Showing papers by "Nicholas X. Fang published in 2010"

Nonlithographic patterning and metal-assisted chemical etching for manufacturing of tunable light-emitting silicon nanowire arrays

Abstract: Semiconductor nanowires have potential applications in photovoltaics, batteries, and thermoelectrics We report a top-down fabrication method that involves the combination of superionic-solid-state-stamping (S4) patterning with metal-assisted-chemical-etching (MacEtch), to produce silicon nanowire arrays with defined geometry and optical properties in a manufacturable fashion Strong light emission in the entire visible and near infrared wavelength range at room temperature, tunable by etching condition, attributed to surface features, and enhanced by silver surface plasmon, is demonstrated

First jump of microgel; actuation speed enhancement by elastic instability

Abstract: Swelling-induced snap-buckling in a 3D micro hydrogel device, inspired by the insect-trapping action of Venus flytrap, makes it possible to generate astonishingly fast actuation. We demonstrate that elastic energy is effectively stored and quickly released from the device by incorporating elastic instability. Utilizing its rapid actuation speed, the device can even jump by itself upon wetting.

A smooth optical superlens

Abstract: We demonstrate a smooth and low loss silver (Ag) optical superlens capable of resolving features at 1/12th of the illumination wavelength with high fidelity. This is made possible by utilizing state-of-the-art nanoimprint technology and intermediate wetting layer of germanium (Ge) for the growth of flat silver films with surface roughness at subnanometer scales. Our measurement of the resolved lines of 30 nm half-pitch shows a full-width at half-maximum better than 37 nm, in excellent agreement with theoretical predictions. The development of this unique optical superlens leads promise to parallel imaging and nanofabrication in a single snapshot.

Solvent-driven polymeric micro beam device

Abstract: The response of current hydrogel devices mainly depends on the diffusion of stimuli. However, diffusion is a slow transport mechanism compared to advection, which therefore limits the response speed of hydrogel devices. To overcome this limitation, we introduce a capillary network and elastic instability mechanism. Particularly, an open surface capillary delivers and distributes solvent, thus triggering the swelling and bending of curved polymeric beams. To demonstrate this concept, we fabricate these polymeric microstructures using projection micro-stereolithography (PµSL). Combined with instability criteria analysis based on static beam theory, this device is designed to exhibit two-way snap-through behavior. Our analysis provides the minimum dimensionless stiffness β for the beam device to snap during solvent actuation. Here, β is a well-defined dimensionless parameter in our analysis that indicates whether the device can provide sufficient axial force to trigger the snap-through of the beam. The actuation displacement can be as high as 45% of the length of the beam. We observe a maximum midpoint speed of 3.1 cm s−1 for a beam 2 mm long—20 times higher than that for a beam without an elastic instability mechanism. This device can be used in artificial muscle and as the key component for fluidic-to-mechanical signal transduction in active micro-fluidic circuits.

Method of forming an array of high aspect ratio semiconductor nanostructures

Abstract: A new method for forming an array of high aspect ratio semiconductor nanostructures entails positioning a surface of a stamp comprising a solid electrolyte in opposition to a conductive film disposed on a semiconductor substrate. The surface of the stamp includes a pattern of relief features in contact with the conductive film so as to define a film-stamp interface. A flux of metal ions is generated across the film-stamp interface, and a pattern of recessed features complementary to the pattern of relief features is created in the conductive film. The recessed features extend through an entire thickness of the conductive film to expose the underlying semiconductor substrate and define a conductive pattern on the substrate. The stamp is removed, and material immediately below the conductive pattern is selectively removed from the substrate. Features are formed in the semiconductor substrate having a length-to-width aspect ratio of at least about 5:1.

Excitation and imaging of resonant optical modes of Au triangular nanoantennas using cathodoluminescence spectroscopy

Abstract: Cathodoluminescence (CL) imaging spectroscopy is an important technique to understand the resonant behavior of optical nanoantennas. The authors report high-resolution CL spectroscopy of triangular gold nanoantennas designed with near-vacuum effective index and very small metal-substrate interface. This design helped in addressing issues related to background luminescence and shifting of dipole modes beyond visible spectrum. Spatial and spectral investigations of various plasmonic modes are reported. Out-of-plane dipole modes excited with a vertically illuminated electron beam showed high-contrast tip illumination in panchromatic imaging. By tilting the nanostructures during fabrication, in-plane dipole modes of antennas were excited. Finite-difference time-domain simulations for electron and optical excitations of different modes showed excellent agreement with experimental results. Their approach of efficiently exciting antenna modes by using low index substrates is confirmed both with experiments and n...

SERS EM field enhancement study through fast Raman mapping of Sierpinski carpet arrays

Abstract: Self-similar Sierpinski carpet fractals were fabricated using a new nanopatterning technique, i.e. solid-state superionic stamping (S4), to study the effect of feature size on plasmon-enhanced Raman scattering. A real-time Raman image showing electromagnetic (EM) field enhancement contrast is demonstrated. The effects were studied and explained in the context of modes of surface plasmon excitation. Additionally, we find that the EM field enhancement supported by the fractals extends further spatially as compared to continuous Ag surfaces. Copyright © 2010 John Wiley & Sons, Ltd.

Non-lithographic patterning and metal-assisted chemical etching for manufacturing of tunable light-emitting silicon nanowire arrays

Abstract: We report a top-down fabrication method that involves the combination of superionic-solid-state-stamping (S4) patterning with metal-assisted-chemical-etching (MacEtch), to produce silicon nanowire arrays with defined geometry and optical properties in a manufacturable fashion.

Sub-diffraction-limited far-field imaging in infrared

Abstract: We investigated a far-field superlens operating at mid-infrared wavelength that allows resolving subwavelength features in the far-field. By utilizing evanescent enhancement provided by surface plasmon excitation of silver nanorods and Moire effect, we numerically demonstrated that subwavelength information of an object can be converted to propagating information. This information can then be captured by conventional optical components. A simple image reconstruction algorithm can restore the subwavelength object. A sub-diffraction-limited resolution of 2.5 μm at 6-μm wavelength is demonstrated.

Plasmon-Assisted Optical Curtains

Abstract: We predict an optical curtain effect, i.e., formation of a spatially invariant light field as light emerges from a set of periodic metallic nano-objects. The underlying physical mechanism of generation of this unique optical curtain can be explained in both the spatial domain and the wave-vector domain. In particular, in each period, we use one metallic nanostrip to equate the amplitudes of lights impinging on the openings of two metallic nanoslits and also shift their phases by π difference. We elaborate the influence on the output effect from some geometrical parameters like the periodicity, the slit height, and so on. By controlling the light illuminated on metallic subwavelength apertures, it is practical to generate optical curtains of arbitrary forms, which may open new routes of plasmonic nanolithography.

Controlled directional growth of silver microwires on a solid electrolyte surface

Abstract: A silver microwire was formed on a silver sulfide surface in a directional and reversible manner. This wire formed upon applying an electric field between a tip-less atomic force microscopy probe placed on the Ag2S surface and a grounded silver electrode embedded in the surface. The process was studied in real-time with optical microscope and a discussion was provided on how the morphological instability was violated. A numerical model based on the mixed ionic-electronic transport was developed to study the observation and a good match of growth rates was found. The fast metal wire growth phenomenon observed here has potential in applications for electronics as well as plasmonic sensors and waveguides

Enhancing Light Coupling With Plasmonic Optical Antennas

Abstract: We describe in this chapter development of plasmonic optical antennas for light concentration and near-field enhancement. A set of bow tie nanoantennas are fabricated and characterized with optical and electron excitation methods. Optical spectroscopy of these subwavelength antennas displays pronounced extinction peaks at resonant wavelength, showing total extinction cross sections as much as 10 times of their physical dimensions. On the other hand, coherent excitation of the bow tie antennas allows tuning the peak wavelength of the scattered light by changing the periodicity. Under dark-field microscopy, we observed the scattered waves from arrays of different bow tie antennas in complete visible spectrum. The local resonant modes of the bow tie antennas are also probed by focused electrons. Such cathodoluminescence spectroscopy reveals the fine details of enhanced field on the optical nanoantennas at resolution down to 20 nm. Finally, we show examples of surface-enhanced Raman spectroscopy on the nanoantennas. Effective designs based on local enhancement and radiation engineering of the plasmonic optical antennas would promise revolutionary changes in highly compact and integrated photonics for photon energy conversion, adaptive sensing, and image processing.

Cathodoluminescence Imaging of Plasmonic Modes of Ag Nanostructures

Abstract: We report cathodoluminescence spectroscopy on Ag triangular nanostructures with specially designed substrates having near-vacuum index and low luminescence. FDTD simulations were carried out to predict the role of substrate and the experimentally observed out-of-plane mode.

Excitation and Imaging of Resonant Optical Modes of Au Triangular Nano-Antennas Using Cathodoluminescence Spectroscopy

Abstract: Cathodoluminescence (CL) imaging spectroscopy is an important technique to understand resonant behavior of optical nanoantennas. We report high-resolution CL spectroscopy of triangular gold nanoantennas designed with near-vacuum effective index and very small metal-substrate interface. This design helped in addressing issues related to background luminescence and shifting of dipole modes beyond visible spectrum. Spatial and spectral investigations of various plasmonic modes are reported. Out-of-plane dipole modes excited with vertically illuminated electron beam showed high-contrast tip illumination in panchromatic imaging. By tilting the nanostructures during fabrication, in-plane dipole modes of antennas were excited. Finite-difference time-domain simulations for electron and optical excitations of different modes showed excellent agreement with experimental results. Our approach of efficiently exciting antenna modes by using low index substrates is confirmed both with experiments and numerical simulations. This should provide further insights into better understanding of optical antennas for various applications.

Thin Film Absorbers Based on Plasmonic Phase Resonances

Abstract: We demonstrate an efficient double-layer light absorber by exciting plasmonic phase resonances. We show that the addition of grooves can cause mode splitting of the plasmonic waveguide cavity modes and all the new resonant modes exhibit large absorptivity greater than 90%. Some of the generated absorption peaks have wide-angle characteristics. Furthermore, we find that the proposed structure is fairly insensitive to the alignment error between different layers. The proposed plasmonic nano-structure designs may have exciting potential applications in thin film solar cells, thermal emitters, novel infrared detectors, and highly sensitive bio-sensors.

Plasmon Assisted Optical Curtains

Abstract: We predict an optical curtain effect, i.e., formation of a spatially invariant light field as light emerges from a set of periodic metallic nano-objects. The underlying physical mechanism of generation of this unique optical curtain can be explained in both the spatial domain and the wave-vector domain. In particular, in each period we use one metallic nanostrip to equate the amplitudes of lights impinging on the openings of two metallic nanoslits and also shift their phases by pi difference. We elaborate the influence on the output effect from some geometrical parameters like the periodicity, the slit height and so on. By controlling the light illuminated on metallic subwavelength apertures, it is practical to generate optical curtains of arbitrary forms, which may open new routes of plasmonic nano-lithography.

Flipped-Fishnet Structure Design for Optical Modulator

Abstract: We investigate a novel design of NIM modulator for optical communication. Numerical studies indicate a strong modulation of fiber-guided signal. Experimental observation verifies simulation results, that shows the good potential for on-fiber small footprint modulator.