Yipei Wang

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.

Hybrid Photon-Plasmon Nanowire Lasers

By near-field coupling a high-gain CdSe nanowire (NW) and a 100-nm-diameter Ag NW, we demonstrate a hybrid photon-plasmon laser operating at 723-nm wavelength at room temperature, with a plasmon mode area of 0.008λ2. This device simultaneously provides spatially separated photonic far-field output and highly localized coherent plasmon modes with sub-diffraction-limited beam size.

Single-mode plasmonic waveguiding properties of metal nanowires with dielectric substrates

Single-mode plasmonic waveguiding properties of metal nanowires with dielectric substrates are investigated using a finite-element method. Au and Ag are selected as plasmonic materials for nanowire waveguides with diameters down to 5-nm-level. Typical dielectric materials with relatively low to high refractive indices, including magnesium fluoride (MgF2), silica (SiO2), indium tin oxide (ITO) and titanium dioxide (TiO2), are used as supporting substrates. Basic waveguiding properties, including propagation constants, power distributions, effective mode areas, propagation distances and losses are obtained at the typical plasmonic resonance wavelength of 660 nm. Compared to that of a freestanding nanowire, the mode area of a substrate-supported nanowire could be much smaller while maintaining an acceptable propagation length. For example, the mode area and propagation length of a 100-nm-diameter Ag nanowire with a MgF2 substrate are about 0.004 μm2 and 3.4 μm, respectively. The dependences of waveguiding properties on geometric and material parameters of the nanowire-substrate system are also provided. Our results may provide valuable references for waveguiding dielectric-supported metal nanowires for practical applications.

2D Materials for Optical Modulation: Challenges and Opportunities

Owing to their atomic layer thickness, strong light-material interaction, high nonlinearity, broadband optical response, fast relaxation, controllable optoelectronic properties, and high compatibility with other photonic structures, 2D materials, including graphene, transition metal dichalcogenides and black phosphorus, have been attracting increasing attention for photonic applications. By tuning the carrier density via electrical or optical means that modifies their physical properties (e.g., Fermi level or nonlinear absorption), optical response of the 2D materials can be instantly changed, making them versatile nanostructures for optical modulation. Here, up-to-date 2D material-based optical modulation in three categories is reviewed: free-space, fiber-based, and on-chip configurations. By analysing cons and pros of different modulation approaches from material and mechanism aspects, the challenges faced by using these materials for device applications are presented. In addition, thermal effects (e.g., laser induced damage) in 2D materials, which are critical to practical applications, are also discussed. Finally, the outlook for future opportunities of these 2D materials for optical modulation is given.

Nanocrystalline tungsten carbide encapsulated in carbon shells

Abstract Nanocrystalline tungsten carbide with an average grain size of 12 nm was synthesized using the ion arc method. The characterization was carried out by means of XRD, TEM, and TGA. The experimental results indicate that the as-synthesized powders are mainly cubic WC 1 − x . Besides, a small amount of α-W 2 C is also observed. These tungsten carbide powders are encapsulated within crystalline graphite. Owing to the ultrafine sizes of the powers, oxidation of the graphite occurs at about 350 °C. The nanocrystalline tungsten carbide transforms into WO 3 at about 670 °C.

Single-Band 2-nm-Line-Width Plasmon Resonance in a Strongly Coupled Au Nanorod.

This paper reports a dramatic reduction in plasmon resonance line width of a single Au nanorod by coupling it to a whispering gallery cavity of a silica microfiber. With fiber diameter below 6 μm, strong coupling between the nanorod and the cavity occurs, leading to evident mode splitting and spectral narrowing. Using a 1.46-μm-diameter microfiber, we obtained single-band 2-nm-line-width plasmon resonance in an Au nanorod around a 655-nm-wavelength, with a quality factor up to 330 and extinction ratio of 30 dB. Compared to an uncoupled Au nanorod, the strongly coupled nanorod offers a 30-fold enhancement in the peak intensity of plasmonic resonant scattering.

Bending effects on lasing action of semiconductor nanowires

High flexibility has been one of advantages for one-dimensional semiconductor nanowires (NWs) in wide application of nanoscale integrated circuits. We investigate the bending effects on lasing action of CdSe NWs. Threshold increases and differential efficiency decreases gradually when we decrease the bending radius step by step. Red shift and mode reduction in the output spectra are also observed. The bending loss of laser oscillation is considerably larger than that of photoluminescence (PL), and both show the exponential relationship with the bending radius. Diameter and mode dependent bending losses are investigated. Furthermore, the polarizations of output can be modulated linearly by bending the NWs into different angles continuously.

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.

Single CdTe Nanowire Optical Correlator for Femtojoule Pulses.

On the basis of the transverse second harmonic generation (TSHG) in a highly nonlinear subwavelength-diameter CdTe nanowire, we demonstrate a single-nanowire optical correlator for femto-second pulse measurement with pulse energy down to femtojoule (fJ) level. Pulses to be measured were equally split and coupled into two ends of a suspending nanowire via tapered optical fibers. The couterpropagating pulses meet each other around the central area of the nanowire, and emit TSHG signal perpendicular to the axis of the nanowire. By transferring the spatial intensity profile of the transverse second harmonic (TSH) image into the time-domain temporal profile of the input pulses, we operate the nanowire as a miniaturized optical correlator. Benefitted from the high nonlinearity and the very small effective mode area of the waveguiding CdTe nanowire, the input energy of the single-nanowire correlator can go down to fJ-level (e.g., 2 fJ/pulse for 1064 nm 200 fs pulses). The miniature fJ-pulse correlator may find applications from low power on-chip optical communication, biophotonics to ultracompact laser spectroscopy.