Newly-Discovered Phase-Change Materials May Be a Boon to Photonics

Last week, scientists at the University of Southampton designed phase-change materials that, as they claim, may "revolutionize optical circuits" and even take the place of conventional electronic parts. 

 

Researcher Dr. Ioannis Zeimpekis poses the in the cleanroom complex. Image used courtesy of the University of Southampton

 

Researchers say this new material exhibits ultra-low loss at telecommunication wavelengths and can be switched with very low power.

 

Phase-Change Materials in Photonics

Traditional electronics for communication consume a significant portion of their energy at the interconnection level, and their bandwidth is directly limited by the communication length. Using photons instead of electrons mitigates these limitations.

This is where the field of photonics comes into the picture. Much of fiber-optic communication occurs in the wavelength region where optical fibers have little transmission loss. At a wavelength of 1550 nm, the loss of any optical fiber is minimum. Phase-change materials are designed and tested at this wavelength to demonstrate benefits for telecommunication applications.

To compete with electronic devices, photonic devices need to be reprogrammable and reconfigurable while also providing denser integration and miniaturization. Research on chalcogenide phase‐change materials suggests that when a phase-change material (PCM) is heated by electrical or optical pulses, it can be crystallized (SET) and re-amorphized (RESET). This not only significantly varies the electrical resistivity but also the optical properties of PCM. 

The change in properties can be exploited for various applications in photonics.

 

A New Phase-Change Material is Discovered

Phase-change materials in photonics allow for very fast switching between states. The researchers at the University of Southampton discovered that out of all the available technologies, using Sb₂S₃ and Sb₂Se₃ as a phase-change material exhibits the lowest losses.

 

Various optical images of silicon chips with thin films of Sb₂S₃ (pictured in a, c, and e) and  Sb₂Se₃ (pictured in b, d, and f).  Image used courtesy of the University of Southampton

 

These materials were deposited on top of optical chips, where a short laser pulse was used to crystalize the material and change the phase of the guided light. The researchers demonstrated this property reversibly thousands of times. Additionally, the material remembers its last state without any applied signals, leading to significant potential power savings. 

Both Sb₂S₃ and Sb₂Se₃ are highly transparent in the telecommunication wavelength of around 1550 nm and have a moderately large refractive index that is well matched to silicon photonics components, hence offering two important advantages compared to conventional phase-change materials such as GST.

When integrated onto silicon waveguides, these materials have a propagation loss that is two orders of magnitudes lower than the commonly-used optical material (GST—Ge₂Sb₂Te₅). 

 

How the New PCM Will Affect Telecommunications

Optoelectronics provides information transport through waveguides and optical fibers, enabling displays, memories, and integrated optical sensors. Electronic systems are used for data storage while photonics have an edge when data is to be transported. 

Data centers nowadays have an ever-increasing need to handle huge amounts of data due to online streaming, cloud storage, and cloud computing. This is especially true during COVID-19 with data centers increasingly overburdened because of stay-in-place order.

Optical interconnects are used at data-centers because they can transfer data at a very high rate with much lower power loss compared to electronics. In fact, this research from the University of Southampton comes on the heels of research from Microsoft and the University College London, which indicates that optical switches may be the answer to extending Moore's law in data centers. 

Engineers in the telecommunication industry have to design devices such as transceivers, routers, and arrayed waveguide gratings (AWG); photonic integrated circuits are useful in such applications. The use of a newly-discovered PCM demonstrates the possibility of reduced power consumption and larger data handling.   

 

Future of Silicon Photonic Circuits

The technology developed at the University of Southampton is compatible with existing silicon photonic circuits, which makes it ready for “technology transfer” to applications that are commercially used. This capability opens doors for neuromorphic computing, allowing a controlled flow of ions/photons to help artificial neurons communicate with each other. You might compare this with current deep neural networks, where the computational complexity is high and has considerable power consumption.

Besides this, this new technology (rapid phase change) in photonics will stimulate the growth of newly emerging applications such as solid-state LiDAR and quantum computing that are currently limited by the performance of the existing materials. 

Another reason phase-change materials (PCMs) show promise is that PCMs, unlike optoelectronic effects in conventional materials (like Si, LiNbO3), the properties of PCMs can change rapidly and dramatically. Furthermore, they can be maintained in non-volatile states without sustained electrical or optical bias.

As such, researchers at MIT, the University of Pennsylvania, the University of Minnesota, Purdue University, and the University of Maryland found that photonic devices utilizing PCMs can feature smaller dimensions and lower power consumption than devices based on traditional optoelectronic materials.

 

Diagram of the various parts of a programmable photonic chip. Image used courtesy of Ghent University's Photonics Research Group

 

“Quantum optical circuits are on the horizon and ultralow loss components are needed to make the next step in controlling and routing quantum information,”says Professor Otto Muskens, head of the integrated nanophotonics group. 

One of the major objectives of photonics research is to bridge the gap between photonics and electronics and bridge a transition between the two fields, eventually outgrowing the limitations of traditional electronics. This breakthrough may be a key in enabling complete photonic integrated circuits (PICs), but this potential replacement of current technologies will likely take many years before we can see a leap forward in photonic computing.