We report a simple and practical method to design and fabricate antireflection (AR) coatings for the emission facet of GaAs-based laser chips suited to operating in the very far infrared (IR) or terahertz (THz) spectral region. Vacuum-deposited silica films about 8 Ým thick serve as a single layer, quarter-wave AR and with reflectivities measured below 0.5% over ~10 cm-1 range in the far IR. Quantum cascade lasers (QCL) thus coated function well at 8-10 K coupled to a tunable, external cavity optical system at ~158 cm-1 (∼4.75 THz).

In this paper we show time-resolved experiment with 35 fs resolution on bismuth single crystal, for which very high sensitivity detection system has been used. Coherent and incoherent lattice dynamics as well as electrons dynamics can be clearly seen into the reflectivity changes. The complex behaviour of the reflectivity could not be explained in the light of the existing theories. Therefore, we developed a new theory, starting from the very basic principle of laser-matter interaction, which is in good agreement with experimental results. Two main results will be shown: the coherent phonon is excited by the temperature gradient; the changes in reflectivity are related directly to the changes in electron-phonon collision frequency.

In this paper we report on the development of micromachined filters for operation at terahertz frequencies. SU8, a negative photodefinable epoxy, is used to define arrays of high aspect ratio rods which are subsequently sputter coated in gold to form the filter. We fabricate and test a filter with a fixed period but variable diameter along the length of the array. By moving the array in the terahertz beam we demonstrate the ability to mechanically tune specific filter characteristics from a single device.

We report our recent progress made on the development of widely-tunable monochromatic THz sources. They have been realized based on difference-frequency generation (DFG) in GaSe, ZnGeP2, and GaP crystals, respectively. Using a GaSe crystal, the output wavelength was tuned continuously in the range from 66.5 µm to 5664 µm (from 150 cm-1 to 1.77 cm-1) with the peak power reaching 389 W. Such a high peak power corresponds to a conversion efficiency of about 0.1% (a photon conversion efficiency of 19%). A further optimization on the THz beam parameter may result in an output peak power of a few kW. Within the range of 100-250 µm the output peak powers were higher than 100 W. We have also investigated THz frequency upconversion in GaSe, ZnGeP2, and GaP crystals. Such a parametric process has a potential for detecting THz pulses at room temperature or just using a thermoelectric cooler. The minimum detectable energy per pulse was measured to be 245 pJ, which corresponds to a noise equivalent energy of 77.5 . A further optimization of the process can reduce this value down to 1 .

We report on a room temperature detector of infrared/terahertz laser radiation allowing to measure and characterized the radiation polarization state with picosecond time resolution.