New Scientific Possibilities
with High Power THz Sources
29 - 30 June 2006 ~ Holiday Inn Runcorn

Learning New Chemistry and Physics with THz Light

ABSTRACT: Terahertz spectroscopy emerged about 14 years ago with the demonstration that nearly single cycle pulses of far-infrared (FIR) radiation could be generated, sent through free space, and subsequently detected in the time-domain. Since then, THz spectroscopy has become an active area with studies ranging from condensed matter physics to gas-phase spectroscopy to biomedical imaging. One of the most unique aspects of THz spectroscopy is that the pulses are of sub-picosecond duration, and it is possible to carry out time-dependent measurements in the FIR on a ultrafast timescales. After a brief description of THz methodology, I will present an overview of optical pump – THz probe spectroscopy. Topics might include: solvent dynamics, transient photoconductivity in CdSe quantum dots, and transient photoconductivity in colloidal nanocrystalline TiO2. If time permits, I may also describe THz emission experiments of intramolecular electron transfer in oriented dye molecules and/or ultrafast demagnetization. Finally, I will reserve 10 to 15 minutes to discuss unique experiments that could be carried out with a high power THz source.

THz Photonics Using Ambient Air*

ABSTRACT: Due to severe water vapor attenuation, the attenuation for THz waves in the atmosphere is higher than 100 dB/km. Consequently, it has been previously considered to be impossible to perform long distance broadband THz wave sensing and spectroscopy.

Ambient air, with a composition of about 78% nitrogen and 21% oxygen, exhibits remarkable performance in the THz photonics through the use of femtosecond laser beams. I will report recent development of nonlinear THz photonics, which generate and detect pulsed THz waves through the use of ambient air as an emitter and sensor. Combined with the demonstrated THz wave emission from ambient air or laser-induced air plasma, this all-air and all-optical approach will enable remote THz wave sensing and spectroscopy in a high humidity atmospheric environment.

The nonlinear THz photonic approach enables remote THz sensing by sending an optical beam to generate and detect THz waves locally, utilizing a lower attenuation at the visible range (<0.01 dB/km). Perhaps the greatest advantages to using air as a THz wave sensor is the flexibility of selecting a sensing location in even the most complicated weather conditions, since air is one of the most readily available resources in free space. Theoretical and experimental results for sensing and spectroscopy applications in ambient air, ionized air, and selected gases will also be presented.

CW THz Spectroscopy of Peptide Nanotubes and Co-solvated Crystals

ABSTRACT: THz radiation interrogates the lowest frequency vibrational modes of the biomolecule. For crystalline solids, these modes include lattice translations, librational (intermolecular) and intramolecular modes. We have obtained THz absorption spectra for a series of small peptides to explore the sensitivity of these nuclear motions to the peptide hydrogen bonding network and the presence of co-crystallized solvents. Crystalline tri-alanine represents a theoretically tractable and diverse system for study because i) the parallel and anti-parallel ß-sheets forms are easily grown ii) the co-crystallized water in the ap sheet is easily removed under vacuum. The THz spectra of the three forms (as well as other amino acids) are shown to be vastly different, illustrating the extreme sensitivity in this region to residue sequence, ß-sheet structure and hydration levels. Results for a series of dipeptides that form nanotubes having either a hydrophobic or hydrophilic core also will be discussed. THz spectra are shown to be sensitive to tube diameter and to various trapped solvents in the tubes. Quantum chemical (DFT/PW91) calculations performed on periodic systems have served to identify the nuclear motions probed in the THz region and to help elucidate the principal deficiencies in classical force field models like CHARMM.
¹National Institute of Standards and Technology, Optical Technology Division, Physics Laboratory, Gaithersburg, MD 20899 USA

Probing THz Dynamics in Devices and Materials with the UCSB Free-electron Laser - From Semiconductor Quantum Structures to Proteins

ABSTRACT: Tunable, high power from the UCSB free-electron lasers (140 GHz to 4.8 THz, 500 W to 10kWatts) is used to explore the terahertz dynamics of devices and materials. The sources are used to full advantage to drive quantum structures far-from equilibrium with intense terahertz radiation. Strong terahertz fields dramatically modify the electrical and optical properties of semiconductor quantum structures; the most striking phenomena are terahertz photon assisted transport and quantum coherent interference of terahertz and optical beams in excitonic systems. Equally important but less dramatic is linear terahertz spectroscopy in systems where high power is used to overcome very low through put. Here we focus on terahertz waveguide spectroscopy of semiconductor quantum structures and recent work on terahertz absorption spectra of proteins in liquid water.

Terahertz research activities at Novosibirsk free electron laser

ABSTRACT: The first stage of Novosiborsk free electron laser emerges monochromatic radiation within spectral range from 0.12 to 0.24 mm with maximum average power 400 W. Some portion of radiation of second and third harmonics is also detected. Second stage of the facility, which is to be constructed in couple of years, will generate radiation in the spectral region of 5 – 300 micrometers at substantially higher power. Present status of the facility and a brief discription of the selected results obtained by users will be presented in this talk. Nowaday, terahertz free electron laser became a user facility. The operational time is distributed between FEL team for works on study and improvement of laser characteristics and internal and external users. Several groups from reseach institutes of Siberian Branch of Russian Academy of Science already started their experiments on the facility. User experiments cover come fieldes of biology, chemistry, semiconductor physics and technology, solis-state physics, detector development, imaging and holography. Several groups are designing equipment to begin experiments in near future, in particular, in atmosphere study and aerodynamics.

THz Scanning Near-field Microspectroscopy Employing Coherent Synchrotron Radiation

ABSTRACT: The spatial resolution of conventional THz imaging systems is diffraction-limited and thus only features with a dimension from hundred micrometers to millimetres are resolvable. This limit can be overcome by utilising near-field imaging techniques to achiev sub-wavelength spatial resolutions. However, extremely brilliant sources are necessary to compensate for intensity losses since most techniques employ apertures to confine the THz radiation at cost of total power.

A THz near-field imaging technique which benefits from the broadband and highly brilliant coherent synchrotron radiation from an electron storage ring is established at BESSY utilizing a detection method based on locking on to the intrinsic time structure of the synchrotron radiation. Together with a Martin-Puplett spectrometer the technique of THz scanning near-field infrared microscopy presented enables spectroscopic mapping of samples at a spatial resolution well below the diffraction limit. Different types of conical near-field probe are discussed. The potential of the technique is exemplified by imaging biological samples.

U. Schade, K. Holldack, G. Staats, D. Schondelmaier

A proposed novel multiplexed near field Terahertz microscope

ABSTRACT: There is burgeoning interest in the possibility that ‘terahertz’ measurements will yield novel biological information. This relates primarily to the fact that proteins and other large molecules have many low frequency modes which fall in the terahertz region and which may give structural information. Additionally there is interest in possible measurements of ‘mechanical’ resonance modes of membranes in structures such as liposomes.

A fundamental problem with terahertz measurements is that, since one terahertz has a wavelength of three hundred microns, diffraction limited imaging can only yield a resolution of the order of half a millimetre, too large for items of biological interest. The solution is well known – scanning near field microscopy – which can yield resolution far below the wavelength. However the solution comes with a problem, namely very slow data rates and hence very long imaging times.

We describe a proposed multiplexed near field microscope which can collect fully spectrally resolved information on up to ~100 pixels simultaneously using one high quality detector. Resolution of a few microns appears feasible (possibly rather better at the top end of the terahertz range.) The microscope is ideally suited for use with coherently enhanced synchrotron radiation sources provided they run in CW mode.

H.N. Rutt, M.M. Al-Makim

Scientific possibilities for a THz facility to study vibrationally mediated enzyme action

ABSTRACT: The physical basis of the catalytic power of enzymes remains contentious despite sustained and intensive research efforts.  The question of whether enzymes have evolved to use quantum tunnelling to the best advantage has provoked a heated debate and is currently a hot topic in the study of enzyme mechanism. That tunnelling occurs is now widely accepted, with conceptual frameworks incorporating protein motion into the enzymic H-tunnelling process. More controversial is a role for compressive motion to promote H tunnelling, a property of enzymes that might be endowed by evolutionary pressure. Some studies have suggested a role for protein motion in facilitating tunnelling. Kinetic studies are in agreement with environmentally coupled models of H tunnelling, but atomic-level insight into the tunnelling event and associated protein motions can only be inferred from high-resolution protein structures and computational analysis. A THz facility coupled to the fourth generation light source (4GLS) offers unprecedented opportunity to analyses fast (sub picosecond) protein vibrations. The experimental identification of fast promoting vibrations and coupled motions will provide a step change in our understanding of the physical basis of enzyme catalysis. The scientific possibilities of a THz facility to study vibrationally mediated enzyme action will be discussed.

• P. Ball, Nature 431, 396 (2004).
• L. Masgrau, A. Roujeinikova, L.O. Johannissen, P. Hothi, J. Basran, K.E. Ranaghan, A. J. Mulholland, M. J. Sutcliffe, N. S. Scrutton & D. Leys Science In press

New science with powerful THz sources

ABSTRACT: One of the reasons why terahertz science has so much potential is the fact that it can explore the interface between atomic physics and condensed state physics. Quantum mechanics is an excellent basis for understanding the dynamics of atoms and simple molecules while the methods of statistical physics, aided by some simple quantisation rules, provides a good foundation for our understanding of crystals and, at least partially, amorphous solids and liquids.

Terahertz is found at the meeting point of the experimental frequencies used in investigating these two concepts, as are systems of between 100 and 10,000 atoms, many of which may not be adequately described by either philosophy. Among the molecules of this size are folded proteins with a complexity that is clearly both dynamic and functional. New classes of co-operative behaviour are likely to be discovered in such complex systems. In its lowest energy configuration a folded structure will have many low frequency (soft) modes associated with large scale bending motions at the folds. Those modes which correspond to movements within a cage will become highly anharmonic when the oscillations reach the confining structure. At room temperature these soft modes will be excited to a sufficient degree that a better description might be of a freely moving body rattling within a cage. This motion will introduce time-shifted correlations between different parts of the structure. It is these correlations that might underlie some of the most important dynamics of such molecules.

The experimental observation of these collective modes will require many identically prepared molecules in which the large-scale motions are coherent. This could be achieved in an experiment which starts with molecules that are cold and therefore in their ground state. These would then be excited with a powerful terahertz pulse that is not resonant with any modes except the soft mode of interest. Probing the molecules as different times after the excitation should lead to the observation of the consequent changes in their dynamics while the ensemble rattles in concert. close

Polarisation sensitive terahertz spectroscopy

ABSTRACT: Terahertz time domain spectroscopy (THz-TDS) is now an established technique for performing linear spectroscopy in the far infrared region of the spectrum.  In general THz-TDS is performed using emitters and detectors of linearly polarised radiation. However, in order to study birefringent and optically active materials properly it is necessary to record the full polarisation state of this radiation. Chiral biomolecules such as proteins have optically active vibronic modes at THz frequencies. Thus polarisation sensitive THz-TDS would offer exciting possibilities for determination of protein structure and function.
We have developed a detector which records the full polarisation state of a THz pulse. The three-electrode photoconductive receiver simultaneously records the electric field of an electromagnetic pulse in two orthogonal directions as a function of time.  In this talk the design and optimisation of this device will be discussed with particular emphasis on its significance for the development of new forms of polarization sensitive terahertz spectroscopy, including THz circular dichrosim spectroscopy.

Artificial Materials for Terahertz Frequency Use: What Might Be Their Potential for High Power Source Applications?

In 1998 Ebbessen et al reported enhanced transmission of light through sub-wavelength apertures in an optically opaque metallic film. The apertures were arranged in an aperiodic two dimensional array. The enhanced transmission, which can be several orders of magnitude above classical predictions, is frequency specific. It is now widely accepted that this effect can be accounted for by surface plasmons, generated for specific frequencies of radiation. There has been little investigation of these, and related, effects at Terahertz frequencies; however, it is clear that there may be some potential for the use of such structures as filters, and in imaging or microscopy systems. Materials of this type, which contain repeated and organized features such holes or pillars, are often termed artificial materials. In such materials the optical constants are determined by the architecture of the repeated structure, as well as the natural material properties. An extension of this concept, the metamaterial has also been proposed, in which the refractive index is negative. There is now good evidence that such materials can be realized at THz and other wavelengths.

In this talk, I shall review recent progress in the design, modelling, production and assessment of some representative artificial structures. I shall also discuss routes to the realization of negative refractive index materials. I shall outline possible uses of artificial materials of various types as field concentrators in microscope and imaging systems, especially for high power source applications.

The High Power THz Programme at Jefferson Lab.

ABSTRACT: We are currently commissioning a broadband THz user facility at Jefferson Lab, which will eventually operate in the range 0.1 - 10 THz with an average power of about 100 Watts, and a peak power of several megawatts. At present the facility produces microjoule pulses of 300fs duration with pulse repetition frequencies up to 75 MHz. We will describe the source and facility, and will then discuss a range of scientific opportunities that it presents. Niche areas that have already been identified are in imaging, pump-probe and non-linear phenomena. However, we will also address interesting scientific instrumentation challenges such as wavefront phase, polarization and propagation, measurements of power, and spectral analysis. Due to the nature of this source, we will also discuss personnel safety factors.

Gwyn Williams, Mike Klopf, George Neil - Jefferson Lab
Alan Todd, Vincent Christina, Hans Bluem - Advanced Energy Systems.

Controlling Light with Intense Coherent THz Pulses

ABSTRACT: The NSLS Source Development Lab operates a photo-injected S-band linac for accelerating sub-picosecond electron bunches with charge approaching 1 nC. These ultrashort electron bunches can be used to produce single-cycle coherent THz pulses with energy up to 100 microjoules. The focused electric field strength is then sufficient to observe a number of nonlinear mixing effects. For example, one of the standard THz detection methods uses the electro-optic effect in ZnTe. The THz electric field induces a refractive index change that is normally sensed as a small phase shift in a co-propagated ultrafast laser pulse. But when the THz field is strong, the modulation of the laser's phase is large and its time dependence becomes evident as significant modifications to the laser's spectral content (spectral shifting and chirping). Other effects, such as THz induced lensing, also become apparent. Together, these effects illustrate how intense THz pulses can be used to control ultrafast light pulses.

In collaboration with D. Arena, T. Watanabe, Y. Shen, J. Murphy, C. Kao and X. Wang. Supported by the U.S. Dep't. of Energy under contract DE-AC02-98CH10886, including BNL LDRD.