H. Schömig

Optical spectroscopy on individual CdSe/ZnMnSe quantum dots

Optical single dot studies in wide-band gap diluted magnetic CdSe/ZnMnSe quantum dots have been performed. Due to the sample design, the photoluminescence energy of the quantum dot signal is energetically below the internal Mn2+ transition, resulting in high quantum efficiencies comparable to nonmagnetic CdSe/ZnSe quantum dots. Magnetic-field- and temperature-dependent measurements on individual dots clearly demonstrate the exchange interaction between single excitons and individual Mn2+ ions, resulting in a giant Zeeman effect and a formation of quasi-zero-dimensional magnetic polarons.

Magnetic imprinting of submicron ferromagnetic wires on a diluted magnetic semiconductor quantum well

Hybrid structures consisting of submicron ferromagnetic dysprosium wires on a diluted magnetic semiconductor quantum well have been prepared and investigated by micro-magnetoluminescence spectroscopy. A magnetic field dependent redshift of the semiconductor band gap just beneath the dysprosium wires with respect to a reference area clearly demonstrates the impact of the magnetic fringe field on the optical properties of the underlying semiconductor.

Optical Spectroscopy on Single Semimagnetic Quantum Dots — Probing the Interaction between an Exciton and Its Magnetic Environment

Single diluted magnetic semiconductor (DMS) quantum dots are studied by means of photoluminescence spectroscopy and magnetoluminescence. The sp-d exchange interaction between a single electron-hole pair and roughly 100 Mn spins within the dot is demonstrated to result in (i) a significant enhancement (more than one order of magnitude) of the emission linewidth and (ii) a strongly modified magnetic field dependence of the polarization degree in a single DMS quantum dot as compared to a non-magnetic reference sample.

Selective thermal interdiffusion using patterned SiO2 masks: An alternative approach to buried CdTe/CdMgTe quantum wires

Buried CdTe/CdMgTe quantum wires with a lateral confinement potential of about 290 meV have been realized. Using electron beam lithography, SiO2 stripes are defined on a single quantum well sample and a subsequent 2 h annealing step in a Zn atmosphere results in a surprisingly strong interdiffusion between Cd and Mg atoms under the capped areas, causing a lateral modulation of the band gap. We obtain, e.g., for a nominal wire width of 100 nm, a lateral subband splitting of more than 8 meV, while the extension of the squared exciton wave function of the ground state is reduced to about 20 nm due to the error function-like potential shape.

Photoluminescence spectroscopy on single CdSe quantum dots in a semimagnetic ZnMnSe matrix

Abstract Photoluminescence spectroscopy with high spatial resolution has been applied to study single CdSe quantum dots embedded in a semimagnetic ZnMnSe matrix. The exchange interaction between the exciton and Mn 2+ ions of the crystal matrix results in the formation of a ferromagnetically aligned spin complex, a quasi-0D excitonic magnetic polaron. We demonstrate, that the energy shift of the EMP with temperature and excitation power, respectively, directly reflects the change of the magnetization, in particular the spin temperature, on a nanometer scale.

Optical spectroscopy on non-magnetic and semimagnetic single quantum dots in external fields

We demonstrate the ability to control the eigenstates in single quantum dots by applying well-defined external fields. Electric fields oriented in-plane as well as perpendicular to the disc-shaped dots allow a modification of the spatial part of the excitonic wavefunction, giving access to the charge distribution in the dot. In contrast, magnetic fields modify the spin part of the wavefunction, resulting in a Zeeman splitting and a diamagnetic shift of the photoluminescence emission. We used the unique property of semimagnetic quantum dots to tailor the effective g-factor, i.e. the sensitivity of the eigenstates to external magnetic fields, by about two orders of magnitude simply by varying the Mn concentration in the dots.

Micromagnetoluminescence on ferromagnet-semiconductor hybrid nanostructures

We present a magneto-optical approach for probing the optical response of a diluted magnetic semiconductor to the fringe field of nanostructured ferromagnets with sub-μm spatial resolution. Scanning across a CdZnMnSe/ZnSe quantum well covered by a single 700 nm wide dysprosium (Dy) wire a redshift of the magnetoluminescence signal is found beneath the ferromagnetic stripe. This on one hand demonstrates the strength of our experimental technique and on the other hand gives an indication of a local band gap modulation due to the giant Zeeman effect caused by the fringe field of ferromagnetic nanostructures.

Laser-controlled magnetization in a single magnetic semiconductor quantum dot

The photoluminescene signal of individual semimagnetic CdSe–Zn0.75Mn0.25Se quantum dots is used to study the magnetization of the Mn2+ spin system in the exchange field of a single exciton. We demonstrate that by increasing the laser excitation power a significant blue shift of the photoluminescence signal occurs. This is attributed to a laser-induced demagnetization, i.e. the laser-generated carriers heat the Mn2+ spin system via spin–flip exchange scattering.