Electrical and Electronic Engineering - Theses

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Start date: 2020 × End date: 2021 × Subject: Graphene ×

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    Nano-Optical Photodetectors Based on Two-Dimensional Materials
    Sefidmooye Azar, Nima ( 2021)
    The discovery of graphene in 2004 opened the door to the wonderful world of two-dimensional (2D) layered materials, and the properties and applications of these materials have been hot research topics ever since. The atomic-level thinness and layered structure of 2D materials give rise to extraordinary properties and enable novel functionalities, and they have exhibited great potential in various fields including electronics and optoelectronics. They are particularly promising for photodetection, and detectors from ultraviolet to terahertz wavelengths have been demonstrated based on these materials. However, low light absorption in 2D materials, which originates from their thin structure, has hindered their widespread application in photodetection. In this thesis, we demonstrate optical nanostructures that can significantly boost the interaction of light with 2D materials and thus improve their photodetection performance. Our focus is on infrared (IR) photodetectors which have applications in a wide range of areas that include biomedical and thermal imaging, telecommunication, spectroscopy, and many other modern technologies. First, we present a hybrid plasmonic structure for enhancing the light absorption in graphene in the long-wave IR (LWIR) spectral region. This structure, consisting of a metallic bull's eye grating and optical nanoantennas, employs surface plasmon polaritons and localized surface plasmons to concentrate light into a monolayer graphene flake with sub-wavelength lateral extent. Optical simulations show that this plasmonic structure provides a 558-fold light absorption enhancement in graphene and a 32-fold enhancement in the detectivity of the LWIR photodetector. It is also found that integrating this structure with an optical cavity substrate further boosts the device performance. Black phosphorus (bP), another 2D layered material with a narrow and direct bandgap of 0.31 eV, has great potential for IR optoelectronics. Nevertheless, the performance of bP-based photodetectors is limited by weak light absorption in bP, resulting from its thinness and optical anisotropy. In the next work, via optical simulations, we demonstrate hybrid plasmonic nanoantenna/optical cavity structures that boost the IR light absorption in multilayer bP through polarization conversion and light intensity enhancement. In a reciprocal manner, these nanostructures enhance the spontaneous emission from bP. Light absorption and emission enhancements of up to 185-fold and 18-fold, respectively, are achieved. Detectivity and electroluminescence efficiency of 2D material-based photodetectors and light-emitting diodes can be significantly enhanced employing these optical nanostructures. Recently, platinum diselenide (PtSe2), a 2D noble-transition-metal dichalcogenide, has also been investigated for IR detection. However, wavelengths up to the short-wave infrared region have been the main focus of these studies. In the last work, we present LWIR photodetectors based on multilayer PtSe2. We utilise a TiO2/Au optical cavity substrate for enhancing the LWIR light absorption in PtSe2. Responsivity values of up to 54 mA/W are obtained at 8.35 um. In addition, these devices show a fast photoresponse with a time constant of 54 ns to white light illumination. This study reveals the potential of multilayer PtSe2 for fast and broadband photodetection from visible to LWIR wavelengths. It also highlights the key role of the substrate in the performance of 2D material-based IR photodetectors.
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    Magnetic mirrors and plasmonic metasurfaces for mid-infrared graphene photodetectors and biosensors
    Ye, Ming ( 2020)
    Graphene is the name given to a monolayer of carbon atoms arranged in a two-dimensional honeycomb lattice. Recently, there has been much interest concerning the use of graphene in photodetectors and biosensors due to its unique electronic and optical properties. Specifically, graphene is an attractive material for developing broadband and high-speed photodetectors because of its gapless band structure and ultrafast carrier dynamics. The high spatial confinement and electrical tunability of mid-infrared (MIR) graphene plasmon have also been used for biosensors which permit the quantification and identification of biomolecule monolayers. However, the realisation of high-performance graphene photodetectors operating in the MIR is hindered by the intrinsically low optical absorption (< 2.3 %) and short carrier lifetime (sub-picosecond) of this material. In addition, the sensitivity of graphene biosensors based on plasmons is limited by the relatively small field enhancement of graphene plasmons compared to that of conventional metal plasmons. In this thesis, we present nano-optical approaches to enhance the performance of graphene-based photodetectors and biosensors operating in the MIR by employing magnetic mirrors and/or plasmonic metasurfaces. First, we propose and experimentally demonstrate a long-wave infrared device that we termed a magnetic mirror, which consists of an array of amorphous silicon cuboids on a gold film. The device is demonstrated to reflect light with high reflectance and zero phase shift. A modified multipole analysis method is devised and employed to interpret the magnetic mirror behaviour. We investigate the use of this device in a graphene photodetector application and show that the light absorption by graphene placed on top can be boosted by more than three orders of magnitude compared to the absorption that would occur were the graphene instead placed on a gold mirror. This is achieved by producing a field distribution with enhanced intensity at the device surface. Second, we design and experimentally demonstrate a mid-wave infrared polarization-independent graphene photodetector via the integration of plasmonic nanoantennas that we term Jerusalem-cross antennas (JC-antennas). The JC-antennas serve to concentrate the incident light onto graphene for strongly enhanced optical absorption, as well as to collect the photocarriers. We demonstrate mid-wave infrared detection both at room temperature and at cryogenic temperatures. Our device also shows a fast and broadband photoresponse that extends to visible and near-infrared wavelengths, thanks to the carrier collection by the JC-antennas. Last, we propose and investigate a biosensor device that combines the strong field confinement and electrical tunability of graphene plasmons with the large field enhancement of metallic nanoantennas. The device consists of an array of plasmonic nanoantennas and graphene nanoslits on a resonant substrate. Systematic electromagnetic simulations are performed to quantify the sensing performance of the proposed device. Our simulations show that the proposed device outperforms designs in which only plasmons from metallic nanoantennas or plasmons from graphene are utilized.