The fine structure splitting of bright exciton states is measured for a range of thermally annealed InGaAs quantum dot (QD) samples with differing degrees of In/ Ga intermixing and also for a dot-in-a-well (DWELL) structure. Magnitudes of the fine structure splitting are determined in polarization-resolved differential transmission experiments from measurements of the period of quantum beats observed in QD exciton dynamics. The splitting is found to decrease in structures with weaker strain: both for In/ Ga intermixed QD's and also in dots surrounded by strain-reducing layers (DWELL's). Our findings pave the way to the achievement of entangled two photon sources based on emission from individual QD's, currently prevented since the fine structure splitting is larger than the radiative linewidth. In many QD systems the breakdown of a pure-spin picture for exciton states is observed and linearly polarized exciton eigenmodes form split by the electron-hole exchange interaction 1-10 (see Fig. 1). This prevents the realization of an entangled two photon source based on individual QD's. 11,12 A splitting smaller than the radiative broadening [of order of a few eV (Ref. 13)] is required for photon entanglement 11 in such sources of potential importance for implementation in quantum information systems. 14The fine structure splitting arises due to the long range electron-hole exchange interaction, and occurs in systems with lowered in-plane symmetry. 4,8,15 Due to the reduced atomistic symmetry of zinc blende semiconductors (C 2v symmetry), fine structure splitting is expected for self-assembled dots with cylindrical symmetry about the growth axis, although in this case splittings of only up to Ϸ9 eV are predicted, 8 considerably smaller than those found in most as-grown dot samples (see below). The splitting will then be enhanced by anisotropy in dot shape occurring during crystal growth. In a similar way it is known that the strain field is anisotropic for self-assembled dots, distinguishing the [110] and ͓−110͔ directions, even for dots of spherical shape. 16,17 The strain leads to piezoelectric fields in the samples, and when included in k · p theory treatments of electronic band structure, leads to markedly enhanced p-state splittings; 17 the s-like ground state is expected to show similarly enhanced splitting when strain and piezoelectric fields are enhanced. In accord with the above arguments, in unstrained dots in a glass matrix, the fine structure is zero. 18 Further support to the above reasoning is given by recent experiments where externally applied uniaxial strain was shown to produce a large splitting of bright excitons in quantum wells. 19Here we present systematic studies of the fine structure splitting in ensembles of InGaAs dots, providing experimental evidence for the factors determining the magnitude of the electron-hole exchange interaction. In particular, we show that E FS can be accurately tuned in InGaAs dots by postgrowth thermal annealing (enhancing the In/ Ga intermixing and reducing strain 20 ...

The temperature dependence of spin coherence in InGaAs quantum dots is obtained from quantum beats observed in polarization-resolved pump-probe experiments. Within the same sample we clearly distinguish between coherent spin dynamics leading to quantum beats and incoherent long-lived spin-memory effects. Analysis of the coherent data using a theoretical model reveals approximately 10 times greater stability of the spin coherence at high temperature compared to that found previously for exciton states in four-wave-mixing experiments by Borri et al. [Phys. Rev. Lett. 87, 157401 (2001)]]. The data on incoherent polarization reveal a new form of spin memory based on charged quantum dots.

The Raman spectra of solid CO2, N2O, N2, and CO have been observed. Each spectrum can be analyzed by assuming an approximately ordered structure isomorphous with that of CO2. The lattice mode intensities have been calculated on the basis of the oriented gas model; they are in good agreement with the measured values and lead to confident assignments of the observed bands. The frequencies of the Eg and two Tg librations are 73.5, 91.5, and 132 cm−1, respectively, for CO2(15°K) and 68, 82, and 124.5 cm−1, respectively, for N2O (15°K). In α-N2 at 16°K two sharp bands are observed at 33.5 cm−1 (Eg) and 37.5 cm−1 (Tg). The other Tg mode may coincide with the Eg. Only one broad, asymmetric lattice band is observed for α-CO, which is located at 47.5 cm−1 at 12°K. The peak frequencies of these bands decrease an average of ∼ 15% on warming from 15°K to the phase transition, and there is a considerable concommitant increase in their widths.

No abstract

Librational lattice modes have been observed at 15°K in solid Cl2 at 83, 100, and 118 cm−1, and at 55, 74, 86, and 101 cm−1 in solid Br2. Their intensities relative to one another can be qualitatively accounted for by the oriented gas model. The stretching frequencies [ν(1–0)] of Cl2 and Br2 are shifted −13 and −20 cm−1, respectively, from their gas phase values and show structure due to isotope splittings and inter-molecular coupling. The spectra of the lattice modes, the Br2 stretching motion, and the Cl2 stretching motion indicate, respectively, strong, intermediate, and weak intermolecular coupling relative to the isotope splittings. Both lattice and internal frequencies indicate stronger intermolecular forces in solid Br2 than in solid Cl2.