DX centers are semiconductor dopants which form deep states due to a large lattice relaxation. At low temperature, the DX centers exhibit persistent photoconductivity. When exposed to light in a spatial pattern, the photocarriers are confined to the illuminated regions by Coulomb interaction with the localised DX centers. The resulting spatial modulation of the free carrier density gives rise to a modulation of both the electrical conductivity and the dielectric constant. We demonstrate both effects by measurements of the conductance anisotropy and optical diffraction of samples exposed to excitation in a striped pattern. Erasure is achieved by thermal annealing. The constrast ratio of the conductivity modulation is greater than 108; in our experiment it is limited to ∼100 by light scattering. We estimate that 100nm resolution is feasible. Optical diffraction efficiencies up to 40% have been demonstrated in a stripe-illuminated thick sample. The persistence of the written patterns at low temperature is potentially useful in high-density data storage applications and the fabrication of erasable submicron devices.