There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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Computational Methods For Electromagnetics

Ultra-high sensitivity sensor has significant application for micro-nano optical devices in terahertz. Here, we propose a simple graphene metasurface, which can achieve obvious graphene plasmon-induced transparency (PIT) phenomenon. We can find that PIT, reflectivity, and absorbance can be effectively tuned by the Fermi level. Moreover, the finite-different time-domain (FDTD) numerical results are well agreement with the coupled mode theory (CMT) results. Interestingly, an ultra-high sensitivity sensor performance based on tunable PIT in terahertz bands can be realized in our proposed metasurface, the sensitivity and Figure of merit (FOM) can reach up to 1.7745 THz/RIU and 23.61, respectively. Hence, these results can provide theoretical guidance for terahertz dynamic integrated photonic devices.

Graphene-based metasurface sensing applications in terahertz band

In this work, we present an efficient polarization conversion device via using a hollow graphene metasurface. The platform can simultaneously realize a series of excellent performances, including the broadband x-to-y cross polarization conversion (CPC) function with near unity polarization conversion ratio (PCR), dual-frequency linear-to-circular polarization conversion (LTC-PC) function, and highly sensitive polarization conversion function manipulation under wide oblique incidence angle range. For instance, the proposed device obtains an x-to-y CPC function with the bandwidth up to 1.83 THz (χPCR ≥98.8%). Moreover, the x-to-y CPC function can be switched to LTC-PC function via artificially tuning the Fermi energy of graphene. The maximal frequency shift sensitivity (S) of polarization conversion function reaches 23.09 THz/eV, suggesting a frequency shift of 2.309 THz for the LTC-PC function when the chemical potential is changed by 0.1 eV. Based on these superior performances, the polarization converter can hold potential applications in integrated and compact devices, such as polarization sensor, switches and other optical polarization control components.

Multi-functional polarization conversion manipulation via graphene-based metasurface reflectors.

Quad-Band Transmissive Metasurface with Linear to Dual-Circular Polarization Conversion Simultaneously

In this paper, using a graphene-based metasurface, we demonstrate a unique design to develop a highly efficient, broadband, mid-infrared cross-polarization converter. The proposed graphene-based metasurface structure comprises periodical ϕ-shaped graphene on the top surface of a noble-metal-backed dielectric silicon dioxide (SiO2). The reported structure converts the incident linearly polarized wave into cross-polarized components with a peak polarization conversion ratio of more than 0.9 over a large band. Furthermore, the metasurface structure exhibits the full width at half-maximum bandwidth of 41.98% with respect to its center frequency of 5.98 THz. The physical insights behind electromagnetic polarization conversion are supported by field distributions and retrieved electromagnetic parameters. The structure works as a broadband cross-polarization converter up to 40° incident angle for both TE and TM polarizations. In addition, the structure is found to be as thin as ∼λ/6 with respect to lowermost frequency of the polarization conversion. The period of the unit cell is ∼λ/24 to support the fact that the structure can be treated as a metasurface.

Graphene-based metasurface for a tunable broadband terahertz cross-polarization converter over a wide angle of incidence.

This paper proposes a tunable graphene-based wideband metasurface for high absorption in the lower mid-infrared frequency region. The unit cell of the proposed structure consists of slot generated out of modified fractal-shaped graphene layer deposited on top of amorphous silicon dioxide (SiO2) grown over 0.1-μm-thick gold layer. The structure exhibits distinctive high absorption greater than 90% absorptivity over the range of 3.69–9.77 THz covering a wideband of 6.08 THz. Incorporation of graphene in the metastructure can tune the absorption bandwidth owing to conductivity modulation of graphene by the control of impurity doping and mechanical stretching. Besides, the absorber is polarization independent under normal incidence because of the fourfold symmetry of the deposited graphene pattern. The designed prototype has been studied under oblique incidences for both transverse electric and transverse magnetic polarizations where wideband absorption has been experienced up to 40° incident angles for both cases. The prototype is found to be as thin as ∼ λ /11 with respect to the lowermost frequency of the absorption. The period of the unit cell is ∼ λ /13 supporting the effective homogeneity condition in subwavelength region. The structure can have applications toward sensing, spectroscopy, etc.

Tunable Graphene-Based Metasurface for Polarization-Independent Broadband Absorption in Lower Mid-Infrared (MIR) Range

In this study, the authors have designed and proposed a monolayer graphene-based metasurface, performing as a broadband polarisation converter over a wide angle of incidence. The unit cell of the metasurface consists of split ring generated over a perforated single layer of graphene grown over metal-backed silicon dioxide. The structure converts incident linearly polarised wave into its cross-polarised component over a wide bandwidth with peak polarisation conversion exhibits value to unity. The plane of polarisation of the incident electromagnetic wave is rotated orthogonally on reflection from the proposed metasurface over full width at half maxima bandwidth around 72.83% compared with centre frequency of 2.16 THz, which can be maintained up to 40° incident angles under both transverse electric and transverse magnetic polarisations. The proposed structure is as thin as ~
 λ
/7.3 with respect to lowermost frequency and the period of the structure is about λ
/10.7. The frequency band of polarisation conversion can be tuned by altering chemical doping as well as the electrostatic grating of the graphene sheet. The structure can find applications toward sensing, spectroscopy etc.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-map.2018.5373

Sambit Kumar Ghosh

Papers

Graphene-based metasurface for a tunable broadband terahertz cross-polarization converter over a wide angle of incidence.

Tunable Graphene-Based Metasurface for Polarization-Independent Broadband Absorption in Lower Mid-Infrared (MIR) Range

Wideband tunable mid-infrared cross-polarisation converter using monolayered graphene-based metasurface over a wide angle of incidence

Transmittive-type triple-band linear to circular polarization conversion in THz region using graphene-based metasurface

Graphene based metasurface with near unity broadband absorption in the terahertz gap