We demonstrate a one-to-one correspondence between the polarization state of a light pulse tuned to neutral exciton resonances of single semiconductor quantum dots and the spin state of the exciton that it photogenerates. This is accomplished using two variably polarized and independently tuned picosecond laser pulses. The first "writes" the spin state of the resonantly excited exciton. The second is tuned to biexcitonic resonances, and its absorption is used to "read" the exciton spin state. The absorption of the second pulse depends on its polarization relative to the exciton spin direction. Changes in the exciton spin result in corresponding changes in the intensity of the photoluminescence from the biexciton lines which we monitor, obtaining thus a one-to-one mapping between any point on the Poincaré sphere of the light polarization to a point on the Bloch sphere of the exciton spin.
We demonstrate control over the spin state of a semiconductor quantum dot exciton using a polarized picosecond laser pulse slightly detuned from a biexciton resonance. The control pulse follows an earlier pulse, which generates an exciton and initializes its spin state as a coherent superposition of its two nondegenerate eigenstates. The control pulse preferentially couples one component of the exciton state to the biexciton state, thereby rotating the exciton's spin direction. We detect the rotation by measuring the polarization of the exciton spectral line as a function of the time difference between the two pulses. We show experimentally and theoretically how the angle of rotation depends on the detuning of the second pulse from the biexciton resonance.
We have examined in detail the optical properties of InGaN quantum wells ͑QWs͒ grown on pyramidal GaN mesas prepared by lateral epitaxial overgrowth ͑LEO͒ in a metalorganic chemical vapor deposition system that resulted in QWs on ͕1-101͖ facets. The effects of In migration during growth on the resulting QW thickness and composition were examined with transmission electron microscopy ͑TEM͒ and various cathodoluminescence ͑CL͒ imaging techniques, including CL wavelength imaging and activation energy imaging. Spatial variations in the luminescence efficiency, QW interband transition energy, thermal activation energy, and exciton binding energy were probed at various temperatures. Cross-sectional TEM was used to examine thickness variations of the InGaN/GaN QW grown on a pyramidal mesa. CL imaging revealed a marked improvement in the homogeneity of CL emission of the LEO sample relative to a reference sample for a conventionally grown In 0.15 Ga 0.85 N/GaN QW. The characteristic phase separation that resulted in a spotty CL image profile and attendant carrier localization in the reference sample is significantly reduced in the LEO QW sample. Spatial variations in the QW transition energy, piezoelectric field, and thermal activation energy were modeled using excitonic binding and transition energy calculations based on a single-band, effective-mass theory using Airy function solutions. Band-edge and effective-mass parameters were first obtained from a strain-and In-composition-dependent k"p calculation for wurtzite In x Ga 1Ϫx N, using a 6ϫ6 k"p Hamiltonian in the ͕1-101͖ representations. The calculations and experiments confirm a facet-induced migration of In during growth, which results in a smooth compositional variation from xϷ0.10 at the bottom of the pyramid to x Ϸ0.19 at the top. We demonstrate the existence of a strong correlation between the observed thermal activation behavior of QW luminescence intensity and the associated exciton binding energy for various positions along the pyramidal InGaN/GaN QWs, suggesting exciton dissociation is responsible for the observed temperature dependence of the QW luminescence in the ϳ150 to 300 K range.
The optical properties of vertically stacked self-assembled GaN / AlN quantum dots ͑QD's͒ grown on Si substrates were studied by means of temporally and spatially resolved cathodoluminescence ͑CL͒. An analysis of the CL spectra, thermal activation energies, and measured decay times of the QD luminescence was performed near stress-induced microcracks, revealing changes in the optical properties that can be attributed to stress-dependent variations in the band edges, the polarization field, and the oscillator strength between electrons and holes. Three-dimensional 6 ϫ 6 k · p calculations of the QD electron and hole wave functions and eigenstates were performed to examine the influence of biaxial and uniaxial stresses on the optical properties in varying proximity to the microcracks.
We have examined in detail the optical properties and carrier capture dynamics of coupled In x Ga 1−x N/GaN multiple and single quantum well ͑MQW and SQW͒ structures that possess various numbers of QWs in the confinement region adjacent to a SQW. The aim is to study the influence of the structure of an InGaN MQW confinement region on carrier transfer and collection into a coupled SQW. By applying in a complementary way temperature-and excitation-dependent cathodoluminescence ͑CL͒ spectroscopy and time-resolved CL measurements, we have analyzed the carrier dynamics and state filling in the SQW and the adjacent MQW. We solved self-consistently the nonlinear Poisson-Schrödinger equation for wurtzite materials including strain, deformation potentials, and piezoelectric field of our In x Ga 1−x N / GaN single and multiple QW structures to obtain the excitation-dependent eigenstates that are used to calculate band filling, excitonic lifetimes, and exciton binding energies. We show that it is possible to treat a coupled In x Ga 1−x N single and multiple QW system in a way that allows for a determination of the quasi-Fermi levels, carrier densities in separated QWs, luminescence efficiencies, thermal activation energies for carrier transfer, and carrier capture and recombination rates. We demonstrate in this unique method an improved determination of the piezoelectric field and In composition x by a non-contact optical means alone. The results demonstrate an enhanced luminescence efficiency and yet decreased carrier capture rate by the SQW as the number of QWs increases in the adjacent MQW confinement region.
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