Nanobridged rhombic antennas supporting both

image: (a) Sketch, topography and near-field imaging of an NBRA dimer. (b) SEIRA spectra of monolayer molecule adsorbed on the NBRA dimer with or without reflector.
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Credit: OEA

In a new post by Opto-electronic advances; DO I 10.29026 / oea.2021.210076, the research groups of Professor Zhong-Qun Tian of Xiamen University, Xiamen, China, and Professor Huigao Duan of Hunan University in Changsha, China, discuss nanobridged rhombic antennas supporting Dipolar and high-order plasmon modes with spatially superimposed hot spots in the mid-infrared.

Mid-infrared antennas (MIRA), often constructed from metals (e.g. range (400 to 4000 cm − 1). MIRAs can act as receiving antennas, thus concentrating mid-infrared beams from space free to nanoscale regions (called hot spots) near the surface of MIRAs. MIRAs can also serve as transmitting antennas to directionally amplify thermal radiation produced by local heating of sources coupled to MIRAs. impressive MIRAs have inspired a wide range of studies of their potential applications for Surface Enhanced Infrared Absorption Spectroscopy (SEIRA) leading to ultra-high sensitivities (up to hundreds of oscillators), for biological sensors and chemicals in the mid-infrared region, for engineering the beamform of quantum cascade lasers and for highly reactive photodetectors with improved absorption and collection efficiency of mid-infrared photocarriers. The core elements for high performance applications are MIRA micro- and nanostructures, but the development of MIRA structures lags far behind that of optical antenna nanostructures in the visible spectral range.

One-arm dipole antenna structures are among the more classic MIRAs, often made of gold rods with resonant wavelengths tunable by adjusting the length of the rods. In addition, double-armed dipole antennas with nanometer-sized spaces (nanogaps), such as gold rod dimers, have also been developed due to the strength of local field enhancement factors in their nanogaps. However, one- or two-armed dipole antennas generally only support the dipole resonance mode, which is a fundamental, narrowband mode with a typical bandwidth of around 200 to 500 cm-1. Usually, single-arm or dual-arm high order modes are generally too low in optical spectra. This feature limits the application requiring multiple resonances in the MIR region.

To obtain multiband MIRAs, several micro- and nanostructures beyond one- and two-armed antennas have been designed, among which are gold nanocrosses, nano-aperture structures, fractal microstructures, log trapezoidal structures. periodicals and dipole antennas of several lengths. These structures could be classified into micro- and nanostructures supporting several dipolar modes. Fundamentally, it is a long-term challenge to develop one- or two-armed antennas supporting fundamental and high-order plasmon modes simultaneously pronounced as a quadrupole mode.

The research group of Professor Zhong-Qun Tian of Xiamen University and Professor Huigao Duan of Hunan University designed and fabricated a multi-scale nanobridged rhombic antenna (NBRA, Fig. 1a) that supported two dominant resonances in the MIR (Fig. 1b), including a charge transfer plasmon band (CTP) and a bridged dipolar plasmon (BDP) band that looks like a quadruple resonance. These assignments are evidenced by scattering-type scanning near-field optical microscopy (s-SNOM) imaging and electromagnetic simulations. In comparison to other nanobridged structures, such as nanobridged disks or rectangles, the NBRA shows distinct multiband resonances in the mid-infrared region in the simulated extinction spectra. In addition, the hot spots of the NBRA are located at the ends of the structure, while the hot spots of the bridged nanodisks or PTC resonance rectangles are dispersed in a dispersive manner. The high order band only occurs with a nanoscale (nanobridge) bridge bonded to one end of the rhombic arm which primarily acts as the inductance and resistance by analysis of the RLC circuit. In addition, the main hot spots associated with the two resonant bands are spatially superimposed, making it possible to amplify the local field for the two bands by multiscale coupling. With large field improvements, multiband detection with high sensitivity to a single layer of molecules is achieved when using SEIRA spectroscopy. This work provides a novel strategy to enable high order modes for the design of multiband MIRAs with both nanobridges and nanogaps for MIR applications such as multiband SEIRAs, IR detectors and beam shaping. quantum cascade lasers in the future.

Article reference : You EM, Chen YQ, Yi J, Meng ZD, Chen Q et al. Nanobridged rhombic antennas supporting both dipolar and high-order plasmon modes with spatially superimposed hot spots in the mid-infrared. Opto-Electron Adv 4, 210076 (2021). do I: 10.29026 / oea.2021.210076

Keywords: Optical antenna / charge transfer plasmon / multiband resonances / for near field scanning optical microscopy / surface enhanced infrared spectroscopy

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Huigao Duan is a full professor at the College of Mechanical and Automotive Engineering at Hunan University, Changsha, China. Professor Duan is the founder of the Micro / Nanofabrication and Microsystems Technology Research Center at Hunan University and co-founder of the Advanced Manufacturing Laboratory for Micro / Nano-Optical Devices (Shenzhen Institute of the University of Hunan). His doctoral thesis was selected to be one of the top 100 doctoral theses in 2012. Prof. Duans’ current research interests include 10nm modeling, nanophotonics, intelligent micro / nanosystems and their relevant applications. Professor Duan has published over 180 peer-reviewed journal articles with citations of over 8,300 times and an H index of 46.

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