Coherent control and enhancement of refractive index in an asymmetric double quantum well

We propose coherent control and enhancement of the refractive index with zero absorption in an n-type asymmetric double quantum well using intersubband transitions. These effects are caused by quantum coherence and interference whereby a strong infrared laser mixes upper conduction subbands of an intersubband transition with an auxiliary subband. In the ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ system considered here, an approximately 10% enhancement of the refractive index accompanied with zero absorption occurs at 10.5 μm, when the system is driven by a 6.2-μm laser field with an intensity of ∼1 ${\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^2$. The interplay between quantum coherence and electron population dynamics not only causes an enhancement of refractive index similar to that in atomic systems, but can also induce both low positive and negative group velocities.