Abstract-Broadband terahertz polarization conversion using metasurfaces (Conference Presentation)

Hou-Tong Chen,  Chun-Chieh Chang,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10756/107560C/Broadband-terahertz-polarization-conversion-using-metasurfaces-Conference-Presentation/10.1117/12.2322496.short

Two-dimensional metamaterials - metasurfaces - offer tremendous opportunities in realizing exotic optical phenomena and functionalities to address the technological challenges encountered in the terahertz frequency regime. By tailoring the resonant response of basic building blocks as well as their mutual interactions, we are able to effectively control of amplitude, phase, and polarization state of terahertz waves. Here we report the realization of highly efficient polarization conversions including: (1) Reflective linear polarization rotation using an array of anisotropic resonators backed with a ground plane; (2) Transmissive linear polarization rotation using an array of anisotropic resonator array sandwiched by two orthogonal gratings; and (3) Reflective linear-to-circular polarization conversion using two cascading arrays of complementary resonators. They operate over a broad bandwidth more than one octave and approaching two octaves in some cases. We further show that the linear polarization rotation is accompanied by a tunable phase discontinuity, which allows us to demonstrate an ultrathin terahertz flat lens enabling diffraction-limited focusing. The broadband linear-to-circular polarization may also find applications including terahertz circular dichroism spectroscopy and excitation of valley polarization in 2D materials.

Abstract-Broadband terahertz polarization conversion using metasurfaces (Conference Presentation)

Hou-Tong Chen,  Chun-Chieh Chang,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10756/107560C/Broadband-terahertz-polarization-conversion-using-metasurfaces-Conference-Presentation/10.1117/12.2322496.short

Two-dimensional metamaterials - metasurfaces - offer tremendous opportunities in realizing exotic optical phenomena and functionalities to address the technological challenges encountered in the terahertz frequency regime. By tailoring the resonant response of basic building blocks as well as their mutual interactions, we are able to effectively control of amplitude, phase, and polarization state of terahertz waves. Here we report the realization of highly efficient polarization conversions including: (1) Reflective linear polarization rotation using an array of anisotropic resonators backed with a ground plane; (2) Transmissive linear polarization rotation using an array of anisotropic resonator array sandwiched by two orthogonal gratings; and (3) Reflective linear-to-circular polarization conversion using two cascading arrays of complementary resonators. They operate over a broad bandwidth more than one octave and approaching two octaves in some cases. We further show that the linear polarization rotation is accompanied by a tunable phase discontinuity, which allows us to demonstrate an ultrathin terahertz flat lens enabling diffraction-limited focusing. The broadband linear-to-circular polarization may also find applications including terahertz circular dichroism spectroscopy and excitation of valley polarization in 2D materials.

Abstract-Resonance coupling and polarization conversion in terahertz metasurfaces with twisted split-ring resonator pairs

Chenyu Li, Chun-Chieh Chang, Qingli Zhou, Cunlin Zhang, and Hou-Tong Chen

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-21-25842

We investigate edge-coupling of twisted split-ring resonator (SRR) pairs in the terahertz (THz) frequency range. Using a simple coupled-resonator model we show that such a system exhibits resonance splitting and cross-polarization conversion. Numerical simulations and experimental measurements agree well with theoretical calculations, verifying the resonance splitting as a function of the coupling strength given by the SRR separation. We further show that a metal ground plane can be integrated to significantly enhance the resonance coupling, which enables the effective control of resonance splitting and the efficiency and bandwidth of the cross-polarization conversion. Our findings improve the fundamental understanding of metamaterials with a view of accomplishing metamaterial functionalities with enhanced performance, which is of great interest in realizing THz functional devices required in a variety of applications.

© 2017 Optical Society of America

Abstract-Demonstration of a highly efficient terahertz flat lens employing tri-layer metasurfaces

Chun-Chieh Chang, Daniel Headland, Derek Abbott, Withawat Withayachumnankul, and Hou-Tong Chen

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-42-9-1867&origin=search

We demonstrate a terahertz flat lens based on tri-layer metasurfaces allowing for broadband linear polarization conversion, where the phase can be tuned through a full 2𝜋$2\pi$ range by tailoring the geometry of the subwavelength resonators. The lens functionality is realized by arranging these resonators to create a parabolic spatial phase profile. The fabricated 124-μm-thick device is characterized by scanning the beam profile and cross section, showing diffraction-limited focusing and ∼68%$\sim 68\%$ overall efficiency at the operating frequency of 400 GHz. This device has potential for applications in terahertz imaging and communications, as well as beam control in general.

© 2017 Optical Society of America

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.