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1. 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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2. On-chip spin-orbit locking of quantum emitters in 2D materials for chiral emission

   https://doi.org/10.1364/OPTICA.463481  

   Ma, Yichen; Zhao, Haoqi; Liu, Na; Gao, Zihe; Mohajerani, Seyed Sepehr; Xiao, Licheng; Hone, James; Feng, Liang; Strauf, Stefan (January 2022, Optica)

   Light carries both spin angular momentum (SAM) and orbital angular momentum (OAM), which can be used as potential degrees of freedom for quantum information processing. Quantum emitters are ideal candidates towards on-chip control and manipulation of the full SAM–OAM state space. Here, we show coupling of a spin-polarized quantum emitter in a monolayer W S e 2 with the whispering gallery mode of a S i 3 N 4 ring resonator. The cavity mode carries a transverse SAM of σ = ± 1 in the evanescent regions, with the sign depending on the orbital power flow direction of the light. By tailoring the cavity–emitter interaction, we couple the intrinsic spin state of the quantum emitter to the SAM and propagation direction of the cavity mode, which leads to spin–orbit locking and subsequent chiral single-photon emission. Furthermore, by engineering how light is scattered from the WGM, we create a high-order Bessel beam which opens up the possibility to generate optical vortex carrying OAM states. 

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3. Light-controlled strong coupling of optical cavity modes spaced by 200 THz

   https://doi.org/10.1364/OL.563882  

   Taneja, Lavanya; Schuster, David I; Simon, Jonathan (January 2025, Optics Letters)

   Cavities have driven significant advances in optical physics and quantum science, with applications ranging from lasers and spectroscopy to quantum information processing, simulation, and metrology. For standard optical cavities, each eigenmode corresponds to a single, well-defined frequency. Here, we present a macroscopic optical Fabry–Pérot cavity whose eigenmodes are coherent superpositions of two frequency modes in the VIS-NIR range. Specifically, we demonstrate strong coupling between 384 THz (780 nm) and 580 THz (516 nm) cavity modes by incorporating an intracavityχ⁽²⁾crystal driven by a non-resonant optical pump at 1529 nm. Strong coupling enables us to demonstrate frequency conversion with an end-to-end, free-space conversion efficiency of 30(1)%, limited by current cavity design and internal cavity losses. We also demonstrate coupling between distinct spatial modes at the two frequencies, extending coherent control to the spatial basis. In conjunction with improved resonator design and low-loss non-linear crystals, we anticipate a factor of >50 increase in two-mode cooperativity for stronger coupling and near-unity conversion efficiency at low pump powers. This platform opens new avenues for cavity-QED experiments, with potential applications spanning cavity-mediated interactions between distinct atomic species, interconnects for quantum networking and modular computing, and spatially multimode cavity physics. 

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4. Cavity-enhanced narrowband spectral filters using rare-earth ions doped in thin-film lithium niobate

   https://doi.org/10.1038/s44310-024-00023-8  

   Zhao, Yuqi; Renaud, Dylan; Farfurnik, Demitry; Jiang, Yuxi; Dutta, Subhojit; Sinclair, Neil; Lončar, Marko; Waks, Edo (December 2024, npj Nanophotonics)

   Abstract On-chip optical filters are fundamental components in optical signal processing. While rare-earth ion-doped crystals offer ultra-narrow optical filtering via spectral hole burning, their applications have primarily been limited to those using bulk crystals, restricting their utility. In this work, we demonstrate cavity-enhanced spectral filtering based on rare-earth ions in an integrated nonlinear optical platform. We incorporate rare-earth ions into high quality-factor ring resonators patterned in thin-film lithium niobate. By spectral hole burning at 4 K in a critically coupled resonance mode, we achieve bandpass filters ranging from 7 MHz linewidth, with 13.0 dB of extinction, to 24 MHz linewidth, with 20.4 dB of extinction. By reducing the temperature to 100 mK to eliminate phonon broadening, we achieve an even narrower linewidth of 681 kHz, which is comparable to the narrowest filter linewidth demonstrated in an integrated photonic device, while only requiring a small device footprint. Moreover, the cavity enables reconfigurable filtering by varying the cavity coupling rate. For instance, as opposed to the bandpass filter, we demonstrate a bandstop filter utilizing an under-coupled ring resonator. Such versatile integrated spectral filters with high extinction ratio and narrow linewidth could serve as fundamental components for optical signal processing and optical memories on-a-chip. 

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5. Optical-parametric oscillation in photonic-crystal ring resonators

   https://doi.org/10.1364/OPTICA.469210  

   Black, Jennifer A.; Brodnik, Grant; Liu, Haixin; Yu, Su-Peng; Carlson, David R.; Zang, Jizhao; Briles, Travis C.; Papp, Scott B. (January 2022, Optica)

   By-design access to laser wavelength, especially with integrated photonics, is critical to advance quantum sensors, such as optical clocks and quantum-information systems, and open opportunities in optical communication. Semiconductor-laser gain provides exemplary efficiency and integration but merely in developed wavelength bands. Alternatively, nonlinear optics requires control of phase matching, but the principle of nonlinear conversion of a pump laser to a designed wavelength is extensible. We report on laser-wavelength access by versatile customization of optical-parametric oscillation (OPO) with a photonic-crystal ring resonator (PhCR). Leveraging the exquisite control of laser propagation provided by a photonic crystal in a traveling-wave ring resonator, we enable OPO generation across a wavelength range of 1234–2093 nm with a 1550-nm pump and 1016–1110 nm with a 1064-nm pump. Moreover, our platform offers pump-to-sideband conversion efficiency of > 10 % and negligible additive optical-frequency noise across the output range. From laser design to simulation of nonlinear dynamics, we use a Lugiato–Lefever framework that predicts the system characteristics, including bidirectional OPO generation in the PhCR and conversion efficiency in agreement with our observations. Our experiments introduce broadband lasers by design with PhCR OPOs, providing critical functionalities in integrated photonics. 

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