GeSbSeTe-based high extinction ratio optical modulator.

In this paper, a design for a high extinction ratio Mach-Zehnder optical modulator is proposed. The switchable refractive index of the germanium-antimony-selenium-tellurium (GSST) phase change material is employed to induce destructive interference between the waves passing through Mach-Zehnder interferometer (MZI) arms and to realize amplitude modulation. A novel, to the best of our knowledge, asymmetric input splitter is designed for the MZI to compensate for unwanted amplitude differences between MZI arms and increase the modulator performance. Three-dimensional finite-difference-time-domain simulations show a very high extinction ratio (ER) and low insertion loss (IL) of 45 and 2 dB, respectively, for the designed modulator at the wavelength of 1550 nm. Moreover, the ER is above 22 dB, and the IL is below 3.5 dB in the wavelength range of 1500-1600 nm. The thermal excitation process of GSST is also simulated using the finite-element method, and the speed and energy consumption of the modulator are estimated.

First molecular detection of SARS-CoV-2 virus in cockroaches.

Coronavirus is one of the main pathogens that primarily targets the human respiratory system. There are several ways to transmit this virus, such as direct contact or droplets spread by coughing or sneezing, and direct contact with fomites and surfaces is another way. This cross-sectional study was conducted in Shiraz, southern Iran, in 2021. 5 locations, including 3 hospitals and 2 dormitories, were selected for the survey. The cockroaches were collected from selected locations and transferred to the Laboratory of Medical Entomology at Shiraz University of Medical Sciences. All specimens were identified morphologically. The external and gastrointestinal washouts of collected samples with sterile phosphate-buffered saline separately were used for molecular analysis. An RT-qPCR assay, which suggests the possible insect­borne transmission, was used. External and gastrointestinal washout of B. germanica from Dastgheyb Dormitory and P. americana from Ali-Asghar Hospital were positive for contamination with the SARS-CoV-2. Cockroaches spread the virus in the environment and contaminate human food and various surfaces of buildings. Their role will be more important in crowded places such as hotels, lodging houses, restaurants, and hospitals; vector control programs should be carried out with more accuracy in such places.

Efficient binary and QAM optical modulation in ultra-compact MZI structures utilizing indium-tin-oxide.

A design for a CMOS-compatible active waveguide is proposed in which the epsilon-near-zero (ENZ) property of the indium-tin-oxide (ITO) is used to induce large variations in the real and imaginary parts of the waveguide effective index. The proposed waveguide comprises a TiN/HfO2/ITO metal-oxide-semiconductor (MOS) structure where the speed and power consumption are significantly improved by the application of the TiN and realization of double accumulation layers in the ITO. Simulations show the insertion loss (IL) of 0.38 dB/µm, extinction ratio (ER) of 11 dB/µm, the energy consumption of 11.87fJ/bit and electrical bandwidth of 280 GHz when the designed waveguide is used as an electro-absorption modulator. The waveguide is then used in an MZI structure to design binary and quadrature-amplitude-modulator (QAM) modulators. For binary modulator, the IL, ER, and VπLπ figures of merit are found to be 1.24 dB, 54 dB, and 6.4 V µm, respectively, which show substantial improvement over previous ITO-based designs. In the QAM design, the symmetry in the real and imaginary parts of the waveguide effective index is employed to obviate the need for additional phase shift elements. This considerably reduces the overall length of the proposed QAM modulator and improves efficiency. Simulations show the energy consumption and bit rate, of 2fJ/bit and 560 Gbps, respectively in a 4-QAM modulator with the overall length of 6.2 µm. The symmetry properties of the proposed waveguide can be further exploited to realize quadrature-phase-shift-keying (QPSK) modulators which here is used in combination with the 4-QAM to propose a design for the more advanced modulation scheme of 16-QAM. The design of ITO-based QAM modulators is here reported for the first time and the abovementioned performance parameters show the unique properties of these modulators in terms of footprint, energy consumption and modulation-speed.

Plasmon-induced transparency sensor for detection of minuscule refractive index changes in ultra-low index materials.

Detection of low-index materials such as aerogels and also detection of refractive index variations in these materials is still a challenging task. Here, a high figure of merit (FOM) sensor based on plasmon-induced transparency (PIT) is proposed for the detection of aerogel refractive index changes. In the proposed PIT sensor, the transparency window in an opaque region arises from the coupling between surface plasmon polariton (SPP) mode and planar waveguide mode. By comprising sub-wavelength grating (SWG) in the planar waveguide region, the maximum of the electric field of waveguide occurs in a low index media. This facilitates detection of the aerogels when they are used as the low index material (sensing material). Application of the subwavelength grating waveguide also improves the sensitivity of the sensor by a factor of six compared to a conventional structure with a homogenous waveguide. The proposed structure has a quality factor of Q ≥ 1800, and a reflection of 86%, and can detect the refractive index changes as low as Δn = 0.002 (around n = 1.0). The lineshape, Q-factor, and resonant wavelength of the transparency spectrum can be controlled by tailoring the structural parameters. Our work also has potential application in switching, filtering, and spectral shaping.

Low power, high speed, all-optical logic gates based on optical bistability in graphene-containing compact microdisk resonators.

CMOS-compatible all-optical logic gates based on optical bistability are designed and numerically characterized. For this, graphene and H-BN-based hybrid plasmonic microdisk/waveguide structures have been used to achieve optical bistability at very low threshold power and with small dimensions. The simulation results and coupled-mode theory calculations show that, by adjusting the radius of the microdisk resonator, the threshold power and response time of the optical bistability can be tuned in a wide range. It is shown that bistable devices with overall dimensions of 2µm×2.2µm can easily be designed having either threshold powers as low as 0.79 µW (microdisk radius of 0.92 µm) or very short fall time and rise times of 1.24 and 1.53 ps (microdisk radius of 0.93 µm). The design procedure for the AND, NAND, OR, NOR, and NOT logic gates is discussed. Simulation results show that the proposed logic gates have much smaller footprints, lower power consumption, and higher speeds with acceptable response time, compared with the previously reported structures.

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.

Design and numerical analysis of a high-performance optical modulator based on Si-VO2 Bragg grating waveguide.

Design and numerical characterization of a high-performance VO2-based optical modulator are proposed. The modulation is achieved by the phase transition of VO2 in a Bragg grating which can be formed by the selective VO2 deposition on a silicon strip waveguide. The interplay of the Bragg reflection and the inherent loss of the metal phase VO2 is used to increase the extinction ratio (ER) while the similarity of the refractive indices of the silicon and insulator phase VO2 resulted in a low insertion loss (IL). ER and IL of the modulator are 34.5 dB and 3.4 dB, respectively, at the wavelength of 1.55 µm, and they are, respectively, above 33 dB and below 3.5 dB across the entire optical C-band. The ER can be improved to 110 dB at the expense of an increased IL of 7.3 dB. The energy consumption and the modulation speed are estimated by considering different VO2 triggering schemes, and it is shown that the energy consumption of 91.7fJ/bit and the speed of 14 THz can be achieved with the proper VO2 stimulation. Furthermore, the robustness of the device performance to fabrication errors is studied by simulating the effect of the variation in different geometrical parameters.

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.

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.

Tunable graphene plasmonic Y-branch switch in the terahertz region using hexagonal boron nitride with electric and magnetic biasing.

A tunable graphene plasmonic Y-branch switch at THz wavelengths is proposed. The effects of magnetic and electric biasing are studied to harness the transmission of the transverse electric and magnetic guided mode resonances. In the structure, hexagonal boron nitride is utilized as a substrate for graphene. The application of hexagonal boron nitride, with the advantages of high mobility and ultralow ohmic loss, introduces a promising alternative substrate for graphene. Analytical and numerical results show that, by slight variation of the doping level in graphene through magnetic and electric biasing, the characteristics of the propagation of the guided mode resonances can be manipulated. A large extinction ratio of 40 dB at a wavelength of 60 µm is obtained. Besides, the proposed switch shows a low insertion loss of about 1 dB and a relatively large optical bandwidth of 1 µm. The electric biasing is of the order of 0.1 mV. Additionally, with the presence of magnetic biasing, a compact switch with a size of 25 µm is achieved. Showing a high extinction ratio, low insertion loss, and compact size, the proposed switch can find potential applications in graphene plasmonics integrated devices.

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.

Ultra-wideband high-speed Mach-Zehnder switch based on hybrid plasmonic waveguides.

In this paper, the distinctive dispersion characteristic of hybrid plasmonic waveguides is exploited for designing ultra-wideband directional couplers. It is shown that by using optimized geometrical dimensions for hybrid plasmonic waveguides, nearly wavelength-independent directional couplers can be achieved. These broadband directional couplers are then used to design Mach-Zehnder-interferometer-based switches. Our simulation results show the ultra-wide bandwidth of ∼260 nm for the proposed hybrid plasmonic-waveguide-based switch. Further investigation of the proposed Mach-Zehnder switch confirms that because of the strong light confinement in the hybrid plasmonic waveguide structure, the switching time, power consumption, and overall footprint of the device can be significantly improved compared to silicon-ridge-waveguide-based Mach-Zehnder switches. For the Mach-Zehnder switch designed by using the optimized directional coupler, the switching time is found to be less than one picosecond, while the power consumption, VπLπ figure of merit, and active length of the device are ∼61 fJ/bit, 85 V×µm, and 30 µm, respectively.

Optical bistability in a one-dimensional photonic crystal resonator using a reverse-biased pn-junction.

Optical bistability provides a simple way to control light with light. We demonstrate low-power thermo-optical bistability caused by the Joule heating mechanism in a one-dimensional photonic crystal (PC) nanobeam resonator with a moderate quality factor (Q ~8900) with an embedded reverse-biased pn-junction. We show that the photocurrent induced by the linear absorption in this compact resonator considerably reduces the threshold optical power. The proposed approach substantially relaxes the requirements on the input optical power for achieving optical bistability and provides a reliable way to stabilize the bistable features of the device.

Transmission line model for extraction of transmission characteristics in photonic crystal waveguides with stubs: optical filter design.

A simple and efficient transmission line model is proposed here to study how the transmission characteristics of photonic crystal waveguides are tailored by introduction of stubs patterned in the photonic crystal lattice. It is shown that band-pass and band-stop optical filters can be easily designed and optimized when stubs of appropriate length are brought in. Since the lengths of the designed stubs are not necessarily integer multiples of the photonic crystal lattice constant, a geometric shift in a portion of the photonic crystal structure is shown to be essential. The proposed model is verified by using a rigorous numerical method. An excellent agreement is observed between the numerical results and the transmission characteristics as extracted by the proposed model.

Transmission-line model to design matching stage for light coupling into two-dimensional photonic crystals.

The transmission-line analogy of the planar electromagnetic reflection problem is exploited to obtain a transmission-line model that can be used to design effective, robust, and wideband interference-based matching stages. The proposed model based on a new definition for a scalar impedance is obtained by using the reflection coefficient of the zeroth-order diffracted plane wave outside the photonic crystal. It is shown to be accurate for in-band applications, where the normalized frequency is low enough to ensure that the zeroth-order diffracted plane wave is the most important factor in determining the overall reflection. The frequency limitation of employing the proposed approach is explored, highly dispersive photonic crystals are considered, and wideband matching stages based on binomial impedance transformers are designed to work at the first two photonic bands.

Geometrical approach in physical understanding of the Goos-Haenchen shift in one- and two-dimensional periodic structures.

The Goos-Haenchen shift of a totally reflected beam at the planar interface of two dielectric media, as if the incident beam is reflected from beneath the interface between the incident and transmitted media, has been geometrically associated with the penetration of the incident photons in the less-dense forbidden transmission region. This geometrical approach is here generalized to analytically calculate the Goos-Haenchen shift in one- and two-dimensional periodic structures. Several numerical examples are presented, and the obtained results are successfully tested against the well-known Artman's formula. The proposed approach is shown to be a fast, simple, and efficient method that can provide good physical insight to the nature of the phenomenon.