Two-photon imaging through a multimode fiber

In this work we demonstrate 3D imaging using two-photon excitation through a 20 cm long multimode optical fiber (MMF) of 350 micrometers diameter. The imaging principle is similar to single photon fluorescence through a MMF, except that a focused femtosecond pulse is delivered and scanned over the sample. In our approach, focusing and scanning through the fiber is accomplished by digital phase conjugation using mode selection by time gating with an ultra-fast reference pulse. The excited two-photon emission is collected through the same fiber. We demonstrate depth sectioning by scanning the focused pulse in a 3D volume over a sample consisting of fluorescent beads suspended in a polymer. The achieved resolution is 1 micrometer laterally and 15 micrometers axially. Scanning is performed over an 80x80 micrometers field of view. To our knowledge, this is the first demonstration of high-resolution three-dimensional imaging using two-photon fluorescence through a multimode fiber

Beyond Vibrationally Mediated Electron Transfer: Coherent Phenomena Induced by Ultrafast Charge Separation

Wave packet propagation succeeding electron transfer (ET) from alizarin dye molecules into the nanocrystalline TiO2 semiconductor has been studied by ultrafast transient absorption spectroscopy. Due to the ultrafast time scale of the ET reaction of about 6 fs the system shows substantial differences to molecular ET systems. We show that the ET process is not mediated by molecular vibrations and therefore classical ET theories lose their applicability. Here the ET reaction itself prepares a vibrational wave packet and not the electromagnetic excitation by the laser pulse. Furthermore, the generation of phonons during polaron formation in the TiO2 lattice is observed in real time for this system. The presented investigations enable an unambiguous assignment of the involved photoinduced mechanisms and can contribute to a corresponding extension of molecular ET theories to ultrafast ET systems like alizarin/TiO2.Comment: This work was supported by the German Research Foundation (DFG) (Hu 1006/6-1, WA 1850/6-1) and European Union projects FDML-Raman (FP7 ERC StG, contract no. 259158) and ENCOMOLE-2i (Horizon 2020, ERC CoG no. 646669

Ultrafast photoinduced electron transfer in coumarin 343 sensitized TiO2-colloidal solution

Photoinduced electron transfer from organic dye molecules to semiconductor nanoparticles is the first and most important reaction step for the mechanism in the so called “wet solar cells” [1]. The time scale between the photoexcitation of the dye and the electron injection into the conduction band of the semiconductor colloid varies from a few tens of femtoseconds to nanoseconds, depending on the specific electron transfer parameters of the system, e.g., electronic coupling or free energy values of donor and acceptor molecules [2–10]. We show that visible pump/ white light probe is a very efficient tool to investigate the electron injection reaction allowing to observe simultaneously the relaxation of the excited dye, the injection process of the electron, the cooling of the injected electron and the charge recombination reaction

Residual effects of nitrogen fertilization on soil nitrogen pools and corn growth

Given the dynamic nature of soil nitrogen (N), inorganic N fertilization to corn (Zea mays L.) has potential to alter N pool balance by creating an accumulation or depletion of soil N. Current corn N recommendations in the common corn-soybean rotation of Indiana strive to find the best N rate that maximizes producer profit. Increasing our understanding of soil N will inform producers if they should adjust fertilizer rates for corn to influence maintenance of organic N and Carbon. Our objective was to determine residual N effects from fertilized corn in a corn-soybean rotation by measuring (1) soil N pools post soybean, (2) soil fertility, (3) growth and yield of corn, and (4) nitrogen removal in rotated corn. Field-scale corn N trials were established in 2006 at 6 Indiana farms with corn-soybean rotations (near cities of West Lafayette, Farmland, Columbia City, Wanatah, Butlerville, and Lafayette). A randomized complete block design assigned six corn N rates (ranging from starter only to above the minimum needed to maximize yield) to each replication. The design was not re-randomized the next corn year. In 2015, two or three of the replications at each location had only starter fertilizer, thus, allowing for determination of cumulative differences in N fertilization on soil N supply. Soil composites of each plot were collected from 0-20 cm, 20-40 cm, and 40-60 cm post corn planting (\u3c9 \u3edays). Initial samples were analyzed for general fertility, inorganic N, and total N. In addition, a 50-day incubation with soils maintained at 25 °C and 33 kPa moisture was used to examine mineralization and nitrification at days 10 and 50 days. Earleaf samples were collected at VT followed by stover and grain samples at maturity; plant samples were analyzed for macro- and micro-nutrient concentrations. Grain yield and total plant N uptake were also determined. Locations were kept separate for statistical analyses. ANOVAs carried out on general soil fertility data revealed minimal N rate effects. At Lafayette, pH for the 0-20 cm soil decreased linearly beginning at N rate 3 (135 kg ha -1); the acidifying effect of the side-dressed urea-ammonium nitrate (UAN) may be responsible for the pH decrease with increasing N rate. This trend was not observed at other locations. There was no N rate effect on day 0 total inorganic N (α ≤ 0.05). As N rate increased, total N decreased linearly from 0.9 to 0.8 g kg-1 (R2 = 0.68) at Columbia City. When the soil was incubated for 50 days, total inorganic N did not vary by N rate. Generally, soil inorganic and organic N decreased with depth from 0 to 60 cm. When corn was grown and the predominate source of N was derived from the soil, no differences were noted in plant N uptake nor yield for any location. Spring 2015 had record-breaking rainfall amounts which certainly contributed to residual N loss. Furthermore, the soil’s natural N supply, location management practice, and crop N demand are probable cause for the variances noted between locations. Overall, we conclude that corn N rate has negligible effects on residual N abundance, soil fertility, uptake, and grain yield for Indiana corn-soybean rotations