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1. Generation of Anisotropic Second‐Harmonic Petal‐Like Beams with NbOBr ₂ Holograms

   https://doi.org/10.1002/adpr.202500319  

   Deka, Jayanta; Gao, Jie; Yang, Xiaodong (April 2026, Advanced Photonics Research)

   The coherent superposition of two vortex beams with opposite topological charges gives rise to the formation of petal‐like beams with unique intensity profiles and azimuthal phase structures, offering great potential for various applications. Here, the generation of anisotropic second‐harmonic petal‐like beams is demonstrated using ultrathin niobium oxide dibromide (NbOBr₂) holograms. The polarization‐dependent Raman measurements are conducted on the NbOBr₂flake to reveal the anisotropic properties of the crystal. By imprinting the beam profile information of the superposed vortex beams on the designed NbOBr₂holograms, highly anisotropic second‐harmonic petal‐like beams with different combinations of the topological charges of vortex beams are produced. The results demonstrated here provide a pathway toward the development of chip‐scale polarization‐sensitive functional optical devices for applications in optical communication, optical trapping, and quantum optics. 

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2. Anisotropic Third‐Harmonic Vortex Beam Generation with Ultrathin Germanium Arsenide Fork Gratings

   https://doi.org/10.1002/lpor.202401519  

   Deka, Jayanta; Gao, Jie; Yang, Xiaodong (April 2025, Laser & Photonics Reviews)

   Abstract Optical vortices have the tremendous potential to increase data capacity by leveraging the extra degree of freedom of orbital angular momentum. On the other hand, anisotropic 2D materials are promising building blocks for future integrated polarization‐sensitive photonic and optoelectronic devices. Here, highly anisotropic third‐harmonic optical vortex beam generation is demonstrated with fork holograms patterned on ultrathin 2D germanium arsenide flakes. It is shown that the anisotropic nonlinear vortex beam generation can be achieved independent of the fork grating orientation with respect to the crystallographic orientation. Furthermore, 2D fork hologram is designed to generate multiple optical vortices having different topological charges with strong anisotropic responses. These results pave the way toward the advancement of 2D material‐based anisotropic nonlinear optical devices for future applications in photonic integrated circuits, optical communication, and optical information processing. 

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3. Ultrafast Microscopy of Plasmonic Vortex Fields

   Zhou, Zhikang (April 2025, University of Pittsburgh)

   Surface Plasmon Polariton (SPP), as a novel information carrier, offers unprecedented opportunity for confining electromagnetic fields that carry orbital angular momentum (OAM) to subwavelength dimensions. In this thesis, I focus experimentally on the generation, manipulation, and spatio-temporal evolution—and theoretically on the analytical modeling—of plasmonic phase singularities, known as plasmonic vortices, at the silver (Ag)/vacuum interface. I image and study the dynamics of plasmonic vortices by interferometric time-resolved multi-photon photoemission electron microscopy (ITR-mP-PEEM). Firstly, I report on the generation, evolution, and topological properties of plasmonic vortices carrying pure geometrically induced orbital angular momentum (OAM), generated by illuminating Archimedean spiral coupling structures with normally incident, linearly polarized light. Next, I present an analytical model describing the generation and evolution of these plasmonic vortices, and based on this model, I further analyze their spatial structure and dynamics. I also derived the spin angular momentum (SAM) of plasmonic vortices, whose textures reveal transient plasmonic spin-Skyrmion topological quasiparticles. In parallel, I also record images of plasmonic vectoral vortex field evolution on the nanometer spatial and femtosecond temporal scale, from which I derive the plasmonic spin Skyrmion boundary and topological charge. The excellent agreement between analytical model and experimental results confirms the topological spin texture at surface plasmon polariton vortex core. To extend the understanding of ITR-PEEM imaging, I perform a simple experiment withv double line coupling structure at the silver/vacuum interface, which reveals an asymmetric cross term between the different components of the SPP field that also appear in the ITR-PEEM imaging. Finally, I approach a novel method to manipulate momentum transport between two plasmonic vortices analytically and experimentally. By tuning the relative distance between two vortex generator structures with same sign and sign of the geometric charge, a conveyor belt-like field could be observed at the center of the device, which can be applied to transport the field, momentum, and energy between two plasmonic vortices. 

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4. Deep tuning of photo-thermoelectricity in topological surface states

   https://doi.org/10.1038/s41598-020-73950-z  

   Huang, Shouyuan; Miotkowski, Ireneusz; Chen, Yong P.; Xu, Xianfan (December 2020, Scientific Reports)

   null (Ed.)

   Abstract Three-dimensional topological insulators have been demonstrated in recent years, which possess intriguing gapless, spin-polarized Dirac states with linear dispersion only on the surface. The spin polarization of the topological surface states is also locked to its momentum, which allows controlling motion of electrons using optical helicity, i.e., circularly polarized light. The electrical and thermal transport can also be significantly tuned by the helicity-control of surface state electrons. Here, we report studies of photo-thermoelectric effect of the topological surface states in Bi 2 Te 2 Se thin films with large tunability using varied gate voltages and optical helicity. The Seebeck coefficient can be altered by more than five times compared to the case without spin injection. This deep tuning is originated from the optical helicity-induced photocurrent which is shown to be enhanced, reduced, turned off, and even inverted due to the change of the accessed band structures by electrical gating. The helicity-selected topological surface state thus has a large effect on thermoelectric transport, demonstrating great opportunities for realizing helicity control of optoelectronic and thermal devices. 

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5. Wafer-scale fabrication of InGaP-on-insulator for nonlinear and quantum photonic applications

   https://doi.org/10.1063/5.0225747  

   Thiel, Lillian; Castro, Joshua E; Steiner, Trevor J; Nguyen, Catherine L; Pechilis, Audrey; Duan, Liao; Lewis, Nicholas; Cole, Garrett D; Bowers, John E; Moody, Galan (September 2024, Applied Physics Letters)

   The development of manufacturable and scalable integrated nonlinear photonic materials is driving key technologies in diverse areas, such as high-speed communications, signal processing, sensing, and quantum information. Here, we demonstrate a nonlinear platform—InGaP-on-insulator—optimized for visible-to-telecommunication wavelength χ(2) nonlinear optical processes. In this work, we detail our 100 mm wafer-scale InGaP-on-insulator fabrication process realized via wafer bonding, optical lithography, and dry-etching techniques. The resulting wafers yield 1000 s of components in each fabrication cycle, with initial designs that include chip-to-fiber couplers, 12.5-cm-long nested spiral waveguides, and arrays of microring resonators with free-spectral ranges spanning 400–900 GHz. We demonstrate intrinsic resonator quality factors as high as 324 000 (440 000) for single-resonance (split-resonance) modes near 1550 nm corresponding to 1.56 dB/cm (1.22 dB/cm) propagation loss. We analyze the loss vs waveguide width and resonator radius to establish the operating regime for optimal 775–1550 nm phase matching. By combining the high χ(2) and χ(3) optical nonlinearity of InGaP with wafer-scale fabrication and low propagation loss, these results open promising possibilities for entangled-photon, multi-photon, and squeezed light generation. 

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