Efficient, low-temperature conversion of infrared light into visible light ~red, orange, green! is reported at single heterojunctions and undoped quantum wells of GaAs and ordered AlxGa 12xInP 2; an increase in photon energy of 700 meV is obtained. The signal originates from the high-band-gap layers and disappears only if the excitation energy is tuned below the GaAs band gap. The intensity of the up-converted photoluminescence ~PL! is found to decrease significantly slower with increasing temperature than that of the regular PL and it remains observable up to 200 K. Interface-induced cold Auger processes along with the presence of trapped states for both electrons and holes in these ordered alloys account for this nonlinear mechanism. A colinear double-beam experiment confirms this.@S0163-1829~96!50132-3# Up-conversion is the observation of radiative transitions at higher energies than that of the excitation source. In general, the observation of up-conversion requires either high temperatures to achieve, for example, ~i! a thermal population of holes for momentum ~k! conserving bulk Auger processes, or ~ii! a thermal population of phonon modes for anti-Stokes Raman lines, 1 or it relies on a nonlinear mechanism, such as two-photon absorption via real or virtual states. In addition, the excited carriers should not immediately relax back to their ground states to enable the observation of radiative transitions from the excited state. Experimental observation of up-conversion at cryogenic temperatures ~cold up-conversion! in solids was reported only rarely and they concerned only small energy gains or high excitation densities. 2‐5 Recently, Kim et al. 6 observed up-conversion ~‘‘anti-Stokes luminesence’’ ! in an asymmetric doublequantum-well system, which they attributed to quantum oscillations. In a recent theoretical paper, Zegrya and Kharchenko 7