Bio: Qiang Yan is an academic researcher from China Academy of Engineering Physics. The author has contributed to research in topics: Terahertz radiation & Parametric statistics. The author has an hindex of 3, co-authored 11 publications receiving 34 citations.
TL;DR: High-resolution and high-quality terahertz (THz) in-line digital holography based on the synthetic aperture method based on a new autofocusing criterion termed “reconstruction objective function” is introduced to obtain the best in-focus reconstruction distance.
Abstract: We demonstrate high-resolution and high-quality terahertz (THz) in-line digital holography based on the synthetic aperture method. The setup is built on a self-developed THz quantum cascade laser, and a lateral resolution better than 70 μm (∼λ) is achieved at 4.3 THz. To correct intensity differences between sub-holograms before aperture stitching, a practical algorithm with global optimization is proposed. To address the twin-image problem for in-line holography, a sparsity-based phase retrieval algorithm is applied to perform the high-quality reconstruction. Furthermore, a new autofocusing criterion termed “reconstruction objective function” is introduced to obtain the best in-focus reconstruction distance, so the autofocusing procedure and the reconstruction are unified within the same framework. Both simulation and experiment prove its accuracy and robustness. Note that all the methods proposed here can be applied to other wavebands as well. We demonstrate the success of this THz synthetic aperture in-line holography on biological and semiconductor samples, showing its potential applications in bioimaging and materials analysis.
TL;DR: An iterative optimization framework, where the conventional object constraint and the L1 sparsity constraint can be combined to efficiently reconstruct the complex amplitude distribution of the sample to achieve 40 μm spatial resolution corresponding to ~0.7λ with a numerical aperture.
Abstract: We demonstrate terahertz (THz) lens-free in-line holography on a chip in order to achieve 40 μm spatial resolution corresponding to ~0.7λ with a numerical aperture of ~0.87. We believe that this is the first time that sub-wavelength resolution in THz holography and the 40 μm resolution were both far better than what was already reported. The setup is based on a self-developed high-power continuous wave THz laser at 5.24 THz (λ = 57.25 μm) and a high-resolution microbolometer detector array (640 × 512 pixels) with a pitch of 17 μm. This on-chip in-line holography, however, suffers from the twin-image artifacts which obfuscate the reconstruction. To address this problem, we propose an iterative optimization framework, where the conventional object constraint and the L1 sparsity constraint can be combined to efficiently reconstruct the complex amplitude distribution of the sample. Note that the proposed framework and the sparsity-based algorithm can be applied to holography in other wavebands without limitation of wavelength. We demonstrate the success of this sparsity-based on-chip holography by imaging biological samples (i.e., a dragonfly wing and a bauhinia leaf).
TL;DR: A sparsity-based extrapolation model based on L1-norm is proposed, where the object mask is not required and the generalized positive absorption constraint can be utilized, which can achieve super-resolution reconstruction completely by numerical postprocessing without any modification to the imaging system.
Abstract: We introduce a generalized extrapolation framework as a constrained optimization problem to enhance the resolution in terahertz digital in-line holography. The alternating minimization method is employed to iteratively extrapolate the hologram beyond the actually detecting area and to reconstruct the complex amplitude distribution of the object wavefront. Within this framework, we propose a sparsity-based extrapolation model based on L1-norm, where the object mask is not required and the generalized positive absorption constraint can be utilized. This work can achieve super-resolution reconstruction completely by numerical postprocessing without any modification to the imaging system. Both the simulation and experiments on the terahertz band demonstrate the feasibility of the proposed algorithms, and the limit of resolution can be extended by a factor of 1.67.
TL;DR: In this article, the seed enhancement of an injection-seeded terahertz (THz)-wave parametric generator (is-TPG) with the help of a theoretical model was studied.
Abstract: We have studied the seed enhancement of an injection-seeded terahertz (THz)-wave parametric generator (is-TPG) with the help of a theoretical model. During simulation, the power of the seed beam varied from 1 W to 1 MW as the initial condition, while the pump power is set to 18.5 MW. When the peak power of the seed beam reaches megawatt level, the output power of THz wave increases rapidly and the peak position for the THz output dramatically moves towards the front of the crystal. The calculated output power of the injection-pulse-seeded TPG (ips-TPG) with 0.3 MW pulsed seed is 3.4 kW in the case of a surface-emitting configuration and 14 kW in the case of a Si-prism coupled output configuration. The enhancement ratios are 3.5 and 1.4 times, respectively, compared with the watt level CW-seeded is-TPG. Guided by the simulation, we experimentally demonstrated an 11 kW peak power Si-prism output coupled ips-TPG. The enhancement ratio is 1.6 times. The energy of the pulsed seed is 30 µJ (0.3 MW). The pump energy is 7.2 mJ, which is about half that of the typical is-TPG with the same output power. Compared with a typical is-TPG, most of the THz power in ips-TPG accumulates in the front of the crystal and thus is easier to collect. One obvious advantage of ips-TPG is that the output THz energy can be increased while keeping the pump power at a safe level.
TL;DR: In this paper, a numerical model for injection-seeded terahertz-wave parametric generators in which the pump depletion, the duration of the pump pulse, as well as the spatial modes of the generator and seed beams are taken into consideration is presented.
Abstract: We have constructed a numerical model for injection-seeded terahertz-wave parametric generators in which the pump depletion, the duration of the pump pulse, as well as the spatial modes of the pump and seed beams are taken into consideration. Compared with other models which only provide the parametric gain under the assumption of low pump depletion, our model can make the predictions on the energy as well as spatial and temporal profiles of the generated terahertz-wave. Besides, the appropriate value of nonlinear coefficient of MgO:LiNbO3 in terahertz regime is difficult to find in the published works. To solve this problem, Miller’s rule is applied for the wavelength scaling of the nonlinear coefficient during calculation. We present a detailed description of the model and show that its predictions agree well with the reported experimental results. The presented model allows the performance to be estimated when designing an injection-seeded terahertz-wave parametric generator.
29 Jul 2019
TL;DR: This theoretical work develops a novel nonlinear ghost imaging approach that conceptually outperforms established single-pixel imaging protocols at inaccessible wavelengths and demonstrates how time-resolved, full-wave acquisition enables accurate spatiotempora acquisition.
Abstract: We propose a new type of THz Ghost-Imaging technique combining nonlinear pattern generation and time-resolved single-pixel measurements. Our approach allows to reconstruct the morphology and spectrum of the sample with deep subwavelength resolution.
TL;DR: A class of THz imaging implementations, named coherent lensless imaging, that reconstruct the coherent response of arbitrary samples with a minimized experimental setup based only on a coherent source and a camera are reviewed.
Abstract: Imaging with THz radiation has proved an important tool for both fundamental science and industrial use. Here we review a class of THz imaging implementations, named coherent lensless imaging, that reconstruct the coherent response of arbitrary samples with a minimized experimental setup based only on a coherent source and a camera. After discussing the appropriate sources and detectors to perform them, we detail the fundamental principles and implementations of THz digital holography and phase retrieval. These techniques owe a lot to imaging with different wavelengths, yet innovative concepts are also being developed in the THz range and are ready to be applied in other spectral ranges. This makes our review useful for both the THz and imaging communities, and we hope it will foster their interaction.
TL;DR: In this paper, the authors present the research status and prospects of several common continuous-wave (CW) terahertz medical imaging systems and applications of THz medical imaging in biological tissues.
Abstract: In the past few decades, the applications of terahertz (THz) spectroscopy and imaging technology have seen significant developments in the fields of biology, medical diagnosis, food safety, and nondestructive testing. Label-free diagnosis of malignant tumours has been obtained and also achieved significant development in THz biomedical imaging. This review mainly presents the research status and prospects of several common continuous-wave (CW) THz medical imaging systems and applications of THz medical imaging in biological tissues. Here, we first introduce the properties of THz waves and how these properties play a role in biomedical imaging. Then, we analyse both the advantages and disadvantages of the CW THz imaging methods and the progress of these methods in THz biomedical imaging in recent ten years. Finally, we summarise the obstacles in the way of the application of THz bio-imaging application technology in clinical detection, which need to be investigated and overcome in the future.
TL;DR: Based on the prior knowledge of the model of Fresnel zone plate with circular diffraction gratings, the pixel density is increased by the bicubic interpolation (BIP) method inside the hologram to enhance the low-frequency terms of the object as mentioned in this paper.
••20 Apr 2019
TL;DR: This work proposes a coherent imaging technique reconstructing two objects, hidden one behind the other, from relative shifts between them, and expects that, in selected applications, the suggested approach can replace a similar phase retrieval technique, namely ptychography.
Abstract: Imaging through scattering materials is of utmost importance, especially for security and biomedical imaging. Unlike the rest of the electromagnetic spectrum, terahertz radiation is a non-ionizing probe and allows imaging deep through non-conducting materials with sub-millimeter resolution. Here, we propose a coherent imaging technique reconstructing two objects, hidden one behind the other, from relative shifts between them. Experimental reconstructions at λ=96.5 μm of amplitude and phase objects hidden behind a glass fabric sample are presented. The hidden objects are retrieved with a lateral resolution of ≈1.5λ and a depth resolution of ≈λ/10. Besides envisioning its use in non-invasive imaging, we anticipate that, in selected applications, the suggested approach can replace a similar phase retrieval technique, namely ptychography.