Abstract
Summary form only given. Mixed type-I/type-II GaAs/AlAs double quantum wells (QW) offer a means of generating electron-hole pairs with a lifetime which can exceed 400 /spl mu/s. For the sample chosen for the measurements discussed here, electrons excited in the /spl Gamma/-valley of the narrow well are transferred to the X-valley in the barrier material and subsequently to the /spl Gamma/-valley of the wider well. The recombination rate of the spatially separated electron-hole pairs is dominated by the tunneling rate of holes from the narrow well into the wider well. This leads to an intrinsically long life-time of the spatially separated electron-hole pairs which enables large carrier densities (7/spl times/10/sup 11/ cm/sup -2/) to be excited with low optical power densities (0.07 W/cm/sup 2/). We have exploited these high carrier lifetimes and densities to build an optically controlled THz-modulator with large contrast ratios for extremely low optical power.
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