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

It has always been a dream of physicists and chemists to follow temporal variations of molecular geometry during a chemical reaction in real time, to “film” them in a way similar as in the everyday life. Unfortunately, chemical events take place on tiny time scales comprised between 10 fs and 100 ps, approximately. Visualizing atomic motions thus remained a dream over two centuries. This is no longer true today, consequence of an immense instrumental development the last decades. Two methods are particularly important. The first of them is ultrafast optical spectroscopy employing the recently developed laser technology. In his breakthrough work A. Zewail was able to show how can this method be used to follow the photoelectric dissociation of gaseous ICN in real time[1,2]. It has later been applied to several other problems, and particularly so to visualize OH..O motions in liquid water[3,4]. Unfortunately, visible light interacts predominantly with outer shell rather than with deeper lying core electrons that most directly indicate molecular geometry. It is thus difficult to convert spectral data into data on molecular geometry. The second method refers to time resolved x-ray diffraction and absorption. As x-rays interact predominantly with deeply lying core electrons which are tightly bonded to the nuclei, converting x-ray data into data relative to molecular geometry is, in principle at least, much more straightforward than in optical spectroscopy. Unfortunately, pulsed x-rays techniques are technically very demanding.

Real Time Visualization of Atomic Motions in Dense Phases

Carrying digital information in traditional copper wires is becoming a major issue in electronic circuits. Optical connections such as fiber optics offers unprecedented transfer capacity, but the mismatch between the optical wavelength and the transistors size drastically reduces the coupling efficiency. By merging the abilities of photonics and electronics, surface plasmon photonics, or 'plasmonics' exhibits strong potential. Here, we propose an original approach to fully understand the nature of surface electrons in plasmonic systems, by experimentally demonstrating that surface plasmons can be modeled as a phase of surface waves. First and second order phase transitions, associated with percolation transitions, have been experimentally observed in the building process of surface plasmons in lattice of subwavelength apertures. Percolation theory provides a unified framework for surface plasmons description.

Experimental evidence of percolation phase transition in surface plasmons generation

We describe a highly sensitive and stable quantum-cascade-laser-based attenuated total reflection (ATR) terahertz sensor for the detection of very low concentration solutions, using a dual-modulation differential approach and ATR geometry. This sensor offers a very high dynamic range and a long-term stability of 40 dB, which extends the potential of terahertz radiation for the analysis of liquid and biological samples. The performance is illustrated by measurements on standard solutions of ions, sugars, and proteins, for concentrations down to 1 µM.

High precision dual-modulation differential terahertz ATR sensor for liquid measurements.

Quasi-optical techniques have been used to efficiently couple freely propagating subpsec pulses of THz electromagnetic radiation into rectangular metal waveguides. We have observed dispersive, low-loss propagation over the frequency band from 0.65 to 3.5 1Hz with typical waveguide cross-section dimensions of the order of 250 μm and lengths of 25 mm. We demonstrate both theoretically and experimentally that it is possible to achieve efficient TE10 single-mode coupling and propagation in a suitably sized rectangular waveguide for an incoming focused, linearly polarized 1Hz pulse with a bandwidth covering many octaves in frequency and overlapping more than 35 waveguide modes. The possible applications enabled by these results will be discussed.

Ultrafast Applications of THz Waveguides

Summary form only given. THz-TDS has now been established as a powerful technique for THz (far-infrared) spectroscopy. To date, THz-TDS applications have predominantly utilized photoconductive antenna for the receiver. However, electro-optic (EO) THz receivers have demonstrated exceptionally high frequency performance, but without continuous spectral coverage due to the complications of strong absorption and dispersion of the electro-optic crystal, phase mismatch between the interrogating light pulse and the propagating THz pulse and a strong frequency dependence of the EO susceptibility. We present a frequency-domain theoretical description of the complete EO detection process, including a frequency-dependent EO nonlinearity. Our result, which only requires the evaluation of a Fourier integral, shows that THz-TDS is also applicable with EO detection, independent of the extensive time-domain pulse distortion of the actual EO measurement.

Guilhem Gallot

Papers

Real Time Visualization of Atomic Motions in Dense Phases

Experimental evidence of percolation phase transition in surface plasmons generation

High precision dual-modulation differential terahertz ATR sensor for liquid measurements.

Ultrafast Applications of THz Waveguides

THz time-domain spectroscopy (THz-TDS) with electro-optic detection