J. Niels Hovenier

The p-Ge terahertz laser-properties under pulsed- and mode-locked operation

The results of a detailed study of the optical output of the p-Ge hot hole terahertz laser for pulsed-locked, as well as for mode-locked operation, is reported in this paper. The recently developed technique to achieve active mode locking is described. Results on the shape of the pulses in the small-signal gain, as well as in the saturated gain regime under mode-locked operation, are given. These will be discussed in the light of new results on time- and wavelength-resolved experiments for normal pulsed operation. Under favorable conditions, it is found that trains of pulses with a full width at half maximum pulsewidth of 100 ps can be produced.

Scattering of sub-millimeter radiation from rough surfaces: absorbers and diffuse reflectors for HIFI and PACS

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.

Terahertz calorimetry: an absolute power meter for terahertz radiation and the absorptivity of the Herschel Space Observatory telescope mirror coating

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%.

Pulsed and mode-locked p-Ge THz laser: wavelength-dependent properties

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.

Noise temperature of a 4.3 THz HEB receiver

We have characterized a heterodyne receiver based on an NbN hot electron bolometer integrated with spiral antenna as mixer and an optically pumped FIR ring laser at 4.3 THz as local oscillator (LO). We succeeded in measuring the receiver output power, responding to the hot/cold load, as a function of bias voltage at optimum LO power. From the resulted receiver noise temperature versus the bias voltage, we found a DSB receiver noise temperature of 3500 K at a bath temperature of 4 K, which is a minimum average value. This is the highest sensitivity reported so far at frequencies above 4 THz.