Bayaner Arigong

Tuneable complementary metamaterial structures based on graphene for single and multiple transparency windows

Novel graphene-based tunable plasmonic metamaterials featuring single and multiple transparency windows are numerically studied in this paper. The designed structures consist of a graphene layer perforated with quadrupole slot structures and dolmen-like slot structures printed on a substrate. Specifically, the graphene-based quadrupole slot structure can realize a single transparency window, which is achieved without breaking the structure symmetry. Further investigations have shown that the single transparency window in the proposed quadrupole slot structure is more likely originated from the quantum effect of Autler-Townes splitting. Then, by introducing a dipole slot to the quadrupole slot structure to form the dolmen-like slot structure, an additional transmission dip could occur in the transmission spectrum, thus, a multiple-transparency-window system can be achieved (for the first time for graphene-based devices). More importantly, the transparency windows for both the quadrupole slot and the dolmen-like slot structures can be dynamically controlled over a broad frequency range by varying the Fermi energy levels of the graphene layer (through electrostatic gating). The proposed slot metamaterial structures with tunable single and multiple transparency windows could find potential applications in many areas such as multiple-wavelength slow-light devices, active plasmonic switching, and optical sensing.

A Fully Symmetrical Crossover and Its Dual-Frequency Application

In this paper, a new fully symmetrical four-port microstrip crossover and its dual-frequency application are presented. The proposed single-band crossover has a simple structure and an easily controlled bandwidth; and its dual-frequency application can provide flexible frequency ratio between the two working frequencies. Moreover, analytical design equations are derived using the even-odd-mode method. The final explicit design equations are concise. To verify the design concepts, both a single-band crossover working at 1 GHz and a dual-frequency crossover working at 1/2.3 GHz are fabricated and tested. The measurement results agree well with the design theory.

A Varactor Based 90

In this paper, a symmetric four-port microwave varactor based 90 ° directional coupler with tunable coupling ratios and reconfigurable responses is presented. The proposed coupler is designed based on the modified structure of a crossover, where varactors are loaded. By applying suitable biasing voltages to the varactors, the power-dividing ratios between the two output ports (i.e., port 2 and port 3) of the coupler can be easily controlled. Moreover, it is found that the realizable power ratio using the proposed structure is very flexible (it could be extremely large or small). Therefore, under the special case when the coupling ratio is tuned to be 1, the proposed coupler is reconfigured to be a crossover. Good isolation and return-loss performance have been maintained for different power-dividing ratios. To theoretically analyze the proposed device, closed-form design equations are derived using the even-odd mode method. Based on these analytical equations, an experimental prototype working at 1 GHz is designed, fabricated, and characterized. The measurement results match well with the simulation and theoretical results, validating the proposed design theory.

^{\circ}

In this paper, we proposed a novel dual-frequency cross polarization converter that can simultaneously work at two frequencies in the reflection mode, which is constructed of an L-shaped perforated graphene sheet printed on a dielectric material backed by a gold ground plane. For the normal incidence, the optical rotation at these two working frequencies originates from the simultaneous excitation of both eigenmodes that are characterized as the results of the localized surface plasmon resonances. Moreover, by merely varying the Fermi energy of the graphene, both working frequencies can be tuned within a large frequency range, which suggests numerous potential optical applications.

Directional Coupler With Tunable Coupling Ratios and Reconfigurable Responses

In this paper, we presented highly efficient reflective cross polarization converters based on metamaterials operating in the infrared regime, which are composed of a dielectric spacer sandwiched between slotted L-shaped metallic nanoantennas and a ground plane. The proposed polarization converters can convert a linearly polarized wave to its cross polarized wave with high polarization conversion ratio (> 0.95) over multiple / broad frequency bands. The resulting multi-band and broadband operations are induced by the localized mode hybridizations between the slot and the original metallic nanoantenna. Furthermore, the performance of the proposed converters under different incident angles is also explored. It is found that the first broad band (or the first two resonant frequencies) of the proposed broadband (or multi-band) converters appears to be independent of the incident angle (up to 47°).

Mid-Infrared Tunable Dual-Frequency Cross Polarization Converters Using Graphene-Based L-Shaped Nanoslot Array

This letter presents a modified Doherty power amplifier (DPA) architecture to release bandwidth limitation of the conventional DPA. The proposed DPA structure eliminates two quarter wavelength impedance inverters used in the conventional DPAs. Instead, both the carrier and peak amplifiers in the proposed DPA are matched to 70 Ω at the output ports, which enables an easier implementation of broadband matching networks. Broadband input matching network (IMN) and output matching network (OMN) are then designed to achieve wideband DPA with enhanced performance. To verify the design concept, a broadband DPA is designed, fabricated, and measured on a RT/Duroid 5880 substrate with 2.2 dielectric constant and 0.787 mm substrate thickness. In the working frequency bands (0.8 to 1.2 GHz), the designed DPA provides 50.8% to 78.5% power-added efficiency (PAE) at full output power, 30.3% to 40.1% PAE at 6 dB of output power back-off (OBO), 10.8 to 14.8 dB gain (the gain variation is within 2.6 dB over different input power levels at each specific frequency), and maximum output power between 40.2 and 42.9 dBm.

Efficient multiband and broadband cross polarization converters based on slotted L-shaped nanoantennas

In this paper, we present an achievable gradient refractive index in bi-continuous holographic structures that are formed through five-beam interference. We further present a theoretic approach for the realization of gradient index devices by engineering the phases of the interfering beams with a pixelated spatial light modulator. As an example, the design concept of a gradient index Luneburg lens is verified through full-wave electromagnetic simulations. These five beams with desired phases can be generated through programming gray level super-cells in a diffractive spatial light modulator. As a proof-of-concept, gradient index structures are demonstrated using synthesized and gradient phase patterns displayed in the spatial light modulator.

Design of GaN Doherty Power Amplifiers for Broadband Applications

In this paper, we propose a reconfigurable surface plasmon polariton (SPP) wave adapter designed by transformation optics, which can control the propagation of SPP waves on un-even surfaces. The proposed plasmonic device is constructed using homogeneously tunable materials (e.g. liquid crystals) so that the corresponding SPP wave transmission can be reconfigured by applying different voltages. Additionally, modified designs optimized for practical fabrication parameters are investigated. Their performance is verified by full-wave simulations. The proposed devices will pave the way towards developing tunable plasmonic devices.

Spatially addressable design of gradient index structures through spatial light modulator based holographic lithography

Gradient index (GRIN) structures have attracted great interests since their invention. Especially, the recent advance in the fields of transformation optics, plasmonics, and nanofabrication techniques has opened new directions for the applications of GRIN structures in nano-photonic devices. In this paper, we apply Luneburg lens and its transformed counterpart to realize efficient coupling to plasmonic nano-waveguides. We first briefly present the general structures of Luneburg lens and generalized Luneburg lens, as well as the design process of flattened Luneburg lens applying quasi-conformal mapping techniques. After that, we study the performance of these lenses for coupling electromagnetic signals to nano-waveguides (the metal-insulator-metal nano-waveguide). Different coupling schemes are investigated. It is found that the proposed Luneburg lens based optical couplers can be used to provide broadband light couplings to plasmonic nano-waveguides under wide incident angles.

Reconfigurable surface plasmon polariton wave adapter designed by transformation optics

In this paper, we present a method for the mathematically formulated phase engineering of interfering laser beams through a spatial light modulator for a holographic fabrication of graded photonic lattices. The desired phases can be programmed at specific locations by assigning gray levels in cellular structures. The method is demonstrated by embedding single-lattice structures or missing lattices in dual-lattice periodic photonic structures. The demonstrated method can be potentially combined with the coordinate transformation technique in transformation optics for the fabrication of graded photonic devices.