Lasing effects based on individual quantum dots have been investigated in optically pumped high-Q micropillar cavities. We demonstrate a lowering of the threshold pump power from a off-resonance value of 37 microW to 18 microW when an individual quantum dot exciton is on-resonance with the cavity mode. Photon correlation studies below and above the laser threshold confirm the single dot influence. At resonance we observe antibunching with g((2))(0) = 0.36 at low excitation, which increases to 1 at about 1.5 times the threshold. In the off-resonant case, g((2))(0) is about 1 below and above threshold.

We present lasing in optically pumped high-Q micropillar cavity lasers with low thresholds and high β factors. The micropillar cavities with diameters between 1.0 and 4.0μm contain a single layer of low density In0.3Ga0.7As quantum dots as active region. Cavity Q factors of up to 23.000 for 4.0μm micropillar cavities and lasing based on less than 70 quantum dots is demonstrated.

The infrared reflectivity of a La0.67Ca0.33MnO3 single crystal is studied over a broad range of temperatures (78-340 K), magnetic fields (0-16 T), and wavenumbers (20-9000 cm −1 ). The optical conductivity gradually changes from a Drude-like behavior to a broad peak feature near 5000 cm −1 in the ferromagnetic state below the Curie temperature TC = 307 K. Various features of the optical conductivity bear striking resemblance to recent theoretical predictions based on the interplay between the double exchange interaction and the Jahn-Teller electron-phonon coupling. A large optical magnetoconductivity is observed near TC. 72.80.Ga, 71.38.+i Perovskite manganites of R (1−x) A x MnO 3 (R: trivalent rare-earth ions, A: divalent alkaline-earth ions, 0.2 ≤ x ≤ 0.5) are materials with interesting electric and magnetic properties which give rise to novel phenomena such as the colossal negative magnetoresistance (CMR). Many structural [1,2], magnetic [3][4][5], and transport [6-8] aspects of a ferromagnetic (FM) metal -paramagnetic (PM) insulator instability, triggered by either temperature or external magnetic field, can be explained in terms of the interplay of the electron-phonon coupling and the double-exchange (DEX) transport mechanism [9,10]. A spin polarized half-metallic behavior exists in the ferromagnetic state, which is believed to be essential for the occurrence of CMR in the perovskite manganites [11][12][13][14]. However, details of the interplay between the Jahn-Teller(JT) type electron-phonon coupling and the DEX mechanism [9,10] are yet to be manifested experimentally. PACSThis work aims at addressing this important issue by correlating the infrared (IR) optical conductivity of a La 0.67 Ca 0.33 MnO 3 single crystal with its magnetic and electron transport properties. The experimental approach involves measurements of the IR reflectivity spectra of a La 0.67 Ca 0.33 MnO 3 single crystal over a broad range of temperatures (78-340 K), magnetic fields (0-16 T), and wavenumbers (20-9000 cm −1 ). In addition, dc magnetization and resistivity measurements are performed on the same sample for comparison. The observed temperature dependence of the optical conductivity bears striking resemblance to the theoretical predictions [9,10] of temperature-dependent competing effects between the JT coupling and the DEX interaction in the ferromagnetic state. The frequency dependence of the optical conductivity σ(ω) varies from the Drude-like behavior at low temperatures to a broad peak feature in the mid-IR at high temperatures. These characteristics of optical conductivity are shown to be determined by the ratio of the electron-phonon coupling to the electron kinetic energy, consistent with theory [9,10].The single crystal of La 0.67 Ca 0.33 MnO 3 was grown by a floating zone method. X-ray diffraction confirms that the sample is single-phased with an orthorhombic structure close to cubic. The temperature-dependent magnetization of this La 0.67 Ca 0.33 MnO 3 single crystal reveals that the Curie temperature T C is at 3...

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Magnetic materials are usually divided into two classes: those with localised magnetic moments, and those with itinerant charge carriers. We present a comprehensive experimental (spectroscopic ellipsomerty) and theoretical study to demonstrate that these two types of magnetism do not only coexist but complement each other in the Kondo-lattice metal, Tb2PdSi3. In this material the itinerant charge carriers interact with large localised magnetic moments of Tb(4f) states, forming complex magnetic lattices at low temperatures, which we associate with self-organisation of magnetic clusters. The formation of magnetic clusters results in low-energy optical spectral weight shifts, which correspond to opening of the pseudogap in the conduction band of the itinerant charge carriers and development of the low- and high-spin intersite electronic transitions. This phenomenon, driven by self-trapping of electrons by magnetic fluctuations, could be common in correlated metals, including besides Kondo-lattice metals, Fe-based and cuprate superconductors.