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

We performed surface and bulk processing experiments on different transparent materials with ultra short laser pulses. The investigations were performed mainly at 800 nm and at pulse widths ranging from 0.2 to 5 ps. We focused our attention on fluence and shot number dependencies to analyze possible incubation effects in the different materials and determine the damage threshold. In the multi- shot experiments we determined strong incubation effects which we attribute to laser-induced defect formation and accumulation. Inside the bulk we were able to generate dots and lines even in sub-micrometers sizes. The structures were analyzed by means of optical microscopy. Laser pulses at a pulse width above ca. 1 ps demonstrate strong self focusing which can be utilized for bulk and rear surface micro structuring. Below a certain pulse width other effects counteract self focusing and beam diffraction and fillamentation seem to dominate. Depending on focusing optics we observe strong differences in the possibility to process the bulk of transparent materials with fs laser pulses which we attribute to the effects in Kerr non- linearity. Also, the consequences of incubation effects on the structuring inside the bulk seem to depend strongly on the pulse width. We discuss the results based on possible technological relevance and the ablation mechanism involved.

Laser-induced incubation in transparent materials and possible consequences for surface and bulk microstructuring with ultrashort pulses

Nadezhda M. Bulgakova', Razvan Stoian^, Arkadi Rosenfeld^, Ingolf V. HerteP^ and Eleanor E.B. Campbell' 'Institute of Thermophysics SB RAS, I Lavrentyev Ave., 630090 Novosibirsk, Russia ^Laboratoire TSI (UMR 5516 CNRS), Universite Jean Monnet, 10 rue Barrouin, 42000 Saint Etienne, France, ^Max-Born-Institut fiir Nichtlineare Optik und Kurzzeitspektroskopie, MaxBorn Str. 2a, D-12489 Berlin, Germany, * Department of Physics, Free University of Berlin, Amimallee 14, 14195 Berlin, Germany, ^Department of Physics, Goteborg University, SE41296 Goteborg, Sweden

https://www.springer.com/cda/content/document/cda_downloaddocument/9780387304526-c2.pdf?SGWID=0-0-45-351713-p107946667

Fast Electronic Transport and Coulomb Explosion in Materials Irradiated with Ultrashort Laser Pulses

Ultrashort pulsed laser ablation of dielectrics has been investigated using ex-situ morphological examinations in combination with in-situ time-of-flight mass spectrometry of the ablated species. Analysis of the energy spectrum of the ablation products provides a wealth of information on the processes occurring during femtosecond laser ablation of materials. The presentation will focus on the case of sapphire (Al 2 O 3 ) and discuss the fundamental processes in ultrashort pulsed laser sputtering. Two different ablation phases have been identified, a gentle phase with low ablation rates and a strong etch phase with higher ablation rates, but with limitation in structure quality. A comparison of the energy and momentum distributions of ejected ions, neutrals and electrons allows one to distinguish between non-thermal and thermal processes that lead to the macroscopic material removal. Fast positive ions with equal momenta are resulting from Coulomb explosion of the upper layers at low fluence and low number of irradiating laser pulses (gentle etch phase). Pump-probe studies with fs laser pulses reveal the dynamics of excitation and electron mediated energy transfer to the lattice. At higher laser fluences or after longer incubation, evidence for phase explosion can be derived from both the morphology of the surface and the results of the in-situ experiments.

Surface and bulk ultra-short pulsed laser processing of transparent materials

Ultrashort laser pulses have considerable potential for micron and sub-micron structuring of several materials. The lower energy impact, the reduction of thermal damage, the elimination of laser-plume interaction, and the exploitation of nonlinear optical effects all contribute to a strong improvement when compared to results using pulse widths in the nanosecond range. Depending on the choice of fluence compared to the damage threshold, with ultra-short laser pulses one is able to generate different types of structures, minimizing the heat affected zone. The damage threshold drops dramatically during the first laser shots, due to defect incubation. This has important consequences for applications, such as laser machining and for the lifetime of optical components. At a fluence below surface damage threshold we were also able to generate bulk modifications of different size and location in a controllable fashion by variation of laser pulse width, energy and number of shots, utilizing the beam narrowing effects during self focusing. A study of the dependence of the structure depth on the square root of the laser power for a given pulse length provides a straightforward method for determining the non-linear index of refraction.

Material processing of dielectrics with femtosecond lasers

Ultrashort pulsed laser ablation of dielectrics has been investigated using ex-situ morphological examinations in combination with in-situ time-of-flight mass spectrometry of the ablated species. Analysis of the energy spectrum of the ablation products provides a wealth of information on the processes occurring during femtosecond laser ablation of materials. The presentation will focus on the case of sapphire (Al2O3) and discuss the fundamental processes in ultrashort pulsed laser sputtering. Two different ablation phases have been identified, a gentle phase with low ablation rates and a strong etch phase with higher ablation rates, but with limitation in structure quality. A comparison of the energy and momentum distributions of ejected ions, neutrals and electrons allows one to distinguish between non-thermal and thermal processes that lead to the macroscopic material removal. Fast positive ions with equal momenta are resulting from Coulomb explosion of the upper layers at low fluence and low number of irradiating laser pulses (gentle etch phase). Pump-probe studies with fs laser pulses reveal the dynamics of excitation and electron mediated energy transfer to the lattice. At higher laser fluences or after longer incubation, evidence for phase explosion can be derived from both the morphology of the surface and the results of the in-situ experiments.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Arkadi Rosenfeld

Papers

Laser-induced incubation in transparent materials and possible consequences for surface and bulk microstructuring with ultrashort pulses

Fast Electronic Transport and Coulomb Explosion in Materials Irradiated with Ultrashort Laser Pulses

Surface and bulk ultra-short pulsed laser processing of transparent materials

Material processing of dielectrics with femtosecond lasers

Surface and bulk ultrashort-pulsed laser processing of transparent materials