Hanspeter Helm

Far-infrared vibrational spectra of all-trans, 9-cis and 13-cis retinal measured by THz time-domain spectroscopy

Low-frequency torsional vibration modes of the conjugated polyene chain of the biologically important chromophore retinal are of major relevance when studying its photoisomerization dynamics. We present absorptivity and dispersion spectra of three diAerent retinal isomers in the far-infrared region between 10 and 100 cm ˇ1 (0.3‐3.0 THz) measured by terahertz time-domain spectroscopy at 298 and 10 K. At low temperatures the observed broad absorption bands resolve into narrow peaks correlating to torsional modes of the molecule. Comparison of the absorption spectra of diAerent retinal isomers enables the approximate localization of the modes within the molecule. Experimental linewidths and transition strengths are reported. ” 2000 Elsevier Science B.V. All rights reserved.

Terahertz time-domain spectroscopy and imaging of artificial RNA

We use terahertz time-domain spectroscopy (THz-TDS) to measure the far-infrared dielectric function of two artificial RNA single strands, composed of polyadenylic acid (poly-A) and polycytidylic acid (poly-C). We find a significant difference in the absorption between the two types of RNA strands, and we show that we can use this difference to record images of spot arrays of the RNA strands. Under controlled conditions it is possible to use the THz image to distinguish between the two RNA strands. We discuss the requirements to sample preparation imposed by the lack of sharp spectral features in the absorption spectra.

Interactions and Dynamics in Ionic Liquids

Precise dielectric spectra have been determined at 25 degrees C over the exceptionally broad frequency range of 0.1 <or= nu/GHz <or= 3000 for the imidazolium-based room-temperature ionic liquids (RTILs) [bmim][BF4], [bmim][PF6], [bmim][DCA], and [hmim][BF4]. The spectra are dominated by a low-frequency process at approximately 1 GHz with a broad relaxation time distribution of the Cole-Davidson or Cole-Cole type, which is thought to correspond to the rotational diffusion of the dipolar cations. In addition, these RTILs possess two Debye relaxations at approximately 5 GHz and approximately 0.6 THz and a damped harmonic oscillation at approximately 2.5 THz. The two higher-frequency modes are almost certainly due to cation librations, but the origin of the approximately 5 GHz mode remains obscure.

Chemical Recognition With Broadband THz Spectroscopy

THz science is developing rapidly in Europe as well as the rest of the world. There is a strong interest in the exploitation of optical technologies in the THz frequency range in virtually all fields of basic and applied sciences of physics, chemistry, biology as well as medicine. Commercial interest in this field has also been growing, spurred by the potential of THz tools in quality control and the biotechnology sector. We will review some contrast mechanisms, which form the basis for real-world applications of THz technology, in particular in the fields of applied chemistry and biotechnology. Whereas narrow bandwidth THz technology may become important for, e.g., real-time imaging at larger standoff distances, we will concentrate on broad bandwidth THz technologies for spectroscopic identification of various substances. It has recently been established that the 0.1-5 THz spectral range contains unique fingerprints of a very large number of crystalline materials, including explosives, illicit drugs as well as most other chemicals in powder form. Since many packaging materials are transparent to THz radiation this fundamental property of crystalline compounds allows remote (contact- free) sensing combined with chemical recognition. On the other hand, the THz spectrum of amorphous systems, including aqueous solutions, contains very little information about the detailed composition of the system. However, under certain conditions it is still possible to learn a great deal about amorphous systems with broadband THz spectroscopy. Amorphous systems of great biotechnological importance include DNA and proteins, both in aqueous solution and as dried matter. We will discuss methods for THz science and technology to attack the very complex problems involved in the extraction of useful new information, which may be difficult, expensive, or impossible to obtain with other methods, from minute amounts of biomaterial.

Terahertz near-field imaging of electric and magnetic resonances of a planar metamaterial

Experimental investigations of the microscopic electric and in particular the magnetic near-fields in metamaterials remain highly challenging and current studies rely mostly on numerical simulations. Here we report a terahertz near-field imaging approach which provides spatially resolved measurements of the amplitude, phase and polarization of the electric field from which we extract the microscopic magnetic near-field signatures in a planar metamaterial constructed of split-ring resonators (SRRs). In addition to studying the fundamental resonances of an individual double SRR unit we further investigate the interaction with neighboring elements.

Chemical sensing and imaging with pulsed terahertz radiation.

AbstractOver the past decade, terahertz spectroscopy has evolved into a versatile tool for chemically selective sensing and imaging applications. In particular, the potential to coherently generate and detect short, and hence, broadband terahertz pulses led to the development of efficient and compact spectrometers for this interesting part of the electromagnetic spectrum, where common packaging materials are transparent and many chemical compounds show characteristic absorptions. Although early proof-of-principle demonstrations have shown the great potential of terahertz spectroscopy for sensing and imaging, the technology still often lacks the required sensitivity and suffers from its intrinsically poor spatial resolution. In this review we discuss the current potential of terahertz pulse spectroscopy and highlight recent technological advances geared towards both enhancing spectral sensitivity and increasing spatial resolution. Online abstract figureArtists view of a terahertz pulse emitted from a photoconductive antenna probing the vibrational modes of a sugar molecule.

Lattice modes mediate radiative coupling in metamaterial arrays

We show that a resonant response with very high quality factors can be achieved in periodic metamaterials by radiatively coupling their structural elements. The coupling is mediated by lattice modes and can be efficiently controlled by tuning the lattice periodicity. Using a recently developed terahertz (THz) near-field imaging technique and conventional far-field spectroscopy together with numerical simulations we pinpoint the underlying mechanisms. In the strong coupling regimes we identify avoided crossings between the plasmonic eigenmodes and the diffractive lattice modes.

Terahertz pulse propagation in the near field and the far field

We present a detailed investigation of the propagation properties of beams of ultrashort terahertz (THz) pulses emitted from large-aperture (LA) antennas. The large area of the emitter is demonstrated to have substantial influence on the temporal pulse profile in both the near field and the far field. We perform a numerical analysis based on scalar and vectorial broadband diffraction theory and are able to distinguish between near-field and far-field contributions to the total THz signal. We find that the THz beam from a LA antenna propagates like a Gaussian beam and that the temporal profile of the THz pulse, measured in the near field, contains information about the temporal and spatial field distribution on the emitter surface, which is intrinsically connected to the carrier dynamics of the antenna substrate. As a result of pulse reshaping, focusing of the THz beam leads to a reduced relative pulse momentum, with implications in THz field-ionization experiments.

Characterization and application of dichroic filters in the 0.1-3-THz region

Low-loss dichroic filters, a subgroup of frequency-selective components, have been characterized by terahertz time-domain spectroscopy in the region from 0.1 to 3 THz and with Fourier transform spectroscopy. The two data sets are fully consistent. The time-domain spectrometer is used to investigate the phase velocity behavior of dichroic filters. The dichroic filters have various applications in frequency mixing, multiplying, and diplexing experiments. In a novel application, cascaded filters were used to limit the terahertz pulse bandwidth and to monitor molecular transitions of atmospheric water vapor in a selected frequency band.

Beam-Profiling and Wavefront-Sensing of THz Pulses at the Focus of a Substrate-Lens

We report combined wavefront detection and beam profiling of single-cycle terahertz (THz) pulses. In our system, the electric field is recorded highly resolved in two spatial and one temporal dimension before and after propagation through an optical component. Using this approach, we examine the imaging properties of a hyperhemispherical silicon lens as it is commonly used in THz dipole antennas. We observe an asymmetric spatiotemporal field dynamic in the focus, which can be attributed to distortion of the incident wavefront in combination with the image properties of the lens. Diffraction on the lens aperture influences the spectral beam profile at the focus. The frequency dependence of the Airy pattern indicates a rapidly degrading Strehl ratio with increasing frequency.