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1. Anisotropic and Temperature‐Tunable Second‐Harmonic Vortex Generation in Ferroelectric NbOCl ₂ Holograms

   https://doi.org/10.1002/lpor.202501523  

   Deka, Jayanta; Yang, Xiaodong; Gao, Jie (January 2026, Laser & Photonics Reviews)

   Abstract Optical vortex beams with helical phase fronts have immense potential to enhance data capacity due to the unbounded values of orbital angular momentum. Chip‐scale platforms for producing vortex beams are of paramount importance for a variety of applications. On the other hand, 2D materials with unique optical properties are essential for developing multifunctional ultrathin photonic devices. Here, anisotropic and temperature‐tunable second‐harmonic vortex beam generation is demonstrated with ultrathin ferroelectric niobium oxide dichloride (NbOCl₂) fork holograms. The polarization‐resolved Raman measurements are performed on the NbOCl₂crystal to understand the anisotropic behavior of the Raman modes. It is demonstrated that the anisotropic and temperature‐tunable second‐harmonic vortex beams can be generated regardless of the relative orientation of the fork gratings with respect to the crystallographic orientation of the NbOCl₂flakes. Furthermore, the Curie temperature of the ferroelectric NbOCl₂crystal is determined according to the measured temperature‐dependent second‐harmonic generation intensities. The results presented here create new opportunities for the development of advanced polarization‐sensitive and temperature‐switchable nonlinear photonic devices used for future applications in integrated photonic devices, quantum optical chips, and optical communication. 

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2. Naturally occurring van der Waals heterostructure lengenbachite with strong in-plane structural and optical anisotropy

   https://doi.org/10.1038/s41699-021-00271-8  

   Dasgupta, Arindam; Yang, Xiaodong; Gao, Jie (December 2021, npj 2D Materials and Applications)

   Abstract Lengenbachite is a naturally occurring layered mineral formed with alternating stacks of two constituent PbS-like and M 2 S 3 -like two-dimensional (2D) material layers due to the phase segregation process during the formation. Here, we demonstrate to achieve van der Waals (vdW) heterostructures of lengenbachite down to a few layer-pair thickness by mechanical exfoliation of bulk lengenbachite mineral. The incommensurability between the constituent isotropic 2D material layers makes the formed vdW heterostructure exhibit strong in-plane structural anisotropy, which leads to highly anisotropic optical responses in lengenbachite thin flakes, including anisotropic Raman scattering, linear dichroism, and anisotropic third-harmonic generation. Moreover, we exploit the nonlinear optical anisotropy for polarization-dependent intensity modulation of the converted third-harmonic optical vortices. Our study establishes lengenbachite as a new natural vdW heterostructure-based 2D material with unique optical properties for realizing anisotropic optical devices for photonic integrated circuits and optical information processing. 

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3. Anisotropic third-harmonic generation of exfoliated As ₂ S ₃ thin flakes

   https://doi.org/10.1364/OE.457858  

   Tripathi, Ravi P.; Yang, Xiaodong; Gao, Jie (January 2022, Optics Express)

   Van der Waals (vdW) materials have recently attracted significant interest in the context of orientation-dependent linear and nonlinear optical properties. Recently, arsenic trisulfide (As 2 S 3 ) or orpiment is identified as a new vdW layered material having anisotropic vibrational and optomechanical responses due to the reduced in-plane crystal symmetry, but its nonlinear optical response is still not well understood yet. Herein, the anisotropic third-harmonic generation (THG) response of mechanically exfoliated As 2 S 3 thin flakes is reported. The polarization-dependent evolution of THG emission from butterfly-shaped pattern to four-lobe pattern is comprehensively explored. Moreover, the third-order nonlinear susceptibility of As 2 S 3 crystal is extracted by analyzing the thickness-dependent THG emission. We anticipate that the discussed results will not only update the existing understanding on the nonlinear light-matter interaction in anisotropic vdW materials, but also promote future applications in integrated photonic circuits, on-chip nonlinear signal processing, and polarization-sensitive optical devices. 

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4. Anisotropic Second‐ and Third‐Harmonic Hermite–Gaussian Beam Generation with NbOI ₂ Holograms

   https://doi.org/10.1002/adpr.202500188  

   Deka, Jayanta; Gao, Jie; Yang, Xiaodong (December 2025, Advanced Photonics Research)

   Hermite–Gaussian (HG) modes are an orthogonal set of solutions to the paraxial wave equation possessing distinct spatial intensity variations, which are important in many optical applications. Here, anisotropic second‐ and third‐harmonic HG beam generation is demonstrated with ultrathin niobium oxide diiodide (NbOI₂) grating holograms to produce the nonlinear HG₀₁and HG₁₀modes. It is shown that the generated second‐harmonic HG modes exhibit a high anisotropy ratio reaching a large value of 14.95, while the simultaneously generated third‐harmonic HG modes have an anisotropy ratio up to 4.85. The relative magnitudes of the second‐ and third‐order nonlinear susceptibility tensor elements of NbOI₂crystal are extracted by analyzing the polarization‐dependent nonlinear emission. These demonstrations provide new opportunities for building functional polarization‐sensitive nonlinear optical devices used for future integrated photonic chips, optical computing, and optical communication. 

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5. Natural 2D layered mineral cannizzarite with anisotropic optical responses

   https://doi.org/10.1038/s41598-022-14046-8  

   Dasgupta, Arindam; Yang, Xiaodong; Gao, Jie (December 2022, Scientific Reports)

   Abstract Cannizzarite is a naturally occurring mineral formed by van der Waals (vdW) stacking of alternating layers of PbS-like and Bi 2 S 3 -like two-dimensional (2D) materials. Although the PbS-type and Bi 2 S 3 -type 2D material layers are structurally isotropic individually, the forced commensuration between these two types of layers while forming the heterostructure of cannizzarite induces strong structural anisotropy. Here we demonstrate the mechanical exfoliation of natural cannizzarite mineral to obtain thin vdW heterostructures of PbS-type and Bi 2 S 3 -type atomic layers. The structural anisotropy induced anisotropic optical properties of thin cannizzarite flakes are explored through angle-resolved polarized Raman scattering, linear dichroism, and polarization-dependent anisotropic third-harmonic generation. Our study establishes cannizzarite as a new natural vdW heterostructure-based 2D material with highly anisotropic optical properties for realizing polarization-sensitive linear and nonlinear photonic devices for future on-chip optical computing and optical information processing. 

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