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Developing Chip-Based Photonic Resonators for UV and Visible Light

Scientists recently created chip-based photonic resonators that function in the ultraviolet (UV) and visible regions of the spectrum and display a record low UV light loss.

Developing Chip-Based Photonic Resonators for UV and Visible Light

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

The newly developed resonators establish a solid foundation for expanding the dimensions, intricacy, and precision of UV photonic integrated circuit (PIC) design.

This advancement holds the potential to facilitate the creation of innovative miniaturized chip-based gadgets, opening up possibilities in fields like spectroscopic sensing, underwater communication, and quantum information processing.

Compared to the better-established fields like telecom photonics and visible photonics, UV photonics is less explored 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.

Chengxing He, Research Team Member, Yale University

In the journal Optics Express, published by the Optica Publishing Group, the scientists detail their discovery of alumina-based optical microresonators. They elucidate how an extraordinary level of low loss at UV wavelengths was attained through the strategic amalgamation of appropriate materials, meticulously optimized design, and precise fabrication techniques.

Our work 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.

Hong Tang, Yale University

Hong Tang headed the research team.

Decreasing Light Loss

The microresonators consisted of premium-quality alumina thin films, meticulously crafted by co-authors Carlo Waldfried and Jun-Fei Zheng from Entegris Inc., employing a scalable atomic layer deposition (ALD) procedure.

Alumina boasts a substantial bandgap of approximately 8 eV, rendering it transparent to UV photons, which possess significantly lower energy levels (~4 eV) than the bandgap. Consequently, UV wavelengths are not absorbed by this material.

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

Chengxing He, Research Team Member, Yale University

To produce the microresonators, scientists employed an etching process on the alumina, forming a structure commonly referred to as a rib waveguide. In this configuration, a slab with a strip atop it generates the light-confining structure.

As the rib's depth increased, the light confinement grew more pronounced, yet it also led to heightened scattering losses. To strike the optimal balance, they utilized simulations to identify the precise etch depth that would attain the necessary light confinement while simultaneously minimizing scattering losses.

Making Ring Resonators

Applying the insights gleaned from their waveguide studies, the researchers went on to construct ring resonators featuring a 400-micron radius. They observed that by maintaining an etch depth exceeding 80 nm in a 400-nm thick alumina film, they could effectively minimize radiation loss to levels of less than 0.06 dB/cm at 488.5 nm and less than 0.001 dB/cm at 390 nm.

Subsequently, after fabricating these ring resonators based on their calculated parameters, the researchers determined their Q factors. They achieved an exceptional Q factor of 1.5 × 106 at 390 nm, within the UV spectrum, and a Q factor of 1.9 × 106 at 488.5 nm, corresponding to visible blue light. Higher Q-factors are indicative of reduced 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.

Chengxing He, Research Team Member, Yale University

He added, “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.”

The research team is currently focused on advancing alumina-based ring resonators with tunability across various wavelengths.

This development holds the potential for achieving precise control over wavelengths or creating modulators through the interaction of two resonators. Furthermore, their objective includes the creation of a UV light source integrated into a photonic integrated circuit (PIC) to establish a comprehensive PIC-based UV system.

Journal Reference

He, C., et al. (2023) Ultra-high Q alumina optical microresonators in the UV and blue bands. Optica.

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