Post-transient relaxation in graphene after an intense laser pulse

High intensity laser pulses were recently shown to induce a population inverted transient state in graphene [T. Li et al. Phys. Rev. Lett. 108, 167401 (2012)]. Using a combination of hydrodynamic arguments and a kinetic theory we determine the post-transient state relaxation of hot, dense, population inverted electrons towards equilibrium. The cooling rate and charge-imbalance relaxation rate are determined from the Boltzmann-equation including electron-phonon scattering. We show that the relaxation of the population inversion, driven by inter-band scattering processes, is much slower than the relaxation of the electron temperature, which is determined by intra-band scattering processes. This insight may be of relevance for the application of graphene as an optical gain medium.Comment: 10 pages, 4 figures, submitted as contribution of the IMPACT Special Topics series of the EP

Focal plane tuning in wide-field-of-view microscope with Talbot pattern illumination

We have developed a focal plane tuning technique for use in focus-grid-based wide-field-of-view microscopy (WFM). In WFM, the incidence of a collimated beam on a mask with a two-dimensional grid of aperture produced the Talbot images of the aperture grid. The Talbot pattern functioned as a focus grid and was used to illuminate the sample. By scanning the sample across the focus grid and collecting the transmission, we can generate a microscopy image of the sample. By tuning the wavelength of the laser, we can tune the focal plane of the WFM and acquire images of different depth into the sample. Images of a green algae microscope slide were acquired at different focal planes for demonstration

Photoinduced femtosecond relaxation of antiferromagnetic orders in the iron pnictides revealed by ultrafast laser ellipsometry

We report ultrafast softening of the antiferromagnetic order, ~150fs after the electron thermalization, which follows a two-step recovery pathway to reveal a distinct interplay of magnetism and the nematic order in iron pnictides

Ultrafast observation of critical nematic fluctuations and giant magnetoelastic coupling in iron pnictides

Many of the iron pnictides have strongly anisotropic normal-state characteristics, important for the exotic magnetic and superconducting behavior these materials exhibit. Yet, the origin of the observed anisotropy is unclear. Electronically driven nematicity has been suggested, but distinguishing this as an independent degree of freedom from magnetic and structural orders is difficult, as these couple together to break the same tetragonal symmetry. Here we use time-resolved polarimetry to reveal critical nematic fluctuations in unstrained Ba(Fe_(1-x)Co_x)_2As_2. The femtosecond anisotropic response, which arises from the two-fold in-plane anisotropy of the complex refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase with a critical divergence approaching the structural transition temperature. The dynamics also reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.Comment: The final, published version in Nature Communications Nat. Commun. 5:3229 doi: 10.1038/ncomms4229 (2014

The Fast-Evolving phy-2 Gene Modulates Sexual Development in Response to Light in the Model Fungus Neurospora crassa

Rapid responses to changes in incident light are critical to the guidance of behavior and development in most species. Phytochrome light receptors in particular play key roles in bacterial physiology and plant development, but their functions and regulation are less well understood in fungi. Nevertheless, genome-wide expression measurements provide key information that can guide experiments that reveal how genes respond to environmental signals and clarify their role in development. We performed functional genomic and phenotypic analyses of the two phytochromes in Neurospora crassa, a fungal model adapted to a postfire environment that experiences dramatically variable light conditions. Expression of phy-1 and phy-2 was low in early sexual development and in the case of phy-2 increased in late sexual development. Under light stimulation, strains with the phytochromes deleted exhibited increased expression of sexual development-related genes. Moreover, under red light, the phy-2 knockout strain commenced sexual development early. In the evolution of phytochromes within ascomycetes, at least two duplications have occurred, and the faster-evolving phy-2 gene has frequently been lost. Additionally, the three key cysteine sites that are critical for bacterial and plant phytochrome function are not conserved within fungal phy-2 homologs. Through the action of phytochromes, transitions between asexual and sexual reproduction are modulated by light level and light quality, presumably as an adaptation for fast asexual growth and initiation of sexual reproduction of N. crassa in exposed postfire ecosystems