No abstract

We present a novel optical switch that consists of a 1D array of plasmonic (Au) nanowires present on a vanadium dioxide ( VO 2 ) thin film layer, which is further present on top of an underlying plasmonic (Au) film. The optical switching action in this nanostructure arises from the semiconductor-to-metal transition of the VO 2 spacer layer, i.e., upon phase transition of VO 2 from its semiconductor state to its metallic state, which can be induced by the application of heat, infrared light, or voltage. The phase transition of the VO 2 layer results in a change in the reflectance signal from the Au nanowire- VO 2 spacer-Au film nanostructure. The differential reflectance signal, i.e., difference in the reflection spectra from this nanostructure when the VO 2 spacer layer is present in its metallic and the semiconductor states, was calculated using RCWA simulations and employed as a measure of switchability. We demonstrate that switchability, as well as the wavelength at which the maximum differential reflectance is observed (i.e., the optimal switching wavelength), can be tuned over a wide spectral regime by changing the structural parameters of these optical switches. Ultrafast switching can be achieved using these optical switches, as the phase transition in the VO 2 film spacer occurs at femto-second time scales. These optical switches can be fabricated using currently available nanofabrication capabilities.

We report a large chiro-optical response from a nanostructured film of aperiodic dielectric helices decorated with ellipsoidal metal nanoparticles. The influence of the inherent fabrication variation on the chiro-optical response of the wafer-scalable nanostructured film is investigated using a computational model which closely mimics the material system. From the computational approach, we found that the chiro-optical signal is strongly dependent on the ellipticities of the metal nanoparticles and the developed computational model can account for all the variations caused by the fabrication process. We report the experimentally realized dissymmetry factor ∼1.6, which is the largest reported for wafer scalable chiro-plasmonic samples till now. The calculations incorporate strong multipolar contributions of the plasmonic interactions to the chiro-optical response from the tightly confined ellipsoidal nanoparticles, improving upon the previous studies carried in the coupled dipole approximation regime. Our analyzes confirm the large chiro-optical response in these films developed by a scalable and simple fabrication technique, indicating their applicability pertaining to manipulation of optical polarization, enantiomer selective identification and enhanced sensing and detection of chiral molecules.

A surface plasmon resonance sensing and imaging platform based on plasmonic non-uniform nano-gratings with narrow groove (sub-10 nm) is presented. In these nanogratings, normally incident optical radiation is directly coupled to surface plasmons without the requirements of any other conventional surface plasmon coupling mechanisms such as prism-based or grating-based coupling. Theoretical analysis of practically realizable plasmonic non-uniform nano-gratings with rounded tops and slanted sidewalls is carried out to numerically to determine reflectance and differential reflectance signals when the localized refractive index of the medium around the gold layer present in these nano-gratings is changed. This change in the localized refractive index can occur due to the binding of biomolecules to the gold layer. Two kinds of plasmonic non-uniform nano-gratings are studied using finite difference time domain (FDTD) modelling: gold nano-gratings (GNGs) and gold-coated silicon nano-gratings (GSNGs). The plasmonic non-uniform nano-gratings being proposed, more specifically the GSNGs, can be easily fabricated with the presently existing nanofabrication and thin film deposition methods as opposed to uniform nano-gratings (with parallel sidewalls) that are very difficult to fabricate. The plasmonic non-uniform nano-gratings with narrow grooves eliminate the strict requirements on the angle of incidence for coupling of light into surface plasmons, which are needed in conventional prism-based coupling mechanisms. By employing FDTD calculations, we demonstrate that these plasmonic non-uniform nano-gratings provide very high differential reflectance amplitude values, which are indicative of high sensitivities of the SPR or SPRi sensors when the localized refractive index around the sensors is varied. Moreover, the sensors being proposed in this paper provide a maximum sensitivity of localized refractive index sensing (i.e. surface sensitivity or SS) of 70 nm/nm with a figure of merit of the localized sensor (FOMS) of 1.5 nm -1 . This sensitivity of localized refractive index sensing is the highest reported thus far in comparison with previously reported plasmonic sensors. Moreover, these plasmonic non-uniform nano-grating based sensors exhibit significantly better performance when compared with conventional SPR or SPRi sensors based on the Kretschmann configuration.

We propose a simple structure of a metamaterial (MM) to achieve near-perfect narrowband absorption. This MM utilizes both a plasmonic antenna and its complimentary structure for trapping electromagnetic fields, without the use of a complete ground plate compared to conventional designs. The simple design opens a new possibility of engineering perfect absorbers that can be easily fabricated, and thus many potential applications in mid-infrared include thermal imaging, energy harvesting, localized biological sensing, optical tweezers, and laser beam shaping.