A functionality switchable meta-device: from perfect reflection to perfect absorption

In this article, we presented our proposal for a metamaterial perfect absorber (MPA) operating in the near-infrared band. Our MPA comprised a trapezoidal silver layer, a substrate layer, and an intermediate alumina layer. The transmission characteristics of MPA were analyzed by the three-dimensional (3D) finite difference time domain method. The simulation results were analyzed by equivalent circuit theory. The results show that the MPA achieved 100% absorption of transverse magnetic (TM) polarization and transverse electric (TE) polarization with resonance wavelength λ = 1492.5 nm. By varying the length of the bottom side of the trapezoid, the MPA’s resonance wavelength could be effectively tuned, and the absorption was maintained at above 99%. With the incidence angle of 0°–55°, the absorption remains above 90% at TE polarization and above 80% at 55°–70°. The absorption at the TM polarization remains above 99% when the incident angle is 0°–70°. A wide angle of incidence and good absorption were achieved. The sensitivity (S) and figure of merit were 228 nm/refractive index unit and 1378 RIU−1, respectively. The proposed absorber with excellent absorption in the range of infrared spectra has a promising application potential in fields such as energy harvesting and infrared sensors.

The laser emission of transverse electric (TE) and transverse magnetic (TM) polarization, in alternating successive spectral ranges, that can occur when external grating diode lasers are wavelength tuned, is theoretically and experimentally investigated. The modulated threshold gain curves of the external cavity for the two polarizations can be made to intersect with each other if a grating is used in such a way as to practically eliminate the difference between the laser diode losses and confinement factors for TE and TM polarization. The result is a sequence of wavelength ranges where the threshold gain is alternately lower for the TE resp. TM polarization. Within each range, a wavelength tunable single mode laser emission is obtained. The phenomenon is described, taking into account the characteristics of the key components of the external cavity, i.e., laser diode structure, antireflective (AR) coating, and grating efficiency. Design tools are proposed for the characteristics of the components necessary for the development of such an external cavity. The theoretical and design concepts are experimentally confirmed in a number of external cavities, using diode lasers emitting around 1.5 /spl mu/m, with different types of antireflective coatings, as well as different grating configurations. >

Frequency Selective Surfaces (FSSs) are engineered periodic structures that function as spatial filters, enabling the selective transmission, reflection, or rejection of electromagnetic (EM) waves. These structures are widely utilized in advanced RF and microwave systems for applications such as filtering, electromagnetic shielding, and stealth technology. In this manuscript, we propose an angularly stable and polarization-insensitive band-stop Frequency Selective Surface (FSS) optimized for applications in the lower Ka-band. The proposed FSS is constructed from a periodic two-dimensional array of compact unit cells, each with dimensions of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5.5 ~\text{mm} \times 5.5 ~\text{mm}$</tex>. This compact configuration supports seamless integration into high-density millimeter-wave (mmWave) systems. Each unit cell incorporates a concentric dual-ring geometry embedded with orthogonally intersecting cross-slots. This unique configuration enhances the EM wave rejection capability of the surface by inducing multiple resonances. As a result, the FSS achieves a wideband stopband response ranging from 20 GHz to 28.5 GHz, corresponding to a notable fractional bandwidth (FBW) of approximately 34%. The dual-ring structure promotes resonance coupling, while the cross-slot inclusions facilitate effective suppression of incident waves, regardless of polarization orientation. The concentric design further contributes to polarization insensitivity, ensuring consistent stopband performance for both vertically and horizontally polarized waves. In addition, the symmetrical layout enhances angular stability, a critical requirement for real-world deployment. To verify the robustness of the design, full-wave electromagnetic simulations were conducted under varying angles of incidence for both Transverse Electric (TE) and Transverse Magnetic (TM) polarizations. The results reveal negligible shifts in the resonance frequency and stable stopband attenuation across angles up to 60°, confirming the angular resilience of the structure. With its compact form factor, wideband rejection characteristics, and strong angular and polarization stability, the proposed FSS is highly suitable for advanced mmWave applications, including EM interference suppression, co-site interference mitigation, and radar cross-section (RCS) reduction in defense, aerospace, and next-generation communication systems.

Nowadays, with the extensive use of mobile phones, the Electromagnetic (EM) radiation penetration from Radio Frequencies (RFs), particularly into the human head, is an issue that needs resolving. Some serious biological hazards occur inside the human body due to RF radiation accumulation. The RF radiation can be minimized by embodying shielding and coating materials on the front side of the mobile handset. The novelty of the proposed work is the use of mathematical analysis in calculating the Specific Absorption Rate (SAR) absorbed by planar four-layer adult and child head models when exposed to mobile smartphone RF radiation. The variation of SAR with the Angle of Incident (AoI) of the EM wave considers Transverse Electric (TE) and Transverse Magnetic (TM) Polarization. The SAR absorption alteration with the AoI of the EM wave is calculated with the help of the shielding effectiveness parameter of the external Polyethylene Terephthalate (PET) shield coated with conductive copper (Cu) mesh, forming a laminated shield using the methodology of the transmission line method. Furthermore, the SAR variation with AoI for both human head models is calculated theoretically at Sub-6 GHz mobile frequencies of 4.5GHz and 3.6GHz. SAR of 7.41e-12 W/kg and 4.41e-11 W/kg is achieved theoretically for adult and child head models respectively, at 89° TE polarization at 4.5GHz.

We investigate dynamic effective anisotropy in photonic crystals (PCs) through a combination of an effective medium theory, which is a high-frequency homogenization (HFH) method explicitly developed to operate for short waves, as well as through numerical simulations and microwave experiments. The HFH yields accurate predictions of the effective anisotropic properties of periodic structures when the wavelength is of comparable order to the pitch of the array; specifically, we investigate a square array of pitch 2 cm consisting of dielectric rods of radius 0.5 cm and refractive index n=6√ within an air matrix. This behaves as an effective medium, with strong artificial anisotropy, at a frequency corresponding to a flat band emerging from a Dirac-like point in transverse magnetic (TM) polarization. At this frequency, highly directive emission is predicted for an electric source placed inside this PC, and this artificial anisotropy can be shown to coincide with a change of character of the underlying effective equation from isotropic to unidirective, with coefficients of markedly different magnitudes appearing in the effective equation tensor. In transverse electric (TE) polarization, we note a second radical change of character of the underlying effective equation, this time from elliptic to hyperbolic, near a frequency at which a saddle point occurs in the corresponding dispersion curves. Delicate microwave experiments are performed in both polarizations for such a PC consisting of 80 rods, and we demonstrate that a directive emission in the form of a + (respectively, an X) is indeed seen experimentally at the predicted frequency 9.5 GHz in TM polarization (respectively, 5.9 GHz in TE polarization). These are clearly dynamic effects since in the quasistatic regime the PC just behaves as an isotropic medium.

Thermal radiation control has attracted increasing attention in a wide range of field, including infrared detection, radiative cooling, thermal management, and thermal camouflage. Previously reported thermal emitters for thermal camouflage presented disadvantages of lacking either tunability or thermal stability. In this paper, we propose a tunable thermal emitter consisting of metal-insulator-metal (MIM) plasmonic metamaterial based on phase-change material Ge2Sb2Te5 (GST) to realize tunable control of thermal radiation in wavelength ranges from 3 μm to 14 μm. Meanwhile, the proposed thermal emitter possesses near unity emissivity at the wavelength of 6.3 μm to increase radiation heat dissipation, maintaining the thermal stability of the system. The underlying mechanism relies on fundamental magnetic resonance and the interaction between the high-order magnetic resonance and anti-reflection resonance. When the environmental background is blackbody, the tunable emitter maintains signal reduction rates greater than 80% in middle-IR and longer-IR regions from 450 K to 800 K and from room temperature to 800 K, respectively. The dependences of thermal camouflage on crystallization fraction of GST, incident angles and polarization angles have been investigated in detail. In addition, the thermal emitter can continuously realize thermal camouflage for various background temperatures and environmental background in atmospheric window in the range of 3–5 μm.

The low light extraction efficiency (LEE) is one of the major factors hindering the application of AlGaN based deep ultraviolet (DUV) light-emitting diodes (LEDs). Here we investigate the LEE of AlGaN based nanowire (NW) DUV LEDs emitting at 275 nm for bare NW, NW integrated with aluminum (Al) bowtie antenna array, and NW with passivation layer under transverse-electric (TE) and transverse-magnetic (TM) polarization. It is observed that by integrating plasmonic Al bowtie antenna array with AlGaN based NW, the LEE up to 83% and 74% can be achieved under TE and TM polarization. In addition, the effect of the three different passivation layer SiO2, SiNx and AlN on the LEE of AlGaN based NW is also analysed, the results suggests that SiO2, which has smaller refractive index than NW core, could extract more photons from the NW and lead to large enhancement of LEE. For SiNx and AlN passivation layer, which has refractive index similar to the NW core, have strong coupling with the NW core, when the thickness of passivation layer satisfy resonance coupling conditions, the LEE could be achieved more than 80% for both TE and TM polarization. These integrated NW/antenna array and NW with passivation layer system can provide guidelines for designing other nano-photonic devices.

An ultrathin single-layer metasurface manifesting both linear cross-polarization conversion (CPC) and linear-to-circular polarization (LP-to-CP) conversion in X-band is presented in this research. The designed metasurface acts as a multifunctional metasurface achieving CPC over a fractional bandwidth of 31.6% (8–11 GHz) with more than 95% efficiency while linear-to-circular polarization conversion is realized over two frequency bands from 7.5–7.7 GHz and 11.5–11.9 GHz. Moreover, the overall optimized structure of the unit cell results in a stable polarization transformation against changes in the incidence angle up to 45° both for transverse-electric (TE) and transverse-magnetic (TM) polarizations. The proposed metasurface with simple structure, compact size, angular stability and multifunctional capability qualifies for many applications in communication and polarization manipulating devices.

Retroreflectors are a critical component in numerous applications, such as target tracking and lifesaving, as they scatter the incident electromagnetic (EM) wave back to the incoming direction. Metasurfaces are utilized to planarize retroreflectors due to their compelling advantages of low profile, lightweight, and the capacity to flexibly customize the wavefront of the electromagnetic wave. Nevertheless, the current metasurface retroreflectors are still constrained by their wide-angle or specific single-polarized excitation capabilities. A reconfigurable retroreflector that is both wide-angle and arbitrarily polarized is proposed in this paper. The retroreflector is composed of an active anisotropic metasurface with a thickness of only 2.5 mm or 0.034 operating wavelength. By adjusting the bias voltages of loaded varactors, the reflective phases of constituent meta-atoms under orthogonal dual-polarization, transverse electric (TE) and transverse magnetic (TM) modes can be dynamically and continuously controlled, ensuring a wide phase coverage until the incident angle reaches <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$50^{\circ}$</tex>. As a result, the coding metasurface is capable of reflecting incident waves backwards for arbitrary polarization, which can be decomposed into TE and TM polarization. This results in efficient dynamic retroreflection over a wide continuous incident angle range from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-50^{\circ}$</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$50^{\circ}$</tex>.

3D simulation of NSOM operation by a nanosphere as a microscope probe using Boundary Element Method

This paper presents novel designs and simulation results of electro-optic modulators in integrated silicon photonics platforms. The electro-optic modulators described in this paper are based on the modulation of the photonic bandgap in a silicon photonic crystal slab waveguide, consisting of a phase change material (germanium selenide) sandwiched in between silicon layers. Three-dimensional finite difference time domain (FDTD) modeling is employed to simulate two configurations of electro-optic modulators based on mode-gap shifting in photonic crystal slab waveguides—one that is designed for operation with the transverse electric (TE) polarization of light and the other for the transverse magnetic (TM) polarization. When the electric field is applied across the germanium selenide layer surrounded by doped silicon layers, there is a change of phase of the germanium selenide layer. A large refractive index contrast between the two phases of the germanium selenide layer—i.e., between the amorphous phase and the crystalline phase—on application of the electric field is responsible for shifting of the mode gap in the dispersion characteristics of the PhC slab waveguide. Our results show excellent performance of the electro-optic modulators in terms of the high On/Off extinction ratio ( &gt; 37 d B in the TE configuration and &gt; 28 d B in the TM configuration), broadband switching capability ( ∼ 100 n m ), low insertion loss in the On state ( &lt; 2 d B ), high range of tunability (by changing the lattice constant and radius of air holes), and low operating voltage ( &lt; 4 V ). The proposed devices can be used as electro-optic modulators in future nanoscale photonic integrated circuits.

We present an analysis of the band structure, the equifrequency surfaces, and the density of states (DOS) for the transverse magnetic (TM) polarization mode of the dielectric superlattice, modeled by means of Dirac-delta functions. This complements a recent article [Phys. Rev E 59, 3624 (1999)] that analyzes the case of transverse electric (TE) polarization. Unfortunately, for this simple model, there is no manifestation of the Brewster effect in the band structure for the TM modes. For large values of the frequency or the grating strength, the equifrequency surfaces essentially degenerate into a set of concentric, hollow, and narrow cylinders centered on the superlattice axis. The DOS is enhanced relative to free space for any frequency and it exhibits discontinuities in the slope at the band edges. These results are relevant to the spontaneous emission by an atom or to dipole radiation in one-dimensional periodic structures. The differences between TE and TM modes are discussed. We take the opportunity to correct an error in the DOS calculation for TE polarization in the article referred above.

This paper presents the design, fabrication, and characterization of a broadband metamaterial absorber capable of optical switching between high-and low-absorbing states. The proposed metasurface exhibits a wide absorption bandwidth ranging from 13 to 17.3 GHz, maintaining a high absorption for wide incidence angles and for both transverse electric (TE) and transverse magnetic (TM) polarizations, demonstrating the polarization-insensitive performance. The metasurface is composed of periodically arranged unit cells integrated with photoresistors that enable tunable electrical resistance under optical illumination. When exposed to light, the photoresistors reduce their resistance, allowing the structure to achieve a high absorption. In the absence of illumination, the increased resistance suppresses the absorption response, effectively switching the metasurface to be a reflective state. This optically controllable behavior provides a simple and efficient approach for realizing dynamically reconfigurable metamaterial absorbers, offering potential applications in adaptive stealth technology, electromagnetic shielding, and tunable sensing systems.

CPW-fed dual-element metamaterial inspired multiband antenna using simple FSS for gain enhancement

Lifetimes are estimated for the two-dimensional resonant optical modes of a dielectric cavity with an equilateral triangular cross section, that are approximately confined by total internal reflection. Exact solutions of a two-dimensional scalar wave equation for triangular geometry with Dirichlet boundary conditions are used to describe approximately the vector fields of the possible transverse electric (TE) and transverse magnetic (TM) modes. Only two-dimensional electromagnetic solutions are considered here, where there is no propagation vector perpendicular to the plane of the triangle (kz = 0). The field properties just inside and outside the cavity boundary are shown to be significantly different for TE and TM field polarizations, the two cases having different dependences on the index mismatch with the exterior. For a given mode specified by particular quantum numbers, TE polarization leads to longer lifetime than TM polarization at high index mismatch, assuming that escape of evanescent boundary waves at the corners is the primary decay process.