Abstract-Plasmon-induced transparency in metamaterials: Active near field coupling between bright superconducting and dark metallic mode resonators

Applied Physics Letters

Wei Cao, Ranjan Singh,  Caihong Zhang, Jiaguang Han,  Masayoshi Tonouchi, Weili Zhang

https://aip.scitation.org/doi/abs/10.1063/1.4819389

Structured plasmonic metamaterial devices offer the design flexibility to be size scaled for operation across the electromagnetic spectrum and are extremely attractive for generating electromagnetically induced transparency and slow-light behaviors via coupling of bright and dark subwavelength resonators. Here, we experimentally demonstrate a thermally active superconductor-metal coupled resonator based hybrid terahertz metamaterial on a sapphire substrate that shows tunable transparency and slow light behavior as the metamaterial chip is cooled below the high-temperature superconducting phase transition temperature. This hybrid metamaterial opens up the avenues for designing micro-sized active circuitry with switching, modulation, and “slowing down terahertz light” capabilities.

Light-powered 3-D printer creates terahertz lens

http://www.nanowerk.com/news2/gadget/newsid=43278.php

(Nanowerk News) From visible light to radio waves, most people are familiar with the different sections of the electromagnetic spectrum. But one wavelength is often forgotten, little understood, and, until recently, rarely studied. It's called terahertz, and it has important applications in imaging and communications."Terahertz is somewhat of a gap between microwaves and infrared," said Northwestern University's Cheng Sun. "People are trying to fill in this gap because this spectrum carries a lot of information."Sun and his team have used metamaterials and 3-D printing to develop a novel lens that works with terahertz frequencies. Not only does it have better imaging capabilities than common lenses, but it opens the door for more advances in the mysterious realm of the terahertz.Supported by the National Science Foundation, the work was published online on April 22 in the journal Advanced Optical Materials ("Additive Manufacturing of a 3D Terahertz GradieRefractive Index Lens")GradieRefractive Index Lens").

                                            The design of Sun's lens with gradient refractive index.

"Typical lenses -- even fancy ones -- have many, many components to counter their intrinsic imperfections," said Sun, associate professor of mechanical engineering at Northwestern's McCormick School of Engineering. "Sometimes modern imaging systems stack several lenses to deliver optimal imaging performance, but this is very expensive and complex."The focal length of a lens is determined by its curvature and refractive index, which shapes the light as it enters. Without components to counter imperfections, resulting images can be fuzzy or blurred. Sun's lens, on the other hand, employs a gradient index, which is a refractive index that changes over space to create flawless images without requiring additional corrective components.There are two major factors that made this new lens possible. First, it is made from a novel metamaterial that exhibits properties not readily available in nature. "Such properties originate from its tiny structures that are much smaller than the terahertz wavelength," said Fan Zhou, the paper's first author and member of Sun's laboratory. "By assembling these tiny structures, we can create specific refractive index distribution."Second, the lens was manufactured with a 3-D printing technique called projection micro-stereo-lithography. The technique enables a scalable, rapid, and inexpensive way to produce the tiny features that are needed for the lens to operate at the terahertz frequency band. The printing technology allowed the researchers to fabricate the metamaterial to precisely fit their designs.

"For printing, we use a photo-polymer in liquid form," Sun said. "When we shine a light on the material, it converts it into a solid. The material forms to the shape of the light, allowing us to create a 3-D structure. You cannot accomplish a gradient index with traditional manufacturing processes."The lens could make terahertz imaging, which is particularly useful for security, cheaper, higher resolution, and more available. While X-rays can detect metal, they cannot detect plastic or chemicals. Terahertz scanners, however, can detect both of items to discover concealed weapons, biological weapons such as anthrax, and plastic explosives. And unlike X-rays, terahertz radiation is completely harmless to humans."This advance means we can unveil previously inaccessible information of some opaque materials in high resolution," said Wei Cao, Sun's collaborator at Oklahoma State University. "This opens up an entirely new technique for a massive range of potential uses from biomedical research to security."

Abstract-Ultrahigh-Q Fano Resonances in Terahertz Metasurfaces: Strong Influence of Metallic Conductivity at Extremely Low Asymmetry

1. Yogesh Kumar Srivastava^1,2, 
2. Manukumara Manjappa^1,2, 
3. Longqing Cong^1,2, 
4. Wei Cao³, 
5. Ibraheem Al-Naib⁴, 
6. Weili Zhang³and
7. Ranjan Singh^1,2,*

Article first published online: 14 DEC 2015

DOI: 10.1002/adom.201500504

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

http://onlinelibrary.wiley.com/doi/10.1002/adom.201500504/abstract


Fano resonances in metasurfaces are important due to their low loss subradiant behavior that allows excitation of high-quality (Q) factor resonances extending from the microwave to the optical regime. High-Q Fano resonances have recently enabled applications in the areas of sensing, modulation, filtering, and efficient cavities for lasing spasers. Highly conducting metals are the most commonly used materials for fabricating the metasurfaces, especially at the low-frequency terahertz region where the DC, Drude, and perfect electric conductivity show similar resonant behavior of the subwavelength meta-atoms. Here, it is experimentally and theoretically demontrated that the Q factor of a low asymmetry Fano resonance is extremely sensitive to the conducting properties of the metal at terahertz frequencies. Large differences in the Q factor and figure of merit of the Fano resonance is observed for perfect electric conductors, Drude metal, and a DC-conducting metal, which is in sharp contrast to the behavior of the inductive–capacitive resonance of meta-atoms at terahertz frequency. Identification of such a low asymmetry regime in Fano resonances is the key to engineer the radiative and nonradiative losses in plasmonic and metamaterial-based devices that have potential applications in the microwave, terahertz, infrared, and the optical regimes.

Abstract-Terahertz metasurfaces with a high refractive index enhanced by the strong nearest neighbor coupling

Siyu Tan, Fengping Yan, Leena Singh, Wei Cao, Ningning Xu, Xiang Hu, Ranjan Singh, Mingwei Wang, and Weili Zhang
https://www.osapublishing.org/oe/abstract.cfm?URI=oe-23-22-29222

The realization of high refractive index is of significant interest in optical imaging with enhanced resolution. Strongly coupled subwavelength resonators were proposed and demonstrated at both optical and terahertz frequencies to enhance the refractive index due to large induced dipole moment in meta-atoms. Here, we report an alternative design for flexible free-standing terahertz metasurface in the strong coupling regime where we experimentally achieve a peak refractive index value of 14.36. We also investigate the impact of the nearest neighbor coupling in the form of frequency tuning and enhancement of the peak refractive index. We provide an analytical circuit model to explain the impact of geometrical parameters and coupling on the effective refractive index of the metasurface. The proposed meta-atom structure enables tailoring of the peak refractive index based on nearest neighbor coupling and this property offers tremendous design flexibility for transformation optics and other index-gradient devices at terahertz frequencies.

© 2015 Optical Society of America

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Abstract-Terahertz metasurfaces with a high refractive index enhanced by the strong nearest neighbor coupling

Siyu Tan, Fengping Yan, Leena Singh, Wei Cao, Ningning Xu, Xiang Hu, Ranjan Singh, Mingwei Wang, and Weili Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-23-22-29222

The realization of high refractive index is of significant interest in optical imaging with enhanced resolution. Strongly coupled subwavelength resonators were proposed and demonstrated at both optical and terahertz frequencies to enhance the refractive index due to large induced dipole moment in meta-atoms. Here, we report an alternative design for flexible free-standing terahertz metasurface in the strong coupling regime where we experimentally achieve a peak refractive index value of 14.36. We also investigate the impact of the nearest neighbor coupling in the form of frequency tuning and enhancement of the peak refractive index. We provide an analytical circuit model to explain the impact of geometrical parameters and coupling on the effective refractive index of the metasurface. The proposed meta-atom structure enables tailoring of the peak refractive index based on nearest neighbor coupling and this property offers tremendous design flexibility for transformation optics and other index-gradient devices at terahertz frequencies.

© 2015 Optical Society of America

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Abstract-Active metasurface terahertz deflector with phase discontinuities.

Xiaoqiang Su, Chunmei Ouyang, Ningning Xu, Wei Cao, Xin Wei, Guofeng Song, Jianqiang Gu, Zhen Tian, John F O'Hara, Jiaguang Han, Weili Zhang

http://www.pubfacts.com/detail/26480376/Active-metasurface-terahertz-deflector-with-phase-discontinuities

Metasurfaces provide great flexibility in tailoring light beams and reveal unprecedented prospects on novel functional components. However, techniques to dynamically control and manipulate the properties of metasurfaces are lagging behind. Here, for the first time to our knowledge, we present an active wave deflector made from a metasurface with phase discontinuities. The active metasurface is capable of delivering efficient real-time control and amplitude manipulation of broadband anomalous diffraction in the terahertz regime. The device consists of complementary C-shape split-ring resonator elements fabricated on a doped semiconductor substrate. Due to the Schottky diode effect formed by the hybrid metal-semiconductor, the real-time conductivity of the doped semiconductor substrate is modified by applying an external voltage bias, thereby effectively manipulating the intensity of the anomalous deflected terahertz wave. A modulation depth of up to 46% was achieved, while the characteristics of broadband frequency responses and constant deflected angles were well maintained during the modulation process. The modulation speed of diffraction amplitude reaches several kilohertz, limited by the capacitance and resistance of the depletion region. The scheme proposed here opens up a novel approach to develop tunable metasurfaces.

Abstract-Active metasurface terahertz deflector with phase discontinuities

Xiaoqiang Su, Chunmei Ouyang, Ningning Xu, Wei Cao, Xin Wei, Guofeng Song, Jianqiang Gu, Zhen Tian, John F. O’Hara, Jiaguang Han, and Weili Zhang
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-23-21-27152

Metasurfaces provide great flexibility in tailoring light beams and reveal unprecedented prospects on novel functional components. However, techniques to dynamically control and manipulate the properties of metasurfaces are lagging behind. Here, for the first time to our knowledge, we present an active wave deflector made from a metasurface with phase discontinuities. The active metasurface is capable of delivering efficient real-time control and amplitude manipulation of broadband anomalous diffraction in the terahertz regime. The device consists of complementary C-shape split-ring resonator elements fabricated on a doped semiconductor substrate. Due to the Schottky diode effect formed by the hybrid metal-semiconductor, the real-time conductivity of the doped semiconductor substrate is modified by applying an external voltage bias, thereby effectively manipulating the intensity of the anomalous deflected terahertz wave. A modulation depth of up to 46% was achieved, while the characteristics of broadband frequency responses and constant deflected angles were well maintained during the modulation process. The modulation speed of diffraction amplitude reaches several kilohertz, limited by the capacitance and resistance of the depletion region. The scheme proposed here opens up a novel approach to develop tunable metasurfaces.

© 2015 Optical Society of America

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Three-dimensional invisibility cloaks functioning at terahertz frequencies

Wei Cao, Fan Zhou, Cheng Sun, Jianqiang Gu, Dachuan Liang, Jiaguang Han, Shuang Zhang and 

Weili Zhang

http://spie.org/x108531.xml

Both the geometric and spectroscopic signatures of an object were completely concealed under 3D terahertz invisibility cloaks made of either homogeneous or inhomogeneous media.

3 June 2014, SPIE Newsroom. DOI: 10.1117/2.1201405.005440

Achieving invisibility cloaking in the terahertz regime has recently garnered a great deal of attention due to unique and promising emerging applications of terahertz technology. Particularly with recent advances in terahertz communications and radar, there has been an increasing demand for cloaking devices functioning at terahertz frequencies. The ultimate goal is to conceal a large object from being observed by terahertz radar in civilian or space communications.

So far, terahertz invisibility cloaks have been experimentally demonstrated in quasi-3D geometry based on both subwavelength building blocks and homogeneous uniaxial crystals.1, 2 These quasi-3D cloaks are also called ground plane or carpet cloaks, and they were proposed to overcome the difficulties of complete cloaks that require dielectric singularity, have high loss, and very narrow bandwidth.3, 4 One of the challenges that remains at terahertz frequencies is finding photo-curable dielectric materials with lower loss that allow for a broader response bandwidth. Indeed, the complete cloak that features 3D and broad bandwidth is the major challenge for the entire electromagnetic spectrum. Here we present experimental demonstrations of quasi-3D terahertz cloaks made from either 3D inhomogeneous metamaterials 1 or homogeneous 2 media.

The inhomogeneous cloak made from dielectric metamaterials was lithographically fabricated using a scalable Projection Microstereolithography (PμSL) process (see Figure 1). The triangular cloaking structure has a total thickness of 4.4mm, comprised of 220 layers of 20μm thickness. The distribution of the varying hole geometry can be clearly identified in the scanning electron microscope (SEM) images. The space underneath the bump is designated as the cloaked region. The cloak operates at a broad frequency range between 0.3 and 0.6THz, and is placed over an α-lactose monohydrate absorber with rectangular shape. The α-lactose monohydrate exhibits a resonant attenuation signature at 0.53THz due to the presence of collective vibrational transition modes. We measured the reflected terahertz wave in four cases: (I) a flat reflective surface, (II) exposed lactose on a reflective surface, (III) a control structure with a reflective bump placed on top of the lactose, and (IV) the cloaking structure placed on top of the lactose. The measurements were carried out in an angular-resolved reflection terahertz time domain spectroscopy (THz-TDS) system. The terahertz cloak in case (IV) conceals both the geometric and spectroscopic features of the lactose, which then closely resembles case (I), demonstrating the successful design of the cloak.

 

Figure 1. Inhomogeneous quasi-3D terahertz cloak. (a) Schematic diagram illustrating the projection micro-stereolithography system fabricating a 3D metamaterial cloaking device. The grayscale of individual pixels within each 85.2×85.2μm unit cell can be adjusted so the holes can be fabricated with sub-pixel precision. (b) Optical and scanning electron microscope images of the fabricated cloaking device. The gradual change in hole size near the bump can be clearly observed.1

We also demonstrated a large-scale terahertz invisibility cloak made from birefringent crystalline sapphire (see Figure 2). This homogeneous cloaking device features a large concealed volume, low loss, and broad bandwidth. In particular, it is capable of hiding objects with a dimension nearly an order of magnitude larger than that of its lithographic counterpart, but without involving complex and time-consuming cleanroom processing. The cloak was made from two 20mm thick high-purity sapphire prisms. The area beneath the cloak is 1.75mm tall, nearly ten times taller than the inhomogeneous counterpart. In addition, the useful bandwidth increased from 0.3-0.6THz to 0.2-1.0THz due to different design approaches and material absorption properties. The volume of the cloaking region is approximately 5% of the whole sample.

 

Figure 2. Measured cloaking effect of a homogeneous terahertz cloak with respect to the relative positions (x-axis) and the frequency (y-axis). The color represents the relative spectral amplitude of (a) cloaking, (b) flat surface reflection, and (c) reference with the same cloak lens. A schematic of the cloak design is shown in (d).2

We characterized the homogeneous cloak using the same terahertz spectroscopy system, except that no test sample was placed in the cloak region. Instead, we used the uncloaked transverse electric polarized beam profile as the reference, which exhibits significant beam splitting: see Figure2(c). The THz-TDS results indicated that the terahertz invisibility cloak successfully concealed both the geometric and spectroscopic signatures of the object, making it undetectable to the observer. As shown in Figure 2, the reflected transverse magnetic beam from the cloak (a) shows nearly the same profile as that reflected by a flat mirror (b). On the other hand, for the transverse electric beam, the detector received two largely separated beam profiles at the left and right side of the cloaking profile, shown in (c).

This straightforward cloaking approach greatly reduces the complexity in design and fabrication of the metamaterial-based cloaks. More importantly, the initial work in the visible regime confirmed that this is a promising way toward practical macroscopic cloaking devices with frequency and incidence angle robustness.

In conclusion, we demonstrated that quasi-3D invisibility cloaks in the terahertz regime, comprised of either homogeneous or inhomogeneous media, can completely conceal both the geometrical and spectroscopic signatures of a rectangular absorber placed beneath them. Our next steps in this direction will explore innovative approaches toward a complete 3D cloak for terahertz wavelengths. The keys will be the transformation optics design, unique metamaterial building blocks, and novel fabrication processing.

The authors thank Y. Bao, C. T. Stuart, and Y. Yang for outstanding contributions in this work. We acknowledge financial support from the U.S. National Science Foundation and the China National Natural Science Foundation.

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Wei Cao

Oklahoma State University

Stillwater, OK

Fan Zhou, Cheng Sun

Northwestern University

Evanston, IL

Jianqiang Gu, Dachuan Liang, Jiaguang Han

Tianjin University

Tianjin, China

Shuang Zhang

University of Birmingham

Birmingham, United Kingdom

Weili Zhang
Oklahoma State University
Stillwater, OK
and
Tianjin University
Tianjin, China

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References:

1. F. Zhou, Y. J. Bao, W. Cao, C. T. Stuart, J. Gu, W. Zhang, C. Sun, Hiding a realistic object using a broadband terahertz invisibility cloak, Sci. Rep. 1, p. 78, 2011.

2. D. Liang, J. Gu, J. Han, Y. Yang, S. Zhang, W. Zhang, Robust large dimension terahertz cloaking, Adv. Mater. 24, p. 916, 2012.

3. J. S. Li, J. B. Pendry, Hiding under the carpet: a new strategy for cloaking, Phys. Rev. Lett. 101, p. 203901, 2008. doi:10.1103/PhysRevLett.101.203901

4. D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, J. B. Starr, D. R. Smith, Metamaterial electromagnetic cloak at microwave frequencies, Science 314, p. 977, 2006.

Abstract-Limitation in thin-film sensing with transmission-mode terahertz time-domain spectroscopy

Withawat Withayachumnankul, John F. O’Hara, Wei Cao, Ibraheem Al-Naib, and Weili Zhang  »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-22-1-972

Thin-film sensing with a film thickness much less than a wavelength is an important challenge in conventional transmission-mode terahertz time-domain spectroscopy (THz-TDS). Since the interaction length between terahertz waves and a sample film is short, a small change in the transmitted signal compared with the reference is considerably obscured by system uncertainties. In this article, several possible thin-film measurement procedures are carefully investigated. It is suggested that an alternating sample and reference measurement approach is most robust for thin-film sensing. In addition, a closed-form criterion is developed to determine the critical thickness, i.e., the minimal thickness of a film unambiguously detectable by transmission-mode THz-TDS. The analysis considers influences from the Fresnel transmission at interfaces and the Fabry-Pérot reflections, in addition to the propagation across the film. The experimental results show that typical THz-TDS systems can detect polymer films with a thickness down to a few microns, two orders of magnitude less than the wavelength. For reasonably accurate characterization, it is recommended that the film thickness be at least ten times above this limit. The analysis is readily extended to biomolecular and semiconductor films. The criterion can be used to estimate the system-dependent performance in thin-film sensing applications, and can help to ascertain whether an alternative terahertz sensing modality is necessary.

© 2014 Optical Society of America

Abstract-Tailoring terahertz plasmons with silver nanorod arrays

Wei Cao, Chunyuan Song, Thomas E. Lanier, Ranjan Singh, John F. O'Hara, William M. Dennis, Yiping Zhao & Weili Zhang

• http://www.nature.com/srep/2013/130503/srep01766/full/srep01766.html

Plasmonic materials that strongly interact with light are ideal candidates for designing subwavelength photonic devices. We report on direct coupling of terahertz waves in metallic nanorods by observing the resonant transmission of surface plasmon polariton waves through lithographically patterned films of silver nanorod (100 nm in diameter) micro-hole arrays. The best enhancement in surface plasmon resonant transmission is obtained when the nanorods are perfectly aligned with the electric field direction of the linearly polarized terahertz wave. This unique polarization-dependent propagation of surface plasmons in structures fabricated from nanorod films offers promising device applications. We conclude that the anisotropy of nanoscale metallic rod arrays imparts a material anisotropy relevant at the microscale that may be utilized for the fabrication of plasmonic and metamaterial based devices for operation at terahertz frequencies.