Near-perfect wide-band absorbers based on one-dimensional photonic crystal structures in 1-20 THz frequencies.

This paper investigates the absorption behavior of one-dimensional (1D) photonic crystal (PhC) structures in the 1-20 THz region. The structures are analyzed by the transfer matrix method to achieve accurate results quickly with ordinary simulation facilities. The simulation results indicate a strong dependence of the absorber performance on the thickness and material of the PhC layers, as well as the frequency and angle of incident light. The combination of silica and titanium (Ti) materials as dielectric and metal layers presents a great choice for broadband high-absorption applications so that this structure can absorb, on average, more than 80% of the normal incident radiation in the studied frequency range. Additionally, this absorber has the lowest dependence on incident light with the angle varying from 0° to 80° compared to identical absorbers with silver, aluminum, gold, chromium, nickel, and tungsten metals. The excellent absorption feature of the Ti-based absorber compared to the other absorbers is attributed to the lower permittivity of Ti (in both real and imaginary parts) in comparison with the other metals. In addition to owning simple and fabrication-friendly structures, 1D PhCs can pave the way to achieve various absorption spectra proportional to the needs of photonics, communications, and aerospace applications.

Ultracompact, high-extinction ratio XOR, OR, and Feynman logic gates based on plasmonic metal-insulator-metal directional couplers.

Three plasmonic logic gates including XOR, OR, and Feynman based on directional couplers operating in the optical communications band are proposed. First, a 3 dB directional coupler based on metal-insulator-metal waveguides is presented and its operation principles are investigated using the coupled mode theory method. Then, XOR and OR logic gates are implemented simultaneously in one structure. The simulations performed by the finite-difference time-domain method show that the extinction ratios for the OR and XOR gates are infinite and 29.3 dB, respectively. The Feynman gate is also implemented using two directional couplers placed parallel to each other. The ER values for outputs P and Q are 7.26 dB and 34.23 dB, respectively. The footprints of the XOR/OR and Feynman structures are less than 8µm 2 and 12µm 2, respectively. The presented gates have a high potential to be used in photonic integrated circuits.

High-performance tunable resonant electro-optical modulator based on suspended graphene waveguides.

The exceptional tunable waveguiding characteristics of graphene surface plasmons have remained unrivaled since it has inspired many electro-optical (EO) devices in terahertz (THz) and mid-infrared (MIR) photonic circuits. We propose and numerically investigate a low-loss, highly extinctive resonant EO modulator based on a suspended graphene plasmonic waveguide. Unlike other resonance-based modulators, the input power has negligible interaction with lossy resonance cavity in on-state, remarkably reducing the losses. Achieving the insertion loss (IL) of 1.3 dB and the extinction ratio (ER) of 22 dB within a footprint less than 3 µm2 substantiates the superiority of the proposed structure. The charge transport simulations are first conducted to calculate the steady-state charge distribution. The three-dimensional finite-difference time-domain (3D-FDTD) method is utilized to monitor the guided wave propagation and modulation properties. We show that the transmission spectrum is highly dependent upon geometric parameters of the structure, and the modulator can be effectively tuned to operate at the desired wavelength by applying a suitable gate voltage. Simulation results show the modulation bandwidth of 71 GHz corresponding to the total capacitance of 4.8 fF within the active area. The novel EO modulator structure has shown great potentiality and flexibility to find other applications in MIR and THz integrated circuits like controllable notch filters and switches.

Graphene-based plasmonic electro-optical SR flip-flop with an ultra-compact footprint.

In this paper, we present a new concept of electro-optical plasmonic Set-Reset flip-flops at mid-infrared frequencies. We use the 3D finite-difference time-domain (FDTD) method to simulate and evaluate our designed flip-flop. In the proposed structure, the propagation of surface plasmon polaritons is controlled by applying an electrostatic field and the switching actions occur in the electrical domain while the output signal is in the form of light. The energy consumed by each switch is 2.5 fJ/bit. In this flip-flop, the no-change state of the flip-flop is realized by using a Bias port. The time response diagram indicates that the minimum extinction ratio of the flip-flop is 14.61 dB. The probability of various errors in the flip-flop state occurring due to the lack of synchronization between the switches is also considered by the FDTD simulations and it is shown that the device has a great performance against errors. Furthermore, the structure has an ultra-compact footprint of 1.62 µm2. Our surveys show that no plasmonic flip-flop has been reported to date.

Ultra-broadband metamaterial absorber based on cross-shaped TiN resonators.

In this paper, a novel broadband plasmonic absorber based on cross-shaped titanium nitride (TiN) resonators in the ultraviolet, visible, and near-infrared regions is presented. The proposed perfect solar absorber consists of periodic arrays of cross-shaped TiN resonators located on a stack of ${{\rm SiO}_2}/{\rm TiN}$SiO2/TiN layers. By using the finite-difference time-domain method, the effects of variations of the thickness and radius of the elliptical metasurface resonators on the absorption are comprehensively investigated. The cross-shaped metamaterial absorber exhibits an averaged absorption of 90%, ranging from 200 to 3000 nm, and shows over 90% absorption from 200 to 2500 nm. Furthermore, the proposed absorber indicates absorption efficiency over 80% for an oblique incidence up to 50 deg for both TE- and TM-polarized light. These features make the proposed solar absorber usable in many solar-based applications, imaging, and thermal emitting.

High-extinction ratio and ultra-compact two-bit comparators based on graphene-plasmonic waveguides.

Electro-optical 1-bit and 2-bit comparators based on graphene plasmonic waveguides are proposed. Surface plasmon polaritons are stimulated by radiation of a TM-polarized light with a wavelength of 15 µm and their propagation is controlled by applying electrical signals which represent the binary numbers that need to be compared. The results of comparing two numbers are obtained in the form of light at the output ports. Finite-difference time-domain simulation results show that the minimum extinction ratios (ERs) for 1-bit and 2-bit comparators are 9.69 dB and 6.13 dB, respectively. Also, the 1-bit and 2-bit structures have footprints of ${0.42}\;\unicode{x00B5}{\rm m}^2$0.42µm2 and ${0.9}\;\unicode{x00B5}{\rm m}^2$0.9µm2, respectively. Our studies show that the optical comparators with these ultra-compact dimensions have not been reported so far. The presented structures benefit from high ER and ultra-compact footprint, which make them suitable for use in optical signal processing and photonic computing.

Dielectric-loaded graphene-based plasmonic multilogic gate using a multimode interference splitter.

In this paper, we propose a multilogic gate (MLG) by utilizing a dielectric-loaded graphene-based plasmonic waveguide (DLGPW) in the mid-infrared spectral region. The proposed MLG is composed of DLGPW-based switches and multimode interference (MMI) splitters and supports three logical operations-AND, XNOR, and NOR-simultaneously. First, by proper control of the graphene surface conductivity, a graphene plasmonic on-off switch is presented, and then a 3 dB MMI splitter based on a DLGPW for a wavelength of 7.8 µm is designed and investigated. Our studies show that electro-optical logic gates based on DLGPWs have not been reported to date. The structure of the presented MLG is very simple and CMOS-compatible. The calculated minimum extinction ratios (ER) for AND, XNOR, and NOR logic gates are 17.53 dB, 53.43 dB, and 17.53 dB, respectively. Also, by some modifications, this structure can act as a NAND logic gate with an ER of 55.22 dB. Compact footprint, high ER, and easiness of on-chip implementation are some advantages of the presented MLG.