and Challenges:
Advantages:
Transparent from the near-infrared to the far-infrared (2 to 14 µm).
Compatible with silicon photonics for hybrid integration.
Challenges:
Fabrication processes are less mature than SOI and SiN.
Higher optical losses compared to SOI and SiN.
Aluminium Oxide - AlO: The High-power Handler
Brief History: Aluminium oxide is a relative newcomer, gaining attention for its compatibility with emerging quantum technologies, the incorporation of amplifying materials, and wide transparency range into the UV part of the spectrum.
Key Applications: Aluminium oxide is being explored for trapped-ion quantum computing and other advanced quantum and sensing applications that require visible or even UV wavelengths, where a number of key ions have electronic transitions ideal for quantum computing.
Advantages and Challenges:
Advantages:
High transparency in the ultraviolet to mid-infrared (0.2 to 5 µm).
Compatible with deposition techniques that enable integration with other materials, such as rare earth metals that enable amplification.
Specifically tailored for high-power optical applications.
Challenges:
Still in the early stages of development for photonic integration.
No volume-level wafer processing available yet.
Choosing the Right Material for Your Application
Selecting a material platform depends on several factors, including:
Transparency Window: Ensure the material is transparent at the operating wavelength of your device,
Refractive Index Contrast: High contrast allows for compact devices but may increase propagation losses,
Nonlinear and Electro-Optic Properties: Applications like frequency conversion or modulators require materials with strong nonlinear responses,
Fabrication Maturity: Consider the availability of foundry services and the possibility of scaling to volume production,
Integration Requirements: Many modern applications require hybrid integration of active and passive materials, and
Packaging: Appreciating packaging considerations for your device from the start gives you the best chance of getting your technology into the real world.
The Future of Material Integration
The photonics industry is moving toward heterogeneous integration, where multiple materials are combined in a single platform to leverage their unique strengths. For instance:
SOI can be integrated with InP lasers for on-chip light sources.
SiN can be integrated with SOI layers to utilise low-loss propagation with integrated modulation and detection.
TFLN layers for modulation are being integrated with SiN waveguides for high-speed, low-loss data processing.
Emerging platforms like GeOSi and AlO are poised to play roles in applications like quantum sensing and mid-infrared photonics.
As the field evolves, the ability to engineer bespoke material stacks tailored to specific applications will unlock new possibilities for integrated photonics.
What we can do for you
Our team specialises in helping companies develop integrated photonic products from idea to MVP. Our expertise across the increasingly diverse number of available platforms and wavelengths helps ensure you develop the best product, utilising the best platform for your application.
If you’re navigating the complexities of platform selection for your new technology, let us guide you to success. Reach out today via our contact form.
