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In this article, a new tunable absorber and polarizer using one-dimensional anisotropic graphene photonic crystal (1D AGPC) is proposed for terahertz applications. The optical properties of the 1D AGPC structure is obtained through the transfer matrix method (TMM) calculations. In our design, a single defect layer is introduced in the 1D AGPC structure to increase its absorption. The absorption coefficient is further modified by optimizing the AGPC parameters such as the number of photonic crystal periods, the defect layer thickness, and the chemical potential of graphene. The modified structure can provide perfect absorption with a narrow bandwidth of 0.05 THz. On the other hand, considering the anisotropic optical properties of graphene, polarization dependence of the absorption coefficient is investigated. This feature of the structure can be employed to realize a tunable terahertz polarizer by adjusting incident angle at θ0 = 80°. In this way, high tunable polarization extinction ratio of 6.1 dB to 11.63 dB with respectively very narrow bandwidth of 0.031 THz to 0.023 THz are obtained. The maximum insertion losses within the tunable frequency range are as low as 0.023 dB and 0.031 dB. Hence, our design may have potential applications in narrow-band optoelectronic devices at the terahertz frequency range. [ABSTRACT FROM AUTHOR]

Si3N4 has emerged as a prominent material for expanding the capability of silicon photonics to wavelengths below < 1   μm. However, realizing an efficient optical modulator, a key building block for any integrated optics platform, remains a major challenge in Si3N4 mainly because this material has a vanishing Pockels coefficient. Here, we propose a compact Si3N4 based optical modulator by using a thin VO2 layer on top of a Si3N4 strip waveguide where amplitude modulation is achieved via phase transition of the VO2 layer. To reduce the actuation time of the temperature-induced VO2 phase transition, a mono-layer graphene microheater is designed for the active Si3N4 VO2 waveguide. Our simulations indicate a high extinction ratio of ~ 8.28 dB/μm with an insertion loss of ~ 2.8 dB/μm at the design wavelength of 850 nm for the proposed modulator and wideband operation in the wavelength range of 800–900 nm. It is shown that employing the electrical and thermal properties of graphene not only leads to a significant reduction of the power consumption of the device but also, decreases the actuation time compared to previous modulators based on the thermal phase transition of the VO2. [ABSTRACT FROM AUTHOR]

A fractional-order silicon hybrid plasmonic temporal integrator with considerably wide full-width at half- maximum (FWHM) bandwidth of ∼1 THz, higher than the previously reported photonic temporal integrators, and noticeably compact footprint of ∼5 μm × 3 μm is proposed. The presented plasmonic temporal integrator is implemented based on a double-ring add-drop resonator structure with ring radii of 1.5 μm. The 3-D finite-difference time-domain (3D-FDTD) simulation results show that the fractional-order of the integrator is 0.64 and it is capable of processing the ultrashort pulses with the duration of 450 fs. The integration time window is estimated as 520 fs, while the achieved energy efficiency is as high as −1.92 dB. The compact size and wide bandwidth of the proposed integrator makes it suitable for ultrafast optical signal processing applications. As an example, the functionality of the designed integrator for solving ordinary differential equations (ODEs) is investigated. [ABSTRACT FROM AUTHOR]

We propose and design a high-efficiency forward and backward second harmonic generation (SHG) in nonlinear polymer filled silicon slot waveguide known as silicon-organic hybrid (SOH) waveguides. The spatial distribution of second-order nonlinearity, χ(2), of waveguide is altered by periodically poling of nonlinear polymer to achieve quasi-phase matching (QPM) between fundamental frequency and second harmonic waves. The simulations show that first-order QPM SHG with high conversion efficiency of η ≍ 7.2% can be achieved in the optimized structure with relatively small input power of 25 mW. This level of improvement is the result of high confinement of SOH waveguide as well as possible fabrication of very small period length in silicon photonics which could not be realized in traditional nonlinear materials such as LiNbO3. Our simulation results show ∼17% efficiency with only 25 mW input power for forward SHG being comparable to the other platforms even with smaller device length and lower pump power. We expect this device will open up its application in the fields of all-optical signal processing, optical (mirrorless) parametric amplification, low-noise detection of mid-IR signal in near-IR windows, and mid-IR sources. [ABSTRACT FROM AUTHOR]

Strong Kerr nonlinearity of graphene is a promising basis for the realization of all-optical nonlinear applications on a silicon chip. However, on one hand, the interaction of light with graphene as a two-dimensional material is weak and, on the other hand, the sign of the Kerr indices in silicon and graphene is different, which results in reverse index changes and consequently, reduces the total nonlinear index shift. In this paper, a slotted silicon nano-beam cavity is designed to both increase the interaction of light with the graphene sheet and simultaneously reduce the nonlinear phenomena in the silicon through the strong light confinement in the slot region and in the vicinity of the graphene layer. The designed cavity supports two TE modes, so it facilitates on-chip coupling of both the pump and signal to the cavity which is essential for final packaging. Optical amplitude modulation (or on/off switching) of the light in the device is theoretically investigated using the perturbation and the temporal coupled-mode theories. A modulation depth of >80% (∼8 dB) is obtained for the low pump power of 50 mW when pumped by a pulse with a 10-ps temporal full-width half-maximum and a 10-GHz repetition rate. The photonic bandwidth of the proposed structure is limited by the photon lifetime of the cavity and is as large as 123 GHz. The device is also highly compact with the overall footprint of ∼8.5 μm2. The combination of low-power, high-speed operation, large modulation depth, and the compactness of the proposed structure makes it a good candidate for the realization of all-optical integrated circuits. [ABSTRACT FROM AUTHOR]

A thermally/electrically tunable graphene on white graphene polarizer is proposed, in which the resonant coupling between the plasmonic surface modes of the structure and the transverse electric (TE) [or transverse magnetic (TM)] polarized incident wave is used to absorb this polarization, while the TM (or TE) polarized incident wave is totally reflected. It is then shown that the thermal and electrical tunability of surface conductivity of graphene can be used to control the optical properties of the proposed polarizer, including the selection of the desired polarization, and adjusting the amplitude of the reflected (desired) polarization. The application of the hexagonal boron-nitride (white graphene) as the substrate of graphene increases the propagation of the surface waves of the structure, which in turn results in the very high polarization extinction ratio of 75 dB. Moreover, the ultra-small insertion loss of 0.022 dB and relatively large bandwidth of ~ 60 nm are calculated for the proposed polarizer. [ABSTRACT FROM AUTHOR]

In this paper, a horizontal slot Si–VO2–Si optical waveguide is proposed and its optical properties are investigated. Numerical simulation results show that the effective index and the propagation loss of the proposed waveguide undergo substantial changes upon the VO2 transition from insulating to metallic phase. The effective index and the propagation loss variations of the proposed waveguide are then maximized by optimizing waveguide dimensions. It is shown that 0.226 change in the effective index (Δn eff = 0.226) and 30 dB/μm change in the propagation loss (Δl dB = 30 dB/μm) are achievable using the optimum dimensions. These extraordinary variations in waveguide properties recommend the proposed waveguide as an excellent candidate for optical active device realization. To investigate these applications, performance parameters of the proposed waveguide are further studied in terms of the transition speed and the power consumption. In these studies, the VO2 phase transition is assumed to be actuated by applying an electric field. Two examples of optical active devices based on the proposed waveguide are then presented: an electro-absorption modulator and a 1 × 2 directional coupler optical switch. Finite-difference time-domain simulation of the proposed devices shows very high extinction ratio of 21 dB along the ultrasmall propagation length of 1 μm, for the proposed electro-absorption modulator, and high extinction ratios of ∼18.5 dB and ∼8.6 dB in off- and on-state of the proposed 1 × 2 switch, which has very small length of ∼6 μm. Further simulations also show interesting properties of the proposed devices in terms of the power consumption, insertion loss, and bandwidth. [ABSTRACT FROM PUBLISHER]

A transmission line model is developed for coupled plasmonic metal–insulator–metal (MIM) waveguides. In the proposed model coupling between electric fields of two plasmonic waveguides is modeled by distributed mutual capacitor while distributed mutual inductor accounts for magnetic field coupling. These mutual elements are determined using propagation constants of supermodes of coupled waveguides. The model is applied to analyze coupled line directional coupler and side-coupled rectangular resonators. The effectiveness of the model is assessed using fully numerical finite-difference time-domain (FDTD) technique. The results have excellent agreement with the numerical methods. [ABSTRACT FROM PUBLISHER]