Abstract-Optical tuning of dielectric properties of La0.7Sr0.3MnO3/SrTiO3 superlattices in the terahertz range

Honglei Cai, Haoliang Huang, Qiuping Huang, Xiang Hu, Jie Zhang, Xiaofang Zhai,  Yalin Lu

Fig. 2 (a) Schematic diagram of the home-made THz-TDS system. The green arrow indicates the excitation under 532 nm continuous waves. (b) Schematic diagram of the home-made OPTP system. Here, λ/4 and W.P. refer to a quarter-wave plate and a Wollaston prism, respectively.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-6-7842&origin=search

Two (La0.7Sr0.3MnO3)n/(SrTiO3)m superlattices with different superlattice period but the same total thickness were deposited on LaAlO3 substrates by pulsed laser deposition. Dielectric properties of these samples were investigated by means of terahertz time-domain spectroscopy (THz-TDS) under external continuous wave green laser excitation and optical-pump terahertz-probe spectroscopy (OPTP) at room temperature. Experimental results show that the real part of the permittivity for both superlattices increases significantly with increasing green laser pump power, which indicates the decrease of the plasma frequency, along with the increase of the electron scattering rate, soft mode eigenfrequency and oscillator strength in the Drude-Lorentz model. Furthermore, it’s observed that the insulating superlattice exhibits a more significant dielectric tunability than the metallic superlattice. Besides, the carrier lifetime of superlattices is much shorter than the La0.7Sr0.3MnO3 thin film in the OPTP measurements, indicating that the electrons excited in the La0.7Sr0.3MnO3 layers may be trapped by the defects located in the interfaces of La0.7Sr0.3MnO3 and SrTiO3 or the SrTiO3 layers. With the optical field-induced tunability of dielectric properties, (La0.7Sr0.3MnO3)n/(SrTiO3)m superlattices show great potential in the actively tunable devices in the THz range.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Multifunctional Hybrid Metasurfaces for Dynamic Tuning of Terahertz Waves

Honglei Cai,  Shi Chen,   Chongwen Zou,   Qiuping Huang,   Yu Liu,  Xiang Hu,   Zhengping Fu,  Yi Zhao,  Hongchuan He,   Yalin Lu,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201800257


Actively tuning optical transmission through hybrid metasurfaces incorporated with multifunctional active media holds great promise for the next generation optical devices. In the terahertz (THz) range, they remain rare due to the lack of dynamic and multifunctional designs and materials. Here, a vanadium dioxide (VO2)‐based hybrid metasurface is proposed to present multifunctional control of THz waves via electrically triggering and ultrafast optical excitation. By minimizing the thermal mass of VO2 and optimizing the VO2 patterns within two side gaps of the asymmetric split‐ring resonators, a hybrid metasurface which can tune the THz wave with an absolute modulation depth up to 54% and a figure of merit as high as 138% is hereby presented. The hybrid metasurface achieves a switching time of 2.2 s under the electrically triggering and offers an ultrafast modulation within 30 ps under the femtosecond pulse excitation. More interestingly, owing to the intrinsic hysteresis behavior of VO2, the hybrid metasurface exhibits distinguishing multistate transmission amplitudes with a single electrical input. In short, this study paves the way for robust multifunctionality in electric‐controlled terahertz switching, photonic memory, and ultrafast terahertz optics.

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