Abstract-A Model of Terahertz Parametric Process Including Spontaneous Parametric Down-conversion

 

Yu Qin, Zeyu Li, Qiang Yan, Xun Zhou, Mingrui Zou,  Weipeng Kong, 

https://link.springer.com/article/10.1007/s10762-021-00778-7

A new numerical model of terahertz parametric process is demonstrated, which takes the spontaneous parametric down-conversion into consideration by adding tens of frequency components in the coupled-wave equations. To set the values of some parameters in this model, the residual pump energy and idler spectra during spontaneous parametric down-conversion are measured experimentally for different pump energy and crystal lengths. Compared with previous models which only involve three waves, the new model is more accurate, especially when the seed power is low or the generated terahertz-wave is in the low frequency region.

Abstract-Theoretical and experimental study on the enhancement of seed injection in terahertz-wave generation

Weipeng Kong, Zeyu Li, Qiang Yan, Mingrui Zou, Xun Zhou, and Yu Qin

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-37-8-2479

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.

© 2020 Optical Society of America

Abstract-Terahertz synthetic aperture in-line holography with intensity correction and sparsity autofocusing reconstruction

Zeyu Li, Ruijiao Zou, Weipeng Kong, Xuemin Wang, Qinghua Deng, Qiang Yan, Yu Qin, Weidong Wu, and Xun Zhou

Synthetic aperture hologram with intensity correction for a dragonfly forewing. (a) Nine normalized sub-holograms with intensity correction. (b) Synthetic aperture hologram composed of (a). (c) Synthetic aperture hologram without intensity correction. (d) Optical image of the dragonfly forewing sample. (e) Amplitude distribution reconstructed from (b) with 20 iterations. (f) Amplitude distribution reconstructed from (c) with 20 iterations. The effect of non-uniform intensity on reconstruction can be seen from the parts marked by the white and blue dotted circles.

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-7-12-1391

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$\sim \lambda$) 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.

© 2019 Chinese Laser Press

Abstract-Sparsity-based continuous wave terahertz lens-free on-chip holography with sub-wavelength resolution

Zeyu Li, Qiang Yan, Yu Qin, Weipeng Kong, Guangbin Li, Mingrui Zou, Du Wang, Zhisheng You, and Xun Zhou

Fig. 1 Schematic layout (a) and real picture (b) of the experiment setup. PM1 and PM2 are gold-coated confocal off-axis parabolic mirrors with the focal length of 50.8 mm and 101.6 mm, respectively. An output THz laser beam of ~10 mm in diameter is expanded and collimated to ~20 mm so that the detector array can be completely covered by THz wave. The wave scattered by the sample forms the object wave while the unscattered part of the illumination forms the reference wave. The resulting interference pattern is called an in-line hologram.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-2-702

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).

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement