Ultrafast Single-Molecule Fluorescence Measured by Femtosecond Double-Pulse Excitation Photon Antibunching

Most measurements of fluorescence lifetimes on the single-molecule level are carried out using avalanche photon diodes (APDs). These single-photon counters are inherently slow, and their response shows a strong dependence on photon energy, which can make reconvolution of the instrument response function (IRF) challenging. An ultrafast time resolution in single-molecule fluorescence is crucial, e.g., in determining donor lifetimes in donor-acceptor couples which undergo energy transfer, or in plasmonic antenna structures, where the radiative rate and non-radiative rates are enhanced. We introduce a femtosecond double-excitation (FeDEx) photon correlation technique, which measures the degree of photon antibunching as a function of time delay between two excitation pulses. In this boxcar integration, the time resolution of fluorescence transients is limited solely by the laser pulse length and is independent of the detector IRF. The versatility of the technique is demonstrated with a custom-made donor-acceptor complex with one donor and two acceptors and with single dye molecules positioned accurately between two gold nanoparticles using DNA origami. The latter structures show , similar to 75-fold radiative-rate enhancement and fluorescence lifetimes down to 19 ps, which is measured without the need for any reconvolution. With the potential of measuring subpicosecond fluorescence lifetimes, plasmonic antenna structures can now be optimized further

Modeling Plant Tissue Growth and Cell Division

Morphogenesis is the creation of form, a complex process requiring the integration of genetics, mechanics, and geometry. Patterning processes driven by molecular regulatory and signaling networks interact with growth to create organ shape, often in unintuitive ways. Computer simulation modeling is becoming an increasingly important tool to aid our understanding of these complex interactions. In this chapter we introduce computational approaches for studying these processes on spatial, multicellular domains. For some problems, such as the exploration of many patterning processes, simulation can be done on static (non-growing) templates. These can range from abstract idealized cells, such as rectangular or hex grids, to more realistic shapes such as Voronoi regions, or even shapes extracted from bio-imaging data. More dynamic processes like phyllotaxis involve the interaction of growth and patterning, and require the simulation of growing domains. In the simplest case growth can be modeled descriptively, provided as an input to the model. Growth is specified globally, and must be designed carefully to avoid conflicts (growing cells must fit together). We present several methods for this that can be applied to shoots, roots, leaves, and other plant organs. However when shape is an emergent property of the model, different cells or areas of the tissue need to specify their growth locally, and physically-based methods (mechanics) are required to resolve conflicts. Among these are mass-spring, finite element, and Hamiltonian-based approaches