TPS spectra 3000 used in topological insulators research at Nanyang Technological University, Singapore

Terahertz conductivity of topological surface states in Bi1.5Sb0.5Te1.8Se1.2

Chi Sin Tang, Bin Xia, Xingquan Zou, Shi Chen, Hong-Wei Ou, Lan Wang, A. Rusydi, Jian-Xin Zhu& Elbert E. M. Chia

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore,

NUSNNI-NanoCore, Department of Physics, National University of Singapore, 117542, Singapore,

Theoretical Division and Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos NM87545, USA.

Scientific Reports 3, 3513 doi:10.1038/srep03513

Abstract

Topological insulators are electronic materials with an insulating bulk and conducting surface. However, due to free carriers in the bulk, the properties of the metallic surface are difficult to detect and characterize in most topological insulator materials. Recently, a new topological insulator Bi_1.5Sb_0.5Te_1.7Se_1.3 (BSTS) was found, showing high bulk resistivities of 1–10 Ω.cm and greater contrast between the bulk and surface resistivities compared to other Bi-based topological insulators. Using Terahertz Time-Domain Spectroscopy (THz-TDS), we present complex conductivity of BSTS single crystals, disentangling the surface and bulk contributions. We find that the Drude spectral weight is 1–2 orders of magnitude smaller than in other Bi-based topological insulators, and similar to that of Bi₂Se₃ thin films, suggesting a significant contribution of the topological surface states to the conductivity of the BSTS sample. Moreover, an impurity band is present about 30 meV below the Fermi level, and the surface and bulk carrier densities agree with those obtained from transport data. Furthermore, from the surface Drude contribution, we obtain a ~98% transmission through one surface layer — this is consistent with the transmission through single-layer or bilayer graphene, which shares a common Dirac-cone feature in the band structure.

For full paper see http://www.nature.com/srep/2013/131217/srep03513/pdf/srep03513.pdf

THz-TDS

THz transmission of the BSTS single crystal was measured using a conventional THz-TDS system (TeraView Spectra 3000) incorporated with a Janis ST-100-FTIR cryostat. The THz signal was generated and detected by photoconductive antennae fabricated on low temperature-grown GaAs films. The aperture diameter is 3.5 mm, allowing for an accurate measurement of the THz signal down to ~0.4 THz. The time-domain electric field of the THz pulse signal is transmitted through the BSTS sample  , while the reference signal  is transmitted through vacuum. 1800 THz traces were taken in 60 seconds for each reference or sample run. The sample holder was moved back and forth between the sample and reference positions by means of a vertical motorized stage with a resolution of 2.5 μm. Fast Fourier Transform (FFT) was then performed on the time-domain THz signal to obtain the amplitude and phase of the THz spectra. Since the THz-TDS detects both the amplitude and phase of the THz signal, there is no need to use the Kramers-Kronig transformation to extract the real and imaginary components of the material optical parameters.

For more information about TeraView and its products visit TeraView or contact us at ian.grundy@teraview.com for more information.

Continuous tuning of a narrow-band terahertz wave in periodically poled stoichiometric LiTaO3 crystal with a fan-out grating structure

Nan Ei Yu1, Myoung-Kyu Oh1, Hoonsoo Kang1, Changsoo Jung1, Bok Hyeon Kim1, Kyu-Sup Lee2, Do-Kyeong Ko2, Shunji Takekawa3 and Kenji Kitamura3

Abstract

Continuous tuning of terahertz waves was demonstrated with seamless change in grating period in a periodically poled stoichiometric lithium tantalite (SLT) crystal. The periodically poled fan-out structure allows wide tunability such as 610 GHz with the bandwidth as narrow as 17 GHz at a carrier frequency of 1.00 THz. Temperature-dependent measurements show a gradual intensity increase of the THz pulses as the temperature decreases. Furthermore, absorption and refractive index of SLT in the THz range were estimated and compared with those of LiNbO3 (LN). The absorption coefficient of the LT crystal at ordinary wave was almost half of that in the LN crystal. SLT could be one of the powerful crystals for high-power THz generation with large optical aperture because of the fewer defects inside the crystal.

Full Article: http://iopscience.iop.org/1882-0786/7/1/012101

Characterisation of Crystalline-Amorphous Blends of Sucrose with Terahertz-Pulsed Spectroscopy: the Development of a Prediction Technique for Estimating the Degree of Crystallinity with Partial Least Squares Regression

I. Ermolina, 

J. Darkwah, 

G. Smith

Abstract

The control of the amorphous and crystalline states of drugs and excipients is important in many instances of product formulation, manufacture, and packaging, such as the formulation of certain (freeze-dried) fast melt tablets. This study examines the use of terahertz-pulsed spectroscopy (TPS) coupled with two different data analytical methods as an off-line tool (in the first instance) for assessing the degree of crystallinity in a binary mixture of amorphous and polycrystalline sucrose. The terahertz spectrum of sucrose was recorded in the wave number range between 3 and 100 cm−1 for both the pure crystalline form and for a mixture of the crystalline and amorphous (freeze-dried) form. The THz spectra of crystalline sucrose showed distinct absorption bands at ∼48, ∼55, and ∼60 cm−1 while all these features were absent in the amorphous sucrose. Calibration models were constructed based on (1) peak area analysis and (2) partial least square regression analysis, with the latter giving the best LOD and LOQ of 0.76% and 2.3%, respectively. The potential for using THz spectroscopy, as a quantitative in-line tool for percent crystallinity in a range of complex systems such as conventional tablets and freeze-dried formulations, is suggested in this study.

This study was performed using TeraView's TPS Spectra 3000 system. (TeraView, Cambridge, UK)

Full Article: http://link.springer.com/article/10.1208/s12249-013-0042-2

Quantitative analysis of visible surface defect risk in tablets during film coating using terahertz pulsed imaging

Masahiro Niwa, 

Yasuhiro Hiraishi

Abstract

Tablets are the most common form of solid oral dosage produced by pharmaceutical industries. There are several challenges to successful and consistent tablet manufacturing. One well-known quality issue is visible surface defects, which generally occur due to insufficient physical strength, causing breakage or abrasion during processing, packaging, or shipping. Techniques that allow quantitative evaluation of surface strength and the risk of surface defect would greatly aid in quality control. Here terahertz pulsed imaging (TPI) was employed to evaluate the surface properties of core tablets with visible surface defects of varying severity after film coating. Other analytical methods, such as tensile strength measurements, friability testing, and scanning electron microscopy (SEM), were used to validate TPI results. Tensile strength and friability provided no information on visible surface defect risk, whereas the TPI-derived unique parameter terahertz electric field peak strength (TEFPS) provided spatial distribution of surface density/roughness information on core tablets, which helped in estimating tablet abrasion risk prior to film coating and predicting the location of the defects. TPI also revealed the relationship between surface strength and blending condition and is a nondestructive, quantitative approach to aid formulation development and quality control that can reduce visible surface defect risk in tablets.

Full Article: http://www.sciencedirect.com/science/article/pii/S0378517313010454#fig0060

TeraView at Photonics West: TeraView will be one of the companies in the UK cluster

TeraView will be one of the companies within the UK Cluster at Photonics West 2014.

Exhibit at SPIE Photonics West, 4 - 6 February 2014, the annual flagship event for the lasers and photonics industry. 

Identification of concealed materials, including explosives, by terahertz reflection spectroscopy

Norbert Palka

Military University of Technology

Abstract

We report on a method for extracting the characteristic features of covered materials, including hexogen, in the range from 0.5 to 1.8 THz. This time-domain spectroscopy-based technique takes into account only part of the signal reflected from a covered sample and analyzes it by using the Fourier transform. The obtained power spectrum has distinctive peaks that correspond to peaks measured in the transmission configuration and can be used for further identification of the materials.

This study was performed using TeraView's TPS Spectra 3000 system. (TeraView, Cambridge, UK)

Full Article: http://opticalengineering.spiedigitallibrary.org/article.aspx?articleid=1787555

CLEO: 2014 - Laser Science to Photonic Applications

The submission deadline for CLEO: 2014 (8-13 June 2014 in San Jose, California, USA) will be on the 22nd of January 2014, 17:00 GMT.

CLEO: 2014 is well known for its high quality, peer-reviewed research, and bridges fundamental science all the way through to technology development and uses of technology and applications. Submissions are being accepted in:

CLEO: QELS- Fundamental Science

CLEO: Science & Innovations

CLEO: Applications & Technology 

Visit the website for topic categories and descriptions.

More information about CLEO: 2014 can be found on the website at www.cleoconference.org.