Photonic Integrated Circuits: Overcoming Challenges in Silicon Photonics

Networking companies have been communicating largely through electrical signals sent via copper wire. The challenge is that the speed of transmission is significantly affected by the interaction between electrons and other atoms traveling through the wire that generates heat.

Increased data traffic demand has revolutionized long-distance communication by substituting electrons with light (photons) sent via optical fiber to transmit data across a wide range of distances. Advances are even made for sending multiple signals through the same fiber by using different light wavelengths that do not interfere with each other.

To merge optical signals with electronic data processing, photonic systems must replace electronic ones, and advances in photonic integrated circuits (PICs) are needed. Photonic technology transmits and manipulates light as light particles or photons. Efforts to develop the next generation of optoelectronic communication systems are moving forward with the development of unique devices for use in ultrafast communication systems.

What are photonics integrated circuits?

Photonic integrated circuits are microchips that contain two or more photonic components functioning as an integrated optical circuit. PICs manipulate photons (light particles) to process and transmit information, unlike electronic integrated circuits that use electrons. This fundamental difference allows PICs to overcome many of the limitations of traditional electronic circuits. These miniaturized optical systems have the ability to transform various industries, from telecommunications to healthcare.

Key Components of Photonic Integrated Circuits

Typical components of a PIC include

● A light source, LASER

● Modulators convert digital electronic signals to digital optical signals.

● Photodetectors convert digital optical signals to digital electronic signals.

● Waveguides are structures that direct and confine the propagation of light.

● Filters

● Multiplexers and demultiplexers.

These elements work together to generate, manipulate, and detect light signals, enabling complex optical functions on a microscopic scale.

Silicon Photonics Integrated Circuits

Silicon photonic integrated technology, as illustrated in Fig. 1, combines optical and electronic components on a single silicon chip. Even if silicon-based computing has already reached its physical limits in terms of speed and scale, many technological advances are still anticipated to be based on this accessible, affordable material.

Fig. 1: Illustration of Silicon Photonics Integrated Circuits Source: MDPI

Advantages

Silicon photonics have advanced significantly and have been the preferred material for nanophotonic devices because of their

● High refractive index

● Proven fabrication process

● Compatibility with CMOS electronics

● Increased availability

● Low cost, etc.

For several reasons, silicon's well-known low absorption range can be viewed as an advantage in waveguides. Signal propagation through the waveguide can be more efficiently accomplished with such low absorption in the near-infrared wavelength range employed in silicon photonics.

This low absorption means less light loss during propagation, allowing for longer transmission distances without appreciable degradation. This is essential to provide high-performance optical communication and energy efficiency without the need for extra amplification, preserve signal integrity, minimize crosstalk between adjacent waveguides, and integrate with other photonic components properly.

Fabrication

Silicon photonic PICs are typically fabricated on silicon-on-insulator (SOI) wafers using processes compatible with CMOS manufacturing. This allows for potential co-integration with electronic circuits and existing semiconductor fabrication infrastructure.

Conventional microfabrication methods, such as photolithography and electron beam lithography, have limitations regarding the resolution and accuracy of the structures they produce. Several new methods enable higher resolution, more complexity, and more control over the dimensions and shapes of nanoscale objects. Extreme ultraviolet lithography (EUVL), direct laser writing, focused ion beam (FIB) milling, and nanoimprint lithography are some examples.

Challenges

The two common challenges faced by silicon photonic integrated circuits are discussed below:

Silicon’s Inability to Emit Light

Silicon's capacity to emit and absorb light is restricted because of its inherent indirect band gap, as shown in Fig. 2. Their primary challenge is the indirect band gap between the valence and conduction bands in silicon, which prevents radiative recombination. Because of this, light emission in monocrystalline silicon is challenging to achieve.

Fig. 2: Diagrammatic illustration of direct and indirect bandgap Source: Rakesh Kumar, Ph.D.

Research using non-silicon III-V compound semiconductors (such as InP and GaAs) and attempts to create silicon photonic devices that are compatible with CMOS technology by employing material and design modifications have been made.

Lack of a Pockels Effect

The lack of a Pockels effect in silicon is a significant challenge for silicon photonics, particularly for developing efficient electro-optic modulators. The Pockels effect is the use of an electric field to change the refractive index of a material (how fast light goes through a certain object). The Pockels effect is commonly used in other materials for high-speed, low-power electro-optic modulation. Its absence in silicon limits the options for creating efficient modulators.

Overcoming Challenges

By combining various approaches in a creative manner, it will be possible to overcome the constraints of silicon in nanophotonic devices, which include

● Utilizing the most recent developments in materials

● Nanofabrication

● Modified structure

● Modified geometry

Utilizing Alternative Material Sources

Silicon is frequently employed in photonics devices, but its ability to emit and absorb light is limited, as discussed above. Using complementary materials with superior optical properties is a potential option. , such as

● III-V semiconductors like indium phosphide (InP) and gallium arsenide (GaAs)

● 2D materials, including gallium nitride (GaN) and graphene

● Transition metal dichalcogenides (TMDCs)

By incorporating these materials into silicon-based electronics, hybrid architectures with higher performance can be made.

Modified structure

For certain silicon-based device limitations, a new device architecture provides an improved solution.

● Photonic crystal or plasmonic waveguides can be used in place of conventional optical waveguides and can provide higher confinement and reduced losses.

● Similarly, light sources like quantum dots or nanowire LEDs, which can offer higher efficiency and lower power consumption, can replace conventional light emitters like LEDs or laser diodes.

Nanoscale structures have the ability to manipulate light in ways that are not possible with traditional optics. For instance, photonic crystals can produce photonic bandgaps, which stop light at specific wavelengths from passing through the substance. Similarly, electromagnetic fields in very small volumes can be enhanced and contained using plasmonic structures. The architecture used will be determined by the application, performance needs, and suitability of the materials and technology used.

Modified geometry

A silicon device's design can be engineered to imitate several physical features, often only possible with alternative materials. Altering a structure's dimensions and geometry can also manipulate light. Altering a structure's dimensions and geometry can also manipulate light. For instance, silicon waveguides are typically formed like rectangles to reduce the number of optical modes passing through them.

Modulating the waveguide's geometry will allow for the control of light dispersion and polarization. Utilizing specifically designed nanostructures, such as nanowires, nanotubes, and nanodisks, is another illustration of light confinement. The light can then be contained in incredibly small amounts.

However, these efforts have several challenges, and research is being done to solve them and improve the performance of silicon photonic integrated circuits.

Summarizing the Key Points

● Photonic integrated circuits utilize light for data transmission, offering significant advantages over traditional electrical signals, including higher speed and reduced heat generation.

● Key challenges in silicon photonics include its inability to emit light due to an indirect band gap and the absence of a Pockels effect for efficient electro-optic modulation.

● Enabling hybrid architectures that improve performance in silicon-based devices can improve performance, and alternative materials like III-V semiconductors and 2D materials can enhance optical properties.

Reference

Avi Karsenty, “Overcoming Silicon Limitations in Nanophotonic Devices by Geometrical Innovation: Review,” IEEE Photonics Journal 15, no. 4 (August 1, 2023): 1–19, https://doi.org/10.1109/jphot.2023.3295077.

Lee Carroll et al., “Photonic Packaging: Transforming Silicon Photonic Integrated Circuits into Photonic Devices,” Applied Sciences 6, no. 12 (December 15, 2016): 426, https://doi.org/10.3390/app6120426.

Asianometry, “Silicon Photonics: The Next Silicon Revolution?,” June 16, 2022, https://www.youtube.com/watch?v=29aTqLvRia8.