Abstract-Flexible and giant terahertz modulation based on ultra strain sensitive conductive polymer composites

Qiwu Shi, Ke Tian, Hongfu Zhu, Zi-run Li, Li-Guo,  Zhu Hua,  Deng Wanxia Huang, Qiang Fu,

https://pubs.acs.org/doi/10.1021/acsami.9b21890

Dynamic tuning of terahertz (THz) wave has a great potential application for smart THz devices, such as switching, modulation, sensor, and so on. However, realization of flexible THz modulation with high efficiency is rarely observed, which is nearly absent from the booming development and demands on flexible electronics. Here, we report a flexible THz modulation based on conductive polymer composites composed of thermoplastic polyurethane (TPU) and conductive particles (Ni). By designing the additive content of Ni particles, such flexible layer exhibits resistivity change of 6-7 orders under tensile strain, due to the formation of electron transport channel provided by in situ evolution of Ni network. It could be used to dynamically control THz transmission with giant modulation depth of around 96%, at high strain operation (up to around 58.5%). Moreover, these characteristics are demonstrated to be available for highly tension sensitive THz spectroscopy and imaging. This work opens up a connection between flexible polymer based composites and THz dynamic device. It proposes an unprecedented flexible THz modulation with giant tuning efficiency, and provides a scheme for contactless and passive tension sensor.

Abstract-Electro-optic sampling of optical pulses and electron bunches for a compact THz-FEL source

Bang Wu, 

Zhe Zhang, 

Lei Cao,  

Qiang Fu, 

Yongqian Xiong

https://www.sciencedirect.com/science/article/pii/S1350449518302895

A systematic one dimensional analytical model describing the full electromagnetic propagation in electro-optic (EO) diagnostic is proposed and designed for a terahertz FEL source at HUST. Our model contains two main aspects: the propagation of the THz pulse (1.5–6 THz) along the transport line and the THz EO process in the detection crystal. The diffraction, Fabry–Perot and focusing effects are included in the THz propagation process. The phase mismatch, the frequency-dependent EO coupling coefficient and geometrical overlap between probe and THz pulses as well as the group velocity dispersion and finite duration of the probe pulse are considered in the THz EO process. The calculation shows that the diffraction in the TPX window and the Fabry–Perot effect in the GaP crystal are crucial for retrieving the details of the THz optical pulse. While for electron bunches, the detected Coulomb electric field will be significantly broadened due to the relatively low relativistic Lorenz factor (γ=10–20$\mathrm{}\mathrm{}$). Therefore, a convenient deconvolution algorithm is utilized to reconstruct THz electric field from the distorted EO signals, where the truncated singular value decomposition method is employed. Our algorithm is verified to work well even with a low signal-to-noise ratio (20 dB) in the future measurement.

Abstract- Terahertz Electro-Optic Sampling in Thick ZnTe Crystals Below the Reststrahlen Band With a Broadband Femtosecond Laser

Bang Wu,  Lei Cao,   Zhe Zhang,   Qiang Fu,   Yongqian Xiong

https://ieeexplore.ieee.org/document/8319525/

The method of electro-optic (EO) sampling of terahertz (THz) pulses in thick EO crystals leads to waveform distortions due to effects of phase mismatch, dispersive propagation, and absorption. In this paper, we demonstrate theoretically and experimentally that EO sampling with a broadband femtosecond (fs) laser could significantly eliminate these distortions below the Reststrahlen band. Our simulation results show that the oscillations and dips in the signals of EO response function of thick crystals are smoothed out by the broadband spectrum of the femtosecond laser. In the experiment, we use a laser with a bandwidth of 100 nm and a low temperature GaAs photoconductive antenna to generate typical THz pulses (0.1-3 THz). The measurement results confirm that a {110}-oriented 3-mm-thick ZnTe crystal is an attractive EO material for sampling THz pulse below 3 THz and agree with simulation results. This technique is particularly useful in areas where a large time window is needed, for example in the THz time-domain spectroscopy system.

Abstract-Terahertz magnetoplasmon-polaritons with nonlocal corrections for lossy two dimensional electron gas in GaN-based heterostructures

Lei Cao,  Qiang Fu, Bang Wu and Yongqian Xiong

http://iopscience.iop.org/article/10.1088/1361-648X/aa80c6

The high electron concentration in GaN-based heterostructures necessitate the consideration of nonlocal corrections in the magnetoconductivity for the study of magnetoplasmon-polaritons (MPPs) in the lossy two dimensional electron gas system with grating couplers. With the complete set of Maxwell equations, the complex dispersion frequency and absorption spectrum are calculated numerically at different magnetic field (B max  =  10 T) and plasmon wavevectors (k max  =  5  ×  107m−1). Taking the AlGaN/GaN structure as a representative case, we observe the cyclotron resonance and its high order harmonics, as well as the MPP modes. The effective aspect ratio of the grating is determined according to the absorption spectrum. Effects of the nonlocal corrections on the dispersion frequency and absorption spectrum are analyzed in detail. The contributions of LO phonons, electron collisions, THz wave polarizations and retardations are also discussed.

Abstract-Energy scaling of terahertz-wave parametric sources

Energy scaling of terahertz-wave parametric sources Guanqi Tang, Zhenhua Cong, Zengguang Qin, Xingyu Zhang, Weitao Wang, Dong Wu, Ning Li, Qiang Fu, Qingming Lu, and Shaojun Zhang  »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-23-4-4144

Optics Express, Vol. 23, Issue 4, pp. 4144-4152 (2015)
http://dx.doi.org/10.1364/OE.23.004144

View Full Text Article

Enhanced HTML    Acrobat PDF (873 KB)

Terahertz-wave parametric oscillators (TPOs) have advantages of room temperature operation, wide tunable range, narrow line-width, good coherence. They have also disadvantage of small pulse energy. In this paper, several factors preventing TPOs from generating high-energy THz pulses and the corresponding solutions are analyzed. A scheme to generate high-energy THz pulses by using the combination of a TPO and a Stokes-pulse-injected terahertz-wave parametric generator (spi-TPG) is proposed and demonstrated. A TPO is used as a source to generate a seed pulse for the surface-emitted spi-TPG. The time delay between the pump and Stokes pulses is adjusted to guarantee they have good temporal overlap. The pump pulses have a large pulse energy and a large beam size. The Stokes beam is enlarged to make its size be larger than the pump beam size to have a large effective interaction volume. The experimental results show that the generated THz pulse energy from the spi-TPG is 1.8 times as large as that obtained from the TPO for the same pumping pulse energy density of 0.90 J/cm² and the same pumping beam size of 3.0 mm. When the pumping beam sizes are 5.0 and 7.0 mm, the enhancement times are 3.7 and 7.5, respectively. The spi-TPG here is similar to a difference frequency generator; it can also be used as a Stokes pulse amplifier.

© 2015 Optical Society of America