Carbon nanodots (C-dots) synthesized by electrochemical ablation and small molecule carbonization, as well as graphene quantum dots (GQDs) fabricated by solvothermally cutting graphene oxide, are three kinds of typical green fluorescence carbon nanomaterials. Insight into the photoluminescence origin in these fluorescent carbon nanomaterials is one of the important matters of current debates. Here, a common origin of green luminescence in these C-dots and GQDs is unraveled by ultrafast spectroscopy. According to the change of surface functional groups during surface chemical reduction experiments, which are also accompanied by obvious emission-type transform, these common green luminescence emission centers that emerge in these C-dots and GQDs synthesized by bottom-up and top-down methods are unambiguously assigned to special edge states consisting of several carbon atoms on the edge of carbon backbone and functional groups with C═O (carbonyl and carboxyl groups). Our findings further suggest that the competition among various emission centers (bright edge states) and traps dominates the optical properties of these fluorescent carbon nanomaterials.

Remote laser in air based on amplified spontaneous emission (ASE) has produced rather well-collimated coherent beams in both backward and forward propagation directions [1-3], opening up possibilities for new remote sensing approaches [4,5]. The remote ASEbased lasers were shown to enable operation either at ~391 and 337 nm using molecular nitrogen or at ~845 nm using molecular oxygen as gain medium, depending on the employed pump lasers. To date, a multi-wavelength laser in air that allows for dynamically switching the operating wavelength has not yet been achieved, although this type of laser is certainly of high importance for detecting multiple hazard gases. In this Letter, we demonstrate, for the first time to our knowledge, a harmonic-seeded switchable multi-wavelength laser in air driven by intense mid-infrared femtosecond laser pulses. Furthermore, population inversion in the multi-wavelength remote laser occurs at an ultrafast time-scale (i.e., less than ~200 fs) owing to direct formation of excited molecular nitrogen ions by strong-field ionization of inner-valence electrons, which is fundamentally different from the previously reported pumping mechanisms based either on electron recombination of ionized molecular nitrogen or on resonant two-photon excitation of atomic oxygen fragments resulting from resonant two-photon dissociation of molecular oxygen. The bright multi-wavelength laser in air opens the perspective for remote detection of multiple pollutants based on nonlinear spectroscopy [6].

A facile and cost-effective preparation of moisture-responsive graphene bilayer paper by focused sunlight irradiation is reported. The smart graphene paper shows moisture-responsive properties due to selective adsorption of water molecules, leading to controllable actuation under humid conditions. In this way, graphene-based moisture-responsive actuators including a smart claw, an orientable transporter, and a crawler paper robot are successfully developed.

Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets, whose recollision energy in few-cycle ionizing laser pulses strongly depends on the optical waveform. Our work demonstrates an efficient and selective way of predetermining fragmentation and isomerization reactions in polyatomic molecules on subfemtosecond time scales.

Laser filamentation generated when intense laser pulses propagate in air has been an attractive phenomenon having a variety of potential applications such as detection and spectroscopy of gases at far distant places. It was discovered recently that the filamentation in air induces ‘lasing', showing that electronically excited N2+ is population-inverted, exhibiting marked contrast to the common understanding that molecular ions generated by intense laser fields are prepared mostly in their electronic ground states. Here, to clarify the mechanism of the population inversion, we adopt few-cycle laser pulses, and experimentally demonstrate that the lasing at 391 nm occurs instantaneously after N2+ is produced. Numerical simulations clarify that the population inversion is realized by the post-ionization couplings among the lowest three electronic states of N2+. Our results shed light on the controversy over the mechanism of the air lasing, and show that this post-ionization coupling can be a general mechanism of the atmospheric lasing.

We experimentally demonstrate the generation of narrow-bandwidth emissions with excellent coherent properties at ∼391 and ∼428 nm from N +) inside a femtosecond filament in air by an orthogonally polarized two-color driver field (i.e. 800 nm laser pulse and its second harmonic). The durations of the coherent emissions at 391 and 428 nm are measured to be ∼2.4 and ∼7.8 ps, respectively, both of which are much longer than the duration of the pump and its second harmonic pulses. Furthermore,

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We investigated the ejection of energetic protons from a series of polyatomic hydrocarbon molecules exposed to 790 nm 27 fs laser pulses. Using multiparticle coincidence imaging we were able to decompose the observed proton energy spectra into the contributions of individual fragmentation channels. It is shown that the molecules can completely fragment already at relatively low peak intensities of a few 10(14)  W/cm(2), and that the protons are ejected in a concerted Coulomb explosion from unexpectedly high charge states. The observations are in agreement with enhanced ionization taking place at many C-H bonds in parallel.