Abstract-Nonvolatile Solid-State Charged-Polymer Gating of Topological Insulators into the Topological Insulating Regime

R. M. Ireland, Liang Wu, M. Salehi, S. Oh, N. P. Armitage, and H. E. Katz

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.9.044003

We demonstrate the ability to reduce the carrier concentration of thin films of the topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by utilizing a nonvolatile electrostatic gating via corona charging of electret polymers. Sufficient electric field can be imparted to a polymer-TI bilayer to result in significant electron density depletion, even without the continuous connection of a gate electrode or the chemical modification of the TI. We show that the Fermi level of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is shifted toward the Dirac point with this method. Using terahertz spectroscopy, we find that the surface chemical potential is lowered into the bulk band gap (approximately 50 meV above the Dirac point and 170 meV below the conduction-band minimum), and it is stabilized in the intrinsic regime while enhancing electron mobility. The mobility of surface state electrons is enhanced to a value as high as approximately $1600{\mathrm{cm}}^2/Vs$ at 5 K.

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Terahertz Spectroscopy Captures Metal Conductivities

Terahertz signals from 0.4 to 2.5 THz can be used in the analysis of thin metal conductors.

Jack Browne
http://www.mwrf.com/test-measurement/terahertz-spectroscopy-captures-metal-conductivities

Thin metal films such as copper are essential to the design and fabrication of high-frequency circuits, such as microstrip and coplanar waveguide (CPW) on almost-as-thin dielectric substrates in the formation of RF/microwave and even millimeter-wave printed-circuit boards (PCBs). While many studies have been performed on the analysis of the properties of dielectric materials in those PCBs, such as dielectric constant and dissipation factor, much remains to be known about the metal conductors.

In quest of that knowledge, researchers from a number of locations—including the School of Mechanical Engineering of China’s Tianjin University and the School of Electrical and Computer Engineering of Oklahoma State University—pooled their resources into the investigation of different types of thin metal films. The researchers used terahertz spectroscopy to study two key material parameters (conductivity and thickness) simultaneously. They investigated three different metal films (aluminum, copper, and silver) and discovered that their measured values of conductivities for the metals were significantly different than the already-known bulk material conductivity values. Their measurements of material thicknesses were consistent with values obtained from other measurement methods.

The studies were performed with the measurement power of the terahertz time-domain spectroscopy (THz-TDS) system at Tianjin University with the aid of grants from China’s National Natural Science Foundation and the Natural Science Foundation of Tianjin Province. The analysis system operates mainly in the spectrum from 0.4 to 2.5 THz. Samples were prepared by depositing metal films on 22-μm-thick Mylar substrates by means of thermal evaporation.

The researchers concluded that the differences in the measured values of conductivities and the known bulk materials could stem from a number of factors, such as defects in film metal (e.g.,  grain boundaries), leading to a reduction in the measured conductivities for those thin metal films compared to bulk metal values. The measurement system and test approach provide convenient means for capturing the two simultaneous conductive metal parameters. The material conductivity and thickness data gathered by the THz-TDS system, while not without some variations, does provide invaluable additional insights to computer-aided-engineering (CAE) design programs, including electromagnetic (EM) simulation software used for circuit designs.

See “Characterization of Thin Metal Films Using Terahertz Spectroscopy,” IEEE Transactions on Terahertz Science and Technology, Vol. 8, No. 2, March 2018, p. 161.

Abstract-Polarization switching in ferroelectric thin film induced by a single-period terahertz pulse

Elena Mishina,  Kirill Grishunin, Vladislav Bilyk, Natalia Sherstyuk, 

https://www.cambridge.org/core/journals/mrs-advances/article/polarization-switching-in-ferroelectric-thin-film-induced-by-a-singleperiod-terahertz-pulse/91CA5CE74306D2C39DC594B0BE31C92E

We report here an experimental study of ultrafast response of the dielectric polarization in (Ba0.8Sr0.2)TiO3 thin films to a strong electric field of a nearly single-cycle THz pulse. The phenomenon of Second Harmonic Generation (SHG) is used as a probe of the polarization in the terahertz pump-optical probe experiment. SHG loops for THz pulses of different amplitudes were obtained. The SHG response is modelled assuming that the ferroelectric material is split into 180-degree domains. It is shown that intuitive model based on forced harmonic oscillator does not fully describe to the observed ultrafast ferroelectric response.

COPYRIGHT: © Materials Research Society 2018 

Abstract-Broadband Terahertz Light-Matter Interaction Enhancement for Precise Spectroscopy of Thin Films and Micro-Samples

Romain Peretti, Flavie Braud, Emilien Peytavit,  Emmanuel Dubois, Jean-François Lampin

https://www.preprints.org/manuscript/201802.0157/v1


In biology molecules and macromolecules like sugars, proteins, DNA, and RNA are of utter importance. Detecting their presence as well as their conformation is still a challenge in many cases. It is well known that the vibrational states of such molecules lie from the infrared to the TeraHertz range. Spectroscopy can be used to detect such compounds and probe their conformation. Still, terahertz spectroscopy on biosample is a challenge for two main reasons: water absorption; and the small size of the samples. The sample volume is smaller than the cube of the TeraHertz wavelength; the light matter interactions are thus extremely reduced. In this paper, we present the design, fabrication, characterization and the first typical uses of a biophotonic device aiming at increasing light matter interaction to enable terahertz spectroscopy of minute samples on a broad band (0.2–2 THz). We demonstrate time domain spectroscopy experiments on few µl samples showing the validity of our approach.