Shahrain bin Mahmood

Free-standing THz electromagnetic metamaterials.

Using micromanufactured S-shaped gold strings suspended in free space by means of window-frames, we experimentally demonstrate an electromagnetic meta-material (EM(3)) in which the metallic structures are no longer embedded in matrices or deposited on substrates such that the response is solely determined by the geometrical parameters and the properties of the metal. Two carefully aligned and assembled window-frames form a bi-layer chip that exhibits 2D left-handed pass-bands corresponding to two different magnetic resonant loops in the range of 1.4 to 2.2 THz as characterized by Fourier transform interferometry and numerical simulation. Chips have a comparably large useful area of 56 mm(2). Our results are a step towards providing EM(3) that fulfill the common notions of a material.

All-metal self-supported THz metamaterial--the meta-foil.

Modern metamaterials face functional constraints as they are commonly embedded in or deposited on dielectric materials. We provide a new solution by microfabricating a completely free-standing all-metal self-supported metamaterial. Using upright S-string architecture with the distinctive feature of metallic transverse interconnects, we form a locally stiff, globally flexible space-grid. Infrared Fourier transform interferometry reveals the typical double-peak structure of a magnetically excited left-handed and an electrically excited right-handed pass-band that is maintained under strong bending and heating, and is sensitive to dielectrics. Exploiting UV/X-ray lithography and ultimately plastic moulding, meta-foils can be mass manufactured cost-effectively to serve as optical elements.

Industrial applications of micro/nanofabrication at Singapore Synchrotron Light Source

SSLS (Singapore Synchrotron Light Source) has set up a complete one-stop shop for micro/nanofabrication in the framework of the LIGA process. It is dubbed LiMiNT for Lithography for Micro and Nanotechnology and allows complete prototyping using the integral cycle of the LIGA process for producing micro/nanostructures from mask design/fabrication over X-ray lithography to electroplating in Ni, Cu, or Au, and, finally, hot embossing in a wide variety of plastics as one of the capabilities to cover a wide range of application fields and to go into higher volume production. The process chain also includes plasma cleaning and sputtering as well as substrate preparation processes including metal buffer layers, plating bases, and spin coating, polishing, and dicing. Furthermore, metrology using scanning electron microscopy (SEM), optical profilometry, and optical microscopy is available. LiMiNT is run as a research lab as well as a foundry. In this paper, several industrial applications will be presented, in which LiMiNT functions as a foundry to provide external customers the micro/nano fabrication services. These services include the fabrication of optical or X-ray masks, of micro/nano structures from polymers or from metals and of moulds for hot embossing or injection moulding.

PATTERNING OF TWO-DIMENSIONAL PHOTONIC CRYSTAL STRUCTURES BY NANOIMPRINT LITHOGRAPHY

We report on the process parameters of nanoimprint lithography for the fabrication of two-dimensional (2D) photonic crystals. The nickel mold with 2D photonic crystal patterns covering an area up to 20 mm2 is produced by electron-beam lithography and electroplating. Periodic pillars as high as 200 nm to 250 nm are produced on the mold with the diameters ranging from 180 nm to 500 nm. Optimization of process parameters is essential to generate high-quality nanoscale patterns and control the residual stress in the mold. The mold is employed for nanoimprinting on the poly-methyl-methacrylate (PMMA) layer spin-coated on the silicon substrate. Periodic air holes are formed in PMMA above its glass-transition temperature and the patterns on the mold are well transferred. This process can be utilized for commercial applications of photonic crystal devices.

Free‐space Electromagnetic Metamaterials From The Far Infrared To The Visible

The development of electromagnetic metamaterials by micro/nanomanufacturing at SSLS has led to matrix‐embedded or substrate‐supported rod‐split‐ring‐based samples reaching left‐handed pass‐bands at 216 THz or 1.39 μm and to free‐space S‐string bi‐layer chips at 2.2 THz. Potential applications of metamaterials range from sub‐wavelength resolution imaging over invisibility cloaking to advanced antennae and are relevant to fields including microscopy, lithography, electromagnetic shielding, and telecommunication.

Micro/nanomanufactured THz electromagnetic metamaterials as a base for applications in transportation

Micro/nanomanufactured electromagnetic metamaterials in the THz spectral range may help extending the use of metamaterials in transportation. S-string based THz metamaterials as manufactured by SSLS, in particular, the meta-foil, provide a promising platform for applications. Special emphasis may be given to antennas being conformal or quickly steerable or tunable for inter-traffic communication. Achievements by SSLS in co-operation with MIT and Zhejiang University are discussed and potential applications outlined.

Self-supported all-metal THz metamaterials

Ideal metamaterials would consist of metal conductors only that are necessary for negative ε and μ. However, most of present-day metamaterials include dielectrics for various support functions. Overcoming dielectrics, we manufactured free-standing THz metamaterials as bi-layer chips of S-string arrays suspended by window-frames at a small gap that controls the resonance frequency. Remaining problems concerning their useful range of incidence angles and the possibility of stacking have been solved by manufacturing the first self-supported free-standing all-metal metamaterials featuring upright S-strings interconnected by metal rods. Large-area slabs show maximum magnetic coupling at normal incidence with left-handed resonances between 3.2 - 4.0 THz. Such metamaterials which we dub the meta-foil represent an ideal platform for including index-gradient optics to achieve optical functionalities like beam deflection and imaging.

Towards large area THz electromagnetic metamaterials

Up to date, electromagnetic metamaterials (EM3) have been mostly fabricated by primary pattern generation via electron beam or laser writer. Such an approach is time-consuming and may have limitations of the area filled with structures. Especially, electron beam written structures are typically confined to areas of a few 100×100 μm2. However, for meaningful technological applications, larger quantities of good quality materials are needed. Lithography, in particular X-ray deep lithography, is well suited to accomplish this task. Singapore Synchrotron Light Source (SSLS) has been applying its LIGA process that includes primary pattern generation via electron beam or laser writer, X-ray deep lithography and electroplating to the micro/nano-manufacturing of high-aspect ratio structures to produce a variety of EM3 structures. Starting with Pendrys split ring resonators, we have pursued structure designs suitable for planar lithography since 2002 covering a range of resonance frequencies from 1 to 216 THz. More recently, string-like structures have also been included. Latest progress made in the manufacturing and characterization of quasi 3D metamaterials having either split ring or string structures over areas of about ≈1 cm2 extension will be described.

Towards 3D Electromagnetic Metamaterials in the THz Range

SSLS has been using its lithography‐based micro/nanofabrication facility LiMiNT (Lithography for Micro and Nanotechnology) and its infrared spectro/microscopy facility ISMI to develop and characterize the first electromagnetic metamaterials having their spectral response in the THz range. Derived from Pendry’s nested‐split‐ring resonator design, these structures require micro/nanofabrication in order to have resonances in the THz range. They exhibit a negative refractive index and hold promise of sub‐diffraction limit imaging. Besides the reduction of the size of the resonating structures to extend the spectral range towards the visible, outstanding issues include the production of high‐aspect‐ratio resonators that are sensitive for the magnetic field in any direction (3D sensitivity) and the capability to produce copious amounts of the electromagnetic metamaterials with a good yield. In this paper, we shall report on first results of 3D EM3 structures made by inclined exposures.

X‐ray‐based Micro/Nanomanufacturing at SSLS — Technology and Applications

Deep and high‐aspect‐ratio micro/nanostructures can be accurately patterned in a variety of resists by proximity lithography using high energy, intense, parallel beams of X‐rays from synchrotron radiation sources. Using so called LIGA technology, high‐aspect‐ratio micro/nanostructures can be produced with vertical dimensions ranging from micrometers to millimeters and horizontal dimensions as small as microns. SSLS has set up its LiMiNT facility (Lithography for Micro/Nanotechnology) and is running it partly as a foundry, partly as a research lab. Under the foundry aspect, work is done for customers in various fields of applications. SSLS’ own research is focusing on the development of devices and artificial composite materials such as electromagnetic metamaterials. In this paper, the technology capabilities of the LiMiNT facility and application examples are presented.