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-Image fusion based on multiscale transform and sparse representation to enhance terahertz images

Qi Mao, Yunlong Zhu, Cixing Lv, Yao Lu, Xiaohui Yan, Dongshan Wei, Shihan Yan, and Jingbo Liu

Schematic diagram of the fusion method based on MST and SR.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-17-25293

High-quality terahertz (THz) images are vital to integrated circuit (IC) manufacturing. Due to the unique sensitivity of THz waves to different materials, the images obtained from the point-spread function (PSF) model have fewer image details and less texture information in some frequency bands. This paper presents an image fusion technique to enhance the resolution of THz IC images. The source images obtained from the PSF model are processed by a fusion method combining a multiscale transform (MST) and sparse representation (SR). The low-pass band is handled by sparse representation, and the high-pass band is fused by the conventional “max-absolute” rule. From both objective and visual perspectives, four popular multiscale transforms—the Laplacian pyramid, the ratio of low-pass pyramids, the dual-tree complex wavelet transform and the curvelet transform—are thoroughly compared at different decomposition levels ranging from one to four. This work demonstrates the feasibility of using image fusion to enhance the resolution of THz IC images.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Convolutional neural network model based on terahertz imaging for integrated circuit defect detections

Qi Mao, Yunlong Zhu, Cixing Lv, Yao Lu, Xiaohui Yan, Shihan Yan, and Jingbo Liu

(a)Schematic of the T-Gauge 5000 system, (b) physical diagram of the THz-TDS system operating in the transmission mode.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-4-5000

Detection of integrated circuit (IC) defects is vital in IC manufacturing. Traditional defect detection methods have relied on scanning electron microscopy and X-ray imaging techniques that are time consuming and destructive. Hence, in this paper we considered terahertz imaging as a label-free and nondestructive alternative. This study aimed to use a convolutional neural network model (CNN) to improve the performance of the terahertz imaging IC detection system. First, we constructed a terahertz imaging IC dataset and analyzed it. Subsequently, a new CNN structure was proposed based on the VGG16 network. Finally, it was optimized based on its structure and dropout rate. The method proposed above can improve IC defects detection accuracy of THz imaging. Most significantly, this work will promote the application of terahertz imaging in practice and provide a foundation to further research in relevant fields.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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.