Yale University: The Ultraviolet Light Loss Of Chip-Scale Photonic Resonators Has Reached A New Low

With the key role of photonics in information communication and quantum computing, the research in the field of ultraviolet light is particularly important. A research team at Yale University has successfully built a chip-based photonic resonator that operates in the ultraviolet (UV) to visible light spectrum and exhibits unprecedented low UV light loss. This new resonator provides a solid foundation for expanding the design size, complexity and fidelity of ultraviolet photonic integrated circuits (PIC), and is expected to advance the application of microchip-based devices in spectral sensing, underwater communications, and quantum information processing.

The chip-scale ring resonator, shown in Figure 1, operates in the ultraviolet to visible spectrum and achieves record low UV light loss. The resonator (small circle in the middle) is shown in blue light.

Chengxing He, a member of the research team at Yale University, said: "Compared with the relatively mature telecommunication photonics and visible photonics, the research of ultraviolet photonics is still relatively small. However, considering the need to use ultraviolet wavelengths in atom/ion based quantum computing to manipulate certain atomic state transitions and activate specific fluorescent molecules for biochemical sensing, exploration in this area is extremely valuable. Our research lays an important foundation for the construction of ultraviolet wavelength photonic circuits."

In the paper, the researchers describe an alumina-based optical micro-resonator and how they achieved unprecedented low losses at ultraviolet wavelengths by combining the right materials with optimized design and fabrication.

Hong Tang, leader of the research team, said: "Our research shows that ultraviolet photonic integrated circuits (UV PICs) have now reached a turning point where light loss is no more severe in the ultraviolet spectrum than in the visible region. This means that all advanced PIC structures previously developed for visible and telecommunication wavelengths, such as frequency combs and injection locking technologies, can now be extended to ultraviolet wavelengths."

DOI:https://doi.org/10.1364/OE.492510

Alumina micro-resonator: reduce light loss

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The microresonator is constructed from a high-quality alumina film prepared by Integris co-authors Carlo Waldfried and Jun-Fei Zheng using advanced atomic layer deposition (ALD) technology. Alumina has a large band gap (about 8 eV), making it transparent to lower energy (about 4 eV) ultraviolet photons, so the material does not absorb ultraviolet light.

The previous record was achieved using aluminum nitride with a band gap of about 6 eV. Unlike single-crystal aluminum nitride, amorphous atomic layers deposited with alumina have fewer defects, are easier to produce, and have lower light loss.

During the fabrication of the microresonator, the researchers etched aluminum oxide to form a structure commonly referred to as a "ribbed waveguide." In this ribbed waveguide, a narrow strip at the top forms a structure that limits the propagation of light. The deeper the rib of the waveguide, the stronger the light constraint, but it also means that the scattering loss increases. To optimize the structure, they used simulation techniques to determine the optimal etching depth, aiming to achieve the ideal beam confinement while minimizing scattering losses.

Ring resonators: Performance evaluation and integration prospects

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The research team applied the experience they gained from studying waveguides to fabricating a ring resonator with a radius of 400 μm. They observed that on aluminum oxide films with a thickness of 400 nm, when the etching depth reaches more than 80 nm, the radiation loss decreases to less than 0.06 dB/cm at 488.5 nm and 0.001 dB/cm at 390 nm.

On a ring resonator built according to these parameters, the researchers evaluated the quality factor Q by measuring the resonant peak width and scanned the optical frequency of the resonator. The results show that the quality factor is as high as 1.5×106 at 390 nm wavelength (UV range) and 1.9×106 at 488.5 nm (visible blue range) (higher quality factor means less light loss).

Compared to PICs designed specifically for visible light or telecommunication wavelengths, UV PICs may have an advantage in the communications field due to their wider bandwidth or being less easily absorbed under certain conditions, such as under water. More notably, the atomic layer deposition technology for the production of alumina is compatible with CMOS technology, which creates the possibility of the fusion of CMOS and amorphous alumina photonics.

Currently, researchers are working on developing alumina-based ring resonators that can be tuned to multiple wavelengths. This will help to achieve precise wavelength control, or to develop modulators by using two interacting resonators. In addition, they plan to develop a UV light source integrated on the PIC to build a complete Pic-based UV system.

Extreme ultraviolet light (EUV) is a subregion in the ultraviolet (UV) range that has a shorter wavelength than other UV subregions and is often used for high-precision technical applications. In order to improve China's research level in the fields of science, technology and application related to extreme ultraviolet light source, and promote the comprehensive development of extreme ultraviolet light source for the world's scientific frontier, national strategic needs, the main battlefield of national economy, information and artificial intelligence, China Laser plans to publish the topic "Extreme Ultraviolet Light Source and Application" in the 7th issue (April) of 2024. Focus on the latest progress and development trend of extreme ultraviolet light source in research and technical application, and promote the training of composite high-quality talents and the construction of related disciplines.