A toroidal dipole in metasurfaces provides an alternate approach for the excitation of high-Q resonances. In contrast to conventional multipoles, the toroidal dipole interaction strength depends on the time derivative of the surrounding electric field. A characteristic feature of toroidal dipoles is tightly confined loops of oscillating magnetic field that curl around the fictitious arrow of the toroidal dipole vector.

In recent years, the stunning performance of transition metal dichalcogenides (TMDCs) has been utilized in the area of field effect transistors, integrated circuits, photodetectors, light generation and harvesting, valleytronics, and van der Waals (vdW) heterostructures. However, the optoelectronic application of TMDCs in realizing efficient, ultrafast metaphotonic devices in the terahertz part of the electromagnetic spectrum has remained unexplored. The most studied member of the TMDC family, i.e., MoS2, shows an ultrafast carrier relaxation after photoexcitation with near‐infrared femtosecond pulse of energy above the bandgap. Here, this study investigates the photoactive properties of MoS2 to demonstrate an ultrasensitive active switching and modulation of the sharp Fano resonances in MoS2‐coated metamaterials consisting of asymmetric split ring resonator arrays. The results show that all‐optical switching and modulation of micrometer scale subwavelength Fano resonators can be achieved on a timescale of hundred picoseconds at moderate excitation pump fluences. The precise and active control of the MoS2‐based hybrid metaphotonic devices open up opportunities for the real‐world technologies and realization of ultrafast switchable sensors, modulators, filters, and nonlinear devices.

Simple designs for polarization independent, metamaterial absorbers at mid-infrared wavelengths and over wide angle of incidence are evaluated computationally. One design consists of an array of circular metallic disks separated from a continuous metallic film by a dielectric film, and shows over 99.9% peak absorbance and a resonant bandwidth of about 0.2 μm wavelengths. The effects of various geometric parameters are analyzed for this metamaterial. Another design consisting of an array of stacked metal-dielectric-metal disks is shown to have an absorbance of over 90% in a comparatively large band of over 1 μm bandwidth, although with a lower peak absorbance of 97%.

Simple periodic structures of stacked metal and dielectric microdisks can display very high absorbance over multiple bands at infrared frequencies (3–10 μm wavelengths). The stack can be envisaged as intersecting tri-layers, each tri-layer composed of metal–dielectric–metal disks that form independent impedance matched resonators, and give rise to large absorbance at different frequencies. Numerical simulations show that dual-band and multi-band absorbers with near unity absorbance on all their bands can be flexibly designed whereby the dielectric materials determine the absorption band of the metamaterial. The multi-band absorber is reasonably polarization insensitive and the absorbance remains large even with large angles of incidence. This approach of multi-layered stacked metamaterials is compared and shown to be superior to another approach to multiple-band metamaterial perfect absorbers having closely packed resonators within a unit cell.

chemical composition and structure, e.g., OH, NH, CO, CC, etc., by measuring the absorption in the infrared spectral region. The experimentally obtained infrared spectra are then compared with widely established databases that enable a rapid identification of materials which is a common procedure in analytical chemistry, biology and medicine. Most organic functional group absorption occurs in the mid-IR range, between 2.5 and 25 µm (4000 and 400 cm −1 ), respectively. The absorption of infrared light can be characterized by the Bouger Lambert-Beer law. According to Bouger Lambert-Beer law, the amount of light absorbed is proportional to the extinction coefficient (α),which depends on the nature and state of the substance, the wavelength (λ) of the radiation, and the absorption path length "d," and is mathematically expressed as I(d) =I 0 e −α( λ )d , where I 0 is the intensity of the beam entering a layer of matter. From this expression, it can explicitly be seen that the absorption decreases exponentially with decreasing thickness of the material and thus it becomes extremely challenging to detect/sense the absorption of the material which is only a few layers thick. In the extreme case of a monolayer, the absorption signal becomes prohibitively weak.In 1980, Hartstein et al. reported the first intensification of infrared-active vibrational modes of organic thin film on a silicon substrate by evaporating a thin metal film (Ag and Au) on to the organic film. [2] The phenomenon of surfaceenhanced infrared absorption (SEIRA) is explained as the coupling between the vibrational modes of the molecules and the enhanced optical fields near the surface of the particles under the assumption that the film is not continuous but consists of metal islands smaller than the wavelength of light. [3][4][5][6][7] Thin metal films consisting of metal islands smaller than the wavelength of light exhibit plasmonic resonances because of the coherent oscillation of free electrons in the metal islands. This resonant excitation and the related near-field enhancement is known as localized surface plasmon resonance. The field enhancement depends largely on the morphology of the metal surface. For metal films consisting of randomly distributed metal islands, the overall response of individual metal island resonators with various sizes results in a broadband resonance with a poor quality factor (Q). Realizing strong plasmon-vibration interactions between infrared-active vibrational bands and resonating plasmonic metasurfaces opens up the possibilities for ultrasensitive label-free detection of chemical and biological agents.The key prerequisites for exploiting strong plasmon-vibration interactions in practical spectroscopy are structures, which provide giant field enhancement that highly depends on the line-width and line-shape of the plasmonic resonances supported by these structures. Here, multiband surface-enhanced infrared absorption (SEIRA) of poly(methyl methacrylate) (PMMA) is demonstrated. The line-width and line-shape of the prop...