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1. Polarization-sensitive optical responses from natural layered hydrated sodium sulfosalt gerstleyite

   https://doi.org/10.1038/s41598-022-08235-8  

   Tripathi, Ravi P. N.; Yang, Xiaodong; Gao, Jie (March 2022, Scientific Reports)

   Abstract Multi-element layered materials have gained substantial attention in the context of achieving the customized light-matter interactions at subwavelength scale via stoichiometric engineering, which is crucial for the realization of miniaturized polarization-sensitive optoelectronic and nanophotonic devices. Herein, naturally occurring hydrated sodium sulfosalt gerstleyite is introduced as one new multi-element van der Waals (vdW) layered material. The mechanically exfoliated thin gerstleyite flakes are demonstrated to exhibit polarization-sensitive anisotropic linear and nonlinear optical responses including angle-resolved Raman scattering, anomalous wavelength-dependent linear dichroism transition, birefringence effect, and polarization-dependent third-harmonic generation (THG). Furthermore, the third-order nonlinear susceptibility of gerstleyite crystal is estimated by the probed flake thickness-dependent THG response. We envisage that our findings in the context of polarization-sensitive light-matter interactions in the exfoliated hydrated sulfosalt layers will be a valuable addition to the vdW layered material family and will have many implications in compact waveplates, on-chip photodetectors, optical sensors and switches, integrated photonic circuits, and nonlinear signal processing applications. 

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2. Integrated photonics on thin-film lithium niobate

   https://doi.org/10.1364/AOP.411024  

   Zhu, Di; Shao, Linbo; Yu, Mengjie; Cheng, Rebecca; Desiatov, Boris; Xin, C_J; Hu, Yaowen; Holzgrafe, Jeffrey; Ghosh, Soumya; Shams-Ansari, Amirhassan; et al (May 2021, Advances in Optics and Photonics)

   Lithium niobate (LN), an outstanding and versatile material, has influenced our daily life for decades—from enabling high-speed optical communications that form the backbone of the Internet to realizing radio-frequency filtering used in our cell phones. This half-century-old material is currently embracing a revolution in thin-film LN integrated photonics. The successes of manufacturing wafer-scale, high-quality thin films of LN-on-insulator (LNOI) and breakthroughs in nanofabrication techniques have made high-performance integrated nanophotonic components possible. With rapid development in the past few years, some of these thin-film LN devices, such as optical modulators and nonlinear wavelength converters, have already outperformed their legacy counterparts realized in bulk LN crystals. Furthermore, the nanophotonic integration has enabled ultra-low-loss resonators in LN, which has unlocked many novel applications such as optical frequency combs and quantum transducers. In this review, we cover—from basic principles to the state of the art—the diverse aspects of integrated thin-film LN photonics, including the materials, basic passive components, and various active devices based on electro-optics, all-optical nonlinearities, and acousto-optics. We also identify challenges that this platform is currently facing and point out future opportunities. The field of integrated LNOI photonics is advancing rapidly and poised to make critical impacts on a broad range of applications in communication, signal processing, and quantum information. 

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3. Nonlinear Optics in Hydrogenated Amorphous Silicon

   https://doi.org/10.1109/JSTQE.2018.2854590  

   Li, Kangmei; Foster, Amy C. (July 2018, IEEE Journal of Selected Topics in Quantum Electronics)

   Nonlinear photonic circuits with the ability to generate and process signals all-optically have emerged in the past decade with superior performance to electronic chips. In particular, crystalline silicon has become a leading platform for integrated nonlinear optics. More recently, hydrogenated amorphous silicon emerged as a promising alternative to crystalline silicon due to its large nonlinearity. In this paper, we review recent research on nonlinear optical interactions in and applications of hydrogenated amorphous silicon nanophotonic devices. This new material platform enables the capability of multilayer CMOS-compatible photonic integrated circuits with low power requirements for high-speed optical signal processing. 

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4. High-precision local transfer of van der Waals materials on nanophotonic structures

   https://doi.org/10.1364/OME.383255  

   Rosser, David; Fryett, Taylor; Saxena, Abhi; Ryou, Albert; Majumdar, Arka (January 2020, Optical Materials Express)

   Prototyping of van der Waals materials on dense nanophotonic devices requires high-precision monolayer discrimination to avoid bulk material contamination. We use the glass transition temperature of polycarbonate, used in the standard dry transfer process, to draw an in situ point for the precise pickup of two-dimensional materials. We transfer transition metal dichalcogenide monolayers onto a large-area silicon nitride spiral waveguide and silicon nitride ring resonators to demonstrate the high-precision contamination-free nature of the modified dry transfer method. Our improved local transfer technique is a necessary step for the deterministic integration of high-quality van der Waals materials onto nanocavities for the exploration of few-photon nonlinear optics on a high-throughput, nanofabrication-compatible platform. 

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5. InGaP χ(2) integrated photonics platform for broadband, ultra-efficient nonlinear conversion and entangled photon generation

   https://doi.org/10.1038/s41377-024-01653-5  

   Akin, Joshua; Zhao, Yunlei; Misra, Yuvraj; Haque, A_K_M Naziul; Fang, Kejie (December 2024, Light: Science & Applications)

   Abstract Nonlinear optics plays an important role in many areas of science and technology. The advance of nonlinear optics is empowered by the discovery and utilization of materials with growing optical nonlinearity. Here we demonstrate an indium gallium phosphide (InGaP) integrated photonics platform for broadband, ultra-efficient second-order nonlinear optics. The InGaP nanophotonic waveguide enables second-harmonic generation with a normalized efficiency of 128, 000%/W/cm²at 1.55μm pump wavelength, nearly two orders of magnitude higher than the state of the art in the telecommunication C band. Further, we realize an ultra-bright, broadband time-energy entangled photon source with a pair generation rate of 97 GHz/mW and a bandwidth of 115 nm centered at the telecommunication C band. The InGaP entangled photon source shows high coincidence-to-accidental counts ratio CAR > 10⁴and two-photon interference visibility > 98%. The InGaP second-order nonlinear photonics platform will have wide-ranging implications for non-classical light generation, optical signal processing, and quantum networking. 

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