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

The formation of laser-induced periodic surface structures (LIPSS, ripples) was studied since four decades for a variety of materials including semiconductors. Recently, it was demonstrated that even a structuring of materials with high-energetic band edges like ZnO is enabled by highly intense femtosecond laser pulses at center wavelengths of 800 nm and pulse durations between 10 fs and 200 fs [1–3]. Because of its transparence and biocompatibility, ZnO is of increasing interest for UV optoelectronics, photovoltaics, spintronics and biosensors. Although the LIPSS phenomenon is well-known for a long time, the discussion about the relevant mechanisms is still controversial. Usually, the formation of periodic ripples is explained by the interference of a scattered surface electromagnetic wave with the incident laser pulse. For high intensity ultrashort laser pulses, however, the laser-material interaction is rather complex because of contributions from nonlinear optical excitation pathways. Here, we report on recent investigations on LIPSS formation in single-crystalline ZnO with two distinct types of ripple structures characterized by lower and higher spatial frequency LIPSS. The results are interpreted in frame of a simple Drude-model including a nonlinear process at the irradiated surface. On the basis of this approach it can be well understood that high frequency ripples are only observed with sub-picosecond laser pulses for a below band-gap excitation of dielectrics and semiconductors.

Mechanisms of femtosecond-laser induced surface structuring of zinc oxide crystals

Summary from only given. Femtosecond laser ablation of dielectrics, especially of brittle materials, often results in specific collateral damage in the form of fracture and exfoliation, as a consequence of the mechanical stress induced in the material. Fast electronic relaxation in solids with strong electron-phonon coupling establishes a guideline for using temporally-shaped pulses to exploit dynamical processes and optimize structuring with respect to the reduction of the residual damage. First effects of a modulated excitation were extracted by studying laser-induced optical damage with temporally tailored pulses. Two extreme cases were considered; /spl alpha/-SiO/sub 2/ with 100 fs electron trapping in self-induced deformations and Al/sub 2/O/sub 3/ where electrons remain quasi-free for tens of ps.

The effect of temporal pulse shaping in ultrafast laser ablation of dielectrics

Bei einem Verfahren zur direkten Mikrostrukturierung von Materialien mittels mindestens eines ultrakurzen Einzelpulses oder einer Pulsfolge mit definiertem Energieeintrag in das Material werden erfindungsgemas zur Vermeidung von Mikrorissen und Spannungen nacheinander mindestens zwei zeitlich geformte Laserpulse oder Pulszuge auf die Oberflache des zu bearbeitenden Materials gerichtet und wird der Abstand zweier aufeinander folgender Pulse oder Pulszuge kleiner oder gleich Pikosekunden eingestellt, sodass der folgende Puls noch in die bewirkte Anderung des ersten Pulses im zu bearbeitenden Material trifft, und Energie und Dauer des Pulses werden in Abhangigkeit vom zu bearbeitenden Material eingestellt. In a method for the direct microstructuring of materials by means of at least one ultrashort individual pulse or a pulse sequence with a defined energy input into the material according to the invention directed at the surface of the material to be machined in order to avoid of micro-cracks and stresses in succession at least two temporally shaped laser pulses or pulse trains, and the distance two successive pulses or pulse trains is less than or equal to set picoseconds, so the following pulse still meets in the induced change of the first pulse in the material to be processed, and energy and duration of the pulse can be adjusted depending on the material to be processed.

Verfahren zur direkten Mikrostrukturierung von Materialien Method for direct microstructure of materials

Summary form only given. Ultrashort laser pulses (with pulse durations below 5 ps) offer the possibility for microstructuring the surface of transparent materials and inside the bulk. The interaction of ultrashort laser pulses at a wavelength of 800 nm with the bulk of transparent materials like fluorides and oxides depends strongly on the pulse duration. To obtain equal conditions for the different experiments the laser beam was focused onto the front surface of the different samples. At a low fluence below surface damage threshold it was possible to initiate damage inside the bulk and with increasing shot numbers the damage track moves towards the surface. The pulse duration was 1.1 ps, characterizing a limit at which the self-focusing driven bulk damage was observable. While at sub-ps pulses no permanent damage was registered, at 5.3 ps only 10 laser shots were necessary to introduce damage inside the bulk of BK7.

Beam propagation of intense ultrashort laser pulses inside transparent dielectric materials for controlled 3D micro structuring

Zum Feinpolieren/-strukturieren warmeempfindlicher dielektrischer Materialien, insbesondere mit einem geringen Warmeausdehnungskoeffizienten, mittels Laserstrahlung wird erfindungsgemas ein Verfahren angegeben, bei dem intensive ultrakurze Laserstrahlung auf eine zu bearbeitende Oberflache des Materials gerichtet wird und die Einwirkzeit der Laserstrahlung auf die Oberflache im Bereich von 10 -13 s bis 10 -11 s und die Energie der Laserimpulse unterhalb der Ablationsschwelle aber ausreichend fur das Entstehen einer Coulombexplosion eingestellt wird. For fine polishing / -strukturieren heat sensitive dielectric materials, in particular with a low coefficient of thermal expansion, by means of laser radiation is specified according to the invention a method is directed at the intensity ultra-short laser radiation on a surface to be worked of the material and the time of action of the laser radiation on the surface in the range of 10 - 13 s, however, set to 10 -11 s and the energy of the laser pulses below the ablation threshold sufficient for the emergence of a Coulomb explosion. Mit dem erfindungsgemasen Verfahren wird ein Materialabtrag im Nanometerbereich mit ultrakurzen Laserpulsen im Pikosekunden- und Subpikosekunden-Bereich realisiert, wobei wahrend eines praablativen Verfahrensschritts (Abtrag unterhalb der Ablationsschwelle) die Materialoberflache fein poliert wird. With the inventive method, material removal is realized in the nanometer range with ultra-short laser pulses in the picosecond and sub-picosecond range, while a praablativen process step (cut below the ablation threshold) the material surface is finely polished. Aufgrund der extrem kurzen Einwirkzeit der Laserstrahlung auf die zu bearbeitende Oberflache findet eine sehr kleine Erwarmung statt, die nur im Bereich von einigen 10 Grad liegt. Because of the extremely short contact time of the laser radiation on the surface to be processed a very small heating takes place, which is only in the range of some 10 degrees.

Verfahren zum Feinpolieren/-strukturieren wärmeempfindlicher dielektrischer Materialien mittels Laserstrahlung A method for fine polishing / -strukturieren heat-sensitive dielectric materials by laser radiation

Arkadi Rosenfeld

Papers

Mechanisms of femtosecond-laser induced surface structuring of zinc oxide crystals

The effect of temporal pulse shaping in ultrafast laser ablation of dielectrics

Verfahren zur direkten Mikrostrukturierung von Materialien Method for direct microstructure of materials

Beam propagation of intense ultrashort laser pulses inside transparent dielectric materials for controlled 3D micro structuring

Verfahren zum Feinpolieren/-strukturieren wärmeempfindlicher dielektrischer Materialien mittels Laserstrahlung A method for fine polishing / -strukturieren heat-sensitive dielectric materials by laser radiation