Probe absorptions in an asymmetric double quantum well

We investigate the probe absorption properties in an asymmetric coupled double quantum-well structure for both the transient and steady-state processes. Beyond the rotating-wave approximation, we directly simulate the transient dynamics with the density-matrix equations. The probe absorption is found to be completely eliminated under the condition of two-photon resonance, without requiring the one-photon resonance condition. More interestingly, our results show that the absorption properties can be tuned by changing the tunnelling barrier. It is well known that the optical dispersive and absorption properties of weak probe fields can be controlled by strong coherently coupled fields under the condition of electromagnetically induced transparency (EIT) [1, 2]. EIT and similar phenomena have been deeply studied in dense atomic gases [1–10] starting from its observation in sodium vapours [11], and disclosing new possibilities for nonlinear optics and quantum information processing. In particular, the suppression of both twoand three-photon absorptions in the four-wave mixing (FWM) and hyper-Raman scattering (HRS) has been analysed and discussed in resonant coherent atomic media [7–9]. Compared with the implementation of EIT in dense atomic gases, it is difficult to realize EIT in solid-state media due to a shorter coherence time constant [1]. Nevertheless, some investigations have shown that it is possible to realize EIT and related phenomena in quantum-well (QW) structures [12–17] and molecular magnets [18]. For example, Sadeghi et al [13] showed that EIT may be obtained in an asymmetric QW with appropriate driving fields, provided that the coherence between the intersubbands considered is preserved for a sufficiently long time. So quantum coherence and interference in QW structures have attracted great interest due to their potentially important applications in optoelectronics and solid-state quantum information science. In fact, phenomena such as Autler–Townes splitting [17], gain without inversion [19], modified Rabi oscillations and controlled population transfer [20, 21], largely enhanced second harmonic generation [22], controlled optical bistability [23, 24], ultrafast all-optical switching [25, 26] and other novel phenomena [27–33] have been theoretically studied and experimentally demonstrated through the intersubband transitions (ISBT) of semiconductor QWs. We take note of the model for an asymmetric semiconductor coupled double quantum well [34], in which the realization of coherent sub-millimetre-wave emission was demonstrated by controlling the quantum interference. Instead of the previous work, in this work we focus on the absorptions of a weak probe field based on the condition of two-photon resonance, and analyse the probe absorption properties for the transient and steady-state processes in such a system. Our results show that the probe absorption can be completely eliminated under the condition of twophoton resonance without requiring the one-photon resonance condition. Different from the three-subband quantum well system [14], the absorption properties in our structure can be tuned by changing the tunnelling barrier. 1. The physical model and equations of motion Let us consider an asymmetric semiconductor coupled double quantum well structure consisting of ten pairs of a 51-monolayer (145 Å) thick wide well and a 35-monolayer (100 Å) thick narrow well, separated by a Al0.2Ga0.8As buffer 0953-4075/09/225501+06$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK J. Phys. B: At. Mol. Opt. Phys. 42 (2009) 225501 W-X Yang et al