Shengfei Feng

Random Laser Based on Waveguided Plasmonic Gain Channels

A waveguide-plasmonic scheme is constructed by coating the matrix of randomly distributed gold nanoisland structures with a layer of dye-doped polymer, which provides strong feedback or gain channels for the emission from the dye molecules and enables successful running of a random laser. Excellent overlap of the plasmonic resonance of the gold nanoislands with the photoluminescence spectrum of the dye molecules and the strong confinement mechanism provided by the active waveguide layer are the key essentials for the narrow-band and low-threshold operation of this random laser. This kind of feedback configuration potentially enables directional output from such random lasers. The flexible solution-processable fabrication of the plasmonic gold nanostructures not only enables easy realization of such a random laser but also provides mechanisms for the tuning and multicolor operation of the laser emission.

A miniaturized sensor consisting of concentric metallic nanorings on the end facet of an optical fiber.

A polarization-independent optical sensor is created by fabricating a concentric gold ring grating with a period of 900 nm on the end facet of an optical fiber. The sensing function of this miniaturized device is realized by sending white light as a probe to the gold rings and collecting the response signal in the back-reflection through the optical fiber. A pronounced peak due to the Rayleigh anomaly of the gold ring grating is observed in the reflection spectrum, the center wavelength of which is sensitive to the change in the environmental refractive index of the fiber end facet. Theoretical analysis not only shows excellent agreement with the experimental results, but also gives insights into the mechanisms of this kind of sensor. Using the center position of the Rayleigh peak as the response signal, a high sensitivity dλ/dn of 900 nm per unity refractive index is realized for this sensor and a resolution of Δn/n ≈ 1% is demonstrated in preliminary experiments. The sensitivity is solely determined by the period of the grating.

Fiber coupled waveguide grating structures

Fabrication and characterization of the miniature device of waveguide grating-structures (WGS) on the end facet of an optical fiber are demonstrated. A layer of ZnO between the fiber and the grating structures serves as the waveguide. The fiber is used to direct the excitation light to the WGS and to carry the signal response back to the detection system. The narrow-band waveguide resonance mode tunable in the visible spectrum can be measured through the fiber in both the transmission and reflection. This nanodevice may be suitable as long-range sensors for the detection of refractive-index changes in nontransparent or toxic liquids.

Solution-processible fabrication of large-area patterned and unpatterned gold nanostructures

Solution-processible fabrication of large-area plasmonic nanostructures using colloidal gold nanoparticles has proven its advantages not only in its simplicity, low cost, high speed, and high flexibility, but also in the realization of some nanoscale optoelectronic devices that cannot be achieved by conventional methods. We demonstrate in this paper a variety of techniques for fabricating different plasmonic nanostructures using solution-processible gold nanoparticles and differently designed annealing processes. Using interference lithography and low- (<300 degrees Celsius) or high-temperature (>350 degrees Celsius) annealing processes, we succeeded in fabricating high-quality one- and two-dimensional metallic photonic crystals which exhibit promising optical responses for the exploration of new optoelectronic devices. Furthermore, using direct high-temperature (>350 degrees Celsius) annealing of a thin film of colloidal gold nanoparticles, we succeeded in fabricating large-area unpatterned isolated gold nano-island structures with a mean diameter tunable from about 35 to 100 nm and a mean height from 20 to 70 nm by controlling the concentration of the gold nanoparticle colloid, the annealing temperature, and the surface properties of the substrate. The corresponding optical response is thus tunable in the visible spectral range by changing the fabrication parameters. This introduces a new lithography-free technique for the preparation of gold nanostructures. These flexible fabrication techniques constitute a systematic route for the realization of plasmonic nanodevices.

Sensors Based on Plasmonic-Photonic Coupling in Metallic Photonic Crystals

An optical sensor based on the coupling between the plasmonic and photonic resonance modes in metallic photonic crystals is investigated. Large-area metallic photonic crystals consisting of periodically arranged gold nanostructures with dimensions down to sub-100 nm are fabricated using solution-processible gold nanoparticles in combination with interference lithography or interference ablation, which introduces a variety of fabrication techniques for the construction of this kind of sensor device. Sensitivity of the plasmonic response of the gold nanostructures to the changes in the environmental refractive index is enhanced through the coupling between the narrow-band photonic resonance mode and the relatively broad-band plasmon resonance, which is recognized as a Fano-like effect and is utilized to explore sensors. Theoretical modeling shows the characterization and the optimization of the sensitivity of this kind of sensor device. Theoretical and experimental results are demonstrated for the approaches to improve the sensitivity of the sensor device.

Waveguide Fabry-Pérot microcavity arrays

Spectroscopic properties of the Fabry-Perot (FP) microcavities arranged periodically on a dielectric waveguide are investigated theoretically. A strong coupling behavior between the waveguide resonance and the FP resonance modes is demonstrated for incident light with TE polarization. The narrow-band waveguide resonance modes are observed as asymmetric transmission peaks within the FP stop band, whereas they become transmission dips (antiresonance) in the FP pass band. The coupled mode transits between these two states as the waveguide resonance mode is tuned between the FP pass and stop bands. This transition can be explained in the Fano-picture and is important for the design of photonic devices with narrow-band and tunable optical response

Theoretical analysis on the tuning dynamics of the waveguide-grating structures

We investigate theoretically the tuning properties of the resonant mode of the waveguide-grating structures (WGS). This intends to understand how tuning mechanisms of the waveguide resonance mode depend on the structural and the geometric parameters of the WGS device, which can be used as guidance for the design of biosensors and other optoelectronic devices. The device parameters studied here include the angle of incidence, the thickness and refractive index of the waveguide, the period of the grating, and the refractive indices of the substrate and the medium on top of the grating. In particular, the control of the tuning rate and the adjustment of the tuning range by optimizing the combination of the relevant parameters provide a practical route for the design of biosensor and optical switch.

Spatial hole burning degradation of AlGaAs/GaAs laser diodes

The degradation of AlGaAs/GaAs laser diodes is studied in detail using laser scanning confocal microscopy, cathodoluminescence images, and x-ray diffraction (XRD) techniques. Our analysis has identified a degradation mechanism that results from the periodic distribution of the carrier density and the near-field intensity originating from periodic spatial hole burning. Based on the XRD measurements, we find that the epitaxial layer enters a polycrystalline phase during degradation due to the dark line defects, and the out-of-plane strain and in-plane compressive stress are induced by degradation.

Enhanced optical response in doubly waveguided plasmonic gratings

A waveguide fabricated on top of a bottom-waveguided gold-nanowire grating device excites additional resonance modes through interaction between the two waveguides, which overlaps the “intrinsic” resonance mode of the bottom waveguide. Thus, the TE polarized (parallel to the gold nanowires) waveguide mode and TM polarized (perpendicular to the nanowires) coupled mode between the waveguide mode and the particle plasmon resonance of the gold nanowires are enhanced significantly in a tunable spectral band. The top waveguide also extends considerably the spectral tuning range of this optical filter device.

Sensitivity enhancement through overlapping simultaneously excited Fano resonance modes of metallic-photonic-crystal sensors

We investigated enhancement of sensitivity of sensors based on metallic photonic crystals through tuning the thickness of the waveguide layer by pulsed laser deposition. Thicker waveguides made of InGaZnO allow double resonance of Fano coupling modes due to plasmonic-photonic interactions. Tuning the angle of incidence enables overlap between these doubly resonant modes, which induces much enlarged and spectrally narrowed sensor signals, leading to significantly enhanced sensitivity of the sensor device. The thickness of the waveguide layer is found to be a crucial structural parameter to improve sensitivity of the MPC sensors.