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1. Experimental Demonstration of a New Oscillator Concept Based on Degenerate Band Edge in Microstrip Circuit

   Dmitry Oshmarin, Ahmed F. (September 2021, arXivorg)

   The first experimental demonstration of an oscillator based on a periodic, resonant microstrip circuit with a degenerate band edge (DBE) is presented. The DBE is a fourth-order exceptional degeneracy of the eigenmodes in a lossless periodic waveguide that is visible in the wavenumber-frequency dispersion diagram, and the periodic microstrip behaves as a frequency selective medium. The presence of the DBE condition and the associated DBE resonance allow for a stable, single-mode oscillation as well as stability with varying the load resistance. 

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2. Degenerate Band Edge Resonances in Air-filled Substrate Integrated Waveguide

   https://doi.org/10.23919/EuCAP48036.2020.9135497  

   Zheng, Tianyu; Casaletti, Massimiliano; Abdelshafy, Ahmed F.; Capolino, Filippo; Ren, Zhuoxiang; Valerio, Guido (March 2020, Proc. of 14th European Conference on Antennas and Propagation (EuCAP))

   The degenerate band edge (DBE) is a special fourth-order degenerate point in a dispersion diagram, where four eigenmodes coalesce to a single degenerate eigenmode. It leads to field enhancement of the Bloch mode and to high quality factors, which are useful for high-Q resonators, oscillators and ultrasensitive sensors. The air-filled substrate integrated waveguide (AFSIW) is a novel form of SIW which is low cost and low loss. We propose a design of an AFSIW supporting a degenerate band edge (DBE). We show the occurrence of the so-called “giant resonance” associated to the DBE and we study how losses influence the DBE. 

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3. Free-standing silicon nitride nanobeams with an efficient fiber-chip interface for cavity QED

   https://doi.org/10.1364/OME.411219  

   Alajlan, Abdulrahman; Khurana, Mohit; Liu, Xiaohan; Cojocaru, Ivan; Akimov, Alexey_V (November 2020, Optical Materials Express)

   We present the design, fabrication and characterization of high quality factor silicon nitride nanobeam PhC cavities at visible wavelengths for coupling to diamond color centers in a cavity QED system. We demonstrate devices with a quality factor of ∼24, 000 (±250) around the zero-phonon line of the germanium-vacancy center in diamond. We also present an efficient fiber-to-waveguide coupling platform for suspended nanophotonics. By gently changing the corresponding effective indices at the fiber-waveguide interface, we achieve a coupling efficiency of ∼96% (±2%) at the cavity resonance. 

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4. Frequency combs in optically injected terahertz ring quantum cascade lasers

   https://doi.org/10.1063/5.0173912  

   Khan, Md Istiak; Xiao, Zhenyang; Addamane, Sadhvikas J.; Burghoff, David (December 2023, APL Photonics)

   Quantum cascade lasers (QCLs) have emerged as promising candidates for generating chip-scale frequency combs in mid-infrared and terahertz wavelengths. In this work, we demonstrate frequency comb formation in ring terahertz QCLs using the injection of light from a distributed feedback (DFB) laser. The DFB design frequency is chosen to match the modes of the ring cavity (near 3.3 THz), and light from the DFB is injected into the ring QCL via a bus waveguide. By controlling the power and frequency of the optical injection, we show that combs can be selectively formed and controlled in the ring cavity. Numerical modeling suggests that this comb is primarily frequency-modulated in character, with the injection serving to trigger comb formation. We also show that the ring can be used as a filter to control the output of the DFB QCL, potentially being of interest in terahertz photonic integrated circuits. Our work demonstrates that waveguide couplers are a compelling approach for injecting and extracting radiation from ring terahertz combs and offer exciting possibilities for the generation of new comb states in terahertz, such as frequency-modulated waves, solitons, and more. 

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5. Photonic circuits for laser stabilization with integrated ultra-high Q and Brillouin laser resonators

   https://doi.org/10.1063/5.0091686  

   Liu, Kaikai; Dallyn, John H.; Brodnik, Grant M.; Isichenko, Andrei; Harrington, Mark W.; Chauhan, Nitesh; Bose, Debapam; Morton, Paul A.; Papp, Scott B.; Behunin, Ryan O.; et al (September 2022, APL Photonics)

   The integration of stabilized lasers, sources that generate spectrally pure light, will provide compact, low-cost solutions for applications including quantum information sciences, precision navigation and timing, metrology, and high-capacity fiber communications. We report a significant advancement in this field, demonstrating stabilization of an integrated waveguide Brillouin laser to an integrated waveguide reference cavity, where both resonators are fabricated using the same CMOS-compatible integration platform. We demonstrate reduction of the free running Brillouin laser linewidth to a 292 Hz integral linewidth and carrier stabilization to a 4.9 × 10 −13 fractional frequency at 8 ms reaching the cavity-intrinsic thermorefractive noise limit for frequencies down to 80 Hz. We achieve this level of performance using a pair of 56.4 × 10 6 quality factor Si 3 N 4 waveguide ring-resonators that reduce the high-frequency noise by the nonlinear Brillouin process and the low-frequency noise by Pound–Drever–Hall locking to the ultra-low loss resonator. These results represent an important step toward integrated stabilized lasers with reduced sensitivity to environmental disturbances for atomic, molecular, and optical physics (AMO), quantum information processing and sensing, and other precision scientific, sensing, and communications applications. 

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