S. Halm

Vertical cavity surface emitting laser action of an all monolithic ZnO-based microcavity

We report on room temperature laser action of an all monolithic ZnO-based vertical cavity surface emitting laser (VCSEL) under optical pumping. The VCSEL structure consists of a 2λ microcavity containing eight ZnO/Zn0.92Mg0.08O quantum wells embedded in epitaxially grown Zn0.92Mg0.08O/Zn0.65Mg0.35O distributed Bragg reflectors (DBRs). As a prerequisite, design and growth of high reflectivity DBRs based on ZnO and (Zn,Mg)O for optical devices operating in the ultraviolet and blue-green spectral ranges are discussed.

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.

Local spin manipulation in ferromagnet-semiconductor hybrids

The authors demonstrate the usage of magnetic fringe fields from nanoscale ferromagnets to locally control the spin degree of freedom in a semiconductor. Fringe fields stemming from Fe∕Tb multilayer ferromagnets induce a local, remanent out-of-plane magnetization in a ZnCdMnSe dilute magnetic semiconductor quantum well, which in turn aligns the spin of photogenerated carriers via sp-d exchange interaction. The authors achieve a local exciton spin polarization of up to ±12% at 4K without the need of an external magnetic field. The spin polarization can be controlled in sign and magnitude via the magnetization of the ferromagnet and is observable up to T=80K.

Strong exciton-photon coupling in a monolithic ZnO/(Zn,Mg)O multiple quantum well microcavity

We report on strong exciton-photon coupling in an epitaxially grown (Zn,Mg)O-based λ-microcavity (MC) containing four 3.5 nm wide ZnO quantum wells (QWs) as active layers. At 5 K, the observed Rabi splitting in absorption is 26 meV, while the inhomogeneous linewidth of A and B excitons in similar QWs without a MC is about 10 meV. The strong coupling regime (SCR) is lost between 150 K and 200 K due to additional homogeneous broadening. From transfer matrix calculations, we deduce that increasing the number of QWs per cavity length can extend the SCR up to room temperature.

Local control of spin polarization in a semiconductor by microscale current loops

We demonstrate a method to electrically manipulate the spin polarization in a semiconductor on a micrometer length scale and a submicrosecond time scale. A variable magnetic field induced by a microscale current loop magnetizes the Mn2+ ions in a CdMnTe/CdMgTe diluted magnetic semiconductor quantum well, and via sp-d exchange interaction polarizes photogenerated electron-hole pairs. A maximum spin polarization degree of ±8.5% is obtained at 4.2 K without external magnetic field. The induced carrier spin polarization and the thermal heating of Mn2+ spins are resolved spatially and temporally by microphotoluminescence measurements.

Manipulation of spin states in a semiconductor by microscale magnets

We present the ability to manipulate spin states in a semiconductor on a sub-micrometre length scale via the magnetic fringe fields of microstructured magnets. Fe/Tb multilayers with remanent out-of-plane magnetization induce a remanent, vertical magnetization in an underlying diluted magnetic semiconductor (DMS), which in turn results in an efficient spin polarization of optically excited charge carriers via sp–d exchange interaction. By optically switching the magnetization of the ferromagnet, the DMS magnetization can be manipulated and the limits of a dynamical interaction between the spin states in the ferromagnet and the magnetic semiconductor are discussed. Moreover, magnetic ion spins of the DMS initially aligned along the sample surface are tipped by optically generated, spin polarized holes, which leads to a coherent spin precession around the total magnetic field. We show that the fringe field of in-plane magnetized Co wires can be utilized to locally modify the coherent dynamics of the semiconductor magnetization and demonstrate how the field inhomogeneity influences the coherent response of the Mn spin ensemble in the DMS.

Laser-induced erasure and reversal of the permanent magnetization in a ferromagnet-dilute magnetic semiconductor hybrid structure

Manipulation of the magnetizations via laser pulse heating is studied for a hybrid structure, where the fringe field of a metallic ferromagnet controls remotely the carrier and magnetic ion spins in a diluted magnetic semiconductor quantum well. A single nanosecond pulse with an energy density of 160 pJ/μm2 is found to be sufficient to erase the ferromagnet magnetization. Applying a reversed external bias field about five times below the ferromagnet coercive field, a complete reversal of the magnetization via optical excitation is demonstrated.

Coherent spin dynamics in Permalloy-GaAs hybrids at room temperature

Time-resolved Kerr rotation is used to study the coherent evolution of spin states in Permalloy-GaAs hybrid structures up to room temperature. Varying the width, distance and orientation of the nanopatterned Permalloy wires allow us to control the degree of frequency shift of the coherent electron spin precession due to the local magnetic fringe field in a wide range. Analytical fringe field calculations describe the measured data at room temperature quite well.

Local Control of Carrier Spin States in a Semiconductor by Microscale Ferromagnetic Wires

Employing fringe fields from microscale Fe/Tb multilayer ferromagnets (FMs) with remanent out‐of‐plane magnetization, we are able to define a local, remanent carrier spin polarization in an underlying ZnCdMnSe dilute magnetic semiconductor quantum well (DMS QW). The fringe fields of the FMs “imprint” a locally varying magnetization into the DMS QW by orienting the magnetic moment of the incorporated Mn2+ ions. Optically excited charge carriers align their spin along the local DMS magnetization due to the s‐pd exchange interaction. Using polarization resolved, magnetic field dependent photoluminescence (PL) spectroscopy we demonstrate a remanent DMS carrier spin polarization of 5 % in the vicinity of ferromagnetic Fe/Tb wire structures.