Showing papers in "Applied Physics A in 2003"

Femtosecond waveguide writing: a new avenue to three-dimensional integrated optics

Abstract: Using tightly focussed femtosecond laser pulses, waveguides can be fabricated inside various glasses and crystals. This technique has the potential to generate not only planar but three-dimensional photonic devices. In this paper we present, to the best of our knowledge, the first true three-dimensional integrated optical device, a 1×3 splitter fabricated in pure fused silica. The optical properties of this device and possibilities for the fabrication of complex high-density integrated optical elements are discussed.

Bulk heating of transparent materials using a high-repetition-rate femtosecond laser

Abstract: Femtosecond laser pulses can locally induce structural and chemical changes in the bulk of transparent materials, opening the door to the three-dimensional fabrication of optical devices. We review the laser and focusing parameters that have been applied to induce these changes and discuss the different physical mechanisms that play a role in forming them. We then describe a new technique for inducing refractive-index changes in bulk material using a high-repetition-rate femtosecond oscillator. The changes are caused by a localized melting of the material, which results from an accumulation of thermal energy due to nonlinear absorption of the high-repetition-rate train of laser pulses.

Exciton binding energies in organic semiconductors

Abstract: The exciton binding energy is one of the key parameters that govern the physics of many opto-electronic organic devices. It is shown that the previously reported values for the exciton binding energies in many organic semiconductors, which differ by more than an order of magnitude, can be consistently rationalized within the framework of the charging energy of the molecular units, with a simple dependence of the exciton binding energy on the length of these units. The implications of this result are discussed.

Dynamics of the plume formation and parameters of the ejected clusters in short-pulse laser ablation

Abstract: The dynamics of the early stages of the ablation plume formation and the mechanisms of cluster ejection are investigated in large-scale molecular dynamics simulations The cluster composition of the ablation plume has a strong dependence on the irradiation conditions and is defined by the interplay of a number of processes during the ablation plume evolution At sufficiently high laser fluences, the phase explosion of the overheated material leads to the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets, gas-phase molecules, and small clusters The ejection of the largest droplets is attributed to the hydrodynamic motion in the vicinity of the melted surface, especially active in the regime of stress confinement Spatially resolved analysis of the dynamics of the plume formation reveals the effect of segregation of the clusters of different sizes in the expanding plume A relatively low density of small/medium clusters is observed in the region adjacent to the surface, where large clusters are being formed Medium-size clusters dominate in the middle of the plume and only small clusters and monomers are observed near the front of the expanding plume Despite being ejected from deeper under the surface, the larger clusters in the plume have substantially higher internal temperatures as compared to the smaller clusters The cluster-size distributions can be relatively well described by a power law Y(N)∼N-τ with exponents different for small, up to ∼15 molecules, and large clusters The decay is much slower in the high-mass region of the distribution

Towards nanostructuring with femtosecond laser pulses

Abstract: Detailed investigations of the possibilities for using femtosecond lasers for the nanostructuring of metal layers and transparent materials are reported. The aim is to develop a simple laser-based technology for fabricating two- and three-dimensional nanostructures with structure sizes on the order of several hundred nanometers. This is required for many applications in photonics, for the fabrication of photonic crystals and microoptical devices, for data storage, displays, etc. Measurements of thermionic electron emission from metal targets, which provide valuable information on the dynamics of femtosecond laser ablation, are discussed. Sub-wavelength microstructuring of metals is performed and the minimum structure size that can be fabricated in transparent materials is identified. Two-photon polymerization of hybrid polymers is demonstrated as a promising femtosecond laser-based nanofabrication technology.

Structure and properties of tio2 surfaces: a brief review

Abstract: Titanium oxides are used in a wide variety of technological applications where surface properties play a role. TiO2 surfaces, especially the (110) face of rutile, have become prototypical model systems in the surface science of metal oxides. Reduced TiO2 single crystals are easy to work with experimentally, and their surfaces have been characterized with virtually all surface-science techniques. Recently, TiO2 has also been used to refine computational ab initio approaches and to calculate properties of adsorption systems. Scanning tunneling microscopy (STM) studies have shown that the surface structure of TiO2(110) is more complex than originally anticipated. The reduction state of the sample, i.e. the number and type of bulk defects, as well as the surface treatment (annealing in vacuum vs. annealing in oxygen), can give rise to different structures, such as two different (1×2) reconstructions, a ‘rosette’ overlayer, and crystallographic shear planes. Single point defects can be identified with STM and influence the surface chemistry in a variety of ways; the adsorption of water is discussed as one example. The growth of a large number of different metal overlayers has been studied on TiO2(110). Some of these studies have been instrumental in furthering the understanding of the ‘strong metal support interaction’ between group-VIII metals and TiO2, as well as low-temperature oxidation reactions on TiO2-supported nanoscopic gold clusters. The growth morphology, interfacial oxidation/reduction reaction, thermal stability, and geometric structure of ultra-thin metal overlayers follow general trends where the most critical parameter is the reactivity of the overlayer metal towards oxygen. It has been shown recently that the technologically more relevant TiO2 anatase phase can also be made accessible to surface investigations.

Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses

Abstract: Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 J cm-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass shows a broad fluorescence band at 630 nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses. Waveguides that support the fundamental mode at 633 nm have been fabricated inside fused silica by scanning the glass along the fs laser beam axis. The index changes are estimated to be approximately 0.07×10-3.

Femtosecond-laser-induced nanostructure formed on hard thin films of TiN and DLC

Abstract: We report observation of nanostructures formed on thin TiN and DLC films that were irradiated by 800- and 267-nm, femtosecond (fs) Ti:sapphire laser pulses at an energy fluence slightly above the ablation threshold. On the ablated thin-film surfaces, the linearly polarized fs pulses produce arrays of fine periodic structures that are almost oriented to the direction perpendicular to the laser polarization, while the circularly polarized light forms fine-dot structures. The size of these surface structures is 1/10–1/5 of the laser wavelength and decreases with a decrease in the laser wavelength.

3-D microstructuring inside photosensitive glass by femtosecond laser excitation

Abstract: We show that a femtosecond laser enables us to produce true three-dimensional (3-D) microstructures embedded in a photosensitive glass, which has superior properties of transparency, hardness and chemical and thermal resistances. The photosensitivity arises from the cerium in the glass. After exposure to a focused laser beam, latent images are written. Modified regions are developed by a post-baking process and then preferentially etched away in a 10% dilute solution of hydrofluoric acid at room temperature. We have measured the critical dose for modification of the photosensitive glass, and fabricated 3-D microstructures with microcells and hollow microchannels embedded in the glass based on the critical dose.

Imaging and writing at the nanoscale with focused visible light through saturable optical transitions

Abstract: Effecting a saturable optical molecular transition with a spatial intensity distribution featuring a local minimum allows the fundamental breaking of the diffraction barrier both in microscopy and in material structuring. If the transition can be repeatedly reverted, as in switchable fluorescent proteins and photochromic compounds, fluorescence imaging and writing is possible with spatial resolution down to the molecular scale.

A simple wet-chemical synthesis and characterization of CuO nanorods

Abstract: Using a simple wet-chemical route, we synthesized CuO nanorods with diameters of ca. 5–15 nm and lengths of up to 400 nm. The purity, crystallinity, morphology, structure features, and chemical composition of the as-prepared CuO nanorods were investigated by powder X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy.

Electro-responsive asymmetric nanopores in polyimide with stable ion-current signal

Abstract: For the preparation of a single asymmetrically shaped nanopore in a polyimide membrane, Kapton foils were irradiated with single heavy ions and subsequently etched from one side in sodium hypochlorite (NaOCl). The other side of the membrane was protected from etching by a stopping medium containing a reducing agent for hypochlorite ions (OCl-). The resulting conical nanopore rectified ion current and exhibited a stable ion-current flow.

Time-resolved spectroscopy measurements of a titanium plasma induced by nanosecond and femtosecond lasers

Abstract: We have studied the plasma induced at the surface of a titanium target following irradiation with femtosecond and nanosecond laser pulses. Time-resolved imaging and spectroscopic measurements allowed us to evidence some features specific to the femtosecond-laser-induced plasma. In this ultrashort interaction regime, we could discriminate between three different velocity populations in the plasma expansion. Coulomb explosion firstly creates highly energetic Ti+ ions, which are followed by atomic neutral titanium and lastly by nanoscale titanium oxide clusters.

Excimer laser polymer ablation: twenty years on

Abstract: Research and development in excimer laser polymer ablation has been actively pursued for some twenty years, driven by interest in the basic science as well as by numerous applications that have emerged for this high-resolution technique for material removal. This paper reviews some of the basic mechanistic aspects of the UV laser–polymer interaction as a prelude to dealing with practical matters related to polymer processing by ablation. Applications in micro-machining and potential areas for future research are briefly covered.

Ferroelectric and dielectric properties of Li-doped ZnO thin films prepared by pulsed laser deposition

Abstract: Zinc oxide is a very important piezoelectric material with lower preparation temperature, simpler structure and composition. By doping with some elements having smaller ionic radii, such as lithium, to substitute the zinc ions, it is expected that the center of the positive charge in a unit cell will not overlap with that of the negative charge in the same unit cell, leading to the appearance of the spontaneous polarization. Thin films of Li-doped ZnO with different compositions (Zn1-xLixOy, x=0.075, 0.1, 0.125 and 0.15) have been prepared on heavily doped Si substrates by a pulsed laser deposition technique. In the films with x=0.1 and x=0.125, ferroelectric P–E hysteresis loops were successfully observed. The remanent polarization and the coercive field of Zn0.9Li0.1Oy and Zn0.875Li0.125Oy were (0.193 μC/cm2, 4.8 kV/cm) and (0.255 μC/cm2, 4.89 kV/cm), respectively. An anomalous point in the dielectric spectrum of the Li-doped ZnO ceramics is observed, showing that the ferroelectric phase transition occurs around 67 °C under 7.5 at. % Li-doped ZnO and 74 °C under 10 at. %. If the remanent polarization of this material can be further increased, it may be used as a ferroelectric material.

Interfaces under ion irradiation: growth and taming of nanostructures

Abstract: We have investigated the synthesis of nanostructures, as well as the control of their size and location by means of ion beams. The phase separation and interface kinetics under ion irradiation give new possibilities for controlling the growth of nanostructures. Additionally, the chemical decomposition of the host matrix by collisional mixing can contribute to the self-organization of nanostructures, especially at interfaces. It is shown how collisional mixing during ion implantation affects nanocrystal (NC) synthesis and how ion irradiation through NCs modifies their size and size distribution. An analytical expression for solute concentration around an ion-irradiated NC was found, which may be written like the well-known Gibbs–Thomson relation. However, parameters have modified meanings, which has a significant impact on the evolution of NC ensembles. “Inverse Ostwald ripening” of NCs, resulting in an unimodal NC size distribution, is predicted, which has been confirmed experimentally for Au NCs in SiO2 and by kinetic lattice Monte Carlo simulations. At interfaces, the same ion-irradiation-induced mechanism may result in self-organization of NCs into a thin δ-layer. Collisional decomposition of SiO2 may enhance the NC δ-layer formation in SiO2 at the Si/SiO2 interface. The distance of the self-organized NC δ-layer from the SiO2/Si interface renders the structure interesting for non-volatile memory applications.

Design, formation and characterization of a novel multifunctional window with VO2 and TiO2 coatings

Abstract: A novel multifunctional window was demonstrated using VO2-based thermochromic films with a TiO2 antireflection coating. A strong enhancement in luminous transmittance of VO2 was calculated using either a single layer or a double layer of TiO2 for antireflection, with the double layer giving the better performance. A TiO2 (25 nm)/VO2 (50 nm)/TiO2 (25 nm) structure, of which the film thickness has been optimized to a maximum integrated luminous transmittance (Tlum) by calculation, was formed on SiO2 glass by sputtering in turn a V target in Ar+O2 for VO2 and a TiO2 target in Ar for TiO2. Optical properties were measured with a spectrophotometer, and the integrated solar and luminous properties were calculated based on the measured spectra. A maximum increase in Tlum by 86% (from 30.9% to 57.6%) was obtained for VO2 after double-layer TiO2 antireflection coating. The deposited VO2 film on SiO2 exhibits good thermochromism with a sharp optical transition at 58.5 °C, which decreased slightly to 57.5 °C after TiO2 coating. The proposed window is the most advanced among those similarly reported in being multifunctional with automatic solar/heat control, ultraviolet stopping and possibly a wide range of photocatalytic functions in addition to a high value of the luminous transmittance.

Polarisation-sensitive optical coherence tomography for material characterisation and strain-field mapping

Abstract: Optical coherence tomography (OCT) is a relatively new imaging technique capable of recording cross-sectional images of transparent and turbid structures with micrometer-scale resolution. Originally developed for biomedical imaging applications, this technique also has a great potential for non-destructive material characterisation and testing. Polarisation-sensitive (PS) OCT is a recent extension of classical OCT that measures and images birefringence properties of a sample, which, however, has not yet been applied to materials science. We present imaging of glass-fibre-enforced epoxy resin compounds and the detection of dry spots, where the epoxy did not properly penetrate the glass-fibre structure. Furthermore, we demonstrate PS-OCT imaging of the birefringence properties of different materials. The mapping of strain fields of samples under uniaxial and non-uniform external stress and the detection of flow patterns in injection-moulded plastic parts could be demonstrated with this technique for the first time.

Structural and optical properties of nanophase zinc oxide

Abstract: Nanophase zinc oxide samples were synthesized by a two-step solid-state reaction method. The phase structure and microstructure were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The vibrational Raman spectra were compared with those from the bulk and their grain size dependence was also examined. Their photoelectric behavior was studied by X-ray photoelectron spectroscopy (XPS). The peaks at 1044.5 and 1021.4 eV were recorded as corresponding to the respective binding energies of Zn 2p1/2 and Zn 2p3/2, and the photoelectron spectrum of O 1s in the as-prepared powder was located at 531.2 eV. A strong visible emission centered at 580 nm was clearly observed in the nanosized zinc oxide at room temperature. Photoluminescence (PL) spectra were investigated as a function of grain size after different heat treatments. The origin of the luminescence is attributed to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band.

Supramolecular architectures and nanostructures at metal surfaces

Abstract: The controlled formation of non-covalent bonds (H-bonding, metal–ligand interactions) is the key ingredient for the fabrication of supramolecular architectures and nanostructures. Upon deposition of molecular building blocks at well-defined surfaces, this issue can be directly addressed. Scanning tunneling microscopy observations are presented, which provide insight into the interaction of functional groups on metal substrates at the molecular level. In particular, carboxylic acids were employed: (4-[(pyrid-4-yl-ethynyl)]-benzoic acid (PEBA), 4-[trans-2-(pyrid-4-yl-vinyl)]-benzoic acid (PVBA) and trimesic acid (1,3,5-benzenetricarboxylic acid, TMA), which could be stabilized in a flat geometry at the surface. By choosing the appropriate substrate material and symmetry, the sensitive balance of intermolecular and molecule–substrate interactions can be tuned to obtain well-defined supramolecular architectures and nanostructures. The head-to-tail hydrogen bonding of the related rod-like species PEBA and PVBA stabilizes molecular rows on Ag(111). The subtle difference in the molecular geometries is reflected in the lateral ordering: While 2-D islanding is encountered with PEBA, 1-D nanogratings of supramolecular chiral H-bonded twin chains evolve for PVBA. The threefold symmetry of TMA in conjunction with the self-complementarity of its exodentate groups accounts for the formation of H-bonded honeycomb networks on Cu(100) at low temperatures. Metal–ligand interactions were probed with PVBA and TMA at Cu surfaces at ambient temperature. Deprotonation of the carboxyl moiety takes place, which readily interacts with Cu adatoms evaporated from step edges. This leads to a head-to-head pairing of PVBA on Cu(111) and cloverleaf-shaped Cu–TMA coordination compounds on Cu(001).

Nanoscale effects in focused ion beam processing

Abstract: Focused ion beams with diameters of 8 to 50 nm are used for material processing in the nanoscale regime. In this paper, effects of the ion beam–solid interaction determining the formation of small structures by ion-beam sputtering and chemically assisted material deposition and etching are investigated. In the case of decreasing feature size, angle-dependent sputtering, a non-constant sputter rate, and scattered ions play an important role. The impact on side-wall angle, aspect ratio, and shape of the bottom of the etched structures is discussed. In beam tail regions, these effects will be especially pronounced, leading to material swelling instead of material removal. Ion beam assisted etching and deposition will face additional effects. For small structures, gas depletion becomes a significant drawback. The impact on gas depletion and the competition with sputtering are discussed.

Transmission electron microscopy studies of femtosecond laser induced modifications in quartz

Abstract: Cross-sectional transmission electron microscopy investigations of femtosecond laser induced sub-micrometer structural modifications inside crystalline quartz were carried out. Modifications from single laser shots and from lines built of overlapping shots were imaged. Both single laser shot modifications and line structures show an amorphous core surrounded by a disturbed crystalline structure. A strong strain field surrounding the central, irradiated, core is responsible for an increase of the refractive index. Finite element method calculations of the strain field show maxima on both sides of the irradiated core, which are in good agreement with optical measurements of the refractive-index change.

Dispersion and alignment of carbon nanotubes in polycarbonate

Abstract: Dispersion and alignment of carbon nanotubes in thermoplastic polymers such as polycarbonate have been studied. Dispersion was accomplished by mixing in a conical twin-screw extruder and alignment was carried out using a fiber-spinning apparatus. The effects of mixing time and fiber draw rates on dispersion and alignment were investigated. Uniform dispersions were produced with relatively short residence times in the extruder. Excellent alignment of carbon nanotubes in nanocomposite filaments was obtained when the fiber draw rate was greater than 70 m/min. The ability to closely control the dispersion and alignment of carbon nanotubes in polymers is expected to lead to the development of nanocomposites with desirable electronic and structural properties.

Contacting single bundles of carbon nanotubes with alternating electric fields

Abstract: Single bundles of carbon nanotubes have been selectively deposited from suspensions onto sub-micron electrodes with alternating electric fields. We show that it is possible to control the trapping of a single bundle by the use of Ag as electrode material which, unlike Au, strongly interacts with the carboxyl functionalized carbon nanotubes. Excellent alignment of the bundles between Au or Ag electrodes occurs at frequencies above 1 kHz, with superior contacts being formed with Ag electrodes.

Ablation and cutting of planar silicon devices using femtosecond laser pulses

Abstract: Although lasers are generally able to machine silicon, the major material in many microsystems applications, doing so without influencing the physical properties of the bulk material remains an important challenge. Ultrafast lasers, in particular, with their potential to precisely ablate all kinds of solid materials, are able to perform such processes with high efficiency and accuracy. This article starts with an overview of the general interaction of ultrafast laser radiation with semiconductors, explaining the absorption processes and different fluence regimes for the ablation of silicon. Major parameter influences, especially for cutting processes in thin silicon, are described. By varying pulse energies, beam shaping methods, the beam polarization, and temperatures, the cutting quality and speed can be significantly influenced. One important quality aspect, besides kerf widths and surface roughness, is the amount of back-side chipping when cutting brittle materials. Achievements in speed enhancement using linear focus shapes are presented, with cutting speeds up to five times higher than by conventional spot-focusing. On the other hand, laser processes that cut with a spot focus offer the possibility of free-shape cutting, which is explained using the example of wafers carrying silicon chips with highly increased package densities.

Ultraviolet photodetection properties of indium oxide nanowires

Abstract: Photoconducting properties of In2O3 nanowires were studied. Devices based on individual In2O3 nanowires showed a substantial increase in conductance of up to four orders of magnitude upon exposure to UV light. Such devices also exhibited short response times and significant shifts in the threshold gate voltage. The sensitivity to UV of different wavelengths was studied and compared. We have further demonstrated the use of UV light as a “gas cleanser” for In2O3 nanowire chemical sensors, leading to a recovery time as short as 80 s.

Extended defects in shallow implants

Abstract: Extended defects are often found after ion implantation and annealing of silicon and they are known to affect dopant diffusion. The article reviews the structure and energetics of the most often found extended defects and describes the mechanisms by which all these defects grow in size and transform during annealing. Defects grow by interchanging the Si atoms they are composed of and thus maintain large supersaturations of free Si interstitials in the region. A model has been developped to describe such an evolution in presence of a free surface. It is shown that after low energy implantation, the surface of the wafer may recombine large amounts of these free Si interstitials, driving defects into dissolution before transformation into more stable forms.

Deep drilling of metals by femtosecond laser pulses

Abstract: Results of recent investigations on deep drilling of metals by femtosecond laser pulses are reported. At high laser fluences, well above the ablation threshold, femtosecond lasers can drill deep, high-quality holes in metals without any post-processing or special gas environment. It is shown that for high-quality drilling of metals, the following processes are important: (1) laser-induced optical breakdown of air containing metal vapor and small metal particles (debris) generated by multi-pulse femtosecond laser ablation, (2) transformation of laser pulses into light filaments, and (3) low-fluence finishing.

Influence of the substrate temperature during deposition on film characteristics of copper phthalocyanine and field-effect transistor properties

Abstract: In this paper, we employ different substrate temperatures during the deposition process and observe a highly ordered structure and strong orientation of copper phthalocyanine (CuPc) molecules on Si/SiO2 by using X-ray-diffraction and transmission electron microscopy analysis. The results show the effect of CuPc morphology at different substrate temperatures on the organic field-effect-transistor performance. When the substrate temperature for deposition of CuPc is 120 °C, a mobility of 3.75×10-3 cm2/V s can be obtained.

Surface nanostructuring of silicon

Abstract: Irradiation with polarized laser light of 248-nm wavelength induces the formation of periodic undulations ∼10-nm-highon flat silicon substrates. The wavelength of these periodic structures is a function of the light wavelength and the angle of incidence of the laser beam. Linear arrays of silicon nanoparticles with fairly uniform size that extended up to a millimeter were formed if the irradiation was performed using polarized light. When non-polarized laser light with the same fluence was used to illuminate an initially flat surface, non-aligned nanoparticle strings were obtained. However, if part of the irradiated area was microstructured, nanoparticle linear arrays resulted in the vicinity of the microstructured region. An analysis on the evolution of these nanostructures is presented. Nanocolumns could be grown on top of every cone of a microstructured surface upon cumulative laser irradiation with non-polarized light, reaching a height of ∼3 μm and a diameter of 100–200 nm. The mechanisms of nanocolumn origin and growth are analyzed.