Yaoguang Ma

Direct Coupling of Plasmonic and Photonic Nanowires for Hybrid Nanophotonic Components and Circuits

We report direct coupling of plasmonic and photonic nanowires using ultracompact near-field interaction. Photon-plasmon coupling efficiency up to 80% with coupling length down to the 200 nm level is achieved between individual Ag and ZnO nanowires. Hybrid nanophotonic components, including polarization splitters, Mach-Zehnder interferometers, and microring cavities, are fabricated out of coupled Ag and ZnO nanowires. These components offer relatively low loss with subwavelength confinement; a hybrid nanowire microcavity exhibits a Q-factor of 520.

Direct measurement of propagation losses in silver nanowires.

Propagation losses of surface plasmons in single silver nanowire waveguides were obtained by measuring light intensity at the end of a silver nanowire. Surface plasmons were excited directly by a tapered fiber. A typical propagation loss of 0.53 dB/εm was obtained.

Semiconductor nanowire lasers

Semiconductor nanowires (or other wire-like nanostructures, including nanoribbons and nanobelts) synthesized by bottom-up chemical growth show single-crystalline structures, excellent geometric uniformities, subwavelength transverse dimensions, and relatively high refractive indices, making these one-dimensional structures ideal optical nanowaveguides with tight optical confinement and low scattering loss. When properly pumped by optical or electrical means, lasing oscillation can be readily established inside these high-gain active nanowires with feedback from endface reflection or near-field coupling effects, making it possible to realize nanowire lasers with miniature sizes and high flexibilities. Also, the wide-range material availability bestows the semiconductor nanowire with lasing wavelength selectable within a wide spectral range from ultraviolet (UV) to near infrared (IR). As nanoscale coherent light sources, in recent years, nanowire lasers have been attracting intensive attention for both fundamental research and technological applications ranging from optical sensing, signal processing, and on-chip communications to quantum optics. Here, we present a review of the status and perspectives of semiconductor nanowire lasers, with a particular emphasis on their optical characteristics categorized in two groups: (1)xa0waveguiding related properties in Sectionxa03, which includes waveguide modes, near-field coupling, endface reflection, substrate-induced effects, and nanowire microcavities, and (2)xa0optically pumped semiconductor nanowire lasers in Sectionxa04, starting from principles and basic types of UV, visible, and near-IR nanowire lasers relying on Fabry–Perot cavities, to advanced configurations including wavelength-tunable, single-mode operated, fiber-coupled, and metal-incorporated nanowire lasing structures for more possibilities. In addition, the material aspects of semiconductor nanowires, including nanowire synthesis and electrically driven nanowire lasers, are briefly reviewed in Sectionsxa02 and 5, respectively. Finally, in Sectionxa06 we present a brief summary of semiconductor nanowire lasers regarding their current challenges and future opportunities.

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.

On-chip three-dimensional high-Q microcavities fabricated by femtosecond laser direct writing

We report on the fabrication of three-dimensional (3D) high-Q whispering gallery microcavities on a fused silica chip by femtosecond laser microfabriction, enabled by the 3D nature of femtosecond laser direct writing. The processing mainly consists of formation of freestanding microdisks by femtosecond laser direct writing and subsequent wet chemical etching. CO(2) laser annealing is followed to smooth the microcavity surface. Microcavities with arbitrary tilting angle, lateral and vertical positioning are demonstrated, and the quality (Q)-factor of a typical microcavity is measured to be up to 1.07 × 10(6), which is currently limited by the low spatial resolution of the motion stage used during the laser patterning and can be improved with motion stages of higher resolutions.

Broadly Defining Lasing Wavelengths in Single Bandgap-Graded Semiconductor Nanowires

Designing lasing wavelengths and modes is essential to the practical applications of nanowire (NW) lasers. Here, according to the localized photoluminescence spectra, we first demonstrate the ability to define lasing wavelengths over a wide range (up to 119 nm) based on an individual bandgap-graded CdSSe NW by forward cutting the NW from CdSe to CdS end. Furthermore, free spectral range (FSR) and modes of the obtained lasers could be controlled by backward cutting the NW from CdS to CdSe end step-by-step. Interestingly, single-mode NW laser with predefined lasing wavelength is realized in short NWs because of the strong mode competition and increase in FSR. Finally, the gain properties of the bandgap-graded NWs are investigated. The combination of wavelength and mode selectivity in NW lasers may provide a new platform for the next generation of integrated optoelectronic devices.

Electrical Tuning of Surface Plasmon Polariton Propagation in Graphene–Nanowire Hybrid Structure

We demonstrate a dynamic surface plasmonic modulation based on graphene-nanowire (grapheme-NW) hybrid structures in the visible light range. A static modulation depth of as high as 0.07 dB/μm has been achieved experimentally. Through careful simulation and systematical experimental investigation, we found that the dual-confinement effect of charge density and electromagnetic energy around the vicinity of the NW will dramatically enhance the light-matter interaction and increase the Fermi level shifting, which are the key roles for bringing the optical response of the device to the visible range. The carrier concentration near the vicinity of a Ag NW is estimated to reach 0.921×10(14) cm(-2) after applying more than 25 V voltages, which is enough to shift the Fermi level for visible light. Furthermore, the modulation behaviors near the Dirac point of monolayer graphene and the singularity of gap-induced bilayer graphene are investigated. Calculated optical conductivity as a function of Fermi level predicts a minimum value near the Dirac point, which is consistent with the experimental results.

Pigtailed CdS nanoribbon ring laser

We demonstrated a pigtailed nanoribbon ring laser constructed with a 400 nm wide and 230 nm thick CdSe nanoribbon. When the 9-μm-diameter ring was irradiated by light from a semiconductor laser (532 nm, 10 ns, 10 Hz) laser, multi longitudinal mode laser emission was observed around 728 nm. The laser output from the pigtail showed strong orientation-dependent polarization, with a maximum polarization ratio of 5.

Multicolour laser from a single bandgap-graded CdSSe alloy nanoribbon.

Multicolour lasing with wavelength varying from 578 nm to 640 nm is realized from a single bandgap-graded CdSSe alloy nanoribbon, by selecting the excited spot at room temperature. Though reabsorption is a serious problem to achieve lasing at short wavelength, multiple scatters on the nanoribbon form localized cavities, and thus lasing at different wavelengths is realized. By increasing the excitation area, multicolour lasing from the same nanoribbon is also observed simultaneously.

Surface plasmon excitation in silver nanowires directly deposited on a laser diode chip

We demonstrate surface plasmon (SP) excitation in silver nanowires directly deposited on the emission facet of a laser diode (LD) chip. Evident light output from the silver nanowires is observed. The output is linear-polarized and is strongly dependent on the nanowire orientation. SP excitation at the central part of a silver nanowire is also observed. The possibility of direct SP excitation on an LD chip may open opportunities for realizing plasmonic and photonic circuits or components with high compactness.