Plasmonic metamaterial based structures for designing of multiband and thermally tunable light absorbers, multiple thermal infrared emitter, and high-contrast asymmetric transmission optical diode
Metamaterials with sub-wavelength nanostructures refer to a class of synthetic materials that possess exotic electromagnetic properties which cannot be observed with natural materials. Negative refractive index, asymmetric light transmission, invisible cloaking, and lasing are examples of these attributes. Among these possible applications, the concept of light confinement and harvesting by subwavelength structures have attracted considerable attention due to their widespread applications ranging from thermal emission, optical modulator, sensing, and photodetectors. Here, we propose and design plasmonic metamaterials with subwavelength structures in four different application areas, namely 1) Hybrid indium tin oxide-Au metamaterial absorber in the visible and near-infrared ranges for selective thermal emissions and sensors. 2) Active tuning from narrowband to broadband absorbers using a sub-wavelength VO2 for optical modulator. 3) A spectrally selective nanoantenna emitter compatible with multiple thermal infrared applications. 4) Diode like high-contrast asymmetric transmission of linearly polarized waves. In the first work, we propose a multi-band Metamaterial Perfect absorber (MPA) with two narrowband absorption responses that are centered on the visible and near-infrared (NIR) wavelengths [773 and 900 , respectively] and a broadband absorptive characteristic in another window in the NIR region [ranging from 1,530 to 2,700 with a bandwidth of 1,170 ]. The MPA comprises a periodic array of self-aligned hybrid indium tin oxide (ITO)-Au split-ring-resonators that are separated from an optically thick bottom reflector with a SiO2 layer. Based on numerical calculations, which are accompanied with a semi-analytical examination, we find that the dual narrowband and broadband responses are attributed to the hybridization of the optical responses of gold as a plasmonic material with the ones of ITO. Note that ITO acts as a low-loss dielectric in the visible range and a lossy plasmonic material in the NIR region. Moreover, due to the applied symmetry in the unit cell of the metamaterial, the proposed MPA represents polarization insensitive and omnidirectional absorptive features. The proposed metastructure can find potential applications in selective thermophotovoltaic devices, thermal emitters, and sensors. In the second work, we propose an MPA with diverse functionalities enabled by vanadium dioxide (VO2) embedded in a metal-dielectric plasmonic structure. For the initial design purpose, a Silicon (Si) nanograting on a Silver (Ag) mirror is proposed to have multiple resonant responses in the near infrared (NIR) region. Then, the insertion of a thin VO2 layer at the right position enables the design to act as an on/off switch and resonance tuner. In the insulator phase of VO2, in which the permittivity data of VO2 is similar to that of Si, a double strong resonant behavior is achieved within the NIR region. By increasing the temperature, the state of VO2 transforms from insulator to metallic so that the absorption bands turn into three distinct resonant peaks with close spectral positions. Upon this transformation, a new resonance emerges and the existing resonance features experience blue/red shifts in the spectral domain. The superposition of these peaks makes the overall absorption bandwidth broad. Although Si has a small thermo-optic coefficient, owing to strong light confinement in the ultrasmall gaps, a substantial tuning can be achieved within the Si nanogratings. Therefore, the proposed hybrid design can provide multi-resonance tunable features to cover a broad range and can be a promising strategy for the design of linearly thermal-tunable and broadband MPAs. Owing to the proposed double tuning feature, the resonance wavelengths exhibits great sensitivity to temperature, covering a broad wavelength range. Overall, the proposed design strategy demonstrates diverse functionalities enabled by the integration of a thin VO2 layer with plasmonic absorbers. In the third work, we design a wavelength-selective nanoantenna emitter based on the excitation of gap-surface plasmon modes using a metal–insulator–metal configuration (silicon dioxide (SiO2) sandwiched between silver (Ag) layers) for satisfying multiple infrared applications. The proposed design, which is called design I, realizes triple narrowband perfect absorptions at the resonance wavelengths of 1524 nm, 2279 nm, and 6000 nm, which perfectly match the atmospheric absorption bands while maintaining relatively low emissivity in the atmospheric transparency windows of 3 − 5 m and 8 − 12 m. Later, the functionality of design I is extended, which is called design II, to include a broadband absorption at the near-infrared region to minimize the solar irradiation reflection from the nanoantenna emitter. Finally, single- and three-layer graphene are introduced to provide a real-time tuning of the infrared signature of the proposed nanoantenna emitter (design II). It is also demonstrated that the three-layer graphene structure can suppress an undesired absorption resonance wavelength related to the intrinsic vibrational modes (optical phonons) of the SiO2 layer by 53.19% compared to 25.53% for the single-layer one. The spectral analysis of design I is validated using both analytical and numerical approaches where the numerical simulation domain is extended for the analysis of design II. The thermal characteristic analyses of design I and design II (without/with graphene layers) reveal that infrared signatures of the blackbody radiation are significantly reduced for the whole wavelength spectrum at least by 96% and 91% within a wide temperature ranging from room temperature to 500 , respectively. In the fourth and final work, we present a narrow-band optical diode with a high-contrast forward-to-backward ratio at the near-infrared (NIR) region. The design has a forward transmission of approximately 88%, and a backward one of less than 3%, yielding a contrast ratio of greater than 14.5 dB at a wavelength of 1550 nm. The structure is composed of a one-dimensional diffraction grating on top of a dielectric slab waveguide, both of which are made of silicon nitride (Si3N4), and all together are placed over a silver (Ag) thin film embedded on a dielectric substrate. Utilizing a dielectric-based diffraction grating waveguide on a thin silver layer leads to the simultaneous excitation of two surface plasmon modes known as long- and short-range surface plasmon polaritons (SPPs) at both interfaces of the metallic layer. The plasmon-tunneling effect, which is the result of the coupling of SPPs excited at the upper interface of the metallic layer to the radiation modes, provides a high asymmetric transmission (AT) property. The spectral response of the proposed high-contrast AT device is verified using both rigorous coupled-wave analysis as an analytical approach and finite difference time domain as a numerical one.