We demonstrate a novel all-optical quantum well light modulator device. The device is all-optical in the sense that one “control” laser beam modulates a “read” beam without the use of an external bias. It exploits the large built-in piezoelectric fields present in strained semiconductor layers grown along a polar axis. The new concept here is the use of piezoelectric barriers as opposed to wells. This enables efficient spatial separation of photocarriers created by the “control” beam. We present room-temperature results showing that an optical “control” power of 70 W/cm2 creates in the heart of the structure a space-charge field of about 30 kV/cm, inducing large spectral shifts in the photoluminescence spectra of a CdHgTe quantum well in the mid-IR spectral region.