We report up-converted photoluminescence in a structure with InAs quantum dots embedded in GaAs. An efficient emission from the GaAs barrier is observed with resonant excitation of both the dots and the wetting layer. The intensity of the up-converted luminescence is found to increase superlinearly with the excitation density. The results suggest that the observed effect is due to a two-step two-photon absorption process involving quantum dot states. Photoluminescence ͑PL͒ up-conversion in

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... onductor heterojunctions ͑HJs͒ and quantum wells ͑QWs͒, i.e., the observation of an emission at energies higher than that of the excitation energy, has attracted much attention in the last few years. 1-8 Up-conversion is a well-known phenomenon in nonlinear optics, but processes like second-harmonic generation and two-photon absorption ͑TPA͒ occur primary at high excitation intensities ͑уkW/cm 2 ͒ and are usually quite inefficient in continuous-wave ͑cw͒ experiments. 9 On the other hand, a highly efficient PL up-conversion at extremely low excitation intensity ͑0.1-10 W/cm 2 ͒ has been observed in semiconductor heterostructures. [1] [2] [3] [4] [5] [6] [7] [8] In these experiments, the carriers photoexcited in the narrow gap material are redistributed into the wide gap material where they undergo recombination and give rise to an up-converted PL ͑UPL͒. For the mechanisms that up convert the carriers, Auger process 1-3 and two-step, two-photon absorption ͑TS-TPA͒ have been suggested. 4-8 In the Auger process, the energy released by the electron-hole recombination is transferred to another electron or hole. The excited carriers are ejected into the barrier and after relaxation to the band edges they recombine radiatively to give UPL. The presence of a heteroboundary lifts the k-conservation requirement in the direction perpendicular to the interface and allows Auger recombination without a thermal threshold. On the other hand, the TS-TPA must be distinguished from the purely TPA in nonlinear optics, because the intermediate state is real and relaxation of the excitation to lower-lying real states may occur before the second photon is absorbed. The up-converted band-to-band luminescence has also been previously observed in bulk semiconductors. 10,11 The effect has been explained by a TS-TPA process involving a deep energy level as an intermediate state. Recently, PL up-conversion has been reported for InP/GaInP self-assembled quantum dots ͑QDs͒ 12 and InP and CdSe colloidal QDs. 13 In Ref. 12, QD emission in the presence of electrical current flowing through the sample has been observed with an optical excitation below the ground state. The electrical current provides electrons, some of which are trapped into the QDs, while the holes are optically excited via deep energy levels localized close to the QDs. In the case of colloidal QDs, the PL up-conversion has been explained by a microscopic mechanism that involves the surface states near the valence band and conduction band edges. 13 Here we report on PL up-conversion in InAs/GaAs selfassembled QDs. We find an efficient PL from the GaAs barrier when the excitation energy is scanned within the absorption band of the InAs dots. The excitation spectrum and the density dependence of the UPL are studied and the mechanism for this emission is proposed. The self-assembled InAs QD structures studied were fabricated by molecular beam epitaxy ͑MBE͒ on ͑100͒ GaAs substrate. 14 At the initial stage of the growth 40 periods of GaAs/AlAs ͑2 nm/2 nm͒ short period smoothing superlattice was grown at the substrate temperature of 630°C, followed by the deposition of a 200-nm-thick GaAs buffer layer at T g ϭ580°C with substrate rotation. The dots were formed by depositing 1.7 monolayers of InAs at T g ϭ530°C. A growth interruption of 30 s was used to narrow down the size distribution. Then the dots were covered at 530°C with a thin GaAs cap of a thickness 1 nm, followed by a second 30 s growth interruption. Atomic force microscopy ͑AFM͒ after such a growth procedure, revealed well defined islands of lateral dimension Ϸ60 nm and height Ϸ1.5 nm. A final 50nm-thick GaAs capping layer was grown at a temperature of 580°C to further protect the QDs layer. The PL and PL excitation ͑PLE͒ measurements were done at low temperature (Tϭ2 K) using a cw Ti-sapphire laser. The luminescence was dispersed by an 0.85 m double monochromator ͑SPEX 1404͒ and detected with a cooled InGaAsP/InP photomultiplier tube ͑Hamamatsu R5509-42͒ using a conventional lock-in techniques. The detection system provided a spectral resolution of 0.2 nm. Figure 1͑a͒ shows the PL spectrum of the sample with an excitation energy above the GaAs band gap (E exc ϭ1.63 eV). The excitation intensity is I exc ϭ80 W/cm 2 . The spectrum involves both the QD emission and the emission of the GaAs barrier. A weak PL from the InAs wetting layer ͑WL͒ is also observed at E WL ϭ1.423 eV. The QD emission can be deconvoluted into three Gaussians with a full width at half maximum ͑FWHM͒ of 23-26 meV and peak energies a͒ Electronic mail: plapa@ifm.liu.se APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 6