In recent years, problems pertaining to the genera tion of terahertz oscillations in dimensionally con fined semiconductor structures have drawn increasing interest. Terahertz radiation was observed during quantum transitions in asymmetric coupled quantum wells [1][2][3], superlattices [4,5], and separate quantum wells as a result of quantum beats between excitons involving both light and heavy holes [1]. The impor tant role of exciton states in the generation of terahertz radiation, especially in the case of pumping in the exciton spectral range, was repeatedly pointed out [1,2,[6][7][8]. Recently, Berstermann et al. [9] observed ter ahertz bands in polariton spectra due to ultrafast mod ulation of the frequency of transitions between micro cavity modes and the exciton level, and Vara Ciro et al.[10] reported on the absorption of this radiation by exciton polaritons in quantum dots.In this Letter, we propose a new mechanism of the generation (amplification) of terahertz radiation in three dimensional (3D) or dimensionally confined semiconductors, which is based on the quantum tran sitions between two exciton and biexciton states under conditions of single photon excitation from the ground state of a crystal.Consider a pulse of resonant laser radiation with a frequency ω 0 corresponding to an exciton transition in a semiconductor crystal, which is incident onto the crystal and excites excitons from the ground state of the crystal (Fig. 1). Let us assume that the exciton level (ex) is micropopulated, as well as the two exciton state (2ex) at the double frequency 2ω 0 . These states were frequently used in the interpretations of experimental results on the four wave mixing in semiconductors [11,12] and in the investigations of two photon absorption by double exciton states [13] and photode cay (photodissociation) of biexcitons [14]. The same states can apparently play an important role in the generation of terahertz radiation. Since a biexciton state biex with the eigenfrequency Ω 0 = 2ω 0 -Ω m is situated below the two exciton state with the eigenfre quency 2ex, the corresponding difference being Ω m , the single photon excitation from the ground state leads to the inversion of populations between the 2ex and biex levels. Therefore, if a weak pulse of terahertz radiation with a frequency of ω 2 = Ω m enters the crys tal under consideration, this radiation will be ampli fied due to the induced reset of inversion.The laws of energy and momentum conservation in the region of a two exciton-biexciton transition can be written as follows:(1) where k 1 = k ph , q, and k 2 are the wave vectors of the exciton (and the exciting photon), biexciton, and ter ahertz radiation photon (wit frequency ω 2 ), respec 2E ex k 1 ( ) E biex q ( ) បω 2 , 2k 1 + q k 2 , + = = Abstract-A new mechanism of the generation (amplification) of terahertz radiation in semiconductors is proposed, which is based on the quantum transitions between two exciton and biexciton under conditions of single photon excitation from the ground state of a cryst...
