Harnessing The Potential Of 4H-SiC In Optomechanical Devices

When it comes to optomechanical applications, silicon carbide (SiC) has significant potential due to its excellent optical and mechanical properties. This relatively novel photonic material has traditionally been used as an abrasive or in power electronics, but past challenges with SiC nanofabrication have limited its use in optomechanical devices due to a weakening in the strength of a light signal known as optical loss.

In a new study led by Carnegie Mellon and the University of Florida, researchers demonstrated a low-loss, ultracompact integrated 4H-SiC optomechanical resonator—the first ever high-performance device of its kind for the 4H polytype.

“My group has been working on 4H-SiC for years,” said Qing Li, associate professor of electrical and computer engineering. “We were mostly focused on the photonic properties, not on the mechanical ones. Professor Philip Feng at the University of Florida has expressed interest in the mechanical characterization of this material, so that’s how this collaboration came about.”

SiC makes for an attractive material for optomechanical applications due to its exceptional thermal stability and mechanical robustness. But it’s these very characteristics that also present challenges in the nanofabrication process, leading to little success of 4H-SiC optomechanical resonators up until now.

Despite their advantages of incorporating both light and mechanical properties, optomechanical devices are subject to an intrinsic material limit based on the inherent characteristics of the material used in their design. This limit dictates the performance of optomechanical devices based on the loss of mechanical energy and limits how high the mechanical quality factor can be for a given material.

“For a mechanical oscillator or resonator, what we care about is this loss,” Li said. “If there is a mechanical wave that has no loss, this oscillation can last forever. But, the material always has some kind of decay.”

Compared to other materials, SiC has a material limit that is one order of magnitude higher than that of silicon and diamond, suggesting there is potential to achieve a reduction in the loss of mechanical vibrational energy—a key factor that determines the potential success of a device’s optomechanical applications.

Size is another important factor when balancing the mechanical and optical properties of an optomechanical resonator. The diameter of a strand of hair is roughly 100 microns, but Li’s device is 25 times smaller with a microdisk radius measuring only four microns. To craft a device of this size, the researchers employ a process called nanofabrication and use a technique known as electron beam lithography, which is able to define tiny features in the disk.

In addition to its size and low-loss, what makes the device so successful is its high mechanical frequency and mechanical quality factor (𝑓𝑚·𝑄𝑚) product. This product serves as a “figure of merit for optomechanical devices,” Li said. The higher the 𝑓𝑚·𝑄𝑚 product, the greater the sensitivity of the device to mechanical vibrations. Researchers achieved an 𝑓𝑚·𝑄𝑚 product of 1.82 ×1013 Hz, which is among the highest reported values of optomechanical devices of this size in an ambient environment at room temperature.

“Our nanofabrication yielded higher optical qualities, and then we were able to characterize mechanical quality factors. To excite this device, you need to use light, which then generates a force to make the mechanical resonator oscillate. This allows us to characterize the mechanical Q,” Li said. “Our success comes from our advanced nanofabrication and years of experience in silicon carbide photonics.”

When considering real-world applications, optomechanical devices can serve as a link in the conversion between mechanical vibrations at microwave frequencies (phonons) to optical frequency signals (photons). This conversion is especially important in quantum network applications and for quantum computers, which require this conversion of quantum information between different domains.

Moving forward, the researchers are looking to unlock the full potential of SiC, as the 𝑓𝑚·𝑄𝑚 product they achieved is still several factors below the intrinsic material limit. Additionally, they are working to craft a disk that is even smaller in size.

“This is just the first step towards the full optomechanical application of silicon carbide,” Li said. “There is still progress to be made. We are looking forward to future studies that build on this work.”