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1. Modeling and Analysis of Terahertz Graphene Switches for On-Wafer Coplanar Transmission Lines

   Trichopoulos, G ; Theofanopoulos, P ( June 2020 , Journal of infrared millimeter and terahertz waves)

   null (Ed.)

   We present an analysis of graphene-loaded transmission line switches for sub-millimeter wave and terahertz applications. As such, we propose equivalent circuit models for graphene-loaded coplanar waveguides and striplines and examine the switching performance under certain parameters. Specifically, we identify the optimum design of graphene switches based on transmission line characteristic impedance, scaling factor, graphene shape, and topology (series or shunt). These parameters are varied to obtain the insertion loss and ON/OFF ratio of each switch configuration. The extracted results act as the design roadmap toward an optimum switch topology and emphasize the limitations with respect to fabrication challenges, parasitic effects, and radiation losses that are especially pronounced in the millimeter wave/terahertz bands. This is the first time that such an in-depth analysis is carried out on graphene-loaded transmission line switches, enabling the development of efficient millimeter wave/terahertz tunable topologies in terms of insertion loss and ON/OFF ratio. Specifically, the optimized switches can be integrated with antennas or employed for the development of tunable phase shifters, leading to the implementation of efficient reconfigurable reflective surfaces (e.g., reflectarrays) or coded phased arrays either for imaging or wireless communication applications. In our models, we use measured graphene values (sheet impedance) instead of theoretical equations, to obtain the actual switching performance. Moreover, the proposed study can be easily expanded to other thin film materials that can be characterized by a sheet impedance including vanadium dioxide and molybdenum disulfide. Finally, the proposed equivalent models are crucial for this in-depth study; since we simulated more than 2,000,000 configurations, a computationally challenging task with the use of full-wave solvers. 

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2. High-performance graphene-integrated thermo-optic switch: design and experimental validation [Invited]

   https://doi.org/10.1364/OME.382856  

   Li, Junying ; Huang, Yizhong ; Song, Yi ; Li, Lan ; Zheng, Hanyu ; Wang, Haozhe ; Gu, Tian ; Richardson, Kathleen ; Kong, Jing ; Hu, Juejun ; et al ( January 2020 , Optical Materials Express)

   The extraordinary optical properties of single-layer graphene have spurred the development of a variety of photonic components. We have previously demonstrated a scalable and versatile platform to facilitate the integration of graphene and other 2-D materials with chalcogenide glass-based planar photonics. In this paper, we detail the design criteria and optimization guidelines towards high-performance graphene-integrated thermo-optic (TO) switches based on the chalcogenide glass-on-graphene platform. Notably, absorption loss of graphene can be reduced to < 20 dB/cm when it is sandwiched inside photonic structures capitalizing on the anisotropic absorption property of graphene. We quantify energy efficiency of the TO switch, showing that the choice of cladding materials plays a critical role in improving device efficiency. Furthermore, we report a record TO switching efficiency of 10 nm/mW via judicious engineering of the overlap between optical mode and thermal profile.

    

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3. A Low Cost Reflect Array for Near-field Millimeter-Wave Beam Focusing Applications

   https://doi.org/10.1109/APUSNCURSINRSM.2018.8609004  

   Molaei, Ali ; Martinez-Lorenzo, Jose ( July 2018 , 2018 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting)

   A low cost millimeter-wave (mm-wave) electronically reconfigurable Reflect Array (RA) has been presented in this paper. Aperture-Coupled Patch (ACP) elements are used to forma 40×40 element reconfigurable RA operating in the 71-74 GHz range of the mm-wave band. A feeding network, integrated with Single-Pole, Single-Throw (SPST) switches is designed to adjust the phase of the reflected field for the ON/OFF state of the switch. The performance of the reconfigurable RA is evaluated by performing beam focusing at the near-field of the array. 

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4. Microwave signal switching on a silicon photonic chip

   https://doi.org/10.1038/s41598-019-47683-7  

   Fang, Cheng-Yi ; Lin, Hung-Hsi ; Alouini, Mehdi ; Fainman, Yeshaiahu ; El Amili, Abdelkrim ( August 2019 , Scientific Reports)

   Microwave photonics uses light to carry and process microwave signals over a photonic link. However, light can instead be used as a stimulus to microwave devices that directly control microwave signals. Such optically controlled amplitude and phase-shift switches are investigated for use in reconfigurable microwave systems, but they suffer from large footprint, high optical power level required for switching, lack of scalability and complex integration requirements, restricting their implementation in practical microwave systems. Here, we report Monolithic Optically Reconfigurable Integrated Microwave Switches (MORIMSs) built on a CMOS compatible silicon photonic chip that addresses all of the stringent requirements. Our scalable micrometer-scale switches provide higher switching efficiency and require optical power orders of magnitude lower than the state-of-the-art. Also, it opens a new research direction on silicon photonic platforms integrating microwave circuitry. This work has important implications in reconfigurable microwave and millimeter wave devices for future communication networks.

    

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5. Simultaneous Extraction of the Grain Size, Single-Crystalline Grain Sheet Resistance, and Grain Boundary Resistivity of Polycrystalline Monolayer Graphene

   https://doi.org/10.3390/nano12020206  

   Park, Honghwi ; Lee, Junyeong ; Lee, Chang-Ju ; Kang, Jaewoon ; Yun, Jiyeong ; Noh, Hyowoong ; Park, Minsu ; Lee, Jonghyung ; Park, Youngjin ; Park, Jonghoo ; et al ( January 2022 , Nanomaterials)

   The electrical properties of polycrystalline graphene grown by chemical vapor deposition (CVD) are determined by grain-related parameters—average grain size, single-crystalline grain sheet resistance, and grain boundary (GB) resistivity. However, extracting these parameters still remains challenging because of the difficulty in observing graphene GBs and decoupling the grain sheet resistance and GB resistivity. In this work, we developed an electrical characterization method that can extract the average grain size, single-crystalline grain sheet resistance, and GB resistivity simultaneously. We observed that the material property, graphene sheet resistance, could depend on the device dimension and developed an analytical resistance model based on the cumulative distribution function of the gamma distribution, explaining the effect of the GB density and distribution in the graphene channel. We applied this model to CVD-grown monolayer graphene by characterizing transmission-line model patterns and simultaneously extracted the average grain size (~5.95 μm), single-crystalline grain sheet resistance (~321 Ω/sq), and GB resistivity (~18.16 kΩ-μm) of the CVD-graphene layer. The extracted values agreed well with those obtained from scanning electron microscopy images of ultraviolet/ozone-treated GBs and the electrical characterization of graphene devices with sub-micrometer channel lengths. 

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