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 semiconductor 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][5][6][7][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 ...

Semiconductor quantum dots (QDs) have been demonstrated viable for efficient light emission applications, in particular for the emission of single photons on demand. However, the preparation of QDs emitting photons with predefined and deterministic polarization vectors has proven arduous. Access to linearly polarized photons is essential for various applications. In this report, a novel concept to directly generate linearly-polarized photons is presented. This concept is based on InGaN QDs grown on top of elongated GaN hexagonal pyramids, by which the predefined elongation determines the polarization vectors of the emitted photons from the QDs. This growth scheme should allow fabrication of ultracompact arrays of photon emitters, with a controlled polarization direction for each individual emitter. Keywords: GaN; InGaN; photoluminescence; polarized emission; quantum dot INTRODUCTION Quantum dots (QDs) have validated their important role in current optoelectronic devices and they are also seen promising as light sources for quantum information applications. An improved efficiency of laser diodes and light-emitting diodes can be achieved by the incorporation of QDs ensembles in the optically active layers.1 In addition, the proposed quantum computer applications rely on photons with distinct energy and polarization vectors, which can be seen as the ultimate demand on photons emitted from individual QDs.2 A common requirement raised for several optoelectronic applications, e.g., liquid-crystal displays, three-dimensional visualization, (bio)-dermatology 3 and the optical quantum computers, 4 is the need of linearly polarized light for their operation. For existing applications, the generation of linearly polarized light is obtained by passing unpolarized light through a combination of polarization selective filters and waveguides, with an inevitable efficiency loss as the result. These losses can be drastically reduced by employment of sources, which directly generate photons with desired polarization directions.Conventional QDs grown via the Stranski-Krastanov (SK) growth mode are typically randomly distributed over planar substrates and possess different degrees of anisotropies. The anisotropy in strain field and/or geometrical shape of each individual QD determines the polarization performance of the QD emission. Accordingly, a cumbersome post-selection of QDs with desired polarization properties among the randomly distributed QDs is required for device integration.

A photoluminescence study of an (A16a)As-GaAs quantum-well-wire array directly grown by molecular-beam epitaxy on a tilted substrate is described. A strong anisotropy was observed in the ratio of the electron-light-hole-exciton peak intensity to the electron-heavy-hole-exciton peak intensity. A theory incorporating the optical selection rule for two-dimensional quantum confinement is found to agree very well with the measured data. These results constitute the first evidence of two-dimensional quantum confinement in artificial wire structures having cross-sectional dimensions in the nanometer range. P ACS numbers: 78.6S.Fa, 73.20.Dx, 78.SS.Cr Low-dimensional structures' having quantum confinement (QC) of two or three dimensions such as quantum-well wires (QWW's) and quantum-well boxes (QWB's) have in the last few years attracted much attention not only for their potential in uncovering new phenomena in solid-state physics but also for their potential device applications. Extremely high electron mobility in QWW's' and high performance of QWW or QWB lasers and modulators are expected from theoretical predictions. Recent experiments in QWB resonanttunneling devices have in fact claimed to demonstrate new structures that are attributed to a zero-dimensional system. Transport measurements in narrow wires have also demonstrated a complete quenching of the Hall eA'ect associated with the one-dimensional quantum transport. Most of these low-dimensional structures have been made by fine lithographical methods such as electronbeam lithography, focused-ion-beam implantation, impurity-induced interdiff'usion, etc. However, the lateral dimensions in such structures have been much larger than the vertical dimensions and still in the submicron-meter range, leading to relatively small separations of subband energies. In most cases, the width broadening of energy levels have been larger than the energy separations, and these lithographical techniques seem to have intrinsic difhculties. However, a new approach to control the nucleation and growth kinetics available with molecular-beam epitaxy (MBE) on vicinal substrates has made possible the direct growth of QWW superlattices. Recently, a two-dimensional band-gap modulation of tilted superlattices (TSL) was successfully demonstrated by use of MBE and organometallicvapor-phase epitaxy. ' In those structures, the lateral dimensions are in the low-nanometer range.It is currently the only technique to make QWW arrays with these dimensions, and initial measurements, as reported here, suggest that it may be an extremely powerful technique for QWW device structures.In this paper, we report a photoluminescence (PL) study on a QWW array prepared directly by MBE, and the observation of a strong optical anisotropy in PL excitation (PLE) spectra: The ratio of electron-light-holeexciton peak intensity (I~,~h) to electron-heavy-holeexciton peak intensity (I~,hh) depends strongly on the polarization orientation of the incident light with respect to the QWW direction. Relative matrix elements of ...

Fabrication of single InGaN quantum dots (QDs) on top of GaN micropyramids is reported. The formation of single QDs is evidenced by showing single sub-millielectronvolt emission lines in microphotoluminescence (μPL) spectra. Tunable QD emission energy by varying the growth temperature of the InGaN layers is also demonstrated. From μPL, it is evident that the QDs are located in the apexes of the pyramids. The fact that the emission lines of the QDs are linear polarized in a preferred direction implies that the apexes induce unidirected anisotropic fields to the QDs. The single emission lines remain unchanged with increasing the excitation power and/or crystal temperature. An in-plane elongated QD forming a shallow potential with an equal number of trapped electrons and holes is proposed to explain the absence of other exciton complexes.

Electrochemical synthesis and physical characterization of ZnO nanoparticles functionalized with four different organic acids, three aromatic (benzoic, nicotinic, and trans-cinnamic acid) and one nonaromatic (formic acid), are reported. The functionalized nanoparticles have been characterized by X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV-vis, and photoluminescence spectroscopy. The adsorption of the organic acids at ZnO nanoparticles was further analyzed and interpreted using quantum-chemical density-functional theory computations. Successful functionalization of the nanoparticles was confirmed experimentally by the measured splitting of the carboxylic group stretching vibrations as well as by the N(1s) and C(1s) peaks from XPS. From a comparison between computed and experimental IR spectra, a bridging mode adsorption geometry was inferred. PL spectra exhibited a remarkably stronger near band edge emission for nanoparticles functionalized with formic acid as compared to the larger aromatic acids. From the quantum-chemical computations, this was interpreted to be due to the absence of aromatic adsorbate or surface states in the band gap of ZnO, caused by the formation of a complete monolayer of HCOOH. In the UV-vis spectra, strong charge-transfer transitions were observed.