Self-referenced ultra-broadband transient terahertz spectroscopy using air-photonics.

Terahertz (THz) air-photonics employs nonlinear interactions of ultrashort laser pulses in air to generate and detect THz pulses. As air is virtually non-dispersive, the optical-THz phase matching condition is automatically met, thus permitting the generation and detection of ultra-broadband THz pulses covering the entire THz spectral range without any gaps. Air-photonics naturally offers unique opportunities for ultra-broadband transient THz spectroscopy, yet many critical challenges inherent to this technique must first be resolved. Here, we present explicit guidelines for ultra-broadband transient THz spectroscopy with air-photonics, including a novel method for self-referenced signal acquisition minimizing the phase error, and the numerically-accurate approach to the transient reflectance data analysis.

Bulk magnetic terahertz metamaterials based on dielectric microspheres.

Rigid metamaterials were prepared by embedding TiO2 microspheres into polyethylene. These structures exhibit a series of Mie resonances where the lowest-frequency one is associated with a strong dispersion in the effective magnetic permeability. Using time-domain terahertz spectroscopy, we experimentally demonstrated the magnetic nature of the observed resonance. The presented approach shows a way for low-cost massive fabrication of mechanically stable terahertz metamaterials based on dielectric microresonators.

Transition between metamaterial and photonic-crystal behavior in arrays of dielectric rods.

Using finite-difference time-domain simulations, we study the interactions of electromagnetic radiation with a square array of dielectric rods parallel to the electric vector. We observe the electric and magnetic Mie resonances which induce intervals of negative effective permittivity and permeability and which contribute to the formation of the photonic band gaps. Owing to the interplay of these phenomena, a narrow spectral range with a negative refractive index can occur. However, this requires the filling fraction of the dielectric to fall into a well defined interval of values and its permittivity to exceed a minimum of about 50. We discuss these phenomena from the perspective of both photonic crystal and metamaterial concepts.

Gouy shift correction for highly accurate refractive index retrieval in time-domain terahertz spectroscopy.

Terahertz spectroscopic measurements are usually performed in focused beam geometry while the standard routine for the retrieval of the sample refractive index assumes plane-wave approximation. In this paper we propose a model for the transmission function which accounts for spatially limited Gaussian terahertz beams. We demonstrate experimentally its validity and applicability for an accurate extraction of the refractive index from experimental data.

Influence of the electron-cation interaction on electron mobility in dye-sensitized ZnO and TiO2 nanocrystals: a study using ultrafast terahertz spectroscopy.

Charge transport and recombination in nanostructured semiconductors are poorly understood key processes in dye-sensitized solar cells. We have employed time-resolved spectroscopies in the terahertz and visible spectral regions supplemented with Monte Carlo simulations to obtain unique information on these processes. Our results show that charge transport in the active solar cell material can be very different from that in nonsensitized semiconductors, due to strong electrostatic interaction between injected electrons and dye cations at the surface of the semiconductor nanoparticle. For ZnO, this leads to formation of an electron-cation complex which causes fast charge recombination and dramatically decreases the electron mobility even after the dissociation of the complex. Sensitized TiO2 does not suffer from this problem due to its high permittivity efficiently screening the charges.

Broadband dielectric terahertz metamaterials with negative permeability.

We present a design of dielectric metamaterials exhibiting a broad range of negative effective permeability in the terahertz spectral region. The investigated structures consist of an array of high-permittivity rods that exhibit a series of Mie resonances giving rise to the effective magnetic response. The spectral positions of resonances depend on the geometrical parameters of the rods and on their permittivity, which define the resonant confinement of the electromagnetic field within the rods. The electromagnetic coupling between the adjacent rods is negligible. With a suitable aspect ratio of the rods, a broadband magnetic response can be obtained.

High tunability of the soft mode in strained SrTiO(3)/DyScO(3) multilayers.

SrTiO(3)/DyScO(3) epitaxial multilayers with variable number and thickness (10-100 nm) of bilayers deposited on DyScO(3) substrates were investigated by means of time-domain terahertz spectroscopy at room temperature. A tensile strain develops in the SrTiO(3) films and shifts the eigenfrequency of the ferroelectric soft mode down by ∼25-45 cm(-1) with respect to the value found for single crystals. In all films the soft mode strongly hardens upon the electrical bias and a linear coupling to a silent excitation of relaxation type at 10 cm(-1) is observed. We show that the change in the THz and sub-THz response of the layers with an increasing field is determined solely by the soft mode eigenfrequency and we propose a phenomenological model describing the origin of the tunability and the peculiar properties of the ferroelectric soft mode in the terahertz spectral range.

Fast one-dimensional photonic crystal modulators for the terahertz range.

Optically controlled one-dimensional photonic crystal structures for the THz range are studied both theoretically and experimentally. A GaAs:Cr layer constitutes a defect in the photonic crystals studied; its photoexcitation by 800 nm optical femtosecond pulses leads to the modulation of the THz beam. Since the THz field can be localized in the photoexcited layer of the photonic crystal, the interaction between photocarriers and THz light is strengthened and yields an appreciable modulation of the THz output beam even for low optical pump fluences. Optimum resonant structures are found, constructed and experimentally studied. The dynamical response of these elements is shown to be controlled by the lifetime of THz photons in the resonator and by the free carrier lifetime. The time response of the structures studied is shorter than 330 ps.

Charge transport in thin layer Na x CoO2 (x ∼ 0.63) studied by terahertz spectroscopy.

Charge transport in Na0.63CoO2 thin film deposited by a spin-coating method was investigated experimentally by time-domain terahertz spectroscopy and theoretically using Monte Carlo calculations of charge response in nano-structured materials. The dominating type of transport mechanism over the entire investigated range of temperatures (20-300 K) is a metallic-like conductivity of charges partly confined in constituting nano-sized grains. Due to the granular character of our thin film, the scattering time at low temperatures is limited by scattering on grain boundaries and the conductivity is strongly suppressed due to capture of a major fraction of charge carriers in deep traps. Nevertheless, our experimental setup and the applied model allowed us to distinguish the parameters related to the grain interior from those influenced by grain boundaries, and to conclude that the metallic type of conductivity is the intrinsic property relevant to single crystal materials.

Long-range and high-speed electronic spin-transport at a GaAs/AlGaAs semiconductor interface.

Spin-valves or spin-transistors in magnetic memories and logic elements are examples of structures whose functionality depends crucially on the length and time-scales at which spin-information is transferred through the device. In our work we employ spatially resolved optical pump-and-probe technique to investigate these fundamental spin-transport parameters in a model semiconductor system. We demonstrate that in an undoped GaAs/AlGaAs layer, spins are detected at distances reaching more than ten microns at times as short as nanoseconds. We have achieved this unprecedented combination of long-range and high-speed electronic spin-transport by simultaneously suppressing mechanisms that limit the spin life-time and the mobility of carriers. By exploring a series of structures we demonstrate that the GaAs/AlGaAs interface can provide superior spin-transport characteristics whether deposited directly on the substrate or embedded in complex semiconductor heterostructures. We confirm our conclusions by complementing the optical experiments with dc and terahertz photo-conductivity measurements.

Quality assurance of physical parameters in radiation oncology at the University Hospital of Basel--a retrospect.

BACKGROUND AND PURPOSE: The necessity for and the benefit of a quality assurance program in radiation oncology are not questioned. Nevertheless, a retrospective analysis of the accumulated results of several years of quality assurance offers the possibility for further optimization. MATERIALS AND METHODS: The results of the physical quality control in radiation treatment planning and on radiation treatment units in the Institute for Radiation Oncology at the University Hospital of Basel for the years 1985, 1991 and 1994 are analyzed and compared mutually. The frequencies of the deviations from the nominal values for the different tests are stated. RESULTS: The relevance of the deviations for the different parameters is rated and the manifested influence of the type and age of the equipment on the results of the quality assurance is discussed. CONCLUSIONS: A condition for the maximum benefit gained from the quality assurance is the oncologist's understanding of the necessity for regular checks and the urgency for eliminating the established deficiencies. In that way the accuracy for the treatment planning, simulation and set-up process and for the realization of the radiation treatment can be increased and the methods can be improved.

[Radiation dose to the head, and particularly to the lens of the eye, during tomography with EMI scanner]. / Uber die Strahlenbelastung des Kopfes, insbesondere der Augenlinsen, bei axialen Tomographie mit dem EMI-Scanner

Radiation dose to the lens of the eye was estimated using thermoluminescence and film dosimetry on patients undergoing computerised axial skull tomography with the EMI scanner, and also on various phantoms. The dose decreases from the lateral margin of the right eye to the lateral margin of the left eye. During conventional three layer tomography, maximal exposure to the eye is about 0.6 R (1.55 X 10 10-4 C.kg-1), produced entirely by scatter. Direct irradiation of the plane of orbits produces maximal exposure rate of 3 R (7.7X 10-4 C.kg-1). Maximal dose to the head depends on skull size and is about 4 R (approximately 10-3 C.kg-1). The local dose dependence of the eyes was confirmed theoretically by drawing up a computer programme, and was related to the method used by the EMI scanner.

[Retention and dosage in radiation synovectomy with yttrium-90-silicate colloid]. / Zur Frage der Retention und der Dosis bei der Radiosynoviorthese mit Yttrium-90-Silikatkolloid.

Investigations on patients and on phantoms were performed to determine the retention and the absorbed dose in radiation synovectomy with 90Y silicate colloid. The electrons of 90Y are totally absorbed within the surrounding tissue and produce bremsstrahlung with a maximum intensity at about 110 keV. After application of 6 mCi 90Y the dose rate of the bremsstrahlung was measured at a distance of 10 cm over 13 knees in a total of 10 patients and was found to range between 0.6 and 1.3 mR/h. During a period of 3 hours up to 3 days after injection practically no removal of the material from the joint was observed. According to approximative calculations the mean dose absorbed in the knee ranged between 3650 and 7300 rad, and the absorbed dose in the synovium between 5500 and 11000 rad.

[Demonstration of the uptake of 35S by chondrosarcomas (author's transl)]. / Nachweis der 35S-Anreicherung in Chondrosarkomen.

Described are two methods of evaluation of the uptake of 35S by chondrosarcomas, i.e. autoradiography and external monitoring of the Bremsstrahlung. A higher concentration of sulfur in the tumor area of three patients was demonstrated after the injection of aliquots of both diagnostic and therapeutic activities. Apart from the simple handling and the quickly obtained results, the external monitoring of the Bremsstrahlung permits quantitative evaluation of the degree of uptake and a simple control over the progress of therapy.

[Campylobacter pylori, gastritis and peptic ulcer]. / Campylobacter pylori, Gastritis und Ulcus pepticum.

In the course of routine gastroduodenoscopic examination of 218 patients bioptic mucosal specimens were examined bacteriologically for the presence of Campylobacter (C.) pylori. The organism was isolated from 52 out of 53 patients (98%) with duodenal ulcer, 7 out of 9 with gastric ulcer (78%), 24 out of 31 with mucosal erosions (77%), 10 out of 10 with duodenitis (100%), 16 out of 16 with chronic active gastritis (100%) and from 40 out of 73 patients (55%) with inactive chronic gastritis. By contrast, all specimens from 26 patients with endoscopically and histologically normal mucosa were negative for this bacterium. The rate of elimination of C. pylori from mucosal specimens was investigated as a first step towards studying the influence of antibiotic therapy upon healing of gastric and duodenal ulcers. For this purpose 30 patients with duodenal ulcers were treated either with ranitidine alone (15) or together with bacampicillin (15), which was shown to be highly active in studies with ampicillin in vitro. After 4 weeks the organism was still found in specimens from all patients treated with ranitidine alone, but also in 12 out of 15 patients given combined therapy. This result demonstrates that systemic antimicrobial chemotherapy with bacampicillin is insufficient to eradicate C. pylori from the stomach and the duodenum.

The efficacy of antimicrobial treatment in Campylobacter pylori-associated gastritis and duodenal ulcer.

The efficacy of various antimicrobial and anti-ulcer agents on the elimination of Campylobacter pylori in duodenal ulcer patients was investigated. Ranitidine, cimetidine, pirenzepine, aluminium phosphate gel as well as combinations of H2-receptor antagonists or pirenzepine + penicillin V, ciprofloxacin, ofloxacin, phenyl-mercuryborate or rifampicin had no influence on C. pylori in vivo. Short term elimination of C. pylori was achieved in 3/15 patients treated with ranitidine + bacampicillin and in 1/5 treated with cimetidine + metronidazole. This elimination was accompanied by a significant reduction of polymorphonuclear infiltration of the antral mucosa. Development of bacterial resistance was observed in patients with additional quinolones, metronidazole and rifampicin but not in patients treated with betalactam antibiotics.