Multi-wavelength and broadband plasmonic switching with V-shaped plasmonic nanostructures on a VO2coated plasmonic substrate.

In this paper, periodic arrays of identicalV-shaped gold nanostructures and variableV-shaped gold nanostructures are designed on top of a gold-coated silicon dioxide (SiO2) substrate with a thin spacer layer of vanadium dioxide (VO2) to realize multi-wavelength and broadband plasmonic switches, respectively. The periodic array of identicalV-shaped nanostructures (IVNSs) with small inter-particle separation leads to coupled interactions of the elementary plasmons of aV-shaped nanostructure (VNS), resulting in a hybridized plasmon response with two longitudinal plasmonic modes in the reflectance spectra of the proposed switches when the incident light is polarized in thex-direction. Thex-direction is oriented along the axis that joins theV-junctions of all VNSs in one unit cell of the periodic array. On exposure to temperature, electric field, or optical stimulus, the VO2layer transforms from its monoclinic semiconducting state to its rutile metallic state, leading to an overall change in the reflectance spectra obtained from the proposed nanostructures and resulting in an efficient multi-wavelength switching action. Finite difference time domain modelling is employed to demonstrate that an extinction ratio (ER) >12 dB at two wavelengths can be achieved by employing the proposed switches based on periodic arrays of IVNSs. Further, plasmonic switches based on variableV-shaped nanostructures-i.e. multiple VNSs with variable arm lengths in one unit cell of a periodic array-are proposed for broadband switching. In the broadband operation mode, we report an ER >5 dB over an operational wavelength range >1400 nm in the near-IR spectral range spanning over all optical communication bands, i.e. theO, E, S, C, LandUbands. Further, it is also demonstrated that the wavelength of operation for these switches can be tuned by varying the geometrical parameters of the proposed switches. These switches have the potential to be employed in communication networks where ultrasmall and ultrafast switches with multi-wavelength operation or switching over a wide operational bandwidth are inevitably required.

Plasmonic switches based on VO2as the phase change material.

In this paper, a comprehensive review of the recent advancements in the design and development of plasmonic switches based on vanadium dioxide (VO2) is presented. Plasmonic switches are employed in applications such as integrated photonics, plasmonic logic circuits and computing networks for light routing and switching, and are based on the switching of the plasmonic properties under the effect of an external stimulus. In the last few decades, plasmonic switches have seen a significant growth because of their ultra-fast switching speed, wide spectral tunability, ultra-compact size, and low losses. In this review, first, the mechanism of the semiconductor to metal phase transition in VO2is discussed and the reasons for employing VO2over other phase change materials for plasmonic switching are described. Subsequently, an exhaustive review and comparison of the current state-of-the-art plasmonic switches based on VO2proposed in the last decade is carried out. As the phase transition in VO2can be activated by application of temperature, voltage or optical light pulses, this review paper has been categorized into thermally-activated, electrically-activated, and optically-activated plasmonic switches based on VO2operating in the visible, near-infrared, infrared and terahertz frequency regions.

Plasmonic "nano-fingers on nanowires" as SERS substrates.

A surface-enhanced Raman scattering (SERS) substrate based on plasmonics-active metallic nano-finger arrays grown on arrays of triangular-shaped metal-coated silicon nanowire arrays is proposed. Finite-difference time-domain modeling is employed to numerically calculate the effect of the inter-finger gap and the growth angle of the nano-fingers on the magnitude of SERS enhancement and the plasmon resonance wavelength. Additionally, the polarization dependence of the SERS signals from these novel substrates has been studied. A protocol for the fabrication of the proposed SERS substrate is also discussed.

Plane wave scattering from a plasmonic nanowire-film system with the inclusion of non-local effects.

In this paper we present a theoretical analysis of the electromagnetic response of a plasmonic nanowire-film system. The analytical solution accounts for both the dispersive as well as non-local nature of the plasmonic media. The physical structure comprises of a plasmonic nanowire made of a plasmonic metal such as gold or silver placed over a plasmonic film of the same material. Such a nanostructure exhibits a spectrum that is extremely sensitive to various geometric parameters such as spacer thickness and nanowire radius, which makes it favorable for various sensing applications. The non-locality of the plasmonic medium, which can be captured using the hydrodynamic model, significantly affects the resonant wavelength of this system for structures of small dimensions (~ less than 5 nm gap between the nanowire and the film). We present an analytical method that can be used to predict the effect of non-locality on the resonances of the system. To validate the analytical method, we also report a comparison of our analytical solution with a numerical Finite Difference Time Domain analysis (FDTD) of the same structure with the plasmonic medium being treated as local in nature.

VO(2) based waveguide-mode plasmonic nano-gratings for optical switching.

In this paper, we present one dimensional plasmonic narrow groove nano-gratings, covered with a thin film of VO(2) (Vanadium Dioxide), as novel optical switches. These narrow groove gratings couple the incident optical radiation to plasmonic waveguide modes leading to high electromagnetic fields in the gaps between the nano-gratings. Since VO(2) changes from its semiconductor to its metallic phase on heating, on exposure to infra-red light, or on application of voltage, the optical properties of the underlying plasmonic grating also get altered during this phase transition, thereby resulting in significant switchability of the reflectance spectra. Moreover, as the phase transition in VO(2) can occur at femto-second time-scales, the VO(2)-coated plasmonic optical switch described in this paper can potentially be employed for ultrafast optical switching. We aim at maximizing this switchability, i.e., maximizing the differential reflectance (DR) between the two states (metallic and semiconductor) of this VO(2) coated nano-grating. Rigorous Coupled Wave Analysis (RCWA) reveals that the switching wavelengths - i.e., the wavelengths at which the values of the differential reflectance between VO(2) (S) and VO(2) (M) phases are maximum - can be tuned over a large spectral regime by varying the nano-grating parameters such as groove width, depth of the narrow groove, grating width, and thickness of the VO(2) layer. A comparison of the proposed ideal nano-gratings with various types of non-ideal nano-gratings - i.e., nano-gratings with non-parallel sidewalls - has also been carried out. It is found that significant switchability is also present for these non-ideal gratings that are easy to fabricate. Thus, we propose highly switchable and wide-spectra VO(2) based narrow groove nano-gratings that do not have a complex structure and can be easily fabricated.

Hybrid nanoparticle-nanoline plasmonic cavities as SERS substrates with gap-controlled enhancements and resonances.

We present hybrid nanoline-nanoparticle plasmonic substrates which allow easily achievable sub-5 nm gaps and a possibility of large-area fabrication. These substrates--based on plasmonic nanocavities formed by arrays of plasmonic nanoparticle (NP) dimers lying inside periodic metal nanolines (NLs)--can be used as tunable surface enhanced Raman scattering (SERS) substrates due to the tunability of cavity modes in the gap regions. Theoretical studies were conducted, using finite difference time domain (FDTD) modeling, to understand the plasmon resonance tunability as a function of gaps in these hybrid plasmonic substrates. The gaps forming the nanocavities include those between nanolines and nanoparticles (NL-NP) and between two nanoparticles (NP-NP). Our analysis reveals that these gaps play a combined role in tuning the resonance wavelength and the magnitude of electromagnetic field enhancement. Moreover, distinct structure-dependent plasmon resonance peaks are present in addition to material-dependent resonance peaks characteristic to the metal involved. Replacing the spherical particle arrays inside the nanolines with nanorod arrays revealed the possibility of tuning the plasmon resonance in the near-infrared regime. This indicates that there is a possibility of tuning the plasmon resonance wavelength to any region of the visible or near-infrared spectrum by changing the size or shape of the particles assembled inside these plasmonic nanolines.