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Abstract. In this paper we report on the observation of response of a Bloch oscillator at room temperature to a THz-field of a frequency larger than the Bloch frequency. The oscillator consisted of a semiconductor superlattice structure, with an applied dc voltage giving rise to a dc electron drift current. Submitting the oscillator to a field at a frequency of 3.3 THz caused a sizeable reduction of the current; the TH2-field was generated by use of intense THz-radiation pulses focused on an antenna coupled to the superlattice. We attribute the THz-field induced reduction of the current to a frequency modulation of the Bloch oscillations of electrons at the frequency of the THz-field, leading to reduction of the electron drift velocity and, consequently, of the current.Keywords: Superlattice; Bloch oscillator; 'hahertz radiation.According to Bloch [I], the electric conductivity by electrons moving in a periodic potential of atoms in a crystal superimposed with a potential of a homogeneous dc electric field is a consequence of scattering of the electrons due to defects and thermal vibrations of the atoms. Zener [2] pointed out that ballistic electrons, i.e. electrons moving without being scattered, should perform oscillations, today denoted as Bloch oscillations, because of the Bragg reflection of the electrons when their kinetic energy corresponds to the upper boundary of the energy band due to the periodic potential. Besides Bloch oscillations, i.e. electron motion within a band, electron transport between different bands separated by forbidden bands was recognized as origin of dielectric breakdown at high dc fields [2]. The eigen frequency of the Bloch oscillation, the Bloch frequency, is determined by the dc voltage across a spatial period of the periodic potential. Esaki and Tsu [3] proposed to study Bloch oscillations in semiconductor superlattices with periods that are, on the one hand, large enough for sustaining, without dielectric breakdown, a sufficiently large voltage across a period and, on the other hand, small enough to allow almost ballistic propagation of electrons over many periods and predicted that under the condition of the occurrence of Bloch oscillations a negative differential conductance should appear in the current-voltage characteristic * Permanent address: Institute for Physics of Microstructures, Russian Academy of Sciences, 46 UIjanov Str., 603600 Nizhny Novgorod, Russia.

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Direct observation of Gunn oscillations up to 20 GHz, induced by picosecond light pulses in an undoped GaAs/AlAs superlattice, is reported. They are obtained in the superlattice growth direction and from 7 K up to room temperature. The frequency is strongly dependent on the applied bias voltage and on the photoexcited carrier density. The oscillation frequency and the mode of operation are modeled by a classical numerical simulation.