Abstract-Temperature-dependent birefringence of lithium triborate, LBO in the THz regime

• Kechao Song, 
• Zhen Tian, 
• Weili Zhang & 
• Mingwei Wang

https://www.nature.com/articles/s41598-017-08626-2?WT.feed_name=subjects_materials-science

Optical properties of lithium triborate (LBO) in the terahertz regime (0.2–2 THz) were characterized using broadband terahertz time-domain spectroscopy. The frequency dependence of refractive index and absorption coefficient of the LBO crystal was experimentally investigated over the temperature range of 77–297 K, which the experimental results indicated that LBO has very low optical absorption coefficient at terahertz frequencies especially for the beam polarization along the crystal’s principal dielectric axis X. Moreover, a giant birefringence was observed, and the refractive index difference between the axis X and Z gradually decreased with decreasing temperature, which is attributed to the behavior of the TO phonon modes of B1 and B2 symmetries at low frequencies at different temperatures. As potential applications, LBO can be exemplarily used as terahertz wave shapers, beam splitters, terahertz wave plates, circular polarizers and other polarization devices.

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

Full Article  |  PDF Article