Coherent and tunable terahertz oscillators, generators, and amplifiers

We have considered feasibilities of using optical parametric oscillation and amplification, and difference-frequency generation for efficiently generating and amplifying terahertz waves in several second-order nonlinear optical materials. They include GaAs, GaP, GaSe, CdSe, LiNbO3 and LiTaO3. The advantage of using birefringence in CdSe and GaSe is tunability of the output terahertz frequency. Furthermore, both CdSe and GaSe can be used to achieve the backward parametric oscillation without any cavity. On the other hand, in periodically-poled LiNbO3 and LiTaO3, one can take advantage of large diagonal elements of second-order nonlinear susceptibility tensor. In the diffusion-bonded-stacked GaAs and Gal? plates, quasi-phase matching can be achieved by alternatively rotating the plates. It is also feasible to achieve phase matching in a single GaAs or GaP plate due to the presence of the absorption tail of polariton modes. In this case, tuning can be achieved by changing the pump wavelength. The advantages of using coherent parametric processes are possibilities of efficiently generating and amplifying temporally-coherent and narrow-linewidth tunable terahertz waves at room temperature. However, all other schemes do not possess these advantages. Compared with a noncollinear configuration, by using the parallel wave propagation configurations, the conversion efficiency can be higher because of longer effective interaction length among all the waves.