Wei-Chih Liu

Glancing angle deposited gold nanohelix arrays on smooth glass as three-dimensional SERS substrates

Gold nanohelix arrays (NHAs) were fabricated on a smooth glass substrate using glancing angle deposition technique. At a deposition angle of 89°, gold NHAs were fabricated by introducing liquid nitrogen to flow under the backside of BK7 glass substrate holder to reduce the temperature of substrate to be around −140 °C under deposition. The spin rate was controlled with respect to the deposition rate to grow three different sized nanohelices. The morphology and optical properties were measured and compared for the three samples. In application, the surface-enhanced Raman scattering (SERS) from each three-dimensional NHA was measured and analyzed with near field simulation.

Self-Shadowing Deposited Pure Metal Nanohelix Arrays and SERS Application.

In this work, one-step glancing angle deposition is utilized to fabricate gold and silver nanohelix arrays (NHAs) on smooth glass substrates. During deposition, the substrate is cooled using liquid nitrogen and rotated with a tunable spin rate. The substrate spin rate is tuned to match the deposition rate to yield a spiral-like helix structure. The morphologies and optical properties of spiral-like Ag and Au NHAs are measured and compared. The polarization-dependent reflectance of Au NHA leads to a strong g-factor. The three-dimensional nanohelical structures are demonstrated to be a highly sensitive surface-enhanced Raman scattering (SERS) substrate.

Densely packed aluminum-silver nanohelices as an ultra-thin perfect light absorber

Metals have been formed into nanostructures to absorb light with high efficiency through surface plasmon resonances. An ultra-thin plasmonic structure that exhibits strong absorption over wide ranges of wavelengths and angles of incidence is sought. In this work, a nearly perfect plasmonic nanostructure is fabricated using glancing angle deposition. The difference between the morphologies of obliquely deposited aluminum and silver nanohelices is exploited to form a novel three-dimensional structure, which is an aluminum-silver nanohelix array on a pattern-free substrate. With a thickness of only 470 nm, densely distributed nanohelices support rod-to-rod localized surface plasmons for broadband and polarization-independent light extinction. The extinctance remains high over wavelengths from 400 nm to 2000 nm and angles of incidence from 0° to 70°.

Optical coating on nano-optical antennas to enhance directional radiation

Abstract. A traditional high-reflection optical coating was applied to enhance the directional radiation of nanoantennas. A highly reflective multilayer upon the top lateral side of a horizontally lying silver nanorod enhances the forward scattering when an optical wave is incident on the bare bottom side of the nanorod. Enhanced forward scattering can thus be observed from a glancing deposited silver nanorod array (NRA). An effective method for coupling the energy of incident light into a NRA involves arranging the NRA in a prism coupling. The highly efficient light coupling effect over a broadband and a wide range of angles results in extra-strong forward light scattering.

Design and deposition of a metal-like and admittance-matching metamaterial as an ultra-thin perfect absorber

A stratiform metamaterial, comprising metal and dielectric thin films, exhibits both near-perfect antireflection and strong light extinction to function as a perfect and ultra-thin light absorber. The equivalent admittance and extinction coefficient of the metamaterial are tailored using a visual method that is based on an admittance diagram. A five-layered metamaterial was designed and deposited with a total thickness of 260 nm on a mirror to exhibit strong and wide angle absorption over wavelengths from 400 nm to 2000 nm. A seven-layered metamaterial with a total thickness of less than 200 nm was designed and deposited to have equivalent admittance around unity and an extinction coefficient that is comparable to that of metal. Such a metal-like metamaterial exhibits low reflectivity so couples most visible light energy into the films and dissipates energy with an equivalent skin depth of less than 55 nm over visible wavelengths.

High efficient light absorption and nanostructure-dependent birefringence of a metal-dielectric symmetrical layered structure

A multilayer that comprises ultra-thin metal and dielectric films has been investigated and applied as a layered metamaterial. By arranging metal and dielectric films alternatively and symmetrically, the equivalent admittance and refractive index can be tailored separately. The tailored admittance and refractive index enable us to design optical filters with more flexibility. The admittance matching is achieved via the admittance tracing in the normalized admittance diagram. In this work, an ultra-thin light absorber is designed as a multilayer composed of one or several cells. Each cell is a seven-layered film stack here. The design concept is to have the extinction as large as possible under the condition of admittance matching. For a seven-layered symmetrical film stack arranged as Ta2O5 (45 nm)/ a-Si (17 nm)/ Cr (30 nm)/ Al (30 nm)/ Cr (30 nm)/ a-Si (17 nm)/ Ta2O5 (45 nm), its mean equivalent admittance and extinction coefficient over the visible regime is 1.4+0.2i and 2.15, respectively. The unit cell on a transparent BK7 glass substrate absorbs 99% of normally incident light energy for the incident medium is glass. On the other hand, a transmission-induced metal-dielectric film stack is investigated by using the admittance matching method. The equivalent anisotropic property of the metal-dielectric multilayer varied with wavelength and nanostructure are investigated here.

Deposited nanohelices on smooth surface: morphology and SERS application(Conference Presentation)

Metal nanohelix arrays have been fabricated using glancing angle deposition. Comparing with method of substrate patterning, nanohelices with average arm width of 32 nm, pitch length of 34 nm and radius of curvature around 12.5 nm grew on a regular seeded layer with period of 79 nm and average seed diameter of 14 nm. In order to mass produce metal nanohelices, smooth substrates were adopted to deposited nanohelix arrays. Due to shadowing effect achieved under substrate cooling, the silver and gold nanohelix arrays could be grown successfully on smooth substrates by well controlling the substrate spin rate with respect to the deposition rate. In this work, the thickness of deposition monitored by quartz monitor was kept at 0.3 nm/s. The substrate was cooled to a low temperature around -10oC. The average arm width, pitch length, radius of curvature and spacer between nanohelices vary with deposition angle are investigated here. The morphology of nanohelix varies with different deposition angles (from 86o to 80o) were also to be investigated. In this work, the average space between adjacent nanohelices and radius of curvature were reduced and increased by increasing the deposition angle, respectively. The average pitch of each nanohelix array was low dependent on the deposition angle. The overlap effect occurs between adjacent nanohelices and the gaps between nanohelices support local field enhancement. The area associated local field enhancement called hot spots. Surface-enhanced Raman scattering (SERS) signals from nanostructured thin films were measured and compared with near-field simulations.

Subwavelength resolution from multilayered structure(Conference Presentation)

Breaking optical diffraction limit is one of the most important issues needed to be overcome for the demand of high-density optoelectronic components. Here, a multilayered structure which consists of alternating semiconductor and dielectric layers for breaking optical diffraction limitation at THz frequency region are proposed and analyzed. We numerically demonstrate that such multilayered structure not only can act as a hyperbolic metamaterial but also a birefringence material via the control of the external temperature (or magnetic field). A practical approach is provided to control all the diffraction signals toward a specific direction by using transfer matrix method and effective medium theory. Numerical calculations and computer simulation (based on finite element method, FEM) are carried out, which agree well with each other. The temperature (or magnetic field) parameter can be tuned to create an effective material with nearly flat isofrequency feature to transfer (project) all the k-space signals excited from the object to be resolved to the image plane. Furthermore, this multilayered structure can resolve subwavelength structures at various incident THz light sources simultaneously. In addition, the resolution power for a fixed operating frequency also can be tuned by only changing the magnitude of external magnetic field. Such a device provides a practical route for multi-functional material, photolithography and real-time super-resolution image.

Obliquely deposited nanohelix array: morphology and circular dichroism

We proposed a glancing angle deposition technique with substrate cooling technique to grow a silver nanohelix array on a glass substrate. The significant circular dichroism (g-factor) can be experimentally observed in the visible region

Metal-dielectric multilayered band-pass filters: induced-transmission for metamaterials

We investigate a metal/dielectric films to achieve a bandpass filter with high transmission peak. Our design method is developed on a normalized admittance diagram. The design can be applied to develop low-loss negative index metamaterial.