Abstract-Near-field terahertz imaging using sub-wavelength apertures without cutoff

Shuchang Liu, Oleg Mitrofanov, and Ajay Nahata

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-3-2728

We demonstrate near-field imaging capabilities of a conical waveguide without cutoff using broadband terahertz (THz) radiation. In contrast to conventional conically tapered waveguides, which are characterized by strong suppression of transmission below the cutoff frequency, the proposed structure consists of two pieces, such that there is an adjustable gap along the length of the waveguide. We also ensure that the sidewalls are thin in the vicinity of the gap. The combination of these geometrical features allow for significantly enhanced transmission at frequencies below the cutoff frequency, without compromising the mode confinement and, consequently, the spatial resolution when used for imaging applications. We demonstrate near-field imaging with this probe simultaneously at several frequencies, corresponding to three regimes: above, near and below the cutoff frequency. We observe only mild degradation in the image quality as the frequency is reduced below the cutoff frequency. These results suggest that further refinements in the probe structure will allow for improved imaging capabilities at frequencies well below the cutoff frequency.

© 2016 Optical Society of America

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Abstract-Graphene-based tunable metamaterial terahertz filters

Kai Yang¹, Shuchang Liu¹, Sara Arezoomandan¹, Ajay Nahata¹ and Berardi Sensale-Rodriguez^1,a)
http://scitation.aip.org/content/aip/journal/apl/105/9/10.1063/1.4894807
1 Department of Electrical and Computer Engineering, The University of Utah, 50 S. Central Campus Dr., Salt Lake City, Utah 84112, USA
a) Electronic mail: berardi.sensale@utah.edu
Appl. Phys. Lett. 105, 093105 (2014); http://dx.doi.org/10.1063/1.4894807

We propose and describe a micro-machined tunable metamaterial terahertz filter based ongraphene. The device structure consists of periodic metallic rings with several gaps where tunablegraphene stripes are located. We demonstrate that the filter resonance frequency can be adjusted easily by varying the conductivity of graphene and implement this by changing the number of stacked graphene layers. Moreover, the proposed design is scalable, in the sense that the resonance frequency tuning can be controlled by scaling the inner and outer radius of the metal rings. Using numerical simulations and terahertz time-domain spectroscopy measurements of the fabricated samples, we show that the resonance frequency of the structure can be altered by 40% (i.e., from ∼0.2 THz to ∼0.12 THz) by simply tuning the conductivity of graphene. Importantly, the active area of the device is ≪0.1% of the total unit cell area, which can boost the device speed upon electrostatic actuation.

Abstract-Injection Molding of Free-Standing, Three-Dimensional, All-Metal Terahertz Metamaterials

Full Paper

Injection Molding of Free-Standing, Three-Dimensional, All-Metal Terahertz Metamaterialshttp://onlinelibrary.wiley.com/doi/10.1002/adom.201400094/abstract

1. Jinqi Wang¹, 
2. Shuchang Liu¹, 
3. Sivaraman Guruswamy² and
4. Ajay Nahata^1,*

Article first published online: 17 APR 2014

DOI: 10.1002/adom.201400094

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Issue

Advanced Optical Materials

Early View (Online Version of Record published before inclusion in an issue)

Fabrication of free-standing two- and three-dimensional terahertz meta­materials is demonstrated via injection molding of gallium, a metal that melts at temperatures just slightly above room temperature. Molds are created by inscribing the desired microchannel geometries in one or two polydimethylsiloxane (PDMS) films using conventional soft lithography techniques and then reversibly bonding the two films together using van der Waals forces. After heating gallium above its melting point (∼30 °C), the liquid metal is injected into the mold. Surprisingly, the metal does not solidify even after cooling the filled mold at −16 °C for 24 h. However, when the liquid metal comes into contact with solid gallium at room temperature, the entire metal device solidifies within the mold immediately. The PDMS films can then be peeled away, yielding a free-standing solid gallium structure. A 2D split ring resonator-based metamaterial is fabricated and three different approaches for creating 3D metamaterials are demonstrated: a multilayer stack, a manually folded structure that maintains its shape after folding, and a directly injection molded 3D structure. The transmission properties of these devices are measured using terahertz time-domain spectroscopy and are shown to not suffer from limitations imposed by substrates.

Abstract-Reconfigurable terahertz metamaterial device with pressure memory

Jinqi Wang, Shuchang Liu, Sivaraman Guruswamy, and Ajay Nahata  »View Author Affiliations

Optics Express, Vol. 22, Issue 4, pp. 4065-4074 (2014)
http://dx.doi.org/10.1364/OE.22.004065

We demonstrate a liquid metal-based reconfigurable terahertz (THz) metamaterial device that is not only pressure driven, but also exhibits pressure memory. The discrete THz response is obtained by injecting eutectic gallium indium (EGaIn) into a microfluidic structure that is fabricated in polydimethylsiloxane (PDMS) using conventional soft lithography techniques. The shape of the injected EGaIn is mechanically stabilized by the formation of a thin oxide surface layer that allows the fluid to maintain its configuration within the microchannels despite its high intrinsic surface energy. Although the viscosity of EGaIn is twice that of water, the formation of the surface oxide layer prevents flow into a microchannel unless a critical pressure is exceeded. Using a structure in which the lateral channel dimensions vary, we progressively increase the applied pressure beyond the relevant critical pressure for each section of the device, enabling switching from one geometry to another (split ring resonator to closed ring resonator to an irregular closed ring resonator). As the geometry changes, the transmission spectrum of the device changes dramatically. When the external applied pressure is removed between device geometry changes, the liquid metal morphology remains unchanged, which can be regarded as a form of pressure memory. Once the device is fully filled with liquid metal, it can be erased through the use of mechanical pressure and exposure to acid vapors.

© 2014 Optical Society of America