Metallic nanorod metamaterials, arrays of vertically aligned nanorods embedded in an alumina matrix (diameter ~80 nm, length 100-250 nm, period ~113 nm), have recently emerged as a flexible platform for applications in photonics, opto-electronics and sensing. The optical constants for these nanostructured materials are directly associated with their crystallinity. Controlling the crystallinity of these metamaterials in a fast manner has presented a new challenge. Here we show a laser annealing with a pulsed Nd:YAG laser (λ = 532 nm, FWHM 15 ns) to rapidly change the crystallinity of the metallic nanorods. The small column X-Ray diffraction characterization shows that not only the crystallinity of the metallic nanorods is changed, but also that evaporation of the metal occurs with laser annealing.
We present two realizations of a highly sensitive platform useful in environmental sensing and diagnostics - a
Fabry-Perot (FP) interferometer - (i) a pair of semi-transparent mirrors integrated into a microfluidic channel
and (ii) a silicon membrane of sub-micrometer thickness. Simple way to make microfluidic channels by (i)
hot-embossing into a sheet of technical grade PMMA and (ii) double-sided tape fixed glass with Au-coated
mirrors are presented. By changing the thickness of the Au coating, the roughness and porosity of mirror surface
is controlled. In turn, this provides a method to tune finesse of the FP cavity to monitor solutions flowwing
between the FP-mirrors. In case of silicon, the FP cavity is formed by coating two sides of a Si-membrane. These
two different approaches to harness a high sensitivity of the FP interferometry are proposed: changes of FP cavity
caused by materials in the channel can be monitored, while the coated membrane is used to monitor the effects
which are induced by membrane's ambiance. The finesse of the FP cavity is optimized for the maximum spectral
sensitivity at the cost of transmitted light intensity in case of microfluidic channel and silicon membrane. Via
optimization of the finesse (in the range 2-5) and overall transmission of a FP-pair (20-60%) practical solutions
are proposed for spectral sensing of (i) refractive index and mechanical channel width's changes in a microfluidic
channel as well as (ii) temperature changes of membrane's environment. Asymmetric thickness of the FP mirrors
can be used to optimize sensitivity.
KEYWORDS: Plasmons, Nanostructures, Near field, Near field optics, Metals, Current controlled current source, Gold, Silicon, Electromagnetism, Nanostructure materials
The excitation of plasmons in metallic nanostructures by light can give rise to pronounced local optical field enhancement with respect to the incident electromagnetic field. The details of these optical near fields depend sensitively on the properties of the nanostructures (material, size and shape), on the light wavelength and polarization, and also on the substrate. In this article we discuss several of these aspects influencing the near-field distribution for a given object and the resulting surface ablation by optical near fields. To this end we use both experimental and simulation techniques. Additionally we will present first results of experiments investigating the light emitted during nanoscale ablation. Finally, we will present an example how plasmon-mediated near-field effects act on the conductance of atomic-size contacts.
Phonons are considered as a most important origin of scattering and dissipation for electronic coherence in
nanostructures. The generation of coherent acoustic phonons with femtosecond laser pulses opens the possibility to
control phonon dynamics in amplitude and phase. We demonstrate a new experimental technique based on two
synchronized femtosecond lasers with GHz repetition rate to study the dynamics of coherently generated acoustic
phonons in semiconductor heterostructures with high sensitivity. High-speed synchronous optical sampling (ASOPS)
enables to scan a time-delay of 1 ns with 100 fs time resolution with a frequency in the kHz range without a moving part
in the set-up. We investigate the dynamics of coherent zone-folded acoustic phonons in semiconductor superlattices
(GaAs/AlAs and GaSb/InAs) and of coherent vibration of metallic nanostructures of non-spherical shape using ASOPS.
The contactless removal of small particles from surfaces by irradiation with intense laser pulses - dubbed laser cleaning
- has been used and studied for nearly two decades. Nevertheless, its applicability and the mechanisms involved are still
under debate. Here we give first a brief overview on relevant processes, and then present measurements of the velocities
of colloidal model particles after detachment under vacuum conditions. We also demonstrate a new Laser Cleaning
approach, by which submicrometer particles are removed by laser irradiation of the rear side of the wafers. The particles
are detached by an acoustic shock wave traveling to the wafer front side after laser ablation of the rear side. Not only is
this promising approach capable of defect free surface cleaning, detailed studies of particle velocities versus laser
fluence also allow insight into the different cleaning mechanisms involved. Furthermore, this technique could be applied
to determine adhesion energies of particles in the future.
A model for ns dry laser cleaning that treats the substrate and particle expansion on a unified basis is suggested. Formulas for the time-dependent thermal expansion of the substrate, valid for temperature-dependent parameters are derived. Van der Waals adhesion, the elasticity of the substrate and particle, as well as particle inertia is taken into account for an arbitrary temporal profile of the laser pulse. Time scale related to the size of the particles and the adhesion/elastic constants is revealed. Cleaning proceeds in different regimes if the duration of the laser pulse is much shorter/longer than this characteristic time. Expressions for cleaning thresholds are provided and compared with experiments on the cleaning of Si surfaces from spherical SiO2 particles with radii between 200 and 2585 nm in vacuum with 248 nm KrF excimer laser and 532 nm frequency doubled Nd-YAG laser. Large discrepancies between the experimental data and theoretical results for KrF laser suggest that ns dry laser cleaning cannot be explained on the basis of thermal expansion mechanism alone.
We report on experiments on the underlying physical mechanisms in the Dry-(DLC) and Steam Laser Cleaning (SLC) process. Using a frequency doubled, Q-switched Nd:YAG laser (FWHMequals8 ns), we removed polystyrene (PS) particles with diameters from 110-2000 nm from industrial silicon wafers by the DLC process. The experiments have been carried out both in ambient conditions as well as in high vacuum (10-6mbar) and the cleaned areas have been characterized by atomic force microscopy for damage inspection. Besides the determining the cleaning thresholds in laser fluence for a large interval of particle sizes we could show that particle removal in DLC is due to a combination of at least three effects: thermal substrate expansion, local substrate ablation due to field enhancement at the particle and explosive evaporation of absorbed humidity from the air. Which effect dominates the process is subject to the boundary conditions. For our laser parameters no damage free DLC was possible, i.e. whenever a particle was removed by DLC we damaged the substrate by local field enhancement. In our SLC experiments we determined the amount of superheating of a liquid layer adjacent to surfaces with controlled roughness that is necessary, in good agreement with theoretical predictions. Rough surfaces exhibited only a much smaller superheating.
H.-J. Muenzer, Mario Mosbacher, M. Bertsch, Oliver Dubbers, F. Burmeister, A. Pack, Reinhold Wannemacher, B.-U. Runge, Dieter Baeuerle, Johannes Boneberg, Paul Leiderer
We present a method for directly imaging the undisturbed near field of a particle resting on a surface. A comparison with numerical computations shows good agreement with the results of our experiments. These results have important consequences for laser-assisted particle removal where field enhancement may cause local surface damage and is one of the physical key processes in this cleaning method. On the other hand, the application of near fields at particles allows structuring of surfaces with structure dimensions in the order of 100 nm and even below.
In the present work, the effects of nanosecond pulsed laser irradiation on oxidized metallic surfaces were investigated, in order to estimate the possibility of using laser techniques for cleaning applications. After oxidation, the oxide layer developed on the metallic surfaces depends on the composition of the substrate and also on oxidation conditions. In this study a modeling system: pure Fe oxidized at 500 degrees C, has been investigated. The influence of the optical properties of the oxide films on the removal mechanisms and the cleaning efficiency of the laser technique was illustrated by performing specific electrochemical treatments.
We study the periodic damage structures that can be produced don the rough-surface of semiconductor when they are irradiated with a single beam of intense laser radiation. We observed a formation of periodic surface structures for porous Si with microscopic surface roughens. In the case of more strong rough Ge we observed an opposite effect to ripple formation: i.e. an effective destruction of resonance Fourier components of the random disturbed surface.
The time-resolved evolution of the cloud of the porous silicon (PS) particles produced by laser ablation is studied in situ by the analysis of the kinetics of photoluminescence (PL) signal. The cloud of the nanometer-sized silicon crystallites had the high enchantment of luminescence quantum efficiency (QE) in the red region of spectra. The slow PL kinetics component, which is due to the localized carriers, decays on a millisecond time scale. We observed high intensity of laser ablation process for light excited PS. We also study the emission of photons from remnants of porous silicon after laser ablation of PS sample. The red light generation was observed in this case of excitation of PS. Time-resolved experiments on the luminescence show that likely there are large lifetime phonons in quantum silicon wires.
We have investigated the correlation of photoluminescence (PL) properties with certain etching conditions and laser annealing of porous silicon (PS). We used the optical second-harmonic generation and photoluminescence methods for studies of IR laser annealing of porous silicon. We observed that IR illumination by series of laser pulses causes decreasing of SH signal and increasing of luminescence efficiency for PS samples.
We report the studies of process of laser annealing of island Ge films on the SI substrates. Based on the time- resolved reflectivity measurements, we obtained the data concerning melting, the dissolution and the resolidification of Ge thin films on the SI after laser annealing with nanosecond laser pulse We observed periodic melting of the interface Ge-Si under an illumination by series of laser pulses that connected with the peculiarity of the solution Ge in Si.
The cleaning of silicon surfaces from submicron dust particles has been studied by means of the 'Steam Laser Cleaning' (SLC) process and compared to 'Dry Laser Cleaning' (DLC) which is used nowadays in many applications. For SLC a thin liquid layer (e.g. a water- alcohol mixture) is condensed onto the substrate, and is subsequently evaporated by irradiating the surface with a short laser pulse. The DLC process, on the other hand, only relies on the laser pulse, without application of a vapor jet. We have systematically investigated the efficiency of these two processes for the removal of well-characterized polymer, silica and alumina particles of various sizes down to 60 nm in diameter, and have also studied the influence of light wavelength and laser pulse duration for nanosecond and picosecond pulses. The results demonstrate that for the gentle cleaning of silicon wafers SLC is a very efficient method and is superior to DLC. An effect which so far has only rarely been taken into account for laser cleaning is the field enhancement under the particles, which can give rise to serious surface damage, in particular when cleaning pulses in the picosecond and femtosecond range in the DLC are applied.
The present study concerns the cleaning of materials using pulsed laser irradiation and it summarizes the most recent results obtained by the collaborative research of different European groups, within the framework of a European program for training and mobility of researchers. A series of pulsed lasers, which emit at various wavelengths (from UV to IR) with short duration of pulse (few nano-, pico- or femto- seconds), is used for the removal of metallic, ceramic and organic pollutants from contaminated solid surfaces of different natures. The scientific results obtained so far are focused on the laser cleaning of silicon wafers from sub-micrometer particles, the theoretical modeling of particles removal mechanism during dry laser cleaning, the removal of oxide layers from oxidized metals and alloys, as well as on the development of laser imaging as a diagnostic tool for the estimation of the efficiency of the proposed cleaning technique.
The continuing trend towards miniaturization of integrated circuits requires increasing efforts and new concepts to clean wafer surfaces from dust particles. We report here about our studies of the `steam laser cleaning' process first described by Tam and coworkers. In order to remove submicron particles from a surface, first a thin liquid layer is condensed onto the substrate from the gas phase, and is subsequently evaporated momentarily by irradiating the surface with a short laser pulse. We have investigated the nucleation and growth of gas bubbles in the liquid, by which the whole process is started, with optical techniques like light scattering and surface plasmon resonance spectroscopy. The experiments indicate that the temperature where nucleation sets in is surprisingly low, which facilitates the application of this phenomenon for cleaning purposes. On the basis of these results and in order to study the cleaning effect for the particularly interesting surface of silicon in a quantitative way, we have deposited well-characterized spherical polymer and silica particles of different diameters from several ten to hundred nanometers on commercial Si wafers and have studied systematically the cleaning efficiency of the explosive evaporation process. The results show that steam laser cleaning is a promising and suitable method for removing sub-micron particles from semiconductor surfaces.
Surface plasmons, excited in thin metal films by means of laser irradiation, can be used as a very sensitive probe to study a wide variety of processes at surfaces and interfaces. We give here an overview over some recent applications. Examples are adsorption and desorption phenomena of physisorbed films, which can be studied with high temporal resolution on a nano-second scale. Another application is the sensitive detection of changes in the optical properties of thin films, e.g. of photo-chromic materials. Furthermore, surface plasmons can be used to monitor pressure variations on short timescales, and are thus suitable, e.g., for investigating and characterizing shock waves impinging onto liquid-solid interfaces. The surface plasmon technique can also be utilized for imaging the lateral structure of a patterned surface with a resolution on the order of a few microns. Surface plasmon spectroscopy is thus a very versatile tool for surface and interface diagnostics.
Light beams whose transverse intensity profile is given by the zero order Bessel function of the first kind, so-called Bessel beams, exhibit a sharp intensity maximum on the optic axis which, quite remarkably, does not spread even over macroscopic propagation distance. It is shown that this quasi non-diffracting behavior can lead to interesting novel pumping schemes in nonlinear optics. In experiments on stimulated Raman and Brillouin scattering of Bessel pump beams, we demonstrate that the frequency of the scattered radiation may be tuned without the use of a resonator, merely by changing the propagation constant of the Bessel beam. Due to the peculiar intensity distribution of the Bessel beam, the scattered beam is of particularly high quality and in the far field becomes intrinsically separated from the pump beam in the far field. Conversion efficiencies are comparable to those achieved with Gaussian beams.
Thin metal films (Cu, Au, Ni) and thin Si films with different thicknesses on fused silica were irradiated with a ns-pulse from a Nd:YAG-laser (lambda equals 532 nm, FWHM 7 ns). In this paper we want to focus on the optical properties of an undercooled metallic liquid and the observed dewetting of the thin films. The ongoing processes were monitored in situ with ns- time-resolved reflectivity measurements. We determined the thresholds for partial and compete melting of these films. Due to the high cooling rates (up to 1011 K/s) different 'frozen' stages of the dewetting phenomenon can be studied in detail with ex situ microscopic investigations, e.g. optical microscopy and scanning near field acoustic microscopy. We present measurements which show that we have observed spinodal dewetting of these thin films.
The transient temperature and pressure field development in the excimer laser-induced vaporization of liquids in contact with a solid surface is studied. A thin silicon film, which has temperature-dependent optical properties, is embedded between an absorbing chromium film and a transparent fused quartz substrate. Static reflectivity measurement is performed to determine the thin film optical properties at elevated temperatures. The transient backward reflectance responses from the silicon layer are compared with heat transfer modeling results. The backward reflectance probe is not affected by the creation of bubbles and is successfully employed for the first time to measure non-intrusively the temperature development during the rapid vaporization process. The optical reflectance probes are applied from the front-side and back-side of the sample simultaneously to monitor the dynamic bubble nucleation behavior and transient temperature development, respectively, at various ambient pressures using a high- pressure cell. The investigation on the effect of ambient pressure on the bubble nucleation threshold combined with the surface temperature measurement determines the thermodynamic state of the superheated metastable liquid at the interface and subsequently the explosion pressure.
The formation of bubbles at a liquid-solid interface due to acoustic cavitation depends particularly on the preconditions of the interface. Here, it is shown that following laser- induced bubble formation at the interface the acoustic cavitation efficiency is strongly enhanced. Optical reflectance measurements reveal that this observed enhancement of acoustic cavitation due to preceding laser-induced bubble formation, which could be termed as a memory effect, decays in a few hundred microseconds. By performing a double-pulse experiment using two excimer lasers the influence of process parameters, such as liquid temperature and salt concentration, on the temporal decay of the memory effect has been studied. An analysis of the experimental results by a diffusion model is presented.
Optical reflectivity and transmissivity measurements have been used to investigate the dynamics of melting and recrystallization of thin films of Si and Ge after laser-annealing with a ns Nd:YAG-laser pulse. We report on temperature dependent changes of the reflectivity of the liquid phase above and below the melting point and on various nucleation and solidification scenarios in thin films, depending on the energy density of the annealing laser.
We have studied the photochromic effect of tungsten oxide thin films, i.e. their coloration by exposure to ultraviolet light, in order to investigate the basic mechanisms of the chromogenic behavior of this material. We have measured the absorptivity of our samples by means of optically excited surface plasmons which enabled us for the first time to resolve the photochromism of layers 10 nm thick on a scale as short as seconds. In contrast to the widely accepted double-charge-injection model, we found that in the case of photochromism, oxygen is exchanged between the sample and the surrounding atmosphere. On the basis of this model, a simple nonlinear rate equation is presented which accounts both for the temporal behavior and for the atmosphere dependence of the absorbance of the films during and after ultraviolet light exposure.
Thin Si films used for solar energy purposes are commonly treated by relatively slow thermal annealing on a time scale of seconds to obtain the proper electrical behavior. We investigate a different approach, in which the films are annealed and/or molten by a frequency doubled Q- switched Nd:YAG laser pulse on a nanosecond time scale. We studied thin polycrystalline Si films of thickness between 43 nm and 259 nm on fused silica and on sapphire substrates. The different thermal conductivities of these substrates lead to different quench rates for the molten Si films. The optical and electrical properties of the Si films were systematically characterized during, respectively after the various annealing conditions. In addition we monitored the solidification process in situ by time-resolved optical measurements. At low energy densities the film is not completely molten by the laser pulse and resolidification takes place at the moving liquid-solid interface. Above a thickness-dependent threshold energy density complete melting is observed and nucleation in the supercooled melt prevails. In the latter case Sameshima and Usui showed that amorphization can be observed for Si films on fused silica up to thicknesses of 36 nm. We found that Si films on sapphire even with a thickness of 80 nm can be amophized. The reproducible threshold values suggest the possibility of lateral structuring.
We describe the use of optically excited surface plasmons to measure the thickness of ultrathin films deposited on gold and silver surfaces with submonolayer resolution. Additional structural information on the film is obtained by looking at the scattering of the surface plasmons. Applications of this method to physisorbed and quench-condensed molecular hydrogen films and to spreading of liquids are presented.
The dynamics of pulsed excimer laser ablation of thin films of organic material from a metal surface was investigated using optically excited surface plasmons. The metal substrate is formed by a 500 A thick silver film adjacent to the base of a glass prism, which serves for optical excitation and detection of the surface plasmons propagating at the silver-vacuum interface. The organic films were prepared by cooling the sample to 77 K and depositing the materials (isopropanol, tetrafluoromethane, acetone) from the vapor phase; ablation of the films was accomplished by KrF excimer laser ( = 248nm). Detection of the surface plasmon resonance allows to monitor the ablation process on a nanosecond time scale and with a resolution far better than a monolayer. At the same time, the surface plasmons provide an in- situ probe for time-resolved measurements of the substrate temperature. This allows to determinate the temperature at which the ablation sets in. For tetrafluoromethane, the ablation temperature was found to be independent of the laser fluence, suggesting a thermal desorption process. On the other hand, for isopropanol and acetone, a strong dependence of the ablation temperature on the fluence was observed. From the large delay between the leading edge of the laser pulse and the onset of ablation, we conclude that ablative photodecomposition is not present in our experiment. There is, however, evidence for laser- induced chemical transformations in the organic films. Solid films of transformed material, which were stable at room-temperature and under atmospheric conditions, were formed during the isopropanol experiments. We suggest these transformation process to be connected to the observed fluence dependence of the ablation temperature.
The behavior of the reflectivity of a Si single crystal during irradiation with two successive Nd:YAG pulses is investigated with ns resolution. The first pulse melts the surface, and therefore the reflection coefficient increases to the value of the metallic liquid at the melting temperature. Upon further heating of the surface with the second pulse, we observe a decrease of the reflection coefficient, resulting from the temperature dependent dielectric function of the molten Si. The largest decrease in the reflectivity that could be reached before damaging the surface amounted to 9% for both wavelengths 633 nm and 488 nm.
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