Abstract-Tunable terahertz phase shifter based on dielectric artificial birefringence grating filled with polymer dispersed liquid crystal

Xin Zhang, Fei Fan, Chun-Yue Zhang, Yun-Yun Ji, Xiang-Hui Wang, and Sheng-Jiang Chang

Schematic diagram in the experiment and experimental data. (a) The structure of the dielectric gradient grating filled with PDLCs in the experiment; (b) The schematic diagram of geometry configuration in the measurement; (c) The time domain signals of + 45° and -45° LP components at 0 V and (d) 80 V measured by THz-TDS system, respectively.

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-10-2-282

An active terahertz (THz) anisotropic manipulation is based on a structure combined polymer dispersed liquid crystal (PDLC) with sub-wavelength dielectric gradient grating. In this structure, the PDLC works as an adjustable anisotropic material due to the change of the optical axis direction induced by applying a biased electric field, while the dielectric grating serves as an artificial high birefringence material. By using an appropriate design, the THz birefringence of this structure can be enhanced or offset under different biased voltages, and the phase shift curve of this structure becomes flatter than that of the pure PDLC cell due to the dispersion manipulation of the grating. Moreover, the experimental results fit with the simulative designing, demonstrating that the phase shift of the structure can vary from π to 0 near 0.8 THz when the electric field increases from 0 to 80V, and this device realizes the function of polarization conversion as a tunable THz half-wave plate. This work exhibits potential applications in THz functional devices, such as actively controlled phase shifters and polarization convertors combined LC with artificial microstructure.

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

Abstract-Photo-induced terahertz near-field dynamics of graphene/InAs heterostructures

Ziheng Yao, Vyacheslav Semenenko, Jiawei Zhang, Scott Mills, Xiaoguang Zhao, Xinzhong Chen, Hai Hu, Ryan Mescall, Thomas Ciavatti, Stephen March, Seth R. Bank, Tiger H. Tao, Xin Zhang, Vasili Perebeinos, Qing Dai, Xu Du, and Mengkun Liu

Fig. 1 Schematic of the near field optical-pump THz-probe (n-OPTP) setup, equally capable of performing optical pump near-field THz emission (n-OPTE) experiments. ①: 800 nm, 300 mW pump pulses pass through the ITO then go through the center of, while parallel to, the THz beam ②. Optical pump ① and THz probe ② are focused onto the AFM tip apex using an off-axis parabolic mirror. ③: THz gate (detection) beam. Tip scattered THz signals can be mapped out in the time domain by modifying the arrival time of THz gate beam ③ to the THz photoconductive antenna detector, thus changing the delay between ② and ③ (t1). Changing the delay between ① and ③ (t2) by modifying arrival time of pump pulse ① probes the photo-excited ultrafast dynamics of the sample (see main text), where in this work, t1 is fixed at the peak position of the scattered THz electric field while t2 is varied to yield n-OPTE (with THz probe ② blocked) and n-OPTP (with THz probe ② unblocked) measurements.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-10-13611

In this letter, we report optical pump terahertz (THz) near-field probe (n-OPTP) and optical pump THz near-field emission (n-OPTE) experiments of graphene/InAs heterostructures. Near-field imaging contrasts between graphene and InAs using these newly developed techniques as well as spectrally integrated THz nano-imaging (THz s-SNOM) are systematically studied. We demonstrate that in the near-field regime (𝜆/6000$\lambda /6000$), a single layer of graphene is transparent to near-IR (800 nm) optical excitation and completely “screens” the photo-induced far-infrared (THz) dynamics in its substrate (InAs). Our work reveals unique frequency-selective ultrafast dynamics probed at the near field. It also provides strong evidence that n-OPTE nanoscopy yields contrast that distinguishes single-layer graphene from its substrate.

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

Abstract-Deep Electrical Modulation of Terahertz Wave Based on Hybrid Metamaterial-Dielectric-Graphene Structure

Liangping Xia, Xin Zhang, Man Zhang, Suihu Dang, Shijian Huang, Yong Tan, Wenjuan Yan, Hong-Liang Cui

https://www.mdpi.com/2076-3417/9/3/507

A terahertz modulation structure based on hybrid metamaterial and graphene is proposed and demonstrated in this work. The metamaterial with a square slit ring array excites terahertz resonance in the slits and enhances the interaction between the terahertz wave and graphene. The graphene layer acting as the active material is tuned by the applied electrical field. With the separation by a dielectric layer between the graphene and the metallic structure, the resonant frequency and transmitted energy are both modulated by the graphene. Experimental result indicates that the modulation depth of the terahertz transmitted amplitude is 65.1% when the applied modulation voltage is tuned 5 V.

Abstract-An air-spaced terahertz metamaterial perfect absorber

Guangwu Duan, Jacob Schalch, Xiaoguang Zhao, Jingdi Zhang, Richard Averitt, Xin Zhang,



https://www.sciencedirect.com/science/article/pii/S0924424717317648

Metamaterial absorbers are typically comprised of a layer of split ring resonators and a ground plane with a dielectric spacer layer that provides structural support and in which absorbed energy is deposited. We address the question “What happens to the absorption if the spacer layer is removed?” through the design, fabrication, and characterization of a terahertz metamaterial absorber with air as the spacer layer. Reflection based terahertz time-domain spectroscopy was employed to measure the absorption and interference theory was used to interpret the results. The surface current in the gold ground plane and split ring resonator (SRR) layer is solely responsible for the absorption in the form of joule heating. In comparison to dielectric spacer layer absorbers, the quality factor is increased by a factor of ~3. The electric field is highly concentrated in the volume between SRR layer and the ground plane offering the potential for novel sensing application if materials can be incorporated into this region (e.g. with microfluidics). In the spirit of this possibility, simulations of the absorption have been performed. The variation of the real part of the permittivity of the spacer material results in an absorption peak frequency shift, while a change in the imaginary part affects the quality factor and amplitude. Ultimately, the high quality factor and the absence of the spacer material provide the air-spacer metamaterial absorber with unique advantages for sensing applications.

Abstract-Imidazolium-grafted graphene oxide via free radical polymerization: An efficient and simple method for an interpenetrating polymer network as electrolyte membrane

Amina Ouadah, 

Tianwei Luo, 

Jing Wang, 

Shuitao Gao, 

Xing Wang, 

Xin Zhang, 

Zhou Fang, 

Zeyu Wu, 

Jie Wang, 

Changjin Zhu, 

https://www.sciencedirect.com/science/article/pii/S0266353817327422

In this work, graphene oxide (GO) is modified via free radical polymerization with butylvinylimidazolium (b-VIB) to produce GO/IM, which is characterized using FTIR spectral analysis, X-ray diffraction (XRD), thermogravimetric analysis (TGA), Raman analysis, X-ray photoelectron spectroscopy (XPS), elemental analysis, and a morphology study with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Further, GO/IM is incorporated for an in-situ polymerization, with the synthesized copolymer para-methyl styrene/butylvinylimidazolium (PMS/b-VIB) and synthesized poly(4,4′-diphenyl ether-5,5′-bibenzimidazole) (DPEBI) as a matrix, giving nanocomposite membranes referred to as GO/IM-X. These nanohybrid membranes possess higher conductivity than the pristine membrane of PMS/b-VIB/DPEBI and the conductivity increases with increasing amount of GO/IM, reaching 78.5 mS cm⁻¹ at 100 °C and 26.5 mS cm⁻¹ 25 °C (chloride conductivity), enhancements of about 14.93% and 33.16% compared to the pristine membrane. Nanocomposite membrane properties were investigated; the swelling ratio and water uptake, ion exchange capacity (IEC), thermal properties via TGA, structure characterization using FTIR, morphology via TEM and mechanical properties. Taken together, these results suggest the present nanohybrid membranes have great potential for use as polymer electrolyte membranes with fuel cell applications.

Abstract-Dimensionality Reduction for Identification of Hepatic Tumor Samples Based on Terahertz Time-Domain Spectroscopy

 Haishun Liu,  Zhenwei Zhang, Xin Zhang,  Yuping Yang, Zhuoyong Zhang, Xiangyi Liu, Fan Wang,  Yiding Han, Cunlin Zhang

https://ieeexplore.ieee.org/document/8333778/

Terahertz time-domain spectroscopy (THz-TDS) combining with chemometrics methods was proposed for the identification of hepatic tumors. Two linear compression methods, principle component analysis and locality preserving projections (LPPs), and a nonlinear method, Isomap, were used to reduce the dimensionality of the measured dataset. Comparing two-dimensional (2-D) data reduced by these three dimensionality reduction techniques, only 2-D Isomap plot could separate the distances between two classes for the THz time-domain data and LPP had capacity of distinguishing two types of samples building on frequency-domain data. The best classification accuracies from 2-D time-domain data were 99.81±0.30% and 99.69±0.61% given by Isomap probabilistic neural network (PNN) and Isomap support vector machine (SVM), respectively, while the best classification results of 2-D frequency-domain data were 100.00±0.00%, 99.75±0.32% provided by LPP-PNN, LPP-SVM. The results showed that Isomap and LPP are appropriate techniques to reflect the nonlinear manifold of the THz data. The THz technology either in time-domain or frequency-domain coupled with Isomap-PNN or LPP-PNN could offer a potential procedure to identify hepatic tumors.

Abstract-Electromechanically tunable metasurface transmission waveplate at terahertz frequencies

Xiaoguang Zhao, Jacob Schalch, Jingdi Zhang, Huseyin R. Seren, Guangwu Duan, Richard D. Averitt, and Xin Zhang

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-3-303

Dynamic polarization control of light is essential for numerous applications ranging from enhanced imaging to material characterization and identification. We present a reconfigurable terahertz metasurface quarter-wave plate consisting of electromechanically actuated microcantilever arrays. Our anisotropic metasurface enables tunable polarization conversion through cantilever actuation. Specifically, voltage-based actuation provides mode-selective control of the resonance frequency, enabling real-time tuning of the polarization state of the transmitted light. The polarization tunable metasurface has been fabricated using surface micromachining and characterized using terahertz time domain spectroscopy. We observe a ∼230  GHz$\sim 230\mathrm{GHz}$ cantilever actuated frequency shift of the resonance mode, sufficient to modulate the transmitted wave from pure circular polarization to linear polarization. Our CMOS-compatible tunable quarter-wave plate enriches the library of terahertz optical components, thereby facilitating practical applications of terahertz technologies.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Quantitative Identification of the Annealing Degree of Apatite Fission Tracks Using Terahertz Time Domain Spectroscopy (THz-TDS)

Hang Wu, Shixiang Wu, Nansheng Qiu, Jian Chang, Rima Bao, Xin Zhang, Nian Liu, Shuai Liu

http://journals.sagepub.com/doi/10.1177/0003702818761668

Apatite fission-track (AFT) analysis, a widely used low-temperature thermochronology method, can provide details of the hydrocarbon generation history of source rocks for use in hydrocarbon exploration. The AFT method is based on the annealing behavior of fission tracks generated by ²³⁸U fission in apatite particles during geological history. Due to the cumbersome experimental steps and high expense, it is imperative to find an efficient and inexpensive technique to determinate the annealing degree of AFT. In this study, on the basis of the ellipsoid configuration of tracks, the track volume fraction model (TVFM) is established and the fission-track volume index is proposed. Furthermore, terahertz time domain spectroscopy (THz-TDS) is used for the first time to identify the variation of the AFT annealing degree of Durango apatite particles heated at 20, 275, 300, 325, 450, and 500 ℃ for 10 h. The THz absorbance of the sample increases with the degree of annealing. In addition, the THz absorption index is exponentially related to annealing temperature and can be used to characterize the fission-track volume index. Terahertz time domain spectroscopy can be an ancillary technique for AFT thermochronological research. More work is urgently needed to extrapolate experimental data to geological conditions.

Abstract- Graphene Terahertz Amplitude Modulation Enhanced by Square Ring Resonant Structure

 Liangping Xia,  Xin Zhang, Dongshan Wei. Hong-Liang Cui, Chunlei Du

http://ieeexplore.ieee.org/document/8141872/

A terahertz amplitude modulator based on graphene on a metallic square ring resonant structure is proposed. By separating the graphene and the metallic structure with a thin organic dielectric layer, both the resonant frequency and the amplitude of the transmission resonant peak are modulated when graphene is electrically tuned by a bias voltage. A maximal amplitude modulation depth of 72% at the frequency of 0.6 THz is achieved for the fabricated Terahertz modulator. An analysis model based on the transmission line theory is built to explore the modulation mechanism. Results of the transmission spectrum and the amplitude modulation indicate a good agreement between the transmission line theoretical predictions and the experimental measurements

Abstract-Terahertz Dispersion Characteristics of Super-aligned Multi-walled Carbon Nanotubes and Enhanced Transmission through Subwavelength Apertures

Yue Wang, Guangwu Duan, Liying Zhang, Lihua Ma, Xiaoguang Zhao,  Xin Zhang

https://www.nature.com/articles/s41598-018-20118-5

The terahertz (THz) dielectric properties of super-aligned multi-walled carbon nanotube (MWCNT) films were characterized in the frequency range from 0.1 to 2.5 THz with terahertz time-domain spectroscopy. The refractive index, effective permittivity, and conductivity were retrieved from the measured transmission spectra with THz incident wave polarized parallel and perpendicular to the orientation of carbon nanotubes (CNTs), and a high degree of polarization dependence was observed. The Drude-Lorentz model combined with Maxwell-Garnett effective medium theory was employed to explain the experimental results, revealing an obvious metallic behavior of the MWCNT films. Moreover, rectangular aperture arrays were patterned on the super-aligned MWCNT films with laser-machining techniques, and the transmission measurement demonstrated an extraordinarily enhanced transmission characteristic of the samples with incident wave polarized parallel to the orientation of the CNTs. Surface plasmon polaritons were employed to explain the extraordinarily enhanced transmission with high accuracy, and multi-order Fano profile was applied to model the transmission spectra. A high degree of agreement was exhibited among the experimental, numerical, and theoretical results.

Abstract-Analysis of the thickness dependence of metamaterial absorbers at terahertz frequencies

Guangwu Duan, Jacob Schalch, Xiaoguang Zhao, Jingdi Zhang, Richard D. Averitt, and Xin Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-3-2242

Metamaterial absorbers typically consist of a metamaterial layer, a dielectric spacer layer, and a metallic ground plane. We have investigated the dependence of the metamaterial absorption maxima on the spacer layer thickness and the reflection coefficient of the metamaterial layer obtained in the absence of the ground plane layer. Specifically, we employ interference theory to obtain an analytical expression for the spacer thickness needed to maximize the absorption at a given frequency. The efficacy of this simple expression is experimentally verified at terahertz frequencies through detailed measurements of the absorption spectra of a series of metamaterials structures with different spacer thicknesses. Using an array of split-ring resonator (SRR) as the metamaterial layer and SU8 as the spacer material we observe that the absorption peaks redshift as the spacer thickness is increased, in excellent agreement with our analysis. Our findings can be applied to guide metamaterial absorber designs and understand the absorption peak frequency shift of sensors based on metamaterial absorbers.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz-driven Luminescence and Colossal Stark Effect in CdSe:CdS Colloidal Quantum Dots

Brandt Pein, Wendi Chang, Harold Young Hwang, Jennifer M Scherer, Igor Coropceanu, Xiaoguang Zhao, Xin Zhang, Vladimir Bulovic, Moungi G. Bawendi, and Keith A. Nelson

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b01837?mi=aayia761&af=R&AllField=nano&target=default&targetTab=std

Optical properties of colloidal semiconductor quantum dots (QDs), arising from quantum mechanical confinement of charge, present a versatile testbed for the study of how high electric fields affect the electronic structure of nanostructured solids. Studies of quasi-DC electric field modulation of QD properties have been limited by electrostatic breakdown processes under high externally applied electric fields, which have restricted the range of modulation of QD properties. In contrast, here we drive CdSe:CdS core:shell QD films with high-field THz-frequency electromagnetic pulses whose duration is only a few picoseconds. Surprisingly, in response to the THz excitation we observe QD luminescence even in the absence of an external charge source. Our experiments show that QD luminescence is associated with a remarkably high and rapid modulation of the QD bandgap, which changes by more than 0.5 eV (corresponding to 25% of the unperturbed bandgap energy). We show that these colossal energy shifts can be explained by the quantum confined Stark effect even though we are far outside the regime of small field-induced shifts in electronic energy levels. Our results demonstrate a route to extreme modulation of material properties and to a compact, high-bandwidth THz detector that operates at room temperature.

Abstract-Terahertz saturable absorption in superconducting metamaterials

George R. Keiser, Jingdi Zhang, Xiaoguang Zhao, Xin Zhang, and Richard D. Averitt
https://www.osapublishing.org/josab/abstract.cfm?uri=josab-33-12-2649

We present a superconducting metamaterial saturable absorber at terahertz frequencies. The metamaterial was designed to have a resonant absorption peak at 0.5 THz for 𝑇=10  K$T=10\mathrm{K}$. The absorber consists of an array of split ring resonators (SRRs) etched from a 100 nm YBa2Cu3O7${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$film. A polyimide spacer layer and gold ground plane are placed above the SRRs using the metamaterial tape concept, creating a reflecting perfect absorber. Increasing either the temperature or incident electric field (𝐸$E$) decreases the superconducting condensate density and corresponding kinetic inductance of the SRRs. This alters the impedance matching in the metamaterial, broadening the resonance and reducing the peak absorption. At low electric fields, the experimental absorption was optimized near 80% at 𝑓=0.47  THz$f=0.47\mathrm{THz}$ for 𝑇=10  K$T=10\mathrm{K}$ and decreased to 20% for 𝑇=70  K$T=70\mathrm{K}$. For 𝐸=40  kV/cm$E=40\mathrm{kV}/\mathrm{cm}$ and 𝑇=10  K$T=10\mathrm{K}$, the peak absorption was 70%, decreasing to 40% at 200 kV/cm, corresponding to a modulation of 43%.

© 2016 Optical Society of America

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