High-Q Si₃N₄ microresonators based on a subtractive processing for Kerr nonlinear optics

Abstract

Microresonator frequency combs (microcombs) are enabling new applications in frequency synthesis and metrologyfrom high-speed laser ranging to coherent optical communications. One critical parameter that dictates the performance of the microcomb is the optical quality factor (Q) of the microresonator. Microresonators fabricated in planar structures such as silicon nitride (Si3N4) allow for dispersion engineering and the possibility to monolithically integrate the microcomb with other photonic devices. However, the relatively large refractive index contrast and the tight optical confinement required for dispersion engineering make it challenging to attain Si3N4 microresonators with Qs > 10 7 using standard subtractive processing methodsi.e. photonic devices are patterned directly on the asdeposited Si3N4 film. In this work, we achieve ultra-smooth Si3N4 microresonators featuring mean intrinsic Qs around 11 million. The cross-section geometry can be precisely engineered in the telecommunications band to achieve either normal or anomalous dispersion, and we demonstrate the generation of mode-locked dark-pulse Kerr combs as well as soliton microcombs. Such high-Qs allow us to generate soliton microcombs with photodetectable repetition rates, demonstrated here for the first time in Si3N4 microresonators fabricated using a subtractive processing method. These results enhance the possibilities for co-integration of microcombs with high-performance photonic devices, such as narrow-linewidth externalcavity diode lasers, ultra-narrow filters and demultiplexers.

High-Q Dispersion-Engineered Si3N4 Microresonators Based on a Subtractive Processing Technique

“…Thus far, it has been unclear whether high Qs in dispersion-engineered Si3N4 microresonators could be attained in a reproducible manner using standard processing techniques. Here, we show mean Qs > 10 million in Si3N4 microresonators using a subtractive processing technique [6]. We also demonstrate for the first-time Si3N4 soliton microcombs operating at 100 GHz fabricated using standard processing techniques.…”

“…After annealing, electron beam lithography and dry etching by inductively coupled plasma reactive ion etching define the core waveguide structures. Specifics of the recipe are detailed in [6]. A TEOS SiO2 layer of 500 nm followed by ~ 2 m of plasma-enhanced chemical vapor deposited silica forms the top cladding.…”

Fundamental optical linewidth of soliton microcombs

“…The SiN microring resonator is fabricated via subtractive processing [53]. The radius of the microresonator used for experiment is 100 m, and the corresponding FSR is about 227.5 GHz.…”

25 GHz soliton microcombs in high-Q Si3N4 racetrack-shaped microresonators

“…Boundary conditions based on start and end points are used to determine the polynomial coefficient. The devices are fabricated using the technique reported in [4]. The height and width of the microresonator waveguide are 740 nm x 2000 nm.…”

“…Our group has recently reported a novel fabrication technique using traditional subtractive processing and electron beam lithography (EBL) to directly pattern the as-deposited Si3N4 film [4]. Compared with DUV stepper lithography, EBL has the ability to define small features down to 8 nm.…”