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Wednesday, April 16, 2014

Synopsis: High-Precision Terahertz Spectroscopy



Frequency-Comb-Assisted Terahertz Quantum Cascade Laser Spectroscopy

S. Bartalini, L. Consolino, P. Cancio, P. De Natale, P. Bartolini, A. Taschin, M. De Pas, H. Beere, D. Ritchie, M. S. Vitiello, and R. Torre
Published April 9, 2014

Trace-gas sensing with high sensitivity and precision in the terahertz regime can be important in environmental monitoring, security, and astrophysics, as well as in tests of fundamental physics. Now, as reported in Physical Review X, a research team has performed the first terahertz spectroscopic measurements using a so-called frequency comb—a technique that allows frequency measurements with extremely high accuracy. As a proof-of-principle, the team measured a rotational transition in a gas molecule (methanol) to a precision of 4 parts in one billion, 10 times better than the previous record. The result is also twice as precise as the theoretically predicted frequency, suggesting the technique could help refine theoretical models.
Saverio Bartalini of the Italian National Institute of Optics (INO-CNR) and the European Laboratory for Non-linear Spectroscopy (LENS) and his colleagues have taken a terahertz system they previously developed and used it for spectroscopy. The researchers focused near-infrared laser pulses into a nonlinear crystal to produce a terahertz comb—a single beam containing thousands of discrete and closely spaced frequencies of light. The comb is referenced to a cesium atomic clock. To provide enough intensity for spectroscopy, they “phase locked” a quantum cascade laser to one of the comb’s “teeth.” The result is an ultrastable source with which they can measure the absorption of a gas sample as they slowly vary the laser frequency. With some simple improvements, the authors believe they can further boost their measurement precision by a factor of 100.  David Ehrenstein


Presentation-Graphene Plasmonics and Terahertz Photonics


Tuesday, April 29, 2014 - 3:30pm
Regents 109
http://www.physics.georgetown.edu/colloquia/2014/4/29/graphene-plasmonics-and-terahertz-photonics

The experimental discovery of two-dimensional (2D) gated graphene in 2004 by Novoselov and Geim is a seminal event in electronic materials science, ushering in a tremendous outburst of scientific activity in the study of electronic properties of this unique two-dimensional material with a gapless Dirac electronic spectrum. The lack of a traditional bandgap makes graphene an exceptionally versatile photonic material, and the ability to dope graphene through metallic contacts and tune the carrier density through the application of a gate opens possibilities for a variety of transformative photonic devices. In particular highly doped graphene has recently been recognized as a powerful plasmonic material that combines many important properties at terahertz (THz) frequencies with the ability of being electrically tunable. Terahertz radiation has uses from security to medicine. Currently, however, THz technology is notoriously underdeveloped. Graphene plasmonics has promise of filling in this conspicuous gap in the electromagnetic spectrum with a robust and radically new technology. Recently, sensitive room temperature THz detectors have been demonstrated that operate on a photo-thermo-electric principle with response times of 10s of femtoseconds. THz absorption in a graphene element raises the temperature of the graphene carriers, which then diffuse to the contacts made of dissimilar metals and produces a photo voltage proportional to the Seebeck coefficient of the graphene. A source of THz radiation based on this photo-thermo-electric effect also looks promising. A graphene element is used as an optical mixer of near IR to generate THz plasmons which are then coupled to free space radiation by an antenna. A review of graphene and these THz developments will be described.

Host: 
 Paola Barbara
Discussion Leader: 
 Paola Barbara

Thesis-Magneto-photonic phenomena at terahertz frequencies



Mostafa Shalaby, 
http://cds.cern.ch/record/1695545
Magneto-terahertz phenomena are the main focus of the thesis. This work started as supporting research for the science of an X-ray laser (SwissFEL). X-ray lasers have recently drawn great attention as an unprecedented tool for scientific research on the ultrafast scale..... To answer this fundamental question, we performed original numerical simulations using a coupled Landau- Lifshitz-Gilbert Maxwell model. ... Those requirements were the motivations for the experiments performed in the second part of the thesis. To shape the terahertz pulses, .... Regarding the field intensities, we followed two approaches. The first deals with field enhancement in nanoslits arrays. We designed a subwavelength structure characterized by simultaneous high field enhancement and high transmission at terahertz frequencies to suit nonlinear sources. The second approach depended on up-scaling the generation from laser-induced plasma by increasing the pump wavelengths. Numerical calculations have also brought to our attention the importance of linear magnetoterahertz effects. In particular, the simulations showed that the ultrafast dynamics could lead to significant rotation of the polarization plane of the triggering terahertz pulse. Motivated by this finding, we focused in the last part of the thesis on the linear effects. We performed three original studies coming out with first demonstrations of broadband non-reciprocal terahertz phase retarders, terahertz magnetic modulators, and the non-reciprocal terahertz isolators. In the first two experiments, we extended the unique properties of the magnetic liquids (Ferrofluids) to the terahertz regime. In the latter experiment, we used a permanent magnet (Ferrite) to experimentally show complete isolation (unidirectional transmission) of the terahertz waves.


Abstract- Chemometrics applied to quantitative analysis of ternary mixtures by Terahertz spectroscopy



Josette EL HADDAD Frederick De Miollis Joyce 
Anal. Chem., Just Accepted Manuscript
DOI: 10.1021/ac500253b
Publication Date (Web): April 16, 2014
Copyright © 2014 American Chemical Society
http://pubs.acs.org/doi/abs/10.1021/ac500253b
Chemometrics was applied to qualitative and quantitative analyses of terahertz spectra obtained in transmission mode. A series of mixtures of three pure products, namely citric acid, D-(-) fructose and α-lactose monohydrate under various concentrations were prepared as pressed pellets with polyethylene as binder. Then terahertz absorbance spectra were recorded by terahertz time domain spectroscopy and analyzed. Firstly, principal component analysis allowed to correctly locating the samples into a ternary diagram. Secondly, quantitative analysis was achieved by partial least square (PLS) regression and artificial neural networks (ANN). The concentrations were predicted with values of relative mean square error lower than 0.9 % for the three constituents. As a conclusion, chemometrics was demonstrated to be very efficient for the analysis of the ternary mixtures prepared for this study.

Tuesday, April 15, 2014

AC/DC for terahertz waves - rectification with picosecond clock rates


MBI – 15.04.2014:

AC/DC for terahertz waves - rectification with picosecond clock rates

Researchers at the Max-Born-Institute in Berlin, Germany discover an ultrafast rectifier for terahertz radiation. In the unit cells of a lithium niobate crystal alternating currents (AC) with a frequency 1000 times higher than that of modern computer systems are transformed into a direct current (DC), thereby generating simultaneously a series of overtones of the terahertz radiation.
When the guitarist Angus Young of the Australian hard rock band AC/DC touches the strings of his electric guitar, a strongly distorted sound rings out from the loudspeaker. The origin of the electronically generated overtones is the rectifying effect in the electronic tubes of the guitar amplifier. In the simplest case an (A)lternating (C)urrent generates a (D)irect (C)urrent, an effect which finds its application in telecommunications at much higher radio or mobile phone frequencies. From a physics point of view the highly interesting question arises: up to which cut-off frequencies can one generate directed currents (DC) and which microscopic mechanisms underlie them?
For the generation of a direct current out of alternating currents the material used must feature a preferred direction. This condition is fulfilled by ferroelectric crystals, in which the spatial separation of positively and negatively charged ions is connected to a static electric polarization. Most ferroelectrics are electric insulators, i.e., low electric fields cannot cause any detectable electric currents in the material. A drastic change of this behavior is observed if one applies for a short period an extremely high electric field in the range of several 100.000 volts per centimeter. At such field strengths, bound electrons, the so called valence electrons can be freed for a short period by means of the quantum mechanical tunneling process leading in turn to a current through the crystal.
Now, researchers at the Max-Born-Institute in Berlin, Germany investigated the properties of such a current for the first time and report their results in the current issue of the journal Physical Review Letters 112.146602 (2014)). Using ultrashort, intense terahertz pulses (1 Terahertz = 1012 Hz, period of a field oscillation 1 picosecond=10-12 seconds) they applied an AC field to a thin lithium niobate (LiNbO3) crystal which causes an electric current in the material. The properties of this current were studied in detail by measuring and analyzing the electric field radiated by the accelerated electrons. Besides an oscillating current with the frequency of the applied terahertz field (2 THz) and several overtones of the latter, the researchers observed the signature of a directed current (DC) along the c-axis the preferred direction of the ferroelectric LiNbO3 crystal.
The rectified current along the ferroelectric c-axis has its origin in the interplay of quantum mechanical tunneling of electrons between the valence and several conduction bands of the LiNbO3 crystal and the deceleration of electrons by friction processes. The tunneling process generates free electrons which in absence of friction would spatially oscillate in time with the applied terahertz field. The friction destroys this oscillatory motion, a mechanism called decoherence. Due to the asymmetry of the tunneling barrier along the ferroelectric c-axis decoherence results in a spatially asymmetric transport, i.e., the tunneling barrier lets pass more electrons from right to left than from left to right. This mechanism is operative within each unit cell of the crystal, i.e., on a sub-nanometer length scale, and causes the rectification of the terahertz field. The effect can be exploited at even higher frequencies, offering novel interesting applications in high frequency electronics.
Fig. 1: Experiment: The high electric field of the intense terahertz pulse accelerates electrons in a lithium niobate LiNbO3 crystal. The hexagonal unit cell contains lithium atoms (green spheres), niobium atoms (blue spheres), and oxygen atoms (red spheres) the latter being arranged on the corners of a unit cell. The crystal lacks inversion symmetry and, thus, shows a ferroelectric polarization along the c-axis.

Fig. 2: During transport along the c-axis, electrons see alternating different distances between lithium and niobium atoms. Moreover, the niobium atoms are not in the center of the oxygen octahedrons. Such geometry leads to asymmetric barriers the electrons have to pass by quantum mechanical tunneling when moving along the c-axis. The electrons are driven through the barriers by the high terahertz AC field. The barrier asymmetry together with decoherence/friction processes result in a spatially asymmetric transport, i.e., the rectification to a DC current.
AC/DC for terahertz waves Fig. 2










Fig. 2 | Fig.: MBI

Moviehttp://www.mbi-berlin.de/en/current/index.html


Original article

C. Somma, K. Reimann, C. Flytzanis, T. Elsaesser, und M. Woerner: High-Field Terahertz Bulk Photovoltaic Effect in Lithium Niobate
Physical Review Letters 112.146602 (2014)

Contact

Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (MBI)
Dr. Michael Wörner, woerner@mbi-berlin.de, Tel.: 0049 30 6392 1470
Carmine Somma, somma@mbi-berlin.de, Tel.: 0049 30 6392 1474
Prof. Dr. Thomas Elsässer, elsaesser@mbi-berlin.de, Tel.: 0049 30 6392 1400