We demonstrate optical modes in InGaAs/GaAs microtubes acting as optical ring resonators. Self-supporting microtubes were fabricated by optical lithography and wet-etching processes utilizing the self-rolling mechanism of strained bilayers. The optical modes were probed by the photoluminescence of InAs quantum dots embedded in the tube's wall. In this novel microtube ring resonator we find a spectrum of sharp modes. They are in very good agreement with the theoretical results for a closed thin dielectric waveguide.

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Spin-wave excitations (magnons) are investigated in a one-dimensional (1D) magnonic crystal fabricated out of Ni80Fe20 nanowires. We find two different magnon band structures depending on the magnetic ordering of neighboring wires, i.e., parallel and antiparallel alignment. At a zero in-plane magnetic field H the modes of the antiparallel case are close to those obtained by zone folding of the spin-wave dispersions of the parallel case. This is no longer true for nonzero H which opens a forbidden frequency gap at the Brillouin zone boundary. The 1D stop band gap scales with the external field, which generates a periodic potential for Bragg reflection of the magnons.

Arrays of field-effect-con fined quantum dots with diameters smaller than 100 nm have been prepared starting from Al v Gaiv As-GaAs heterostructures. In far-infrared spectroscopy, we induce transitions between the 2-meV-separated quantum levels. We observe discrete steps in the gate-voltage dependence of the integrated absorption strength indicating directly the incremental occupation of each dot with TV™ I, 2, 3, and 4 electrons. From the gate-voltage dependence, we can estimate a Coulomb charging energy of about 15 meV. On a very fine scale, we also observe a spectral fine structure for the excitation of the quantum-dot atoms. PACS numbers: 71.45.-d, 73.20.Dx, 73.40.Gk, 73.40.Kp Low-dimensional quantum confined electronic systems in semiconductors have recently attracted much interest. The ultimate limit is a quantum dot, an artificial atom, where the electrons are confined in all three dimensions [1-6]. We have prepared, starting from two-dimensional electron systems in Al Y Gaiv As-GaAs heterostructures, high-precision periodic arrays of quantum dots where electrons are confined by the field effect of a laterally structured gate electrode. The separation between the quantized energy levels is typically 2 meV. In farinfrared (FIR) spectroscopy, we excite transitions between these levels and we observe discrete steps in the gate-voltage (V g ) dependence of the integrated absorption strength. Since the integrated absorption strength isproportional to the number of electrons per dot, this indicates directly the stepwise, discrete charging of each dot with 1, 2, 3, and 4 electrons. This links our experiments to another interesting topic, i.e., single-electron charging effects, which are extensively studied in both small metallic and semiconductor systems [7-12]. From the gatevoltage dependence of the steps, we can estimate the Coulomb charging energy to be about 15 meV. This large value stabilizes a well defined number of electrons in each individual dot of the array and allows us to study the spectral fine structure of quantum-dot atoms.