Jingyi Lou

Single-mode guiding properties of subwavelength-diameter silica and silicon wire waveguides

Single-mode optical wave guiding properties of silica and silicon subwavelength-diameter wires are studied with exact solutions of Maxwells equations. Single mode conditions, modal fields, power distribution, group velocities and waveguide dispersions are studied. It shows that air-clad subwavelength-diameter wires have interesting properties such as tight-confinement ability, enhanced evanescent fields and large waveguide dispersions that are very promising for developing future microphotonic devices with subwavelength-width structures.

Modeling of silica nanowires for optical sensing

Based on evanescent-wave guiding properties of nanowire waveguides, we propose to use single-mode subwavelength-diameter silica nanowires for optical sensing. Phase shift of the guided mode caused by index change is obtained by solving Maxwells equation, and is used as a criterion for sensitivity estimation. Nanowire sensor employing a wire-assembled Mach-Zehnder structure is modeled. The result shows that optical nanowires, especially those fabricated by taper drawing of optical fibers, are promising for developing miniaturized optical sensors with high sensitivity.

Photonic nanowires directly drawn from bulk glasses

High-uniform nanowires with diameters down to 50 nm are directly taper-drawn from bulk glasses. Typical loss of these wires goes down to 0.1 dB/mm for single-mode operation. Favorable photonic properties such as high index for tight optical confinement in tellurite glass nanowires and photoluminescence for active devices in doped fluoride and phosphate glass nanowires are observed. Supporting high-index tellurite nanowires with solid substrates (such as silica glass and MgF2 crystal) and assembling low-loss microcoupler with these wires are also demonstrated. Photonic nanowires demonstrated in this work may open up vast opportunities for making versatile building blocks for future micro- and nanoscale photonic circuits and components.

Fast detection of humidity with a subwavelength-diameter fiber taper coated with gelatin film

A subwavelength-diameter tapered optical fiber coated with gelatin layer for fast relative humidity (RH) sensing is reported. The sensing element is composed of a 680-nm-diameter fiber taper coated with a 80-nm-thickness 8-mm-length gelatin layer, and is operated at a wavelength of 1550 nm. When exposed to moisture, the change in refractive index of the gelatin layer changes the mode field of the guided mode of the coated fiber, and converts a portion of power from guided mode to radiation mode, resulting in RH-dependent loss for optical sensing. The sensor is operated within a wide humidity range (9-94% RH) with high sensitivity and good reversibility. Measured response time is about 70 ms, which is one or two orders of magnitude faster than other types of RH sensors relying on conventional optical fibers or films.

Self-modulated taper drawing of silica nanowires

We report a self-modulated taper-drawing process for fabricating silica nanowires with diameters down to 20?nm. Long amorphous silica nanowires obtained with this top-down approach present extraordinary uniformities that have not been achieved by any other means. The measured sidewall roughness of the wires goes down to the intrinsic value of 0.2?nm, along with a diameter uniformity better than 0.1%. The wires also show high strength and pliability for patterning under optical microscopes. The ability to prepare and manipulate highly uniform silica nanowires may open up new opportunities for studying and using low-dimensional silica material on a nanometre scale.

Microfiber optical sensors: a review.

With diameter close to or below the wavelength of guided light and high index contrast between the fiber core and the surrounding, an optical microfiber shows a variety of interesting waveguiding properties, including widely tailorable optical confinement, evanescent fields and waveguide dispersion. Among various microfiber applications, optical sensing has been attracting increasing research interest due to its possibilities of realizing miniaturized fiber optic sensors with small footprint, high sensitivity, fast response, high flexibility and low optical power consumption. Here we review recent progress in microfiber optical sensors regarding their fabrication, waveguide properties and sensing applications. Typical microfiber-based sensing structures, including biconical tapers, optical gratings, circular cavities, Mach-Zehnder interferometers and functionally coated/doped microfibers, are summarized. Categorized by sensing structures, microfiber optical sensors for refractive index, concentration, temperature, humidity, strain and current measurement in gas or liquid environments are reviewed. Finally, we conclude with an outlook for challenges and opportunities of microfiber optical sensors.

Dispersion shifts in optical nanowires with thin dielectric coatings.

Based on exact solutions of Maxwells equations of a 3-layer-structured cylindrical waveguide, we calculated dispersion shifts in thin-dielectric-coated optical nanowires. Typical parameters of silica and silicon nanowires are used for numerical simulations. It shows that, the dispersion of a nanowire waveguide can be made highly sensitive to the thickness and index of the coating layer, and a thin coat may lead to considerable dispersion shift of the guided light. For example, in a 300-nm-diameter silicon nanowire, a 1% decrease in diameter of the silicon core by oxidation of silicon into silica shell leads to a 34% decrease in dispersion at 1450-nm wavelength. Results presented in this work suggest the possibility of tuning waveguide dispersions of optical nanowires by coating thin dielectric layers.

All-fiber hybrid photon-plasmon circuits: integrating nanowire plasmonics with fiber optics

We demonstrate all-fiber hybrid photon-plasmon circuits by integrating Ag nanowires with optical fibers. Relying on near-field coupling, we realize a photon-to-plasmon conversion efficiency up to 92% in a fiber-based nanowire plasmonic probe. Around optical communication band, we assemble an all-fiber resonator and a Mach-Zehnder interferometer (MZI) with Q-factor of 6 × 10(6) and extinction ratio up to 30 dB, respectively. Using the MZI, we demonstrate fiber-compatible plasmonic sensing with high sensitivity and low optical power.

Modeling of Au-Nanowire Waveguide for Plasmonic Sensing in Liquids

We theoretically demonstrate a plasmonic nanosensor, using Au-nanowire waveguide to measure the refractive-index changes in aqueous solutions. Based on finite element method simulations, waveguiding properties of Au nanowires for plasmonic sensing in liquids are investigated, with Au nanowire diameter down to 10 nm. A plasmonic nanowire Mach-Zehnder interferometer is proposed to measure the phase shift introduced by the index changes of surroundings. We find that, for a typical Au nanowire with 100-nm diameter, the calculated sensitivity is as high as 5.5π/(μm·RIU), and the sensitivity can be increased by reducing the nanowire diameter. Besides, for reference, we have also investigated Au nanowire plasmonic sensing in other liquids including ethylene glycol and index-matching oil. The nanowire plasmonic sensing scheme proposed here represents a high-sensitivity nanosensor with ultra-small footprint, and may open new opportunities for miniaturized sensing platform based on highly confined 1-D waveguiding plasmons.

Modeling of subwavelength-diameter optical wire waveguides for optical sensing applications

Low-loss optical wave guiding along a subwavelength-diameter silica wire leaves a large amount of the guided field outside the solid core as evanescent wave and at the same time maintains the coherence of the light, making it possible to develop sensitive and miniaturized optical sensors for physical, chemical and biological applications. Here we introduce, for the first time to our knowledge, a scheme to develop optical sensors based on evanescent-wave-guiding properties of subwavelength-diameter wires. Optical wave guiding properties of these wires that are pertinent to a waveguide sensor, such as single-mode condition, evanescent field, Poynting vector and optical loss are investigated. By measuring the phase shift of the guided light, we propose a Mach-Zehnder-type sensor assembled with two silica wires. The sensitivity and size of the sensor are also estimated, which shows that, subwavelength-diameter silica wires are promising for developing optical sensors with high sensitivity and small size.