TM-polarized Waves Research Articles

Terahertz polarization beam splitter based on photonic crystal and multimode interference

We design a compact terahertz (THz) polarization beam splitter. Both plane wave expansion method and finite-difference time-domain method are used to calculate and analyze the characteristics of the proposed device. The designed polarization beam splitter can split TE-polarized and TM-polarized THz waves into different propagation directions. The simulation results show that the extinction ratios are larger than 18.36 dB for TE polarization and 13.35 dB for TM polarization in the frequency range from 1.86 THz to 1.91 THz, respectively. The designed polarization beam splitter has the advantages of small size and compact structure with a total size of 4.825 mm×0.400 mm.

Electromagnetic energy within coated cylinders at oblique incidence and applications to graphene coatings.

We address electromagnetic (EM) wave scattering by an infinite coated cylinder at an arbitrary incidence angle. The time-averaged EM energy stored inside the core-shell cylinder is analytically calculated for TM- and TE-polarized incident plane waves. An analytical expression relating the internal energy to the absorption cross section is derived. As an application, the EM energy inside dielectric cylinders coated with isotropic graphene layers epitaxially grown on silicon carbide (SiC) is studied. We find that off-resonance field enhancement occurs in graphene SiC microshells for TM-polarized terahertz waves, a phenomenon that can be explained in terms of Fano resonances.

Multilayer graphene waveguides

We study dispersion properties of TM-polarized electromagnetic waves guided by a multilayer graphene metamaterial. We demonstrate that both dispersion and localization of the guided modes can be efficiently controlled by changing the number of layers in the structure. Remarkably, we find that in the long wavelength limit, the dispersion of the fundamental mode of the N-layer graphene structure coincides with the dispersion of a plasmon mode supported by a single graphene layer, but with N times larger conductivity. We also compare our exact dispersion relations with the results provided by the effective media model.

High-efficiency fused-silica reflection grism

A fused-silica reflection grism (combination of grating and prism) based on the phenomenon of total internal reflection (TIR), and used in the -1st order, is designed and fabricated. The grism is etched directly into the fused-silica prism, which greatly facilitates the use of the TIR grating as no other angle coupling devices are involved. The grating profile is optimized by the use of the rigorous coupled-wave analysis method. Diffraction efficiency of larger than 99% at a wavelength of 980 nm for TM-polarized waves can be theoretically obtained. Two-beam interference lithography and inductively coupled plasma etching techniques are used to manufacture such grism. Diffraction efficiencies of larger than 95% are experimentally demonstrated.

Polarization-resolved evanescent wave scattering from gold-coated tilted fiber gratings

The scatterings of TE- and TM-polarized evanescent wave on the surface of a tilted fiber Bragg grating (TFBG) with a 50 nm thick gold coating were investigated experimentally by observing radiation patterns from discontinuities in the coating. The scattering intensity for TM-polarized light is larger than for TE light when the evanescent wave propagates from the coating towards the discontinuity. The opposite occurs for light propagating from an uncoated section towards the coating edge. However in the latter case the scattering is much weaker. These results confirm that cladding modes with TE and TM polarization can be excited selectively with a TFBG, and that they scatter light differentially at discontinuities. These results are used to propose a simple polarimeter design based on total scattered light intensity monitoring.

A New Efficient Technique for Transient Scattering of Conducting Cylinders Illuminated by a TM-polarized Plane Wave

In this letter, the transient analysis of infinite conducting cylinders in free space is obtained by solving the time-domain electric field integral equation (TD-EFIE) via a time-domain method of moments (TD-MoM). The PEC cylinders with arbitrary cross section are illuminated by a transverse magnetic (TM)-polarized Gaussian plane wave. The temporal basis function is triangular, and the spatial basis function is rectangular. The two-dimensional Green's function is employed to develop the TD-MoM formulation. Two different approximations are used to calculate the self-coupling terms of the impedance matrix. It is shown that the time convolution integrals and self-terms involved in the formulation can be evaluated analytically to increase the accuracy and the stability of the proposed technique.

Engineering of phase matching for mid-infrared coherent anti-Stokes Raman wavelength conversion with orthogonally polarized pump and Stokes waves in silicon-on-sapphire waveguides

The conversion efficiency of mid-infrared wavelength conversion based on coherent anti-Stokes Raman scattering with TE-polarized pump and TM-polarized Stokes waves is theoretically investigated in silicon-on-sapphire (SOS) waveguides. The peak conversion efficiency of -10 dB is obtained when the linear propagation loss is 1 dB/cm at Δk=0; however, it is reduced to -13.6 dB when the linear propagation loss is 2 dB/cm. The phase matching for wavelength conversion with orthogonally polarized pump and Stokes waves can be realized by engineering the birefringence in SOS waveguides, because proper phase mismatch induced by birefringence together with material dispersion-induced phase mismatch can counteract the large phase mismatch induced by waveguide dispersion. Moreover, compared with the phase matching for identically polarized pump and Stokes waves, the phase matching for orthogonally polarized pump and Stokes waves can be realized in a SOS waveguide with much smaller cross section, which reduces the power requirement for optical systems.

Shape identification of anisotropic diffraction gratings for TM-polarized electromagnetic waves

This paper concerns the shape identification problem of anisotropic periodic structures which are known as diffraction gratings. We study the so-called Factorization method as a tool for reconstructing the anisotropic periodic media from measured spectral data involving scattered electromagnetic waves in TM modes. This sampling method provides a simple criterion to compute a picture of shape of diffraction gratings in a rapid way. We propose a rigorous analysis for the method as well as numerical experiments to examine its performance.

Silicon Based One-Dimensional Photonic Crystal as a TM-Mode Filter

The effect of temperature and angle of incidence on the transmission spectra of a one dimensional photonic crystal for TE and TM polarizations has been studied. For a range of angle of incidence from 0° to 60°, the proposed structure works as an omni-directional reflector. For other angles of incidence more than 60°, the structure works as a selective TM-polarization filter i.e. a filter which completely blocks TE-polarized waves but allows certain frequencies of TM-polarized waves. At an angle of incidence of 89°, the structure works as a multi channel optical filter without introducing any defect in the geometry. Also, this multi-channel filter can work as a tunable filter by varying the temperature of the structure and the number of layers in the geometry. Only for two bi-layers the proposed structure works as a single channel filter. Therefore, such types of structure are easier to fabricate and may be cost effective as compared to defect one-dimensional photonic crystals.

Research of SPR Phase Detection for Measuring Ultra Thin Metal Film

SPR phase detection method is adopted in this paper to measure the thickness of nanometer scale metal film. It provided a novel means to calculate the value of thickness through figuring up the relative offset of interference fringes with the TM-polarized and TE-polarized wave reflected from the prism which is covered by a film-substrate step. Simulation results are presented out to illustrate the relations between phase variation and metal film thickness. This method is succeeded in measuring the target metal film with nominal thickness 50 nm and the average measurement value is 50.02 nm. The standard deviation is 0.41 nm and the maximum repeatability error is 0.90 nm.

Wide-Band Optical Mode Converters for Coupling Between Fibers and Silicon Photonic Wires With Large Misalignment Tolerance

A wideband optical mode converter is presented for coupling between fibers and sub-wavelength silicon photonic wires with large misalignment tolerance. This converter consists of a three-dimensional (3-D) SU-8 taper and silicon inverted nanotapers, which are cascaded and show coupling losses of 2.8 dB and 4.1 dB for the TE- and TM-polarized waves launched from a cleaved single mode fiber. The fiber misalignment tolerance for 3-dB transmittance drop is estimated to be ±3 μm.

A Quasi-Yagi Antenna Backed by a Jerusalem Cross Frequency Selective Surface

A quasi-Yagi antenna is developed to operate at 2.4 GHz (ISM band) presenting a low profile and off-axis radiation when packaged over a metal ground plane. The off-axis radiation is realized by incorporating a Jerusalem cross frequency selective surface (JC-FSS) as the ground plane for the antenna. A JC-FSS is preferred because of its frequency stability in the operating band for a large angular spectrum (≈70°) of TE- and TM-polarized incident waves. In this research, the substrate of the antenna flush-mounted on top of the FSS is added to the JC-FSS model and allows for a smaller cell grid. The prepared quasi-Yagi antenna over the JC-FSS offered 260 MHz of functional bandwidth and 54° of beam tilt towards the end-fire direction. To the best of the authors’ knowledge this is the first instance that these two structures are combined for off-axis radiation. Additionally, to support the preferred use of the JC-FSS, the quasi-Yagi is backed by a square patch (SP) FSS for comparison purposes.

Electric field enhancement inɛ-near-zero slabs under TM-polarized oblique incidence

We investigate local field enhancement phenomena in subwavelength, {\epsilon}-near-zero (ENZ) slabs that do not exploit Fabry-P\'erot resonances. In particular, we study the linear response of engineered metamaterial slabs of finite thickness based on plasmonic nanoshells that show an ENZ band in the visible range, and naturally occurring materials (e.g., SiO2) that also display ENZ properties, under oblique, TM-polarized plane wave incidence. We then introduce active gain material in engineered metamaterial slabs that adds peculiar spectral and angular features to transmission, reflection, and absorption properties, and leads to a further local field enhancement. These findings are supported by two theoretical studies: First, a simple interface between two semi-infinite media, namely free space and a generic ENZ medium; then, an ENZ slab of finite thickness, with the aim of understanding the system's behavior when varying the ENZ properties as well as the incident angle. For either case we report three distinct physical conditions for which we explain spectral and angular features that might result in strong field enhancement. The gain-assisted metamaterial implementation has the potential of triggering and enhancing low-threshold nonlinear phenomena thanks to the large local fields found at specific frequency and angular bands.

Hyperbolic secant lens with a slit for subwavelength light focusing

A modeling by FDTD method shown, that binary photonic crystal secant hyperbolic micro lens made of Si (n0 = 3,47), with size of 2×5 mm with a slit width of 50 nm and length of 250 nm focuses TM-polarized flat wave in a focal spot near exit surface of lens. Focal spot FWHM size is λ/23, diffractive efficiency of the lens is 44%, and the focal intensity is 10 times more then the intensity of the incident light.

New Absorbing Boundary Conditions and Analytical Model for Multilayered Mushroom-Type Metamaterials: Applications to Wideband Absorbers

An analytical model is presented for the analysis of multilayer wire media loaded with 2-D arrays of thin material terminations, characterized in general by a complex surface conductivity. This includes the cases of resistive, thin metal, or graphene patches and impedance ground planes. The model is based on the nonlocal homogenization of the wire media with additional boundary conditions (ABCs) at the connection of thin (resistive) material. Based on charge conservation, new ABCs are derived for the interface of two uniaxial wire mediums with thin imperfect conductors at the junction. To illustrate the application of the analytical model and to validate the new ABCs, we characterize the reflection properties of multilayer absorbing structures. It is shown that in such configurations the presence of vias results in the enhancement of the absorption bandwidth and an improvement in the absorptivity performance for increasing angles of an obliquely incident TM-polarized plane wave. The results obtained using the analytical model are validated against full-wave numerical simulations.

Focusing Properties of the Visible Light Wave through Plasmonic Lenses with Subwavelength Chirped Slits

Focusing properties of a new kind of plasmonic lenses are investigated in the visible wavelength range through a subwavelength metallic chirped slit arrays which have the same depth but chirped widths. The chirped widths of slits are like a piece-wise-linear distribution which will be approximated by linearly increasing the width of a subwavelength feature and can build up a required phase front for focusing. We analyzed the focusing characteristics of different metallic lenses (silver and gold, respectively) with chirped widths that are obtained by generalizing the relevant phase delay for TE- and TM-polarized incident waves, for different f-numbers of lenses and for different material thickness, respectively. Meanwhile, the comparison of the metallic and dielectric lenses is also presented. The results of calculations show that, the metallic lenses are more sensitive to the polarization of incidence wave than that of dielectric lenses, and can get narrower full-width half-maximum (FWHM) beam width than that of dielectric lenses for TM-polarized incident waves, respectively. No matter which f-number we choose, the FWHM of dielectric lenses are higher than the plasmonic lenses, and the plasmonic lenses can get a higher focal resolution than dielectric lenses do. This kind of plasmonic lenses should have a good potential for applications in photonic and plasmonic integrated devices, sensing, and nano-optical manipulations, etc.

Compact terahertz wave polarization beam splitter based on self-collimating photonic crystals

In this article, a compact terahertz wave polarization beam splitter is proposed for terahertz systems. The design is based on the self-collimation effect in two-dimensional photonic crystals. A ring-type photonic crystal which consists of air rings in silicon background is employed in the design to get a broader bandwidth. The self-collimation for both TE and TM mode is achieved by manipulating both the outer and the inner radii of the rings. A defect region is embedded into the photonic crystal to separate the TE- and TM-polarized wave. The proposed polarization beam splitter has a very compact size of 659μm×659μm. The plane wave expansion method is used to obtain the equi-frequency contours and the band diagrams. The finite-difference time-domain method is used to simulate the light propagation in the structure. The polarization extinction ratio of about 36.23dB for TM-polarized wave and 35.98dB for TE-polarized wave are reported. The corresponding normalized transmissions at TM port and TE port are of about 92.87% and 93.15%, respectively. The impact of the incident angle on the polarization extinction ratio and normalized transmission are also studied.

Active Focusing of Light in Plasmonic Lens via Kerr Effect

We numerically demonstrate the performance of a plasmonic lens composed of an array of nanoslits perforated on thin metallic film with slanted cuts on the output surface. Embedding Kerr nonlinear material in nanoslits is employed to modulate the output beam. A two dimensional nonlinear-dispersive finite-difference time-domain (2D N-D-FDTD) method is utilized. The performance parameters of the proposed lens such as focal length, full-width half-maximum, depth of focus and the efficiency of focusing are investigated. The structure is illuminated by a TM-polarized plane wave and a Gaussian beam. The effect of the beam waist of the Gaussian beam and the incident light intensity on the focusing effect is explored. An exact formula is proposed to derive electric field E from electric flux density D in a Kerr-Dispersive medium. Surface plasmon (SPs) modes and Fabry-Perot (F-P) resonances are used to explain the physical origin of the light focusing phenomenon. Focused ion beam milling can be implemented to fabricate the proposed lens. It can find valuable potential applications in integrated optics and for tuning purposes.

Green function for hyperbolic media

We revisit the problem of the electromagnetic Green function for homogeneous hyperbolic media, where longitudinal and transverse components of the dielectric permittivity tensor have different signs. We analyze the dipole emission patterns for both dipole orientations with respect to the symmetry axis and for different signs of dielectric constants, and show that the emission pattern is highly anisotropic and has a characteristic crosslike shape: the waves are propagating within a certain cone and are evanescent outside this cone. We demonstrate the coexistence of the conelike pattern due to emission of the extraordinary TM-polarized waves and elliptical pattern due to emission of ordinary TE-polarized waves. We find a singular complex term in the Green function, proportional to the $\ensuremath{\delta}$ function and governing the photonic density of states and Purcell effect in hyperbolic media.

Inverse Optimization of Plasmonic and Antireflective Grating in Thin Film PV Cells

This work addresses inverse optimization of three dimensional front and back surface texture grating specifications, for the purpose of shaping the absorptivity spectrum of silicon thin film cells in targeted ways. Periodic plasmonic gratings with dimensions comparable or less than the incident light wavelength are known to enhance light absorption. We consider surface patterning of amorphous silicon (a-Si) thin films using front and/or back metallic nanostrips and ITO coatings, and show that wideband enhancement in unpolarized absorptivity spectrum can be achieved when back reflectors are used. The overall short circuit current enhancement using such structures is significant and can be as high as 97%. For TM-polarized wave it can be even higher as reported in previous work. In this work however, we focus on the optimization for the more realistic unpolarized radiation which is of significantly higher complexity. In addition, optimization is done with respect to two objective functions independently: spectral absorptivity and gain-bandwidth product of the absorptivity spectrum.