Researchers have manufactured chip based optical resonators that can operate in the ultraviolet (UV) and visible light regions of the spectrum

Figure: Researchers have created a chip based ring resonator that operates in the ultraviolet and visible light ranges and exhibits record low UV loss. The resonator (small circle in the middle) is displayed as blue light.

Researchers have created chip based photonic resonators that can operate in the ultraviolet (UV) and visible regions of the spectrum and exhibit record low UV loss. The new resonator lays the foundation for increasing the size, complexity, and fidelity of UV photonic integrated circuit (PIC) design, which can enable new microchip based devices for applications such as spectral sensing, underwater communication, and quantum information processing.

Compared to more mature fields such as telecommunications photonics and visible photonics, the exploration of ultraviolet photonics is relatively limited, despite the need for ultraviolet wavelengths to access certain atomic transitions and excite certain fluorescent molecules used for biochemical sensing in atomic/ion based quantum calculations, "said Cheng He, a research team member at Yale University. Our work has laid a solid foundation for constructing photonic circuits that operate at ultraviolet wavelengths.

In Optics Express, researchers described optical microresonators based on alumina and how they achieved unprecedented low losses at UV wavelengths by combining the correct materials with optimized design and manufacturing.

Our work indicates that UV PICs have reached a critical point where the optical loss of waveguides is no longer more severe than that of visible light, "said Hong Tang, the head of the research team. This means that all interesting PIC structures developed for visible and telecommunications wavelengths, such as frequency combs and injection locking, can also be applied to ultraviolet wavelengths.

Reduce light loss
The micro resonator is made of high-quality aluminum oxide film, which was prepared by Integras using a highly scalable atomic layer deposition (ALD) process. The large bandgap (~8eV) of alumina makes it transparent to ultraviolet photons with much lower energy (~4eV) than the bandgap. Therefore, this material does not absorb ultraviolet rays.

The previous record was completed using aluminum nitride, with a bandgap of~6eV, "he said. Compared to single crystal aluminum nitride, amorphous ALD alumina has fewer defects and is less difficult to manufacture, which helps us achieve lower losses.

In order to manufacture micro resonators, researchers etched aluminum oxide to create what is commonly known as a ridge waveguide, in which a plate with a strip at the top forms a structure that restricts light. The deeper the spine, the stronger the constraint, but the greater the scattering loss. They use simulation to find the appropriate etching depth to achieve the required light constraints while minimizing scattering losses.

Manufacturing ring resonators
The researchers applied the knowledge they learned from waveguides to manufacturing circular resonators with a radius of 400 microns. They found that in a 400 nm thick alumina film, when the etching depth exceeds 80 nm, the radiation loss at 488.5 nm can be suppressed to less than 0.06 dB/cm, and the radiation loss at 390 nm can be suppressed to less than 0.001 dB/cm.

After manufacturing a ring resonator based on these calculations, researchers determined the Q-factor by measuring the width of the resonant peak, while scanning the optical frequency injected into the resonator. They found that the Q-factor reached a record high of 1.5 e6 at 390 nanometers (in the ultraviolet part of the spectrum), and 1.9 e6 at 488.5 nanometers (in the wavelength of visible blue light). The higher the q factor, the less optical loss.

He said, "Compared to PICs of visible light or telecommunication wavelengths, UV PICs may have advantages in communication due to their larger bandwidth or under conditions where other wavelengths are absorbed (such as underwater)." In addition, the atomic layer deposition process used for manufacturing alumina is compatible with CMOS, paving the way for the integration of CMOS and amorphous alumina based photonics.

Researchers are currently working on developing ring resonators based on alumina, which can be tuned to different wavelengths. This can be used to achieve precise wavelength control or to create modulators by using two mutually interfering resonators. They also want to develop an integrated PIC UV light source to form a complete PIC based UV system.

Source: Laser Network