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

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.

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. >

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.

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.

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.

Polarization-sensitive terahertz reflective multi-channel beam separation by cascaded configuration

We investigate the localization properties of surface waves created at the interface between a truncated 1D photonic crystal and homogeneous medium in the presence of a chiral cap layer using the transfer matrix method. The numerical results show that the interface can support surface waves with both transverse electric and transverse magnetic polarizations. We demonstrate that the surface waves can be engineered by varying the chirality parameter of the cap layer, which plays an important role in controlling and localization of surface states. It is shown that the effect of a chirality parameter on surface waves with transverse electric polarization is more remarkable compared with surface waves with transverse magnetic polarization. In addition, the effect of thickness and material type of the cap layer and the termination layer of a photonic crystal on surface waves is investigated. The results show that these parameters have considerable effect on modification of surface states. These findings may open new opportunities in the design of cavity-based devices such as high-Q sensors and perfect absorbers.

Broadband, all-angle negative refraction (AANR) effects and imaging with subwavelength resolution have been analyzed in a honeycomb lattice of silicon rods in air background, as well as in the inverse structure of air holes in a silicon wafer. In the rod lattice, it was possible to obtain AANR effects for transverse-magnetic (TM) polarization in the second band (TM2) with a bandwidth as large as ∼23% after optimizing the ratio of feature size to the lattice constant as 0.44 and a super-lensing effect with a binary coherent source resolution of 0.68λ. On the other hand, the hole lattice shows AANR for both TM2 (bandwidth of 14%) and TE2 (bandwidth of 15.3%) bands with an additional advantage of polarization-independent AANR of 5% bandwidth. The hole lattice also shows a super-lensing behavior, which helps in imaging with a subwavelength resolution of 0.19λ for a single-point source and 0.80λ with a binary source for TM polarization while it is 0.48λ and 0.97λ, respectively, for transverse-electric (TE) polarization. The conditions for perfect imaging have been studied, and the phenomenon of dual negative refraction for both TE and TM polarization is also discussed.

Polarization-independent nonreciprocal radiation in polar dielectric-Weyl semimetal planar heterostructure

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.

Improvement of direct sampling method in transverse electric polarization

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.

We modeled the effects of surface roughness (SR) on a near-infrared guided-mode resonance (GMR) biosensor. A power spectral density function was used to describe the SR with spatial autocorrelation integration through a rigorous coupled-wave analysis (RCWA), and a peak wavelength value (PWV) shift in reflectance was observed under transverse electric (TE) and transverse magnetic (TM) polarization. Sub-nanometer SR caused significant PWV shifts, leading to invalid spectral recognition. In addition to random roughness, we studied the PWV reflectance shift for SR models such as sinusoidal, rectangular, and effective medium types. All SR models showed a larger PWV shift for TM polarization than for TE polarization. Owing to SR, PWV reflectance decreased abruptly for TM polarization at small angular deviations from normal incidence. However, the symmetrically splitting of the reflectance spectra for TE polarization implied a high angular tolerance to fabrication errors. Compared to vertical sidewall roughness in our prior work, the SR is much more deleterious for GMR biosensing and should be appropriately controlled.

When an oscillating foil is placed in a uniform and steady flow, due to its heave and pitching motion, it produce a downstream flow at trailing edge, which leads to a forward force or thrust. Many fishes or insects use this phenomenon to move in air/water. It has proved that the heave and pitch frequency is very important parameter to produce thrust which should be chosen sharply. In this paper a 2D airfoil with oscillating movement was simulated and power/thrust coefficient was calculated. An unsteady numerical model based on boundary element method (BEM) was developed to simulate the flow around the foil. Two different conditions (with different angles of attack, =5, 10) were chosen for the flapping airfoil. For each condition, the foil was oscillated with 8 different frequencies to draw the thrust and power coefficients. The results were compared with experimental data and a finite volume-based solver. It could be seen that however FVM is more precisions to simulate such a hydrodynamics cases but for lifting bodies objects such as airfoil, BEM offers reasonable accuracy. In addition, one of the best advantages of BEM is its speed in calculation. Considering BEM just used 100 boundary cells in simulations, it is about 30 times faster than FVM, which means BEM could be used in rough estimations and preliminary calculations.

Resonant Bessel-beam launchers (BBLs)are radiating devices constituted by a cylindrical metallic cavity with a partially reflecting sheet (PRS) on top. Millimeter-wave resonant BBLs typically exhibit transverse magnetic (TM) polarization due to the use of coaxial probes as feeders and homogenized metasurfaces as PRS. Launchers showing either a purely transverse electric (TE) or a hybrid (quasi-TE) polarization have recently been proposed for realizing wireless power transfer (WPT) links in the radiative near-field region at millimeter waves. The former are obtained by means of a radial slot array as a feeder and a homogenized metasurface as a PRS. The latter are obtained by using a loop antenna as a feeder and an annular strip grating in the homogenization limit as radiating aperture. In this work, based on an original semi-analytical model, such a metasurface is demonstrated to show a dichroic behavior. This interpretation explains the improvement in terms of polarization purity with respect to more nondichroic conventional homogenized metasurfaces. The behavior of the annular strip grating under a pure TM polarization is tested with a coaxial feeder, whereas its behavior under a pure TE polarization is tested by means of the radial slot array feeder. Results confirm the validity of the proposed analysis, which is finally exploited to evaluate the WPT performance.