Terahertz radiation detection with a cantilever-based photoacoustic sensor.

We report the photoacoustic (PA) response in the terahertz (THz) range by employing a detection process actuated with a silicon cantilever pressure sensor and a carbon-based radiation absorber. The detection relies on the mechanical response of the cantilever, when the volume of the carrier gas inside the PA cell expands with the heat produced by the radiation absorber. The detector interferometrically monitors the movement of the cantilever sensor to generate the PA signal. We selected the absorber material with the highest THz responsivity for detailed studies at 1.4 THz (214 µm wavelength). The observed responsivities of two different radiation absorbers are nearly the same at 1.4 THz and agree within 10% with responsivity values at 0.633 µm wavelength. The results demonstrate the potential of covering with a single PA detector a broad spectral range with approximately constant responsivity, large dynamic range, and high damage threshold.

Planar hyperblack absolute radiometer.

The absolute responsivity of a planar cryogenic radiometer fabricated from micromachined silicon and having carbon nanotubes, as the absorber and thermistor were measured in the visible and far infrared (free-field terahertz) wavelength range by means of detector-based radiometry. The temperature coefficient of the thermistor near 4.8 K and noise equivalent power were evaluated along with independent characterization of the window transmittance and specular reflectance of the nanotube absorber. Measurements of absolute power by means of electrical substitution are compared to the German national standard and the uncertainty of the radiometer responsivity as a function of wavelength is summarized.

Absolute terahertz power measurement of a time-domain spectroscopy system.

We report on, to the best of our knowledge, the first absolute terahertz (THz) power measurement of a photoconductive emitter developed for time-domain spectroscopy (TDS). The broadband THz radiation emitted by a photoconductor optimized for the excitation with 1550-nm femtosecond pulses was measured by an ultrathin pyroelectric thin-film (UPTF) detector. We show that this detector has a spectrally flat transmission between 100 GHz and 5 THz due to special conductive electrodes on both sides of the UPTF. Its flat responsivity allows the calibration with a standard detector that is traceable to the International System of Units (SI) at the THz detector calibration facility of PTB. Absolute THz power in the range from below 1 µW to above 0.1 mW was measured.

Traceable terahertz power measurement from 1 THz to 5 THz.

The metrology institute in Germany, the Physikalisch-Technische Bundesanstalt (PTB), calibrates the spectral responsivity of THz detectors at 2.52 THz traceable to International System of Units. The Terahertz detector calibration facility is equipped with a standard detector calibrated against a cryogenic radiometer at this frequency. In order to extend this service to a broader spectral range in the THz region a new standard detector was developed. This detector is based on a commercial thermopile detector. Its absorber was modified and characterized by spectroscopic methods with respect to its absorptance and reflectance from 1 THz to 5 THz and at the wavelength of a helium-neon laser in the visible spectral range. This offers the possibility of tracing back the THz power responsivity scale to the more accurate responsivity scale in the visible spectral range and thereby to reduce the uncertainty of detector calibrations in the THz range significantly.

Optical methods for power measurementof terahertz radiation.

Precision power measurements of terahertz (THz) radiation are required to establish metrological applications in the THz spectral range. However, traceability to the International System of Units (SI) has been missing in the THz region in the past. The Physikalisch-Technische Bundesanstalt (PTB), as the national metrology institute of Germany, determines the spectral responsivity of detectors for THz radiation by using two complementary optical methods: source- and detector-based radiometry. Both approaches have been successfully prototyped, and a pyroelectric THz detector with a well-defined aperture is used to verify the consistency of the two independent calibration methods. These primary investigations led to the design of a new measurement facility for the determination of THz radiant power and the responsivity calibration of THz detectors traceable to the SI.

The duration of capture and restraint during anesthesia and euthanasia influences glucocorticoid levels in male golden hamsters.

Deep litter has been shown to decrease stereotypic wire-gnawing in male golden hamsters, suggesting that increased litter depth may be associated with decreased chronic stress levels. To determine the relationship between litter depth and stress levels in hamsters, the authors measured serum levels of corticosterone, cortisol, and ACTH in male golden hamsters kept in cages with three different depths of litter. The duration of handling the hamsters significantly increased the concentrations of corticosterone, cortisol, and the ratio of cortisol/corticosterone. It took longer to catch hamsters housed in cages with deep litter and the ACTH levels were higher in these hamsters. The positive effect of the enrichment (deep litter) was diminished by methodological problems during handling/anesthesia.

Level set approach for the simulation of focused ion beam processing on the micro/nano scale.

The level set method, introduced by Osher and Sethian (1988 J. Comput. Phys. 79 12-49), has recently become popular in the simulation of etching, deposition and photolithography processes in semiconductor manufacturing, as it is a highly robust and accurate computational technique for tracking moving interfaces. In this paper, the level set approach is applied to focused ion beam fabrication, allowing for the first time the simulation of targets with sub-regions that change their connectivity during processing. It is implemented in the code AMADEUS-level set (advanced modelling and design environment for sputter processes), which is capable of simulating surface topography changes in two dimensions taking re-deposition fluxes into account. We present two examples of comparisons between simulation and experiment that demonstrate the predictive capability of the code.

Simulation-based approach for the accurate fabrication of blazed grating structures by FIB.

Accurate direct fabrication of diffractive gratings is an important task in optical engineering. Several methods have been reported to realize optical diffractive gratings on a silicon substrate using focused ion beams. A method, however, is necessary to improve the overall shape and dimensional accuracy. In this paper a simulation-based technique is presented taking into account redeposition fluxes. First, the influence of the process parameters on the blazed grating structure is studied experimentally. Then the process parameters for a structure with a planar sidewall, a maximum depth of 200 nm, and an opening width of 350 nm are determined. The approach is finally verified by comparing the designed with the fabricated structure. The method may be readily extended to various micro/nano structures in optics.