We report the observation of surface-enhanced Raman scattering (SERS) from a chemically etched ZnSe surface using 4-mercaptopyridine (4-MPy) as probe molecules. A thin film of ZnSe is grown by molecular beam epitaxy (MBE) and then etched using a strong acid. Protrusions of hemiellipsoidal nanoparticles are observed on the surface. Using the results of the Mie theory, we controlled the size of the nanoparticles to overlap significantly with maximum efficiency of near-field plasmon enhancement. In the Raman spectrum, we observe large enhancements of the a1, b1, and b2 modes when 4-MPy molecules are adsorbed on the surface using a 514.5 nm laser for excitation, indicating strong charge-transfer contributions. An enhancement factor of (2 × 106) is observed comparable to that of silver nanoparticles. We believe this large enhancement factor is an indication of the coupled contribution of several resonances. We propose that some combination of surface plasmon, charge transfer, and band-gap resonances is most likely the contributing factor in the observed Raman signal enhancement because all three of these resonances lie close to the excitation wavelength.

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The Aharonov-Bohm (AB) effect is typically discussed for a quantum charged particle moving along a trajectory enclosing a magnetic flux. There, however, exists a possibility of the AB effect associated with an overall neutral quasi-particle that possesses a radial electric dipole moment (e.g., an exciton in quantum ring or cylindrical type-II quantum dot (QD)). Excitons in type-II QDs are particularly interesting, due to relatively larger spatial separation of charged particles. We present results of magneto-photoluminescence (magneto-PL) studies on stacked submonolayer type-II ZnTe/ZnSe QDs. The AB phase reveals itself in magneto-PL of type-II QDs since, due to the cylindrical symmetry, the exciton ground state initially has a zero orbital angular momentum, which changes to higher values with increasing magnetic field. This transition of the angular momentum to a non-zero value with increasing magnetic field is observed in two ways: (i) in the changes of the exciton ground state energy and (ii) in the quenching of the excitonic PL intensity due to PL selection rules. In the figure we show effects of the applied magnetic field on the integrated intensity of the PL as a function of the magnetic field. The broad peak at ~1.42 T is assigned to the AB transition. To explain the observations, we first point out that single electron density calculations show that the electron, in the absence of strain, will be located either above or below the dot and, therefore, no AB signature is expected. In our case, the stacked character of the systems ensures that the electron's wave-function is "pushed" to the side of the dot due to electron-electron interaction, independent of stress, whereas cylindrical geometry nicely defines the ring-like trajectory for an electron. We thus explain the results as a motion of an electron around an entire stack of QDs, one of which is occupied by a hole (see inset in the figure). In addition we shall also show how the excitonic AB effect can be used to measure size of type-II excitons with sub-nanometer precision and discuss the role of the built-in electric field. Finally, we will show some very recent results of temperature dependent magneto-PL, which we use to investigate quantum phase coherence, without 'worrying about contacts' and show that decoherence is perfectly explained by 1-D ballistic theories. Light refreshments will be served.

451 3136As part of our development of an epitaxial lift-off process, utilising a sacrificial magnesium sulphide (MgS) layer, we have developed a MgS-rich ZnMgSSe alloy which provides excellent carrier confinement and resists both oxidation and acid attack. Here the optical transmission of the alloy has been measured and its bandgap determined as a direct transition at 4.19 AE 0.04 eV. Its composition has also been determined by X-ray interference (XRI) and comparison with simulations. For a range of alloy samples we obtain compositions of the Zn 1Àx Mg x S y Se 1Ày layers which are (x, y) ¼ (0.80 AE 0.02, 0.645 AE 0.025). Using the alloy bandgap and composition we have determined direct bandgap transition energy for MgS by extrapolation. This is found to be 4.78 AE 0.14 eV.