Terahertz (THz) imaging, benefiting from THz radiation's capabilities of non-ionizing and penetration of non-conducting materials, serves as a cutting-edge non-destructive evaluation technology. One of the major challenges in photonics is complex amplitude wavefront sensing, the amplitude image indicates the absorption properties, while the phase image reveals the refractive and thickness information, thus simultaneously determining that the amplitude and phase distributions of the wavefront are highly desirable for applications ranging from bioimaging to material characterization. Due to the long wavelength of the THz wave, the imaging resolution is also one of the key considerations for THz applications.
To achieve THz full-field, high-resolution, and complex amplitude imaging, a THz research group from the Research Center of Laser Fusion at China Academy of Engineering Physics (CAEP) carried out THz synthetic aperture in-line holography. The research results are published in Photonics Research, Vol. 7, Issue 12, 2019 (Zeyu Li, et al., Terahertz synthetic aperture in-line holography with intensity correction and sparsity autofocusing reconstruction).
In this work, a high-resolution and high-quality THz lensless in-line holographic setup was established based on a self-developed THz quantum cascade laser (THz-QCL). To enhance the spatial resolution limited by the numerical aperture (NA) of the array detector, the synthetic aperture method was adopted, a practical global optimization algorithm was proposed to correct the intensity differences among sub-holograms, and a lateral resolution better than 70 μm (∼λ) at 4.3 THz was achieved. To overcome the twin-image problem for in-line holography, a sparsity-based iterative phase retrieval algorithm was used to give high-quality reconstructions. Moreover, to obtain the best in-focus reconstruction distance, a new autofocusing criterion based on the “reconstruction objective function” was introduced into in-line holography for the first time, so the autofocusing procedure and the reconstruction were unified within the same framework. They demonstrated the success of the THz synthetic aperture in-line holography on biological and semiconductor samples, showing its potential applications in bioimaging and materials analysis. Note that the proposed approaches can be applied to other wavebands as well, such as visible light and X-ray band.
Prof. Xun Zhou from the research group believes that this work is of great significance to promote the development and application of THz imaging technology; Prof. Weidong Wu believes that the proposed sparsity-based autofocusing phase retrieval algorithm effectively alleviates the twin-image problem and automatically obtains the optimal in-focus distance, providing new ideas for reconstruction in Gabor in-line digital holography.
Future work will focus on THz laser optimization, THz 3D holography, and THz imaging application.
为实现上述目标，中国工程物理研究院激光聚变研究中心太赫兹研究团队开展了太赫兹合成孔径同轴数字全息成像研究。相关结果发表在Photonics Research 2019年第7卷第12期上 (Zeyu Li, et al., Terahertz synthetic aperture in-line holography with intensity correction and sparsity autofocusing reconstruction)。
该项研究基于其自主研发的太赫兹量子级联激光器，构建了太赫兹无透镜同轴全息成像装置。为突破面阵探测器孔径限制，提高成像分辨率，采用合成孔径方法，提出一种全局最优化算法对子孔径强度进行校正，实现子孔径全息图无缝融合，在4.3 THz（λ = 69.7 μm）实现70 μm（~λ）波长级横向空间分辨率。为抑制同轴全息零级像与共轭像干扰，利用基于稀疏约束的相位复原算法，实现了高质量全息重建。为获得最佳重建距离，首次在同轴数字全息中引入基于重建目标函数的自聚焦判据，将同轴全息自动聚焦与数字再现统一在同一优化框架下。基于此装置，该团队开展了生物和半导体样品的太赫兹成像实验，证明了其在生物医学成像和材料分析领域的应用潜力。值得一提的是，该项研究提出的方法和算法并不局限于太赫兹波段，可直接应用于X射线和可见光波段。