Browsing by Author "Ullah, Kaleem"
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Item Melting-free integrated photonic memory with layered polymorphs(Nanophotonics, 2024-01-31) Ullah, Kaleem; Li, Qiu; Li, Tiantian; Gu, TingyiChalcogenide-based nonvolatile phase change materials (PCMs) have a long history of usage, from bulk disk memory to all-optic neuromorphic computing circuits. Being able to perform uniform phase transitions over a subwavelength scale makes PCMs particularly suitable for photonic applications. For switching between nonvolatile states, the conventional chalcogenide phase change materials are brought to a melting temperature to break the covalent bonds. The cooling rate determines the final state. Reversible polymorphic layered materials provide an alternative atomic transition mechanism for low-energy electronic (small domain size) and photonic nonvolatile memories (which require a large effective tuning area). The small energy barrier of breaking van der Waals force facilitates low energy, fast-reset, and melting-free phase transitions, which reduces the chance of element segregation-associated device failure. The search for such material families starts with polymorphic In2Se3, which has two layered structures that are topologically similar and stable at room temperature. In this perspective, we first review the history of different memory schemes, compare the thermal dynamics of phase transitions in amorphous-crystalline and In2Se3, detail the device implementations for all-optical memory, and discuss the challenges and opportunities associated with polymorphic memory.Item Micro-dispenser-based optical packaging scheme for grating couplers(Optics Letters, 2023-04-13) Uddin, S. M. Zia; Gupta, Ellen; Rahim, Masudur; Wang, Zi; Du, Yang; Ullah, Kaleem; Arnold, Craig B.; Mirotznik, Mark; Gu, TingyiDue to their sub-millimeter spatial resolution, ink-based additive manufacturing tools are typically considered less attractive than nanophotonics. Among these tools, precision micro-dispensers with sub-nanoliter volumetric control offer the finest spatial resolution: down to 50 µm. Within a sub-second, a flawless, surface-tension-driven spherical shape of the dielectric dot is formed as a self-assembled µlens. When combined with dispersive nanophotonic structures defined on a silicon-on-insulator substrate, we show that the dispensed dielectric µlenses [numerical aperture (NA) = 0.36] engineer the angular field distribution of vertically coupled nanostructures. The µlenses improve the angular tolerance for the input and reduces the angular spread of the output beam in the far field. The micro-dispenser is fast, scalable, and back-end-of-line compatible, allowing geometric-offset-caused efficiency reductions and center wavelength drift to be easily fixed. The design concept is experimentally verified by comparing several exemplary grating couplers with and without a µlens on top. A difference of less than 1 dB between incident angles of 7° and 14° is observed in the index-matched µlens, while the reference grating coupler shows around 5 dB contrast.Item Photocatalytic Oxidation in Few-Layer Tellurene for Loss-Invariant Integrated Photonic Resonance Trimming(Advanced Optical Materials, 2023-05-05) Mao, Dun; Wang, Yixiu; Lee, Hwaseob; Chang, Lorry; Wang, Feifan; Wu, Darren; Xiao, Yahui; Sun, Boshu; Ullah, Kaleem; Booksh, Karl; Wu, Wenzhuo; Gu, TingyiTwo-dimensional (2D) materials with unique physicochemical properties promote photocatalytic activities. As the 2D material composites research studies the statistical average of complex catalytic behaviors, an integrated photonic platform allows for clean and single flake level photo-catalytic investigations with precisely quantified photocatalytic activities. In this paper, fluence-dependent photo-oxidation in two-dimensional Tellurene (2D Te) is tracked by the evanescently coupled micro-resonator. Nearly 32% of oxidation is achieved in ≈10 nm 2D Te flake, compared to only 4.5% oxidation in a 30 nm sample, probed by the resonance shift in silicon micro-ring resonators substrate. The wider bandgap in the few layers of 2D Te allows faster charge transfer to adsorbed oxygen for a more efficient photocatalytic redox reaction. The photo-oxidation in hybrid 2D Te results in invariant lineshapes of optical transmission resonance for wavelength trimming (more than 3× resonance bandwidth). The low threshold power, near-infrared, and in-waveguide resonance trimming scheme is compatible with most integrated photonic setups for easy fixing of the nanofabrication-induced random resonance deviation for integrated photonic circuit applications in wavelength-division-multiplexing systems and spin qubits quantum computing.Item Recent advances in photonics of three-dimensional Dirac semimetal Cd3As2(Advanced Photonics Nexus, 2022-11-14) Zhou, Renlong; Ullah, Kaleem; Hussain, Naveed; Fadhali, Mohamed M.; Yang, Sa; Zubair, Muhammad; Iqbal, Muhammad FaisalDue to their unusual features in condensed matter physics and their applicability in optical and optoelectronic applications, three-dimensional Dirac semimetals (3D DSMs) have garnered substantial interest in recent years. In contrast to monolayer graphene, 3D DSM exhibits linear band dispersion despite its macroscopic thickness. Therefore, being a bulk material, it is easy to make nanostructures with 3D DSM, just as one normally does with metals such as gold and silver. Among 3D DSMs, cadmium arsenide (Cd3As2) is quite famous and considered an excellent 3D DSM due to its chemical stability in air and extraordinary optical response. In this review, advances in 3D DSM Cd3As2 fabrication techniques and recent progress in the photonics of 3D DSM Cd3As2 are given and briefly reviewed. Various photonic features, including linear and nonlinear plasmonics, optical absorption, optical harmonic generation, and ultrafast dynamics, have been explored in detail. It is expected that Cd3As2 would share an excellent tunable photonic response like graphene. We envision that this article may serve as a concise overview of the recent progress of photonics in 3D DSM Cd3As2 and provides a compact reference for young researchers.