Fuxing Gu

Single MoO3 nanoribbon waveguides: good building blocks as elements and interconnects for nanophotonic applications.

Exploring new nanowaveguide materials and structures is of great scientific interest and technological significance for optical and photonic applications. In this work, high-quality single-crystal MoO3 nanoribbons (NRs) are synthesized and used for optical guiding. External light sources are efficiently launched into the single MoO3 NRs using silica fiber tapers. It is found that single MoO3 NRs are as good nanowaveguides with loss optical losses (typically less than 0.1 dB/μm) and broadband optical guiding in the visible/near-infrared region. Single MoO3 NRs have good Raman gains that are comparable to those of semiconductor nanowaveguides, but the second harmonic generation efficiencies are about 4 orders less than those of semiconductor nanowaveguides. And also no any third-order nonlinear optical effects are observed at high pump power. A hybrid Fabry-Pérot cavity containing an active CdSe nanowire and a passive MoO3 NR is also demonstrated, and the ability of coupling light from other active nanostructures and fluorescent liquid solutions has been further demonstrated. These optical properties make single MoO3 NRs attractive building blocks as elements and interconnects in miniaturized photonic circuitries and devices.

Frequency-resolved optical gating measurement of ultrashort pulses by using single nanowire.

The use of ultrashort pulses for fundamental studies and applications has been increasing rapidly in the past decades. Along with the development of ultrashort lasers, exploring new pulse diagnositic approaches with higher signal-to-noise ratio have attracted great scientific and technological interests. In this work, we demonstrate a simple technique of ultrashort pulses characterization with a single semiconductor nanowire. By performing a frequency-resolved optical gating method with a ZnO nanowire coupled to tapered optical microfibers, the phase and amplitude of a pulse series are extracted. The generated signals from the transverse frequency conversion process can be spatially distinguished from the input, so the signal-to-noise ratio is improved and permits lower energy pulses to be identified. Besides, since the nanometer scale of the nonlinear medium provides relaxed phase-matching constraints, a measurement of 300-nm-wide supercontinuum pulses is achieved. This system is highly compatible with standard optical fiber systems, and shows a great potential for applications such as on-chip optical communication.

Palladium-Coated Silica Microfiber Knots for Enhanced Hydrogen Sensing

Pd-coated microfiber knot resonators (MKR) are fabricated for hydrogen sensing by measuring the shift and absorption at the resonant wavelengths of the microfiber knots. Compared with existing fiber-optic hydrogen sensors, the Pd-coated MKRs exhibit enhanced sensitivity using small interaction lengths, because the recirculation of resonant light within the MKRs significantly accumulate the hydrogen-Pd interaction. It is found that microfiber diameter of the knot is a critical factor to achieve high sensitivity.

Above-Bandgap Surface-Emitting Frequency Conversion in Semiconductor Nanoribbons With Ultralow Continuous-Wave Pump Power

Semiconductors have large optical nonlinear susceptibilities especially in the spectral range above material bandgaps. However, optical frequency conversion encounters large absorption above bandgaps and difficulty using common phase-matching techniques. The frequency conversion bandwidths are thus limited. Here, frequency up-conversion far above the bandgaps using surface emissions from semiconductor nanoribbons is demonstrated, wherein nanoscale waveguiding tightly confines fundamental waves for decreasing pump powers, and above-bandgap absorption is greatly decreased in nanoscale waveguide thickness. By using CdSe nanoribbons, efficient 532- and 404-nm second-harmonic and 459-nm sum-frequency generations are obtained with the continuous-wave pump power less than 100 μW. The normalized efficiency of 532-nm second-harmonic generation is about 2 × 10-5 mm-1 at pump power of 300 μW. Attractive features such as tunable spatial distribution and highly polarization are observed. A broadband emission with a full width half maximum of ~10 nm is attained by frequency summing a continuous-wave laser and an amplified spontaneous emission source.

Mode tailoring in subwavelength-dimensional semiconductor micro/nanowaveguides by coupling optical microfibers.

Benefitted from large fraction of evanescent wave and high endface reflectivity, we have realized mode tailoring in subwavelength-dimensional semiconductor micro/nanowaveguides (MN-WGs) by coupling optical silica microfibers. By investigating the reflection spectra, it was found that the microfiber tips could offer effective reflection and can been used to continuously and reversibly tune the interference wavelengths by changing the contact points with the MN-WGs. The measured extinction ratio in the interference patterns was as high as ~10 dB. In addition, tunable free spectral range of photoluminescence emissions and humidity sensing were also demonstrated. Its advantages of non-destructively tuning, simple fabrication, easy interrogation, and remote monitoring, offer great possible prospects for developing miniature tunable lasers, sensors, and biological endoscopy.

Surface-enhanced fluorescence in metal nanoparticle-doped polymer nanofibers via waveguiding excitation

We report a waveguiding excitation-based approach for surface-enhanced fluorescence. As high as 17-fold enhanced fluorescence intensity of Rhodamine 6G molecules is realized by gold nanoparticles embedded in polymer nanofibers. The enhancement results not only from the spatial confinement of light by the nanofibers but also from the wavelength match among the excitation laser, the localized surface plasmon resonance of nanoparticles, and the absorption band of dyes. On the basis of the enhancement and high-efficient waveguiding regime, the required excitation power for detectable fluorescence is decreased to the 20 nW level, which is about 50 times lower than that by free-space excitation. These fluorophore/nanoparticle-doped nanofibers may find applications in compact and energy-efficient optical devices of chemical analysis and biosensing.

Nanoantenna-assisted ultra-narrow resonances based on coupling of localized plasmons and whispering-gallery modes

Free-space light is efficiently coupled into and from the palladium nanoantenna-microfiber whispering-gallery cavity systems. A measured full width at half-maximum of 3.2 nm at 622.7 nm and enhanced sensitivity to hydrogen detection are obtained.

Polymer Micro/Nanofibre Waveguides for Optical Sensing Applications

This chapter focuses on polymer micro/nanofibre (PMNFs) waveguides and their applications in sensing applications. The PMNFs are functionalized by doping with dyes or blending with solvated polymers before the drawing process. Based on the evanescent wave-coupling technique, the excitation light is efficiently coupled into the PMNFs using silica-fibre tapers and guided along the long-length PMNF waveguides. Due to the tight confinement, the interaction of light with PMNFs is significantly enhanced. Intriguing advantages such as enhanced excitation efficiency, low excitation power operation and high photostability are obtained. On the basis of the optical response when exposed to specimens, functionalized PMNFs are used for humidity, NO2, and NH3 detection with high sensitivity and fast response. By using a simple and low-cost nanoimprinting technique, PMNF Bragg gratings are also demonstrated for strain sensing with a high sensitivity of −2.5 pm/μe.

Metal single-nanowire plasmonic sensors

Metal nanowires including Pd nanoparticle-coated Au nanowires, polyacrylamide film-supported Ag nanowires, and single-crystal Pd nanowires are used for hydrogen and humidity plasmonic sensing, with higher sensitivity and faster response than those in conventional photonic nanowires.