We investigate the possibility of using InGaAs-A1AsSb-InP coupled quan- Sl-S[7-3'2 742 7"21In Eqs. (5) To understand the role of the pump-probe coherence in the probe response of the QW, we show in Fig. 2 the THz gain spectra at different pump intensities obtained by neglecting pump-probe coherent interactions. Namely, the calculated optical gain in Fig. 2 results purely from the pump-induced population inversion.The pump photon energy used in Fig. 2 is ha,,p = E4x=0.7904 el" (1.57 #m) that is close to the wavelength of the conventional communication lasers. We see from Fig. 2 that the THz gain increases as the pump intensity is increased, and the peak gain appears at the probe photon energy equal to the transition energy E4a. It is also evident that the increase in the maximum gain becomes slower when the pump intensity becomes larger (see the inset of Fig. 2). This gain saturation behavior is a direct result of population inversion saturation at high pump intensities, as illustrated in Fig. 3. Note that there is a small difference in the pump intensity dependence of the population inversion for positive and negative pump detunings as shown in Fig. 3. This is due to the fact that slightly more electrons are pumped onto subband Ea as the pump photon energy gets closer to Ea. The saturation behavior of the population inversion is pump frequency dependent.In Fig. 4 we show the THz gain spectra for different pump intensities calculated with the pump-probe coherence included. As in Fig. 2, the pump photon energy is ha2p = E41.
ComparingFigs. 2 and 4 we see that, for small pump intensities (say less than 0.2 MW/cm_), the coherent pump-probe interaction enhances the THz gain. This is so because in the pumpprobe scheme considered in this paper, the resonant Raman scattering in the QW gives rise to a Raman gain in addition to the gain due to the population inversion. Thus one would expect that the THz gain increases with an increase in the pump intensity because the Raman gain is increased as well. However, we see from Fig. 4 that the peak THz gain first increasesand then decreaseswhen the pump intensity is increased. As the pump intensity is furtherlv increased,the gain approachesto a constant value. We also seefrom Fig. 4 that the peak location in the gain spectra is blueshiftedwith increasingthe pump intensity, which is not shown in Fig. 2. This is becauseof the pump-probe interaction induced shift effect that is neglectedin Fig. 2.In summary, wehaveproposedto usea near-infraredpumped InGaAs-A1AsSb-InPtriple quantum well to obtain THz gain. Owing to the large conduction band offset of the QW structure, we showthat the THz radiation can be generatedwith a near-infrareddiode lasers around 1.55 #m. We optimized the QW design to enhancethe pump-induced population inversionbetweenthe two lasing subbandsby adding a subband with an energy separation to the lower lasing subband closeto the LO phononener_'. The THz gain is calculated by including the pump-probe coherent interaction. We showedthat resonant Raman scatte...
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