Hyperbolic metamaterial (HMM) has paved the way for sub-diffraction focusing inside the HMM due to the propagation of large momentum wave vectors in the HMM. However, these high momentum K modes exponentially decay outside the HMM which results in decaying of the focusing resolution in the near field of the HMM. In this work, we introduce both a HMM and a hypergrating structures for sub-wavelength focusing in air. Hypergrating is a structure that combines a HMM with a grating surface. The proposed structure consists of upper metallic slit integrated on HMM based multilayer of doped/intrinsic InAs with lower intrinsic InAs grating surface. HMM based multilayer of doped/intrinsic InAs has the advantage of tuning the focusing wavelength in the mid-IR range. The proposed structure has reported sub-wavelength focusing in air with value reaching 0.08 λ. Hypergrating structure shows focusing resolution enhancement of 0.08λ as compared to 0.15λ for a HMM without lower grating, both at wavelength of 7.3μm. The focusing resolution outside the hypergrating structure is much higher than that is observed in the HMM only due to the introduced lower grating. This structure demonstrates a good candidate for sub -wavelength IR imaging application in air.
In the realm of intensive research on metamaterials, particularly, the two-dimensional analogue, known as metasurfaces have attracted researchers due to their lower losses, high efficiencies and low cost as compared to plasmonic metasurfaces. Dielectric metasurfaces (DMs) have been widely reported to experience magnetic and electric dipole Mie type resonances, in which, upon tuning these two resonances, dielectric metasurfaces can exhibit spatially varying optical responses, phases and polarizations of scattered fields. Recently, dielectric metasurfaces have been used for color printing application with very high color vibrancy. However, the fundamental building blocks essential for the realization of optical metasurfaces are designed with uniform dimension nano structures, resonating at particular wave length, thus printing image only with particular color. In order to be able to cover the whole optical regime, the metasurface needs to be designed with tunable optical response to be able to print images with multiple colors. In this work, we report a cubic TiO<sub>2</sub> metasurface which experience magnetic and electric dipole resonances in the optical regime. We are able to tune the reflection peak of both resonances using Nematic liquid crystal (LCs). LCs are anisotropic materials with controlled orientation based upon different applied voltages. Changing the orientation of the LC allows for tuning the resultant of the electric field component of the LC and thus the reflection peak of the metasurface can be tuned across the optical regime. We report a tunable DM for optical filters application using single dimension designed metausrfcae with efficiency close to 99 % covering three colors in the visible range: red, orange and green.
Energy conservation techniques have been widely explored in recent years for several applications: IR camouflage, solar absorbers and for IR thermal harvesting as well. While many absorbers have been demonstrated using plasmonic metal nanoparticles, surface texturing and low density broad band absorbers, they still encounter inevitable drawbacks. The state of art absorbers are either suffering instability over time or bulkiness which limit their practical application. Metamaterials have provided a significant improvement overcoming the aforementioned challenges through introducing ultra-broad band absorbers. However, the urge for CMOS compatible sub-wave length absorber that can be integrated for opto/electronic devices is still a major challenge. We demonstrate a mid IR silicon absorber using doped Silicon/Silicon Hyperbolic Metamaterial (HMM) integrated with sub-wave length Si grating. HMMs are characterized by their hyperboloid dispersion momentum space that provides large density of photonic states. By applying sub-wavelength grating on HMM, light from free space can be coupled to high propagation wave vectors of the hyperbolic modes upon breaking the momentum mismatch restriction, leading to noticeable absorption. We are able to show that an all Si based designed HMM is capable to achieve absorption across the mid IR wavelength range reaching absorption <i>(A)</i> of value 0.9.This proposed CMOS compatible Si-based absorber serves as good candidate for IR thermal harvesting application for on chips purposes.