There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

One-dimensional (1D) ZnO nanostructures have been studied intensively and extensively over the last decade not only for their remarkable chemical and physical properties, but also for their current and future diverse technological applications. This article gives a comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods. We will cover the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronic, and energy harvesting devices.

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One-dimensional ZnO nanostructures: Solution growth and functional properties

We review recent advances in laser cell surgery, and investigate the working mechanisms of femtosecond laser nanoprocessing in biomaterials with oscillator pulses of 80-MHz repetition rate and with amplified pulses of 1-kHz repetition rate. Plasma formation in water, the evolution of the temperature distribution, thermoelastic stress generation, and stress-induced bubble formation are numerically simulated for NA=1.3, and the outcome is compared to experimental results. Mechanisms and the spatial resolution of femtosecond laser surgery are then compared to the features of continuous-wave (cw) microbeams. We find that free electrons are produced in a fairly large irradiance range below the optical breakdown threshold, with a deterministic relationship between free-electron density and irradiance. This provides a large ‘tuning range’ for the creation of spatially extremely confined chemical, thermal, and mechanical effects via free-electron generation. Dissection at 80-MHz repetition rate is performed in the low-density plasma regime at pulse energies well below the optical breakdown threshold and only slightly higher than used for nonlinear imaging. It is mediated by free-electron-induced chemical decomposition (bond breaking) in conjunction with multiphoton-induced chemistry, and hardly related to heating or thermoelastic stresses. When the energy is raised, accumulative heating occurs and long-lasting bubbles are produced by tissue dissociation into volatile fragments, which is usually unwanted. By contrast, dissection at 1-kHz repetition rate is performed using more than 10-fold larger pulse energies and relies on thermoelastically induced formation of minute transient cavities with lifetimes <100 ns. Both modes of femtosecond laser nanoprocessing can achieve a 2–3 fold better precision than cell surgery using cw irradiation, and enable manipulation at arbitrary locations.

Mechanisms of femtosecond laser nanosurgery of cells and tissues

Ablation of metal targets by Ti:sapphire laser radiation is studied. The ablation depth per pulse is measured for laser pulse durations between 150 fs and 30 ps and fluences ranging from the ablation threshold ∼0.1 J/cm2 up to 10 J/cm2. Two different ablation regimes are observed for the first time. In both cases the ablation depth per pulse depends logarithmically on the laser fluence. A simple theoretical model for a qualitative description of the experimental results is presented.

Ablation of metals by ultrashort laser pulses

This paper reviews a new field of direct femtosecond laser surface nano/microstructuring and its applications. Over the past few years, direct femtosecond laser surface processing has distinguished itself from other conventional laser ablation methods and become one of the best ways to create surface structures at nano- and micro-scales on metals and semiconductors due to its flexibility, simplicity, and controllability in creating various types of nano/microstructures that are suitable for a wide range of applications. Significant advancements were made recently in applying this technique to altering optical properties of metals and semiconductors. As a result, highly absorptive metals and semiconductors were created, dubbed as the “black metals” and “black silicon”. Furthermore, various colors other than black have been created through structural coloring on metals. Direct femtosecond laser processing is also capable of producing novel materials with wetting properties ranging from superhydrophilic to superhydrophobic. In the extreme case, superwicking materials were created that can make liquids run vertically uphill against the gravity over an extended surface area. Though impressive scientific achievements have been made so far, direct femtosecond laser processing is still a young research field and many exciting findings are expected to emerge on its horizon.

Direct femtosecond laser surface nano/microstructuring and its applications

Optimization of the Energy Deposition in Glasses with Temporally-Shaped Femtosecond Laser Pulses

The availability of ultra short (ps and sub-ps) pulsed lasers has stimulated a growing interest in exploiting the enhanced flexibility of femtosecond and/or picosecond laser technology for micro-machining. The high peak powers available at relatively low single pulse energies potentially allow for a precise localization of photon energy, either on the surface or inside (transparent) materials. Three dimensional micro structuring of bulk transparent media without any sign of mechanical cracking has been demonstrated. In this study, the potential of ultra short laser processing was used to modify the cladding-core interface in normal fused silica wave guides. The idea behind this technique is to enforce a local mismatch for total reflection at the interface at minimal mechanic stress. The laser-induced modifications were studied in dependence of pulse width, focal alignment, single pulse energy and pulse overlap. Micro traces with a thickness between 3 and 8 μm were generated with a spacing of 10 μm in the sub-surface region using sub-ps and ps laser pulses at a wavelength of 800 nm. The optical leakage enforced by a micro spiral pattern is significant and can be utilized for medical applications or potentially also for telecommunications and fiber laser technology.

Ultrashort laser pulse processing of wave guides for medical applications

Ultrafast laser radiation usually induces rarefaction and negative refractive index changes in glasses with high thermal expansion. Acting on the laser-induced heat source, tailored intensity envelopes can generate in turn thermo-mechanical compaction. This leads to positive refractive index changes and to potential creation of light guiding structures. Using additional spatial wavefront corrections, the compaction mechanisms can be preserved at arbitrary depths.

Designing laser-induced refractive index changes in "thermal" glasses

The invention relates to a method for microstructuring an optical waveguide having a first cross-sectional region with a first refractive index, a second cross-sectional region with a second refractive index, and a boundary region in the transition from the first to the second cross-sectional region, in which the optical waveguide is exposed to laser radiation in the form of at least one ultra-short single pulse or a sequence of pulses with a defined energy input, whereby the radiant exposure takes place in such a manner that a modification of at least one optical property of the optical waveguide takes place at at least one defined portion of the boundary region.

Microstructuring of an optical waveguide for producing functional optical elements

The formation of laser induced periodic surface structures (LIPSS) is to a large extent of self-organizing nature and
in its early stages essentially influenced by optical scattering. The evolution of related mechanisms, however, has
still to be studied in detail and strongly depends on materials and laser parameters. Excitation with highly intense
ultrashort pulses leads to the creation of nanoripple structures with periods far below the fundamental wavelength
because of opening multiphoton excitation channels. Because of the drastically reduced spatial scale of such laser
induced periodic nanostructures (LIPNS), a particular influence of scattering is expected in this special case. Here
we report on first investigations of femtosecond-laser induced nanostructuring of sputtered titanium dioxide (TiO2)
layers in comparison to bulk material. The crucial role of the optical film quality for the morphology of the resulting
LIPNS was worked out. Typical periods of nanoripples were found to be within the range of 80-180 nm for an
excitation wavelength of 800 nm. Unlike our previously reported results on bulk TiO2, LIPNS in thin films appeared
preferentially at low pulse numbers (N=5-20). This observation was explained by a higher number of scattering
centers caused by the thin film structure and interfaces. The basic assumptions are further supported by
supplementary experiments with polished and unpolished surfaces of bulk TiO2 single crystals.

Arkadi Rosenfeld

Papers

Optimization of the Energy Deposition in Glasses with Temporally-Shaped Femtosecond Laser Pulses

Ultrashort laser pulse processing of wave guides for medical applications

Designing laser-induced refractive index changes in "thermal" glasses

Microstructuring of an optical waveguide for producing functional optical elements

Scattering-controlled femtosecond-laser induced nanostructuring of TiO2 thin films