CRYOGENIC NEAR-FIELD SCANNING OPTICAL MICROSCOPY: QUANTUM DOTS, CHARGE-ORDERED DOMAINS, AND FERROMAGNETIC NUCLEATION
Kolb, Paul Walter
Drew, Howard D
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Imaging of GaAs quantum dots (QDs) and the transition from charge-ordered insulator (COI) to ferromagnetic metal (FMM) in Nd<sub>½</sub>Sr<sub>½</sub>MnO<sub>3</sub> was achieved with sub-wavelength spatial resolution by means of near-field scanning optical microscopy (NSOM). To perform these imaging experiments, a cryogenic NSOM was developed. This instrument is compatible with high magnetic fields, has a novel capacitive sensor for xy-position measurement, and uses a novel light collection scheme with a parabolic mirror. The NSOM was used to image GaAs QDs formed by monolayer islands in a 2 nm thick Al<sub>0.3</sub>Ga<sub>0.7</sub>As/GaAs/Al<sub>0.3</sub>Ga<sub>0.7</sub>As quantum well (QW) by means of photoluminescence (PL). Discrete spectra associated with the QDs are observed and studied. Individual QDs are imaged with a resolution of 150 nm. Quantum coupling of some nearby QDs is suggested by the observation of PL with identical energy emanating from two sources spatially separated by 300 nm. Modeling these candidate coupled QDs as monolayer islands with a "dumbbell" shape leads to a consistent description. PL excitation experiments were attempted to confirm the model but failed because of the low throughput of the near-field probes. The NSOM was also used to image solid state structural phase transformations in Nd<sub>½</sub>Sr<sub>½</sub>MnO<sub>3</sub> which is known to exhibit two-phase coexistence. In particular, the transition from the low temperature COI to FMM which occurs at about 160 K was studied as a function of temperature. Both phases are shown to possess optical anisotropy in far and near-field imaging. Upon increasing temperature, the FMM phase is observed by far-field imaging to nucleate preferentially on variant boundaries and sometimes on twin boundaries, initially growing slowly and coexisting with the COI phase from about 155 K to 170 K. The NSOM is used to both image and measure the topography of FMM and COI twins. Polarization-dependent features are observed near the twin boundaries in the FMM and COI phases and are attributed to stress-induced birefringence. Upon increasing temperature near the transition to the FMM phase, polarization-dependent features appear on COI twin boundaries that are attributed to nucleation of the FMM phase.