Abstract-Through-substrate terahertz time-domain reflection spectroscopy for environmental graphene conductivity mapping

Applied Physics Letters

 Hungyen Lin,  Oliver J. Burton, Sebastian Engelbrecht, Kai-Henning Tybussek,  Bernd M. Fischer,  Stephan Hofmann

Schematic of the reflection-based THz-TDS to measure the complex conductivity of graphene through the substrate in a nitrogen purge environment.

https://aip.scitation.org/doi/abs/10.1063/1.5135644

We demonstrate how terahertz time-domain spectroscopy (THz-TDS) operating in reflection geometry can be used for quantitative conductivity mapping of large area chemical vapor deposited graphene films through silicon support. We validate the technique against measurements performed using the established transmission based THz-TDS. Our through-substrate approach allows unhindered access to the graphene top surface and thus, as we discuss, opens up pathways to perform in situ and in-operando THz-TDS using environmental cells.

H.L. acknowledges financial support from the EPSRC (Grant No. EP/R019460/1). S.H. acknowledges funding from the EPSRC (Grant No. EP/K016636/1, GRAPHTED). We also thank Dr. Philipp Braeuninger-Weimer for useful discussion. Additional data for this article are available at https://doi.org/10.17635/lancaster/researchdata/336.

Abstract-Crystallization Caught in the Act with Terahertz Spectroscopy: Non-Classical Pathway for l-(+)-Tartaric Acid

Amin Soltani, Denis Gebauer, Lennart Duschek, Bernd M. Fischer,  Helmut Cölfen, Martin Koch,

http://onlinelibrary.wiley.com/doi/10.1002/chem.201702218/full#references


Crystal formation is a highly debated problem. This report shows that the crystallization of l-(+)-tartaric acid from water follows a non-classical path involving intermediate hydrated states. Analytical ultracentrifugation indicates solution clusters of the initial stages aggregate to form an early intermediate. Terahertz spectroscopy performed during water evaporation highlights a transient increase in the absorption during nucleation; this indicates the recurrence of water molecules that are expelled from the intermediate phase. Besides, a transient resonance at 750 GHz, which can be assigned to a natural vibration of large hydrated aggregates, vanishes after the final crystal has formed. Furthermore, THz data reveal the vibration of nanosized clusters in the dilute solution indicated by analytical ultracentrifugation. Infrared spectroscopy and wide-angle X-ray scattering highlight that the intermediate is not a crystalline hydrate. These results demonstrate that nanoscopic intermediate units assemble to form the first solvent-free crystalline nuclei upon dehydration.

Abstract-Terahertz near-field imaging of dielectric resonators

Wendy S. L. Lee, Korbinian Kaltenecker, Shruti Nirantar, Withawat Withayachumnankul, Markus Walther, Madhu Bhaskaran, Bernd M. Fischer, Sharath Sriram, and Christophe Fumeaux

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-4-3756

As an alternative to metallic resonators, dielectric resonators can increase radiation efficiencies of metasurfaces at terahertz frequencies. Such subwavelength resonators made from low-loss dielectric materials operate on the basis of oscillating displacement currents. For full control of electromagnetic waves, it is essential that dielectric resonators operate around their resonant modes. Thus, understanding the nature of these resonances is crucial towards design implementation. To this end, an array of silicon resonators on a quartz substrate is designed to operate in transmission at terahertz frequencies. The resonator dimensions are tailored to observe their low-order modes of resonance at 0.58 THz and 0.61 THz respectively. We employ a terahertz near-field imaging technique to measure the complex near-fields of this dielectric resonator array. This unique method allows direct experimental observation of the first two fundamental resonances.

© 2017 Optical Society of America

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Abstract-Carbon Nanotube concentration and distribution determination with terahertz waves

Ole Peters ;  Bernd M. Fischer ;  Martin Koch

http://ieeexplore.ieee.org/document/6104861/?reload=true

Carbon Nanotubes show high potential as an additive for polymers to improve mechanical properties and conductivity. For their employment in compounds, a homogenous distribution is crutial. We present THz spectroscopy as a tool for Nanotube content and homogeneity measurements.

Abstract-Gouy phase shift of a tightly focused, radially polarized beam

Korbinian J. Kaltnenecker, Jacob C. König-Otto, Martin Mittendorff, Stephan Winnerl, Harald Schneider, Manfred Helm, Hanspeter Helm, Markus Walther, and Bernd M. Fischer

https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-3-1-35&id=335319

Radially polarized beams represent an important member of the family of vector beams, in particular due to the possibility of using them to create strong and tightly focused longitudinal fields, a fundamental property that has been exploited by applications ranging from microscopy to particle acceleration. Since the properties of such a focused beam are intimately related to the Gouy phase shift, proper knowledge of its behavior is crucial. Terahertz microscopic imaging is used to extract the Gouy phase shift of the transverse and longitudinal field components of a tightly focused, radially polarized beam. Since the applied terahertz time-domain approach is capable of mapping the amplitude and phase of an electromagnetic wave in space, we are able to directly trace the evolution of the geometric phase as the wave propagates through the focus. We observe a Gouy phase shift of 2𝜋 for the transverse and of 𝜋 for the longitudinal component. Our experimental procedure is universal and may be applied to determine the geometric phase of other vector beams, such as optical vortices, or even arbitrarily shaped and polarized propagating waves.

© 2016 Optical Society of America

Terahertz Imaging Modalities of Ancient Egyptian Mummified Objects and of a Naturally Mummified Rat

The Anatomical Record

Volume 298,  Issue 6, pages 1135–1143, June 2015

1. Lena Öhrström¹, 
2. Bernd M. Fischer^2,3, 
3. Andreas Bitzer^4,5, 
4. Jan Wallauer^4,5, 
5. Markus Walther^4,5 and
6. Frank Rühli^1,*

Article first published online: 22 MAY 2015

DOI: 10.1002/ar.23143

© 2015 Wiley Periodicals, Inc.

Issue The Anatomical Record The Anatomical Record Volume 298, Issue 6, pages 1135–1143, June 2015

During the last few years, terahertz (THz) imaging has been used to investigate artwork and historic artifacts. The application of THz imaging to mummy investigations is very attractive since it provides spectroscopic information over a broad frequency range and its radiation has proven to be harmless to human cells. However, compared with the current standard imaging methods in mummy imaging—X-ray and computed tomography (CT)—it remains a novel, emerging technique whose potential still needs to be fully evaluated. Here, ancient Egyptian mummified objects as well as a naturally mummified rat have been investigated by two different THz imaging systems: a broadband THz time domain imaging system and an electronic THz scanner. The obtained THz images are compared with conventional CT, X-ray, and magnetic resonance images. While the broadband THz time domain setup permits analyses of smaller samples, the electronic THz scanner allows the recording of data of thicker and larger samples at the expense of a limited spectral bandwidth. Terahertz imaging shows clear potential for mummy investigations, although currently CT imaging offers much higher spatial resolution. Furthermore, as commercial mobile THz scanners become available, THz imaging could be applied directly in museums or at excavation sites. Anat Rec, 298:1135–1143, 2015. © 2015 Wiley Periodicals, Inc.

Abstract-Metamaterial fibres for subdiffraction imaging and focusing at terahertz frequencies over optically long distances

• Alessandro Tuniz,
• Korbinian J. Kaltenecker,
• Bernd M. Fischer,
• Markus Walther,
• Simon C. Fleming,
• Alexander Argyros
• & Boris T. Kuhlmey

• http://www.nature.com/ncomms/2013/131028/ncomms3706/full/ncomms3706.html

Using conventional materials, the resolution of focusing and imaging devices is limited by diffraction to about half the wavelength of light, as high spatial frequencies do not propagate in isotropic materials. Wire array metamaterials, because of their extreme anisotropy, can beat this limit; however, focusing with these has only been demonstrated up to microwave frequencies and using propagation over a few wavelengths only. Here we show that the principle can be scaled to frequencies orders of magnitudes higher and to considerably longer propagation lengths. We demonstrate imaging through straight and tapered wire arrays operating in the terahertz spectrum, with unprecedented propagation of near field information over hundreds of wavelengths and focusing down to 1/28 of the wavelength with a net increase in power density. Applications could include in vivo terahertz-endoscopes with resolution compatible with imaging individual cells.