The correct numerical calculation of the resonance characteristics and, principally, the quality factor Q of contemporary photonic and plasmonic resonant systems is of utmost importance, since Q ...defines the bandwidth and affects nonlinear and spontaneous emission processes. Here, we comparatively assess the commonly used methods for calculating Q using spectral simulations with commercially available, general-purpose software. We study the applicability range of these methods through judiciously selected examples covering different material systems and frequency regimes from the far-infrared to the visible. We take care in highlighting the underlying physical and numerical reasons limiting the applicability of each one. Our findings demonstrate that in contemporary systems (plasmonics, 2D materials) Q calculation is not trivial, mainly due to the physical complication of strong material dispersion and light leakage. Our work can act as a reference for the mindful and accurate calculation of the quality factor and can serve as a handbook for its evaluation in guided-wave and free-space photonic and plasmonic resonant systems.
Higher-order digital modulation formats are demonstrated by electrically inducing free-carrier concentration changes in thin films of transparent conducting oxides, integrated into well-established ...silicon-photonic waveguiding architectures. The proposed near-infrared modulators employ as physical platforms the silicon-rib and silicon-slot waveguides, exploiting the highly dispersive and carrier-dependent epsilon-near-zero behavior of transparent conducting oxides to modulate the optical carrier. Advancing the existing studies on conventional amplitude modulation, phase-shift keying formats are investigated in this paper, using a rigorous and physically consistent modeling framework that seamlessly combines solid-state physics with Maxwell wave theory through carrier-dependent material models. The designed in-line modulators achieve Vπ L products in the order of 0.1 Vmm, two orders of magnitude lower than their respective all-silicon or lithium niobate counterparts, accompanied by an insertion loss of about 3 dB/π. Switching speeds in the order of 50 GHz are feasible along with a potential for sub-pJ/symbol energy consumption, meeting the demands for on-chip optical modulation.
A side-coupled disk-waveguide system based on a long-range hybrid plasmonic waveguiding configuration is theoretically investigated for Kerr bistability and self-pulsation. The nonlinear response is ...studied with a theoretical framework combining perturbation theory and temporal coupled-mode theory, where all relevant effects, including two-photon absorption (TPA) as well as free-carrier dispersion (FCD) and absorption (FCA), are taken into account. We show that bistable operation with input powers as low as 40 mW is possible; a consequence of the significant reduction in resistive losses made possible by the long-range waveguiding principle. The effect of TPA, FCD, and FCA on the nonlinear response is thoroughly assessed and it is shown that carrier lifetime must drop to 8 ps in order to suppress free-carrier effects and obtain a high-quality bistable response, which is subsequently exploited for demonstrating ultrafast memory operation with low power requirements and high extinction ratio between states. Finally, by appropriate tuning of the carrier lifetime, FCD can lead to self-pulsation. The operating regimes in the detuning-power space are identified and the characteristics of the spontaneous oscillation discussed.
We present the study of a proof-of-concept integrated device that can be used as a nonlinear broadband isolator. The device is based on the asymmetric loading of a highly-confining silicon-slot ...photonic coupler with graphene layers, whose ultrafast and low-threshold saturable absorption can be exploited for nonreciprocal transmission between the cross-ports of the coupler. The structure is essentially a non-Hermitian system, whose exceptional points are briefly discussed. The nonlinear device is modeled with a coupled Schrödinger equation system whose validity is checked by full-vector finite element-based beam-propagation method simulations in CW. The numerically computed performance reveals a nonreciprocal intensity range (NRIR) in the vicinity of 100 mW peak power with a bandwidth spanning tens of nanometers, from CW down to ps-long pulses. Finally, the combination of saturable absorption and self-phase modulation (Kerr effect) in graphene is studied, indicating the existence of two NRIR with opposite directionality.
Longitudinal 2 × 2 thermo-optic switches based on dielectric-loaded plasmonic circuitry are thoroughly studied using vectorial numerical tools, i.e., an eigenmode solver and a beam propagation ...method, implemented with the finite-element technique. The switching configurations studied are based on the Mach-Zehnder interferometer (MZI) and a novel multimode interference (MMI) design. Both components offer improved performance, with respect to comparable microring-based ones, exhibiting larger bandwidth and output port extinction ratio, allowing values up to 30 dB. The reference MZI configuration has better overall performance when compared to the MMI, mainly in terms of available bandwidth.
We employ graphene saturable absorption for the theoretical demonstration of saturable absorption mirrors based on planar silicon photonic Bragg gratings. Two geometries are investigated, that of a ...silicon wire grating and a silicon slot grating, showcasing the increased light-matter interaction of the high-confinement slot waveguide. The gratings are designed in the linear regime for single mode operation, low footprint and broadband operation in the near infrared optical communications frequency range. The saturable absorption effect is introduced through the saturation of graphene's interband surface conductivity, and we discuss the necessary biasing conditions and applicability of our CW approach on ps-long pulses. We also rigorously include other, possibly detrimental, nonlinear effects, such as silicon's Kerr effect and two photon absorption, and graphene's Kerr effect. These effects are proven to have a negligible impact on the operation of the graphene saturable absorber mirror, thanks to the much lower power threshold of graphene's saturable absorption. Finally, we calculate the nonlinear reflectance and transmittance of the graphene-enhanced Bragg gratings and demonstrate that they can provide high modulation depths at low saturation powers, both highly valued characteristics of saturable absorber mirrors for mode locking applications in view of next-generation integrated pulsed photonic light sources.
A nanophotonic passively Q‐switched lasing element in the near infrared is proposed and theoretically investigated. It consists of a silicon‐rich nitride disk resonator enhanced with the contemporary ...MoS2/WSe2$\left(\text{MoS}\right)_{2} / \left(\text{WSe}\right)_{2}$ hetero‐bilayer and a graphene monolayer to provide gain and saturable absorption, respectively. The two‐dimensional materials are placed on top of the disk resonator and are separated by a spacer of hexagonal boron nitride. MoS2/WSe2$\left(\text{MoS}\right)_{2} / \left(\text{WSe}\right)_{2}$ emits at 1128 nm due to the radiative recombination of interlayer excitons after being optically pumped at 740 nm. Optical pumping is conducted in a guided‐wave manner aiming at achieving a high overall efficiency by critically coupling to a cavity mode near the pump transition. The response of the proposed pulsed laser is assessed by utilizing a coupled‐mode theory framework fed with linear finite‐element method simulations, rigorously derived from the Maxwell–Bloch equations. Following a meticulous design process and exploiting the guided pumping scheme, an ultralow lasing threshold of just 24.2 $24 . 2 \text{ } \mu \text{W} $μW is obtained. Overall, the Q‐switched laser delivers pulsed light inside an integrated bus waveguide with mW peak power, ps duration, and GHz repetition rates requiring sub‐mW continuous wave pumping. These properties are highly promising for communication applications and highlight the potential of two‐dimensional materials for nanophotonic light sources.
A nanophotonic passively Q‐switched integrated laser relying on two‐dimensional materials is proposed and thoroughly studied. The optically pumped gain is provided by a transition metal dichalcogenide hetero‐bilayer (MoS2/WSe2$\left(\text{MoS}\right)_{2} / \left(\text{WSe}\right)_{2}$) and the saturable absorption by a graphene monolayer. Following a meticulous design process, mW peak power, ps pulse duration, and GHz repetition rates are achieved, while requiring sub‐mW pump power.
We propose metal-semiconductor-metal cavity arrays as active elements of electrically tunable metasurfaces operating in the terahertz spectrum. Their function is based on reverse biasing the Schottky ...junction formed between top metal strips and the n-type semiconductor buried beneath. A gate bias between the strips and a back metal reflector controls the electron depletion layer thickness thus tuning the Drude permittivity of the cavity array. Using a rigorous multiphysics framework which combines Maxwell equations for terahertz waves and the drift-diffusion model for describing the carrier behavior in the semiconductor, we find a theoretically infinite extinction ratio, insertion loss of around 10%, and picosecond intrinsic switching times at 1 THz, for a structure designed to enter the critical coupling regime once the depletion layer reaches the bottom metal contact. We also show that the proposed modulation concept can be used for devices operating at the higher end of the terahertz spectrum, discussing the limitations on their performance.
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