Tuning of resonance spacing over whole free spectral range based on Autler-Townes splitting in a single microring resonator

Tunable frequency matching for efficient four-wave-mixing Bragg scattering in microrings

We propose and theoretically study a tunable frequency matching method for four-wave-mixing Bragg-scattering frequency conversion in microring resonators. A tunable coupling between the clockwise and counterclockwise propagating modes in the resonators was designed to introduce adjustable mode splitting, thus compensating for the frequency mismatching under different wavelengths. Using a silicon nitride ring resonator as an example, we showed that the tuning bandwidth approaches 35 number of FSRs. Numerical simulations further revealed that the phase-matching strategy is valid under different wavelength combinations and is robust to variations in waveguide geometry and fabrication. These results suggest promising applications in high-efficiency frequency conversion, integrated nonlinear photonics, and quantum optics.

Phase-controlled dual-wavelength resonance in a self-coupling whispering-gallery-mode microcavity

We report a novel, to the best of our knowledge, way to achieve phase-controlled dual-wavelength resonance based on whispering-gallery-mode (WGM) microcavities experimentally. With the help of a feedback waveguide, not only two optical pathways but also a unidirectional coupling between counter-propagating waves are formed, which is the requirement of all-optical analogues of electromagnetically induced transparency and Autler-Townes splitting. By adjusting the accumulating phase introduced from the fiber waveguide, we observe the signal lineshape changes from symmetric to asymmetric, i.e., the resonant transmission and extinction ratio of two splitting modes can be controlled, which brings a new degree of freedom to the WGM resonator system. These results may boost the development of quantum state control and pave the way for reconfiguring devices such as narrow-band filters.

Subwavelength hole defect assisted microring resonator for a compact rectangular filter

We propose and demonstrate a subwavelength hole defect assisted microring resonator (SHDAMR) structure. With the manipulated modal coupling between two degenerate counterpropagating modes induced by a subwavelength hole defect embedded in the microring waveguide, the SHDAMR structure shows a rectangular resonance lineshape instead of the Lorentzian resonance lineshape of a conventional microring. As a proof of concept, the SHDAMR structure is fabricated on the Si₃N₄ waveguide platform, for achieving a rectangular filter with a 3-dB bandwidth of 2.03 GHz and an improved shape factor. The demonstrated SHDAMR structure shows the advantages of compact footprint, simplified tunability, and large tolerance of fabrication errors, showing great potential for various applications.

Autler-Townes splitting biosensing based on a nonuniform photonic crystal waveguide with feedback loop

Aimed at a high-performance biosensor for sensing biological molecules quickly and accurately, a novel nonuniform photonic crystal waveguide (NUPhCW) with a feedback loop is systematically investigated based on the mode-splitting effect. Thanks to the careful design of the nonuniform holes of NUPhCW with a feedback loop, the Autler-Townes splitting occurs in our device, and the spacing of two splitting resonance modes changes with the environmental refractive index, which is beneficial for achieving biosensing scenarios. Further, this integrated structure allows for self-referencing detection for immunity against environmental noise. Sensitivity of the NUPhCW is achieved at ∼117  nm/RIU, which is 2.6 times more than that of the common microring resonator, indicating that our proposed device structure would be promising for high-quality optical label-free sensing.

Tunable coupling-induced resonance splitting in a self-coupled silicon ring cavity with robust spectral characteristics

We propose and demonstrate a self-coupled micro-ring resonator for resonance splitting by mutual mode coupling of a cavity mode and a counter-propagating mode in a silicon-on-insulator platform. The resonator is constructed with a self-coupling region that can excite a counter-propagating mode. We experimentally study the effect of self-coupling on the resonance splitting, resonance extinction, and quality-factor evolution and stability. Based on the coupling, we achieve 72% of the free spectral range (FSR) splitting for a cavity with an FSR of 2.1 nm with <5% variation in the cavity quality factor. The self-coupled resonance splitting shows a highly robust spectral characteristic that can be exploited for sensing and optical signal processing.