Upon femtosecond laser irradiation, a bright, broadband photoluminescence is observed from graphene at frequencies well above the excitation frequency. Analyses show that it arises from radiative recombination of a broad distribution of nonequilibrium electrons and holes, generated by rapid scattering between photoexcited carriers within tens of femtoseconds after the optical excitation. Its highly unusual characteristics come from the unique electronic and structural properties of graphene.Graphene is an sp2-hybridized carbon monolayer that has many uncommon attributes, from exotic electrical 1 and thermal 2 transport to extraordinary mechanical properties. 3 It also exhibits unusual linear optical behavior, which shows a universal absorption constant 4 and can be effectively controlled by electrical gating. 5 The nonlinear optical response of graphene could be even more interesting: it is well known that unusual nonlinear optical phenomena can arise from low-dimensional confinement of carriers as has been demonstrated extensively in semiconductor quantum well structures. 6 Moreover, graphene has a unique band structure that could give rise to extraordinary nonlinear optical properties, yet such aspects have been overlooked so far.In this Rapid Communication, we report the observation of an unusually bright, broadband nonlinear photoluminescence (PL) generated in graphene upon femtosecond laser irradiation. It has a significant blueshifted component covering the entire visible spectrum when excited by near-infrared photons, and has very different characteristics from, for example, those of two-photon photoluminescence (TPPL) in noble metals. 7 Our analysis suggests that the graphene nonlinear PL arises from a broad distribution of nonequilibrium electron-hole (e-h) gas created via rapid scattering between a high density of photoexcited carriers. Although this mechanism is not limited to graphene, its two-dimensional (2D) nature and unusual band structure dramatically enhance the PL efficiency and bandwidth.Our experiments were performed with a 76 MHz Ti:Sapphire oscillator pumping an optical parametric oscillator with ~150 fs output pulses tunable within ~1.4-2.2 eV. As shown schematically in Fig. 1(a)_, the beam was focused on the sample at normal incidence, and the PL was collected in the backscattered direction. The signal was then detected by either a spectrograph equipped with a silicon charge-coupled device for monitoring spectra, or a single-photon counting silicon avalanche photodiode for detecting the integrated signal. The continuous-wave _cw_ Raman spectra were taken with a heliumblueshifted

Rapid evolution of miniaturized, automatic, robotized, function-centered devices has redefined space technology, bringing closer the realization of most ambitious interplanetary missions and intense near-Earth space exploration. Small unmanned satellites and probes are now being launched in hundreds at a time, resurrecting a dream of satellite constellations, i.e., wide, all-covering networks of small satellites capable of forming universal multifunctional, intelligent platforms for global communication, navigation, ubiquitous data mining, Earth observation, and many other functions, which was once doomed by the extraordinary cost of such systems. The ingression of novel nanostructured materials provided a solid base that enabled the advancement of these affordable systems in aspects of power, instrumentation, and communication. However, absence of efficient and reliable thrust systems with the capacity to support precise maneuvering of small satellites and CubeSats over long periods of deployment remains a real stumbling block both for the deployment of large satellite systems and for further exploration of deep space using a new generation of spacecraft. The last few years have seen tremendous global efforts to develop various miniaturized space thrusters, with great success stories. Yet, there are critical challenges that still face the space technology. These have been outlined at an inaugural International Workshop on Micropropulsion and Cubesats, MPCS-2017, a joint effort between Plasma Sources and Application Centre/Space Propulsion Centre (Singapore) and the Micropropulsion and Nanotechnology Lab, the G. Washington University (USA) devoted to miniaturized space propulsion systems, and hosted by CNR-Nanotec—P.Las.M.I. lab in Bari, Italy. This focused review aims to highlight the most promising developments reported at MPCS-2017 by leading world-reputed experts in miniaturized space propulsion systems. Recent advances in several major types of small thrusters including Hall thrusters, ion engines, helicon, and vacuum arc devices are presented, and trends and perspectives are outlined.

Spectral superbroadening of ultrashort-pulse propagation in a nonlinear medium is calculated by solving the nonlinear-wave equation more exactly. The effects of four-wave mixing and pulse deformation on phase modulation are included. The results yield an asymmetric Stokes-anti-Stokes broadening in fair agreement with the experimental observation.

Photoluminescence and electroluminescence from a Si/SiO2 superlattice have been measured. They show similar characteristics and exhibit an inhomogeneously broadened photoluminescence band peaked at 2.06 eV. The excitation spectrum indicates that excitations occur in the Si layers. The insensitivity of the luminescence spectrum and decay to temperature and excitation wavelength suggests that luminescence originates from transitions between localized defect states. These localized states are most likely defect states residing at the Si/SiO2 interfaces, because there should be a significant concentration of defects at the interface and SiO2 due to the large lattice mismatch and the amorphous state. The close proximity of these states offers a more rapid transition path for the excited electrons. An energy band diagram of the superlattice is constructed based on our results.

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