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1. Shallow-etched thin-film lithium niobate waveguides for highly-efficient second-harmonic generation

   https://doi.org/10.1364/OE.395545  

   Zhao, Jie; Rüsing, Michael; Javid, Usman_A; Ling, Jingwei; Li, Mingxiao; Lin, Qiang; Mookherjea, Shayan (June 2020, Optics Express)

   High-fidelity periodic poling over long lengths is required for robust, quasi-phase-matched second-harmonic generation using the fundamental, quasi-TE polarized waveguide modes in a thin-film lithium niobate (TFLN) waveguide. Here, a shallow-etched ridge waveguide is fabricated in x-cut magnesium oxide doped TFLN and is poled accurately over 5 mm. The high fidelity of the poling is demonstrated over long lengths using a non-destructive technique of confocal scanning second-harmonic microscopy. We report a second-harmonic conversion efficiency of up to 939 %.W⁻¹(length-normalized conversion efficiency 3757 %.W⁻¹.cm⁻²), measured at telecommunications wavelengths. The device demonstrates a narrow spectral linewidth (1 nm) and can be tuned precisely with a tuning characteristic of 0.1 nm/°C, over at least 40 °C without measurable loss of efficiency. 

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2. Efficient on-chip green light generation via frequency upconversion in SiN–transfer-printed LN hybrid waveguides

   https://doi.org/10.1364/OL.560034  

   Sumon, Md_Saiful_Islam; Vorobev, Artem_S; Ghosh, Samir; Atar, Fatih_Bilge; Maraviglia, Nicola; Nikor, Sk_Shafaat_Saud; Devarapu, Ganga_Chinna_Rao; Maddi, Chiranjeevi; Faruque, Imad_I; Dwivedi, Sarvagya; et al (May 2025, Optics Letters)

   We demonstrate efficient on-chip green light generation via frequency upconversion in silicon nitride–thin-film lithium niobate (SiN-TFLN) hybrid waveguides, obtained by transfer printing LN coupons on selected areas of photonic integrated circuits (PICs). By utilizing modal phase matching (MPM), our devices achieve a high normalized conversion efficiency of 42.5% W⁻¹cm⁻²in a single-pass, 2.4-mm-long waveguide configuration. The SiN–LN transition in the waveguide inherently facilitates mode conversion, transforming a higher-order second-harmonic mode into a fundamental TE mode, ensuring coherent, narrow-linewidth, green light emission. Our waveguide platform gives rise to a wavelength shift of ∼1 nm for every 10 nm of waveguide width variation and temperature-induced wavelength tuning of ∼0.02 nm/°C. 

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3. Extreme Polarization Anisotropy in Resonant Third‐Harmonic Generation from Aligned Carbon Nanotube Films

   https://doi.org/10.1002/adma.202304082  

   Zhu, Song; Li, Wenkai; Yu, Shengjie; Komatsu, Natsumi; Baydin, Andrey; Wang, Fakun; Sun, Fangyuan; Wang, Chongwu; Chen, Wenduo; Tan, Chuan_Seng; et al (September 2023, Advanced Materials)

   Abstract Carbon nanotubes (CNTs) possess extremely anisotropic electronic, thermal, and optical properties owing to their 1D character. While their linear optical properties have been extensively studied, nonlinear optical processes, such as harmonic generation for frequency conversion, remain largely unexplored in CNTs, particularly in macroscopic CNT assemblies. In this work, macroscopic films of aligned and type‐separated (semiconducting and metallic) CNTs are synthesized and polarization‐dependent third‐harmonic generation (THG) from the films with fundamental wavelengths ranging from 1.5 to 2.5 µm is studied. Both films exhibited strongly wavelength‐dependent, intense THG signals, enhanced through exciton resonances, and third‐order nonlinear optical susceptibilities of 2.50 × 10⁻¹⁹ m² V⁻²(semiconducting CNTs) and 1.23 × 10⁻¹⁹ m² V⁻²(metallic CNTs), respectively are found, for 1.8 µm excitation. Further, through systematic polarization‐dependent THG measurements, the values of all elements of the susceptibility tensor are determined, verifying the macroscopically 1D nature of the films. Finally, polarized THG imaging is performed to demonstrate the nonlinear anisotropy in the large‐size CNT film with good alignment. These findings promise applications of aligned CNT films in mid‐infrared frequency conversion, nonlinear optical switching, polarized pulsed lasers, polarized long‐wave detection, and high‐performance anisotropic nonlinear photonic devices. 

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4. Second-harmonic optical vortex conversion from WS2 monolayer

   https://doi.org/10.1038/s41598-019-45424-4  

   Dasgupta, Arindam; Gao, Jie; Yang, Xiaodong (June 2019, Scientific Reports)

   Abstract Wavelength, polarization and orbital angular momentum of light are important degrees of freedom for processing and encoding information in optical communication. Over the years, the generation and conversion of orbital angular momentum in nonlinear optical media has found many novel applications in the context of optical communication and quantum information processing. With that hindsight, here orbital angular momentum conversion of optical vortices through second-harmonic generation from only one atomically thin WS₂monolayer is demonstrated at room temperature. Moreover, it is shown that the valley-contrasting physics associated with the nonlinear optical selection rule in WS₂monolayer precisely determines the output circular polarization state of the generated second-harmonic vortex. These results pave the way for building future miniaturized valleytronic devices with atomic-scale thickness for many applications such as chiral photon emission, nonlinear beam generation, optoelectronics, and quantum computing. 

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5. MgSiP ₂ : An Infrared Nonlinear Optical Crystal with a Large Non‐Resonant Phase‐Matchable Second Harmonic Coefficient and High Laser Damage Threshold

   https://doi.org/10.1002/adom.202301060  

   He, Jingyang; Guan, Yingdong; Trinquet, Victor; Brunin, Guillaume; Wang, Ke; Robinson, Robert; Zu, Rui; Yoshida, Suguru; Lee, Seng_Huat; Wang, Yu; et al (July 2023, Advanced Optical Materials)

   Abstract Superior infrared nonlinear optical (NLO) crystals are in urgent demand in the development of lasers and optical technologies for communications and computing. The critical challenge is to find a crystal with large non‐resonant phase‐matchable NLO coefficients and high laser damage threshold (LDTs) simultaneously, which however scale inversely. This work reports such a material, MgSiP_2,that exhibits a large second harmonic generation (SHG) coefficient ofd₁₄≈d₃₆= 89 ± 5 pm V⁻¹at 1550 nm fundamental wavelength, surpassing the commercial NLO crystals AgGaS₂, AgGaSe₂, and ZnGeP₂. First principles theory reveals the polarizability and geometric arrangement of the [SiP₄] tetrahedral units as the origin of this large nonlinear response. Remarkably, it also exhibits a high LDT value of 684 GW cm⁻², which is six times larger than ZnGeP₂and three times larger than CdSiP₂. It has a wide transparency window of 0.53–10.35 µm, allowing broadband tunability. Further, it is Type I and Type II phase‐matchable with large effective SHG coefficients ofd_eff,I≈80.2 pm V⁻¹andd_eff,II≈73.4 pm V⁻¹. The outstanding properties of MgSiP₂make it a highly attractive candidate for optical frequency conversion in the infrared. 

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