Detection peculiarities of an un-cooled (room temperature) 8x8 pixel array designed to image broadband THz radiation
were investigated. Each pixel consists of a thin conductive film absorber on a dielectric membrane with thermopile
temperature readout. It was designed and tested for four combinations of two different types of absorber and thermopile
materials. The photo-response profile, determined by scanning the pixels through the focus of a THz laser beam, was
wider than expected from a 2-D convolution of the Gaussian beam and the absorber surface. Also the time response did
depend on the position of the beam relative to the pixel. Simulations show that those properties are due to the fact that
also the thermopiles absorb THz radiation. For the best composition of absorber and thermopile, the responsivity, the
noise equivalent power, and the bandwidth were estimated to be of 28 V/W, 5x10-9 W/Hz1/2 and 50 Hz, respectively.
A recent study initiated by the European Space Agency aimed at identifying the most promising technologies to significantly improve on the generation of coherent electromagnetic radiation in the THz regime. The desired improvements include, amongst others, higher output powers and efficiencies at increasingly higher frequencies, wider tunability and miniaturization. The baseline technologies considered revolve around Photomixing and novel laser based technologies compared to all electronic techniques. Some of the most significant findings will be presented together with technological developments and experimental results selected for medium to short term development. These technologies include advanced p-i-n photomixer with superlattice structures and, THz quantum cascade lasers. Recent results achieved in these fields will be put into the potential perspective for the respective technology in the future.
A new calorimetric absolute power meter has been developed for THz radiation. This broad band THz power meter measures average power at ambient temperature and pressure, does not use a window, and is insensitive to polarization and time structure of THz radiation. The operation of the power meter is based on the calorimetric method: in order to determine the power of a beam of THz radiation, the beam is used to illuminate a highly absorbing surface with known BRDF characteristics until a stable temperature is reached. The power in the incident beam can then be determined by measuring the electric power needed to cause the sample temperature rise. The new power meter was used with laser calorimetry to measure the absorptivity, and thus the emissivity, of aluminum-coated silicon carbide mirror samples produced during the coating qualification run of the Herschel Space Observatory telescope to be launched by the European Space Agency in 2007. The samples were measured at 77 Kelvin to simulate the operating temperature of the telescope in its planned orbit around the second Lagrangian point, L2, of the Earth-Sun system. The absorptivity of both clean and dust-contaminated samples was measured at 70, 118, 184 and 496 mm and found to be in the range 0.2 - 0.8%.
Absorbing coatings for the HIFI and PACS spectrometers aboard the Herschel platform have been developed and optically characterized. Using radiation from an optically pumped far-infrared laser at wavelengths in the 90 - 900 μm range, the specular as well as the diffuse reflection - characterized by the Bi-directional Reflection Distribution Function - have been determined. The influence of polarization has been addressed too. Moreover, the absorption of non-absorbing diffusely reflecting surfaces, to be used for integrating spheres, has been determined using a low temperature calorimetric method.
Specular and diffuse reflectance (BRDF) of black absorbing coatings, meant to be used for the HIFI instrument aboard the FIRST satellite, has been studied in the sub-millimeter region (0.1 < (lambda) < 0.9 mm). These coatings have to meet space qualification requirements and must be usable for at least the overall wavelength band (0.1 - 0.6 mm) covered by the HIFI spectrometer. Existing materials, coatings obtained from other research groups and home made samples have been studied. Optical characterization of these coatings has been performed at wavelengths of 96.5 micrometers , 118.8 micrometers , 184.3 micrometers , 496 micrometers and 889 micrometers , for a large range of directions of incident and reflected light and for different polarization directions. A limited number of reflectance measurements at cryogenic temperatures have been carried out too. A simple experimental set up to study the effect of double scattering has been constructed to investigate the accuracy of numerical simulations based on experimental BRDF values. Data show that the best samples (home made) have BRDF values below about 2.10-2 Sr-1 throughout the wavelength range of interest, quite independent of directions of incidence, reflection and polarization. The Total Hemispherical Reflection of such a coating will then be 0.06.
Wavelength dependent properties of the p-Ge THz laser are reported for pulsed as well as for mode locked operation. The original small mirror laser outcoupler has been replaced by a mesh outcoupler, resulting in clear improvements of laser action. The optical output has been analyzed using a grating spectrometer and fast Schottky diode detectors. FOr 0.25 <EQ B <EQ 0.6T, 170-185 micrometers emission occurs. Laser action starts at short wavelength; during the pulse, longer wavelength components gain intensity, until simultaneous emission across the whole band occurs. With the mesh outcoupler instead of a small mirror, the small signal gain is found to increase, for instance from 0.015 cm-1 to 0.04 cm-1 at 172 micrometers . With the rf field modulation applied, 770 MHz mode locking of the laser is achieved at 172 micrometers , yielding a train of 100 ps FWHM pulses. For 0.5 <EQ B <EQ 1.4T, 75-120 micrometers emission is observed, dependent on both B and E field. Time-and wavelength dependence is complicated; often an oscillatory behavior of spectral components is seen. Although this effect complicates the formation of stable pulse trains under mode locked conditions, 140 ps pulses have been produced.
The p-Ge hot hole laser is as yet the only solid state tunable laser with a strong emission in the THz frequency range. Monte Carlo simulations have shown that modelocking of the laser on the intervalence band transition should be possible by gain modulation through the application of an appropriate rf electric field. Recently we did observe for the first time the generation of 200 picosecond pulses in the high frequency (approximately equals 100 cm-1) emission range. Now also pulses as short as 100 ps have been observed in the low frequency regime (approximately equals 50 cm-1). A detailed study of the wavelength dependent optical output of the laser has been started now for (normal) pulsed -- as well as for active mode locked operation. Results on pulse shape and small signal gain in the low frequency (equals low magnetic field) regime are given.
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