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1. Generalized analytical and numerical modeling of optical second harmonic generation in anisotropic crystals and complex heterostructures using ♯SHAARP package

   https://doi.org/10.1117/12.2676223  

   Zu, Rui; Wang, Bo; He, Jingyang; Weber, Lincoln; Wang, Jianjun; Chen, Long-Qing; Gopalan, Venkatraman (October 2023, SPIE)

   Liu, Zhiwen; Psaltis, Demetri; Shi, Kebin (Ed.)

   Optical Second Harmonic Generation (SHG) is a nonlinear optical effect widely used for nonlinear optical microscopy and laser frequency conversion. The closed-form analytical solution of the nonlinear optical responses is essential for evaluating the optical responses of new materials whose optical properties are unknown a priori. Many approximations have therefore been employed in the existing analytical approaches, such as slowly varying approximation, weak reflection of the nonlinear polarization, transparent medium, high crystallographic symmetry, Kleinman symmetry, easy crystal orientation along a high-symmetry direction, phase matching conditions and negligible interference among nonlinear waves, which may lead to large errors in the reported material properties. To avoid these approximations, we have developed an open-source package named Second Harmonic Analysis of Anisotropic Rotational Polarimetry (♯SHAARP) for single interface (si) and in multilayers (ml) for homogeneous crystals. The reliability and accuracy are established by experimentally benchmarking with both the SHG polarimetry and Maker fringes predicted from the package using standard materials. SHAARP.si and SHAARP.ml are available through GitHub https://github.com/Rui-Zu/SHAARP and https://github.com/bzw133/SHAARP.ml, respectively. 

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2. Optical second harmonic generation in anisotropic multilayers with complete multireflection of linear and nonlinear waves using ♯SHAARP.ml package

   https://doi.org/10.1038/s41524-024-01229-2  

   Zu, Rui; Wang, Bo; He, Jingyang; Weber, Lincoln; Saha, Akash; Chen, Long-Qing; Gopalan, Venkatraman (March 2024, npj Computational Materials)

   Abstract Optical second harmonic generation (SHG) is a nonlinear optical effect widely used for nonlinear optical microscopy and laser frequency conversion. Closed-form analytical solution of the nonlinear optical responses is essential for evaluating materials whose optical properties are unknown a priori. A recent open-source code, ♯SHAARP.si, can provide such closed form solutions for crystals with arbitrary symmetries, orientations, and anisotropic properties at asingleinterface. However, optical components are often in the form of slabs, thin films on substrates, and multilayer heterostructures with multiple reflections of both the fundamental and up to ten different SHG waves at each interface, adding significant complexity. Many approximations have therefore been employed in the existing analytical approaches, such as slowly varying approximation, weak reflection of the nonlinear polarization, transparent medium, high crystallographic symmetry, Kleinman symmetry, easy crystal orientation along a high-symmetry direction, phase matching conditions and negligible interference among nonlinear waves, which may lead to large errors in the reported material properties. To avoid these approximations, we have developed an open-source package named Second Harmonic Analysis of Anisotropic Rotational Polarimetry in Multilayers (♯SHAARP.ml). The reliability and accuracy are established by experimentally benchmarking with both the SHG polarimetry and Maker fringes using standard and commonly used nonlinear optical materials as well as twisted 2-dimensional heterostructures. 

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3. Perspectives on epitaxial InGaP for quantum and nonlinear optics

   https://doi.org/10.1063/5.0241224  

   Akin, Joshua; Zhao, Yunlei; Haque, A_K_M Naziul; Fang, Kejie (December 2024, Applied Physics Letters)

   Nonlinear optical materials are essential for the development of both nonlinear and quantum optics and have advanced recently from bulk crystals to integrated material platforms. In this Perspective, we provide an overview of the emerging InGaP χ(2) nonlinear integrated photonics platform and its experimental achievements. With its exceptional χ(2) nonlinearity and low optical losses, the epitaxial InGaP platform significantly enhances a wide range of second-order nonlinear optical effects, from second-harmonic generation to entangled photon pair sources, achieving efficiencies several orders of magnitude beyond the current state of the art. Moreover, the InGaP platform enables quantum nonlinear optics at the few- and single-photon levels via passive nonlinearities, which has broad implications for quantum information processing and quantum networking. We also examine the current limitations of the InGaP platform and propose potential solutions to fully unlock its capabilities. 

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4. Post-2000 nonlinear optical materials and measurements: data tables and best practices

   https://doi.org/10.1088/2515-7647/ac9e2f  

   Vermeulen, Nathalie; Espinosa, Daniel; Ball, Adam; Ballato, John; Boucaud, Philippe; Boudebs, Georges; Campos, Cecília L. A. V.; Dragic, Peter; Gomes, Anderson S. L.; Huttunen, Mikko J.; et al (May 2023, Journal of Physics: Photonics)

   Abstract In its 60 years of existence, the field of nonlinear optics has gained momentum especially over the past two decades thanks to major breakthroughs in material science and technology. In this article, we present a new set of data tables listing nonlinear-optical properties for different material categories as reported in the literature since 2000. The papers included in the data tables are representative experimental works on bulk materials, solvents, 0D–1D–2D materials, metamaterials, fiber waveguiding materials, on-chip waveguiding materials, hybrid waveguiding systems, and materials suitable for nonlinear optics at THz frequencies. In addition to the data tables, we also provide best practices for performing and reporting nonlinear-optical experiments. These best practices underpin the selection process that was used for including papers in the tables. While the tables indeed show strong advancements in the field over the past two decades, we encourage the nonlinear-optics community to implement the identified best practices in future works. This will allow a more adequate comparison, interpretation and use of the published parameters, and as such further stimulate the overall progress in nonlinear-optical science and applications. 

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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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