Physikalisch-Technische Bundesanstalt (PTB) has more than 20 years of experience in the calibration of space-based instruments using synchrotron radiation to cover the ultraviolet (UV), vacuum UV (VUV), and x-ray spectral range. Over the past decades, PTB has performed calibrations for numerous space missions within scientific collaborations and has become an important partner for activities in this field. New instrumentation at the electron storage ring, metrology light source, creates additional calibration possibilities within this framework. A new facility for the calibration of radiation transfer source standards with a considerably extended spectral range has been put into operation. The commissioning of a large vacuum vessel that can accommodate entire space instruments opens up new prospects. Finally, an existing VUV transfer calibration source was upgraded to increase the spectral range coverage to a band from 15 to 350 nm.
At the soft X-ray free electron laser FLASH, multiphoton ionization of free atoms has been studied by ion time-of-flight spectroscopy. Depending on the multiphoton mechanism, the ionization processes are influenced in different ways by the FEL pulse duration. This feature has been used, e.g., to measure the pulse duration of FLASH in the femtosecond regime by non-linear autocorrelation. In the present contribution, the impact of pulse duration on multiphoton ionization is discussed with an emphasis on the distinction between sequential and non-sequential processes, and collective electron excitation as well.
Metal-Semiconductor-Metal photodiodes were fabricated on epitaxially grown AlxGa1-xN on Si(111). The Aluminium
content of the layers grown by means of molecular beam epitaxy (MBE) was 50, 80 and 100%, respectively. The
processing was performed by standard microelectronic fabrication techniques like photolithography, wet and dry etching
(RIE) and physical and chemical vapor deposition (PVD,CVD). The devices were characterized under illumination in a
wavelength range from 400 to 185nm to determine the cut-off wavelength defined by the band-gap energy. Typical
figures of merit like spectral responsivity R quantum efficiency &eegr; and specific detectivity D* have been extracted from
the measurement data.
Reported are the first calculations and experimental results of the deposition of EUV multilayer coatings that actively suppress the reflectance in the VUV wavelength range. In the undesired 100-200 nm band a factor of five reduction was achieved for one single optical element, while only a minor loss of 4.5% reflectance for λ = 13.5 nm, the operating wavelength of EUVL, was found.
For the determination of absolute photon fluxes from high-intense, pulsed VUV and soft X-ray sources like free-electron lasers, a gas-monitor detector system based on the photoionization of rare gases was developed. A prototype system was successfully used for the characterization of VUV free-electron laser radiation at the TESLA test facility (phase 1) in Hamburg. Pulse-resolved measurements at peak powers of more than 100 MW at a wavelength of 87 nm were demonstrated. In order to provide a photon-beam diagnostic of VUV-FEL radiation during phase 2 of the TTF project, a set of four new detectors has been constructed, based on the prototype. The new detector system can be used not only for intensity measurement and monitoring, but also for measuring the beam position. The detector set was calibrated in the Radiometry Laboratory of the Physikalisch-Technische Bundesanstalt at the electron storage ring BESSY II. The calibration was performed using spectrally-dispersed synchrotron radiation at low intensities and a semiconductor photodiode as a transfer standard.
Characterization of optical materials and components is one of the major tasks for the Radiometry Laboratory of the Physikalisch-Technische Bundesanstalt, Germany's national metrology institute, at the synchrotron radiation source BESSY II. Using spectrally dispersed synchrotron radiation, reflectometry measurements have been performed on highly pure CaF2 crystals in the VUV spectral region between 90 nm and 130 nm wavelength in the vicinity of the absorption edge. Here, the optical constants are influenced by an excitonic resonance directly correlated to the recently found anisotropy of the crystal at 157-nm wavelength. To investigate temperature-dependent effects, the reflectometer sample holder has been equipped with a heater/cooler stage, which currently enables measurements at stable temperatures in the range between -50° C and 80° C.
Solar ultraviolet imaging instruments in space pose most demanding requirements on their detectors in terms of dynamic range, low noise, high speed, and high resolution. Yet UV detectors used on missions presently in space have major drawbacks limiting their performance and stability. In view of future solar space missions we have started the development of new imaging array devices based on wide band gap materials (WBGM), for which the expected benefits of the new sensors - primarily visible blindness and radiation hardness - will be highly valuable. Within this initiative, called “Blind to Optical Light Detectors (BOLD)”, we have investigated devices made of AlGa-nitrides and diamond. We present results of the responsivity measurements extending from the visible down to extreme UV wavelengths. We discuss the possible benefits of these new devices and point out ways to build new imaging arrays for future space missions.
In this paper we present arguments for understanding the phenomenon of optical anisotropy in a perfectly cubic crystal such as CaF2. To simplify the discussion we review the basic arguments which seem to preclude any optical anisotropy in a cubic crystal. We discuss the range of validity and define clear conditions for deviations of optical isotropy in cubic crystals. Length and energy scales involved in the problem of radiation-matter interaction for the DUV wavelength range around 157 nm are discussed. These scaling arguments naturally force us to focus on the role of absorption processes at higher photon energies (i.e. smaller wavelengths). Especially the role of a strong, dispersing absorption, in the case of CaF2 caused by exciton excitation, is emphasized. Recent measurements of the anisotropy of the exciton resonance in CaF2 are described and discussed in terms of the small optical anisotropy.