Delocalized Electric Field Enhancement through Near-Infrared Quasi-BIC Modes in a Hollow Cuboid Metasurface.

The two main problems of dielectric metasurfaces for sensing and spectroscopy based on electromagnetic field enhancement are that resonances are mainly localized inside the resonator volume and that experimental Q-factors are very limited. To address these issues, a novel dielectric metasurface supporting delocalized modes based on quasi-bound states in the continuum (quasi-BICs) is proposed and theoretically demonstrated. The metasurface comprises a periodic array of silicon hollow nanocuboids patterned on a glass substrate. The resonances stem from the excitation of symmetry-protected quasi-BIC modes, which are accessed by perturbing the arrangement of the nanocuboid holes. Thanks to the variation of the unit cell with a cluster of four hollow nanocuboids, polarization-insensitive, delocalized modes with ultra-high Q-factor are produced. In addition, the demonstrated electric field enhancements are very high (103-104). This work opens new research avenues in optical sensing and advanced spectroscopy, e.g., surface-enhanced Raman spectroscopy.

Multifunctional THz Graphene Antenna with 360∘ Continuous ϕ-Steering and θ-Control of Beam.

A novel graphene antenna composed of a graphene dipole and four auxiliary graphene sheets oriented at 90∘ to each other is proposed and analyzed. The sheets play the role of reflectors. A detailed group-theoretical analysis of symmetry properties of the discussed antennas has been completed. Through electric field control of the chemical potentials of the graphene elements, the antenna can provide a quasi-omnidirectional diagram, a one- or two-directional beam regime, dynamic control of the beam width and, due to the vertical orientation of the dipole with respect to the base substrate, a 360∘ beam steering in the azimuth plane. An additional graphene layer on the base permits control of the radiation pattern in the θ-direction. Radiation patterns in different working states of the antenna are considered using symmetry arguments. We discuss the antenna parameters such as input reflection coefficient, total efficiency, front-to-back ratio, and gain. An equivalent circuit of the antenna is suggested. The proposed antenna operates at frequencies between 1.75 THz and 2.03 THz. Depending on the active regime defined by the chemical potentials set on the antenna graphene elements, the maximum gain varies from 0.86 to 1.63.

Graphene THz filter-switch dividers based on dipole-quadrupole and magneto-optical resonance effects.

We propose and analyze a multifunctional THz graphene-based component with graphene elements placed on a dielectric substrate. The structure of the device consists of a disc shaped resonator coupled to three graphene waveguides that excite the dipole or quadrupole resonance of surface plasmon polaritons in the resonator. The graphene resonator can be magnetized by a DC magnetic field. This device fulfills filtering of the input signal and can be used as a power divider and also as a switch. The division mechanism of the T-junction can be provided by application of a DC magnetic field or by changing the Fermi energy of the graphene resonator via an electrostatic field. Some peculiarities of the two mechanisms are discussed. Numerical simulations show that for a central frequency of 7.12 THz, devices in the OFF state have the two output ports isolated from the input port at a central frequency of about 27 dB provided by the dipole mode resonance. In the ON state and the division regime, the transmission to the output ports is around -(4÷5)dB in the 3-dB bandwidth of about 12%.

Dielectric-Loaded Waveguides as Advanced Platforms for Diagnostics and Application of Transparent Thin Films.

An alternative approach to classical surface plasmon resonance spectroscopy is dielectric-loaded waveguide (DLWG) spectroscopy, widely used in the past decades to investigate bio-interaction kinetics. Despite their wide application, a successful and clear approach to use the DLWGs for the one-step simultaneous determination of both the thickness and refractive index of organic thin films is absent in the literature. We propose here, for the first time, an experimental protocol based on the multimodal nature of DLWGs to be followed in order to evaluate the optical constants and thickness of transparent thin films with a unique measurement. The proposed method is general and can be applied to every class of transparent organic materials, with a resolution and accuracy which depend on the nature of the external medium (gaseous or liquid), the geometrical characteristics of the DLWG, and the values of both the thickness and dielectric constant of the thin film. From the experimental point of view, the method is demonstrated in a nitrogen environment with an accuracy of about 3%, for the special case of electroluminescent thin films of Eu3+ß-diketonate complexes, with an average thickness of about 20 nm. The high value of the refractive index measured for the thin film with the Eu(btfa)3(t-bpete) complex was confirmed by the use of a spectroscopic model based on the Judd-Ofelt theory, in which the magnetic dipole transition 5D0 → 7F1 (Eu3+) for similar films containing Eu3+ complexes is taken as a reference. The DLWGs are finally applied to control the refractive index changes of the organic thin films under UVA irradiation, with potential applications in dosimetry and monitoring light-induced transformation in organic thin films.

Graphene-based multifunctional three-port THz and long-wave infrared components.

Two graphene-based T-shaped multifunctional components for THz and long-wave infrared regions are proposed and analyzed. The first component can serve as a divider, a switch, and a dynamically controllable filter. This T-junction presents a circular graphene resonator and three graphene waveguides with surface plasmon-polariton waves connected frontally to the resonator. The resonator can be adjusted to work with dipole, quadrupole, or hexapole modes. The graphene elements are deposited on a SiO2 (silica) and Si (silicon) two-layer substrate. The dynamical control and switching of the component are provided by the electrostatic field, which defines the graphene Fermi energy. Numerical simulations show that the first component in the division regime (which is also the ON regime) has a transmission coefficient of -4.3dB at the central frequency for every two output ports, and the FWHM is 9.5%. In the OFF regime, the isolation of the two output ports from the input one is about -30dB. The second component is a T-junction without a resonator, which fulfills the function of the divider-switch in more than an octave frequency band.

Dynamically controllable graphene terahertz splitters with nonreciprocal properties.

Two novel graphene-based nonreciprocal four-port splitters for the terahertz region are proposed. The input power is divided between two output ports, whereas the input port is isolated from the output ports due to the presence of the fourth port. The splitters consist of a circular graphene resonator and four graphene waveguides coupled to the resonator. These elements are placed on the two-layer dielectric substrate. The central part of the splitter is under a biasing DC magnetic field normal to the graphene layer. The surface plasmon-polariton wave in the input port excites the dipole resonance in the resonator. The splitters have the following parameters: the input power is divided between two output ports almost equally with -4.4 dB. The input port is isolated from two output ports by -15 dB. The bandwidth is 4.0% with a central frequency of 7.4 THz. The biasing DC magnetic field is 0.8 T, and the Fermi energy of graphene ϵF=0.15 eV. Changing the Fermi energy by electrostatic gating allows one to dynamically control the central frequency of the splitters.

Ultrawideband graphene three-port circulator for THz region.

The suggested circulator is formed by a concave pattern graphene junction and three waveguides symmetrically connected to it. The graphene is supported by SiO2/Si layers. The circulation behavior is based on the nonsymmetry of the graphene conductivity tensor which appears due to magnetization by a DC magnetic field applied normally to the graphene plane. The symmetrical mode propagating in the nonmagnetized graphene waveguide, is transformed in magnetized region to an edge-guided one providing the propagation from one port to another port and isolating the third port. The device characteristics depend on the physical parameters of the graphene junction, its dimensions and parameters of the substrate. We discuss a choice of these parameters to maximize the frequency band and isolation level and to minimize the losses and the applied DC magnetic field. The theoretical arguments are confirmed by full-wave computations. In an example, we demonstrate that the circulator can have the frequency band of 42% (from 2.75 THz to 4.2 THz), with the isolation higher than 17 dB and the insertion losses better that 2 dB, provided by the biasing DC magnetic field 1.5 T and the chemical potential of graphene 0.15 eV.

Limits of the Effective Medium Theory in Particle Amplified Surface Plasmon Resonance Spectroscopy Biosensors.

The resonant wave modes in monomodal and multimodal planar Surface Plasmon Resonance (SPR) sensors and their response to a bidimensional array of gold nanoparticles (AuNPs) are analyzed both theoretically and experimentally, to investigate the parameters that rule the correct nanoparticle counting in the emerging metal nanoparticle-amplified surface plasmon resonance (PA-SPR) spectroscopy. With numerical simulations based on the Finite Element Method (FEM), we evaluate the error performed in the determination of the surface density of nanoparticles σ when the Maxwell-Garnett effective medium theory is used for fast data processing of the SPR reflectivity curves upon nanoparticle detection. The deviation increases directly with the manifestations of non-negligible scattering cross-section of the single nanoparticle, dipole-dipole interactions between adjacent AuNPs and dipolar interactions with the metal substrate. Near field simulations show clearly the set-up of dipolar interactions when the dielectric thickness is smaller than 10 nm and confirm that the anomalous dispersion usually observed experimentally is due to the failure of the effective medium theories. Using citrate stabilized AuNPs with a nominal diameter of about 15 nm, we demonstrate experimentally that Dielectric Loaded Waveguides (DLWGs) can be used as accurate nanocounters in the range of surface density between 20 and 200 NP/µm², opening the way to the use of PA-SPR spectroscopy on systems mimicking the physiological cell membranes on SiO2 supports.

Two-color surface plasmon resonance nanosizer for gold nanoparticles.

We study the potentialities of a two-color Surface Plasmon Resonance (SPR) spectroscopy nanosizer by monitoring the assembling of a colloidal dispersion of citrate stabilized gold nanoparticles (AuNPs) on SiO2 surface. When the AuNPs/water composite's optical density layer is negligible and the electron mean-free path limitation is taken into account in the AuNPs' dielectric constant;s formulation, the surface density σ of the nanoparticle array and the statistical mean size of the nanoparticles can be straightly determined by using two-color SPR spectroscopy in the context of Maxwell's Garnett theory. The optical method, demonstrated experimentally for AuNPs with a nominal mean diameter of 15 nm, can, theoretically, be extended to bigger nanoparticles, based on a simple scaling relation between the extinction cross section of the single nanoparticle σext and the surface density σ. The experimental results, comparable to those obtained by AFM, transmission electron microscopy and dynamic light scattering technique, establish a novel insight on the SPR spectroscopy's potential to accurately characterize nanomaterials.

Two-dimensional photonic-crystal-based double switch-divider.

We propose and investigate a new multifunctional component, consisting of a T-junction of three waveguides in 2D photonic crystal with a square lattice. One waveguide is the input port, while the other two serve as output ports. This component can fulfil three functions: First, it can switch OFF the two output ports; second, our component can be used as a 3 dB divider of the input power; and third, it can switch ON any one of the two output ports. Changing the regime is achieved by a DC magnetic field that magnetizes a cylindrical ferrite resonator placed in the T-junction. We present an analysis of the scattering matrices of the component and calculated frequency characteristics in the low terahertz region. In the frequency band of about 1 GHz with a central frequency of f=98.46 GHz, the device has the following parameters: isolation of the output ports from the input port in the first regime is better than -30 dB, division of the input signal is about (-3.8±1.0) dB in the second regime, and isolation in the regime switch ON, where any one of the two output ports is higher than -15 dB and the insertion loss is lower than -2.0 dB.

Planar THz electromagnetic graphene pass-band filter with low polarization and angle of incidence dependencies.

We propose and analyze a graphene electromagnetic filter for the terahertz (THz) region. The filter represents a planar square array of graphene elements. A unit cell of the array is formed by two coaxial graphene rings placed on the opposite sides of a thin dielectric substrate. The two electromagnetically coupled rings resonate with dipole plasmonic modes. The rings have slightly different dimensions and consequently different yet close individual resonant frequencies. At a frequency lying between these two resonances, the currents in the two interacting rings have opposite directions. This leads to a suppression of the reflected from the array waves and consequently to a high transmission through the array. For the chemical potential of the graphene µc=0.6 eV, the calculated quality factor of this resonant mode is Q=5 at the frequency f=0.8 THz. At this frequency, the reflection coefficient of the array equals -36 dB and the transmission peak which is defined by the graphene losses is -1.8 dB. We show that the frequency position of the transmission peak can be varied in a wide range by the graphene chemical potential.

Magneto-optical resonator switches in two-dimensional photonic crystals: geometry, symmetry, scattering matrices, and two examples.

We discuss different geometrical structures of optical switches based on two-dimensional photonic crystals with hexagonal geometry of the unit cell and a magneto-optical resonator. Transition between the states on and off in these switches is achieved by an external DC magnetic field. The input and output waveguides can be front-front, side-side, or front-side coupled to the resonator and these different types of coupling can lead to different mechanisms of switching. Analysis of symmetry and scattering matrices of the switches is based on magnetic group theory. Two examples of switches with 60° and 120° bends and their characteristics are also presented.

Possible mechanisms of switching in symmetrical two-ports based on 2D photonic crystals with magneto-optical resonators.

We analyze possible mechanisms of switching in two-ports based on 2D photonic crystals (PhCs) with a magneto-optical resonator. The input and output waveguides can be side or front coupled with the resonator. The resonator operates with a dipole mode. In the switch with front coupling in the nonmagnetic state the standing dipole mode provides equal nonzero wave amplitudes in the input and output waveguides and therefore transmission of the signal from the input to output waveguides. This is the state on. The applied magnetic field normal to the plane of the PhC rotates the standing dipole mode by 90° setting the nodes in the input and output waveguides. This corresponds to the state off. On the contrary, in the switch with side coupling and nonmagnetized resonator, the standing dipole mode excited by a wave in the input waveguide has its node in the output waveguide. Therefore, the signal is reflected from the input port. This corresponds to the state off of the switch. Magnetization by a DC magnetic field produces a rotating dipole pattern in the cavity. Due to this rotating, the mode signal passes from the input port to the output one and this is the state on.

Optical component: nonreciprocal three-way divider based on magneto-optical resonator.

We suggest and analyze a new compact nonreciprocal optical component based on a magneto-optical (MO) resonator. This component fulfills simultaneously two functions, namely, equal division of the input signal between three output ports and isolation of the input port from output ones. Using group theory, we analyze the scattering matrix of this symmetrical component. Our numerical results for one of the possible schemes of the divider based on 2D photonic crystal with MO material demonstrate that, at the central frequency, the division of the signal between the three output ports is about -6.4 dB. The variation of the division levels in the output ports in this band is (-6.4±0.4) dB. For two of the output ports, the calculated bandwidth for the level -20 dB of the isolation is around 219 GHz at the wavelength 1.55 µm. For the third output port, the isolation at the central frequency is about -6 dB.

Compact optical switch based on 2D photonic crystal and magneto-optical cavity.

A compact optical switch based on a 2D photonic crystal (PhC) and a magneto-optical cavity is suggested and analyzed. The cavity is coupled to two parallel and misaligned PC waveguides and operates with dipole mode. When the cavity is nonmagnetized, the dipole mode excited by a signal in the input waveguide has a node in the output waveguide. Therefore, the input signal is reflected from the cavity. This corresponds to the state off of the switch. Normal to the plane of the PhC magnetization by a dc magnetic field produces a rotation of the dipole pattern in the cavity providing equal amplitudes of the electromagnetic fields in the input and the output waveguides. This corresponds to the state on with high transmission of the input signal. Numerical calculations show that at the 1.55 µm wavelength the device has the insertion loss -0.42 dB in the on state, the isolation -19 dB in the off state and the switch off and on ratio P(on)/P(off) about 72. The frequency band at the level of -15 dB of the resonance curve in off state is about 160 GHz.

Enhancement of Faraday and Kerr rotations in three-layer heterostructure with extraordinary optical transmission effect.

We theoretically investigate and optimize a multilayer planar structure with enhanced magneto-optical (MO) activity. The extraordinary optical transmission observed in a periodically perforated metal plate with subwavelength holes is used to produce a higher MO activity. We consider a three-layer structure that consists of a dielectric layer placed between the perforated metal and the MO layers. In this structure, we obtained the enhancement of Faraday rotation by three times and the Kerr effect by an order as compared to the published results.

Compact three-port optical two-dimensional photonic crystal-based circulator of W-format.

A three-port optical circulator of W-format based on 2D photonic crystal is presented. This circulator is more compact as compared to the known one of the traditional Y-format. The W-circulator does not have rotational symmetry. As a consequence, the frequency responses for different port excitation are slightly different, as it is shown by symmetry analysis and by numerical simulations.

Nonreciprocal optical divider based on two-dimensional photonic crystal and magneto-optical cavity.

We suggest and analyze a new nonreciprocal optical device based on two-dimensional photonic crystal and a magneto-optical cavity that simultaneously fulfills two functions: division of the input signal and isolation of the input port from two output ones. At the central frequency, the division of the signal between the output ports is -3 dB and the isolation of the input port from the output ones is about -25 dB. For the level -20 dB of this isolation, the calculated bandwidth is around 100 GHz at the wavelength 1.5 µm.