Hung-Ying Chen

Plasmonic Nanolaser Using Epitaxially Grown Silver Film

Going Green with Nanophotonics Plasmons are optically induced collective electronic excitations tightly confined to the surface of a metal, with silver being the metal of choice. The subwavelength confinement offers the opportunity to shrink optoelectronic circuits to the nanometer scale. However, scattering processes within the metal lead to losses. Lu et al. (p. 450) developed a process to produce atomically smooth layers of silver, epitaxially grown on silicon substrates. A cavity in the silver layer is capped with a SiO insulating layer and an AlGaN nanorod was used to produce a low-threshold emission at green wavelengths. An atomically smooth layer of silver enhances the performance of nanophotonic devices. A nanolaser is a key component for on-chip optical communications and computing systems. Here, we report on the low-threshold, continuous-wave operation of a subdiffraction nanolaser based on surface plasmon amplification by stimulated emission of radiation. The plasmonic nanocavity is formed between an atomically smooth epitaxial silver film and a single optically pumped nanorod consisting of an epitaxial gallium nitride shell and an indium gallium nitride core acting as gain medium. The atomic smoothness of the metallic film is crucial for reducing the modal volume and plasmonic losses. Bimodal lasing with similar pumping thresholds was experimentally observed, and polarization properties of the two modes were used to unambiguously identify them with theoretically predicted modes. The all-epitaxial approach opens a scalable platform for low-loss, active nanoplasmonics.

Green, yellow, and orange defect emission from ZnO nanostructures: Influence of excitation wavelength

ZnO commonly exhibits luminescence in the visible spectral range due to different intrinsic defects. In order to study defect emissions, photoluminescence from ZnO nanostructures prepared by different methods (needles, rods, shells) was measured as a function of excitation wavelength and temperature. Under excitation at 325nm, needles exhibited orange-red defect emission, rods exhibited yellow defect emission, while shells exhibited green defect emission. Obvious color change from orange to green was observed for needles with increasing excitation wavelengths, while nanorods (yellow) showed smaller wavelength shift and shells (green) showed no significant spectral shift. Reasons for different wavelength dependences are discussed.

InGaN/GaN nanorod array white light-emitting diode

Conventional InGaN/GaN light-emitting diodes based on planar quantum wellstructures do not allow for efficient long-wavelength operation beyond the blue region due to a strong quantum confined Stark effect in lattice-mismatched polar InGaNquantum wells. Here we overcome the limitation by using self-assembled GaNnanorod arrays as strain-free growth templates for thick InGaN nanodisks. In combination with enhanced carrier localization and high crystalline quality, this approach allows us to realize full-color InGaN nanodisk emitters. By tailoring the numbers, positions, and thicknesses of polychromatic nanodisk ensembles embedded vertically in the GaNnanorod p - n junction, we are able to demonstrate natural white (color temperature ∼ 6000 K ) electroluminescence from InGaN/GaN nanorod arrays.

Inorganic Gyroid with Exceptionally Low Refractive Index from Block Copolymer Templating

Nanoporous polymers with gyroid nanochannels can be fabricated from the self-assembly of degradable block copolymer, polystyrene-b-poly(l-lactide) (PS-PLLA), followed by the hydrolysis of PLLA blocks. A well-defined nanohybrid material with SiO2 gyroid nanostructure in a PS matrix can be obtained using the nanoporous PS as a template for sol-gel reaction. After subsequent UV degradation of the PS matrix, a highly porous inorganic gyroid network remains, yielding a single-component material with an exceptionally low refractive index (as low as 1.1).

Structure and photoluminescence properties of epitaxially oriented gan nanorods grown on si(111) by plasma-assisted molecular-beam epitaxy

The authors show that vertically c-axis-aligned GaN nanorod arrays grown by plasma-assisted molecular-beam epitaxy are epitaxially oriented on Si(111) substrates and their crystal structure corresponds to a fully relaxed wurtzite lattice. At later growth stage, these GaN nanorods exhibit the tendency to coalesce into nanorod bundles. Low-temperature photoluminescence spectrum from 1-μm-long GaN nanorods consists of intense exciton lines of strain-free bulk GaN and additional lines at ∼3.21 and ∼3.42eV (Y7 and Y2). The Y7 line is attributed to the excitons trapped along the dislocations at the boundaries of coalesced GaN nanorods, while the Y2 line has its origin in the interface defects at the GaN∕Si(111) interfaces.

Layer-by-Layer Assembly of Three-Dimensional Colloidal Supercrystals with Tunable Plasmonic Properties

We present a simple and efficient method for synthesizing large-area (>1 cm(2)), three-dimensional (3D) gold and silver nanoparticle supercrystal films. In this approach, Janus nanoparticle (top face solvent-phobic and bottom face solvent-philic) films with an arbitrary number of close-packed nanoparticle monolayers can be formed using layer-by-layer (LbL) assembly from suspensions of thiolate-passivated gold or silver colloids. Furthermore, we demonstrate that these films can act as true 3D plasmonic crystals with strong transverse (intralayer) and longitudinal (interlayer) near-field coupling. In contrast to conventional polyelectrolyte-mediated LbL assembly processes, this approach allows multiple longitudinal coupling modes with a conspicuous spectral dependence on the layer number. We have found a universal scaling relation between the spectral position of the reflectance dips related to the longitudinal modes and the layer number. This relation can be understood in terms of the presence of a plasmonic Fabry-Perot nanocavity along the longitudinal direction that allows the formation of standing plasmon waves under plasmon resonance conditions. The realization of 3D plasmonic coupling enables broadband tuning of the collective plasmon response over a wide spectral range (visible and near-IR) and provides a pathway to designer plasmonic metamaterials.

Near-infrared photoluminescence from vertical InN nanorod arrays grown on silicon: Effects of surface electron accumulation layer

We demonstrate that vertically aligned InN nanorods can be grown on Si(111) by plasma-assisted molecular-beam epitaxy. Detailed structural characterization indicates that individual nanorods are wurtzite InN single crystals with the growth direction along the c axis. Near-infrared photoluminescence (PL) from InN nanorods can be clearly observed at room temperature. However, in comparison to the InN epitaxial films, the PL efficiency is significantly lower. Moreover, the variable-temperature PL measurements of InN nanorods exhibit anomalous temperature effects. We propose that these unusual PL properties are results of considerable structural disorder (especially for the low-temperature grown InN nanorods) and strong surface electron accumulation effects.

Far-Field Optical Imaging of a Linear Array of Coupled Gold Nanocubes: Direct Visualization of Dark Plasmon Propagating Modes

Plasmonic nanoantenna arrays hold great promise for diffraction-unlimited light localization, confinement, and transport. Here, we report on linear plasmonic nanoantenna arrays composed of colloidal gold nanocubes precisely assembled using a nanomanipulation technique. In particular, we show the direct evidence of dark propagating modes in the plasmon coupling regime, allowing for transport of guided plasmon waves without far-field radiation losses. Additionally, we demonstrate the possibility of plasmon dispersion engineering in coupled gold nanocube chains. By assembling a nanocube chain with two sections of coupled nanocubes of different intercube separations, we are able to produce the effect of a band-pass nanofilter.

Single InGaN nanodisk light emitting diodes as full-color subwavelength light sources

Subwavelength electroluminescent sources with spatial, spectral, and polarization controlling capabilities are critical elements for optical imaging and lithography beyond the diffraction limit. Here, we show that the electroluminescence from single, strain-free InGaN nanodisks embedded in self-assembled GaN p - n nanorods can span the entire visible spectrum with a large linear polarization ratio ( ∼ 0.85 ) . Furthermore, this unique nanodisk-in-nanorod geometry enables the realization of the ultrasmall footprint light-emitting diodes(LEDs) to be used as subwavelength light sources. Using these nano-LEDs, we are able to demonstrate near-field, subwavelength photolithography by controlling the exposure time and light intensity from single InGaN nanodisks at chosen wavelengths.

Polarized photoluminescence from single GaN nanorods: Effects of optical confinement

By measuring linearly polarized photoluminescence (PL) from single, isolated gallium nitride (GaN) nanorods with the rod diameters in the subwavelength regime (30-90 nm), we present clear evidence for size dependence of polarization anisotropy. The maximum polarization ratio at room temperature (approximately 0.9 with emission and excitation light polarized parallel to the long axis of nanorod) occurs at the rod diameter of approximately 40 nm. The experimental data are compared with the recent theoretical model proposed for thick semiconductor nanowires. It is concluded that the optical confinement effects in this size regime play an important role in the observed giant polarization anisotropy. Furthermore, we have performed a temperature-dependent study of polarized PL to show the importance of internal emission anisotropy at low temperatures.