Processing power generally only makes the news when Apple launches its latest iPhone, but researchers from TMOS, the ARC Centre of Excellence for Transformative Meta-Optical Systems have made a breakthrough in the fundamental science that drives processors with the development of on-chip light sources for photonic integrated circuits.

This breakthrough could lead to near-lightspeed communication on your phone, quantum computing power in your hand, as well as highly sophisticated artificial intelligence as your 24/7 know-it-all companion.

For a long time, data transfer was simply the transfer of electrons through copper wires in technology like the telegraph. Later, these same electron-based signals would travel through silicon-based electronic chips within devices such as the computer. The use of light to transmit information instead of electrons was first implemented into fiber optic cables that carried information for relatively long distances before the signals were then converted into electronic ones in devices. Then scientists created the photonic chip, also known as a photonic integrated circuit, which uses photons to transmit and process information. Photonic chips consume much less power and have a smaller footprint than electronic chips. Since information is now processed and transmitted by photons, photonic chips can increase data processing speed and increase data transmission.

However, the potential of photonic chips has been limited by the lack of miniaturized light sources that can be built into them. Until now, photonic chips have required bulky external lasers which are then coupled on. The size of these external lasers results in photonic chips with limited integration density, meaning that there’s a limit to the number of optical components that can physically fit on a microchip and as a result, the processing power of those chips is limited.

In research published in ACS Nano, TMOS researchers describe a solution to the on-chip light source problem that will significantly increase the integration density of photonic chips and lead to vastly higher processing powers. This will enable emerging technologies such as highly sophisticated AI and quantum computing to operate on smaller devices, such as mobile phones.

The researchers have grown microring lasers using a bottom-up approach. These microring lasers are only five micrometers in diameter with quantum wells inside each ring. These quantum wells enable the microring lasers to operate at wavelengths suitable for information telecommunications. They can adjust the wavelength of the lasers by controlling the thickness and composition of the quantum well. Each of the 2mm x 2mm samples they fabricated contained approximately 1000 microring lasers with fabrication yield exceeding 80%, demonstrating chip-scale manufacturing capability.

Previous attempts at microring lasers for communication purposes used top-down or transfer methods of fabrication, where the microring laser was etched out of a substance, similar to how statues are carved out of marble. These methods created rough surfaces on the quantum wells that made them highly inefficient, and limited the device performance, especially for laser dimensions. The TMOS researchers used a method called selective area-metal organic chemical vapour deposition (SA-MOCVD) to simultaneously ‘grow’ thousands of highly efficient lasers from the bottom layer up.

Lead author Wei Wen Wong says, “Recent decades have seen an exponential growth in data capacities of photonic chips, however on-chip light sources that enable high integration density of these photonic integrated circuits have remained elusive, primarily due to fabrication challenges. Our method successfully grows quantum wells with excellent crystal quality and morphology that conforms to the microring cavity. Importantly, it does not require any post-growth cavity etching.

“We solve numerous long-standing issues in the community, making this a breakthrough towards the realization of integrated micro-lasers and the huge step forward in miniaturizing devices for data-demanding technologies such as quantum computing.”

These microring lasers have a tunable wavelength emission in the telecommunication O-band, which is compatible with the wavelength used by other devices in the data transfer chain, such as 5g cell towers. Importantly, they have an efficacy of over 80% across the device. The impact of this technology will include faster internet speeds, faster computing, and an explosion in the Internet of Things, which requires enormous amounts of data transfer.

The results were underpinned by a rigorous analysis by researchers at The University of Manchester led by Dr. Stephen Church and Dr. Patrick Parkinson, who used an AI algorithm to measure the light output from the thousands of microrings and construct a dataset that shows the consistency of the growth process across the entire chip.

Wong says, “The contribution from the team at Manchester University allows us to demonstrate the high quality of the full sample set, rather than just a few individual examples, which gives an added layer of confidence in the scalability of our fabrication process.”

TMOS Chief Investigator Hark Hoe Tan says, “The next steps for this research will be to fabricate these lasers that can be electrically powered and also on silicon wafers as many photonic chips are made on this platform.

For more information about this research, contact connect@tmos.org.au

Bottom-up, Chip-Scale Engineering of Low Threshold, Multi-Quantum-Well Microring Lasers 

Wei Wen Wong, Naiyin Wang, Bryan D. Esser, Stephen A. Church, Li Li, Mark Lockrey, Igor Aharonovich, Patrick Parkinson, Joanne Etheridge, Chennupati Jagadish, and Hark Hoe Tan.

ACS Nano July 14, 2023

Integrated, on-chip lasers are vital building blocks in future optoelectronic and nanophotonic circuitry. Specifically, III–V materials that are of technological relevance have attracted considerable attention. However, traditional microcavity laser fabrication techniques, including top-down etching and bottom-up catalytic growth, often result in undesirable cavity geometries with poor scalability and reproducibility. Here, we utilize the selective area epitaxy method to deterministically engineer thousands of microring lasers on a single chip. Specifically, we realize a catalyst-free, epitaxial growth of a technologically critical material, InAsP/InP, in a ring-like cavity with embedded multi-quantum-well heterostructures. We elucidate a detailed growth mechanism and leverage the capability to deterministically control the adatom diffusion lengths on selected crystal facets to reproducibly achieve ultrasmooth cavity sidewalls. The engineered devices exhibit a tunable emission wavelength in the telecommunication O-band and show low-threshold lasing with over 80% device efficacy across the chip. Our work marks a significant milestone toward the implementation of a fully integrated III–V materials platform for next-generation high-density integrated photonic and optoelectronic circuits.