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1. Supercontinuum generation by saturated second-order nonlinear interactions

   https://doi.org/10.1063/5.0158926  

   Jankowski, Marc ; Langrock, Carsten ; Desiatov, Boris ; Lončar, Marko ; Fejer, M. M. ( November 2023 , APL Photonics)

   We propose a new approach to supercontinuum generation and carrier-envelope-offset detection based on saturated second-order nonlinear interactions in dispersion-engineered nanowaveguides. The technique developed here broadens the interacting harmonics by forming stable bifurcations of the pulse envelopes due to an interplay between phase-mismatch and pump depletion. We first present an intuitive heuristic model for spectral broadening by second-harmonic generation of femtosecond pulses and show that this model agrees well with experiments. Then, having established strong agreement between theory and experiment, we develop scaling laws that determine the energy required to generate an octave of bandwidth as a function of input pulse duration, device length, and input pulse chirp. These scaling laws suggest that future realization based on this approach could enable supercontinuum generation with orders of magnitude less energy than current state-of-the-art devices.

    

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2. Chip-based self-referencing using integrated lithium niobate waveguides

   https://doi.org/10.1364/OPTICA.392363  

   Okawachi, Yoshitomo ; Yu, Mengjie ; Desiatov, Boris ; Kim, Bok Young ; Hansson, Tobias ; Lončar, Marko ; Gaeta, Alexander L. ( June 2020 , Optica)

   The measurement and stabilization of the carrier–envelope offset frequency$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$via self-referencing is paramount for optical frequency comb generation, which has revolutionized precision frequency metrology, spectroscopy, and optical clocks. Over the past decade, the development of chip-scale platforms has enabled compact integrated waveguides for supercontinuum generation. However, there is a critical need for an on-chip self-referencing system that is adaptive to different pump wavelengths, requires low pulse energy, and does not require complicated processing. Here, we demonstrate efficient$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$stabilization of a modelocked laser with only 107 pJ of pulse energy via self-referencing in an integrated lithium niobate waveguide. We realize an$f\text{-}2f$interferometer through second-harmonic generation and subsequent supercontinuum generation in a single dispersion-engineered waveguide with a stabilization performance equivalent to a conventional off-chip module. The$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$beatnote is measured over a pump wavelength range of 70 nm. We theoretically investigate our system using a single nonlinear envelope equation with contributions from both second- and third-order nonlinearities. Our modeling reveals rich ultrabroadband nonlinear dynamics and confirms that the initial second-harmonic generation followed by supercontinuum generation with the remaining pump is responsible for the generation of a strong$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$signal as compared to a traditional$f\text{-}2f$interferometer. Our technology provides a highly simplified system that is robust, low in cost, and adaptable for precision metrology for use outside a research laboratory.

    

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3. Exploiting Ultralow Loss Multimode Waveguides for Broadband Frequency Combs

   https://doi.org/10.1002/lpor.202000353  

   Ji, Xingchen ; Jang, Jae K. ; Dave, Utsav D. ; Corato‐Zanarella, Mateus ; Joshi, Chaitanya ; Gaeta, Alexander L. ; Lipson, Michal ( December 2020 , Laser & Photonics Reviews)

   Low propagation loss in high confinement waveguides is critical for chip‐based nonlinear photonics applications. Sophisticated fabrication processes which yield sub‐nm roughness are generally needed to reduce scattering points at the waveguide interfaces to achieve ultralow propagation loss. Here, ultralow propagation loss is shown by shaping the mode using a highly multimode structure to reduce its overlap with the waveguide interfaces, thus relaxing the fabrication processing requirements. Microresonators with intrinsic quality factors (Q) of 31.8 ± 4.4 million are experimentally demonstrated. Although the microresonators support ten transverse modes only the fundamental mode is excited and no higher order modes are observed when using nonlinear adiabatic bends. A record‐low threshold pump power of 73 µW for parametric oscillation is measured and a broadband, almost octave spanning single‐soliton frequency comb without any signatures of higher order modes in the spectrum spanning from 1097 to 2040 nm (126 THz) is generated in the multimode microresonator. This work provides a design method that can be applied to different material platforms to achieve and use ultrahigh‐Qmultimode microresonators.

    

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4. Dispersion-engineered χ(2) nanophotonics: a flexible tool for nonclassical light

   https://doi.org/10.1088/2515-7647/ac1729  

   Jankowski, Marc ; Mishra, Jatadhari ; Fejer, M. M. ( September 2021 , Journal of Physics: Photonics)

   This article reviews recent progress in quasi-phasematched${\chi }^{(2)}$nonlinear nanophotonics, with a particular focus on dispersion-engineered nonlinear interactions. Throughout this article, we establish design rules for the bandwidth and interaction lengths of various nonlinear processes, and provide examples for how these processes can be engineered in nanophotonic devices. In particular, we apply these rules towards the design of sources of non-classical light and show that dispersion-engineered devices can outperform their conventional counterparts. Examples include ultra-broadband optical parametric amplification as a resource for measurement-based quantum computation, dispersion-engineered spontaneous parametric downconversion as a source of separable biphotons, and synchronously pumped nonlinear resonators as a potential route towards single-photon nonlinearities.

    

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5. Back-conversion suppressed parametric frequency conversion for ultrawide bandwidth and ultrahigh efficiency devices

   https://doi.org/10.1117/12.2548361  

   Moses, Jeffrey ; Flemens, Noah ; Ding, Xiaoyue ; Schunemann, Peter G. ; Schepler, Kenneth L. ( March 2020 , SPIE LASE, 2020, San Francisco, California, United States)

   For as widely used a tool as nonlinear optical frequency conversion is for both science and industry, it remains widely limited in eciency and bandwidth (and ultimately also in cost) due to the fundamental problem of backconversion in the nonlinear evolution dynamics. This review paper covers new developments and capabilities in frequency conversion devices, including optical up- and down-converters and ampliers, based on nonlinear evolution dynamics in which back-conversion is suppressed. One such approach is adiabatic frequency conversion, in which the dynamics of rapid adiabatic passage replace the regular cyclic conversion evolution in phase-matched sum- and dierence-frequency generation. This approach has enabled devices far surpassing the conventional eciency-bandwidth trade-o. For example, in chirped quasi-phase matched quadratic crystals, microjouleenergy single-cycle mid-infrared pulses were generated with arbitrary pulse shaping capability, presenting a source with unique features for nonlinear spectroscopy and strong-eld physics applications. We review new developments in the use of optical bers as a cubic nonlinear platform for the same concept, utilizing a tapered core diameter or a pressure gradient to allow up- and down-conversion with ultra-wide bandwidth and high eciency. We also review a newly introduced concept for high eciency optical parametric amplication, via a novel approach for suppressing back-conversion in optical parametric amplication by simultaneously phasematching the idler wave for second harmonic generation. Keywords: Adiabatic wave mixing, ecient optical parametric amplication, octave-spanning 

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