S. M. Maniam

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.

THz meta-foil – a platform for practical applications of metamaterials

The meta-foil, an all-metal fully self-supported locally stiff and globally flexible metamaterial, is presented. Its architecture is based on an array of parallel S-strings interconnected by transverse metal rods that are periodically repeated along the strings. For the present samples, this period is about once or twice the length of an S, which is 31 µm. The resonance frequency of the left-handed pass-band is 3.2 THz or somewhat higher depending on geometry. The meta-foil is manufactured by UV or X-ray lithography-based microfabrication. In the end, it may be produced cost-effectively by plastic moulding. The physical function is explained by numerical simulation and equivalent circuit theory. The spectra are measured by Fourier transform infrared spectroscopy. Maximum transmission is achieved at normal incidence with a cosine-square-like decrease with incidence angle. A change of geometrical parameters alters the resonance frequency as expected. The spectra are also rather sensitive to surrounding dielectrics, indicating a sensor capability. As the meta-foil can be bent, a cylindrical hyperlens set-up is discussed as a forthcoming application to sub-wavelength resolution imaging.

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.

Quantitative investigation of phase retrieval from X-ray phase-contrast tomographic images

X-ray phase-contrast tomographic microimaging is a powerful tool to reveal the internal structure of opaque soft-matter objects that are not easily seen in standard absorption contrast. In such low Z materials, the phase shift of X-rays transmitted can be important as compared to the absorption. An easy experimental set up that exploits refractive contrast formation can deliver images that are providing detailed structural information. Applications are abundant in fields including polymer science and engineering, biology, biomedical engineering, life sciences, zoology, water treatment and filtration, membrane science, and micro/nanomanufacturing. However, available software for absorptive contrast tomography cannot be simply used for structure retrieval as the contrast forming effect is different. In response, CSIRO has developed a reconstruction code for phase-contrast imaging. Here, we present a quantitative comparison of a micro phantom manufactured at SSLS with the object reconstructed by the code using X-ray images taken at SSLS. The phantom is a 500 μm thick 800 μm diameter cylindrical disk of SU-8 resist having various eccentric cylindrical bores with diameters ranging from 350 μm to 40 μm. Comparison of these parameters that are well known from design and post-manufacturing measurements with reconstructed ones gives encouraging results.

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.

Manufacturing Metamaterials Using Synchrotron Lithography

The function of metamaterials relies on their resonant response to electromagnetic waves in characteristic spectral bands. To make metamaterials homogeneous, the size of the basic resonant element should be less than 10% of the wavelength. For the THz range up to the visible, structure details of 50 nm to 30 μm are required as are high aspect ratios, tall heights, and large areas. For such specifications, lithography, in particular, synchrotron radiation deep X-ray lithography, is the method of choice. X-ray masks are made via primary pattern generation by means of electron or laser writing. Several different X-ray masks and accurate mask-substrate alignment are necessary for architectures requiring multi-level lithography. Lithography is commonly followed by electroplating of metallic replica. The process can also yield mould inserts for cost-effective manufacture by plastic moulding. We made metamaterials based on rod-split-rings, split-cylinders, S-string bi-layer chips, and S-string meta-foils. Left-handed resonance bands range from 2.4 to 216 THz. Latest is the all-metal self-supported flexible meta-foil with pass-bands of 45% up to 70% transmission at 3.4 to 4.5 THz depending on geometrical parameters.