New chip-based photonic resonators with low UV losses

Researchers from Yale University have created chip-based photonic resonators that operate in the ultraviolet (UV) and visible regions of the spectrum and exhibit a record low UV light loss. The resonators could help increase the size, complexity and fidelity of UV photonic integrated circuit (PIC) design, which could enable new miniature chip-based devices for applications such as spectroscopic sensing, underwater communication and quantum information processing.

Research team member Chengxing He from Yale University said UV photonics is less explored compared to telecom photonics and visible photonics, even though UV wavelengths are needed to access certain atomic transitions in atom/ion-based quantum computing and to excite certain fluorescent molecules for biochemical sensing. “Our work sets a good basis toward building photonic circuits that operate at UV wavelengths,” He said.

In the journal Optics Express, the researchers described the alumina-based optical microresonators and how they achieved low loss at UV wavelengths by combining the right material with optimised design and fabrication. The research demonstrates that UV PICs have reached a critical point where light loss for waveguides is no longer significantly worse than their visible counterparts. This means that all the interesting PIC structures developed for visible and telecom wavelengths, such as frequency combs and injection locking, can be applied to UV wavelengths as well.

The microresonators were made from alumina thin films that the researchers prepared using a scalable atomic layer deposition (ALD) process. Alumina’s large bandgap of ~8 eV makes it transparent to UV photons, which have a lower (~4 eV) energy than the bandgap. Therefore, UV wavelengths are not absorbed by this material.

“The previous record was accomplished with aluminium nitride, which has a bandgap of ~6 eV. Compared to single crystal aluminium nitride, amorphous ALD alumina has fewer defects and is less challenging to fabricate, which helped us to achieve lower loss,” He said.

To create the microresonators, researchers etched the alumina to create a rib waveguide, in which a slab with a strip on top creates the light-confining structure. As the rib becomes deeper, the confinement becomes stronger but so does the scattering loss. The researchers used simulation to find the right etch depth to achieve the necessary light confinement while minimising the scattering loss.

The researchers applied what they learned from waveguides to create ring resonators with a 400 micron radius, and found that the radiation loss can be suppressed to less than 0.06 dB/cm at 488.5 nm and less than 0.001 dB/cm at 390 nm when the etch depth was more than 80 nm in a 400-nm-thick alumina film. After fabricating ring resonators based on these calculations, the researchers determined their Q factors by measuring the width of resonance peaks while scanning the light frequency injected into the resonator. They found a quality (Q) factor of 1.5 x 106 at 390 nm (in the UV portion of the spectrum) and a Q factor of 1.9 x 6 at 488.5 nm (a wavelength for visible blue light). Higher Q factors indicate less light loss.

“Compared to PICs in visible or telecom wavelengths, UV PICs may find an edge in communications due to the larger bandwidth or in conditions where other wavelengths get absorbed, such as underwater. Also, the fact that the atomic layer deposition process used to create the alumina is CMOS compatible paves the way for CMOS integration with amorphous alumina-based photonics,” He said.

Image caption: Researchers created a chip-based ring resonator that operates in the ultraviolet and visible regions of the spectrum and exhibits a record low UV light loss. The resonator (small circle in the middle) is shown with blue light. Image credit: Chengxing He, Yale University.