Volume 24, Issue 4, September 2012
Index of content:
- SPECIAL ISSUE: GENERATION OF SUB-100 nm STRUCTURES BY NONLINEAR LASER-MATERIAL INTERATION
- Technical Articles
24(2012); http://dx.doi.org/10.2351/1.3695174View Description Hide Description
Nanojoining, a burgeoning research area, becomes a key manufacturing of complicated nanodevices with functional prefabricated components. In this work, various nanojoining methods are first reviewed. For nanojoining of Ag/Au nanoparticles, three methods are investigated comparatively. Thermal annealing shows a two-step solid state diffusion mechanism. Laser annealing by millisecond pulses displays the thermal activated solid state diffusion. Meanwhile, two effects have been identified in femtosecond laser irradiation with different laser intensities: photofragmentation at rather high intensity (∼1014 W/cm2) and nanojoining at low intensity (∼1010 W/cm2). The photofragmentation forms a large number of tiny nanoparticles with an average size of 10 nm. Control over irradiation conditions at intensities near 1010 W/cm2 results in nanojoining of most of the nanoparticles. This nanojoining is obtained through a nonthermal melting and a surface fusion welding. Joined Aunanoparticles are expected to have numerous applications, such as probes for surface enhance Raman spectroscopy.
Morphology of nanoscale structures on fused silica surfaces from interaction with temporally tailored femtosecond pulses24(2012); http://dx.doi.org/10.2351/1.3697950View Description Hide Description
Laser control of two basic ionization processes on fused silica, i.e., multiphoton ionization and avalanche ionization, with temporally asymmetric pulse envelopes is investigated. Control leads to different final electron densities/energies as the direct temporal intensity profile and the time inverted intensity profile address the two ionization processes in a different fashion. This results in observed different thresholds for material modification on the surface as well as in reproducible lateral structures being an order of magnitude below the diffraction limit (down and below 100 nm at a numerical aperture of 0.5). In this contribution, the morphology of the resulting structures is discussed.
24(2012); http://dx.doi.org/10.2351/1.4704853View Description Hide Description
Particle based near-field nanostructuring is an excellent possibility to overcome the optical diffraction limit in laser based material processing. In the near-field of microspheres which are irradiated with pulsed laser radiation, it is possible to generate nanoholes with diameters below 100 nm using a laser wavelength of 800 nm. To improve this approach, it is possible to position the microparticles with an optical trap to generate arbitrary structure geometries. In this paper, the authors describe the basic principle of optical trap assisted nanostructuring and present simulational and experimental results demonstrating the potential of this innovative nanoscale optical material processing technology.
Development of functional sub-100 nm structures with 3D two-photon polymerization technique and optical methods for characterization24(2012); http://dx.doi.org/10.2351/1.4712151View Description Hide Description
Investigations of two-photon polymerization (TPP) with sub-100 nm in the structuring resolution are presented by using photosensitive sol-gel material. The high photosensitivity of this material allows for TPP using a large variety in laser pulse durations covering a range between sub-10 fs and ≈140 fs. In this study, the authors demonstrate TPP structuring to obtain sub-100 nm in resolution by different approaches, namely, by adding a cross-linker to the material and polymerization with sub-10 fs short pulses. Additionally, a simulation and model based characterization method for periodic sub-100 nm structures was implemented and applied in an experimental white light interference Fourier-Scatterometry setup.
24(2012); http://dx.doi.org/10.2351/1.4712171View Description Hide Description
Second-harmonic generation (SHG) of light at nanoparticles provides the possibility to generate light (of a desired frequency) in-situ instead of introducing it by focusing an external light beam. Our theoretical study provides steering SHG light through the superposition of the radiation from a number of nanoparticles which are arranged in a circle. The authors assume cone-shaped or rod-shaped nanoparticles. Their radiation can be modeled as radiating dipoles. The superposition of their fields yields a “hot spot” with a full width at half-maximum of around 100 nm. Even more important, the position of the hot spot within the circular arrangement of nanoantennas can be adjusted in the xy plane simply by changing the incident angle of the exciting beam.
24(2012); http://dx.doi.org/10.2351/1.4712658View Description Hide Description
The formation of laser-induced periodic surface structures (LIPSS) in different materials (metals, semiconductors, and dielectrics) upon irradiation with linearly polarized fs-laser pulses (τ ∼ 30–150 fs, λ ∼ 800 nm) in air environment is studied experimentally and theoretically. In metals, predominantly low-spatial-frequency-LIPSS with periods close to the laser wavelength λ are observed perpendicular to the polarization. Under specific irradiation conditions, high-spatial-frequency-LIPSS with sub-100-nm spatial periods (∼λ/10) can be generated. For semiconductors, the impact of transient changes of the optical properties to the LIPSS periods is analyzed theoretically and experimentally. In dielectrics, the importance of transient excitation stages in the LIPSS formation is demonstrated experimentally using (multiple) double-fs-laser-pulse irradiation sequences. A characteristic decrease of the LIPSS periods is observed for double-pulse delays of less than 2 ps.
24(2012); http://dx.doi.org/10.2351/1.4716046View Description Hide Description
An overview of laser-assisted techniques developed in our group for fabricatingcarbonnanostructures, including two-dimensional graphene, one-dimensional carbon nanotubes, and zero-dimensional carbon nanoonions, is presented. Unique laser-material interactions provide versatile possibilities in fabricatingcarbonnanostructures, including localized heating, direct laser writing, tip-enhanced optical near-field effect,polarization, ablation, resonant excitation, precise energy delivery, and mask-free direct patterning. Rapid single-step fabrication of graphene patterns was achieved using laser directing writing. Parallel integration of single-walled carbon nanotubes was realized by making use of tip-enhanced optical near-field effect. High-quality carbon nanoonions were obtained through laser resonant excitation of precursor molecules.
24(2012); http://dx.doi.org/10.2351/1.4718561View Description Hide Description
The authors investigated the formation of periodic subwavelength structures, so-called nanogratings, in the volume of fused silica. These self-organized structures emerge upon irradiation with ultrashort laser pulses, undergoing three distinct stages of growth from randomly distributed nanostructures to extended domains with uniform periodicity. The experiments revealed that the cumulative action of subsequent laser pulses is mediated by dangling-bond type defects. On shorter time scales, transient self trapped excitons may significantly enhance the formation process. Nanogratings exhibit an extremely large temperature stability up to 1150 °C. In combination with the possibility to precisely tune their form birefringence, nanogratings provide a powerful tool to realize, thermally stable complex phase elements.
24(2012); http://dx.doi.org/10.2351/1.4718858View Description Hide Description
Low mean powers of 1–10 mW are sufficient for material nanoprocessing when using femtosecond laser microscopes. In particular, near infrared 12 fs laser pulses at peak TW/cm2 intensities, picojoule pulse energies, and 85 MHz repetition rate have been employed. Three-dimensional two-photonlithography as well as direct multiphotonablation have been performed. Subwavelength sub-100 nm cuts have been realized in photoresists, silicon wafers, glass, polymers, metals, and biological targets. When reducing the mean power to the microwatt range, nondestructive two-photonimaging was performed with the same setup taking advantage of the broad laser emission spectrum. Multiphoton microscopes based on low-cost ultracompact sub-20 fs laser sources may become novel nonlinear optical tools for highly precise nanoprocessing and two-photonimaging.
24(2012); http://dx.doi.org/10.2351/1.4719936View Description Hide Description
Fast and nondestructive laser light scatteringmeasurements characterize the quality of surfaces with sub-100 nm structures during the production process. The aim is to detect defective surface structures. The measurement procedure is based on a primary comparison of measuredspeckle patterns with a multitude of numerically calculated speckle patterns caused by defect-free and defective surface structures. The comparison shows characteristic light scattering effects, which have to be detected by the in-process measurement setup. An efficient rigorous algorithm, implemented on a graphics processing unit, calculates the scattered light intensity distributions within reasonable computing times. The measurements are performed with an angle resolved light scatteringmeasurement setup. The measuring procedure is applied to zinc oxide nanograss surfaces.
Direct laser writing-mediated generation of standardized topographies for dental implant surface optimization24(2012); http://dx.doi.org/10.2351/1.4728135View Description Hide Description
The functionalization of dental implants, aiming at the improvement of long-term acceptance, is of pivotal interest in dental research. Bone, connective tissue, and oral epithelium are in direct contact to the implant surface and exhibit distinct requirements for proper growth and differentiation. The authors applied direct laser writing and atomic layer deposition for the generation of TiO2-coated micro and nanostructures which were subsequently tested for colonization and growth behavior of SaOs-2 cells, an osteosarcoma cell line revealing osteoblastic properties. Structures composed of rigid posts and flexible rods provide a matrix, which—when spaced adequately—favor the three-dimensional growth and proliferation of SaOs-2 cells. The results provide a proof of concept for the optimization of dental implant surfaces using generic techniques which deliver highly standardized structure motifs supporting the biological functions of the tissues affected.
24(2012); http://dx.doi.org/10.2351/1.4730803View Description Hide Description
Loading microgels with bioactive nanoparticles (NPs) often requires multiple synthesis and purification steps, and organic solvents or precursors that are difficult to remove from the gel. Hence, a fast and aqueous synthesis procedure would facilitate the synthesis of inorganic–organic hybrid microgels. Two microgel compounds were hybridized with laser-generated zinc oxide (ZnO) NPs prepared in a single-step procedure. ZnO NPs were formed by laser ablation in liquid, while the polymermicrogels were synthesized in-situ inside the ablation chamber. Further, the authors report the preparation of two different microgel systems. The first one was produced without the use of chemical initiator forming hydrogels with ZnO NPs and diffuse morpholgy. Typical microgelcolloids were also synthesized via a conventional chemical method in a preheated reaction chamber. The existence of microgelcolloids partially loaded with ZnO NPs was confirmed in a transmission electron microscopy investigation. Fourier transform infrared spectroscopic measurements and dynamic light scattering verify the formation of polymercolloids. These initial results indicate the application potential of laser ablation in microgel precursor solution for the fabrication of polymeric carriers for inorganic nanoparticles. Preliminary biological tests using zinc chloride demonstrated negative dose effects on primary cell culture with zinc concentrations above 200 μM but no noticeable influence at 100 μM.
24(2012); http://dx.doi.org/10.2351/1.4730804View Description Hide Description
Laser interference patterning is a versatile tool for the fabrication of nano patterns. For this study, regular nano line patterns with feature sizes between 100 and 1000 nm were produced on polymers polyimide, polyetheretherketone, and polydimethylsiloxane.Cell culture experiments with B35 neuronal cells revealed the alignment of cellular extensions along nano grooves of different feature sizes. Especially, when feature depth exceeds a distinct threshold (aspect ratio > 0.6), more than 50% of cells are oriented parallel, i.e., within angles of 0°–30° to the direction of the line pattern. The presented techniques enable new materials to be processed and offer a promising approach for nerve repair in the central nervous system.
Materials and technologies for fabrication of three-dimensional microstructures with sub-100 nm feature sizes by two-photon polymerization24(2012); http://dx.doi.org/10.2351/1.4730807View Description Hide Description
The fabrication of sub-100 nm feature sizes in large-scale three-dimensional (3D) geometries by two-photon polymerization requires a precise control of the polymericreactions as well as of the intensity distribution of the ultrashort laser pulses. The authors, therefore, investigate the complex interplay of photoresist, processing parameters, and focusing optics. New types of inorganic– organic hybrid polymers are synthesized and characterized with respect to achievable structure sizes and their degree of crosslinking. For maintaining diffraction-limited focal conditions within the 3D processing region, a special hybrid optics is developed, where spatial and chromatic aberrations are compensated by a diffractive optical element. Feature sizes below 100 nm are demonstrated.
24(2012); http://dx.doi.org/10.2351/1.4731304View Description Hide Description
The optical near fields in close vicinity to plasmonic nanoscale objects show a considerable enhancement of the electrical field and are localized to dimensions much less than the wavelength of light. The authors show that an ablation process caused by the near-field enhancement of femtosecond laser pulses pattern the substrate below gold nanotriangles is a way to image the near-field distribution with a resolution below 20 nm. The mechanism of ablation studied by pulsed x-ray scattering reveals the nonthermal nature of the process.
24(2012); http://dx.doi.org/10.2351/1.4732594View Description Hide Description
Silicon wafers were structured with a femtosecond laser on the cm2 scale with high spatial frequency laser-induced periodic surface structures. These areas are characterized by regular parallel ripples with a period of the order of 100 nm. The particular ripple spacing is determined by the illumination wavelength of the tunable femtosecond laser. The cellular reaction to the structuredsilicon wafers and to the same materials, coated with calcium phosphate nanoparticles by electrophoretic deposition, was studied using L929 fibroblasts, human mesenchymal stem cells, and epithelial cells. The cells adhered uniformly to structured and unprocessed areas after seeding but significantly preferred the unstructured silicon after 48 h. This behavior disappeared after coating the structuredsurface with calcium phosphate nanoparticles.
Experimental and theoretical investigation on fs-laser-induced nanostructure formation on thin gold films24(2012); http://dx.doi.org/10.2351/1.4734048View Description Hide Description
In this paper, the authors report on the formation of nanobumps and nanojets on thin goldfilms, induced by single fs-laser pulse irradiation. Experimental results on the structure size and shape depending on the pulse energy and the pulse duration are presented. For the first time, the process of short laser pulse nanostructuring on thin metal films was modeled by molecular dynamic simulations on the scale directly accessible in the experiments. Additionally, pump-probe experiments were performed for in-situ visualization of the structure formation.