The dynamical response of a paramagnetic spin system to the exchange field of quasi-zero-dimensional electron-hole pairs in semiconductor quantum dots is investigated by time-resolved spectroscopy. The spin response time is extracted from the transient spectral shift of the photoluminescence signal caused by the dynamical spin alignment of magnetic ions incorporated in the crystal matrix. The formation of this ferromagnetically aligned spin complex is demonstrated to be surprisingly stable as compared to bulk systems, even at elevated temperatures and high external magnetic fields.

Statistical fluctuations of the magnetization are measured on the nanometer scale. As the experimental monitor we use the characteristic photoluminescence signal of a single electron-hole pair confined in one magnetic semiconductor quantum dot, which sensitively depends on the alignment of the magnetic ion spins. Quantitative access to statistical magnetic fluctuations is obtained by analyzing the linewidth broadening of the single dot emission. Our all-optical technique allows us to address a magnetic moment of only approximately equal 100 micro(B) and to resolve statistical changes on the order of a few micro(B).

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Integrated magnetic spectroscopy of dilute magnetic semiconductors (invited)This paper reviews the electrical, magnetic, and optical properties of diluted magnetic semiconductors (sometimes also referred to as "semi magnetic" semiconductors). These materials are ternary semiconductor alloys whose lattice is made up in part of substitutional magnetic ions. Cd,_,Mn,Te and Hg,_,Mn,Te are examples of such systems. As semiconductors, these alloys display interesting and important p~o~ertles, su.ch as the variation of the energy gap and of effective mass with composition. They also exhibit magnetlc properties which are interesting in their own right, e.g., a low temperature spin glass transition and magnon excitations. Most importantly, however, the presence of substitutional magnetic ions in these alloys leads to spin-spin exchange interaction between the localized magnetic moments and the band electro~s. This in tu~n has rather important consequences on band structure and on donor and acceptor states, leadmg to dramatlc effects in quantum transport, impurity conduction, and magneto-optics. Specifically, the presence 0: exchan~e interaction results in extremely large and temperature dependent g-factors of electrons and holes; m glgantlc values of Faraday rotation; in anomalously large negative magneto resistance; and in the formation of the bound magnetic polaron.

A femtosecond-resolved Faraday spectroscopy has been developed to directly monitor spin dynamics in magnetically tunable semiconductor quantum wells. Tunable terahertz quantum beating of the optical polarization is observed from coherent excitation of the spin states Zeeman split by a single ultrathin magnetic tunneling barrier. Subsequent spin-Aip scattering of photoinjected spin-polarized excitons deposits a magnetic "imprint" in the barrier which is orientation dependent and persists for orders of magnitude longer than the carrier lifetime.