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1. Waveguide-integrated mid-infrared photodetection using graphene on a scalable chalcogenide glass platform

   https://doi.org/10.1038/s41467-022-31607-7  

   Goldstein, Jordan ; Lin, Hongtao ; Deckoff-Jones, Skylar ; Hempel, Marek ; Lu, Ang-Yu ; Richardson, Kathleen A. ; Palacios, Tomás ; Kong, Jing ; Hu, Juejun ; Englund, Dirk ( July 2022 , Nature Communications)

   The development of compact and fieldable mid-infrared (mid-IR) spectroscopy devices represents a critical challenge for distributed sensing with applications from gas leak detection to environmental monitoring. Recent work has focused on mid-IR photonic integrated circuit (PIC) sensing platforms and waveguide-integrated mid-IR light sources and detectors based on semiconductors such as PbTe, black phosphorus and tellurene. However, material bandgaps and reliance on SiO₂substrates limit operation to wavelengthsλ ≲ 4 μm. Here we overcome these challenges with a chalcogenide glass-on-CaF₂PIC architecture incorporating split-gate photothermoelectric graphene photodetectors. Our design extends operation toλ = 5.2 μm with a Johnson noise-limited noise-equivalent power of 1.1 nW/Hz^1/2, no fall-off in photoresponse up tof = 1 MHz, and a predicted 3-dB bandwidth off_3dB > 1 GHz. This mid-IR PIC platform readily extends to longer wavelengths and opens the door to applications from distributed gas sensing and portable dual comb spectroscopy to weather-resilient free space optical communications.
2. Mid-infrared nonlinear optics in thin-film lithium niobate on sapphire

   https://doi.org/10.1364/OPTICA.427428  

   Mishra, Jatadhari ; McKenna, Timothy P. ; Ng, Edwin ; Stokowski, Hubert S. ; Jankowski, Marc ; Langrock, Carsten ; Heydari, David ; Mabuchi, Hideo ; Fejer, M. M. ; Safavi-Naeini, Amir H. ( June 2021 , Optica)

   Periodically poled thin-film lithium niobate (TFLN) waveguides have emerged as a leading platform for highly efficient frequency conversion in the near-infrared. However, the commonly used silica bottom-cladding results in high absorption loss at wavelengths beyond 2.5 µm. In this work, we demonstrate efficient frequency conversion in a TFLN-on-sapphire platform, which features high transparency up to 4.5 µm. In particular, we report generating mid-infrared light up to 3.66 µm via difference-frequency generation of a fixed 1 µm source and a tunable telecom source, with normalized efficiencies up to$200\mathrm{\%<\#comment/>}/\mathrm{W}{\mathrm{c}\mathrm{m}}^2$. These results show TFLN-on-sapphire to be a promising platform for integrated nonlinear nanophotonics in the mid-infrared.
3. Epitaxial mid-IR nanophotonic optoelectronics

   https://doi.org/10.1063/5.0086774  

   Nordin, L. ; Wasserman, D. ( May 2022 , Applied Physics Letters)

   There are a range of fundamental challenges associated with scaling optoelectronic devices down to the nano-scale, and the past decades have seen significant research dedicated to the development of sub-diffraction-limit optical devices, often relying on the plasmonic response of metal structures. At the longer wavelengths associated with the mid-infrared, dramatic changes in the optical response of traditional nanophotonic materials, reduced efficiency optoelectronic active regions, and a host of deleterious and/or parasitic effects makes nano-scale optoelectronics at micro-scale wavelengths particularly challenging. In this Perspective, we describe recent work leveraging a class of infrared plasmonic materials, highly doped semiconductors, which not only support sub-diffraction-limit plasmonic modes at long wavelengths, but which can also be integrated into a range of optoelectronic device architectures. We discuss how the wavelength-dependent optical response of these materials can serve a number of different photonic device designs, including dielectric waveguides, epsilon-near-zero dynamic optical devices, cavity-based optoelectronics, and plasmonic device architectures. We present recent results demonstrating that the highly doped semiconductor class of materials offers the opportunity for monolithic, all-epitaxial, device architectures out-performing current state of the art commercial devices, and discuss the perspectives and promise of these materials for infrared nanophotonic optoelectronics.
4. Broadband ultraviolet-visible frequency combs from cascaded high-harmonic generation in quasi-phase-matched waveguides

   https://doi.org/10.1364/JOSAB.427086  

   Rutledge, Jay ; Catanese, Anthony ; Hickstein, Daniel D. ; Diddams, Scott A. ; Allison, Thomas K. ; Kowligy, Abijith S. ( July 2021 , Journal of the Optical Society of America B)

   High-harmonic generation (HHG) provides short-wavelength light that is useful for precision spectroscopy and probing ultrafast dynamics. We report efficient, phase-coherent harmonic generation up to the ninth order (333 nm) in chirped periodically poled lithium niobate waveguides driven by phase-stable$\le <\#comment/>12\mathrm{n}\mathrm{J}$, 100 fs pulses at 3 µm with 100 MHz repetition rate. A mid-infrared to ultraviolet-visible conversion efficiency as high as 10% is observed, among an overall 23% conversion of the fundamental to all harmonics. We verify the coherence of the harmonic frequency combs despite the complex highly nonlinear process. Accommodating the extreme spectral bandwidth, numerical simulations based on a single broadband envelope equation with only quadratic nonlinearity give estimates for the conversion efficiency within approximately 1 order of magnitude over a wide range of experimental parameters. From this comparison between theory and experiment, we identify a dimensionless parameter capturing the competition between three-wave mixing and group-velocity walk-off of the harmonics that governs the cascaded HHG physics. We also gain insights into spectral optimization via tuning the waveguide poling profile and pump pulse parameters. These results can inform cascaded HHG in a range of different platforms.
5. Cubic-wavelength mode volume photonic crystal nanobeam cavities in a monolithic CMOS platform

   https://doi.org/10.1364/OL.481483  

   Al Qubaisi, Kenaish ; Schiller, Mark ; Zhang, Bohan ; Onural, Deniz ; Naughton, Michael J. ; Popović, Miloš A. ( February 2023 , Optics Letters)

   We report on the design, fabrication, and experimental characterization of photonic crystal (PhC) nanobeam cavities with the smallest footprint, largest intrinsic quality factor, and smallest mode volume to be demonstrated to date in a monolithic CMOS platform. Two types of cavities were designed, with opposite spatial mode symmetries. The opposite mode symmetry, combined with evanescent coupling, allows the nanobeam cavities to be used in reflectionless topologies, desirable in complex photonic integrated circuits (PICs). The devices were implemented and fabricated in a 45 nm monolithic electronics–photonics CMOS platform optimized for silicon photonics (GlobalFoundries 45CLO) and do not require any post-processing. Quality factors exceeding 100 000 were measured for both devices, the highest, to the best of our knowledge, among fully cladded PhC nanobeam cavities in any silicon-on-insulator (SOI) platform. Additionally, the ability of the cavities to confine light into small mode volumes, of the order of (λ/n)³, was confirmed experimentally using near-field scanning optical microscopy (NSOM). These types of cavities are an important step toward realizing ultra-low energy active devices required for the next generation of integrated optical links beyond the current microring resonator-based links and other CMOS PICs.