Sher-Yi Chiam

Increased frequency shifts in high aspect ratio terahertz split ring resonators

The resonance of split ring resonators (SRRs) is known to shift upon the addition of a dielectric overlayer, a feature useful for practical applications. Here, we demonstrate that the frequency shift is enlarged by increasing the SRR height, thereby potentially enhancing sensitivity and tunability. We fabricated SRRs resonating at terahertz frequencies using a focused proton beam. This resulted in SRRs nearly 10 μm high, with smooth and vertical sidewalls. Terahertz time domain spectroscopy was used for characterization. Upon applying a dielectric overlayer (ϵ=2.7), a resonance located at 640 GHz shifted by nearly 120 GHz. Simulations also indicate a widening frequency shift as SRR height increases.

Controlling metamaterial resonances via dielectric and aspect ratio effects

We study ways to enhance the sensitivity and dynamic tuning range of the fundamental inductor-capacitor (LC) resonance in split ring resonators (SRRs) by controlling the aspect ratio of the SRRs and their substrate thickness. We conclude that both factors can significantly affect the LC resonance. We show that metafilms consisting of low height SRRs on a thin substrate are most sensitive to changes in their dielectric environment and thus show excellent potential for sensing applications.

Tailoring the slow light behavior in terahertz metasurfaces

We experimentally study the effect of near field coupling on the transmission of light in terahertz metasurfaces. Our results show that tailoring the coupling between the resonators modulates the amplitude of resulting electromagnetically induced transmission, probed under different types of asymmetries in the coupled system. Observed change in the transmission amplitude is attributed to the change in the amount of destructive interference between the resonators in the vicinity of strong near field coupling. We employ a two-particle model to theoretically study the influence of the coupling between bright and quasi-dark modes on the transmission properties of the system and we find an excellent agreement with our observed results. Adding to the enhanced transmission characteristics, our results provide a deeper insight into the metamaterial analogues of atomic electromagnetically induced transparency and offer an approach to engineer slow light devices, broadband filters, and attenuators at terahertz frequencies.

The fabrication of x-ray masks using proton beam writing

We have developed a simplified method of fabricating x-ray masks for deep x-ray lithography by using proton beam writing (PBW) without subsequent soft x-ray copying steps. Combining direct PBW and subsequent electroplating, x-ray masks with gold absorber patterns of up to 11 µm height and with vertical and smooth sidewalls were fabricated. The smallest size in the absorber pattern is less than 0.5 µm in this work. The masks were used for x-ray lithography with synchrotron radiation, and 870 µm SU-8 structures with smooth sidewalls were produced. This fabrication method is promising to be an important alternative to conventional methods for x-ray mask making.

Characterization of interfacial reactions between ionized metal plasma deposited Al–0.5 wt.% Cu and Ti on SiO2

Abstract It was reported that the reaction between Al and Ti takes place and Al3Ti compound is formed during the annealing at 500°C. Annealing at higher temperatures, such as 550 and 600°C, the Al3Ti compound transforms to Al5Ti2. It is believed that the Al5Ti2 is thermodynamically stable comparing with Al3Ti. In the present research, the interfacial reactions in Al–0.5 wt.% Cu/Ti/SiO2/Si structure have been investigated in the samples prepared by ionized metal plasma (IMP) and then annealed at various temperatures from 200 to 600°C for 30 min in Argon ambient. The results obtained by Rutherford backscattering spectroscopy and transmission electron microscopy show that there is a Ti layer (52 nm in thickness) between Al5Ti2 and SiO2 and there is no formation of the ternary compound — AlxTiySiz, which is detrimental in the contact metallization layer. It indicates that the Ti layer deposited by IMP technique acts as a barrier to retard the reaction between Al5Ti2 and SiO2 and consequentially protect the contact metallization layer.

Quantitative analysis of cascade impactor samples – revisited

Abstract Concentrations of aerosols collected in Singapore during the three months long haze period that affected the whole South-East Asian region in 1997 are reported. Aerosol samples were continuously collected by using a fine aerosol sampler (PM2.5) and occasionally with a single orifice cascade impactor (CI) sampler. Our results show that in the fine fraction (

Channeling contrast microscopy of GaN and InGaN thin films

Abstract The recent development of blue and green light emitting diodes (LED) based on single quantum well structures made from GaN and related materials (AlGaN, InGaN) has created many efforts to achieve a complete characterisation of devices grown under various conditions. Here we report on CCM measurements on GaN thin films (d=0.7–3.0 μm) grown by metal organic vapour phase epitaxy (MOVPE) and on 500 A InGaN films grown epitaxially on top of the GaN thin films. The samples were analysed by broad beam channeling and channeling contrast microscopy (CCM), using 1–2 MeV H+ and He+ ions. Generally, very low minimum yields were found (χmin=2–4%), indicating nearly perfect crystal structures. The susceptibility to ion–beam induced damage was assessed by random and channeled 1 MeV He+ irradiation and subsequent CCM analysis. CCM also revealed the presence μm-sized regions in the InGaN films with increased In signal strength. The channeling PIXE data for 500 A thin films are found to be in excellent agreement with the corresponding RBS results, allowing the determination of channeling yields of elements for which RBS data is difficult to obtain.

High aspect ratio metamaterials for enhanced tunability and sensitivity

Using a proton beam based lithography process, we fabricate and study high aspect ratio metamaterials, revealing distinct 3-dimensional resonances. Increased aspect ratio also leads to enhanced tunability and sensitivity for practical applications.

Study of interfacial reactions in ionized metal plasma (IMP) deposited Al-0.5%wtCu/Ti/SiO2/Si structure

Interfacial reactions in Al-0.5%wtCu/Ti/SiO2/Si structure have been investigated up to the annealing temperature of 600°C for 30 min in Argon ambient. Annealing temperature at above 500°C, Al alloy and Ti start to react and produce Al3Ti, which was already reported. Annealing at higher temperatures (550°C, and 600°C) made Al3Ti transformed into Al5Ti2, which is thermodynamically more stable than Al3Ti. The unreacted 52 nm thick Ti which existed underneath of Al5Ti2 might lead to retardation of the reaction between Al5Ti2 and the underlying SiO2. Hence, the formation of ternary compound (AlxTiySiz) which is believed to be detrimental to the contact metallization layers was protected.