Magnetic-field enhancement in gold nanosandwiches

Using dispersive finite-difference time-domain (D-FDTD) simulations, we show that a pair of gold nanodisks stacked in a 'sandwich'-like (end-fire) configuration produces a large enhancement of the magnetic field when irradiated with a plane optical wave, if the distance between the nanodisks is optically small. The effect, which can be rationalized in terms of a magnetic dipole resonance, is due the excitation of a hybridized asymmetric plasmon mode, in which the induced electrical dipoles in the two disks oscillate out-of-phase. The strong magnetic response, together with the simple morphology, suggests that Au nanosandwiches are suitable elementary building blocks for optical metamaterials that exhibit negative refraction.

All-optical logic gates based on nonlinear plasmonic ring resonators

A nonlinear plasmonic T-shaped switch based on a square-shaped ring resonator is simulated by the finite-difference time-domain numerical method. Three optical logic gates-a NOT, with one T-shaped switch, and AND and NOR gates, each with two cascaded T-shaped switches-are proposed. The nonlinear Kerr effect is utilized to show the performance of our proposed logic gates. The values of transmission at the ON and OFF states of NOT and NOR gates are 70% and less than 0.6% of the input lightwave, respectively, while these values for the AND gate are 90% and less than 30%, respectively.

Multi-functional high-efficiency reflective polarization converter based on an ultra-thin graphene metasurface in the THz band

In this study, an ultra-thin reflective metasurface is proposed for polarization conversion in the terahertz band. Each unit cell of metasurface is composed of graphene ribbons lying diagonally on silicon substrate. A reflective metal is also placed at the bottom of the structure. Our polarization converter works as a linear polarization converter (LPC) and linear to circular polarization converter (LTC-PC) by variation of the chemical potential of graphene, which can actively be changed by chemical doping or electrical bias of the graphene. The working bandwidth of LPC changes by adjusting the chemical potential of the graphene. The LPC structure has more than 99% polarization conversion ratio in the frequency range of 0.83-0.92 THz, even by changing the angle of incident wave up to 45°, the results are still acceptable. The LTC-PC has less than 3dB axial ratio (AR) in the frequency range of 0.6-0.67 THz for left-handed circularly polarized (LHCP) waves and 0.72-0.97 THz for right-handed circularly polarized (RHCP) waves. To verify the simulation results, an equivalent circuit model based on the structure performance is proposed. Equivalent circuit model results agree very well with the simulation results. Due to the fabrication feasibility, ultra-thin thickness, incident angle insensitive, and high efficiency, our structure has great potential in state-of-the-art technologies such as imaging, sensing, communication, and other optical applications.

Graphene-based tunable terahertz and infrared band-pass filter

This paper presents a sheet of graphene as a simple band-pass filter in terahertz and infrared frequencies. The central frequency and quality factor of this band-pass filter can be tuned by changing the physical parameters, such as the substrate thickness, gate voltage, temperature, and conductivity of the graphene. The effects of these parameters on surface plasmon polariton waves and filter specifications are numerically depicted.

Ultra-compact all-optical plasmonic switch for three telecommunication windows using a nonlinear Kerr material and Fano resonance

In this study, an all-optical plasmonic switch based on a metal-insulator-metal (MIM) waveguide coupled to two rectangular cavities that are perpendicularly connected to each other through a vertical stub is proposed and analyzed both theoretically and numerically. Rectangular cavities are filled with a nonlinear Kerr material, and the switching operation is achieved by applying a high-intensity pump input into the MIM waveguide to obtain nonlinear cross-phase modulation (XPM) effect. The proposed structure is designed so that it can realize the switching operation at each of the three telecommunication windows of 850, 1310, and 1550 nm. Realizing the switching operation at these three wavelength bands is accomplished by the Fano resonance. In fact, the Fano resonance is utilized to create a band-stop area that is crucial for building a suitable OFF state for the switching operation at two of the three telecommunication windows of 1310 and 1550 nm. The theoretical and numerical results are obtained using the transmission-line model (TLM) and the finite difference time domain (FDTD) method, respectively, the results of which comply well. The proposed ultra-compact all-optical switch has significant applications in photonic integrated circuits (PICs).

Broadband near-perfect terahertz absorber in single-layered and non-structured graphene loaded with dielectrics

In this work, we have done an extensive study on broadband near-perfect absorbers consisting of single-layered and non-structured graphene loaded with periodical arrays of dielectric bricks, including square and elliptical bricks. We also propose and investigate circular cylinder, rectangular brick, and racecourse dielectric structure. Moreover, the calculated ${z}$z component of the electric field enables us to understand the physical mechanism of resonance absorption. Furthermore, we also studied and proposed a new absorber with periodical arrays of stepped rectangle dielectric structure. We could achieve a broadband absorption from 1.6 to 4.2 THz, with a bandwidth of 2.6 THz and absorption over 90%. The proposed absorber is also tunable; the tunability of the terahertz (THz) broadband absorber is achieved via changing the external gate voltage to modify the Fermi energy of graphene. Also, we compared the results and absorption spectra of different dielectric structures. This THz metamaterial structure can be used in different THz applications such as cloaking, sensing, detection, and imaging.

High-efficiency tunable plasmonically induced transparency-like effect in metasurfaces composed of graphene nano-rings and ribbon arrays and its application

In this paper, a plasmonically induced transparency (PIT)-like phenomenon in a metasurface composed of a periodic graphene ring and ribbon arrays is studied in the terahertz region. We used the Lorentz oscillator model to analyze the metasurface physically and theoretically. This PIT-like effect can be tuned by alternation of the chemical potential and dimension of the nano-graphene ring and ribbon. The resonance frequency of the PIT-like phenomenon is not sensitive to the incident lightwave angle. As an application of the structure, a refractive index sensor is proposed and simulated. Furthermore, we propose a metasurface composed of a double ring and graphene ribbon to realize the PIT-like effect with three dips. Our results express an appropriate approach for the expansion of mid-infrared absorbers and sensors.

Lossless propagation in metal-semiconductor-metal plasmonic waveguides using quantum dot active medium

In this paper, we analyze and simulate the lossless propagation of lightwaves in the active metal-semiconductor-metal plasmonic waveguides (MSMPWs) at the wavelength range of 1540-1560 nm using a quantum dot (QD) active medium. The Maxwell's equations are solved in the waveguide, and the required gains for achieving lossless propagation are derived. On the other hand, the rate equations in quantum dot active regions are solved by using the Runge-Kutta method, and the achievable optical gain is derived. The analyses results show that the required optical gain for lossless propagation in MSMPWs is achievable using the QD active medium. Also, by adjusting the active medium parameters, the MSMPWs loss can be eliminated in a specific bandwidth, and the propagation length increases obviously.

Graphene-based polarization-sensitive metasurfaces for integrated optics applications

In this study, polarization detection of the incident wave with amplitude and frequency is achieved by proposed graphene-based metasurfaces for integrated optics applications. Metasurfaces consist of elliptical nanodisks of graphene as the meta-atoms. Synthesis of the metasurfaces is based on the analytical approach of the equivalent conductivity method. The elliptical nanodisks play an asymmetric meta-atom role with respect to the polarization of the incident waves. This concept is applied to design polarization selective metasurfaces. Two types of polarization-sensitive metasurfaces are designed. One of them has unique nanodisks that change the absorbance coefficient by varying the polarization of the incident wave in an absorber structure. The second one has three types of elliptical nanodisks with various dimensions and orientations that distinguish the polarization. This metasurface is used in a demultiplexing structure to select a desired wave with a specific frequency.

Wideband and multi-frequency infrared cloaking of spherical objects by using the graphene-based metasurface

The ultrathin graphene metasurface is proposed as a mantle cloak to achieve wideband tunable scattering reduction around the spherical (three-dimensional) objects. The cloaking shell over the metallic or dielectric sphere is structured by a periodic array of graphene nanodisks that operate at infrared frequencies. By using the polarizability of the graphene nanodisks and equivalent conductivity method, the metasurface reactance is obtained. To achieve the cloaking shell for both dielectric and conducting spheres, the metasurface reactance as a function of nanodisks dimensions, graphene's Fermi energy, and permittivity of the surrounding areas can be tuned from the inductive to capacitive situation. Inhomogeneous metasurfaces including graphene nanodisks with different radii provide wideband invisibility due to extra resonances. We could significantly increase the 3-dB bandwidth more than the homogenous case by simpler realistic designs compared to the multi-layer structures. The analytical results are confirmed with full-wave numerical simulations.

Magneto-optic surface plasmon polariton modulator based on refractive index variations

In this paper, we have proposed a magneto-optic (MO) surface plasmon polariton (SPP) modulator based on variations of refractive index. For description of modulator operation, we have analyzed the MO effects in the insulator-metal-insulator (IMI) SPP slab waveguides in a transversal configuration in which the applied magnetic field is parallel to the interfaces and normal to the wave propagation direction. We have derived an exact dispersion relation by considering MO effects for one of the side layers by the separation of variables method. The cut-off conditions have been studied for the SPP modes guided by IMI structures as a function of the variations of the dielectric constants of the side layers. We have shown that the SPP modes always propagate in a symmetric structure and the SPP odd modes do not have a cut-off dielectric constant in an asymmetric structure. Also, we have shown that in an asymmetric IMI configuration, the SPP even mode has a cut-off effective dielectric constant for all metal layer thicknesses. These configurations can be used to design active devices, such as switches and modulators to be used in photonic integrated circuits.

Simulation of nerve fiber based on anti-resonant reflecting optical waveguide

Light and optical techniques are widely used for the diagnosis and treatment of neurological diseases as advanced methods. Understanding the optical properties of nervous tissue and nerve cells is vital. Using light sources in these methods raises significant challenges, such as finding the place of light transmission in nerve fibers that could be an appropriate substrate for neural signaling. The myelinated axons are a promising candidate for transmitting neural signals and light due to their waveguide structures. On the other hand, with the emergence of diseases such as multiple sclerosis and disorders within the production and transmission of nerve signals, because of the demyelination, understanding the properties of the myelinated axon as a waveguide is obtaining additional necessity. The present study aims to show that the myelinated axon's refractive index (RI) profile plays an essential role in transmitting the beams in it. According to the nerve fiber, RI profile and its similarity to depressed core fiber with lower RI of the core compared to the cladding, the behaviors of the nerve fiber based on anti-resonant reflecting optical waveguide structure are investigated by taking into account the realistic optical imperfections. Light launching to the myelin sheath and axon is shown by introducing the axon and myelin sheath as a waveguide in the presence of both axon and myelin with bends, myelin sheath variation, and node of Ranvier.

Analysis of the multi-spectral inhomogeneous metasurfaces consisting of different arrays of components

The analytical method to study the multi-spectral inhomogeneous metasurfaces with various components is presented. Because of symmetrical distribution of different components, we can find the effective polarizability of the inhomogeneous metasurfaces. This polarizability provides equivalent conductivity of each metasurface with two and three different nanodisk arrays. Full-wave simulations confirm the analysis of inhomogeneous metasurfaces. In a metasurface, symmetrical distribution of components with a unique periodicity is limited to three types in a hexagonal combination. Then we extend the proposed approach to partly symmetric inhomogeneous metasurfaces in subwavelength scale for four different nanodisks. Also, monolayer and multi-layer absorbers consisting of inhomogeneous metasurfaces with graphene nanodisks are designed as the examples of wideband applications of this method in infrared regime.

SPM and XPM nonlinear effects in plasmonic directional couplers, considering the ponderomotive metal nonlinearity

In this paper, a two-dimensional nonlinear plasmonic directional coupler (2D-NPDC), with 90° waveguide bends, has been numerically analyzed by the finite-difference time-domain (FDTD) method, considering the nonlinear response of metal due to the ponderomotive force. It has been shown that the required switching power of the 2D-NPDC is 0.05% of that when only the dielectric is nonlinear and the nonlinearity of metal is neglected. Also, the cross-phase modulation (XPM) nonlinear effect has been investigated, which the power for switching is decreased significantly compared to the one with the self-phase modulation (SPM) effect.

Design of a flat-gain multipumped distributed fiber Raman amplifier by particle swarm optimization

The pumping scheme of multipumped distributed fiber Raman amplifiers is optimized by a powerful method called particle swarm optimization. By use of particle swarm optimization, we optimize both pump powers and frequencies of multipumped Raman amplifiers with a high number of pumps. Particle swarm optimization is a fast and effective method, and it surpasses other optimization methods, such as the genetic algorithm, for optimizing fiber amplifiers. It is shown that the computational efficiency of particle swarm optimization is significantly better than that of the genetic algorithm, reducing the time of computation to one third, and its implementation is more straightforward. A gain bandwidth of 92.1 nm and a gain variation of 0.49 dB in the range of 1524.5-1616.6 nm are obtained by this method, using ten backward pumps in a 60-km-long amplifier. The gain variation reduction is due to the inclusion of pump frequencies in the optimization process.

High sensitivity tapered fiber refractive index biosensor using hollow gold nanoparticles

A localized surface plasmon resonance (LSPR) sensor based on tapered optical fiber (TOF) using hollow gold nanoparticles (HAuNPs) for measuring the refractive index (RI) is presented. This optical fiber sensor is a good candidate for a label-free RI biosensor. In practical biosensors, bioreceptors are immobilized on nanoparticles (NPs) that only absorb specific biomolecules. The binding of these biomolecules to the receptors changes the local RI around the sensor and this change is detected by the transmittance spectrum of the fiber. Fast, accurate, easy and low-cost disease diagnosis are the advantages of optical fiber biosensors. In this paper, the structure theory is reviewed and the sensor is simulated by the finite difference time domain (FDTD) method and the finite element method (FEM) and the effect of the thickness and diameter of the HAuNPs and the waist diameter of the TOF is investigated. For the structure with HAuNPs thickness (2.5 nm), diameter (50 nm), and the fiber waist diameter of 10 μm, the wavelength sensitivity of 489.8 nm/RIU and full width at half maximum (FWHM) of 50 nm are obtained, which are better than those specifications in some other LSPR fiber sensors. In addition, the sensitivity of the sensor increases about 2-3 times compared to those of sensors with the same structure. Although there are many parameters in human blood that can change its RI, in practical work, the special bioreceptors on the sensor can deactivate other markers except the specific cancer markers, which changes the effective RI. Therefore, this optical fiber sensor is used for label-free detecting the RI of cancer cells and can be used as a biosensor for the detection of early stages of cancers in a non-invasive way, just using human blood samples.

Analytical synthesis of high-Q bilayer all-dielectric metasurfaces with coupled resonance modes

We propose a structure of bilayer dielectric metasurfaces consisting of silicon nano-cuboids with high-quality (Q) transmittance due to the coupling effect between Fabry-Perot and Mie resonances. The synthesis of the structure is done by using a novel straightforward analytical method, to the best of our knowledge, based on finding the equivalent parameters of the dielectric metasurfaces. Considering the dielectric metasurface as an array of meta-atoms with dipole moments addresses the theoretical calculation of the equivalent parameters of the metasurface. Because the main aspect of the analytic manner is precisely finding these equivalent parameters, providing effective polarizabilites of a limited array of meta-atoms instead of polarizabilities of one meta-atom is presented. The calculated equivalent parameters are used to synthesize bilayer dielectric metasurfaces with specific distance. The design activates Fabry-Perot resonances, and coupling these modes with Mie resonances of silicon nano-cuboids causes a band-pass filtering effect with high-Q transmittance. One can tune these transmittances by changing the properties of the structures and tailor them for usage in many optical applications, such as sensing, narrow-band filters, and detectors.

Near-infrared absorbers based on the heterostructures of two-dimensional materials

Although the conductance and dielectric function of graphene can be tuned by applying external voltage, the tunability is less than 3%. Hybridizing graphene with other two-dimensional transition metal dichalcogenides (TMDs) can improve the adjustability and tunability of the optical properties of graphene-based structures at near-infrared frequencies. In this paper, we theoretically compute the dielectric function of graphene-MoTe₂-graphene and graphene-MoTe₂-graphene heterostructures utilizing the quantum electrostatic heterostructure (QEH) model, which is an ab-initio method. Utilizing the QEH results, we propose a hyper crystal (HC) absorber at near-infrared frequencies. Hence, we use the transfer matrix method to investigate our proposed absorber analytically. Moreover, we simulate the graphene-TMD-graphene (G-TMD-G) absorbers by the numerical finite difference time domain method. The results of the numerical solution are consistent with those of the analytical method. Due to the dependency of the Fermi level of graphene on the direct bandgap of the TMDs, the dielectric function of the G-TMD-G heterostructure can be tuned and enhanced further by changing the number of TMD layers. Finally, we demonstrate that the full absorption of the heterostructures can be achieved at different frequencies for transverse magnetic polarization. Since the thicknesses of the layers in the HC are lower than the wavelength of the light, no diffracted bands are ubiquitous, and the absorption can be observed for a wide range of incidence angles and bandwidths at near-infrared frequencies. Because of utilizing graphene-based HCs, in addition to the feasibility of design compared to the complex metasurfaces, the absorption bandwidth is significant for a wide range of incidence angles. This kind of HC absorber can be used in the design of sensitive optical devices, such as tunable filters, detectors, and photovoltaic applications.

Optimum design of a hybrid erbium-doped fiber amplifier/fiber Raman amplifier using particle swarm optimization

We propose and optimize a hybrid erbium-doped fiber amplifier/fiber Raman amplifier (EDFA/FRA). A large number of parameters of a wide-band hybrid amplifier consisting of an erbium-doped fiber amplifier (EDFA) and a fiber Raman amplifier (FRA) have been optimized using an effective and fast global optimization method called particle swarm optimization. Two types of hybrid EDFA/FRA with six- and 10-pumped FRAs have been optimized. A large number of variables affect the hybrid EDFA/FRA performance, thus we need a global optimization method to be able to deal with these variables. Particle swarm optimization helps us to find optimum parameters of a hybrid EDFA/FRA and reduce the gain spectrum variations to 2.91 and 2.03 dB for the six and 10 pumped FRAs, respectively. The optimum design supports the amplification of 60 signal channels in the wavelength range of 1529.2-1627.1 nm for a wavelength-division multiplexing system.

Reduction of the pump power threshold in the nonlinear all-optical photonic crystal directional coupler switches

In this paper, the cross-phase modulation Kerr nonlinear effect is utilized to induce nonlinearity and a change in the coupling length by a high-intensity pump signal in the nonlinear photonic crystal directional coupler. We have analyzed and modified the nonlinear square-lattice directional couplers for the first time, to the best of our knowledge, and have improved the power consumption for inducing the nonlinear switching. We have shown that according to the slower group velocity and the increased confinement of the TM modes with lower frequency in the region with a higher dielectric constant in a square lattice, the required switching pump power of the structure is less than that of the same triangular structure.