Exciton

Semiconductor microcavities, in which strong coupling of excitons to confined photon modes leads to the formation of exciton–polariton modes, have increasingly become a focus for the study of spontaneous coherence, lasing and condensation in solid state systems. This review discusses the significant experimental progress to date, the phenomena associated with coherence which have been observed and also discusses in some detail the different theoretical models that have been used to study such systems. ...

We report the experimental detection of novel zero-resistance states induced by electromagnetic wave excitation in ultra high mobility GaAs/AlGaAs heterostructure devices, at low magnetic fields, B, in the large filling factor limit. Vanishing resistance is observed in the vicinity of B = [4/(4j+1)] B_f, where Bf =2π f m^*/e, where m^* an the effective mass, e is the charge, and f is the microwave frequency. The dependence of the effect is reported as a function of f, the temperature, the power, the current, and other experimental variables.[1] [1] R. G. Mani, J. H. Smet, K. von Klitzing, V. Narayanamurti, W. B. Johnson, and V. Umansky, Nature 420, 646 (2002); cond-mat/0303034, 0305507, 0306388, 0310474, 0311010.

We have examined ultrafast carrier dynamics and light amplification in ZnO nanowires following subpicosecond excitation at room temperature. We performed time- and wavelength-resolved pump-probe transmission and gain measurements on a 'forest' of 100- to 500-nm thick and 20-μm long nanowires, epitaxially grown on a sapphire wafer. Measurements were done using 267-nm pump pulses for direct, but inhomogeneous excitation, and 800-nm pulses to achieve homogeneous excitation via three-photon absorption. At the highest fluences, both for 267-nm and 800-nm pump pulses, a degenerate electron-hole plasma (EHP) is generated with carrier densities of 1025 m-3 or higher. We observed strong amplification of the probe, accompanied by a rapid decay (~ 1.5 ps) of the charge carriers. Below ~ 1025 m-3, the EHP becomes non-degenerate and the decay much slower. A dip in the pump-probe signal was observed, caused by ionization of probe exciton-polaritons by the pump. This effect allows for a measurement of the exciton-polariton dispersion relation and enhanced light-matter interaction in ZnO nanowires.

We develop a microscopic theory of a strong electromagnetic radiation interaction with bilayer graphene where an energy gap is opened by a static electric field perpendicular to graphene planes. We show that an adiabatic changing on time of the gate potentials (that leads to the resonance of the energy gap with electromagnetic field) may produce full inversion of the electron population between valence and conduction bands. Quantum kinetic equations for density matrix are obtained by the use of a tight-binding approach within second quantized Hamiltonian in an intense laser field and taking into account Coulomb correlations between particles. Excitonic absorption in graphene systems (monolayer and bilayer) with opened energy gap is investigated for different values of the gap and the parameters describing the band structure.

The reflection spectrum of the 0–0 component of the first singlet transition of tetracene crystal has been measured for temperatures ranging from 300 to 1.7 °K. A discontinuous change in the Davydov splitting occurs near 186 °K signifying the occurrence of a phase transition. At the lowest temperatures two anomalous reflectivity minima are observed inside the b‐polarized stopping band that are very similar to structure in the corresponding reflection band of anthracene crystal. The reflectivity minima are tentatively assigned to transitions to surface exciton states lying approximately 5 cm−1 and 160 cm−1 above the b‐polarized exciton transition at 5349 A. The other structure within the reflection band is interpreted as a coupling of the polariton to vibrational modes of intra‐ and intermolecular origins.

Quantum Division The notion of quantum mechanics is that variables are expressed as integer values. In quantum fluids, for instance, vortices are quantized in terms of the polarization and phase shifts observed as multiples of 2π, that is, full rotations of each variable around the vortex core. Theoretical work has predicted that in some instances there should be half-quantum vortices, where circumventing the vortex core is characterized by just π rotations of the phase and polarization. By observing exciton-polariton condensates Lagoudakis et al. (p. 974 ) present evidence for the existence of these long-predicted half-quantum vortices.

Zinc oxide (ZnO) nanostructures are very attractive in various optoelectronic applications such as light emitting devices. A fabrication process of these ZnO nanostructures which gives a good crystalline quality and being compatible with that of micro-fabrication has significant importance for practical application. In this work ZnO films with different thicknesses were deposited by RF-sputtering on vertically aligned multiwalled carbon nanotube (MWCNTs) template in order to obtain ZnO nanorods. The obtained hybrid structures (ZnO/MWCNTs) were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and time resolved photoluminescence spectroscopy (PL). Results show that the ZnO/MWCNTs have a nanorod structure like morphology with a good crystalline quality of the deposited ZnO on the MWCNTs. PL measurements reveal an enhancement of the band edge signal of ZnO/MWCNTs which is three times of magnitude higher compared to the ZnO film deposited on silicon. Moreover, the intensity enhancement varies as function of the ZnO thickness. Such hybrid structures are promising for optoelectronic application, such as blue–violet sources.