Dielectric Pillars Research Articles

Waveguiding characteristics of a metal-clad dielectric grating for terahertz applications

Dielectric gratings (DGs) have recently been introduced as a new platform for guiding dielectric-based geometrically-induced surface plasmons (SPs) called spoof SPs (SSPs). In this paper, we report on the modal analysis of a metal-clad DG waveguide and compare its performance with other counterparts including the dielectric image guide (IG) and the metal grating (MG) waveguide. The proposed DG waveguide is made up of a periodic array of high-resistivity silicon (HR-Si) pillars on a metal plate with a metal cladding at a proper distance above the structure. By modal analysis of 2D and 3D structures, we show that the presence of the metal cladding enhances the modal field confinement without sacrificing the bandwidth. In addition, modal characteristics of the structure at hand in the presence of a thin substrate are studied. It is found that the lateral field confinement and bending loss can be impaired in the presence of a substrate. We also study the shape of dielectric pillars and show that nearly the same performance can be achieved by cubic and cylindrical pillars. Finally, some waveguide devices such as a bend, a directional coupler, and a ring resonator are also implemented and studied to assess the applicability of the proposed metal-clad DG waveguide.

Nanoscale Noncoplanar Beam Splitters With Tunable Split Ratio

Beam splitters, especially those with tunable split ratios, play significant role in interferometers, spectrometers, and communication systems. To this end, we proposed a polarization-controllable beam splitter based on dielectric metasurface composed of an array of dielectric pillars. The dielectric metasurface presents phase gradients along two orthogonal directions, say, x- and y- axis, with each phase gradient applicable to one of two orthogonal linear polarizations. Due to the orthogonality of the dual phase gradients in both position and polarization, an elliptically polarized incident beam can be diffracted into two refracted beams, one lies within xoz plane and the other in yoz plane. More importantly, the split ratio of the two refracted beams is continuously tunable by changing the ellipticity of incident beam. Under extreme case, x- or y- polarized incident beam is refracted into one beam, but in different planes for different kind of linear polarization. Besides, the refracted angle of each refracted beam in respective refracted plane can be easily set by adjusting the phase difference of adjacent rows or columns of metasurface. With same method, a polarization-controllable dual-wavelength beam splitter is also achieved on the basis of phase-gradient metasurface. Such a noncoplanar beam splitter with tunable split ratios may play significant role in novel miniature optical devices and systems.

Quasi-triply-degenerate states and zero refractive index in two-dimensional all-dielectric photonic crystals.

We introduce the concept of a quasi-triply-degenerate state (QTDS) and demonstrate its relation to an effective zero refractive index (ZRI) in a two-dimensional (2D) square lattice photonic crystal (PC) of all dielectric pillars. A QTDS is characterized by a triple band structure (TBS), wherein two of the bands manifest a linear dispersion around the Γ-point, i.e. a Dirac-like cone, while the third is a flat zero refractive index (ZRI) band with a frequency that is degenerate with one of the other bands. Significantly, we find that while triple degeneracy of the bands is not observed, the three bands approach one another so close that the observable properties of PCs adapted to the QTDS frequency perform as expected of a ZRI material. We closely examine the ZRI band at the Γ-point and show that by varying the PC material and structure parameters, the ZRI band behavior extends over a wide range of dielectric refractive indices enabling materials made with polymeric constituents. Moreover, the ZRI characteristics are robust and tolerant over a range of frequencies. Furthermore, the computational screening we employ to identify QTDS parameters enables the rational design of low-loss 2D ZRI materials for a broad range of photonic applications, including distributing a common reference phase, cloaking and focusing light.

Universal numerical calculation method for the Berry curvature and Chern numbers of typical topological photonic crystals.

Chern number is one of the most important criteria by which the existence of a topological photonic state among various photonic crystals can be judged; however, few reports have presented a universal numerical calculation method to directly calculate the Chern numbers of different topological photonic crystals and have denoted the influence of different structural parameters. Herein, we demonstrate a direct and universal method based on the finite element method to calculate the Chern number of the typical topological photonic crystals by dividing the Brillouin zone into small zones, establishing new properties to obtain the discrete Chern number, and simultaneously drawing the Berry curvature of the first Brillouin zone. We also explore the manner in which the topological properties are influenced by the different structure types, air duty ratios, and rotating operations of the unit cells; meanwhile, we obtain large Chern numbers from-2 to 4. Furthermore, we can tune the topological phase change via different rotation operations of triangular dielectric pillars. This study provides a highly efficient and simple method for calculating the Chern numbers and plays a major role in the prediction of novel topological photonic states.

Spoof plasmons enable giant Raman scattering enhancement in Near-Infrared region.

Exceptionally strong enhancement of the Raman signal exceeding eight orders of magnitude for near-infrared (1064 nm) excitation is demonstrated for an array of dielectric submicron pillars covered by a relatively thick metal layer. The microstructure is designed to support 'spoof' plasmon-polariton excitations with resonant frequencies significantly below the fundamental surface plasmon resonance. Experiments reveal a relatively narrow range of spatial parameters for the optimal resonant scattering enhancement. They include a period close to the excitation wavelength, a specific ratio of the pillar planar size to the period, and optimal heights of both the pillars and the covering silver metal layer. The realized microstructures can be produced by fab-compatible photolithography techniques, and their outstanding sensing possibilities open the venue for the biomedical applications.

Design of nonuniform substrate dielectric lens antennas using 3D printing technology

AbstractWith the latest developments in 3D printing technologies and decreases in their costs, these prototyping methods are now being used extensively in many applications for fast, low‐cost, and precise prototyping. In this work, 3D printing technology had been used for prototyping of nonuniform substrate dielectric lens antennas. For this, two study cases had been taken into consideration: (a) design of a dielectric lens antenna with 25 nonuniform height dielectric pillars with constant dielectric value, and (b) design of a dielectric lens antenna with 25 pillars of nonuniform dielectric constant value but equal height. Both of the designs, first, were modeled in 3D EM simulation environments and then were prototyped via 3D printing technology. Both the simulations and measured results of the prototyped antennas were compared and found that they were agreeable. Both of the antenna designs achieved a measured gain level of almost 17.4 dBi at 10 GHz, with a return loss of less than −10 dB. Thus, as it can be observed from the experimental results, the proposed 3D printing‐based manufacturing process is an efficient solution for the realization of high‐performance nonuniform substrate dielectric lens antennas that are either difficult or impractical with conventional prototyping methods.

Topological beam-splitting in photonic crystals.

We create a passive wave splitter, created purely by geometry, to engineer three-way beam splitting in electromagnetism in transverse electric and magnetic polarisation. We do so by considering arrangements of Indium Phosphide dielectric pillars in air, in particular we place several inclusions within a cell that is then extended periodically upon a square lattice. Hexagonal lattice structures are more commonly used in topological valleytronics but, as we discuss, three-way splitting is only possible using a square, or rectangular, lattice. To achieve splitting and transport around a sharp bend we use accidental, and not symmetry-induced, Dirac cones. Within each cell pillars are either arranged around a triangle or square; we demonstrate the mechanism of splitting and why it does not occur for one of the cases. The theory is developed and full scattering simulations demonstrate the effectiveness of the proposed designs.

Nanohole-based phase gradient metasurfaces for light manipulation

The commonly accepted approach for metasurface design utilizes nanopillars with varying diameters. In this study, contrary to usual design approach, we propose and design highly efficient, broadband and polarization-independent nanohole all-dielectric metasurfaces operating in the visible spectrum. High focusing efficiency above 70% is achieved between 450 and 700 nm wavelength region with a numerical aperture (NA) value of 0.60. Moreover, focusing efficiency is succeeded higher than 47% with NA = 0.85 for a design wavelength of 532 nm. Nanohole metasurfaces exhibit less chromatic aberration (<18%) compared to nanopillar based metasurfaces. The nanohole array metasurfaces is investigated under the oblique illumination condition and its performance is found to be satisfactory in a wide range of incidence angles. Furthermore, nanohole and nanopillar metasurfaces are analyzed and their performances are compared for different incidence angles, NAs and operating wavelengths. It is shown that contrary to dielectric pillars commonly deployed in the design of metasurfaces, nanoholes with varying diameters allow phase changes with better performances.

Electrowetting-Dominated Instability of Cassie Droplets on Superlyophobic Pillared Surfaces.

The liquid-air interface of Cassie droplets on superhydrophobic/superlyophobic surfaces has been directly captured with a high-precision laser displacement meter. The measured profile of the interface shape and the critical voltage with which the Cassie-to-Wenzel transition occurs are compared against those from numerical simulations of the electric field coupled with the interface shape. Under the applied voltage, the collapsing behavior of water, glycerol, and hexadecane droplets on SU-8, CYTOP, and overhanging Si/SiO2 pillars has been uniquely identified depending on the liquid properties, the pillar geometry, and the pillar material. It is shown that, with increasing voltage, the contact angle at the three-phase contact line approaches the maximum advancing angle along the pillar sidewalls, above which the depinning from the pillar edge leads to a slide-down motion. The slide-down instability is dominant over the pull-in instability both on dielectric pillars and conductive overhanging pillars examined in the present study. It is indicated that the collapsing behavior on the present overhanging pillars is closely related to contact angle saturation in electrowetting and stick-slip motion of the contact line.

Numerical Analysis of the LDMOS With Side Triangular Field Plate

A Lateral Double-diffused Metal Oxide Semiconductor (LDMOS) with side triangular field plate (STFP) is proposed for improving the breakdown voltage ( BV ) and reducing the specific on-resistance ( $R_{on,sp}$ ). The main feature of the novel LDMOS is the STFPs at both ends of the drift region, and they are fabricated into the dielectric pillars. With the introduction of the STFPs, the electric field peaks at the P-well/N-drift and N+/N-drift junctions are reduced effectively. The STFPs together with the dielectric pillars modulate the surface and vertical electric field distributions, which enhances the BV . Meanwhile, the doping concentration of the silicon pillars in the drift region is optimized and thus reduces the $R_{on,sp}$ . The simulation results indicate that the BV of 379 V and the $R_{on,sp}$ of 37.3 $\text{m}\Omega \cdot $ cm2 are achieved by the STFP-LDMOS. The figure of merits ( FOM ) of the STFP-LDMOS is 2.7 times compared with the conventional LDMOS without STFPs. The STFP-LDMOS demonstrates a great trade-off between the $R_{on,sp}$ and BV .

Hypersonic Surface Phononic Bandgap Demonstration in a CMOS-Compatible Pillar-Based Piezoelectric Structure on Silicon

The promise of surface phononic crystals (PnCs) for $e.g.$ rf signal processing in wireless communication (like your mobile phone) has not been realized, due to the complexity of acoustic wave propagation in such structures, plus the lack of a low-loss, CMOS-compatible platform. The authors identify a wide, hypersonic surface phononic band gap in a pillar-based surface PnC. The significance of their platform lies in the reduced phononic material loss in dielectric pillars, and the use of CMOS-compatible AlN. This allows dense integration of low-loss hypersonic devices with electronics on the same die, enabling many exciting applications of surface PnCs.

All-Dielectric anti-reflective metasurface at visible frequencies

We analytically study the broadband antireflection of a square array of dielectric pillars with a glass substrate. The presented condition exploits the optical response of the nanoparticles to achieve complete cancellation of the reflected field by means of destructive interference.

Two-Dimensional Multimode Terahertz Random Lasing with Metal Pillars

Random lasers employing multiple scattering and interference processes in highly disordered media have been studied for several decades. However, it remains a challenge to achieve a broadband multimode random laser with high scattering efficiency, particularly at long wavelengths. Here, we develop a new class of strongly multimode random lasers in the terahertz (THz) frequency range in which optical feedback is provided by multiple scattering from metal pillars embedded in a quantum cascade (QC) gain medium. Compared with the dielectric pillars or air hole approaches used in previous random lasers, metal pillars provide high scattering efficiency over a broader range of frequencies and with low ohmic losses. Complex emission spectra are observed with over 25 emission peaks across a 0.4 THz frequency range, limited primarily by the gain bandwidth of the QC wafer employed. The experimental results are corroborated by numerical simulations that show the lasing modes are strongly localized.

A New Microfabrication Method for Ion-Trap Chips That Reduces Exposure of Dielectric Surfaces to Trapped Ions

Accumulated electrostatic charges on the dielectric surfaces of ion traps are known to induce stray fields, leading to ion micromotions. In typical microfabricated ion-trap chips, metal electrodes are electrically isolated using thick dielectric pillars, which can accumulate stray charges on their sidewalls. This letter presents a new microfabrication method for ion-trap chips that reduces the exposure of dielectric surfaces to trapped ions. The dielectric pillars are fabricated with large T-shaped overhangs, and the sidewalls and top surfaces are coated with AlCu (1%) films. The bottom sides of the overhang parts provide electrical isolation. To prevent oxidation of the AlCu (1%) films, the electrode surfaces are coated with an additional Au film. The fabricated chips were implemented to trap 174Yb+ ions, and the laser-induced stray fields were measured. The results indicated that the trap chip fabricated by the newly developed method generates significantly smaller stray fields as compared with previous chips. [2017-0233]

3D analytical model for the SOI LDMOS with alternating silicon and high-k dielectric pillars

In this paper, a 3D analytical model for the SOI LDMOS with alternating silicon and high-k dielectric pillars (HK LDMOS) is presented. By solving the 3D Poisson’s equation, the surface potential and electric field distribution are derived. A criterion for obtaining the optimal breakdown voltage and drift region doping concentration is obtained. The analytical results are well matched with the numerical results, which confirms the model validity. Based on these models and the numerical simulation, the electric field modulation mechanism and the breakdown characteristics of HK LDMOS are investigated.

Combined dielectric and plasmon resonance for giant enhancement of Raman scattering

Combined dielectric/metal resonators for colossal enhancement of inelastic light scattering are developed and their properties are investigated. It is shown that a record enhancement factor of 2 × 108 can be obtained using these structures. The dielectric resonators are fabricated on Si/SiO2 substrates where periodic arrays of square 10- to 200-nm-high dielectric pillars are produced via electron-beam lithography and plasma etching. The lateral size a of the pillars varies between 50 and 1500 nm, and their period in the array is 2a. To make a combined dielectric/metal resonator, a nanostructured layer of silver is deposited onto the fabricated periodic dielectric structure by thermal evaporation. It is established that, for a fixed height of the dielectric pillars, the Raman scattering enhancement factor experiences pronounced oscillations as a function of the period (and size) of the pillars. It is shown that these oscillations are determined by the modes of the dielectric resonator and governed by the relation between the excitation laser wavelength and the planar size of the dielectric pillars.

Slanted gold mushroom array: a switchable bi/tridirectional surface plasmon polariton splitter.

Surface plasmon polaritons (SPPs) show great promise in providing an ultracompact platform for integrated photonic circuits. However, challenges remain in easily and efficiently coupling light into and subsequently routing SPPs. Here, we theoretically propose and experimentally demonstrate a switchable bi/tridirectional beam splitter which can simultaneously perform both tasks. The photonic device consists of a periodic array of slanted gold 'mushrooms' composed of angled dielectric pillars with gold caps extruding from a periodic array of perforations in a gold film. The unidirectional coupling results from the interference of the in-plane guided modes scattered by a pair of dislocated gold gratings, while the output channel is determined by the polarization of the incident beam. This device, in combination with dynamic polarization modulation techniques, has the potential to serve as a router or switch in plasmonic integrated circuits.

Photonic crystal carpet: Manipulating wave fronts in the near field at 1.55μm

Ground-plane cloaks, which transform a curved mirror into a flat one, and recently reported at wavelengths ranging from the optical to the visible spectrum, bring the realm of optical illusion a step closer to reality. However, all carpet-cloaking experiments have thus far been carried out in the far-field. Here, we demonstrate numerically and experimentally that a dielectric photonic crystal (PC) of a complex shape made of a honeycomb array of air holes can scatter waves in the near field like a PC with a at boundary at stop band frequencies. This mirage effect relies upon a specific arrangement of dielectric pillars placed at the nodes of a quasi-conformal grid dressing the PC. Our carpet is shown to work throughout the range of wavelengths 1500nm to 1650nm within the stop band extending from 1280 to 1940 nm. The device has been fabricated using a single- mask advanced nanoelectronics technique on III-V semiconductors and the near field measurements have been carried out in order to image the wave fronts's curvatures around the telecommunication wavelength 1550 nm.

Novel Low-Resistance Current Path UMOS With High-K Dielectric Pillars

A low specific on-resistance (Ron,sp) UMOS with high permittivity (HK) dielectric pillars underneath the p body region (HK UMOS) is proposed and investigated. Its drift region uniquely consists of two narrow and highly-doped n pillars and one lightly doped n- pillar, which is parallel to the HK dielectric pillars. First, the highly-doped n pillars offer low resistance current paths in the ON-state while the n- region sustains high voltage in the OFF-state. Second, the HK dielectric causes an enhanced self-adapted lateral assistant depletion of the n pillars, which allows to keep a higher doping concentration of the n pillars and thus further reduces the Ron,sp. Third, the HK dielectric enhances the vertical field strength in high-voltage blocking state, leading to an improved breakdown voltage (BV). Compared with a conventional UMOS at the highest figure-of-merit, the HK UMOS with kD=200 not only decreases the Ron,sp by 67%, but also increases the BV by 12%.

Search of Extremely Sensitive Near-Infrared Plasmonic Interfaces: A Theoretical Study

The sensitivity of the wavelength position of localized surface plasmon resonance (LSPR) in metal nanostructures to local changes in the refractive index has been widely used for label-free detection strategies. Tuning the optical properties of the nanostructures from the visible to the infrared region is expected to have a drastic effect on the refractive index sensitivity. Here, we theoretically investigate the optical response of a newly designed plasmonic interface to changes in the bulk refractive index by the finite difference time domain method. It consists of a structured interface, where the planar interface is superposed with dielectric pillars 30 nm in height and 125 nm in length with a separation distance of 15 nm. The pillars are covered with U-shaped gold nanostructures of 50 nm in height, 125 nm in length, and 5 nm of gold base thickness. The whole structure is finally covered with a 5-nm thick dielectric layer of n2 = 2.63. This plasmonic structure shows bulk refractive index sensitivities up to 1750 nm/RIU (RIU : refractive index unit) in the near infrared (λ = 2621 nm). The enhanced sensitivity is a consequence of the extremely enhanced electrical field between the gold nanopillars of the plasmonic interface.