Researchers have developed chip-based photonic resonators capable of operating in both the ultraviolet (UV) and visible parts of the spectrum, boasting an impressive record low UV light loss. These groundbreaking resonators pave the way for expanding the scope, complexity, and precision of UV photonic integrated circuit (PIC) designs. The potential applications are diverse, spanning spectroscopic sensing, underwater communication, and quantum information processing.
Chengxing He, a member of the research team from Yale University, highlighted the relative underexploration of UV photonics compared to more established fields like telecom photonics and visible photonics. UV wavelengths play a crucial role in accessing specific atomic transitions for atom/ion-based quantum computing and exciting fluorescent molecules in biochemical sensing. He emphasized that their work lays a strong foundation for constructing photonic circuits operating at UV wavelengths.
In their publication in Optics Express, the researchers delve into the details of alumina-based optical microresonators and their achievement of remarkably low UV light loss. This feat results from a combination of suitable materials, optimized designs, and precise fabrication techniques.
Hong Tang, the leader of the research team, emphasized that UV PICs have now reached a pivotal point where light loss in waveguides is no longer significantly worse than their visible counterparts. This breakthrough implies that all the intriguing PIC structures developed for visible and telecom wavelengths, such as frequency combs and injection locking, can seamlessly transition to UV wavelengths.
Decreasing light loss
The microresonators in question were crafted from high-quality alumina thin films, meticulously prepared by co-authors Carlo Waldfried and Jun-Fei Zheng of Entegris Inc. These films were produced through a highly scalable atomic layer deposition (ALD) process. Alumina possesses a substantial bandgap of approximately 8 electronvolts (eV), rendering it transparent to UV photons, which have significantly lower energy levels at around 4 eV. Consequently, UV wavelengths encounter no absorption when passing through this material.
Chengxing He explained that the previous record relied on aluminum nitride, which boasts a bandgap of roughly 6 eV. In contrast, amorphous ALD alumina exhibits fewer defects and proves less challenging to fabricate compared to single crystal aluminum nitride. These advantages contributed to achieving lower loss levels.
The creation of these microresonators involved an etching process on the alumina to form what’s commonly referred to as a rib waveguide. In this configuration, a slab with a strip on top establishes the light-confining structure. As the rib’s depth increases, so does the strength of light confinement, but it also leads to greater scattering loss. Researchers employed simulations to pinpoint the ideal etch depth that would balance the desired light confinement with minimal scattering loss.
Making ring resonators
Building upon their insights from waveguides, the researchers applied their knowledge to craft ring resonators boasting an impressive 400-micron radius. They made a noteworthy discovery: by ensuring an etch depth of more than 80 nanometers in a 400-nanometer thick alumina film, they were able to suppress radiation loss to a remarkable degree. At 488.5 nanometers, the radiation loss dwindled to less than 0.06 dB/cm, and at 390 nanometers (within the UV spectrum), it plummeted to less than 0.001 dB/cm.
The team then set out to determine the quality (Q) factors of these fabricated ring resonators. They did this by measuring the width of resonance peaks while systematically varying the injected light frequency. Their findings were extraordinary, with a record-breaking Q factor of 1.5 × 106 at 390 nanometers (UV) and a Q factor of 1.9 × 106 at 488.5 nanometers (visible blue light). Higher Q factors signify significantly reduced light loss.
Chengxing He noted that UV photonic integrated circuits (PICs) might offer advantages in communication, thanks to their broader bandwidth, as well as in scenarios where other wavelengths are susceptible to absorption, such as underwater environments. Moreover, the compatibility of the atomic layer deposition process used for alumina with complementary metal-oxide-semiconductor (CMOS) technology opens doors to CMOS integration with amorphous alumina-based photonics.
The researchers are currently focused on developing alumina-based ring resonators with tunable properties to accommodate various wavelengths. This capability could enable precise wavelength control or the creation of modulators using two resonators that interfere with each other. Additionally, they aim to design a PIC-integrated UV light source, ultimately forming a complete PIC-based UV system.