Ralph Müller

Towards traceable radiometry in the terahertz region

PTB and DLR join their expertise and experience in optical radiometry and in THz techniques to perform what is to our knowledge the first traceable measurement of radiant power of a THz quantum cascade laser and the first absolute calibration of a THz radiation detector against a cryogenic radiometer (CR). A total standard uncertainty of 7.3% was achieved at a frequency of 2.5?THz corresponding to a wavelength of 120??m. This uncertainty is dominated by the limited knowledge of the absorptance of the CR cavity. All other uncertainty contributions including those arising from diffraction are only 2%.

Characterization of detectors for extreme UV radiation

Accurate measurements of the radiant power and other quantities are a prerequisite for the development and optimization of suitable radiation sources for extreme ultraviolet (EUV) lithography. Photodiodes are established as easy-to-operate detectors also in the EUV range. The calibrations at the Physikalisch-Technische Bundesanstalt are based on the comparison of the detector to be calibrated with the cryogenic electrical substitution radiometer as a primary detector standard using monochromatized synchrotron radiation at the soft x-ray radiometry beamline in the spectral range from 1 nm up to 25 nm. The spectral responsivity is measured with a relative uncertainty of 0.3% or better. For the dissemination of these high-accuracy calibrations, we investigated the stability and linearity of silicon n-on-p junction photodiodes under intense EUV irradiation in ultra-high vacuum. The maximum current in linear operation (1% relative saturation) depends on the size of the photon beam and ranges from about 3 mA for a 6 mm photon beam diameter to 0.2 mA for a 0.25 mm diameter spot. Diodes with diamond-like carbon or a TiSiN top layer proved to be stable up to a radiant exposure of about 100 kJ cm−2. Furthermore, examples of the calibration of spectrally and spatially resolving radiometric tools for EUV-source characterization are presented.

Current capabilities at the Metrology Light Source

The Physikalisch-Technische Bundesanstalt (PTB) has set up the 630?MeV electron storage ring Metrology Light Source (MLS) in close cooperation with the Helmholtz-Zentrum Berlin (HZB). This electron storage ring has been in regular user operation since April 2008. It is dedicated to synchrotron-radiation-based metrology and technological developments in the far-IR/THz, IR, UV, VUV and EUV spectral ranges, with the use as primary source standard as the key activity. In a permanent process of improvement, the storage ring itself was optimized regarding its regular performance (beam current and lifetime) as well as for special operations (e.g. variable electron energies and electron bunch lengths). The measurement capabilities at the seven different beamline ports were set up sequentially, first in the UV/VUV and IR spectral ranges. This first phase of instrumentation set-up will be finished in 2011 by completing the beamlines for EUV metrology, for the calibration of radiation sources and for the application of undulator radiation.

The Metrology Light Source operated as a primary source standard

The Metrology Light Source (MLS)?the dedicated electron storage ring of the Physikalisch-Technische Bundesanstalt (PTB), designed for metrology and technological applications in the spectral range from the far IR to the VUV?started user operation in April 2008. The MLS is used as a primary source standard from the NIR to the VUV spectral region and is therefore equipped with all the instrumentation necessary to measure with low uncertainty the storage ring parameters and the geometrical parameters needed for the calculation of the spectral photon flux according to the Schwinger theory. It can be operated at any electron beam energy between 105?MeV and 630?MeV and at electron beam currents varying from 1?pA (one stored electron) up to 200?mA, which allows conditions to be tailor-made for special applications.

Status of the IR and THz beamlines at the Metrology Light Source

The low-energy electron storage ring Metrology Light Source (MLS), a dedicated synchrotron radiation source, is in user operation since April 2008 at operating energies ranging from 105 MeV up to 630 MeV. It provides coherent THz radiation when it is operated in a low alpha mode with short electron bunches. At the MLS, two bending magnet beamlines optimized for the MIR and the THz spectral range are operational. We report the status of these two beamlines and present first results of the IR and THz measurements at the MLS.

IR and THz Beamlines at the Metrology Light Source of the PTB

The low‐energy electron storage ring Metrology Light Source (MLS), a dedicated synchrotron radiation source, is in user operation since April 2008. As a special option it provides coherent synchrotron radiation (CSR) in the THz range when it is operated in a low‐α mode with short electron bunches. At the MLS three beamlines dedicated to the use of IR and THz radiation were designed, built and commissioned.

Terahertz spectral computed tomography

The novel technique of spectral THz computed tomography is demonstrated at an example of a small plastic figure. The data are reconstructed by conventional filtered back-projection. The reconstruction are discussed.

The Metrology Light Source — the New Dedicated Electron Storage Ring of PTB

The Physikalisch‐Technische Bundesanstalt (PTB) is currently constructing a low‐energy electron storage ring in the close vicinity of BESSY II where PTB operates a laboratory for synchrotron‐radiation‐based metrology, mainly in the X‐ray spectral region. The new storage ring, which will be called ‘Metrology Light Source’ (MLS), will mainly be dedicated to metrology and technological development in the UV, EUV and VUV spectral range and will thus fill the gap in the spectral range that has opened up since the shut‐down of BESSY I. Moreover, the MLS will deliver intense radiation in the IR and FIR/THz spectral range. The MLS can be operated with parameters optimized for special calibration tasks, which, at a multi‐user facility such as BESSY II is rarely possible. The electron energy can be tuned in the range from 200 MeV up to 600 MeV and the electron beam current can be adjusted from 1pA (single electron) up to 200 mA. All relevant storage ring parameters can be measured with high accuracy, thus making th...

Pulse energy measurements of extreme ultraviolet undulator radiation

A detector system based on the photoionization of rare gases at low particle densities has been developed for absolute photon flux measurements of highly intense and extremely pulsed radiation in the VUV and EUV spectral range. Due to its wide dynamic range, the device can be calibrated with spectrally dispersed synchrotron radiation at low photon intensities but applied with high power sources. The detector is free of degradation and almost transparent, and therefore suitable for intensity monitoring. We describe here the application of the detector for flux measurements at a beamline for undispersed deflected undulator radiation in the PTB Radiometry Laboratory at the electron storage ring BESSY II. In the single-bunch operation mode of BESSY II, measurements of the pulse energy down to 3 nJ for single EUV pulses have been performed. This demonstrates the capability of the gas detector for a broad range of applications with pulsed EUV sources.

THz power measurement traceable to the International System of Units

THz power measurements traceable to the International System of Units were missing in the past. Investigations of two optical methods source- and detector-based radiometry led to the design of a new calibration facility for THz detectors by a molecular gas laser and a room-temperature radiometer.