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

An infrared laser spectroscopic experiment is described permitting a visualization of molecular rotations in liquid water. One first selects hydrogen bonds of a given OH..O length by pumping the system with an appropriate infrared frequency; the rotational dynamics are analyzed next by measuring its rotational anisotropy. The procedure requires a careful theoretical analysis, realized by adopting the correlation function approach. This experiment also permits to visualize the passage from the inertial rotation to the rotational diffusion. The lifetime of OH..O bonds in water is rotation-limited.

Laser spectroscopic visualization of molecular rotations in liquid water

We demonstrated the direct and noninvasive imaging of functional neurons, 1 as well as auricular heart muscle
electrical activity 2 by Ionic Contrast Terahertz (ICT) near-field microscopy. This technique provides quantitative
measurements of ionic concentrations in both the intracellular and extracellular compartments and opens
the way to direct noninvasive imaging of neurons during electrical, toxin, or thermal stresses. Furthermore,
neuronal activity results from both a precise control of transient variations in ionic conductances and a much
less studied water exchange between the extracellular matrix and the intraaxonal compartment. The developed
ICT technique associated with a full three-dimensional simulation of the axon-aperture near-field system allows
a precise measurement of the axon geometry and therefore the direct visualization of neuron swelling induced by
temperature change or neurotoxin poisoning. This technique should then provide grounds for the development
of advanced functional neuroimaging methods based on diffusion anisotropy of water molecules.

Ionic contrast terahertz near-field imaging of axonal activity and water fluxes

Using attenuated total reflection (ATR) in the terahertz domain [1], we demonstrate non-invasive, non-staining real time measurements of cytoplasm leakage during permeabilization of live epithelial cells. We show that terahertz modalities are more sensitive than fluorescence microscopy which is the reference optical technique for electropermeabilization. We also propose analytical models for the influx and efflux of non-permeant molecules through permeabilized cell membranes by electropermeabilization or by detergent.

Guilhem Gallot

Papers

Laser spectroscopic visualization of molecular rotations in liquid water

Ionic contrast terahertz near-field imaging of axonal activity and water fluxes

Probing Living Cells by Terahertz Attenuated Total Reflection. Application to Permeabilization Dynamics