Abstract-Giant enhancement of THz-frequency optical nonlinearity by phonon polariton in ionic crystals

 

Yao Lu, Qi Zhang, Qiang Wu, Zhigang Chen, Xueming Liu,  Jingjun Xu

Experimental setup and illustration of velocity matching between pump pulse and generated THz wave.

https://www.nature.com/articles/s41467-021-23526-w

The field of nonlinear optics has grown substantially in past decades, leading to tremendous progress in fundamental research and revolutionized applications. Traditionally, the optical nonlinearity for a light wave at frequencies beyond near-infrared is observed with very high peak intensity, as in most materials only the electronic nonlinearity dominates while ionic contribution is negligible. However, it was shown that the ionic contribution to nonlinearity can be much larger than the electronic one in microwave experiments. In the terahertz (THz) regime, phonon polariton may assist to substantially trigger the ionic nonlinearity of the crystals, so as to enhance even more the nonlinear optical susceptibility. Here, we experimentally demonstrate a giant second-order optical nonlinearity at THz frequency, orders of magnitude higher than that in the visible and microwave regimes. Different from previous work, the phonon-light coupling is achieved under a phase-matching setting, and the dynamic process of nonlinear THz generation is directly observed in a thin-film waveguide using a time-resolved imaging technique. Furthermore, a nonlinear modification to the Huang equations is proposed to explain the observed nonlinearity enhancement. This work brings about an effective approach to achieve high nonlinearity in ionic crystals, promising for applications in THz nonlinear technologies.

Abstract-Enhanced on-chip terahertz sensing with hybrid metasurface/lithium niobate structures

Applied Physics Letters

Ride Wang, Qiang Wu, Yaqing Zhang, Xitan Xu, Qi Zhang, Wenjuan Zhao, Bin Zhang, Wei Cai, Jianghong Yao, Jingjun Xu

(a) Schematic of THz detection of an analyte using a microrod array metasurface as an on-chip sensor. A column of y-polarized dipoles located inside the LN waveguide is used to excite THz waves (blue oscillation signal). The thickness of the SiO2 layer is h = 2 μm. The inset shows the detailed design parameters: p, a, l and g are 20, 10, 55, and 15 μm, respectively. (b) Enhanced field confined to the surface of the composite structure. (c) and (d) Distribution of the field components Ey and Ez at f = 0.529 THz.

https://aip.scitation.org/doi/abs/10.1063/1.5087609

Recognizing special molecules is crucial in many biochemical processes, and thus, highly enhanced sensing methods are in high demand. In this work, we designed a microrod array metasurface with a SiO2-loaded subwavelength lithium niobate waveguide as a unique platform for enhanced experimental fingerprint detection of lactose. The metasurface could lead to strong surface wave modes due to the near-field coupling of the spoof localized surface plasmon, which also could provide a stronger interaction length between light and matter. The selectivity was remarkable in the transmission spectrum at an intrinsic characteristic frequency of 0.529 THz with a thin layer of lactose, while it was faint while transmitting terahertz (THz) waves normally through a lactose layer of the same thickness. Together with the ability to freely design the shape of the metasurface and the electromagnetic properties, we believe that this platform can function as an elegant on-chip-scale enhanced THz sensing platform.

Abstract-Real-space and real-time imaging of THz wave confinement and standing wave in a Fabry-Perot resonator

Chongpei Pan, Yane Wang, Yao Lu, Qi Zhang, Wenjuan Zhao, Qiang Wu,  Jingjun Xu,

https://www.osapublishing.org/abstract.cfm?uri=cleo_qels-2017-JW2A.103&origin=search

We fabricate a Fabry-Perot resonator in a LiNbO3 subwavelength slab and investigate the spatiotemporal evolution of terahertz pulses in the structure via time-resolved imaging system. The wave confinement and standing wave modes are clearly observed.

© 2017 OSA

Abstract-Surface enhancement of THz wave by coupling a subwavelength LiNbO3 slab waveguide with a composite antenna structure

Qi Zhang, Jiwei Qi, Qiang Wu, Yao Lu, Wenjuan Zhao, Ride Wang, Chongpei Pan, Shibiao Wang,  Jingjun Xu

https://www.nature.com/articles/s41598-017-17712-4?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+srep%2Frss%2Fcurrent+%28Scientific+Reports%29&utm_content=Google+Feedfetcher

Highly intense terahertz electromagnetic field and efficiently surface localized terahertz field in subwavelength volumes are of vital importance for terahertz photonics integration, also will greatly accelerate the development for integrated applications in biochemical sensing, imaging, terahertz spectroscopy, enhancement of nonlinear effects and even quantum research. In this paper, we achieved large terahertz field enhancement and surface field localization through depositing a pair of Au composite antennas on a LiNbO3 subwavelength slab waveguide, which can serve as an excellent on-chip platform for terahertz research and application. The antennas consist of two opposing tip-to-tip triangles separated by a gap, and each triangle combines with a strip antenna. Time-resolved imaging and finite-difference time-domain method were used to resolve the characteristics of the designed antennas experimentally and simulatively. Through these methods, we demonstrated outstanding abilities of the platform: leading to a large electric field enhancement, concentrating almost full terahertz energy on the waveguide’s surface when they are resonant with the terahertz waves and tunable resonant frequency. These abilities make the subwavelength waveguide coupling with the composite antennas be able to sever as a good integrated device to identify terahertz-sensitive small objects, or an excellent platform to terahertz spectroscopy and quantum research.

Abstract-Direct visualization of light confinement and standing wave in THz Fabry-Perot resonator with Bragg mirrors

Chongpei Pan, Qiang Wu, Qi Zhang, Wenjuan Zhao, Jiwei Qi, Jianghong Yao, Chunling Zhang, W. T. Hill, and Jingjun Xu

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-9-9768

We report for the first time the ability to perform time resolved imaging of terahertz (THz) waves propagating within a Fabry-Perot resonator on a LiNbO3 slab. Electro-optic effect is used to record the full spatiotemporal evolution of THz fields inside the resonator. In addition to revealing the real-space behavior, the data further demonstrate the confinement and the standing wave modes of THz in the cavity in frequency domain. The experimental results are in good agreement with numerical simulations. Using the coherent imaging technique to gain real-time information about a resonator system provides a unique path to study the physics of optical cavity.

© 2017 Optical Society of America