Abstract-Terahertz characterization of two-dimensional low-conductive layers enabled by metal gratings

 

Prashanth Gopalan, Yunshan Wang,  Berardi Sensale-Rodriguez, 

https://www.nature.com/articles/s41598-021-82560-2

While terahertz spectroscopy can provide valuable information regarding the charge transport properties in semiconductors, its application for the characterization of low-conductive two-dimensional layers, i.e., σs <  < 1 mS, remains elusive. This is primarily due to the low sensitivity of direct transmission measurements to such small sheet conductivity levels. In this work, we discuss harnessing the extraordinary optical transmission through gratings consisting of metallic stripes to characterize such low-conductive two-dimensional layers. We analyze the geometric tradeoffs in these structures and provide physical insights, ultimately leading to general design guidelines for experiments enabling non-contact, non-destructive, highly sensitive characterization of such layers.

Abstract-A Computational Design Framework for Efficient, Fabrication Error-Tolerant, Planar THz Diffractive Optical Elements

Sourangsu Banerji,  Berardi Sensale-Rodriguez,

https://www.nature.com/articles/s41598-019-42243-5

We demonstrate ultra-thin (1.5-3λ0), fabrication-error tolerant efficient diffractive terahertz (THz) optical elements designed using a computer-aided optimization-based search algorithm. The basic operation of these components is modeled using scalar diffraction of electromagnetic waves through a pixelated multi-level 3D-printed polymer structure. Through the proposed design framework, we demonstrate the design of various ultrathin planar THz optical elements, namely (i) a high Numerical Aperture (N.A.), broadband aberration rectified spherical lens (0.1 THz–0.3 THz), (ii) a spectral splitter (0.3 THz–0.6 THz) and (iii) an on-axis broadband transmissive hologram (0.3 THz–0.5 THz). Such an all-dielectric computational design-based approach is advantageous against metallic or dielectric metasurfaces from the perspective that it incorporates all the inherent structural advantages associated with a scalar diffraction based approach, such as (i) ease of modeling, (ii) substrate-less facile manufacturing, (iii) planar geometry, (iv) high efficiency along with (v)broadband operation, (vi) area scalability and (vii) fabrication error-tolerance. With scalability and error tolerance being two major bottlenecks of previous design strategies. This work is therefore, a significant step towards the design of THz optical elements by bridging the gap between structural and computational design i.e. through a hybrid design-based approach enabling considerably less computational resources than the previous state of the art. Furthermore, the approach used herein can be expanded to a myriad of optical elements at any wavelength regime.

Abstract-Ultrafast THz modulators with WSe2 thin films [Invited]

Prashanth Gopalan, Ashish Chanana, Sriram Krishnamoorthy, Ajay Nahata, Michael A. Scarpulla, and Berardi Sensale-Rodriguez

Fig. 1 (a) XRD scans indicating highly crystalline WSe2 films showing only the (002) family of planes. (b) Raman mode at 251.2 cm−1, which agrees with reports in [24] for multilayer films. (c) UV-vis absorption measurements of WSe2 thin films indicating a strong excitonic absorption peak at ~740 nm. Based on this, optical excitation for OPTP and TRTS were chosen at 800 nm and 400 nm. (d) SEM image of WSe2 thin film shows grain size of ~1 µm.

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-9-2-826

Tungsten diselenide (WSe2) thin films exhibit ultrafast carrier recombination lifetimes, which makes them promising candidates for high speed modulators. With pulsed optical excitation, they could be used to realize all-optical, frequency agile, terahertz devices. Looking into the potential of this material for such applications, time-resolved terahertz spectroscopy can provide significant insight into its free carrier and exciton dynamics such as recombination lifetimes, photo-induced conductivity and decay pathways. In this study, we measure transient terahertz conductivity and photo-generated carrier lifetimes in custom-grown large-area WSe2thin-films. We discuss its dependence on grain size and number of layers. By analyzing the tradeoffs between carrier-lifetimes, photo-generated conductivity, grain size, and the number of layers, we show that the response of these films can be tailored by controlling the growth parameters. Customizing the film terahertz response can enable large modulation without the need for integration with bulk semiconductors, as widely reported in the literature, thereby achieve high terahertz photoconductivity and high-speed operation. Across samples, our measurements show carrier decay timescale on the order ~10 to 100 ps and a transient conductivity that shows non-Drude behavior. This deviation from a Drude response is dominant within the first few picoseconds (<10 ps) before changing into a Drude like free-carrier response at longer delays. Based on our grown films, we experimentally demonstrate a metamaterial terahertz modulator with WSe2 as the only active element, attaining ~40% modulation depth.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Manifestation of kinetic-inductance in spectrally-narrow terahertz plasmon resonances in thin-film Cd3As2

Ashish Chanana, Neda Loftizadeh, Hugo O. Condori Quispe, Prashanth Gopalan, Joshua R. Winger, Steve Blair, Ajay Nahata, Vikram Deshpande, Michael A. Scarpulla, Berardi Sensale-Rodriquez,

https://arxiv.org/abs/1811.04306

Three-dimensional (3D) semimetals have been predicted and demonstrated to have a wide variety of interesting properties associated with its linear energy dispersion. In analogy to two-dimensional (2D) Dirac semimetals, such as graphene, Cd3As2, a 3D semimetal, has shown ultra-high mobility, large Fermi velocity, and has been hypothesized to support plasmons at terahertz frequencies. In this work, we demonstrate synthesis of high-quality large-area Cd3As2 thin-films through thermal evaporation as well as the experimental realization of plasmonic structures consisting of periodic arrays of Cd3As2 stripes. These arrays exhibit sharp resonances at terahertz frequencies with associated quality-factors (Q) as high as ~ 3.7. Such spectrally-narrow resonances can be understood on the basis of a large kinetic-inductance, resulting from a long momentum scattering time, which in our films can approach ~1 ps at room-temperature. Moreover, we demonstrate an ultrafast tunable response through excitation of photo-induced carriers in optical pump / terahertz probe experiments. Our results evidence that the intrinsic 3D nature of Cd3As2 provides for a very robust platform for terahertz plasmonic applications. Overall, our observations pave a way for the development of myriad terahertz (opto) electronic devices based on Cd3As2 and other 3D Dirac semimetals, benefiting from strong coupling of terahertz radiation, ultrafast transient response, magneto-plasmon properties, etc. Moreover, the long momentum scattering time, thus large kinetic inductance in Cd3As2, also holds enormous potential for the re-design of passive elements such as inductors and hence can have a profound impact in the field of RF integrated circuits

Abstract-Comparison of unit cell coupling for grating‐gate and high electron mobility transistor array THz resonant absorbers

Journal of Applied Physics

Ajay Nahata, Ashish Chanana, Berardi Sensale-Rodriguez, Debdeep Jena, Hugo O. Condori Quispe, Huili Grace Xing, Jimy Encomendero, Mingda Zhu, Nicole Trometer, 

https://aip.scitation.org/doi/abs/10.1063/1.5032102

We report experimental studies on the excitation of synchronized plasmon resonances in AlGaN/GaN High Electron Mobility Transistor (HEMT) arrays. In contrast to the commonly employed grating-gate configurations, the analyzed structure contains periodically patterned ohmic contacts to the two-dimensional electron gas, which are laid-out parallel to the gate fingers. In this structure, the terahertz to plasmon coupling mechanism is fundamentally different from that in grating-gate configurations. Whereas the grating-gate configuration constitutes a coupled resonant system in which the resonance frequency depends on the grating periodicity, when periodical ohmic contacts are incorporated, the system behaves as a synchronized resonant system in which each unit cell is effectively independent. As a result, in a HEMT-array, the resonance is no longer set by the periodicity but rather by the gate and the ungated region length. Experimental results of fabricated samples compare well with numerical simulations and theoretical expectations. Our work demonstrates that the proposed approach allows: (i) more efficient excitation of high order plasmon modes and (ii) superior overall terahertz to plasmon coupling, even in configurations having less number of devices per unit area. From this perspective, our results reveal a simple way to enhance the terahertz to plasmon coupling and thus improve the performance of electron plasma wave-based devices; this effect can be exploited, for example, to improve the response of HEMT-based terahertz detectors.

Abstract-Comparison of unit cell coupling for grating‐gate and high electron mobility transistor array THz resonant absorbers

Journal of Applied Physics

Hugo O. Condori Quispe,   Ashish Chanana, Jimy Encomendero,Mingda Zhu, Nicole Trometer,  Ajay Nahata, Debdeep Jena,   Huili Grace Xing,Berardi Sensale-Rodriguez,

https://aip.scitation.org/doi/10.1063/1.5032102

We report experimental studies on the excitation of synchronized plasmon resonances in AlGaN/GaN High Electron Mobility Transistor (HEMT) arrays. In contrast to the commonly employed grating-gate configurations, the analyzed structure contains periodically patterned ohmic contacts to the two-dimensional electron gas, which are laid-out parallel to the gate fingers. In this structure, the terahertz to plasmon coupling mechanism is fundamentally different from that in grating-gate configurations. Whereas the grating-gate configuration constitutes a coupled resonant system in which the resonance frequency depends on the grating periodicity, when periodical ohmic contacts are incorporated, the system behaves as a synchronized resonant system in which each unit cell is effectively independent. As a result, in a HEMT-array, the resonance is no longer set by the periodicity but rather by the gate and the ungated region length. Experimental results of fabricated samples compare well with numerical simulations and theoretical expectations. Our work demonstrates that the proposed approach allows: (i) more efficient excitation of high order plasmon modes and (ii) superior overall terahertz to plasmon coupling, even in configurations having less number of devices per unit area. From this perspective, our results reveal a simple way to enhance the terahertz to plasmon coupling and thus improve the performance of electron plasma wave-based devices; this effect can be exploited, for example, to improve the response of HEMT-based terahertz detectors.

Abstract-Efficient Design of Diffractive THz Lenses for Aberration Rectified Focusing via Modified Binary Search Algorithm

Sourangsu Banerji, Ashish Chanana, Hugo Condori, Ajay Nahata, and Berardi Sensale-Rodriguez

https://www.osapublishing.org/abstract.cfm?uri=cleo_at-2018-JW2A.76&origin=search

We experimentally demonstrate thin, error tolerant, and aberration corrected 3D printed diffractive 1D and 2D THz lenses for focusing THz beams with NA > 0.25 and an average efficiency greater than 75% (for all cases). Our work has relevance to THz lens design, and in general, THz imaging systems.

© 2018 The Author(s)

Abstract-THz characterization and demonstration of visible-transparent/terahertz-functional electromagnetic structures in ultra-conductive La-doped BaSnO3 Films

Sara Arezoomandan, Abhinav Prakash, Ashish Chanana, Jin Yue, Jieying Mao, Steve Blair, Ajay Nahata, Bharat Jalan,  Berardi Sensale-Rodriguez

https://www.nature.com/articles/s41598-018-22038-w

We report on terahertz characterization of La-doped BaSnO3 (BSO) thin-films. BSO is a transparent complex oxide material, which has attracted substantial interest due to its large electrical conductivity and wide bandgap. The complex refractive index of these films is extracted in the 0.3 to 1.5 THz frequency range, which shows a metal-like response across this broad frequency window. The large optical conductivity found in these films at terahertz wavelengths makes this material an interesting platform for developing electromagnetic structures having a strong response at terahertz wavelengths, i.e. terahertz-functional, while being transparent at visible and near-IR wavelengths. As an example of such application, we demonstrate a visible-transparent terahertz polarizer.

Abstract-Graphene-based reconfigurable terahertz plasmonics and metamaterials

Sara Arezoomandan^a, 

Hugo O. Condori Quispe^a, 

Nicholas Ramey^b, 

Cesar A. Nieves^c, 

Berardi Sensale-Rodriguez

http://www.sciencedirect.com/science/article/pii/S000862231630985X

This work discusses and compares two proposed practical approaches for realizing graphene-based reconfigurable terahertz metamaterials, namely: graphene-only plasmonic structures, and graphene/metal hybrid structures. From rigorous theoretical analysis, full-wave electromagnetic numerical simulations, as well as supporting experiments, several reconfigurable structures are analyzed and compared in terms of their: (i) Quality-factor, (ii) Extinction-ratio, (iii) Unit-cell dimensions, and (iv) Resonance-frequency tunability-range. From this analysis it is observed that at terahertz frequencies, although typically possessing larger unit-cell dimensions and being limited by a restricted resonance-frequency tunability-range, reconfigurable metamaterials based on graphene/metal hybrid structures can provide much larger quality-factors, extinction levels, and, when reconfigured, smaller extinction-level degradation than graphene-only plasmonic structures. As a result, when analyzed in the context of reconfigurable terahertz metamaterials, graphene might result attractive as a reconfigurable media providing tunability to otherwise passive metallic structures rather than as a reconfigurable plasmonic material per-se.

Abstract-Near-field Enhancement and Optimal Performance in Metamaterial Terahertz Modulators Based on 2D-materials

Sara Arezoomandan, Rusen Yan, Prashanth Gopalan, Kun Tian, Ashish Chanana, Ashutosh Tiwari, Huili Grace Xing, Ajay Nahata, and Berardi Sensale-Rodriguez

https://www.osapublishing.org/abstract.cfm?uri=LAOP-2016-LW2B.4

In this work we analyze metamaterial terahertz modulators consisting of 2D-material/metal hybrid-structures. Influence of the near-field enhancement in the modulator performance is discussed. Devices are fabricated and tested using graphene and MoS2 as active materials.

© 2016 OSA

PDF Article

Abstract-Terahertz amplification in RTD-gated HEMTs with a grating-gate wave coupling topology

Hugo O. Condori Quispe1, Jimy J. Encomendero-Risco2,3, Huili Grace Xing2,3 and Berardi Sensale-Rodriguez1


1 The University of Utah, Salt Lake City, Utah 84112, USA
2 University of Notre Dame, Notre Dame, Indiana 46556, USA
3 Cornell University, Ithaca, New York 14853, USA
Appl. Phys. Lett. 109, 063111 (2016); http://dx.doi.org/10.1063/1.4961053
http://scitation.aip.org/content/aip/journal/apl/109/6/10.1063/1.4961053?TRACK=RSS

We theoretically analyze the operation of a terahertz amplifier consisting of a resonant-tunneling-diode gated high-electron-mobility transistor (RTD-gated HEMT) in a grating-gate topology. In these devices, the key element enabling substantial power gain is the efficient coupling of terahertz waves into and out of plasmons in the RTD-gated HEMT channel, i.e., the gain medium, via the grating-gate itself, part of the active device, rather than by an external antenna structure as discussed in previous works, therefore potentially enabling terahertz amplification with associated power gains >40 dB.

Abstract-Exceptional Terahertz Wave Modulation in Graphene Enhanced by Frequency Selective Surfaces

Rusen Yan, Sara Arezoomandan, Berardi Sensale-Rodriguez, and Huili Grace Xing

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/acsphotonics.5b00639

Publication Date (Web): February 11, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.5b00639

In this paper we theoretically and experimentally analyze the design tradeoffs in terahertz metamaterial modulators consisting of hybrid graphene/metal stacked structures. In these devices graphene is used as the active medium whereas a passive metallic frequency selective surface is employed to enhance the light-matter interaction in graphene. When altering the structure key geometrical parameters, we observe a close match between our experiments and theory, showing that it is possible to achieve an optimal tradeoff between modulation depth, insertion loss, and speed in these devices. Moreover, a transmission line based compact mathematical model is introduced in order to explain our experimental observations and predict the performance limits in these structures.

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-Graphene-based electrically reconfigurable deep-subwavelength metamaterials for active control of THz light propagation

• Sara Arezoomandan, 
• Kai Yang, 
• Berardi Sensale-Rodriguez

http://link.springer.com/article/10.1007%2Fs00339-014-8693-8

This work studies the terahertz light propagation through

graphene-based reconfigurable metasurfaces where the unit cell dimensions are much smaller than the terahertz wavelength. The proposed devices, which poses deep-subwavelength unit cell and active region dimensions can operate as amplitude and/or phase modulators in certain specific frequency bands determined by the device geometry. Reconfigurability is attained via electrostatically tuning the optical conductivity of patterned graphene layers, which are strategically located in each unit cell. The ultra-small unit cell dimensions can be advantageous for beam shaping applications.

Prof. Sensale Receives NSF CAREER Award

December 23, 2013

Prof. Sensale

http://www.ece.utah.edu/prof-sensale-receives-nsf-career-award/

A new ECE faculty member, Prof. Berardi Sensale Rodriguez, has received a five-year NSF CAREER Award entitled, “CAREER: THz active metamaterials employing thin-film semiconductors.”

Objective: The objective of this program is to provide a solid foundation for Prof. Sensale’s long-term research goal of developing active solid-state terahertz devices that can be employed in compact, low cost communication and imaging systems. Based on the enhanced light-matter interaction in thin-film semiconductor-based metamaterials, this proposal aims to develop devices for: a) terahertz beam steering, based on the linear properties of these materials, and b) terahertz generation via mixing and/or higher harmonic generation, based on the materials nonlinearities, thereby targeting two of the central limitations in existing terahertz technology.

Intellectual Merit: The intellectual merit is to foster the development of novel terahertz optoelectronic devices, which can form the basis for a wide range of applications. Moreover, these studies are expected to improve the current understanding about the terahertz properties of two thin-film semiconductors: vanadium dioxide and graphene, especially in terms of their non-linear properties. Since both materials can be built on arbitrary substrates, their low cost is expected to lead to market applications. Broader Impact: The broader impacts are to create some of the components necessary to make complex terahertz systems and to make these more readily available. It is expected that the developed technologies will be transformative to a broad range of scientific and application-oriented communities. Moreover, undergraduate and graduate students will be trained in the emergent fields of terahertz and nanomaterials, and they are an integral element of the designed outreach activities to disseminate the new discoveries to the general audience, school teachers and students.

Abstract-Terahertz imaging employing graphene modulator arrays

Berardi Sensale-Rodriguez, Subrina Rafique, Rusen Yan, Mingda Zhu, Vladimir Protasenko, Debdeep Jena, Lei Liu, and Huili Grace Xing »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-21-2-2324

In this paper we propose and experimentally demonstrate arrays of graphene electro-absorption modulators as electrically reconfigurable patterns for terahertz cameras. The active element of these modulators consists of only single-atom-thick graphene, achieving a modulation of the THz wave reflectance > 50% with a potential modulation depth approaching 100%. Although the prototype presented here only contains 4x4 pixels, it reveals the possibility of developing reliable low-cost video-rate THz imaging systems employing single detector.

Dr. Daniel Mittleman gives Terahertztechnology readers a glimpse of activites at TeraNano meeting in Japan

Hi Randy,

I’m writing from the TeraNano meeting, to give you a summary of just a few of the highlights. This conference is an annual review associated with the PIRE (Partnerships for International Research and Education) program at RiceUniversity which is sponsored by the National Science Foundation (among others). This program is run by Professor Junichiro Kono (my colleague at Rice) and Professor Masa Tonouchi from Osaka University. It funds student exchange between the US and Japan, and has been a very successful vehicle for a lot of fruitful research collaborations. It focuses on the science at the overlap between terahertz spectroscopy and nano-materials, hence the name ‘TeraNano’.

Consistent with the mission of NSF-funded activities, the subject matter here this week is very much basic research. A lot of the research being discussed here involves the use of terahertz to study carbon nanostructures, especially graphene (a single layer of carbon atoms in a hexagonal lattice). This is clearly the ‘material of the moment’, since there is a lot of optimism associated with potential applications in electronics, transparent displays, opto-electronics, and so on. It turns out that a lot of the interesting possible applications of graphene can be studied using terahertz techniques, so there is a natural overlap between the two fields. One nice highlight here was the talk by Berardi Sensale-Rodriguez (University of Notre Dame) on the use of graphene as a modulator for terahertz radiation. This device could eventually be used to switch terahertz beams on and off electrically at high speed, a capability which is currently not easy to do by other means. Beyond graphene, I would also mention an interesting talk by Ben Murdin (University of Surrey), who is investigating the use of single-atom impurities in silicon for quantum computing – can we use one phosphorus atom to store one bit of information in its quantum state? If so, then controlling those individual bits could be accomplished using terahertz radiation. My own talk was inspired by my group’s recent work on using parallel metal plates for guiding terahertz waves. We’ve been thinking about how to engineer the amount of radiation that emerges from the wave guide at the output end, and we now have some nice results on minimizing this output, making a (nearly) perfect reflector. This could lead to a waveguide-based resonator, where the radiation is trapped inside the waveguide by these nearly perfect reflections at all output facets.

This afternoon, we have also heard several talks on the integration of terahertz science and technology into educational missions at various levels. Of course, the TeraNano program has ongoing self-evaluation of its effectiveness at the undergraduate and graduate level. But there is also now an effort to bring simple terahertz experiments into high schools, which can help to inspire students to pursue STEM fields when they get to college.

I would also like to put in a small advertisement for a fledgling activity at Rice University. Several of my colleagues and I are organizing an effort to initiate a new seminar series at Rice in the topical areas of terahertz, infrared, and millimeter-wave science and technology. We hope to launch this seminar series early next year, to have regularly scheduled speakers at Rice in these fields. I would welcome suggestions of possible speakers from the readers of your blog, and of course anybody who is in the Houston area would be welcome to attend any of the talks.

Regards,

Dan Mittleman

Abstract-Efficient terahertz electro-absorption modulation employing graphene plasmonic structures

http://arxiv.org/abs/1211.4176
 Berardi Sensale-Rodriguez, Rusen Yan, Mingda Zhu, Debdeep Jena, Lei Liu, Huili Grace Xing

We propose and discuss terahertz electro-absorption modulators based on graphene plasmonic structures. The active device consists of a self-gated pair of graphene layers, which are patterned to structures supporting THz plasmonic resonances. These structures allow for efficient control of the effective THz optical conductivity, thus absorption, even at frequencies much higher than the Drude roll-off in graphene where most previously proposed graphene-based devices become inefficient. Our analysis shows that reflectance-based device configurations, engineered so that the electric field is enhanced in the active graphene pair, could achieve very high modulation-depth, even ~100%, at any frequency up to tens of THz.

Abstract-A New Class of Electrically Tunable Metamaterial Terahertz Modulators

Rusen Yan, Berardi Sensale-Rodriguez, Lei Liu, Debdeep Jena, Huili Grace Xing

http://arxiv.org/abs/1210.7793
Switchable metamaterials offer unique solutions for efficiently manipulating electromagnetic waves, particularly for terahertz waves, which has been difficult since naturally occurring materials rarely respond to terahertz frequencies controllably. However, few terahertz modulators demonstrated to date exhibit simultaneously low attenuation and high modulation depth. In this letter we propose a new class of electrically-tunable terahertz metamaterial modulators employing metallic frequency-selective-surfaces (FSS) in conjunction with capacitively-tunable layers of electrons, promising near 100% modulation depth and < 15% attenuation. The fundamental departure in our design from the prior art is tuning enabled by self-gated electron layers that is independent from the metallic FSS. Our proposal is applicable to all possible electrically tunable elements including graphene, Si, MoS2, oxides etc, thus opening up myriad opportunities for realizing high performance switchable metamaterials over an ultra-wide terahertz frequency range.