Generation of continuous-wave THz radiation by use of quantum interference

Abstract

We propose a scheme for generation of a continuous-wave THz radiation, based on a nonlinear interaction of three electromagnetic (EM) waves in atomic media with a /spl Lambda/ configuration of levels, where the frequency of a transition between two lower metastable states |1> and |2> is in the THz range and transitions |1>-|3> and |2>-|3> to the upper state are driven by optical fields. An important ingredient of the scheme is a preparation of atoms, by virtue of a quantum interference, in the coherent superposition immune to the radiation-the state. As a consequence, the EM fields propagate through the medium with very weak dissipation of energy. Therefore the process may be considered as parametric difference-frequency generation, where the THz frequency is the difference between two optical ones. We have obtained an analytical solution of the EM propagation problem which demonstrates that the intensities oscillate along the propagation path in the form of Jacobi elliptic functions. This allows us to predict that the photon conversion efficiency approaches unity in this technique, and to estimate the optical length at which the energy transfer from the optical field into the THz one is maximum. The analytical solution is confirmed by numerical calculations taking into account Doppler broadening and relaxation of the dark state. These calculations show that the efficiency of the THz generation remains very high in real situations, if the input optical intensity is larger than the intensity threshold necessary for the dark state preparation. Thus, a practical realization of the proposed scheme requires large optical length (large atom density), fairly large input intensities of the optical fields and small decay rate.