T.O. Klaassen

Terahertz heterodyne receiver based on a quantum cascade laser and a superconducting bolometer

We report the first demonstration of an all solid-state heterodyne receiver that can be used for high-resolution spectroscopy above 2THz suitable for space-based observatories. The receiver uses a NbN superconducting hot-electron bolometer as mixer and a quantum cascade laser operating at 2.8THz as local oscillator. We measure a double sideband receiver noise temperature of 1400K at 2.8THz and 4.2K, and find that the free-running QCL has sufficient power stability for a practical receiver, demonstrating an unprecedented combination of sensitivity and stability.

Beam patterns of terahertz quantum cascade lasers with subwavelength cavity dimensions

The need to reach single-mode lasing and minimize at the same time the electrical dissipation of cryogenically operated terahertz quantum cascade lasers may result in small and subwavelength cavity dimensions. To assess the influence of such dimensions on the shape of the laser emission, we have measured the beam pattern of two metal-metal cavity quantum cascade lasers. The patterns show regular angular intensity variations which depend on the length of the laser cavity. The physical origin of these features is discussed in terms of interference of the coherent radiation emitted by end and side facets of the laser bar.

Phase locking and spectral linewidth of a two-mode terahertz quantum cascade laser

We have studied the phase locking and spectral linewidth of an ? 2.7?THz quantum cascade laser by mixing its two lateral lasing modes. The beat signal at about 8?GHz is compared with a microwave reference by applying conventional phase lock loop circuitry with feedback to the laser bias current. Phase locking has been demonstrated, resulting in a narrow beat linewidth of less than 10?Hz. Under frequency stabilization we find that the terahertz line profile is essentially Lorentzian with a minimum linewidth of ? 6.3?kHz. Power dependent measurements suggest that this linewidth does not approach the Schawlow-Townes limit.

Stimulated terahertz emission from group-V donors in silicon under intracenter photoexcitation

Frequency-tunable radiation from the free electron laser FELIX was used to excite neutral phosphorus and bismuth donors embedded in bulk monocrystalline silicon. Lasing at terahertz frequencies has been observed at liquid helium temperature while resonant pumping of odd parity impurity states. The threshold was about two orders of magnitude below the value for photoionization pumping. The influence of nonequilibrium intervalley TO phonons on the population of excited Bi impurity states is discussed.

Cryogenic far-infrared laser absorptivity measurements of the Herschel Space Observatory telescope mirror coatings

Far-infrared laser calorimetry was used 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 K to simulate the operating temperature of the telescope in its planned orbit about the second Lagrangian point, L2, of the Earth-Sun system. Together, the telescopes equilibrium temperature in space and the emissivity of the mirror surfaces will determine the far-infrared-submillimeter background and thus the sensitivity of two of the three astronomical instruments aboard the observatory if stray-light levels can be kept low relative to the mirror emission. Absorptivities of both clean and dust-contaminated samples were measured at 70, 118, 184, and 496 microm. Theoretical fits to the data predict absorptivities of 0.2-0.4% for the clean sample and 0.2-0.8% for the dusty sample, over the spectral range of the Herschel Space Observatory instruments.

Gain of the mode locked p-Ge laser in the low field region

Following the earlier observation of active mode locking in the high field region of the Voigt configured p-Ge intervalence band laser, presently mode locking in the low field region is also reported. The experimental results on the effective small signal gain for active- as well as for self-mode locked operation are given.

Reflectance measurements on submillimeter absorbing coatings for HIFI

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.

Terahertz gain on shallow donor transitions in silicon

Small signal gain measurements of optically excited terahertz silicon lasers are reported. Two types of lasers, Si:P and Si:Bi, were investigated. They were optically excited with radiation from a free electron laser or a CO2 laser. The experiments were performed with an oscillator-amplifier scheme where one sample serves as a laser while the other one is an amplifier. In case of the free electron laser the pump frequency corresponds to intracenter excitation of the 2p0 or 2p± states of the P and Bi Coulomb centers, and the gain was determined for the 2p0?1s(E), 2p0?1s(T2) transitions in Si:P and the 2p±?1s(E) transition in Si:Bi. Pumping with a CO2 laser leads to photoexcitation of the Coulomb centers. In this case the gain was determined for the 2p0?1s(T2) of Si:P transition. The gain for intracenter pumping is in the range 5?10?cm?1 while for photoexcitation the gain is considerably less, namely ? 0.5?cm?1. The experimental results are analyzed and found to be in good agreement with theoretical calculations based on balance equations.

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.

Generation of subnanosecond high power far infrared pulses using a FEL pumped passive resonator

Abstract By cavity-dumping a small passive, external resonator, subnanosecond far infrared pulses with enhanced peak powers were created. The resonator was pumped by radiation produced by the UC Santa Barbara free electron laser and subsequently cavity-dumped using a fast optical semiconductor switch.