Abstract-Frequency‐Agile Temporal Terahertz Metamaterials

Prakash Pitchappa,   Abhishek Kumar,   Haidong Liang,   Saurav Prakash,   Nan Wang,   Andrew A. Bettiol,   Thirumalai Venkatesan,  Chengkuo Lee,   Ranjan Singh,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.202000101

Spatiotemporal manipulation of electromagnetic waves has recently enabled a plethora of exotic optical functionalities, such as non‐reciprocity, dynamic wavefront control, unidirectional transmission, linear frequency conversion, and electromagnetic Doppler cloak. Here, an additional dimension is introduced for advanced manipulation of terahertz waves in the space‐time, and frequency domains through integration of spatially reconfigurable microelectromechanical systems and photoresponsive material into metamaterials. A large and continuous frequency agility is achieved through movable microcantilevers. The ultrafast resonance modulation occurs upon photoexcitation of ion‐irradiated silicon substrate that hosts the microcantilever metamaterial. The fabricated metamaterial switches in 400 ps and provides large spectral tunability of 250 GHz with 100% resonance modulation at each frequency. The integration of perfectly complementing technologies of microelectromechanical systems, femtosecond optical control and ion‐irradiated silicon provides unprecedented concurrent control over space, time, and frequency response of metamaterial for designing frequency‐agile spatiotemporal modulators, active beamforming, and low‐power frequency converters for the next generation terahertz wireless communications

Abstract-Reconfigurable MEMS Fano metasurfaces with multiple-input–output states for logic operations at terahertz frequencies

Manukumara Manjappa, Prakash Pitchappa, Navab Singh, Nan Wang, Nikolay I. Zheludev, Chengkuo Lee,  Ranjan Singh,

https://www.nature.com/articles/s41467-018-06360-5

A broad range of dynamic metasurfaces has been developed for manipulating the intensity, phase and wavefront of electromagnetic radiation from microwaves to optical frequencies. However, most of these metasurfaces operate in single-input–output state. Here, we experimentally demonstrate a reconfigurable MEMS Fano resonant metasurface possessing multiple-input–output (MIO) states that performs logic operations with two independently controlled electrical inputs and an optical readout at terahertz frequencies. The far-field behaviour of Fano resonance exhibits XOR and XNOR operations, while the near-field resonant confinement enables the NAND operation. The MIO configuration resembling hysteresis-type closed-loop behaviour is realized through inducing electromechanically tuneable out-of-plane anisotropy in the near-field coupling of constituent resonator structures. The XOR metamaterial gate possesses potential applications in cryptographically secured terahertz wireless communication networks. Furthermore, the MIO features could lay the foundation for the realization of programmable and randomly accessible metamaterials with enhanced electro-optical performance across terahertz, infrared and optical frequencies.

Abstract-Active Control of Resonant Cloaking in a Terahertz MEMS Metamaterial

Manukumara Manjappa,   Prakash Pitchappa,   Nan Wang,   Chengkuo Lee, Ranjan Singh,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201800141



Metamaterials exhibiting exotic optical properties have played a significant role over the years in guiding the concept of invisibility cloaking from the realm of being fiction to reality. However, due to the difficulties in fabricating the 3D cloaking devices and lack of exotic plasmonic materials at terahertz (THz) frequencies, the experimental realization of cloaking phenomenon in the THz spectrum is challenging. In this work, a new mechanism for invisibility cloaking based on the resonant scattering cancellation technique in a 2D nonconcentric composite metamaterial device, consisting of a split ring resonator (SRR) and a microelectromechanical system (MEMS) reconfigurable closed ring resonator (CRR) at THz frequencies is reported. A strong magnetic interaction between the SRR and CRR eliminates the scattering effects from the SRR at its fundamental eigen mode frequency, thereby making it invisible to the incident THz wave. Further, by voltage actuation of MEMS‐reconfigurable CRR, an active switching between the visible and cloaked states of SRR structure is demonstrated. The proposed technique provides a simple design and technique for realizing invisibility cloaks by utilizing the resonant near‐field interactions in the subwavelength structures across microwave to optical frequencies, thereby circumventing the need for materials with complex geometry and exotic properties.

Abstract-Active Phase Transition via Loss Engineering in a Terahertz MEMS Metamaterial

Longqing Cong, Prakash Pitchappa, Chengkuo Lee, Ranjan Singh

http://onlinelibrary.wiley.com/doi/10.1002/adma.201700733/abstract

Controlling the phase of local radiation by using exotic metasurfaces has enabled promising applications in a diversified set of electromagnetic wave manipulation such as anomalous wavefront deflection, flat lenses, and holograms. Here, we theoretically and experimentally demonstrate an active phase transition in a micro-electromechanical system-based metadevice where both the phase response and the dispersion of the metamaterial cavity are dynamically tailored. The phase transition is determined by the radiative and the absorptive losses in a metal–insulator–metal cavity that obeys the coupled-mode theory. The complete understanding of the phase diagram in a reconfigurable configuration would open up avenues for designing multifunctional metadevices that can be actively switched between different phases leading to a plethora of applications in polarization control, beam deflectors, and holographic metamaterials.

Abstract-Metamaterials: Active Control of Electromagnetically Induced Transparency Analog in Terahertz MEMS Metamaterial

1. Prakash Pitchappa^1,2,4, 
2. Manukumara Manjappa³, 
3. Chong Pei Ho^1,3,4, 
4. Ranjan Singh³, 
5. Navab Singh⁴ and
6. Chengkuo Lee^1,2,*

http://onlinelibrary.wiley.com/doi/10.1002/adom.201670021/abstract
Article first published online: 20 APR 2016

DOI: 10.1002/adom.201670021

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Issue 

Coupled mode resonators provide an access route for the realization of electromagnetically induced transparency (EIT) analogues in the terahertz spectral region. On page 541, C. Lee and co-workers experimentally realize a microelectromechanical system (MEMS)-based metamaterial to provide both active modulation of EIT intensity and spectral tuning of the EIT peak. This is achieved through selective reconfiguration of bright and dark mode resonators, respectively. An order of magnitude change in delay-bandwidth product is achieved.

Abstract-Microelectromechanically tunable multiband metamaterial with preserved isotropy

Prakash Pitchappa,

Chong Pei Ho,

You Qian,

Lokesh Dhakar,

Navab Singh

& Chengkuo Lee

http://www.nature.com/srep/2015/150626/srep11678/full/srep11678.html
We experimentally demonstrate a micromachined reconfigurable metamaterial with polarization independent characteristics for multiple resonances in terahertz spectral region. The metamaterial unit cell consists of eight out-of-plane deformable microcantilevers placed at each corner of an octagon ring. The octagon shaped unit cell geometry provides the desired rotational symmetry, while the out-of-plane movable cantilevers preserves the symmetry at different configurations of the metamaterial. The metamaterial is shown to provide polarization independent response for both electrical inductive-capacitive (eLC) resonance and dipolar resonance at all states of actuation. The proposed metamaterial has a switching range of 0.16 THz and 0.37 THz and a transmission intensity change of more than 0.2 and 0.7 for the eLC and dipolar resonances, respectively for both TE and TM modes. Further optimization of the metal layer thickness, provides an improvement of up to 80% modulation at 0.57 THz. The simultaneously tunable dual band isotropic metamaterial will enable the realization of high performance electro-optic devices that would facilitate numerous terahertz applications such as compressive terahertz imaging, miniaturized terahertz spectroscopy and next generation high speed wireless communication possible in the near future.

Abstract-Electrothermally actuated microelectromechanical systems based omega-ring terahertz metamaterial with polarization dependent characteristics

Chong Pei Ho¹, Prakash Pitchappa¹, Yu-Sheng Lin¹, Chia-Yi Huang², Piotr Kropelnicki³ and Chengkuo Lee^1,a)

 Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore
2 Department of Applied Physics, Tunghai University, No. 1727, Sec. 4, Taiwan Boulevard, Taichung 40704, Taiwan
3 Excelitas Technologies, 8 Tractor Road, Singapore 627969, Singapore
a) Author to whom correspondence should be addressed. Electronic mail: elelc@nus.edu.sg.
Appl. Phys. Lett. 104, 161104 (2014); http://dx.doi.org/10.1063/1.4871999

^{http://scitation.aip.org/content/aip/journal/apl/104/16/10.1063/1.4871999}

We present the design, simulation, fabrication, and characterization of a continuously tunable Omega-ring terahertz metamaterial. The tunability of metamaterial is obtained by integratingmicroactuators into the metamaterial unit cell. Electrothermal actuation mechanism is used to provide higher tuning range, larger stroke, and enhanced repeatability. The maximum achieved tuning range for the resonant frequency is around 0.30 THz for the input power of 500 mW. This shows the potential of using electrothermally actuated microactuators based tunablemetamaterial design for application such as filters, absorbers, sensors, and spectral imagers.