Periodic nanostructures on Si(100) surfaces generated by high-repetition rate sub-15 fs pulsed near-infrared laser light.

Nanoscale rifts and ripples at a periodicity of 130 nm were generated on Si(100) surfaces immersed in water using tightly focused 800 nm 12 fs pulsed 85 MHz laser light at subnanojoule pulse energies. At radiant exposure close to the ablation threshold rifts were typically 20-50 nm in width and 70 nm in depth running perpendicular to the laser polarization. On increase of the irradiance, the rifts broadened and formed periodic ripples, whereas at highest exposure, a random nanoporous surface topology emerged. Rift and ripple formation is explained by laser-induced standing surface plasma waves, which result in periodic variation of dissipation and ablation.

Sub-100 nm structuring of indium-tin-oxide thin films by sub-15 femtosecond pulsed near-infrared laser light.

In magnetron sputtered indium-tin-oxide thin films of varying oxygen content, nanostructures were formed using tightly focused high-repetition rate near-infrared sub-15 femtosecond pulsed laser light. At radiant exposure well beyond the ablation threshold, cuts of 280-350 nm in width were generated. Illumination close to the ablation threshold resulted in periodic cuts of typically 20 nm in width at periodicities between 50 nm and 180 nm, as well as single sub-20 nm cuts. Subthreshold exposure, in combination with hydrochloric acid etching, yielded nanowires of 50 nm minimum lateral dimensions.

Small-hole waveguides in silicon photonic crystal slabs: efficient use of the complete photonic bandgap.

We investigate photonic crystal waveguides that are formed by holes of reduced diameter within a hexagonal lattice of cylindrical airholes in thin freestanding silicon slabs. The waveguides operate in both an even-symmetry bandgap and a partial gap of odd-symmetry modes that form a complete two-dimensional bandgap under the light line. The operating frequency is tuned by the small-hole diameter to fit within the range of both bandgaps and to match a free-space wavelength of 1550 nm. Their narrow bandwidth and low group velocity of light propagation renders the waveguides useful as filters or sensing elements. Because of the strong dependence of the waveguide mode characteristics on structural changes, the highest-precision lithographic fabrication techniques must be applied.

Planar cavity modes in void channel polymer photonic crystals.

Planar dielectric microcavities embedded in woodpile void channel photonic crystals with stop bands in the stacking direction ranging from 4.3 to 4.8 microm in wavelength were generated by femtosecond-laser direct writing in a solid polymer. Infrared transmission spectra revealed fundamental and second-order modes crossing the stop gap region with a free spectral range of 430 nm on varying the microcavity size from 0.3 to 2.25 microm. Supercell calculations confirmed the cavity size dependence of highly localized cavity modes, whereas the angle of incidence was accounted for using a simple Fabry-Perot model.

A new slant on photonic crystal fibers.

We present the novel use of microstructured optical fibers not as "light-pipes", but in a transverse geometry to manipulate the light propagating across the fiber. Fundamental and higher-order bandgaps were observed experimentally in this geometry using a number of techniques. The comparison of the measured spectra with photonic band structure and Finite-Difference Time-Domain simulations provide strong evidence that the spectral features are a result of the periodic nature of the fiber microstructure in the transverse direction.

Fluorescence resonance energy transfer analysis of protein-protein interactions in single living cells by multifocal multiphoton microscopy.

Fluorescence resonance energy transfer (FRET) resolved by multifocal multiphoton microscopy (MMM) was successfully used to measure transport phenomena in living cells. We expressed different pairs of CFP-/YFP-fusion proteins involved in retrograde Golgi-to-ER transport to analyze sorting of the occupied KDEL-receptor into retrograde transport vesicles triggered by application of the external cholera toxin mutant CTXK63. FRET observed as a sensitized emission of the acceptor was confirmed by acceptor photobleaching and the dequenching of the donor was measured. FRET-MMM data obtained from single cells were compared with bulk cell experiments employing spectrofluorimetry. The importance of controlling the degree of overexpression of CFP-/YFP-fusion proteins for FRET analysis is stressed in this article. Using MMM we showed for the first time that FRET can be measured across the Golgi membrane. Finally, FRET-MMM records performed continuously over 2 h allowed to analyze intracellular retrograde transport and sorting events and to discuss these mechanisms on a single cell level.

Near-infrared photonic crystals with higher-order bandgaps generated by two-photon photopolymerization.

Three-dimensional photonic crystals with bandgaps of 1.5-2.3 mum in wavelength and with gap/midgap ratios of as much as 18% were generated efficiently by two-photon photopolymerization in a liquid resin. From 0.5-1.1-mW femtosecond-pulsed 540-nm light, woodpile structures consisting of 40 layers of elliptically shaped rods spaced at 350-500 nm were fabricated by focusing with a 1.3-N.A. objective. The high degree of correlation in these structures allowed the suppression of infrared transmission by as much as 50% as well as the observation of higher-order bandgaps. We also investigated the decrease in the gap wavelength on reduction of layer spacing, in-plane rod spacing, and rod size.