Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time-resolved detection of the electric field of ultrashort radiation bursts in the far-infrared, this technique has become known as terahertz time-domain spectroscopy (THz-TDS). In this review article the authors describe the technique in its various implementations for static and time-resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid-state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.

Terahertz spectroscopy and imaging – Modern techniques and applications

This review provides a perspective on the recent developments in the transmission of light through subwavelength apertures in metal films. The main focus is on the phenomenon of extraordinary optical transmission in periodic hole arrays, discovered over a decade ago. It is shown that surface electromagnetic modes play a key role in the emergence of the resonant transmission. These modes are also shown to be at the root of both the enhanced transmission and beaming of light found in single apertures surrounded by periodic corrugations. This review describes both the theoretical and experimental aspects of the subject. For clarity, the physical mechanisms operating in the different structures considered are analyzed within a common theoretical framework. Several applications based on the transmission properties of subwavelength apertures are also addressed.

http://digital.csic.es/bitstream/10261/47904/1/Light%20passing.pdf

Light passing through subwavelength apertures

Sources and systems for far-infrared or terahertz (1 THz = 10(12) Hz) radiation have received extensive attention in recent years, with applications in sensing, imaging and spectroscopy. Terahertz radiation bridges the gap between the microwave and optical regimes, and offers significant scientific and technological potential in many fields. However, waveguiding in this intermediate spectral region still remains a challenge. Neither conventional metal waveguides for microwave radiation, nor dielectric fibres for visible and near-infrared radiation can be used to guide terahertz waves over a long distance, owing to the high loss from the finite conductivity of metals or the high absorption coefficient of dielectric materials in this spectral range. Furthermore, the extensive use of broadband pulses in the terahertz regime imposes an additional constraint of low dispersion, which is necessary for compatibility with spectroscopic applications. Here we show how a simple waveguide, namely a bare metal wire, can be used to transport terahertz pulses with virtually no dispersion, low attenuation, and with remarkable structural simplicity. As an example of this new waveguiding structure, we demonstrate an endoscope for terahertz pulses.

Metal wires for terahertz wave guiding

Despite long study, a molecular picture of the mechanism of water reorientation is still lacking Using numerical simulations, we find support for a pathway in which the rotating water molecule breaks a hydrogen bond (H-bond) with an overcoordinated first-shell neighbor to form an H-bond with an undercoordinated second-shell neighbor The H-bond cleavage and the molecular reorientation occur concertedly and not successively as usually considered This water reorientation mechanism involves large-amplitude angular jumps, rather than the commonly accepted sequence of small diffusive steps, and therefore calls for reinterpretation of many experimental data wherein water rotational relaxation is assumed to be diffusive

https://science.sciencemag.org/content/sci/311/5762/832.full.pdf

A molecular jump mechanism of water reorientation.

Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

https://iopscience.iop.org/article/10.1088/0034-4885/70/8/R02/pdf

Imaging with terahertz radiation

The properties of the solvation shell surrounding biomolecules in a solution are fundamental to understand the modifications in the dynamics of the water molecules by peptides and proteins. The dynamics of the hydrogen bonding network typically occurs at the picosecond time scale, so terahertz spectroscopy is a unique tool to investigate the solvation shell. Here, we present the terahertz measurements of the refractive index and extinction coefficient of solutions of biomolecules of various molecular weights. We observe a clear correlation between the terahertz dielectric properties and the weight of the molecules. A three-component model is developed to analyze the relative contributions of the solute and the solvation shell to the total dielectric values. We find that the amino acids and short peptides (small molecules) domains are mainly governed by the solvation shell, whereas the solute properties are also implied in the protein domain (big molecules).

Relative Contributions of Core Protein and Solvation Shell in the Terahertz Dielectric Properties of Protein Solutions.

Infra-red spectroscopy of hydrogen bonded systems is shortly reviewed Both experimental and theoretical aspects of the problem are covered, but the accent is put on the theory The first part of the paper refers to the classical, whereas its second part covers the time-resolved spectroscopy of the OH stretching band Modern theories of vibrational band shapes are presented; they are all based on the density matrix approach of statistical mechanics and employ the correlation function formalism Using this technique, molecular dynamics of hydrogen bonds can be monitored up to time scales of the order of a few tens of femtoseconds

Infra-Red Spectra of Hydrogen Bonded Systems: Theory and Experiment

https://hal.archives-ouvertes.fr/hal-02928129/document

Dynamics of Cell Membrane Permeabilization by Saponins Using Terahertz Attenuated Total Reflection.

We developed an extended Fano model describing the Extraordinary Electromagnetic Transmission (EET) through arrays of subwavelength apertures, based on terahertz transmission measurements of arrays of various hole size and shapes. Considering a frequency-dependent coupling between resonant and non-resonant pathways, this model gives access to a simple analytical description of EET, provides good agreement with experimental data, and offers new parameters describing the influence of the hole size and shape on the transmitted signal.

Extended Fano model of Extraordinary Electromagnetic Transmission through subwavelength hole arrays in the terahertz domain.

We demonstrate that in near field imaging, interaction between light and sample can be divided into two main areas: the true near field and the contrast near field domain. We performed extensive numerical simulations in order to identify the limits of these areas, and to investigate contrast near field imaging in which much easier propagation calculation can be achieved. Finally, we show an application with terahertz axonal imaging.

https://hal-polytechnique.archives-ouvertes.fr/hal-00868228/document

Guilhem Gallot

Papers

Relative Contributions of Core Protein and Solvation Shell in the Terahertz Dielectric Properties of Protein Solutions.

Infra-Red Spectra of Hydrogen Bonded Systems: Theory and Experiment

Dynamics of Cell Membrane Permeabilization by Saponins Using Terahertz Attenuated Total Reflection.

Extended Fano model of Extraordinary Electromagnetic Transmission through subwavelength hole arrays in the terahertz domain.

True near field versus contrast near field imaging