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.

Simultaneous all-optical sampling and quantization scheme based on the nonlinear Kerr effect in an ultra-compact silicon waveguide grating structure.

In this paper, a novel method for simultaneous all-optical sampling and quantization is presented, to the best of our knowledge. This study utilizes the nonlinear Kerr effect in a silicon-based waveguide grating to realize nonlinear filters in which the sampling pulses are affected by the input signal. Compatible with complementary metal-oxide-semiconductor fabrication, in this ultra-compact two-bit quantizer, the footprint is reduced to 90µm2 which, to the best of our knowledge, is the least among recent similar studies.

Compact, broadband, and low-loss multimode optical switch based on phase-change material for mode division multiplexing systems.

Mode-division multiplexing (MDM) technology is one of the suitable approaches to increase data transmission capacity in photonic integrated circuits. Multimode optical switches play an important role in MDM interconnection networks. In this article, we present a multimode on-off optical switch using Ge2Sb2Se4Te1 phase-change material for the first two TE modes (TE0 and TE1) and the first two TM modes (TM0 and TM1) in a wide wavelength range between 100 nm to around 1550 nm. The 3D finite-difference time-domain simulations indicate that for each propagating mode across the bandwidth, the insertion loss and extinction ratio are less than 0.80 dB and more than 20.21 dB, respectively. The proposed switch has a compact footprint of 10.7µm×3µm. The presented switch also tolerates a ±20nm change in the waveguide width, a ±10nm silicon waveguide height deflection, and a ±5nm GSST thickness variation with an insertion loss lower than 0.91 dB and an extinction ratio higher than 19.04 dB.

Ultracompact temporal integrator using graphene-based long-range hybrid plasmonic waveguides.

A microdisk-resonator add-drop temporal integrator, composed of a long-range hybrid plasmonic waveguide, with graphene as the central layer, is proposed for the first time, to the best of our knowledge. The integrator benefits from a considerable integration time of ∼5.55ps, which is about 11 times longer than our previously proposed plasmonic integrator, and also is fairly comparable with the integration time of a microring-based integrator with a ring radius of 47.5 µm. Based on 3D-finite-difference time-domain simulations, the integrator, with a significantly compact footprint of ∼4µm×3µm, shows the FWHM of 53 GHz. The presented graphene-based temporal integrator, with a highly miniaturized footprint and satisfactory integration time, may find applications in ultrafast plasmonic-based signal processing systems.

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.

Compact, high-performance, and fabrication friendly two-mode division multiplexer based on a silicon bent directional coupler.

In response to the increasing demands of the capacity enhancement of optical communication, a compact and high-performance silicon mode division multiplexer is proposed that multiplexes the fundamental and first-order transverse magnetic modes. The device structure is based on an asymmetric bent directional coupler with an ultrasmall coupling length of 3.67 µm. Utilizing single-layer silicon waveguides with the same heights allows the proposed device to be fabricated using a single-step CMOS-compatible fabrication process, which provides a cost-effective design in comparison with the previously reported structures. The three-dimensional finite-difference time-domain simulation results confirm that the device has a low loss of 0.87 dB, low crosstalk of ${-}{21.8}\;{\rm dB}$-21.8dB, and high mode conversion efficiency of 98.3% at the communication wavelength of 1.55 µm. Furthermore, the device shows a broad bandwidth of about 110 nm, completely covering the C and L bands with crosstalk less than ${-}{10}\;{\rm dB}$-10dB. Moreover, it is shown that the proposed mode (de)multiplexer is fabrication tolerant for the coupling gap variation of ${-}{40}\;{\rm nm} \lt \Delta {g} \lt {23}\;{\rm nm}$-40nm<Δg<23nm and the waveguide width variation of ${-}{25}\;{\rm nm} \lt \Delta {W} \lt {25}\;{\rm nm}$-25nm<ΔW<25nm for a low loss of ${ \lt }- {1.67}\;{\rm dB}$<-1.67dB and low crosstalk of ${ \lt }- {10}\;{\rm dB}$<-10dB.

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.

Compact and broadband 2 × 2 optical switch based on hybrid plasmonic waveguides and curved directional couplers.

In this paper, a compact, broadband, and CMOS compatible ${2} \times {2}$2×2 optical switch based on hybrid plasmonic waveguides (HPWs) and curved directional coupler is presented. The proposed coupler consists of a combination of straight and curved hybrid plasmonic waveguides. By using the ability of HPWs for confining the light wave in a sub-wavelength scale and the curved structure to decrease the wavelength dependence of the directional coupler, a 3 dB power splitter with a ${3}\,\,\textrm{dB} \pm 0.5$3dB±0.5 bandwidth of about 410 nm and a footprint of ${7.6}\,\,\unicode{x00B5}\textrm{m} \times {2}\,\,\unicode{x00B5}\textrm{m}$7.6µm×2µm is achieved. By exploiting this optimal power splitter in a Mach-Zehnder interferometer structure, a ${2} \times {2}$2×2 electro-optic switch with a broad bandwidth of 400 nm and a small footprint of ${33}\,\,\unicode{x00B5}\textrm{m} \times {2.5}\,\,\unicode{x00B5}\textrm{m}$33µm×2.5µm is realized, which denotes the widest bandwidth compared to the previously reported similar structures. The three-dimensional finite-difference time-domain simulation results show a switching speed of 0.15 THz for the proposed optical switch, while the extinction ratio, power consumption, and insertion loss are 20 dB, 95.2 fJ/bit, and 4 dB, respectively, at the central wavelength of 1550 nm.

Ultracompact and ultrabroadband mode division multiplexer based on an Au nanocube array assisted directional coupler.

An ultracompact and ultrabroadband two-mode (de)multiplexer based on an asymmetric directional coupler for mode division multiplexing is proposed. The device structure consists of a pair of silicon waveguides with an array of plasmonic Au nanocubes sandwiched in the coupling region. The coupling region length of the directional coupler is decreased to 1 µm for coupling of the fundamental transverse magnetic (TM) mode to the first order mode by excitation of the surface plasmon polaritons. This is the shortest length reported for multiplexing of the TM modes until now, to the best of our knowledge. The proposed mode (de)multiplexer has a low loss of 0.72 dB and low crosstalk of -28.3dB at the communication wavelength of 1.55 µm. Also, the 3D finite-difference time-domain simulation results show that a broad bandwidth of 190 nm is realized with crosstalk less than -10dB and the insertion loss lower than 1.29 dB. Furthermore, impact of the fabrication tolerances on the performance of the proposed (de)multiplexer is studied in detail.

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.

Deep subwavelength confinement and threshold engineering in a coupled nanorods based spaser.

In recent years, extensive efforts have been made for design and fabrication of low threshold spasers or plasmonic nanolasers at a deep subwavelength scale. Plasmonic nanolasers with coupled-nanorods structure can realize this purpose due to energy concentration in nano size volumes and effective amplification mechanisms. In this study, a group of structures based on metallic and CdS coupled nanorods are designed and analyzed using the finite element method (FEM). By changing the lateral adjacent surfaces of the metal and semiconductor nanorods through utilizing regular polygons as the cross sections of the nanorods, different characteristics of the plasmonic nanolaser are investigated. Simulation results show that the mode area normalized by the diffraction limit area is as low as 0.0062 in the structures based on hexagonal metallic core with circular semiconductor nanorods while structures based on circular Ag core with hexagonal CdS nanorods can provide a low threshold gain as 1.310 µm-1. Also, it is shown that if ZnO be used as the semiconductor gain material instead of CdS, a normalized mode area of almost one tenth can be attained in a structure with dodecagonal metallic core and circular ZnO nanorods.

Compact and broadband data exchange for mode-division-multiplexed networks.

Recently, mode-division multiplexing (MDM) technology has been used to enable high-bandwidth data transmission. Information swapping or data exchange is an essential function in such high-capacity networks. This paper presents a small, broadband on-chip optical data exchange between two multiplexed modes on the silicon-on-insulator platform. The device consists of a three-waveguide coupler in which the middle waveguide has a hybrid plasmonic structure and the two other ones are silicon waveguides. The total length and the width of the proposed structure are 15.25 µm and 1.0885 µm, respectively. The 3D finite-difference-time-domain simulation results show that the insertion loss is less than 2 dB for a broad bandwidth of 100 nm, over which the crosstalk is lower than -14.7 dB and -21.9 dB for TM0 to TM1 and TM1 to TM0 conversion, respectively.

Design of ultracompact tunable fractional-order temporal differentiators based on hybrid-plasmonic phase-shifted Bragg gratings.

The design and the simulation of tunable fractional-order temporal differentiators based on Si-hybrid plasmonic phase-shifted Bragg gratings are proposed in this paper, where strong light confinement in the hybrid plasmonic waveguide is employed to significantly reduce the overall length of the differentiators. According to 2D- and 3D-FDTD simulation results, the proposed structures with overall lengths of less than 8 µm can provide arbitrary differentiation order and differentiation bandwidths as high as 1.6 THz. The differentiation order and the bandwidth of the proposed structures can be tuned in relatively wide ranges by changing the geometrical parameters of the structures. For example, the differentiation order can be changed from 0.57 to 0.97 by changing the number of the Bragg grating periods in a 3D differentiator structure. Furthermore, it is shown that using an electro-optical polymer as the low-index material of the hybrid plasmonic waveguide, the differentiation order and the central frequency of the proposed differentiators can be actively tuned through applying a proper actuating electrical field (voltage) to the structure. This property, along with the ultracompact footprint and wide bandwidth of the proposed differentiators, suggest their application in ultrafast all-optical signal-processing systems.

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.

Compact fabrication-tolerant subwavelength-grating-based two-mode division (de)multiplexer.

Regarding the importance of bandwidth and capacity expansion in communication systems, a novel mode division (de)multiplexer based on subwavelength grating (SWG) is proposed. SWG-based devices could have smaller sizes, be much more fabrication-tolerant, and have much wider bandwidths due to the reduced confinement of the field and dispersion. It is also feasible to reduce the loss of a SWG-based device by tuning the duty cycle of the grating. Owing to these properties of SWGs, we have designed a compact fabrication-tolerant two-mode division (de)multiplexer. A flat-top transmission of >0.89 (loss <0.5 dB) is obtained over 65 nm from 1500 to 1565 nm, completely covering the entire C-band used for dense wavelength division multiplexing. Moreover, the cross-talk is <-10 dB for a broad bandwidth of ∼120 nm over which the loss of the complete device including both multiplexer and demultiplexer is <1 dB. Therefore, the proposed device is promising for high-capacity optical communications in both conventional and entirely SWG-based silicon photonic circuits.

Analytical investigation of surface plasmon excitation on a graphene sheet using four-wave mixing.

In the present paper, the general conditions for exciting graphene surface plasmon polaritons (GSPPs) on a suspended graphene using nonlinear optics are investigated. The approach uses the Green's function analysis to derive GSPP fields generated under the basis of momentum conservation using four-wave mixing (FWM). Since the incident beam polarization is challenging in the nonlinear excitation of GSPPs, the significant target of this paper has been set to achieve the conditions for the third-order susceptibility tensor and the wave vectors so that the incident beams with varied polarizations are able to excite GSPPs. Nonlinear optics, in particular FWM, is utilized to compensate the mismatch between the free-space and GSPPs wave vectors. In addition, it avoids the need for applying any patterning or lithography on graphene or its substrate.

Spectral response, dark current, and noise analyses in resonant tunneling quantum dot infrared photodetectors.

Reduction of dark current at high-temperature operation is a great challenge in conventional quantum dot infrared photodetectors, as the rate of thermal excitations resulting in the dark current increases exponentially with temperature. A resonant tunneling barrier is the best candidate for suppression of dark current, enhancement in signal-to-noise ratio, and selective extraction of different wavelength response. In this paper, we use a physical model developed by the authors recently to design a proper resonant tunneling barrier for quantum infrared photodetectors and to study and analyze the spectral response of these devices. The calculated transmission coefficient of electrons by this model and its dependency on bias voltage are in agreement with experimental results. Furthermore, based on the calculated transmission coefficient, the dark current of a quantum dot infrared photodetector with a resonant tunneling barrier is calculated and compared with the experimental data. The validity of our model is proven through this comparison. Theoretical dark current by our model shows better agreement with the experimental data and is more accurate than the previously developed model. Moreover, noise in the device is calculated. Finally, the effect of different parameters, such as temperature, size of quantum dots, and bias voltage, on the performance of the device is simulated and studied.

Fabrication-friendly subwavelength-structure-assisted waveguide for dispersion engineering.

A subwavelength structure deposited on top of a silicon strip is utilized as a novel tool for dispersion engineering. The equivalent refractive index of the subwavelength structure can be tailored through adjusting its period and duty cycle. As finding suitable materials with both appropriate refractive index and fabrication compatibility is one of the main difficulties in dispersion engineering, the possibility of refractive index engineering is the most significant advantage of the proposed waveguide. It can be beneficial for controlling the properties of the fundamental quasi-TM mode and consequently its dispersion characteristics without any concern about material compatibility. Utilizing this waveguide geometry, a wide and flattened low-dispersion bandwidth can be achieved. Moreover, high anomalous and normal dispersion is realizable without any degradation in dispersion flatness over bandwidth. Therefore, the proposed waveguide structure is promising for dispersion tailoring in both linear and nonlinear applications.

Swing effect of spatial solitons propagating through Gaussian and triangular waveguides.

We report the behavior of spatial optical solitons propagating through inhomogeneous waveguides with Gaussian, single triangular, and double-triangular refractive index profiles. In a given Gaussian profile, as the soliton amplitude decreases below a certain value, its behavior deviates from that of a particlelike soliton. Dependence of the swing period of a spatial soliton in a single triangular index profile on its amplitude, eta, is less significant than that in a Gaussian profile. We also report the interacting behavior of two solitons propagating simultaneously through a waveguide with a double-triangular index profile. Furthermore, we present the effects of the solitons' initial phase factors and amplitude on their behavior.

Dense wavelength-division multiplexing dispersion compensators based on chirped and apodized Fibonacci structures: CA-FC(j,n).

Chromatic dispersion compensation is an essential feature of high speed dense wavelength-division multiplexing (DWDM) systems. We propose a dispersion compensator structure whose characteristics meet the optical DWDM system requirements. The proposed structure is based on Fibonacci quasi-periodic multilayer structures composed of layers with large index differences. Studying the dispersive properties of Fibonacci structures with generation numbers j=3 and 4, and calculating group delay (GD) and group velocity dispersion (GVD) of their reflection bands, we have demonstrated that to have a smooth GD and almost a constant GVD in each band of a DWDM system, one needs not only to suitably chirp the structure refractive index profile, but also must properly apodize it. We also demonstrate the possibility of achieving high slope GDs and large GVDs by means of high order Fibonacci structures with thicker layers. Finally, by varying the layer dimensions and refractive indices as well as Fibonacci's order, one can design DWDM dispersion compensators suitable for distances as long as 220 km.