Pouya Dastmalchi

Slow-light enhanced subwavelength plasmonic waveguide refractive index sensors.

We introduce slow-light enhanced subwavelength scale refractive index sensors which consist of a plasmonic metal-dielectric-metal (MDM) waveguide based slow-light system sandwiched between two conventional MDM waveguides. We first consider a MDM waveguide with small width structrue for comparison, and then consider two MDM waveguide based slow light systems: a MDM waveguide side-coupled to arrays of stub resonators system and a MDM waveguide side-coupled to arrays of double-stub resonators system. We find that, as the group velocity decreases, the sensitivity of the effective index of the waveguide mode to variations of the refractive index of the fluid filling the sensors as well as the sensitivities of the reflection and transmission coefficients of the waveguide mode increase. The sensing characteristics of the slow-light waveguide based sensor structures are systematically analyzed. We show that the slow-light enhanced sensors lead to not only 3.9 and 3.5 times enhancements in the refractive index sensitivity, and therefore in the minimum detectable refractive index change, but also to 2 and 3 times reductions in the required sensing length, respectively, compared to a sensor using a MDM waveguide with small width structure.

Plasmonic coaxial waveguide-cavity devices.

We theoretically investigate three-dimensional plasmonic waveguide-cavity structures, built by side-coupling stub resonators that consist of plasmonic coaxial waveguides of finite length, to a plasmonic coaxial waveguide. The resonators are terminated either in a short or an open circuit. We show that the properties of these waveguide-cavity systems can be accurately described using a single-mode scattering matrix theory. We also show that, with proper choice of their design parameters, three-dimensional plasmonic coaxial waveguide-cavity devices and two-dimensional metal-dielectric-metal devices can have nearly identical transmission spectra. Thus, three-dimensional plasmonic coaxial waveguides offer a platform for practical implementation of two-dimensional metal-dielectric-metal device designs.

Plasmonic switches based on subwavelength cavity resonators

We design and optimize highly compact plasmonic switches with high modulation depth and moderate insertion loss, consisting of a metal-dielectric-metal waveguide coupled to a subwavelength cavity resonator. We consider a multisection cavity resonator which comprises multiple sections of varying widths. We find that the optimal structure is a perturbation of the maximum size cavity obtained by reducing the width of the middle section in order to tune the resonant wavelength of the cavity. In addition, the on-resonance modulation depth of the optimized multisection cavity switch is greatly enhanced with respect to a conventional Fabry–Perot cavity switch. We use a single-mode scattering matrix theory to account for the behavior of these systems.

Slow-light enhanced nanoscale plasmonic waveguide sensors and switches

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.

Efficient design of nanoplasmonic waveguide devices using the space mapping algorithm.

We show that the space mapping algorithm, originally developed for microwave circuit optimization, can enable the efficient design of nanoplasmonic waveguide devices which satisfy a set of desired specifications. Space mapping utilizes a physics-based coarse model to approximate a fine model accurately describing a device. Here the fine model is a full-wave finite-difference frequency-domain (FDFD) simulation of the device, while the coarse model is based on transmission line theory. We demonstrate that simply optimizing the transmission line model of the device is not enough to obtain a device which satisfies all the required design specifications. On the other hand, when the iterative space mapping algorithm is used, it converges fast to a design which meets all the specifications. In addition, full-wave FDFD simulations of only a few candidate structures are required before the iterative process is terminated. Use of the space mapping algorithm therefore results in large reductions in the required computation time when compared to any direct optimization method of the fine FDFD model.

Analytical method for the sensitivity analysis of active nanophotonic devices

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.

Plasmonic coaxial waveguides: cavity-based devices and slit-based couplers

We investigate 3D plasmonic coaxial waveguide devices. Optical response of stub resonators coupled to a coaxial waveguide is investigated. Also, slit-based structures for coupling free space light into plasmonic coaxial waveguides are introduced.

Subwavelength Plasmonic Two-Conductor Waveguides

This article reviews advances in the development of subwavelength plasmonic two-conductor waveguides. First, the properties of two-dimensional (2D) plasmonic two-conductor waveguides, which are commonly referred to as metal–dielectric–metal (MDM) waveguides, are discussed. This is followed by a review of the properties of three-dimensional (3D) plasmonic two-conductor waveguides, such as plasmonic slot waveguides and plasmonic coaxial waveguides. Following the discussion of plasmonic two-conductor waveguiding geometries, the properties of components based on these waveguides are reviewed. More specifically, we focus on bends and splitters that are essential components of optical integrated circuits. The properties of such bends and splitters in 2D and 3D plasmonic two-conductor waveguides are discussed. Finally, the fabrication and optical characterization methods for 2D and 3D plasmonic two-conductor waveguides are presented. Keywords: plasmonics; surface plasmons; waveguides

Unidirectional reflectionless propagation and slow-light enhanced sensing with plasmonic waveguide-cavity systems

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.

Nanoscale devices based on plasmonic coaxial waveguide resonators

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.