Thursday, 3 January 2019

Insights into the structural dynamics of poly lactic-co-glycolic acid at terahertz frequencies

Shmool, Talia A., and J. Axel Zeitler. "Insights into the structural dynamics of poly lactic-co-glycolic acid at terahertz frequencies." Polymer Chemistry (2019).
Graphical abstract: Insights into the structural dynamics of poly lactic-co-glycolic acid at terahertz frequenciesAbstract
The mechanical properties of an amorphous copolymer are directly related to the dynamic processes occurring at the molecular level. Poly lactic-co-glycolic acid (PLGA) is a biodegradable copolymer, and in this work we investigate the dynamics of PLGA and its glass transition behaviour by performing variable temperature terahertz time-domain spectroscopy (THz-TDS) experiments. We correlate PLGA dynamics, as measured at terahertz frequencies, their temperature dependence, molecular weight (MW), lactide toglycolide ratio, and free volume. The THz-TDS data can be used to detect two distinct transition processes, Tβand Tg,α. To complement our analysis, we use dynamic mechanical analysis (DMA) to probe the β- and α-relaxation processes in PLGA, and compare the results obtained from the DMA experiments with those obtained using THz-TDS. The values of Tg,α, as determined from the THz-TDS experiments, are in good agreement with Tg,DSC, determined by calorimetric measurements, and with Tg,DMA, determined by the DMA measurements. We attribute Tβ to the change in dipole moments associated with the β-relaxation process, originating from the local rotation of C–O macromolecular chain segments, and Tg,α to the change in dipole moments due to large segmental motion of the copolymer backbone associated with the α-relaxation process. By showing experimental temperature-dependent results from THz-TDS and DMA for PLGA, we demonstrate that the behaviour of PLGA is in line with the potential energy surface interpretation by Goldstein as well as the free volume theory of Fox and Flory, and that both concepts together can be used to understand amorphous polymers.

for full paper see 

… 0.50 mm in thickness. 1.2 Experimental setup. The THz-TDS spectra were acquired using a commercial TeraPulse 4000 instrument across the spectral range of 0.2–2.2 THz (TeraView, Cambridge, UK). The sample temperature … 

Thursday, 27 December 2018

Large phase modulation of THz wave via an enhanced resonant active HEMT metasurface

Zhang, Yaxin, Yuncheng Zhao, Shixiong Liang, Bo Zhang, Lan Wang, Tianchi Zhou, Wei Kou et al. "Large phase modulation of THz wave via an enhanced resonant active HEMT metasurface." Nanophotonics (2018).

Abstract
Terahertz (THz) science and technology promise unique applications in high-speed communications, high-accuracy imaging, and so on. To keep up with the demand for THz systems, THz dynamic devices should feature large phase shift modulation and high speed. To date, however, only a few devices can efficiently manipulate the phase of THz waves. In this paper, we demonstrate that efficient phase modulation of THz waves can be addressed by an active and enhanced resonant metamaterial embedded with a nanostructured 2D electron gas (2DEG) layer of a GaN high electron mobility transistor (HEMT). The enhanced resonant metaunit couples the traditional dipolar and inductance-capacitance resonances together to realize a coupling mode with enhanced resonance. Embedded with the nanostructured 2DEG layer of GaN HEMT, the resonance intensity and surface current circuit of the enhanced resonant mode in the metamaterial unit can be dynamically manipulated by the electrical control of the carrier distribution and depletion of the 3 nm 2DEG, leading to a phase shift greater than 150° in simulation. In the dynamic experiments, a 137° phase shift was achieved with an external controlling voltage of only several volts in the THz transmission mode. This work represents the first realization of a phase shift greater than 100° in a dynamic experiment in transmission mode using an active metamaterial structure with only a single layer. In addition, given the high-speed modulation ability of the HEMT, this concept provides a promising approach for the development of a fast and effective phase modulator in THz application systems.
for full paper see https://www.degruyter.com/view/j/nanoph.ahead-of-print/nanoph-2018-0116/nanoph-2018-0116.xml
This group uses a terahertz spectrometer supplied by TeraView, Cambridge, UK  for more information visit their new web site at www.teraview.com

Monday, 24 December 2018

Complex Permittivity Measurement of Paraffin Phase-Change Material at 26 GHz–1.1 THz Using Time-Domain Spectroscopy

Ghassemiparvin, Behnam, and Nima Ghalichechian. "Complex Permittivity Measurement of Paraffin Phase-Change Material at 26 GHz–1.1 THz Using Time-Domain Spectroscopy." Journal of Infrared, Millimeter, and Terahertz Waves (2018): 1-9.

Abstract
We report complex permittivity measurement of hexatriacontane films at the frequency range of 26 GHz–1.1 THz. Hexatriacontane (C36H74) has a melting point of 75 °C that exhibits a 15% volumetric change which is crucial in developing low-loss RF microactuators with large displacement. In this work, we employ time-domain spectroscopy to measure the transmission coefficient of the paraffin samples in the frequency range of 0.3–1.1 THz. In order to extract the dielectric constant and accurately estimate the small values of loss tangent, we developed a propagation model which measured data are fitted to through a new least-squares minimization method. A Debye relaxation model is used to model the frequency dependence of the permittivity. Described method is rapidly convergent with minimum amount of signal processing. This method can be used to determine the complex permittivity of the materials by devising an appropriate function for the frequency dependence of the complex permittivity. Transmission through 20 samples of paraffin with various thicknesses is measured and the average permittivity is found to be 2.25 with standard deviation of 0.028. The loss tangent is monotonically increasing with frequency and the maximum value is 6.32 × 10− 3 at 1.1 THz. Our study demonstrates that paraffin is a low-loss dielectric which makes it an attractive candidate for development of electro-thermo-mechanical actuators for sub-millimeter- and millimeter wave (mmW) variable capacitors, low-loss reconfigurable antennas, and phase shifters.

This group uses a terahertz spectrometer supplied by TeraView, Cambridge, UK  for more information visit their new web site at www.teraview.com

Friday, 21 December 2018

Tuning of Topological Dirac States via Modification of van der Waals Gap in Strained Ultrathin Bi2Se3 Films

Yang, Won Jun, Chang Woo Lee, Da Sol Kim, Hyun Sik Kim, Jong Hyeon Kim, Hwan Young Choi, Young Jai Choi, Jae Hoon Kim, Kyungwha Park, and Mann-Ho Cho. "Tuning of Topological Dirac States via Modification of van der Waals Gap in Strained Ultrathin Bi2Se3 Films." The Journal of Physical Chemistry C 122, no. 41 (2018): 23739-23748.

Abstract Image

Robust massless Dirac states with helical spin textures were realized at the boundaries of topological insulators such as van der Waals (vdW) layered Bi2Se3 family compounds. Topological properties of massless Dirac states can be controlled by varying the film thickness, external stimuli, or environmental factors. Here, we report single-crystal-quality growth of ultrathin Bi2Se3films on flexible polyimide sheets and manipulation of the Dirac states by varying the vdW gap. X-ray diffraction unambiguously demonstrates that under uniaxial bending stress the vdW gap substantially changes with interatomic-layer distances unaltered. Terahertz and photoelectron spectroscopy indicate tuning of the number of quantum conducting channels and of work function, by the stress, respectively. Surprisingly, under compressive strain, transport measurements reveal dimensional crossover and suppressed weak antilocalization. First-principles calculations support the observation. Our findings suggest that variation of vdW gap is an effective means of tuning the Fermi level and topological Dirac states for spintronics and quantum computation.

For full see https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.8b06296
This group uses a terahertz spectrometer supplied by TeraView, Cambridge, UK  for more information visit their new web site at www.teraview.com

Friday, 16 November 2018

Flexible terahertz and FarIR polymer‐supported filter

Sanphuang, Varittha, and Niru K. Nahar. "Flexible terahertz and FarIR polymer‐supported filter." Microwave and Optical Technology Letters.


Abstract

We present the design, fabrication, and measurement of a terahertz (THz) polymer‐supported filter. A periodic structure, frequency selective surfaces (FSS) is employed to provide bandpass characteristic for this filter design with polyimide (PI) film as a substrate. This PI film (13 μm thick) is very thin and thus provides a flexible support to the filter. The average extracted dielectric constant ε r is 3.35 and the average refractive index n s is 1.84. The final filter structure was fabricated using standard microfabrication process. The measured transmission response shows high percentage of transmission (~80%) with the peak at 0.58 THz and sharp roll‐off that covers the broad range of frequency from 0.1 to 1 THz (wavelength of 300 μm‐3000 μm).


For full paper see https://onlinelibrary.wiley.com/doi/abs/10.1002/mop.31414

For more information about TeraView visit www.teraview.com