Yalin Su

Guiding of Long-Range Hybrid Plasmon Polariton in a Coupled Nanowire Array at Deep-Subwavelength Scale

A novel type of hybrid plasmonic waveguiding structure that integrates semiconductor and metallic nanowires has been proposed and investigated at telecommunication wavelengths. Semiconductor nanowires symmetrically placed on both sides of a metallic nanowire provide an additional degree of freedom for tuning the characteristics of the plasmonic nanowire mode. Theoretical analysis reveals that at appropriate geometrical parameters, the symmetric hybrid plasmonic mode of the waveguide could achieve subwavelength mode confinement with ultra-long propagation distance (even exceeding the millimeter range). Such a hybrid plasmonic nanowire structure could facilitate ultra-strong light-matter interaction between semiconductor and metal materials, and enable important applications in nanolasers and nonlinear photonics.

Highly Confined Hybrid Plasmonic Modes Guided by Nanowire-Embedded-Metal Grooves for Low-Loss Propagation at 1550 nm

A waveguiding configuration consisting of a semiconductor nanowire embedded in a dielectric-coated V-shaped metal groove is presented. The modal properties of the fundamental quasi-TE hybrid plasmonic mode are investigated at the wavelength of 1550 nm. Simulation results reveal that by tuning the size of the nanowire, the hybridization between the dielectric mode, and plasmonic mode could be effectively controlled. Through appropriate design, the hybrid mode could be strongly localized in the nanowire and the gap regions on each side, featuring both tight-mode confinement and low propagation loss. Besides, the compromise between confinement and loss could also be balanced by controlling the angle or depth of the metal groove. Moreover, it is found that the hybrid mode could exist for a wide geometrical parameter range, even when the corresponding metal groove by itself does not support a guided channel plasmon polariton mode. The proposed hybrid structure is technologically simple and compatible with planar fabrication methods while avoiding alignment errors.

Dielectrics Covered Metal Nanowires and Nanotubes for Low-Loss Guiding of Subwavelength Plasmonic Modes

Two types of surface plasmon polariton waveguides consisting of square metal nanowires or nanotubes covered by low-high index dielectric layers are presented and their guiding characteristics are investigated numerically at the telecom wavelength of 1550 nm. Numerical analysis reveals that depending on the sizes of the covered low-high index dielectric layers, the nanowire based waveguides can be tuned to provide subwavelength confinement of the plasmonic modes with low propagation loss. Even enhanced optical confinement could be achieved by further increasing the nanowire size or replacing the metal nanowires by nanotubes. Consideration of directional coupling between two identical such plasmonic waveguides reveal ultra-low-crosstalk can be realized with relatively small separation distances. These waveguiding structures, compatible with modern fabrication methods, potentially enable the realization of numerous ultra-compact integrated photonic components.

Hybrid differential interrogation method for sensitive surface plasmon resonance measurement enabled by electro-optically tunable SPR sensors.

A novel detection method enabled by electro-optically tunable waveguide-coupled surface plasmon resonance sensors is demonstrated. Both the WCSPR response of sensor and the interrogation light are varied simultaneously in this hybrid scheme. Modulation and demodulation of the sensors response are achieved by applying a high-frequency AC electrical signal and electrically filtering the detected signal. Scanning the incident angle at a lower speed yields an angular dependent response. Theoretical analyses and experimental results show that the angular-dependent signal is closely related to the derivative of the SPR reflectivity with a sharp, linear jump near the minimum of the SPR peak. Thus, simple linear-fitting and zero-finding algorithms can be used to locate the SPR angle, and sophisticated data processing algorithms and electronic hardware can be avoided.

Hybrid plasmon polariton guiding with tight mode confinement in a V-shaped metal/dielectric groove

Plasmonic waveguides consisting of metallic grooves filled with low- and high-index dielectrics are proposed and the optical properties of guided plasmonic modes are investigated at a wavelength of 1550 nm. Numerical simulations reveal that the quasi-TE-like fundamental hybrid plasmonic mode exhibits strong localization near the low-index dielectric gap, along with pronounced local field enhancement and relatively small mode area. Moderate propagation loss can be achieved as well, corresponding to a propagation distance of around tens to hundreds of microns. The proposed hybrid plasmonic structure is compatible with the fabrication techniques of traditional channel plasmon polariton waveguides, and these structures could be employed as interesting building blocks for highly integrated photonic circuits.

Hybrid plasmonic waveguide incorporating an additional semiconductor stripe for enhanced optical confinement in the gap region

A hybrid plasmonic waveguide consisting of a thin high-index dielectric stripe embedded inside the gap between a metallic substrate and a semiconductor ridge is presented for the purpose of enhanced optical confinement in the gap. By engineering the key geometrical parameters of the stripe, both of the power ratios resided inside the whole gap and the silicon ridge can be enhanced greatly. A power confinement ratio as large as 0.54 in the overall gap region is achievable, for a structure with a 200 nm-wide, 90 nm-thick silicon-stripe embedded in the center of a 100 nm-thick silica gap, which is nearly 50% improvement over that of the corresponding conventional hybrid waveguide. Meanwhile, with the introduction of the 90 nm-thick silicon stripe, the effective mode area of the waveguide exhibits a reduction of 50%‐60% with a reasonable propagation length around 25‐65 m for different stripe widths. A study on the influence of possible fabrication imperfections reveals that the modal property is quite robust and highly tolerant to these errors. Such a hybrid plasmonic waveguide with enhanced optical confinement and moderate modal loss may enable the realization of ultra-compact passive components, nanolasers with low pumping thresholds, and other potential applications.

Hybrid plasmonic structures based on CdS nanotubes: a novel route to low-threshold lasing on the nanoscale

Nanowires and nanotubes could become important building blocks in advanced photonic systems owing to their fascinating optoelectronic properties and high compatibility with versatile chemical synthetic methods. Many intriguing studies have been enabled by applying these nanostructures in the construction of various types of active and passive photonic components. Successful examples are the recent demonstration of semiconductor and plasmonic lasers based on CdS nanowires (Duan et al 2003 Nature 421 241?5, Oulton et al 2009 Nature 461 629?32, Ma et al 2010 Nature Mater. 10 110?13), which generate and deliver intense coherent light down to and even below the diffraction-limited scale. Here in this paper, by carrying out a numerical investigation of a novel hybrid plasmonic structure that consists of a CdS nanotube sitting above a metal substrate separated by a nanometric MgF2 layer, we show theoretically that nanotube-based plasmonic structures can also act as highly efficient lasing sources. Optical properties of such a laser configuration including modal behaviour and the lasing threshold is investigated with regard to the variation of key geometrical parameters. Simulation results reveal that the employment of a CdS nanotube may result in improved optical performance compared with the conventional CdS-nanowire-based plasmon laser. Reduced lasing threshold with mitigated modal loss can be achieved simultaneously under carefully engineered geometries. We also explore the feasibility of combining nanowire- and nanotube-based active and passive components for on-chip integrations. As a simple demonstration, monolithic integration of a CdS nanotube laser with a CdS-nanowire-based passive component is shown numerically on a single chip. We expect that these studies could lay the foundations for nanotube- and nanowire-based hybrid integrated photonic components and circuits.

Sensitive voltage interrogation method using electro-optically tunable SPR sensors

A novel voltage interrogation method using electro-optically tunable waveguide-coupled surface plasmon resonance sensors is demonstrated. Before measurements, we use a bicell photodetector to detect the reflectance from the sensor and take the differential signal from the photodetector as the resonance condition. For different analytes, by scanning the DC voltage on the waveguide layer of the sensor chip, the resonance condition can be maintained the same. Under this condition, we record the values of this voltage, namely the resonant voltage. Theoretical calculations and experimental results show the resonant voltage has a highly linear and sensitive response to analytes refractive index. This method is simple in configuration, and complicated signal processing algorithms can be avoided.

Metal-coated hollow nanowires for low-loss transportation of plasmonic modes with nanoscale mode confinement

Two types of plasmonic waveguiding structures based on hollow dielectric nanowires are proposed and their modal properties are investigated numerically at a wavelength of 1550 nm. The first type of waveguide consists of a high-index hollow nanowire covered directly by a thin metallic film. Depending on the size of the hollow nanowire, such a waveguide could support a plasmonic mode with lower propagation loss than the metal-coated nanowire structures without a hollow core. To further reduce the propagation loss, a second type of waveguide is proposed, which includes an additional low-index silica buffer layer between the metal layer and the hollow nanowire. Simulations reveal that the additional low-index buffer could enable strong hybridization between the dielectric mode and the plasmonic mode, which leads to even lower propagation loss while maintaining nanoscale confinement similar to that of the first type of waveguide. Both of the proposed waveguides are feasible using modern fabrication methods and could facilitate potential applications in integrated photonic components and circuits.