Abstract-Eigenmode hybridization enables lattice-induced transparency in symmetric terahertz metasurfaces for slow light applications

Joshua A. Burrow, Riad Yahiaoui, Andrew Sarangan, Jay Mathews, Imad Agha, and Thomas A. Searles

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-44-11-2705

Traditional lattice-induced transparency demonstrations are activated by varying the meta-atom lattice constant such that the plasmonic and lattice modes interfere. Here we report on the observation of enhanced coupling between two eigenmodes (first- and second-order) by varying the lattice parameter in a symmetric split ring resonator design. The higher-order quasi-dark mode blueshifts, introducing strong coupling with the fundamental bright mode for periods above 345 μm. Numerical simulations are verified experimentally and supplemented with a two-oscillator model showing good agreement. Furthermore, larger positive group delay values are achieved in the vicinity of the transparency window with minimal absorption, indicating a strong potential for slow light applications.

© 2019 Optical Society of America

Abstract-Influence of symmetry breaking on Fano-like resonances in high Figure of Merit planar terahertz metafilms

Joshua A. Burrow, Riad Yahiaoui, Wesley Sims, Zizwe A. Chase, Viet Tran, Andrew Sarangan, Jay Mathews, Willie S. Rockward, Imad Agha, Thomas A. Searles

https://arxiv.org/abs/1812.02063

It is well established that nearly all high-quality (Q) Fano-like resonances in terahertz (THz) metasurfaces broaden as asymmetry increases, resulting in a decline of Q-factor and an increase in the resonance intensity. Therefore, in order to determine the optimal design for applications in THz sensing, a Figure of Merit (FoM) is required. Previous studies have identified the asymmetry regimes at which the peak FoM occurs for various, specific unit cell geometries. However to date, there is no systematic comparison of the resulting FoMs for common and novel geometries. Here, a THz planar metafilm featuring split ring resonators with four distributed capacitive gaps is investigated to compare three unique methods of implementing asymmetry: (1) adjacent L-bracket translation, (2) capacitive gap translation and (3) increasing gap width. The results obtained find that by translating two gaps and increasing the bottom gap width of the unit cell, the high-Q Fano-like resonances are 6× higher than the FoM for the fundamental dipole mode. This work further informs the design process for THz metasurfaces and as such will help to define their applications in photonics and sensing.

Abstract-Angular dispersions in terahertz metasurfaces: Physics and applications

Meng Qiu, Min Jia, Shaojie Ma, Shulin Sun, Qiong He,  Lei Zhou,

https://journals.aps.org/prapplied/accepted/20070AedD5e1680fa1d60312978c2b8f3fcbaf4b7

Angular dispersion --- the response of a metasurface strongly depends on the impinging angle --- is an intrinsic property of metasurfaces, but its physical origin remains obscure which also hinders its applications in metasurface design. Here, we establish a theory to quantitatively describe such intriguing effects in metasurfaces, and verify it by both experiments and numerical simulations on a typical terahertz metasurface. The physical understanding gained motivates us to propose a new strategy to design meta-devices exhibiting impinging-angle-dependent multi-functionalities. As an illustration, we design a polarization-control meta-device that can behave as a half or quarter wave plate under different excitation angles. Our results not only reveal the physical origin of the angular dispersion, but also point out a new degree of freedom to manipulate light, which are important for designing meta-devices facing versatile application requests.

Abstract-Prototyping of Terahertz Metasurface by One-step Lithographically Defined Templating

 Shaolin Zhou,  Songlin Mu,  Salahuddin Raju, Clarissa Prawoto,  Xuexuan Ruan,  KungBo Ng, Mansun Chan,

https://ieeexplore.ieee.org/document/8340782/

To bypass the sophisticated procedure in existing techniques to produce the terahertz (THz) metasurface filter, a one-step lithographically defined templating method is proposed for rapid prototyping. Essentially, a simplified photolithography process is exploited and optimized first to release one freestanding template perforated with desired features, without any sacrificial layer or external force. The key idea is to impart an adequate degree of hydrophobicity to substrate to tune adhesion between substrate and template for a spontaneous release. Then the template-directed metallization by electroless plating gives rise to an intact freestanding metal mesh framework of THz metasurface filter. Spectral characterization by the THz frequency-domain spectroscopy shows good spectral selection behavior, which agrees adequately well with the numerical simulation by HFSS.

Abstract-Electromagnetically induced transparency control in terahertz metasurfaces based on bright-bright mode coupling

Riad Yahiaoui, Joshua A. Burrow, Sirak M. Mekonen, Andrew Sarangan, Jay Mathews, Imad Agha, Thomas A. Searles

http://arxiv-export-lb.library.cornell.edu/abs/1712.08733

We demonstrate a classical analogue of electromagnetically induced transparency (EIT) in a highly flexible planar terahertz metamaterial (MM) comprised of three-gap split ring resonators. The keys to achieve EIT in this system are the frequency detuning and hybridization processes between two bright modes coexisting in the same unit cell as opposed to bright-dark modes. We present experimental verification of two-bright mode coupling for a terahertz EIT-MM in the context of numerical results and theoretical analysis based on a coupled Lorentz oscillator model. In addition, a hybrid variation of the EIT-MM is proposed and implemented numerically in order to dynamically tune the EIT window by incorporating photosensitive silicon pads in the split gap region of the resonators. As a result, this hybrid MM enables the potential active optical control of a transition from the on-state (EIT mode) to the off-state (dipole mode).

Abstract-Polarization-dependent electromagnetic responses of ultrathin and highly flexible asymmetric terahertz metasurfaces

Joshua A. Burrow, Riad Yahiaoui, Andrew Sarangan, Imad Agha, Jay Mathews, and Thomas A. Searles

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-26-32540

We report the polarization-dependent electromagnetic response from a series of novel terahertz (THz) metasurfaces where asymmetry is introduced through the displacement of two adjacent metallic arms separated by a distance δ. For all polarization states, the symmetric metasurface exhibits a low quality (Q) factor fundamental dipole mode. By breaking the symmetry, we experimentally observe a secondary dipole-like mode with a Q factor nearly 9× higher than the fundamental resonance. As δ increases, the fundamental dipole mode f1redshifts and the secondary mode f2 blueshifts creating a highly transmissive spectral window. Polarization-dependent measurements reveal a full suppression of f2 for all asymmetries at θ ≥ 60°. Furthermore, at δ ≥ 60 μm, we observe a polarization selective electromagnetic induced transparency (EIT) for the fundamental mode. This work paves the way for applications in filtering, sensing and slow-light devices common to other high Q factor THz metasurfaces with EIT-like response.

© 2017 Optical Society of America under the terms of the OSA Open Access Publishing 

Abstract-Polarization-dependent electromagnetic responses of ultrathin and highly flexible asymmetric terahertz metasurfaces

Joshua A. Burrow, Riad Yahiaoui, Andrew Sarangan, Imad Agha, Jay Mathews, Thomas A. Searles

(Submitted on 18 Oct 2017)

https://arxiv.org/abs/1710.06714

We report the polarization-dependent electromagnetic response from a series of novel terahertz (THz) metasurfaces where asymmetry is introduced through the displacement of two adjacent metallic arms separated by a distance δ. For all polarization states, the symmetric metasurface exhibits a low quality (Q) factor fundamental dipole mode. By breaking the symmetry, we experimentally observe a secondary dipole-like mode with a Q factor nearly 9× higher than the fundamental resonance. As δ increases, the fundamental dipole mode f1 redshifts and the secondary mode f2 blueshifts creating a highly transmissive spectral window. Polarization-dependent measurements reveal a full suppression of f2 for all asymmetries at θ≥60∘. Furthermore, at δ≥60 μm, we observe a polarization selective electromagnetic induced transparency (EIT) for the fundamental mode. This work paves the way for applications in filtering, sensing and slow-light devices common to other high Q factor THz metasurfaces with EIT-like response.

Researchers Create Hidden Images with Commercial Inkjet Printers

http://www.businesswire.com/news/home/20161208005710/en/Researchers-Create-Hidden-Images-Commercial-Inkjet-Printers

Images printed with silver and carbon reveal their secrets only with the right type of illumination; for use in security-related applications

WASHINGTON--(BUSINESS WIRE)--Researchers have developed a way to use commercial inkjet printers and readily available ink to print hidden images that are only visible when illuminated with appropriately polarized waves in the terahertz region of the electromagnetic spectrum. The inexpensive method could be used as a type of invisible ink to hide information in otherwise normal-looking images, making it possible to distinguish between authentic and counterfeit items, for example.

We used silver and carbon ink to print an image consisting of small rods that are about a millimeter long and a couple of hundred microns wide,” said Ajay Nahata from the University of Utah, leader of the research team. “We found that changing the fraction of silver and carbon in each rod changes the conductivity in each rod just slightly, but visually, you can't see this modification. Passing terahertz radiation at the correct frequency and polarization through the array allows extraction of information encoded into the conductivity.”

In The Optical Society's journal for high impact research, Optica, the researchers demonstrated their new method to hide image information in an array of printed rods that all look nearly identical. They used the technique to conceal both grayscale and 64-color QR codes, and even embedded two QR codes into a single image, with each code viewable using a different polarization. To the naked eye the images look like an array of identical looking lines, but when viewed with terahertz radiation, the embedded QR code image becomes apparent.

“Our very easy-to-use method can print complex patterns of rods with varying conductivity,” said Nahata. “This cannot easily be done even using a multimillion dollar nanofabrication facility. An added benefit to our technique is that it can be performed very inexpensively.”

Printing metamaterials

The new technique allows printing of different shapes that form a type of metamaterial – synthetic materials that exhibit properties that don’t usually exist in nature. Although there is a great deal of interest in manipulating metamaterials to better control the propagation of light, most techniques require expensive lithography equipment found in nanofabrication facilities to pattern the material in a way that produces desired properties.

Nahata and his colleagues previously developed a simple method to use an off-the-shelf inkjet printer to apply inks made with silver and carbon, which can be purchased from specialty stores online. They wanted to see if their ink-jet printing technique could create various conductivities, a parameter that is typically difficult to modify because it requires changing the type of metal applied at each spatial location. To do this using standard lithography would be time consuming and expensive because each metal would have to be applied in a separate process.

“As we were printing these rods we saw that, in many cases, we couldn't visually tell the difference between different conductivities,” said Nahata. “That led to the idea of using this to encode an image without the need for standard encryption approaches.”

Creating hidden images

To see if they could use the method to encode information, the researchers printed three types of QR codes, each 72 by 72 pixels. For one QR code they used arrays of rods to create nine different conductivities, each coding for one gray level. When they imaged this QR code with terahertz illumination, only 2.7 percent of the rods gave values that were different from what was designed. The researchers also used rods printed in a cross formation to create two separate QR codes that could each be read with a different polarization of terahertz radiation.

The team then created a color QR code by using non-overlapping rods of three different lengths to create each pixel. Each pixel in the image contained the same pattern of rods but varied in conductivity. By arranging the rods in a way that minimized errors, the researchers created three overlapping QR codes corresponding to RGB color channels. Because each pixel contained four different conductivities that could each correspond to a color, a total of 64 colors was observed in the final image. The researchers said they could likely achieve even more than 64 colors with improvements in the printing process.

“We have created the capability to fabricate structures that can have adjacent cells, or pixels, with very different conductivities and shown that the conductivity can be read with high fidelity,” said Nahata. “That means that when we print a QR code, we see the QR code and not any blurring or bleeding of colors.”

With the very inexpensive (under $60) printers used in the paper, the technique can produce images with a resolution of about 100 microns. With somewhat more expensive but still commercially available printers, 20-micron resolution should be achievable. Although the researchers used QR codes that are relatively simple and small, the technique could be used to embed information into more complex and detailed images using a larger canvas.

Nahata’s team used terahertz radiation to read the coded information because the wavelengths in this region are best suited for imaging the resolution available from commercial inkjet printers. The researchers are now working to expand their technique so the images can be interrogated with visible, rather than terahertz, wavelengths. This challenging endeavor will require the researchers to build new printers that can produce smaller rods to form images with higher resolutions.

The researchers are also exploring the possibility of developing additional capabilities that could make the embedded information even more secure. For example, they could make inks that might have to be heated or exposed to light of a certain wavelength before the information would be visible using the appropriate terahertz radiation.

Paper: A. Chanana, A. Paulsen, S. Guruswamy, A. Nahata, “Hiding Multi-level Multi-color Images in Terahertz Metasurfaces,” Optica, 3, 12, 1466 (2016).

DOI: 10.1364/optica.3.001466

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-Graphene-based terahertz metasurface with tunable spectrum spli

Graphene-based terahertz metasurface with tunable spectrum splitting 

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-41-16-3799

Zhaoxian Su, Xuan Chen, Jianbo Yin, and Xiaopeng Zhao

We design a tunable terahertz metasurface, which consists of two different trapezoid graphene ribbons patterned in opposite directions on a gold film, separated by a thin dielectric spacer. The two kinds of graphene ribbons can cover a nearly 2𝜋 phase shift with high reflection efficiency in different spectral regions so that the metasurface can reflect different frequency waves to totally different directions. By changing the Fermi level of the graphene ribbons, the response frequency of the proposed metasurface can be adjusted, and as a result, tunable spectrum splitting can be realized. The present metasurface provides a powerful way to control terahertz waves and has potential applications in wide-angle beam splitters.

© 2016 Optical Society of America

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Abstract-Tailoring the slow light behavior in terahertz metasurfaces

Manukumara Manjappa¹, Sher-Yi Chiam², Longqing Cong¹, Andrew A. Bettiol³,Weili Zhang⁴ and Ranjan Singh^1,a)


http://scitation.aip.org/content/aip/journal/apl/106/18/10.1063/1.4919531
1 Center for Disruptive Photonic Technologies, Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
2 NUS High School of Math and Science, 20 Clementi Avenue 1, Singapore, Singapore 129957
3 Department of Physics, National University of Singapore, Science Drive 3, Singapore, Singapore 117542
4 School of Electrical and Computer Engineering, Oklahoma State University, 202 Engineering South, Stillwater, Oklahoma 74078, USA
a) Electronic mail: ranjans@ntu.edu.sg
Appl. Phys. Lett. 106, 181101 (2015); http://dx.doi.org/10.1063/1.4919531

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 transmissionproperties 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 themetamaterial analogues of atomic electromagnetically induced transparency and offer an approach to engineer slow light devices, broadband filters, and attenuators at terahertz frequencies.