Achieving active control of the flow of light in nanoscale photonic devices is of fundamental interest in nanophotonics. For practical implementations of active nanophotonic devices, it is important to determine the sensitivity of the device properties to the refractive index of the active material. Here, we introduce a method for the sensitivity analysis of active nanophotonic waveguide devices to variations in the dielectric permittivity of the active material. More specifically, we present an analytical adjoint sensitivity method for the power transmission coefficient of nanophotonic devices, which is directly derived from Maxwell’s equations, and is not based on any specific numerical discretization method. We show that in the case of symmetric devices the method does not require any additional simulations. We apply the derived theory to calculate the sensitivity of the power transmission coefficient with respect to the real and imaginary parts of the dielectric permittivity of the active material for both two-dimensional and three-dimensional plasmonic devices. We consider Fabry-Perot cavity switches consisting of a plasmonic waveguide coupled to a cavity resonator which is filled with an active material with tunable refractive index. To validate our method, we compare it with the direct approach, in which the sensitivity is calculated numerically by varying the dielectric permittivity of the active material, and approximating the derivative using a finite difference. We find that the results obtained with our method are in excellent agreement with the ones obtained by the direct approach. In addition, our method is accurate for both lossless and lossy devices.
We design a non-parity-time-symmetric plasmonic waveguide-cavity system, consisting of two metal-dielectric-metal stub resonators side coupled to a metal-dielectric-metal waveguide, to form an exceptional point, and realize unidirectional reflectionless propagation at the optical communication wavelength. We also show that slow-light-enhanced ultra-compact plasmonic Mach-Zehnder interferometer sensors, in which the sensing arm consists of a waveguide system based on a plasmonic analogue of electromagnetically induced transparency, lead to an order of magnitude enhancement in the refractive index sensitivity compared to a conventional metal-dielectric-metal plasmonic waveguide sensor. Finally, we show that plasmonic coaxial waveguides offer a platform for practical implementation of plasmonic waveguide-cavity systems.
Waveguide-resonator systems are particularly useful for the development of several integrated photonic devices, such as
tunable filters, optical switches, channel drop filters, reflectors, and impedance matching elements. In this paper, we
introduce nanoscale devices based on plasmonic coaxial waveguide resonators. In particular, we investigate threedimensional
nanostructures consisting of plasmonic coaxial stub resonators side-coupled to a plasmonic coaxial
waveguide. We use coaxial waveguides with square cross sections, which can be fabricated using lithography-based
techniques. The waveguides are placed on top of a silicon substrate, and the space between inner and outer coaxial
metals is filled with silica. We use silver as the metal. We investigate structures consisting of a single plasmonic coaxial
resonator, which is terminated either in a short or an open circuit, side-coupled to a coaxial waveguide. We show that the
incident waveguide mode is almost completely reflected on resonance, while far from the resonance the waveguide mode
is almost completely transmitted. We also show that the properties of the waveguide systems can be accurately described
using a single-mode scattering matrix theory. The transmission and reflection coefficients at waveguide junctions are
either calculated using the concept of the characteristic impedance or are directly numerically extracted using full-wave
three-dimensional finite-difference frequency-domain simulations.
In this paper, we introduce slow-light enhanced nanoscale plasmonic waveguide devices for manipulating light at the
nanoscale. In particular, we investigate nanoplasmonic metal-dielectric-metal (MDM) waveguide structures for highsensitivity
sensors. Such plasmonic waveguide systems can be engineered to support slow-light modes. We find that, as
the slowdown factor increases, the sensitivity of the effective index of the mode to variations of the refractive index of
the material filling the structures increases. Such slow-light enhancements of the sensitivity to refractive index variations
lead to enhanced performance of active plasmonic devices such as sensors. We consider Mach-Zehnder interferometer
(MZI) sensors in which the sensing arm consists of a slow-light waveguide based on a plasmonic analogue of
electromagnetically induced transparency (EIT). We show that a MZI sensor using such a waveguide leads to
approximately an order of magnitude enhancement in the refractive index sensitivity, and therefore in the minimum
detectable refractive index change, compared to a MZI sensor using a conventional MDM waveguide.
We show that the space-mapping algorithm, originally developed for microwave circuit optimization, can enable the efficient optimization of nanoplasmonic devices. Space-mapping utilizes a physics-based coarse model to approximate a fine model accurately describing a device. The main concept in the algorithm is to find a mapping that relates the fine and coarse model parameters. If such a mapping is established, we can then avoid using the direct optimization of the computationally expensive fine model to find the optimal solution. Instead, we perform optimization of the computationally efficient coarse model to find its optimal solution, and then use the mapping to find an estimate of the fine model optimal. In this paper, we demonstrate the use of the space mapping algorithm for the optimization of metal dielectric- metal plasmonic waveguide devices. In our case, the fine model is a full-wave finite-difference frequency domain (FDFD) simulation of the device, while the coarse model is based on the characteristic impedance and transmission line theory. We show that, if we simply use the coarse model to optimize the structure without space mapping, the response of the structure obtained substantially deviates from the target response. On the other hand, using space mapping we obtain structures which match very well the target response. In addition, full-wave FDFD simulations of only a few candidate structures are required before the optimal solution is reached. In comparison, a direct optimization using the fine FDFD model in combination with a genetic algorithm requires thousands of full-wave FDFD simulations to reach the same optimal.