Plasmonics: Manipulating Light at the Subwavelength Scale

Subwavelength conductor-gap-silicon plasmonic waveguides along with compact S-bends and Y-splitters were theoretically investigated and experimentally demonstrated on a silicon-on-insulator platform. A thin SiO2 gap between the conductor layer and silicon core provides subwavelength confinement of light while a long propagation length of 40 microm was achieved. Coupling of light between the plasmonic and conventional silicon photonic waveguides was also demonstrated with a high efficiency of 80%. The compact sizes, low loss operation, efficient input/output coupling, combined with a CMOS-compatible fabrication process, make these conductor-gap-silicon plasmonic devices a promising platform for realizing densely-integrated plasmonic circuits.

Buttcoupling is the most efficient way to excite surface plasmon polariton (SPP) waves at dielectric/metal interfaces in order to realize applications in broadband and ultra-compact integrated circuits (IOCs). We propose a reasonable waveguide structure to efficiently excite and collect the SPP waves supported in a plasmonic trench waveguide in the long-haul telecommunication wavelength range. Our simulation results show that the coupling efficiency between the dielectric strip waveguides and a plasmonic trench waveguide can be optimized, which is dominated by the zigzag propagation path length in the dielectric strip loaded on the metal substrate. It is noted that nearly a 100% coupling efficiency can be achieved when the distance between the excitation source and the plasmonic waveguide is about 6.76 μm.

Multimode interference effect in plasmonic subwavelength waveguides and an ultra-compact power splitter

Cotton fabrics were dyed with three commercial vat dyes in order to provide camouflage in Vis-NIR regions and imitate reflectance profile of greenish leaves. To investigate the effect of nano particles on camouflage properties of dyed fabric, nano particles of TiO2 were applied on dyed fabrics using pad-dry-cure method. The nano TiO2 padded dyed fabrics were investigated using scanning electron microscopy (SEM). Reflectance curves of coated dyed samples with different concentrations of nano TiO2 were measured. Results showed that in both of the standard shades, nano TiO2 increases the reflectance value in NIR region and with increasing the concentration of nano TiO2, the reflectance curves of samples tend to show the maximum reflection of greenish leaves (deciduous leaves) in NIR region. Chromatic values (CIE1976 L*, a*, and b*) and color difference (according to CIECMC color difference ΔE*cmc (2:1)) of each of the coated samples were measured using the reflection spectrophotometer. By considering the influence of white color of nano TiO2 on green shade of dyed cotton fabrics and increasing the color difference (between coated samples and the standard shades of the 1948 U.S army pattern) in visible range, optimum concentrations of nano TiO2 for development of camouflage properties on cotton fabrics in both NIR and visible region were determined. These values for NATO and forest green shades were 0.75 % and 0.5 %, respectively. Fastness results showed that both of the samples have acceptable color fastness. The effect of washing and exposure to light on camouflage properties of coated dyed fabrics in visible (in term of chromatic values) and NIR region (in term of spectral reflectance) was investigated when those were coated in optimum concentrations of nano TiO2. The results showed that the effect of after treatments (washing and exposure to light) on surface color spectral characteristics and camouflage properties was inconsiderable.

The field of plasmonics has substantially affected the study of light-matter interactions at the subwavelength scale. However, dissipation losses still remain an inevitable obstacle in the development of plasmonic-based wave propagation. Although different materials with moderate losses are being extensively studied, absorption arguably continues to be the key challenge in the field. Here, we theoretically and numerically investigate a different route toward the reduction of loss in propagating plasmon waves. Rather than focusing on a material-based approach, we take advantage of structural dispersion in waveguides to manipulate effective material parameters, thus leading to smaller losses. The potential of this approach is illustrated with two examples: plane-wave propagation within a bulk epsilon-near-zero medium and surface plasmon polariton propagation at the interface of a medium with negative permittivity. We provide the recipe for a practical implementation at mid-infrared frequencies. Our results might represent an important step toward the development of low-loss plasmonic technologies.

Metallic nanostructures are able to interact with an incident electromagnetic field at subwavelength scales by plasmon resonance which involves the collective oscillation of conduction electrons localized at their surfaces. Among several possible applications of this phenomenon, the theoretical prediction is that optical circuits connecting multiple plasmonic elements will surpass classical electronic circuits at nanoscale because of their much faster light-based information processing. However, the placement and coupling of metallic elements smaller than optical wavelengths currently remain a formidable challenge by top-down manipulations. Here, we show that organic supramolecular triarylamine nanowires of ≈1 nm in diameter are able to act as plasmonic waveguides. Their self-assembly into plasmonic interconnects between arrays of gold nanoparticles leads to the bottom-up construction of basic optical nanocircuits. When the resonance modes of these metallic nanoparticles are coupled through the organic nanowires, the optical conductivity of the plasmonic layer dramatically increases from 259 to 4271 Ω(-1)·cm(-1). We explain this effect by the coupling of a hot electron/hole pair in the nanoparticle antenna with the half-filled polaronic band of the organic nanowire. We also demonstrate that the whole hybrid system can be described by using the abstraction of the lumped circuit theory, with a far field optical response which depends on the number of interconnects. Overall, our supramolecular bottom-up approach opens the possibility to implement processable, soft, and low cost organic plasmonic interconnects into a large number of applications going from sensing to metamaterials and information technologies.

Gain enhancement in a V-shaped plasmonic slot waveguide for efficient loss compensation at the subwavelength scale

Extraction of the anisotropy factor and refractive index of biological tissue in the near-infrared region from diffusion approximation in the spatial frequency domain

Hybrid systems composed of multiple quantum emitters coupled with plasmonic waveguides are promising building blocks for future integrated quantum nanophotonic circuits. The techniques that can super-resolve and selectively excite contiguous quantum emitters in a diffraction-limited area are of great importance for studying the plasmon-mediated interaction between quantum emitters and manipulating the single plasmon generation and propagation in plasmonic circuits. Here we show that multiple quantum dots coupled with a silver nanowire can be controllably excited by tuning the interference field of surface plasmons on the nanowire. Because of the period of the interference pattern is much smaller than the diffraction limit, we demonstrate the selective excitation of two quantum dots separated by a distance as short as 100 nm. We also numerically demonstrate a new kind of super-resolution imaging method that combines the tunable surface plasmon interference pattern on the NW with the structured illumination microscopy technique. Our work provides a novel high-resolution optical excitation and imaging method for the coupled systems of multiple quantum emitters and plasmonic waveguides, which adds a new tool for studying and manipulating single quantum emitters and single plasmons for quantum plasmonic circuitry applications.

We explore two main representative fields of plasmonics: plasmonic focusing and waveguiding. We overview and discuss several methods of focusing and waveguiding electromagnetic fields in subwavelength regime using plasmonics.

At low frequencies outside the plasmonic range, strongly confined surface waves can be achieved on periodically structured metal surfaces, thereby allowing for the design of compact electromagnetic guiding devices. Here, we propose an approach to realize highly efficient transmission of spoof surface plasmons around 90-degree sharp bends on ultrathin metallic films in the microwave regime. We demonstrate that by judiciously engineering the structure, the dispersion relation can be designed to reduce the scattering. Furthermore, the reflection can be suppressed by proper structural decoration at the bending corner. A one-dimensional scattering theory is employed to understand and verify the transmission properties of our waveguide bend structure. Our design scheme is not restricted to the specific structure we propose here but can be applied to other guiding components built up on two dimensional metal surfaces.

Semiconductor thin films of CuMnS have been deposited onto conductive fluorine-doped tin oxide (FTO) glass substrate using an electrodeposition method to investigate their properties for possible applications. Copper sulfate, manganese sulfate and Thiourea were precursors used for sources of copper, manganese and sulphur ions respectively. The concentration of manganese ions was varied while keeping deposition voltage and time constant at 0.6 and 100 s, respectively. The films were characterized for optical, structural and morphological properties. The results obtained showed that the absorbance of the films is high in the visible (VIS) and near-infrared (NIR) regions but decreases towards NIR. The films transmittance is low in the VIS but increased in the NIR regions. The extinction coefficient is low in the VIS and NIR regions and decreases as concentration of manganese ion increased. The refractive index is high and initially increased slightly from 4.49 to 4.68 in the mid-VIS region while manganese concentration increased from 0.05 to 0.15 M and then decreased to the value of 2.73 as concentration of manganese ion increased further. The optical conductivity is high throughout the VIS and NIR regions while the optical bandgap energy is in the range of 1.5 to 2.05 eV and increases as manganese ion concentration increased. The XRD analysis showed that the deposited thin films of CuMnS are crystalline with average crystallite size and micro-strain in the range of 15.86 - 24. 45 nm and 3.97×10–3 - 6.13×10–3, respectively. The SEM results showed that the films are composed of particle sizes that are spherical in shape, uniform in sizes and densely packed together and consequently make the film surface rough. These properties exhibited by the films make them good materials for applications in photovoltaic calls, solar control coatings, photothermal applications and many other electronic devices that require high temperatures.
 HIGHLIGHTS
 
 This paper focused on the study of the effect of manganese ion concentration on the chalcogenide semiconductor thin films of CuMnS for possible device applications
 Electrodeposition method was used to fabricate the semiconductor thin films of CuMnS
 Optical, Structural and morphological properties of the thin films were characterized
 The deposited thin films of CuMnS were found to have good applications for photovoltaic cells and other optoelectronic device fabrications
 
 GRAPHICAL ABSTRACT

A novel approach to find plasmonic waveguide modes is presented in this paper. The method is based on rational function fitting of spectral Green’s function (RFF-SGF) of slab waveguide with the use of modified VECTFIT algorithm. Dispersion equation solutions including complex modes of structures with material/metal loss can be obtained with no extra complexity during the poles and residues extraction in the fitting approach. Moreover, this method can be used in multilayered plasmonic waveguides straightforwardly which are widely utilized in nanophotonic devices. The main advantage of this method lies in its simple implementation, speed as well as controllable accuracy. Surface plasmon and also surface wave modes of various plasmonic slab waveguides are investigated, and excellent agreements with rigorous methods are shown.

The characteristics of long-range hybrid plasmonic modes guided by multilayer metal–dielectric planar waveguides are investigated at the telecom wavelength. These multilayer structures are formed by sandwiching thin metallic stripes into horizontal silicon slot-like waveguides. Comprehensive numerical studies regarding the geometric parameters’ effects on the modal properties reveal that, by properly choosing the dimensions of the metal stripe and the low-index gaps between the stripe and the silicon layers, the symmetric hybrid modes supported by the structures could feature simultaneously ultra-long propagation distance (several centimeters) and subwavelength mode size. Consideration of possible fabrication imperfections shows that the optical performances of the waveguides are quite robust and highly tolerant to these errors. The presented multilayer plasmonic structures greatly extend the capabilities of conventional long-range surface plasmon polariton waveguides by successfully confining light into a subwavelength scale while maintaining the key advantage of enabling ultra-low-loss propagation, which could facilitate potential applications in ultra-long-range plasmon waveguiding and realizations of compact, high-performance photonic components, as well as building optically integrated circuits with complex functionalities.

Bringing photonics and electronics into a common integration platform can unleash unprecedented performance capabilities in data communication and sensing applications. Plasmonics were proposed as the key technology that can merge ultra-fast photonics and low-dimension electronics due to their metallic nature and their unique ability to guide light at subwavelength scales. However, inherent high losses of plasmonics impede their practical deployment in PICs. To ameliorate this limitation, selective integration of individual nano-plasmonic devices on low-loss photonic platforms is considered, allowing for enhanced chip-scale functionalities with realistic power budgets. Likewise, highly-efficient and fabrication-tolerant optical interfaces for co-planar plasmonic and photonic waveguides become essential, bridging these two "worlds" and ease combined high-volume manufacturing. In this work, a TM-mode butt-coupled interface for stoichiometric Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and gold-based thin-film plasmonic waveguides is proposed aiming to be utilized for bio-sensing applications. Following a systematic design process, this new configuration has been analyzed through 3D FDTD numerical simulations demonstrating interface insertion losses of 2dB at the wavelength of 1550 nm, with increased fabrication tolerance compared to silicon based waveguide alternatives.