Siew Lang Teo

AlGaN/GaN two-dimensional-electron gas heterostructures on 200 mm diameter Si(111)

This Letter reports on the epitaxial growth, characterization, and device characteristics of crack-free AlGaN/GaN heterostructures on a 200 mm diameter Si(111) substrate. The total nitride stack thickness of the sample grown by the metal-organic chemical vapor deposition technique is about 3.3 ± 0.1 μm. The structural and optical properties of these layers are studied by cross-sectional scanning transmission electron microscopy, high-resolution x-ray diffraction, photoluminescence, and micro-Raman spectroscopy techniques. The top AlGaN/GaN heterointerfaces reveal the formation of a two-dimensional electron gas with average Hall mobility values in the range of 1800 to 1900 cm2/Vs across such 200 mm diameter GaN on Si(111) samples. The fabricated 1.5 μm-gate AlGaN/GaN high-electron-mobility transistors exhibited the drain current density of 660 mA/mm and extrinsic transconductance of 210 mS/mm. These experimental results show immense potential of 200-mm diameter GaN-on-silicon technology for electronic devi...

High Aspect Subdiffraction-Limit Photolithography via a Silver Superlens

Photolithography is the technology of choice for mass patterning in semiconductor and data storage industries. Superlenses have demonstrated the capability of subdiffraction-limit imaging and been envisioned as a promising technology for potential nanophotolithography. Unfortunately, subdiffraction-limit patterns generated by current superlenses exhibited poor profile depth far below the requirement for photolithography. Here, we report an experimental demonstration of sub-50 nm resolution nanophotolithography via a smooth silver superlens with a high aspect profile of ~45 nm, as well as grayscale subdiffraction-limit three-dimensional nanopatterning. Theoretical analysis and simulation show that smooth interfaces play a critical role. Superlens-based lithography can be integrated with conventional UV photolithography systems to endow them with the capability of nanophotolithography, which could provide a cost-effective approach for large scale and rapid nanopatterning.

Damping of the acoustic vibrations of individual gold nanoparticles.

In this letter, the ultrafast vibrational dynamics of individual gold nanorings has been investigated by femtosecond transient absorption spectroscopy. Two acoustic vibration modes have been detected and identified. The influence of the mechanical coupling at the nanoparticle/substrate interface on the acoustic vibrations of the nano-objects is discussed. Moreover, by changing the environment of the nanoring, we provide a clear evidence of the impact of the surrounding medium on the damping of the acoustic vibrations. Such results are reported here for the first time on individual nanoparticles. This work points out a new sensing method based on the sensitivity of the acoustic vibration damping to the surrounding medium.

Acousto-plasmonic and surface-enhanced Raman scattering properties of coupled gold nanospheres/nanodisk trimers.

This work is devoted to the fundamental understanding of the interaction between acoustic vibrations and surface plasmons in metallic nano-objects. The acoustoplasmonic properties of coupled spherical gold nanoparticles and nanodisk trimers are investigated experimentally by optical transmission measurements and resonant Raman scattering experiments. For excitation close to resonance with the localized surface plasmons of the nanodisk trimers, we are able to detect several intense Raman bands generated by the spherical gold nanoparticles. On the basis of both vibrational dynamics calculations and Raman selection rules, the measured Raman bands are assigned to fundamental and overtones of the quadrupolar and breathing vibration modes of the spherical gold nanoparticles. Simulations of the electric near-field intensity maps performed at the Raman probe wavelengths showed strong localization of the optical energy in the vicinity of the nanodisk trimers, thus corroborating the role of the interaction between the acoustic vibrations of the spherical nanoparticles and the surface plasmons of the nanodisk trimers. Acoustic phonons surface enhanced Raman scattering is here demonstrated for the first time for such coupled plasmonic systems. This work paves the way to surface plasmon engineering for sensing the vibrational properties of nanoparticles.

Plasmonic properties of gold ring-disk nano-resonators: fine shape details matter

Using numerical simulations, we demonstrate that fine shape details of gold nanoring-disks are responsible for significant modifications of their localized surface plasmon properties. The numerical results are supported by optical transmission measurements and by atomic force microscopy. In particular, we found that, depending on the ring wall sharpness, the spectral shift of the ring-like localized surface plasmon resonance can be as large as few hundred nanometers. These results shed the light on the strong sensitivity of the surface plasmon properties to very small deviations of the ring and disk shapes from the ideally flat surfaces and sharp edges. This effect is particularly important for tailoring the surface plasmon properties of metallic nanostructures presenting edges and wedges for applications in bio- and chemical sensing and for enhancement of light scattering.

InGaN∕GaN light emitting diodes on nanoscale silicon on insulator

The authors report on the fabrication of InGaN∕GaN-based light emitting diodes (LEDs) on nanoscale silicon-on-insulator (SOI) substrates. The LED structures are grown on (111)-oriented 45nm thick SOI overlayer by metal organic chemical vapor deposition. Square-shaped mesa patterns are created by standard LED processing steps including multiple-mask photolithography, inductive coupled plasma etching, and contact metallization. Due to the high reflective Si∕SiO2 beneath AlN buffer and high refractive contrasts at the interfaces, the authors observed multiple interference peaks from LEDs on SOI and such effect resulted in an increased integrated electroluminescence intensity when compared to LED structures fabricated on bulk Si(111).

Gold nanoring trimers: a versatile structure for infrared sensing

In this work we report on the observation of surface plasmon properties of periodic arrays of gold nanoring trimers fabricated by electron beam lithography. It is shown that the localized surface plasmon resonances of such gold ring trimers occur in the infrared spectral region and are strongly influenced by the nanoring geometry and their relative positions. Based on numerical simulations of the optical extinction spectra and of the electric near-field intensity maps, the resonances are assigned to surface plasmon states arising from the strong intra-trimer electromagnetic interaction. We show that the nanoring trimer configuration allows for generating infrared surface plasmon resonances associated with strongly localized electromagnetic energy, thus providing plasmonic nanoresonators well-suited for sensing and surface enhanced near-infrared Raman spectroscopy.

Photonic crystal structures with ultrahigh aspect ratio in lithium niobate fabricated by focused ion beam milling

Lithium niobate (LiNbO3, LN) is an important material which is widely applied in fabricating photonic and acoustic devices. However, it is difficult to either wet etch or dry etch LN due to the material’s properties. Here, the authors report novel pattern fabrication based on LN using focused ion beam (FIB) milling. When an array of small holes is etched, a severe tapering problem is observed as is common, but by replacing the nanocylindrical hole array with a nanoring structure, the authors obtain photonic crystals with an aspect ratio of up to 50:1 (2 μm total etching depth and 40 nm gap aperture). Dense nanorod arrays with sub-30-nm ultrasmall gaps and more than 2.5 μm etching depth are also achieved with FIB milling.

Dual wavelength sensing based on interacting gold nanodisk trimers

Fabrication and surface plasmon properties of gold nanostructures consisting of periodic arrays of disk trimers are reported. Using electron beam lithography, disk diameters as small as 96 nm and gaps between disks as narrow as 10 nm have been achieved with an unprecedented degree of control and reproducibility. The disk trimers exhibit intense visible and infrared surface plasmon resonances which are studied as a function of the disk diameter and of the pitch between trimers. Based on simulations of the optical extinction spectra and of the electric near-field intensity maps, the resonances are assigned to a single trimer response and to collective surface plasmon excitations involving electromagnetic interaction between the trimers. The sensing properties of the disk trimers are investigated using various coating media. The reported results demonstrate the possible use of gold disk trimers for dual wavelength chemical sensing.

Nanoelectromechanical-systems-controlled bistability of double-coupled photonic crystal cavities

In this Letter, we report an approach to controlling the bistability of double-coupled photonic crystal cavities with a nanoelectromechanical comb drive, in which the optical force and thermo-optic effect form a feedback mechanism to the effective index of the cavities, and the gap width between the cavities is steered by the comb drive. A model based on temporal coupled mode theory is established to analyze this approach. Hysteresis loops characterizing the bistability are experimentally achieved by sweeping the gap width forward and in reverse. In addition, the experiments also demonstrate that the bistability is tunable by varying the input light power.