Hybridization-induced resonances with high-quality factor in a plasmonic chipscale ring-disk nanocavity

Plasmonic resonators have drawn more attention due to the ability to confine light into subwavelength scale. However, they always suffer from a low-quality (Q) factor owing to the intrinsic loss of metal. Here, we numerically propose a plasmonic resonator with ultra-high Q factor based on plasmonic metal–insulator-metal (MIM) waveguide structures. The resonator consists of a disk cavity surrounded by a concentric ring cavity, possessing an ultra-small volume. Arising from the plasmon hybridization between plasmon modes in the disk and ring cavity, the induced bonding hybridized modes have an ultra-narrow full width at half maximum (FWHM) as well as ultra-high Q factors. The FWHM can be nearly 1 nm and Q factor can be more than 400. Furthermore, such a device can act as a refractive index sensor with an ultra-high figure of merit (FOM). This work provides a novel approach to design plasmonic high-Q-factor resonators and has potential on-chip applications such as filters, multi-spectral sensors and nanolasers

Chipscale plasmonic modulators and switches based on metal-insulator-metal waveguides with Ge

We introduce phase-change material Ge2Sb2Te5 (GST) into metal–insulator–metal (MIM) waveguide systems to realize chipscale plasmonic modulators and switches in the telecommunication band. Benefitting from the high contrast of optical properties between amorphous and crystalline GST, the three proposed structures can act as reconfigurable and nonvolatile modulators and switches with excellent modulation depth 14 dB and fast response time in subnanosecond while possessing small footprints, simple frameworks, and easy fabrication. We provide solutions to design active devices in MIM waveguide systems and can find potential applications in more compact all-optical circuits for information processing and storage

Novel on-chip applications using silicon photonics

The emerging field of silicon photonics offers solutions to designing CMOScompatible optical devices. By taking advantage of the immense fabrication infrastructure offered by the silicon industry, it would be possible to design optical structures that are smaller, faster, less-power consuming and cheaper than traditional, non-silicon-based optical devices. In this dissertation, the design and performance testing of two novel silicon photonic structures are presented: 1) Silicon nanowire ridge waveguide for sensing applications at the optical transmission frequency. 2) Doped silicon plasmonic structures for negative-index, epsilon-near-zero and sensing applications at the mid-infrared. For the first design, silicon nanowires are arranged in a ridge shape to act as a sensitive medium. Most conventional optical sensors rely on evanescent field detection, where only the tail of the incident wave contributes to the sensing process. Silicon nanowires, on the other hand, allow a large portion of the light wave to be present in the low-index-region, which is comprised of the gaps between the nanowires. This feature provides an opportunity for the optical wave to be vastly affected by the refractive index of the gaps between the nanowires. Thus, by introducing materials-under-test, or analytes, to the gaps, the optical signal response is heavily altered, hence, providing a much larger sensitivity than evanescent field sensors. Previous literature has reported on rib-shaped silicon nanowire sensors. We show that the proposed design is more superior in several areas. Firstly, simulations show that ridge-shaped sensors respond slightly more strongly to refractive-index changes than rib-shaped sensors. This could be due to their slightly larger optical overlap with their surroundings, provided by the inherently larger dimensions of the ridge shape. They can also detect up to a 1e-8 refractive-index-changes in the surrounding environment. Secondly and more importantly, single-mode operation, which is usually mandatory for most optical sensor configurations, can be guaranteed using more flexible dimensions for the ridge-shaped nanowire waveguide than for its rib-shaped counterpart. This provides a fabrication convenience, since devices with larger dimensions are generally cheaper and easier to manufacture. Additionally, since the ridge waveguide sports a wide low-thickness region, it can cover the substrate and safe-guard it against erosion during the fabrication process. To further characterize our design, the ridge-shaped silicon nanowire waveguide was put in a bimodal interferometer sensor configuration. The sensitivity obtained through FDTD simulations proved to be very high; its value was comparable to recently reported sensitivities using much larger footprints. This very high sensitivity-to-footprint ratio, along with the CMOS compatibility of the proposed design, deems it as a suitable candidate for on-chip integration. In the second portion of this dissertation, we aim to model and characterize highconfinement plasmonic devices using doped silicon in the mid-infrared range. The midinfrared range is home to some important applications such as molecular sensing, environmental monitoring and security applications. Traditional plasmonic devices possess some much desired features that may be utilized in the mid-infrared. These include hosting a high surface sensitivity, and having the ability to guide and confine light through subwavelength structures, including sharp bends. However, much of these features are exclusive to the near-infrared and visible frequencies, where the plasma resonance of conventional plasmonic materials lie. This is because conventional materials tend to suffer from low confinement in the mid-infrared region, and are rendered inconvenient for applications that require high confinement such as sensing and on-chip communications. For this reason and for its CMOS compatibility, doped silicon plasmonics seems to be a viable solution. We demonstrate that plasma resonance tunability can be achieved through controlling the doping level. Moreover, the dispersion characteristics of doped silicon devices were analyzed for different applications at the mid-infrared region, and displayed valuable phenomenon such as negative dispersion, which can be utilized for slow light and metamaterial applications, and epsilon-near-zero characteristics, that can be used for extraordinary transmission. The mid-infrared dispersion was studied thoroughly for multiple structures, including the slot structure and the rectangular shell structure. The potential for biological and environmental sensing for the aforementioned structures, as well as for a doped silicon nanoparticle was investigated and very high sensitivity was achieved. In addition, the performance of the slot structure in the negative dispersion region was established through using the waveguide as a slow light medium. Moreover, plasmonic structures that can be used for on-chip light-guiding, such as bends and junctions were evaluated. FDTD simulations showed superior performance through successful lightsplitting in gaps as wide as 1μm

Plasmofluidic Disk Resonators

Waveguide-coupled silicon ring or disk resonators have been used for optical signal processing and sensing. Large-scale integration of optical devices demands continuous reduction in their footprints, and ultimately they need to be replaced by silicon-based plasmonic resonators. However, few waveguide-coupled silicon-based plasmonic resonators have been realized until now. Moreover, fluid cannot interact effectively with them since their resonance modes are strongly confined in solid regions. To solve this problem, this paper reports realized plasmofluidic disk resonators (PDRs). The PDR consists of a submicrometer radius silicon disk and metal laterally surrounding the disk with a 30-nm-wide channel in between. The channel is filled with fluid, and the resonance mode of the PDR is strongly confined in the fluid. The PDR coupled to a metal-insulator-silicon-insulator-metal waveguide is implemented by using standard complementary metal oxide semiconductor technology. If the refractive index of the fluid increases by 0.141, the transmission spectrum of the waveguide coupled to the PDR of radius 0.9 mu m red-shifts by 30 nm. The PDR can be used as a refractive index sensor requiring a very small amount of analyte. Plus, the PDR filled with liquid crystal may be an ultracompact intensity modulator which is effectively controlled by small driving voltageopen

Integrating vernier spectrum with fano resonance for high sensitivity of an all-optical sensor

Vernier and Fano resonances are promising approaches for enhancing the sensitivity of an all-optical sensor. A theoretical analysis was performed to integrate a Fano-like resonance shape with a Vernier resonance by considering the presence of partially reflective end facets at a double microring resonator waveguide. The system was developed based on scattering matrix and optical transfer function. The all-pass racetrack microring resonator (ARMRR) and the double racetrack microring resonator (DRMRR) were compared with and without the end facet at the waveguide to analyze the dynamic change of the output resonance spectrum. The spectrum was analyzed based on the free spectral range and resonance pattern. The resonator systems were applied to a refractive index-based sensing protocol, which was operated by a resonance wavelength shift with a refractive index change. The sensitivity was optimized by varying the configuration parameters such as the radius of the ring, the distance between the end facet, and the coupling coefficients. Integrating Vernier spectrum with Fano resonance improved the sensitivity for ARMRR configuration by 5.16% and the sensitivity for DRMRR configuration by 6.31%. The recorded limit of detection (LOD) of the DRMRR was 3.30 × 10-

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.Comment: Accepted for publication in Reports on Progress in Physic