Abstract-An Electrically Tunable Terahertz Plasmonic Device Based on Shape Memory Alloys and Liquid Metals

Hui Zhou, Ting Zhang, Sivaraman Guruswamy, Ajay Nahata,

http://onlinelibrary.wiley.com/doi/10.1002/adom.201700684/full

An electrically tunable terahertz (THz) plasmonic device is designed and fabricated using liquid metals (eutectic gallium indium) and shape memory alloy wires (Flexinol). The liquid metal is injected into the voids of a poly(dimethyl) siloxane microfluidic mold forming a periodic array of subwavelength apertures, while the wires are inserted into the elastomer below the metal plane. When a DC voltage is applied to the wires, they contract via Joule heating, reducing the aperture periodicity and blueshifting the transmission resonances of the device. When the voltage is removed, the wires cool and elongate back to their original length, allowing the transmission spectrum to return to its original state. The magnitude of this change depends upon the applied voltage. The device is shown to thermally cycle between the relaxed state and the fully contracted state reproducibly over at least 500 thermal cycles. The asymmetric geometry of the device and the contraction process yield transmission properties that are unexpected: two closely spaced resonances, where both resonances correspond to the same scattering indices, and an increase in the transmission amplitude of the lowest order resonance upon contraction. Numerical simulations are used to understand these features.

Abstract-Hiding multi-level multi-color images in terahertz metasurfaces

Ashish Chanana, Andrew Paulsen, Sivaraman Guruswamy, and Ajay Nahata
https://www.osapublishing.org/optica/abstract.cfm?uri=optica-3-12-1466

Our work presents a novel technique to encode information onto terahertz metasurfaces comprised of geometrically identical unit cell arrays. Previous demonstrations on metasurfaces or frequency-selective surfaces have shown interesting concepts to engineer electromagnetic radiation, but such designs often require a spatial arrangement of geometrically varying unit cells, either by shape, size, orientation, etc. In some cases, the output response can be mapped by examining the arrangement of atoms. Here, we show that by fabricating an array of resonant structures that are nominally identical visually, but where individual structures can have different conductivities, we can hide image information that is revealed when imaged using the appropriate terahertz frequency and polarization. This is achieved because changes in the structure’s conductivity correspond to changes in the depth of the resonant absorption observed in transmission. Using the simplest unit cell consisting of a single dipole, we create images that have up to 9 different discernible gray levels when interrogated at a single frequency. When a slightly more complex cross structure is used in the unit cell, 36 discernible levels are encoded in the image using two different polarizations. Finally, when the unit cell consists of multiple dipoles designed for multiple frequencies, we observe 64 unique colors in an encoded image. We believe our results present a unique approach for hiding information that could be applied to security-related applications.

© 2016 Optical Society of America

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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-Terahertz Plasmonic Structures Based on Spatially Varying Conductivities

1. 
   

1. 
   

1. Barun Gupta¹, 
2. Shashank Pandey¹,
3. Sivaraman Guruswamy², 
4. Ajay Nahata^1,*

Article first published online: 7 MAR 2014

DOI: 10.1002/adom.201400018

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

http://onlinelibrary.wiley.com/doi/10.1002/adom.201400018/abstract

Terahertz plasmonic structures are demonstrated in which the conductivity of the metallic film is varied spatially in order to further enhance the response. Using a commercially available inkjet printer, in which one cartridge is filled with conductive silver ink and a second cartridge is filled with resistive carbon ink, computer generated drawings of plasmonic structures are printed in which the individual printed dots can have differing amounts of the two inks, thereby creating a spatial variation in the conductivity. The silver ink has a DC conductivity that is only a factor of six lower than bulk silver, while the carbon ink acts as a lossy dielectric at THz frequencies. Both inks sinter at room temperature immediately after contact with the plastic film. Using a periodic array of subwavelength apertures as a test structure, patterns printed with different fractional amounts of the two inks show dramatically different enhanced optical transmission properties. These differences arise from changes in the propagation loss properties as a function of conductivity. This data is used to design and fabricate aperture arrays in which the conductivity varies spatially. The resulting plasmonic effect is found to dramatically alter the spatial beam profile of the transmitted THz radiation, as measured by THz imaging.

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