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We report spectroscopic measurements of the THz radiation generated by the free electron laser (FEL) FLARE using electron spin resonance at high magnetic fields, which show that the 150 micropulses circulating in the resonator of the FEL have a large degree of coherence. This feature can be exploited for highresolution ESR and spin echo with subμs time resolution.

The user FELIX Facility Nijmegen offers four FEL beam lines, FLARE from 0.2 to 3 THz, FELIX-2 from 12 to 75 THz, FELIX-1 from 2 to 12 THz and FELICE, a special beam line for intra-cavity experiments from 3-75 THz.

We report on the experimental and theoretical study of the Reststrahlen Band assisted photocurrents in epitaxial grown graphene on SiC. We show that excitation of graphene with infrared radiation results in a dc current. We demonstrate that photocurrent in response to linearly polarized radiation exhibit a resonance enhancement in the frequency range of the Reststrahlen Band of the SiC substrate. By contrast the photocurrent excited by circularly polarized radiation is suppressed in the same spectral range. The developed theory is in agreement with the data and reveals a strong influence of the Reststrahl Band on the high frequency transport in graphene.

Transmission spectra of wet human teeth and dentin slices, together with blood of different flow rates were investigated over a wide spectral range, from visible to terahertz (THz) light. The results make it possible to find the optimum light frequency for an all-optical determination of pulpal blood flow and, consequently, for clinically diagnosis of tooth vitality.

We demonstrate THz photon echoes from silicon doped with phosphorus donors. We provide experimental evidence showing that the echo emerges at the predicted angle and that it arrives at our detector at the predicted time. We use this to demonstrate coherent control of the silicon donor states.

Stimulated terahertz emission at different frequencies between 4.5 THz and 6.4 THz has been realized from a single silicon crystal doped simultaneously by two hydrogenlike donor centers, which were optically excited on intracenter transitions at low lattice temperature.

Under optical pumped by mid-infrared lasers, silicon crystals doped by group-V donor centers reveal stimulated emission in terahertz range. Different features of the population inversion mechanisms in the silicon lasers allow covering the frequency range from 1 to 7 THz with a few tenths of mW pulse peak powers at low lattice temperatures.