Plasmonic nanoantennas on VO2 films for active switching of near-field intensity and radiation from nanoemitters.

In this paper, we propose novel plasmonic switches based on plasmonic nanoantennas lying on top of a thin film of a phase change material such as vanadium dioxide (VO2), such that the near-field properties of these nanoantennas can be actively switched by varying the phase of the VO2 film. We employ finite difference time domain (FDTD) simulations to first demonstrate that the near-field intensity in the vicinity of the plasmonic nanoantennas can be substantially switched by changing the phase of the vanadium dioxide film from the semiconductor state to the metallic state. We demonstrate that a ring-bowtie nanoantenna (RBN) switch can switch the near-field intensity by ∼ 59.5 times and ring-rhombus nanoantenna (RRN) switch can switch the near-field intensity by a factor of ∼ 80.8. These values of the maximum intensity switching ratios are substantially higher than those previously reported in the literature. In addition, we optimize the various geometrical parameters of the plasmonic switches to maximize the intensity switching ratio and to understand how the different parameters affect the performance of the plasmonic switches. In this paper, we also show that the intensity of emission from a nanoemitter placed in the gap between the two arms of a plasmonic nanoantenna can be significantly switched by changing the phase of the VO2 film between its semiconductor state and the metallic state. To quantify the switching of emission from the nanoemitters placed in the near-field of the nanoantennas, we define and calculate a parameter, called FESR, the ratio of fluorescent enhancement factors in the on-state and off-state of the plasmonic switch. The maximum fluorescence enhancement switching ratio (FESR) of ∼ 163 is obtained for the RBN switch and FESR of ∼ 200 is obtained for RRN switch. The plasmonic switches being proposed by us can be easily fabricated by employing the conventional nanofabrication and thin film deposition processes.

Steerable plasmonic nanoantennas: active steering of radiation patterns using phase change materials.

In this article, we describe active steering of radiation patterns by complex nanoantenna structures - called steerable nanoantennas (SNs) - formed by combining multiple Yagi-Uda nanoantennas and thin films of a phase change material (VO2). The radiation patterns of these nanoantennas can be actively steered by tunably changing the phase of the VO2 thin films from the semiconductor phase to the metallic phase. Moreover, the nanoantennas enable steering of the radiation patterns 'in the plane' of the nanoantennas. We demonstrate that the radiation pattern's maximum achievable steering is 90° for a two-element steerable nanoantenna when the phase of the VO2 thin film is changed from the semiconductor phase to the metallic phase. Moreover, it was observed that the radiation pattern of the steerable nanoantennas being proposed in our paper can be designed to be much more directed than previously reported steerable nanoantennas. By employing a four element steerable nanoantenna, we also demonstrate a full 360° active steering of the radiation pattern. This steerable nanoantenna consists of four coplanar Yagi-Uda nanoantennas, with each Yagi-Uda nanoantenna being present inside a VO2 thin film but each individually addressable VO2 thin film being separated from the other VO2 films by an air gap. We demonstrate that the radiation pattern can be tunably steered in 12 different directions using this four element steerable nanoantenna depending on the states of the four VO2 thin films. The steerable nanoantennas can find applications in areas such as tunable on chip plasmonic interconnects, networks on chip, or for selective excitation of fluorophores on a sensor chip.

Tunable optical switching in the near-infrared spectral regime by employing plasmonic nanoantennas containing phase change materials.

We propose the design of switchable plasmonic nanoantennas (SPNs) that can be employed for optical switching in the near-infrared regime. The proposed SPNs consist of nanoantenna structures made up of a plasmonic metal (gold) such that these nanoantennas are filled with a switchable material (vanadium dioxide). We compare the results of these SPNs with inverted SPN structures that consist of gold nanoantenna structures surrounded by a layer of vanadium dioxide (VO2) on their outer surface. These nanoantennas demonstrate switching of electric-field intensity enhancement (EFIE) between two states (On and Off states), which can be induced thermally, optically or electrically. The On and Off states of the nanoantennas correspond to the metallic and semiconductor states, respectively of the VO2 film inside or around the nanoantennas, as the VO2 film exhibits phase transition from its semiconductor state to the metallic state upon application of thermal, optical, or electrical energy. We employ finite-difference time-domain (FDTD) simulations to demonstrate switching in the EFIE for four different SPN geometries - nanorod-dipole, bowtie, planar trapezoidal toothed log-periodic, and rod-disk - and compare their near-field distributions for the On and Off states of the SPNs. We also demonstrate that the resonance wavelength of the EFIE spectra gets substantially modified when these SPNs switch between the two states.

Tapered fiber nanoprobes: plasmonic nanopillars on tapered optical fiber tips for large EM enhancement.

Employing finite difference time domain simulations, we demonstrate that electromagnetic field enhancement is substantially greater for tapered optical fibers with plasmonic nanostructures present on their tips as compared with non-tapered optical fibers having those plasmonic nanostructures, or with tapered optical fibers without the plasmonic nanostructures. We also carried out fabrication of plasmonic nanostructures on optical fiber tips.