Debabrata Sikdar's research while affiliated with Indian Institute of Technology Guwahati and other places

Metamaterials opened a new realm to control light–matter interactions at sub-wavelength scale by engineering meta-atoms. Recently, the integration of several emerging nonlinear materials with metamaterial structures enables ultra-fast all-optical switching at the nanoscale and thus brings enormous possibilities to realize next-generation optical communication systems. This Letter presents a novel, to the best of our knowledge, design of plasmonic metamaterials for high-contrast femtosecond all-optical switching. We leverage magnetic plasmon (MP) resonance combined with the nonlinear effects of an epsilon-near-zero (ENZ)-material. The proposed design comprises a periodic array of two closely spaced Au-nanogratings deposited on an optically thick Au-substrate to excite MP-resonance. To enable a dynamically tunable resonance, the nanogrooves in meta-atoms are filled with an ENZ-material, cadmium-oxide (CdO). The intraband transition-induced optical nonlinearities in the ENZ-medium are studied using a two-temperature model. The MP-resonance ensures strong light–matter interactions enabling enhancement of the nonlinearities of the proposed structure. We observe that the pump-induced refractive index change in the CdO layer causes a redshift of the MP-resonance dip wavelength in the reflectance spectrum, leading to a high modulation depth of 0.83 at 1.55 µm. With an ultra-fast response time of 776 fs while maintaining a low pump-fluence of 75 µJ/cm², the proposed metamaterial could help in realizing switches for next-generation optical computation systems.

Recently, there has been a great deal of interest in devices which effectively shield near-infrared light with an additional feature of external field tunability, particularly for energy-saving applications. This article...

Magnetic resonance imaging (MRI) is one of the prominent diagnostic tools which uses non-invasive modalities for clinical imaging of human body parts. Signal-to-noise ratio (SNR), the key figure of merit that defines the quality of any MRI scan, can be boosted by increasing either the applied static magnetic field (B0)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$({B}_{0})$$\end{document} from the scanner’s electromagnet or the oscillating radiofrequency (RF) magnetic field (B1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${B}_{1}$$\end{document}) from the transceiver coil. However, higher RF field intensity inside scanners could bring adverse effects like inhomogeneity of transmitted RF magnetic field, increase in tissue heating (characterized as specific absorption rate (SAR): 3.2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$3.2$$\end{document}W/kg—safety limit) due to stronger RF electric field hotspots and, thus, raising potential safety concerns for patients. Metasurfaces are artificial structures that can enhance magnetic fields in their near-field region, thus find applications in boosting the SNR of MRI without stepping up B0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${B}_{0}$$\end{document}. However, most of the reported metasurfaces for 1.5\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1.5$$\end{document}T MRI are bulky and cannot conform to human body parts with different anatomies. Here, we propose a metasurface-inspired flexible structure that can be wrapped on patients’ body parts with different curvatures for boosting the SNR of 1.5\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1.5$$\end{document}T MRI scans. An equivalent circuit model, formulated for elucidating electromagnetic behavior of the proposed metasurface-inspired wrap, has validated the reflection characteristics obtained from full-wave simulations. The proposed design is investigated in flattened and different wrapped conditions on the phantoms/bio-models mimicking human properties for estimating the enhancement in received magnetic field (B1-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${B}_{1}^{-}$$\end{document}) and SNR at 1.5\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1.5$$\end{document}T MRI. A boost of ∼8\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim8$$\end{document} times in B1-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${B}_{1}^{-}$$\end{document} as well as SNR enhancement factor is observed on the surface of metasurface-wrapped bio-model under excitation of transceiver birdcage coil while maintaining SAR well under the safety limit. Numerical analyses for the conformed shapes of metasurface show that the proposed wrap could be used as a “wearable add-on” inside 1.5\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1.5$$\end{document}T MRI transceiver arrays for significant SNR enhancement in scans of different body parts, such as head, legs, etc.

Tunable metasurfaces provide enormous degrees-of-freedom for dynamic control of light–matter interactions at sub-wavelength scales. However, the realization of polarization- and angle-insensitive active nanophotonic devices with low energy-consumption and high modulation depth remains a challenge. This letter reports gap plasmon resonance based electro-tunable metasurface for optical intensity modulation. The proposed metasurface features a 2D array of Au-nanograting on top of indium-tin-oxide (ITO)–Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> –Au stack. The external electrical biasing accumulates the free-carriers of ITO at ITO–Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> interface, which results in an electro-tunable resonance response in the reflectance spectrum. With a 2V electrical biasing, we observe the resonance dip shifts to the vicinity of 1.55 μm wavelength. Thus, a modulation depth of ~25 dB at 1.55 μm can be achieved for random polarization-angle of incident light. For the first time, the proposed modulator can operate over a wide range of incidence angle (up to 50 degrees) for both x- and y-polarized light. With ~303 fJ/bit energy-consumption and ~900 Mbps modulation speed, the proposed electro-tunable metasurface could help to realize polarization- and angle-independent active flat optics for nanophotonic systems.

Control of polarization states of light is crucial for any photonic system. However, conventional polarization-controlling elements are typically static and bulky. Metasurfaces open a new paradigm to realize flat optical components by engineering meta-atoms at sub-wavelength scale. Tunable metasurfaces can provide enormous degrees-of-freedom to tailor electromagnetic properties of light and thus have the potential to realize dynamic polarization control in nanoscale. In this study, we propose a novel electro-tunable metasurface to enable dynamic control of polarization states of reflected light. The proposed metasurface comprises a two-dimensional array of elliptical Ag-nanopillars deposited on ITO (indium-tin-oxide)–Al 2 O 3 –Ag stack. In unbiased condition, excitation of gap plasmon resonance- in the metasurface leads to rotation of x-polarized incident light to orthogonally polarized reflected light (i.e. ypolarized) at 1.55 μm. On the other hand, by applying bias-voltage, we can alter the amplitude and phase of the electric field components of the reflected light. With 2V applied bias, we achieved a linearly polarized reflected light with a polarization angle of –45 ⁰ . Furthermore, we can tune the epsilon-near-zero (ENZ) wavelength of ITO at the vicinity of 1.55 μm wavelength by increasing the bias to 5 V, which reduces y-component of the electric field to a negligible amplitude, thus, resuting in an x-polarized reflected light. Thus, with an x-polarized incident wave, we can dynamically switch among the three linear polarization states of the reflected wave, allowing a tri-state polarization switching (viz. y-polarization at 0 V, – 45 ⁰ linear polarization at 2 V, and x-polarization at 5 V). The Stokes parameters are also calculated to show a real-time control over light polarization. Thus, the proposed device paves the way towards realization of dynamic polarization switching in nanophotonic applications.

This paper presents a metasurface based reflection mode band-reject filter for future 6G applications. The unit cell of the metasurface is a sub-wavelength modified cross-slot structure with a square patch at the center placed over a Rogers substrate. Numerical simulation shows a narrow band-reject response having a center frequency of 285 GHz with a 3-dB bandwidth of 18.5 GHz. As the metasurface is rotationally symmetric, it also exhibits polarization-independent characteristics. A parametric study is also conducted to provide suitable guidance for designing such metasurface based reflection mode band-reject filters for any Terahertz frequency in the 6G band.