Elliptical Nanowires Research Articles

Optimization of light absorption in ultrathin elliptical silicon nanowire arrays for solar cell applications

The present study proposes an elliptical silicon array for broadband absorption inside ultra-thin silicon solar cells (SCs). The simulation results suggest that breaking the arrangement of the nanowires (NWs) modifies the absorption intensities. The efficient design of irregular NWs can contribute to improved absorption of such structures in comparison with ordinary structures. Single elliptical NWs exert significant optical antenna effects, which enable higher absorption of particular frequencies. However, in the case of NW arrays enhanced absorption is the outcome of trade off between an optical antenna and diffraction effects. In this study, a structure composed of a semi-periodic NW is suggested which can achieve improved performance in ultra-thin silicon SCs.

Elliptical nanowire FET: Modeling the short-channel subthreshold current caused by interface-trapped-charge and its evaluation for subthreshold logic gate

Based on the quasi-3D potential approach, quasi-3D scaling theory, drift-diffusion model, and equivalent flat-band voltage shift, a short-channel subthreshold current caused by the interface-trapped-charge is modeled for the elliptical nanowire FET. With the subthreshold current, the logic swing (LS) of the subthreshold logic gate composed of elliptical nanowire FET is theoretically evaluated. It indicates that the positive interface-trapped-charge can degrade the high output voltage (V OH ) due to its increased/decreased the subthreshold current of N-FET/P-FET. However, the negative interface-trapped-charge can decrease/increase the subthreshold current of N-FET/P-FET, which hence deteriorates the low output voltage (V OL ). Both degradation of the subthreshold current and LS (i.e., ΔI sub and ΔLS) caused by short-channel effects (SCEs) can be well-controlled by the properly selected scaling factor of α. With the minimum scaling factor, the minimum LS degradation for the subthreshold logic gate can be uniquely determined. • Providing the robust interface-trapped-charged-degraded subthreshold model for the elliptical MOSFET. • Examining how the short-channel-effects together with the interface-trapped-charge-effects affect the logic swing of the subthreshold logic gate.

Efficient guiding mid-infrared waves with graphene-coated nanowire based plasmon waveguides

A graphene-coated nanowire (GCNW) based plasmon waveguide is proposed to achieve outstanding guiding performance in the mid-infrared band. The modal properties of the fundamental graphene plasmon mode and modal dependence on the elliptical nanowire size, thickness and width of the low-index dielectric layer, chemical potential of graphene and the operating frequency are revealed in detail by using finite element method. Results show that the low-index layer has a significant influence on modal properties, and a propagation length over 13 μm along with an extremely small normalized mode size (~10−6) could be obtained simultaneously, which is more than two orders of magnitude smaller compared with free-standing GCNW and GCNW dimers. Besides, study on crosstalk characteristics between the adjacent structures indicates the proposed waveguide exhibits very low crosstalk. Due to these excellent transmission characteristics, the proposed configuration could be used to fabricate various functional photonic devices for the future nanoscale photonic integrated circuits.

Mode characteristics of asymmetric graphene-coated elliptical dielectric nano-parallel wires waveguide

An asymmetric graphene-coated elliptical dielectric nano-parallel wires’ waveguide is proposed. By using the multipole method, in the two elliptic cylindrical coordinate systems, firstly, the longitudinal component of the electric field and the magnetic field are expressed by Mathieu functions, then the corresponding angular and radial components are obtained by Maxwell’s equations. The graphene is regarded as a zero-thickness interface with surface conductivity, and the boundary conditions are applied to these interfaces by the point-matching method. A linear algebraic equation system is obtained finally. The effective refractive indices and the field distributions of modes can be obtained by numerically solving the equation. The six lowest order modes supported by the proposed structure are classified, and the dependence of the characteristics of these modes, separately, on the working wavelength, the graphene Fermi energy and waveguide structure parameters are studied. The real part of the effective refractive index, the propagating length, and the quality factor are used to judge the performance of the waveguide. The results reveal that the characteristics of these modes can be greatly changed by altering the working wavelength of the waveguide, the Fermi energy of graphene, and the spacing between nanowires. When the length of the semi-major and the semi-minor axes of the nanowires are modified, the real part of the effective refractive index, the propagating length, and the quality factor can only be changed finely. At the same time, the results obtained by the multipole method are completely consistent with the results from the finite element method. By comparing the performances among the fundamental mode supported by the single graphene-coated elliptical dielectric nanowire, the symmetric graphene-coated elliptical dielectric nano-parallel wires, and the asymmetric graphene-coated elliptical dielectric nano-parallel wires by the means of the FEM based on commercial software (COMSOL), we find that the performances of the proposed waveguide in this paper are superior to those of the other two waveguides. This work can provide a theoretical basis for the design, fabrication, and application of asymmetric graphene-coated elliptical dielectric nano-parallel wires’ waveguide. The proposed structure is expected to be used in the mode conversion and coupling in the future devices.

涂覆石墨烯层的双椭圆形纳米线模式特性

涂覆石墨烯层的双椭圆形纳米线模式特性

Tunable reducibility of Brillouin zone and bandgap width in elliptical nanowire arrays

Rotating elliptical nanowire arrays as two-dimensional photonic crystals has been proposed and studied in this Letter. The analysis of the four lowest energy bands and the first bandgap width of some examples illustrates that the rotation and configuration of the primitive cell can have effects on the reducibility of the Brillouin zone. As the central element’s orientation changes, the irreducible Brillouin zone could be expanded to the whole first Brillouin zone. Special attention has been paid to the nanowire arrays with adjacent elements perpendicular to each other, and the irreducible Brillouin zone unexpectedly retracted back to the 1/8 of the first Brillouin zone though the symmetry of elements is lower than that of the square lattice. Meanwhile, the first bandgap width of the perpendicular array can be adjusted by the rotation of each primitive element.

Graphene-Coated Elliptical Nanowires for Low Loss Subwavelength Terahertz Transmission

Graphene has been recently proposed as a promising alternative to support surface plasmons with its superior performances in terahertz and mid-infrared range. Here, we propose a graphene-coated elliptical nanowire (GCENW) structure for subwavelength terahertz waveguiding. The mode properties and their dependence on frequency, nanowire size, permittivity and chemical potential of graphene are studied in detail by using a finite element method, they are also compared with the graphene-coated circular nanowires (GCCNWs). Results showed that the ratio of the long and short axes (b/a) of the elliptical nanowire had significant influence on mode properties, they also showed that a propagation length over 200 μm and a normalized mode area of approximately 10−4~10−3 could be obtained. Increasing b/a could simultaneously achieve both long propagation length and very small full width at half maximum (FWHM) of the focal spots. When b/a = 10, a pair of focal spots about 40 nm could be obtained. Results also showed that the GCENW had a better waveguiding performance when compared with the corresponding GCCNWs. The manipulation of Terahertz (THz) waves at a subwavelength scale using graphene plasmon (GP) may lead to applications in tunable THz components, imaging, and nanophotonics.

Coupling characteristics of surface plasmons in coupled elliptical nanowires

Modal properties of a plasmon guiding structure consisting of two parallel metallic nanowires embedded in a dielectric material have been analyzed employing the finite element method. The coupling characteristics of the surface plasmon supermode reveal extreme yet linearly changing confinement for a moderate range of nanowire separation. The potentialities of this structure in different nanophotonic applications have been explored in this study based on modal characteristics variation with the alteration of both the operating wavelength and the cross-sectional geometry.

An Optimized Structure for Enhancing Optical Absorption of Solar Energy in Elliptical GaAs Nanowire Array Solar Cell

The enhanced optical absorption of solar energy in a solar cell using nanostructured materials is a very demanding and important research area. Hence, an optimized structure to increase the optical absorption of solar energy in an elliptical GaAs (Gallium Arsenide) nanowire array solar cell was proposed in this paper. The influence of geometric parameters on the optical absorption of elliptical GaAs nanowire solar cells was investigated by exploiting the finite difference in time domain simulations. Based on the design and analysis, the elliptical GaAs nanowire array performed better than any circular GaAs nanowire. It was found that the enhanced performance was due to the reduced reflection from the top surface and a reduced transmission from the bottom surface of the elliptical GaAs nanowire array. The structural parameters of the elliptical GaAs nanowire were optimized by calculating the short circuit current density (Jsc) for different geometries over the solar spectrum. It was demonstrated that there was an increase of 9.4% in the Jsc compared with the circular GaAs nanowire under the optimized conditions.

Theoretical study on the stimulated Brillouin scattering in a sub-wavelength anisotropic waveguide: acousto-optical coupling coefficients and effects of transverse anisotropies

A theoretical study on the stimulated Brillouin scattering (SBS) in a sub-wavelength anisotropic waveguide is conducted. The optical, photoelastic and mechanical anisotropies of the waveguide materials are all taken into account. First, the integral formulae for calculating the acousto-optical coupling coefficients (AOCCs) due to the photoelastic and moving interface effects in SBS are extended to an optically anisotropic waveguide. Then, with the extended formulae, the SBSs in an elliptical nanowire with strong transverse anisotropies are simulated. In the simulations, the elastic fields are computed with the inclusion of mechanical anisotropy. Observable effects of the strong transverse anisotropies are found in numerical results. Most notably, the SBS gains of some elastic modes are found to be very sensitive to the small misalignment between the waveguide axes and the principal material axes. Detailed physical interpretations of this interesting phenomenon are provided. This interesting phenomenon implies an attractive way for more sensitive tuning of the SBS gain without significantly changing the phononic frequency.

Simulation analysis of a temperature sensor based on photonic crystal fiber filled with different shapes of nanowires

We propose a photonic crystal fiber (PCF)-based surface plasmons resonance (SPR) temperature sensor in this paper, and using the dual function of PCF filled with different shapes of nanowires and analyte to realize temperature sensing and have designed an experimental setup. Different shapes of nanowires with the mixed liquid are filled into the proposed octagon PCF. The simulation results indicate that not only structure parameters of PCF but also the shapes of filled nanowires play a key role to the sensing. A blue-shift in the SPR resonant peak is shown with the temperature increases. Different loss spectra present that a higher confinement loss for low temperature and the octagon PCF filled with elliptical nanowires shows a more obvious peak shift and higher sensitivity, which is about 500pm/°C and the corresponding sensor resolution is 2.0×10−2°C. This PCF sensing system has great practical value and significance for its advantage of high sensitivity.

Elliptical silicon nanowire photodetectors for polarization-resolved imaging

Polarization-resolved imaging offers many advantages over conventional imaging because it provides additional information on materials and scenes. In this study, we present an image sensor pixel for polarization-resolved imaging based on an all-silicon nanowire device. As the structure has an intrinsically polarization-dependent response, it is not necessary to employ a polarizer. We fabricate pixels consisting of etched vertical silicon nanowires with elliptical cross-sections that incorporate vertical p-i-n junctions. Our photocurrent measurement reveals that the spectral responsivities are dependent on the polarization state of incident light. Polarization-resolved imaging is performed with fabricated devices. This approach is different from conventional approaches using polarization filters because absorbed light in the elliptical nanowire is converted to photocurrent while light absorbed by a polarization filter is discarded.

Broadband absorption enhancement in elliptical silicon nanowire arrays for photovoltaic applications.

Semiconducting nanowire arrays have emerged as a promising route toward achieving high efficiencies in solar cells. Here we propose a perpendicular elliptical silicon nanowire (PEE-SiNW) array for broadband light absorption in thin film silicon solar cells. Simulation results reveal that light absorption enhancement is originated from the split of the principal modes as well as the excitation of high order modes caused by the asymmetry of the elliptical nanowires and the enhanced mode coupling between adjacent elliptical nanowires attained by the appropriate arrangement of nanowires. An ultimate efficiency of 29.1% is achieved for the optimal PEE-SiNW array, which is 16.4% higher than that of the circular SiNW array with the same fill fraction.

Performance improvement in Mach–Zehnder interferometer-based refractive index sensor using elliptical photonic nanowires

We propose a modified Mach–Zehnder interferometer design based on elliptical silica photonic nanowires. The use of the interferometer as an evanescent field-based refractive index (RI) sensor was numerically investigated. Single-mode operation, maintaining polarization and very high sensitivity, is achieved at short optical wavelengths by simply using elliptical nanowires. The proposed sensor is capable of determining the RI of benzene solutions with different concentrations in water and detecting a RI variation of the order of 10−6 RI units in only a 1-mm length sensitive area. Extremely high sensitivity of 4.63 rad/μm is achieved using an 800 nm elliptical silica nanowire diameter. The operating wavelengths (λ = 650 nm and 970 nm) were chosen to avoid high water absorption. The sensor is shown to be an alternative solution to small circular-nanowire-based sensor whose core size needs to be significantly reduced below 400 nm to achieve comparable performance.

Rigorous Optical Modeling of Elliptical Photonic Nanowires

We analyze the optical properties including chromatic dispersion, birefringence, and nonlinear coefficient dependence on the ellipticity of photonic nanowires. We propose a linear approximation to determine the equivalent-circular photonic nanowire exhibiting similar optical characteristics with the elliptical nanowire. We find strong birefringence up to the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> in elliptical photonic nanowires that could be very attractive for optical fiber sensors and stable combs. We also investigate the effect of the ellipticity on the supercontinuum generation which is found to be detrimental to the spectral broadening.

Fano-Like Resonances in Asymmetric Homodimer of Gold Elliptical Nanowires

The Fano-like resonance properties of the asymmetric dimer for gold nanowires (GNWs) with elliptical section have been investigated based on the 2-D finite element method (2D-FEM). It is found that the reduced symmetry in the GNWs dimer will cause a distinct Fano dip in the absorption spectrum, which arises from the coherent coupling between the superradiant bright mode and the subradiant dark mode. With the increase in the rotation angle of one GNW, the Fano dip in the absorption of the asymmetric dimer enhances first and then reduces. We have further found that the Fano dip in the absorption becomes wider and deeper with the decrease in the separation between two GNWs, whereas the Fano minimum shows a distinct red shift. The tunability in Fano-like resonance of the asymmetric dimer for the GNWs may provide effective applications in sensing and plasmon-induced transparency.

The influence of wire shape on surface plasmon mode distribution

The influence of the nanowire shape on the excitation of surface plasmon polaritons at metallic nanowire arrays is studied numerically. For a system of silver nanowires housed on a polymer substrate, nanowires with rectangular and elliptical cross sections are compared. It was found that in the case of rectangular nanowires the excitation efficiency is higher for surface plasmons at the polymer–metal interface than for surface plasmons at the air–metal interface. Conversely, in the case of elliptical nanowires the air–metal plasmon modes are stronger. Further, it is noted that the nanowire shape directly influences the position of the surface plasmon resonance.

Jahn-Teller Distortions and the Supershell Effect in Metal Nanowires

A stability analysis of metal nanowires shows that a Jahn-Teller deformation breaking cylindrical symmetry can be energetically favorable, leading to stable nanowires with elliptic cross sections. The sequence of stable cylindrical and elliptical nanowires allows for a consistent interpretation of experimental conductance histograms for alkali metals, including both the electronic shell and supershell structures. It is predicted that for gold, elliptical nanowires are even more likely to form since their eccentricity is smaller than for alkali metals. The existence of certain metastable superdeformed nanowires is also predicted.

Electronic shell effects and the stability of alkali nanowires

Experimental conductance histograms for Na nanowires are analyzed in detail and compared to recent theoretical results on the stability of cylindrical and elliptical nanowires, using the free-electron model. We find a one-to-one correspondence between the peaks in the histograms and the most stable nanowire geometries, indicating that several of the commonly observed nanowires have elliptical cross sections.