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

We demonstrated the direct and noninvasive imaging of functional neurons 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 conductance 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. We
also developed Terahertz Attenuated Total Reflection (ATR) devices perfectly suited for studying cell layers.
Inserted in a terahertz time-domain system, and using a high resistivity low loss silicon prism to couple the
terahertz wave into the sample, the detection scheme is based on the relative differential spectral phase of two
orthogonal polarizations. Biological sample imaging as well as subwavelength (λ/16) longitudinal resolution are
demonstrated.

Ionic contrast terahertz near field imaging

In this paper we study the rise to the surface, radial motion and disappearance of the bubbles created by a water jet pouring into a container, in particular their density at the surface as a function of the impact velocity. We first focus on their emergence radius at the surface which follows a log-normal distribution. Next, we establish experimentally a law relating the bubble velocity to the distance to the jet. We also investigate their disappearance, caused at low density mainly by explosion, and at high density predominantly by coalescence. Finally, we build an accurate model for the density of bubbles.

/pdf/spatial-distribution-of-air-bubbles-created-by-an-imping-2iyfdial.pdf

Spatial distribution of air bubbles created by an imping water jet into a free water surface

We present a new Terahertz imaging scheme based on total internal reflection that allows the study of aqueous samples while improving the longitudinal resolution. We were able to image a frog neuron using the spectral phase of the THz pulses using this technique.

Total internal reflection Terahertz imaging

We present two polarizing elements designed to produce linear and circular polarization for broadband, pulsed THz waves The linear polarizer is made out silicon wafers arranged at Brewster's angle and the circular polarizer is made out of a silicon prism using total internal reflection differential phase-shift effect

Achromatic polarizing elements for pulsed THz waves

The coupling between the surface plasmons of two overlapping arrays of orthogonally oriented subwavelength elliptical holes has been demonstrated by terahertz spectroscopy. Experimental data are compared to a simple theoretical model and numerical simulations.

Guilhem Gallot

Papers

Ionic contrast terahertz near field imaging

Spatial distribution of air bubbles created by an imping water jet into a free water surface

Total internal reflection Terahertz imaging

Achromatic polarizing elements for pulsed THz waves

Coupling between surface plasmons in subwavelength hole arrays