Abstract-Bilayer Metasurfaces for Dual- and Broadband Optical Antireflection

Li Huang, Chun-Chieh Chang , Beibei Zeng, John Nogan, Sheng-Nian Luo,  Antoinette J. Taylor,  Abul K. Azad, Hou-Tong Chen,

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.7b00471?journalCode=apchd5

Optical antireflection has long been pursued for a wide range of applications, but existing approaches encounter issues in the performance, bandwidth, and structure complexity, particularly in the long-wavelength infrared regime. Here we present the demonstration of bilayer metasurfaces that accomplish dual- and broadband optical antireflection in the terahertz and mid-infrared spectral ranges. By simply tailoring the structural geometry and dimensions, we show that subwavelength metal/dielectric structures enable dramatic reduction of Fresnel reflection and significant enhancement of transmission at a substrate surface, operating either at two discrete narrow bands or over a broad bandwidth up to 28%. We also use a semianalytical interference model to interpret the obtained results, in which we find that the dispersion of the constituent structures plays a critical role in achieving the observed broadband optical antireflection.

Abstract-Displacement Current Mediated Resonances in Terahertz Metamaterials

1. Chao Liu¹, 
2. Kriti Agarwal¹, 
3. Yuping Zhang²,
4. Dibakar Roy Chowdhury³, 
5. Abul K. Azad²and
6. Jeong-Hyun Cho^1,*

Version of Record online: 18 MAY 2016

DOI: 10.1002/adom.201600196

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

http://onlinelibrary.wiley.com/doi/10.1002/adom.201600196/abstract

Terahertz metamaterials (THz MMs) have been proven to be good candidates for chemical, biological, temperature, strain, and position sensing. However, currently developed thin-metal-film-based split ring resonator (SRR) MMs have relatively low quality factor (Q-factors), leading to a poor sensitivity, which is one of the obstacles for development of sensors. In order to enhance the Q-factor, novel THz MMs, nanopillar-based MMs, are designed, fabricated, and characterized. The nanopillar-based MMs excite the inductive-capacitive resonance via desplacement currents, showing a significantly enhanced Q-factor around 450, which is about 30 times higher than typical thin-metal-film-based MMs. Nanopillar-based MMs also show 17 times larger frequency shift compared to the metal-film-based MMs when the permittivity of the ambient dielectric properties of the MMs changes. Due to high Q-factor and large frequency shift, the nanopillar-based THz MMs utilizing displacement current have great potential for highly sensitive chemical and biomaterial detection as well as frequency-agile THz devices.

Abstract-Terahertz metamaterials for linear polarization conversion and anomalous refraction

Nathaniel K. Grady, Jane E. Heyes, Dibakar Roy Chowdhury, Yong Zeng, Matthew T. Reiten, Abul K. Azad, Antoinette J. Taylor, Diego A. R. Dalvit, Hou-Tong Chen
Polarization is one of the basic properties of electromagnetic waves conveying valuable information in signal transmission and sensitive measurements. Conventional methods for advanced polarization control impose demanding requirements on material properties and attain only limited performance. Here, we demonstrate ultrathin, broadband, and highly efficient metamaterial-based terahertz polarization converters that are capable of rotating a linear polarization state into its orthogonal one. Based on these results we create metamaterial structures capable of realizing near-perfect anomalous refraction. Our work opens new opportunities for creating high performance photonic devices and enables emergent metamaterial functionalities for applications in the technologically difficult terahertz frequency regime.

Abstract-Terahertz Metamaterials for Linear Polarization Conversion and Anomalous Refraction

1. 
   

1. Nathaniel K. Grady¹, 
2. Jane E. Heyes¹, 
3. Dibakar Roy Chowdhury¹, 
4. Yong Zeng², 
5. Matthew T. Reiten¹, 
6. Abul K. Azad¹,
7. Antoinette J. Taylor¹, 
8. Diego A. R. Dalvit², 
9. Hou-Tong Chen¹,*

+Author Affiliations

http://www.sciencemag.org/content/early/2013/05/15/science.1235399.abstract
Polarization is one of the basic properties of electromagnetic waves conveying valuable information in signal transmission and sensitive measurements. Conventional methods for advanced polarization control impose demanding requirements on material properties and attain only limited performance. Here, we demonstrate ultrathin, broadband, and highly efficient metamaterial-based terahertz polarization converters that are capable of rotating a linear polarization state into its orthogonal one. Based on these results, we create metamaterial structures capable of realizing near-perfect anomalous refraction. Our work opens new opportunities for creating high-performance photonic devices and enables emergent metamaterial functionalities for applications in the technologically difficult terahertz frequency regime.

Abstract-Active control of electromagnetically induced transparency analogue in terahertz metamaterials

http://www.nature.com/ncomms/journal/v3/n10/full/ncomms2153.html

Jianqiang Gu, Ranjan Singh, Xiaojun Liu, Xueqian Zhang, Yingfang Ma,, Shuang Zhang,  Stefan A. Maier, Zhen Tian, Abul K. Azad,, Hou-Tong Chen,Antoinette J. Taylor,Jiaguang Han, Weili Zhang

Recently reported metamaterial analogues of electromagnetically induced transparency enable a unique route to endow classical optical structures with aspects of quantum optical systems. This method opens up many fascinating prospects on novel optical components, such as slow light units, highly sensitive sensors and nonlinear devices. In particular, optical control of electromagnetically induced transparency in metamaterials promises essential application opportunities in optical networks and terahertz communications. Here we present active optical control of metamaterial-induced transparency through active tuning of the dark mode. By integrating photoconductive silicon into the metamaterial unit cell, a giant switching of the transparency window occurs under excitation of ultrafast optical pulses, allowing for an optically tunable group delay of the terahertz light. This work opens up the possibility for designing novel chip-scale ultrafast devices that would find utility in optical buffering and terahertz active filtering

Abstract: Dynamically reconfigurable terahertz metamaterial through photo-doped semiconductor

http://apl.aip.org/resource/1/applab/v99/i23/p231101_s1?isAuthorized=no

Dibakar Roy Chowdhury¹, Ranjan Singh¹, John F. O’Hara^1,2, Hou-Tong Chen¹, Antoinette J. Taylor¹, and Abul K. Azad¹

¹Center for Integrated Nanotechnologies, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
²School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA

We demonstrate reconfigurable terahertz metamaterial (MM) in which constituent resonators can be switched from split-ring resonators (SRRs) to closed-ring resonators via optical excitation of silicon islands strategically placed in the split gap. Both the fundamental and the third-order resonance modes experience monotonic damping due to increasing conductive losses in the photo-doped silicon region. More importantly, increasing the optical fluence (>200 μJ/cm²) results in the excitation of the second-order resonance mode, which is otherwise forbidden in a split-ring resonator for the incidence polarization in our experiments. Such dynamical control of metamaterial resonances could be implemented in active terahertz devices to achieve additional functionalities.

© 2011 American Institute of Physics