However the phenomena of titanium and Ti alloys laser processing seems to be well studied, there are still numerous cases of application, as functionalization of medical devices surface, being undiscovered. The task of antibacterial properties of the surface on account of TiO2 covering for medical applications is both demanded and not well studied, while the solution itself is complicated and fundamentally beguiling. Here we demonstrate a novel approach of fast and cheap laser-based technique to obtain TiO2 covering on the titanium mesh for controlled bone augmentation in oral implantology.
I will report on our recent progress in the development of laser printing technologies for fabrication of complex metallic and dielectric nanoparticle structures. Fabrication, characterization, and applications of the generated nanoparticle arrays in nanophotonics, plasmonics, and optical sensing will be demonstrated and discussed.
This paper reports ex-situ preparation of conductive polymer/single-walled carbon nanotubes (SWNTs) nanocomposites by adding high conductive SWNTs to the polymer matrix. Sonication methods were used to disperse the SWNTs in the polymer. The conductivity of the nanocomposites is tuned by increasing the concentration of SWNTs. Furthermore, we present two-photon polymerization (2PP) method to fabricate structures on the basis of conductive photosensitive composites. The conductive structures were successfully generated by means of 2PP effect induced by a femtosecond laser.
A theoretical approach, allowing analyzing the role of multipole modes in the extinction and scattering spectra of
arbitrary shaped nanoparticles, is developed in the framework of the discrete dipole approximation. The proposed
method can be used to control separately the positions of different multipole resonances as a function of nanoparticle
sizes, shapes and irradiation conditions. The main attention is given to the first multipole modes including magnetic
dipole and electric quadrupole moments. The magnetic quadrupole and electric octupole modes can also be involved in
the consideration. The method is applied to nonspherical Si nanoparticles with multipole responses in the visible optical
range, allowing a decomposition of single extinction (scattering) peaks into their constituting multipole contributions.
The unique property of Si nanoparticles to support magnetic optical response opens new ways for the construction of
novel nanooptical elements and can be particularly important for solving the problem of metamaterials with magnetic
properties in the visible spectral range.
We report on the fabrication of inverted Yablonovite-like three-dimensional photonic crystals by nonlinear optical
nanolithography based on two-photon polymerization of a zirconium propoxide hybrid organic-inorganic material with
Irgacure 369 as photo-initiator. Advantage of this material is ultra-low shrinkage that guaranty high fabrication fidelity.
Images of the fabricated structure are obtained with a scanning electron microscope. The photonic crystal consists of
three sets of nearly cylindrical structural elements directed along the three lattice vectors of the fcc lattice and cross each
other at certain angles to produce inverted Yablonovite geometry. To investigate photonic properties of the inverted
Yablonovite structures, we calculate the photonic band structure for ten lowest-frequency electromagnetic modes. In
contrast to the direct Yablonovite structure that has a complete photonic band gap between the second and third bands,
we find no complete photonic band gaps in the inverted Yablonovite lattice. This situation is opposite to the case of fcc
lattice of close-packed dielectric spheres in air that has a complete photonic band gap only for the inverted geometry.
The optical properties of regular nanoparticle arrays consisting of spherical semiconductor and noble metal nanoparticles
are providing interesting aspects for the development of novel and powerful sensor concepts. In this contribution, we
demonstrate femtosecond laser-induced transfer of metallic and semiconductor thin films as a unique tool for realizing
controllable structures of any desired configuration of exactly spherical nanoparticles, having diameters between 40 nm
and 1500 nm. The optical properties of nanoparticles and nanoparticle arrays fabricated by this new approach are
investigated spectroscopically and by scattering of surface plasmon-polaritons (SPPs). SPP-scattering constitutes a novel
method to obtain insight into the contribution of different multipole moments to the scattering properties of the particles.
Furthermore, the particles can be combined with 3D photonic structures fabricated using two-photon polymerization,
providing new approaches to the development of nanophotonic devices and 3D metamaterials. Here, we demonstrate an
optical sensor with a sensitivity of 365 nm/RIU and a figure of merit of 21.5 in the visible spectral range.
Ultrashort pulsed lasers are increasingly used in micromachining applications. Their short pulse lengths lead to well
defined thresholds for the onset of material ablation and to the formation of only very small heat affected zones, which
can be practically neglected in the majority of cases. Structure sizes down to the sub-micron range are possible in almost
all materials - including heat sensitive materials. Ultrashort pulse laser ablation - even though called "cold ablation" - in
fact is a heat driven process. Ablation takes place after a strong and fast temperature increase carrying away most of the
heat with the ablated particles. This type of heat convection is not possible when reducing the laser fluence slightly
below the ablation threshold. In this case temperature decreases slower giving rise to heat-induced material deformations
and melt dynamics. After cooling down protruding structures can remain - ablation-free laser surface structuring is
possible. Structure formation is boosted on thin metal films and offers best reproducibility and broadest processing
windows for metals with high ductility and weak electron phonon coupling strength. All approaches to understand the
process formation are currently based only on images of the final structures. The pump-probe imaging investigations
presented here lead to a better process understanding.
Recently Fourier-Scatterometry has become of increasing interest for quantitative wafer metrology. But also in other
fields the fast and precise optical characterization of periodical gratings of sub 100 nm size is of great interest. We
present the application of Fourier-Scatterometry, extended by the use of the coherent properties of white light for the
characterization of sub-wavelength periodic gratings of photosensitive material structured by two-photon polymerization.
First a simulation-based sensitivity comparison of Fourier-Scatterometry at one fixed wavelength, Fourier-Scatterometry
using a white light light source and also additionally using a reference-branch for white-light-interference has been
carried out. The investigated structures include gratings produced by two-photon polymerization of photosensitive
material and typical semiconductor test gratings. The simulations were performed using the rigorous-coupled-waveanalysis
included in our software package MicroSim. A sensitivity comparison between both methods is presented for
the mentioned structure types. We also show our experimental implementation of the measurement setup using a whitelight-
laser and a modified microscope with a high-NA (NA: 0.95) objective as well as a Linnik-type reference branch
for the phase sensitive measurements. First measurements for the investigation of the performance of this measurement
setup are presented for comparison with the simulation results.
Treatments by femtosecond Ti-sapphire laser (wavelength is 800 nm, pulse duration is <30 fs) and nanosecond KrF
excimer laser (wavelength is 248 nm, pulse duration is 25 ns) were performed to study the impact of laser fluence, pulse
duration and energy of photons on the process of chemical and structural reconstructions in GeOx and GeO2:Ge
heterolayers. The solid GeOx films are metastable and decompose into two phases - Ge and GeO2. Nanosecond KrF laser
irradiation produces significant shrinkage of GeOx films and GeO2:Ge heterolayers caped by thin SiNxOy or SiO2 films.
Pulse annealing by both types of lasers stimulate as the process of nanoclusters forming and crystallization of initial
amorphous Ge-nanoclusters in the GeO2 matrix. The possibility to use the solid germanium monoxide (GeO(s)) and
GeO2:Ge heterolayers (without cap layers) as nano-resist in laser nanolithography was demonstrated. The new
nanostructured material was formed - the layers of nanofoam GeO2 which can be obtained only with application of
femtosecond laser pulse treatments.
We study the guiding properties of laser-written dielectric-loaded surface plasmon polariton waveguides
(DLSPPWs). The guiding structures such as straight waveguides, S-bends, Y-splitter, resonant filters, and Mach-
Zehnder interferometers are realized by two-photon induced polymerization of commercial photolithographic
resists. The height of the components can be adjusted by spin-coating of the material. Minimum widths of 400
nm of the DLSPPWs fabricated directly on thin metal films can be achieved. Replica molding of polymer
surface structures allows a further reduction of the DLSPPW width down to 200 nm. The DLSPPWs are
characterized by leakage radiation microscopy in the visible and near infrared spectral region. We demonstrate
the possibility to selectively excite different modes in the waveguides. Fourier-plane imaging allows a direct
observation of the excited modes of the DLSPPWs. The simultaneous excitation of fundamental and higherorder
modes results in a mode-beating, providing the possibility to control the splitting ratio of guided SPPs in
Y-splitters. The experimental results are supported by theoretical modelling using the finite-difference time
domain method.
We study both, theoretically and experimentally the light-to-surface plasmon polariton (SPP) and SPP-to-SPP scattering
using the Green's function method and leakage radiation microscopy. The scattering structures are fabricated by
nonlinear lithography and laser induced transfer (LIT). SPPs are exited on dot- and ridge-like surface structures. We
demonstrate symmetric or asymmetric excitation of SPP beams and show that the SPP excitation efficiency strongly
depends on the component of the excitation field perpendicular to the metal surface. By adjusting the angle of the
incident beam to the maximum of the total electric field component perpendicular to the metal surface, the scattering
efficiency of light on a single nanoparticle into SPPs can be increased by a factor of 200. The SPP beams allow studying
scattering properties of perfectly spherical gold particles with diameters between 200 nm and 1600 nm fabricated by LIT
of liquid gold droplets. For these diameters, the description of scattering of electromagnetic waves with optical
frequencies has to take into account higher-order terms. Leakage radiation microscopy provides the unique possibility to
observe scattering features attributed to magnetic dipole and electric quadrupole contributions in the 2D scattering
patterns of SPPs. The results are supported by numerical modelling using the Green's tensor approach.
Introduction: Titanium implants can be regarded as the current gold standard for restoration of sound transmission in the middle ear following destruction of the ossicular chain by chronic inflammation. Many efforts have been made to improve prosthesis design, while less attention had been given to the role of the interface. We present a study on chemical nanocoating on microstructured titanium contact surface with bioactive protein.
Materials and Methods: Titanium samples of 5mm diameter and 0,25mm thickness were structured by means of a Ti:Sapphire femtosecond laser operating at 970nm with parallel lines of 5μm depth, 5μm width and 10μm inter-groove distance. In addition, various nanolayers were applied to titanium samples by aminosilanization, to which Star-Polyethylene glycole (Star-PEG) molecules plus biomarkers (e.g. RGD peptide sequence) were linked.
Results: Chondrocytes could be cultured on microstructured surfaces without reduced rate of vital / dead cells compared to native surfaces. Chondrocytes also showed contact guidance by growing along ridges particularly on 5μm lines. On nanocoated titanium samples, first results showed a strong effect of Star-PEG suppressing unspecific protein absorption, while RGD peptide sequence did not promote chondrocyte cell growth.
Discussion: According to these results, the idea of promoting cell growth on titanium prosthesis contact surfaces compared to non-contact surfaces (e.g. prosthesis shaft) by nanocoating is practicable. However, relative selectivity induced by microstructures for growth of chondrocytes compared to fibrocytes is subject to further evaluation.
A parallel processing of two-photon polymerization structuring is demonstrated with spatial light modulator. Spatial light
modulator generates multi-focus spots on the sample surface via phase modulation technique controlled by computer
generated hologram pattern. Each focus spot can be individually controlled in position and laser intensity with computer
generated hologram pattern displayed on spatial light modulator. The multi-focus spots two-photon polymerization
achieves the fabrication of asymmetric structure. Moreover, smooth sine curved polymerized line with amplitude of 5
μm and a period of 200 μm was obtained by fast switching of CGH pattern.
We present our investigations into the design and fabrication of a complex shape, readily assembled micro check-valve
using the two-photon polymerization technique and a hybrid material. A computational fluid dynamics study has been
carried out in order to evaluate the flow performance of the valve under blood pressures exhibited in healthy human
veins. The fabricated micro-valves exhibit good dimensional accuracy when compared to the CAD-created valve design
and the capability of an internal moving component to perform its intended function.
In this work, we prepare and optically characterize novel, titanium-containing hybrid materials that can be
structured three-dimensionally using two-photon polymerization. We investigate the effect on the structurability
of the increase of titanium isopropoxide and methacrylic acid content in this photosensitive composite. We
show that while it is possible to make transparent thin films with titanium isopropoxide molar content as high as
90%, three-dimensional structures can be made only when the titanium isopropoxide molar content is less than
50%. We measure the refractive index of different titanium isopropoxide: methacrylic acid concentrations in the
composite. We show a linear increase of the composite refractive index with titanium isopropoxide
concentration, while the increase of the methacrylic acid content does not it.
The objective of this study was to optimize titanium surfaces by means of Ti:Sapphire femtosecond laser to improve the
attachment of human cartilage cells on titanium prosthesis in middle ear surgery. The application of microstructures on
titanium samples was evaluated and the influence of these microstructures on human auricular chondrocytes was studied
in-vitro. After establishing the ear chondrocyte cell culture, cells were seeded on titanium platelets with selected
microstructure patterns. Whereas the phenotype of cells seeded on unstructured titanium was similar to cells grown on
standard tissue culture surfaces, the morphology of chondrocytes grown on structured titanium samples was influenced
by the pattern. For future titanium middle ear prosthesis structural optimizations will be developed to promote
chondrocyte growth and adhesion while impeding fibrocyte proliferation to avoid scarring on implant interfaces.
Introduction: While a variety of materials has been evaluated for replacement of human middle ear ossicles following inflammation, titanium and its alloys have shown excellent sound transmission properties and biocompatibility. However, cartilage thickness at the tympanic membrane interface deteriorates over time, while fibrous tissue formation may dislodge the titanium prosthesis. This study was performed to evaluate the effect of microstructures on titanium surfaces in contact with adjacent biological tissue.
Materials and Methods: Titanium samples of 5mm diameter and 0,25mm thickness were structured by means of a Ti:Sapphire femtosecond laser operating at 970nm. The structures applied were lines of parabolic shape (cross-sectional) of 5µm (parallel), 5µm (cross-hatch) and 10µm width (parallel). The inter-groove distance between two maxima was exactly twice the line width.
Results: Lines smaller than 5µm were not feasible due to the natural irregularity of the basic material with pits and level changes of up to 2µm. The process showed little debris and constant microstructure shape over the whole structured area (2x2mm). The resulting debris was examined for toxic by-products on human fibrobcytes and chondrocytes.
Discussion: The results show that microstructures can be applied on titanium surfaces for human implantation with reproducible and constant shapes. Further studies will focus on cell culture which has suggested a relative selectivity for chondrocyte compared to fibrocyte growth in earlier studies with selected microstructures.
Excitation, focusing, and directing of surface plasmon polaritons (SPPs) with curved chains of bumps located
on a metal surface is investigated both experimentally and theoretically. We demonstrate that, by using a
relatively narrow laser beam (at normal incidence) interacting only with a portion of a curved stripe or chain
of nanoparticles, one can excite an SPP beam whose divergence and propagation direction are dictated by the
incident light spot size and its position along the structure. It is also found that the SPP focusing regime is
strongly influenced by the chain inter-particle distance. Extensive numerical simulations of the configuration
investigated experimentally are carried out for a wide set of system parameters by making use of the Green's
tensor formalism and dipole approximation. Comparison of numerical results with experimental data shows
good agreement with respect to the observed features in SPP focusing and directing, providing the guidelines
for a proper choice of the system parameters. It was found that the focusing regime of SPPs is strongly
influenced by the chain inter-bump distance, so that the focusing and directing effects with optimal properties
can be obtained only when the chain inter-bump distance is smaller than the SPP wavelength. Following the
experimental conditions, we have studied the role of the size of light spot exciting SPPs. Spectral dependence
of the focusing waist is also numerically studied for gold surface taking into account the ohmic loss.
The lifetime and the efficiency of EUV source collector optics will have direct impact on the cost effectiveness of
the EUV lithography semiconductor production. Therefore the collector optics was identified as a critical issue
in EUVL. To continually improve and optimize the source collector optics precise measurements of the optics
characteristics are required as well as life time studies.
Adequate measuring equipment must be designed to perform these measurements on-site, at-wavelength and
under realistic conditions. Moreover, the measurement accuracy must be sufficient to allow the detection of small
changes in reflectivity and homogeneity of the EUV source collector optics. This makes it possible to predict
the lifetime of an EUV source collector operated with a high power EUV source after a fraction of the specified
pulse number, e.g. after a couple of hours.
A reflectometer for the investigation of full EUV source collectors was developed, designed and set up. As an
EUV source a commercial microfocus EUV tube was used. This source is particularly suitable for application in
metrology, as it is very stable in its output parameters (namely power, spectrum, spot size, spot position), and
it does not emit debris.
The radiation cone emitted by the EUV source is tailored with a Schwarzschild objective to the spectral and
geometrical requirements of the Wolter-shell EUV source collector optics
At the time of this contribution the measurement system presented here is in operation since 18 months,
and a large number of collector measurements was performed. It is used for the quality control of factory-new
EUV source collectors as well as for repeated measurements within the context of lifetime tests. With this
device the efficiency of debris filters, which prevent the EUV source collector optics from contamination, could
be determined and improved.
The concept of the device and the implementation together with measurement results are described.
Rapid advance of nanostructuring technologies offers new possibilities for flexible and low-cost fabrication of plasmonic
components and devices. In this contribution, we study applications of laser-based nonlinear lithography for the fabrication of dielectric surface-plasmon-polariton (SPP)-structures. These structures can be used for localization, guiding, and manipulation of SPPs on a subwavelength scale. Effective excitation of SPPs on dielectric structures and focusing of the generated SPPs are studied. The characterization of the SPP structures is performed by plasmon leakage radiation microscopy. Laser-based nonlinear lithography,
e.g. two-photon polymerization technique, allows the
fabrication of dielectric waveguides, splitters, and couplers directly on metal surfaces. The fabricated dielectric structures
on metal films are demonstrated to be very efficient for the excitation of SPPs. Using these structures, excitation,
focusing, and guiding will be demonstrated.
The detection of damages at optics for the extreme ultraviolett (EUV) requires precise tools for at-wavelength-metrology.
Excellent stability of the probe radiation is a precondition for precise measurements. As an EUV-source
we use an electron-based microfocus EUV-tube. This EUV-source is debris-free, and it provides a output
of up to 300&mgr;W at 13.5 nm. The metrology setup benefits from the very good long-time stability and spatial
stability of this tube. Optical samples were characterized in reflectivity and transmission. Optical defects of
EUV-optics were analyzed at-wavelength. The incidence angle of the EUV-radiation was varied from grazing
incidence to nearly normal incidence. Our reflectivity measurements were compared with a calibrated synchrotron
measurement at the German national metrology institute (PTB). The absolute accuracy of the reflectivity
measurement was found to be better than 3% for any incidence angle. The reproducibility of the measurement
was found to be better than 0.5%. Investigations are performed to further improve the reproducibility and
absolute accuracy. The metrology setup is flexible, thus it can be fit to different types of measurement for
different applications. The concept of the metrology setup is discussed and recent results are presented. The
devices can be purchased from the Laser Zentrum Hannover e.V.
One of the rapidly advancing femtosecond laser technologies is three-dimensional micro- and nanostructuring by two-photon
polymerization (2PP) technique. This technique allows the fabrication of any computer-generated 3D structure by
direct laser "recording" into the volume of a photosensitive material. Because of the threshold behavior and nonlinear
nature of the 2PP process, a resolution beyond the diffraction limit can be realized by controlling the laser pulse energy
and number of applied pulses. Many different applications of 2PP technique are discussed.
Renewed and growing interest in the field of surface plasmon polaritons (SPPs) comes from a rapid advance of
nanostructuring technologies. The desired nanostructures are usually fabricated by electron- or ion-beam lithography. An
alternative approach is the application of two-photon polymerization (2PP) or nonlinear lithography. Both these
technologies are based on nonlinear absorption of near-infrared femtosecond laser pulses. With 2PP, the fabrication of
three-dimensional micro-objects and photonic crystals with a resolution down to 100 nm is possible. In this contribution,
we study applications of advanced femtosecond laser technologies for the fabrication of SPP structures. We demonstrate
that resulting structures can be used for excitation, guiding, and manipulation of SPPs on a subwavelength scale.
Characterization of these structures is performed by detection of the plasmon leakage radiation (LR). 2PP allows the
fabrication of dielectric waveguides, splitters, and couplers directly on metal surfaces. The fabricated dielectric structures
are also very efficient for the excitation of SPPs. Using these structures, excitation and focusing of the resulting plasmon
field can be achieved.
Developers and users of EUV-optics need precise tools for the characterization of their products. Often a
measurement accuracy of 0.1% or better is desired to detect and study slow-acting aging effect or degradation by
organic contaminants. To achieve a measurement accuracy of 0.1% an EUV-source is required which provides an
excellent long-time stability, namely power stability, spatial stability and spectral stability. Naturally, it should
be free of debris. An EUV-source particularly suitable for this task is an advanced electron-based EUV-tube.
This EUV source provides an output of up to 300 μW at 13.5 nm. Reflectometers benefit from the excellent long-time stability of this tool. We design and set up different
reflectometers using EUV-tubes for the precise characterisation of EUV-optics, such as debris samples, filters,
multilayer mirrors, grazing incidence optics, collectors and masks. Reflectivity measurements from grazing
incidence to near normal incidence as well as transmission studies were realised at a precision of down to 0.1%.
The reflectometers are computer-controlled and allow varying and scanning all important parameters online.
The concepts of a sample reflectometer is discussed and results are presented. The devices can be purchased
from the Laser Zentrum Hannover e.V.
We study the application of two-photon absorption of near infrared femtosecond laser pulses and nonlinear maskless femtosecond laser lithography for the fabrication of dielectric and metallic SPP-structures, being used for localization, guiding, and manipulation of SPPs on a subwavelength scale. Resolutions down to 100 nm are already achievable. Characterization of these structures is performed by detection of the plasmon leakage radiation. Nonlinear lithography allows the fabrication of dielectric waveguides, splitters, and couplers directly on metal surfaces, e. g. by two-photon polymerization. The dielectric structures on metal films are demonstrated to be very efficient for the excitation of SPPs. Using these structures, excitation and focusing of the resulting plasmon field can be achieved. Results on the fabrication and characterization of metallic SPP-structures and components on dielectric substrates fabricated by nonlinear femtosecond laser lithography will be presented and discussed.
Infrared, femtosecond laser pulses are ideal for the fabrication of 3D structures in transparent media. Due to the low
absorption cross-section, 2 or more photons are necessary for absorption. This multi-photon effect limits the affected
volume to the focal area allowing for sharp features on the order of the wavelength of light. One possible multi-photon
reaction is the photo-destruction (ablation, decomposition, etc.) or photo-polymerization of materials. Using these
techniques, 3D photonic components can be realized.
A photonic band gap template has been created with a monodisperse polystyrene (PS) spheres (diameter ~ 624 nm).
We have used ultrafast laser pulses to remove spheres (introduce defined defects) at the surface to gain a fuller
understanding of the laser-material interaction. To optimally focus inside the bulk, an index matching material must be
infiltrated. By using a photosensitive material, two-photon polymerization can be used to harden the material
surrounding the spheres and insert defects inside the bulk. With proper placement of defects, 3D photonic components,
i.e., waveguides, splitters, and filters, can be created.
Two-photon polymerization (2PP) is a novel technology which allows the fabrication of complex three-dimensional (3D)
microstructures and nanostructures. The number of applications of this technology is rapidly increasing; it includes the
fabrication of 3D photonic crystals [1-4], medical devices, and tissue scaffolds [5-6].
In this contribution, we discuss current applications of 2PP for microstructuring of biomedical devices used in drug
delivery. While in general this sector is still dominated by oral administration of drugs, precise dosing, safety, and
convenience are being addressed by transdermal drug delivery systems. Currently, main limitations arise from low
permeability of the skin. As a result, only few types of pharmacological substances can be delivered in this manner [7].
Application of microneedle arrays, whose function is to help overcome the barrier presented by the epidermis layer of
the skin, provides a very promising solution. Using 2PP we have fabricated arrays of hollow microneedles with different
geometries. The effect of microneedle geometry on skin penetration is examined. Our results indicate that microneedles
created using 2PP technique are suitable for in vivo use, and for integration with the next generation of MEMS- and
NEMS-based drug delivery devices.
The recently developed two-photon polymerisation technique is used for the fabrication of two- and three-dimensional
structures in photosensitive inorganic-organic hybrid material (ORMOCER), in SU8 , and in positive tone resist with
resolutions down to 100nm. In this contribution we present applications of this powerful technology for the realization of
micromechanical systems and microoptical components. We will demonstrate results on the fabrication of complex
movable three-dimensional micromechanical systems and microfluidic components which cannot be realized by other
technologies. This approach of structuring photosensitive materials also provides unique possibilities for the fabrication
of different microoptical components such as arbitrary shaped microlenses, microprisms, and 3D-photonic crystals with
high optical quality.
In this contribution, we demonstrate multi-photon femtosecond laser lithography for the fabrication and rapid prototyping of plasmonic components. Using this technology different dielectric and metallic SPP-structures can be fabricated in a low-cost and time-efficient way. Resolution limits of this technology will be discussed. Investigations of the optical properties of the fabricated SPP-structures by far-field leakage radiation microscopy will be reported.
The science of surface plasmon polaritons (SPPs) has attracted a lot of attention in the last years. In this contribution, we study applications of two-photon absorption of femtosecond laser radiation for the fabrication of dielectric and metallic SPP-structures, which can be used for localization, guiding, and manipulation of SPPs. Dielectric SPP components, e.g. waveguides, bends and splitters are fabricated on gold films. SPP properties are investigated by scanning optical near-field microscopy (SNOM), indicating guiding and reflection of SPPs by polymer lines. SPP excitation on dielectric line and point structures is observed by far-field microscopy. Results on plasmon focussing and on the fabrication and characterization of metallic SPP-structures and components on dielectric substrates will be presented and discussed.
Conventional microlens arrays consist of a repetitive arrangement of a unit cell on a fixed pitch. In a chirped array, the inflexibility of a regular structure has been overcome. Here, the array consists of individually shaped lenses which are defined by a parametric description of the cells optical function. We propose different fabrication methods for chirped microlens arrays and present experimentally obtained data. Reflow of photoresist is an established technology for the fabrication of microlenses with superior optical performance. For the generation of a chirped microlens array the photolithographic mask for patterning the resist to be melted has to be chirped. We present an algorithm for mask generation with an example of an ultra-thin camera objective. Inherent to the reflow process stringent limitations to viable surfaces apply. For achieving more arbitrary surfaces, laser lithography and also 2-photon polymerization are employed. In both methods the structures are decomposed into pixels. In laser lithography the local height is converted into an intensity value for the exposure. This variable dose writing locally changes the solubility of the resist in the development process leading to the required surface profile. We propose a writing scheme enabling structure heights of several ten microns with sufficient height discretization. 2-photon polymerization is a rapid prototyping method. Here, a small volume of a UV-curing organic-inorganic co-polymer is hardened in the tight focus of the writing beam. The volume pixel to be exposed is addressed by piezoelectric translation stages. Experimentally obtained structures and performed tests of the optical quality are presented.
Conventional lithography is a leading high-throughput patterning method for mass production. But the dramatically increasing cost of lithographic equipment and mask sets, which is a consequence of pushing optical lithography to its limits, makes alternative, maskless lithographic techniques attractive. Femtosecond lasers have been found suitable for processing of a wide range of materials with sub-micrometer resolution. The limit of achievable structure sizes is predicted to be below 100 nm. Therefore, it is attractive to use this technique for maskless lithography. In this paper, first results on super-resolution femtosecond laser lithography showing great potential for future applications are presented.
Photonic and plasmonic structures and devices have attracted a lot of attention during the last years, promising many
scientific and technological applications, e. g. in information technology, sensing and detection, and optical data storage.
Especially, the rapidly growing field of surface plasmon polaritons (SPP) promises unique integration of microelectronic
devices with optics on the microscale. In this contribution, we discuss applications of advanced femtosecond
laser technologies for the fabrication of high quality 2D and 3D photonic and plasmonic structures. We present our
results on the construction of 3 dimensional photonic crystals by two-photon-polymerization (2PP) technique and optical
characterization of these crystals. Furthermore, we show the capability of 2PP in building of 2-dimensional structures for
guiding and manipulation of SPPs. Focusing of SPPs at dielectric structures fabricated by 2PP on metal surfaces is
demonstrated. Waveguides, splitters, and couplers for application in plasmonics are also fabricated by this technique.
A novel compact EUV-reflectometer recently developed is presented. The designconcept relies on a flexible approach, thus this reflectometer can be set up as a compact table-top tool for a specified task as well as a full all-purpose reflectometer. As an EUV-source an electron-based microfocus EUV-tube is used. This EUV source is debris-free and provides a typical output of 30μW at 13.5 nm. The reflectometer benefits from the very good long-time stability and spatial stability of this tube. Reflectivity measurements from grazing incidence to nearly normal incidence as well as transmission studies can be realized in the same setup at a typical precision of measurements of 0.5%. A precise computer-controlled positioning unit allows to vary and scan all important parameters online, allowing for example complex surface scans and angle variations. The concept of the reflectometer is discussed and recent results are presented. This device can be purchased from the Laser Zentrum Hannover e.V.
Rapid progress in ultrafast laser systems opened many exciting possibilities for high-resolution material processing. These laser systems allow to control and deliver optical energy and laser pulses in time and space with unprecedented precision. It is not surprising that these high-quality optical pulses have revolutionized microfabrication technologies. Femtosecond lasers enabled processing of a wide range of materials (including heat sensitive and thermo reactive) with a sub-micrometer resolution. At present, nearly arbitrary shaped 2D and 3D structures can be produced by direct write photofabrication techniques using femtosecond laser pulses. In this paper we present a brief review of our recent progress in femtosecond (maskless, direct-write, nonlinear) laser lithography and 3D photofabrication technique.
Compact extreme ultraviolet (EUV) laboratory sources are strongly required for the fast on-site characterization of optical components and for the precise calibration of EUV diagnostic instruments. The "EUV tube" promises to become an important tool for these applications. This source is based on the transfer of advanced microfocus x-ray tube technology into the EUV spectral range. This allows the realization of a flexible, debris-free, and long-term stable EUV source. Silicon targets are used to generate radiation at 13.5 nm. Detailed characteristics of the source performance are reported and different examples for off-synchrotron at-wavelength metrology are presented.
Two applications of an electron-based EUV-tube are presented: the set up of a grazing incidence EUV reflectometer with high reproducibility and accuracy, and our works towards the realization and application of a Schwarzschild objective for EUV imaging. Both applications benefit form the use of the table-top EUV-source, which is debris-free, long-term stable, and compact.
Novel ultrashort EUV and hard x-ray sources have been developed
and characterized. Radiation pulses were produced by combining
femtosecond laser technology with a specially designed EUV and
x-ray diode. At first, ultrashort electron pulses are generated by
photoemission from a photocathode. Then, these electron pulses are
accelerated over a short distance towards an high-Z anode. EUV
radiation is produced with a silicon anode via L-shell emission,
hard-x-rays are generated with a copper anode.
Measurements of the pulse duration were performed for hard-x-rays
using an advanced streak camera. For high electron pulse charges
(several pC), hard-x-ray pulse durations of less than 10 ps were
observed.
In this contribution we present the concept of our EUV and hard
x-ray source. A wide range of experimental parameters is
investigated: Different cathode and anode materials were tested,
femtosecond laser systems with different parameters are used, the
dependence of the radiation pulse length on experimental
parameters e.g. laser energy and acceleration voltage is studied.
Direct-write micro- and nanostructuring laser technologies are very important for the fabrication of new materials and multifunctional devices. Using tightly focused femtosecond laser pulses one can produce submicrometer holes and periodic structures in metals, semiconductors, and dielectrics on arbitrarily shaped surfaces. The achievable structure size is not restricted by the diffraction limit. It is determined by material properties and the laser pulse stability. We report investigations of possibilities to use femtosecond laser pulses for nanostructuring of different materials.
Results of ablation of different materials by femtosecond and picosecond laser pulses are compared. Advantages and disadvantages of both laser systems are discussed. Two most important criteria, processing speed and quality of the fabricated structures, are addressed. High repetition rate picosecond lasers allow high speed cutting of thin metal foils and silicon wafers, whereas for micro-drilling it is more advantageous to use femtosecond laser systems.
Recently, an electron-based ultrashort hard-x-ray source has been developed at the Laser Zentrum Hannover e.V. In this source x-ray pulses are produced by combining femtosecond laser technology with a specially designed x-ray diode. At first, ultrashort electron pulses are generated by photoemission from a photocathode. Then, these electron pulses are accelerated over a short distance towards a high-Z anode. Hard-x-rays are produced via Bremsstrahlung and characteristic line emission. Now detailed measurements of the hard-x-ray pulse duration have been performed using an advanced streak camera. The streak camera has a sub-picosecond time resolution in the keV range. With this camera hard-x-ray pulse durations of less than 5 ps were observed for electron pulse charges of the order of several pC. In this contribution we present our results on the x-ray pulse duration measurements and their dependence on different experimental parameters. A comparison with theoretical simulations is given.
During the last decade it has been proven that focused femtosecond laser pulses are an ideal tool for micro- and sub-micro-structuring of all kinds of materials. Due to the high intensities that can be achieved in ultrashort pulses they can be applied for machining transparent media within the volume by means of multi-photon absorption. Besides ablative methods, multi-photon absorption can also lead to photo-polymerization of light-sensitive resins, i.e. two-photon polymerization. In this paper we present our latest results on the fabrication of 3D microstructures by means of two-photon polymerization.
A compact electron-based microfocus EUV/soft-x-ray source for applications in metrology and microscopy is developed. The source concept is based on the transfer of advanced microfocus x-ray tube technology into the EUV/soft-x-ray spectral range. This allows the realization of a flexible, debris-free, and long-term stable source. Detailed characteristics of the source performance are reported and different applications of the soft-x-ray tube in the field of at-wavelength metrology are presented.
Recently, an electron-based ultrashort hard-x-ray source has been developed at the Laser Zentrum Hannover e.V. In this source x-ray pulses are produced by combining femtosecond laser technology with a specially designed x-ray diode. At first, ultrashort electron pulses are generated by photoemission from a photocathode. Then, these electron pulses are accelerated over a short distance towards a high-Z anode. Hard-x-rays are produced via Bremsstrahlung and characteristic line emission.
Now detailed measurements of the hard-x-ray pulse duration have been performed using an advanced streak camera. The streak camera has a sub-picosecond time resolution in the keV range. With this camera hard-x-ray pulse durations of less than 10 ps were observed for electron pulse charges of the order of several pC.
In this contribution we present our results on the x-ray pulse duration measurements and their dependence on different experimental parameters. A comparison with theoretical simulations is given.
A compact electron-based extreme ultraviolet (EUV) source for advanced at-wavelength mirror metrology is developed. The source concept is based on the transfer of advanced microfocus x-ray tube technology into the EUV spectral range. This allows the realization of a flexible, debris-free, and long-term stable EUV source. In the EUV tube, silicon targets are used to generate radiation at 13.5 nm. Detailed characteristics of the source performance are reported and different applications of the EUV tube in the field of at-wavelength mirror metrology are presented.
Micro- and nanostructuring are very important for the fabrication of new materials and multifunctional devices. Existing photo-lithographic technologies can only be applied to a limited number of materials and used on plane surfaces. Whereas, microstructuring with femtosecond laser pulses has established itself as an excellent and universal tool for micro-processing, it is still unclear what are the limits of this technology. It is of great interest to use this technique also for nanostructuring. With tightly focused femtosecond laser pulses one can produce sub-micrometer holes and structures whose quality depends on the material. We present new results on nanostructuring of different materials with femtosecond laser pulses in an attempt to make this an universal technology, and discuss its reproducibility, and further prospects for quality control.
Investigations of possibilities for nanostructuring with femtosecond laser pulses of different materials are reported. The aim is to develop a simple laser-based technology for the fabrication of 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. Sub-wavelength structuring of metals by direct femtosecond laser ablation is performed. The band gap dependence of the minimum structure size for transparent materials is identified.
A compact electron-based extreme ultraviolet (EUV) source for at-wavelength metrology is developed. The source concept is based on the transfer of advanced microfocus x-ray tube technology into the EUV spectral range. This allows the realization of a flexible, debris-free, and long-term stable EUV source. In the EUV tube, silicon targets are used to generate radiation at 13.5 nm. Detailed characteristics of the source performance are reported and different applications of the EUV tube in the field of at-wavelength metrology are presented.
A commercial extreme ultraviolet (EUV) source for at-wavelength metrology is developed. The source concept is based
on the transfer of advanced microfocus x-ray tube technology into the EUV spectral range. This concept allows the
realization of a compact, debris-free, and long-term stable EUV source. In the EUV tube, silicon targets are used to
generate radiation at 13.5 nm. Detailed characteristics of the source performance are reported and different applications
of the source in the field of at-wavelength metrology are presented.
The development of suitable radiation sources for extreme ultraviolet lithography (EUVL) is a major challenge. For the optimization of these sources and for the determination of the parameters needed for the system design and the system integration these sources have to be characterized in terms of the absolute in-band power, the spectral distribution in the EUV spectral region and the out-of-band spectral regions, the spatial distribution of the emitting volume and the angular distribution of the emission. For improving the lifetime of such sources, generally accepted as one key risk with EUVL, another task, the debris emitted from sources under development has to be investigated. Therefore, JENOPTIK Mikrotechnik GmbH is co-operating with the Laser Laboratorium Goettingen, the Physikalisch-Technische Bundesanstalt (PTB) and the AIXUV GmbH in developing ready-for-use metrology tools for EUVL source characterization and optimization. The set of the tools employed for EUV-source characterization is presented in detail as well as concepts for calibration and measurement procedures.
A review of our progress in the realization of an ultrashort-pulse laser-driven hard-x-ray source based on the combination of a femtosecond laser system with an x-ray diode is given. New results on the development of electron-based compact EUV sources for "at-wavelength" metrology are presented. Detailed investigations of spectral, spatial, and temporal characteristics of both sources are performed and possible applications are discussed.
It is shown that heating of electrons due to the inverse bremsstrahlung absorption of high-power short-pulse laser radiation results in parametric generation of ion-acoustic waves. The range of wave numbers where the amplitude of the ion-acoustic oscillations increases by more than an order of magnitude is determined.
A compact extreme ultraviolet (EUV) source for metrology is developed. This source is based on an extension of conventional x-ray tube technology into the EUV spectral range. As in an ordinary x-ray tube, electrons are generated by a filament, accelerated in a high-voltage electric field toward an anode, and focused onto a solid target. In this "EUV tube" silicon targets are used to generate radiation at 13.5 nm. Absolute conversion efficiencies from electrons into EUV photons are measured. Illustrations of spectral and spatial properties are given and investigations of the long-term stability of the EUV emission are performed. Possibilities for a power scaling into the milliwatt range are discussed.
Two-photon polymerization (2PP) of photosensitive inorganic-organic hybrid polymers (ORMOCERs, developed at the Frauenhofer Institut fuer Silicatforschung) is demonstrated as a very promising approach for the fabrication of complicated three-dimensional micro- and nanostructures. These materials are produced by sol-gel synthesis with
molecular level mixing of different components. It is remarkable that properties of the hybrid polymers can be tuned from those that are characteristic for organic polymers to those that are similar to inorganic glasses. They have negative resist behaviour and can be used as storage-stable, liquid photo-polymerizable resins. When Ti:sapphire femtosecond laser pulses are tightly focused into the volume of this resin (which is transparent in the infrared) they can initiate two-photon polymerization process transferring liquid into solid state. This process is confined to a highly localized area at
the focal point due to the quadratic dependence of the two-photon absorption rate on the laser intensity. When the laser focus is moved through the resin in three dimensions, the polymerization occurs along the trace of the focus. This allows to fabricate any computer-generated 3D structure by direct laser "recording" into the volume of the ORMOCER. The non-irradiated liquid resin can be dissolved in alcohol leaving the polymerized copy of the computer model.
Compared to conventional photo-lithography which is a planar processing, two-photon polymerization is a real three-dimensional
volume microfabrication technique. This technology can be used for rapid prototyping and low-cost fabrication of artificial micro- and nanostructured components which are required for different applications in optics, medicine, and biology. Numerous examples such as photonic crystals, micromechanical and microoptical devices are
presented.
By observing the TEA CO2 laser generation dynamics during the intracavity processing of reflecting surfaces, physical mechanisms responsible for the fabrication of periodic sub-micrometer structures are studied.
Recent progress in the development of a compact, high-repetition rate, ultrashort-pulse laser-driven hard-x-ray source based on the combination of a femtosecond laser system with an x-ray diode is reported. The x-ray source is characterized in terms of spectral and spatial properties. Hard-x-ray fluxes exceeding 2x1010 photons/s (emitted in 4π sr) are realized at a repetition rate of 250 kHz. A comparison with available laser-plasma hard-x-ray sources is presented. Numerical modeling is performed which proves that picosecond and sub-picosecond hard-x-ray pulses can be produced with this source. Further prospects and possible applications of the femtosecond laser-driven x-ray diode are outlined.
Generation of extreme ultraviolet (EUV) radiation from solid targets is studied and a compact EUV source for small-scale lithographic applications and EUV metrology is development. This source is based on a transfer of conventional x-ray tube technology into the EUV spectral range. As in an ordinary x-ray tube, electrons are generated by a tungesten filament and accelerated in a high-voltage electric field towards a solid target. In the demonstrated "EUV tube" beryllium and silicon targets are used to generate radiation at 11.4 nm and 13.5 nm, respectively. The absolute converstion efficiencies into EUV photons are measured. At 13.5 nm an EUV power of 34μW or 2×1012 photon/s (in 2% bandwidth and a solid angle of 2π sr) is demonstrated. Prospects for a further power scaling of the EUV source are discussed.
The development of a simple laser-based technology for the fabrication of two-dimensional nanostructures with a structure size down to one hundred nanometers is reported. The ability to micro- and nano-structure is very important for the fabrication of new materials and multifunctional microdevices. Photolithographic technologies can be applied only for plane surfaces. Using femtosecond laser pulses one can fabricate 100 nm structures on arbitrary 3D-surfaces of metals and dielectrics. In principle, the minimum achievable structure size is determined by the diffraction limit of the optical system and is of the order of the radiation wavelength. However, this is different for material processing with ultrashort laser pulses. Due to a well-defined threshold character of material processing with femtosecond lasers one can beat the diffraction limit by using tightly focused femtosecond laser pulses and by adjusting laser parameters slightly above the processing threshold. In this case only the central part of the beam can modify the material and it becomes possible to produce sub-wavelength structures. In this presentation, sub-wavelength microstructuring of metals and fabrication of periodic nanostructures in transparent materials are demonstrated as promising femtosecond laser-based nanofabrication technologies.
Two-photon polymerization (2PP) of photosensitive inorganic-organic hybrid polymers (ORMOCERs, developed at the Frauenhofer Institut fur Silicatforschung) is demonstrated as a very promising approach for the fabrication of complicated three-dimensional micro- and nanostructures. These materials are produced by sol-gel synthesis with molecular level mixing of different components. It is remarkable that properties of the hybrid polymers can be tuned from those that are characteristic for organic polymers to those that are similar to inorganic glasses. They have negative resist behaviour and can be used as storage-stable, liquid photo-polymerizable resins.
When Ti:sapphire femtosecond laser pulses are tightly focused into the volume of this resin (which is transparent in the infrared) they can initiate two-photon polymerization process transferring liquid into solid state. This process is confined to a highly localized area at the focal point due to the quadratic dependence of the two-photon absorption rate on the laser intensity. When the laser focus is moved through the resin in three dimensions, the polymerization occurs along the trace of the focus. This allows to fabricate any computer-generated 3D structure by direct laser 'recording' into the volume of the ORMOCER. The non-irradiated liquid resin can be dissolved in alcohol leaving the polymerized copy of the computer model. Compared to conventional photo-lithography which is a planar processing, two-photon polymerization is a real three-dimensional volume microfabrication technique. These technologies can be used for rapid prototyping and low-cost fabrication of artificial micro- and nanostructured components which are required for different applications in optics, medicine, and biology. Numerous examples such as photonic crystals, micromechanical and microoptical devices will be demonstrated in this presentation.
Investigations on two-photon polymerization of inorganic-organic hybrid materials initiated by femtosecond Ti:sapphire laser pulses are performed. The applied resins are designed for ultraviolet photo-lithography and contain photo-initiators sensitive to 390 nm radiation. These resins exhibit exceptionally good mechanical, optical, and chemical properties and can be microstructured by laser-enforced transition from liquid to solid state. These materials are transparent in the infrared, therefore, by tightly focusing femtosecond laser pulses into the volume of a liquid resin, two-photon polymerization can be initiated in a small focal region inside the liquid. First applications of this technique for the fabrication of three-dimensional microstructures and photonic crystals in inorganic-organic hybrid polymers with a structure size down to 200 nm and a periodicity of 450 nm are discussed.
Generation of extreme ultraviolet (EUV) radiation from solid targets is studied and a compact EUV source for small-scale lithographic applications and EUV metrology is developed. This source is based on a transfer of conventional x-ray tube technology into the EUV spectral range. As in an ordinary x-ray tube, electrons are generated by a tungsten filament and accelerated in a high-voltage electric field toward a solid target. In the demonstrated "EUV tube" beryllium and silicon targets are used to generate radiation at 11.4 and 13.5 nm, respectively. The absolute conversion efficiencies into EUV photons at 13.5 nm are measured. Prospects for a further power scaling of the EUV source are discussed.
A compact, high-repetition rate, ultrashort-pulse laser-driven hard-x-ray source based on the combination of a femtosecond laser system with an x-ray diode is demonstrated. A comparison with available laser-plasma hard-x-ray sources is presented. Hard-x-ray fluxes exceeding 1010 photons/s (emitted in 4π) are realized at a repetition rate of 250 kHz. Numerical modeling is performed which proves that picosecond and sub-picosecond hard-x-ray pulses can be produced with this source.
Fabrication of deep high-quality holes in different materials is required for many industrial applications. In this presentation we provide results of detailed investigations on deep drilling of metals by femtosecond laser pulses. We identify most important parameters which should be fulfilled for the fabrication of high-quality holes and efficient femtosecond laser ablation, discuss the role played by the laser-induced optical breakdown, debris, and laser pulse repetition rate.
The results of theoretical studies are reported for threshold characteristics of a metal ablation by picosecond and femtosecond laser pulse. Two possible mechanisms of the laser ablation at laser fluence F ≤ Fth are considered: thermal mechanism of ablation connected with a kinetics of a metal-vacuum surface evaporation and the mechanism of ablation connected with a hydrodynamics of a dense matter. The analysis has been made within the framework of a two-temperature model of metals for femtosecond and picosecond region of laser pulse duration and the extended of a two-temperature model of the metal in the case when the surface temperature Ti more than the critical temperature of metals. Analytical expressions for the ablation-threshold fluency Fth as well as the threshold values of the lattice temperature and the characteristic time of lattice temperature decay td(Fth) are obtained. This analytical description is in satisfactory agreement with particular numerical calculations.
Nonlinear plasma effects important for femtosecond-scale heating dynamics are considered. It is shown that heating of electrons due to the inverse bremsstrahlung absorption of high-power short-pulse laser radiation results in parametric generation of ion-acoustic waves. The range of wave numbers where the amplitude of the ion-acoustic oscillations in-creases by more than an order of magnitude is determined. A self-similar description of electron and phonon kinetics in metals with a sharp gradient of the electron temperature is developed. It is shown that the Cherenkov generation of nonequilibrium phonons results in suppression of the electron heat flux and rapid disintegration of the metal lattice.
At the Laser Zentrum Hannover investigations of possibilities to use femtosecond laser pulses for direct ablative writing and microstructuring of solid materials have been started in 1995. Since then considerable progress in understanding and in the application of different femtosecond technologies has been obtained. At present, we are able to produce high quality microstructuring and large area patterning of solids with structure sizes between one and ten micrometers. By using tightly focused femtosecond pulses it is possible to produce even sub-micrometer structures. In this paper we pursue the goal to find and characterize the limits of femtosecond laser micromachining. Detailed investigations of possibilities to use femtosecond lasers for the sub-wavelength microstructuring of metals and for fabrication of periodic structures in transparent materials with the scale length of the order of several hundreds nanometers are reported.
Using tightly focused femtosecond laser pulses waveguides are fabricated inside glasses and crystalline materials. The guiding and attenuation properties at different wavelengths as well as the micro morphology of the irradiated samples are studied. We demonstrate the fabrication of single- and multi-mode waveguides with damping losses well below 1 dB/cm in fused silica. In crystalline quartz we found that the irradiated area has become amorphous due to the absorption of the laser radiation. In this case waveguiding is observed in a stress-induced region surrounding the irradiated, amorphous area.
A compact indirect laser-driven ultrashort electron and hard-x-ray source based on the combination of a high-repetition rate femtosecond laser system with a conventional x-ray tube is demonstrated. The influence of laser parameters on thermionic electron emission and on the hard-x-ray generation efficiency is studied. This source has an outstanding performance in terms of average power, simplicity, handling, and applicability as compared to the sources based on high-power laser-produced plasmas.
Possibilities to fabricate sub-micron structures in thin metal films, metal coatings, and glass substrates using femtosecond laser pulses are systematically studied. Structures are produced by direct femtosecond-pulse laser ablation of solid targets at atmospheric pressure. Tight focusing and imaging techniques are applied. Dependencies of the structure size on laser pulse energy and pulse number are investigated.
Metal ablation taking into account the hydrodynamics of a dense ablated material with ion temperature close to critical is considered. An extended two-temperature model taking into account hydrodynamic plasma expansion and degeneracy of the electron gas is developed. The new version of the RAPID code is used to perform calculations of ablation rates for several metal targets under conditions where the electron degeneracy is important.
The applicability of hydrodynamic models for theoretical description of UV laser ablation of polymers is studied. The plume formation is considered as a first-kind phase transition. In case of strongly absorbing polymers this phase transition occurs as a surface evaporation, and in case of weakly absorbing polymers as a bulk evaporation. The vapor plume is assumed to be transparent for laser radiation, and its expansion is described by the isoentropic hydrodynamic equations. New analytical expressions for ablation (etch) depths per pulse are obtained, which are in a good agreement with the available experimental data.
Laser ablation of metals by femto- and picosecond pulses is analytically and numerically studied within the framework of different models for the ablated material. Within the plasma model ablation is initiated by high-power thermal and hydrodynamic waves which propagate into the irradiated material. Analytical expressions for the thermal ablation and for the ablation by the shock wave are obtained. Numerical simulations with the computer code RAPID are in a good agreement with analytical results.
Metal ablation taking into account hydrodynamics of a dense ablated material with ion temperature close to the critical one is considered. The extended two-temperature model taking into account the hydrodynamic plasma expansion, degeneracy of the electron gas, cold pressure of ions and interaction between the electron and ion subsystems (nonideality of the metal plasma) is developed. The new version of the RAPID code is used to perform calculations of ablation rates for several metal targets under conditions where the electron degeneracy is important.
Thermal ablation of a metal surface by low-energy ultrashort laser pulses is considered theoretically. The temporal dynamics of the surface electron and lattice temperatures is studied within the framework of the two-temperature model and for different temperature dependencies of the characteristics of the metal (electron-relaxation time, heat capacity, thermal conductivity). The approximation of evaporation into a vacuum is used to determine the ablation depth. Analytical expressions for the ablation-threshold fluence Fth as well as the threshold values of the lattice temperature Tth(Fth) and the characteristic time of lattice temperature decay td(Fth) are obtained. This analytical description is in satisfactory agreement with particular numerical calculations.
Different regimes of heat propagation in metals irradiated by subpicosecond laser pulses are studied on the basis of two-temperature diffusion model. New analytical solutions for the heat conduction equation, corresponding to the different temperature dependences of the electron thermal conductivity (formula available n paper), are found. It is shown that in case of a strong electron-lattice nonequilibrium, the heat penetration depth grows linearly with time, lT varies direct as t, in opposite to the ordinary diffusionlike behavior, lT varies direct as t1/2. Moreover, the heat propagation velocity decreases with increasing laser fluence.
The generation of ion-acoustic waves in metals irradiated by ultrashort laser pulses is studied. It is shown that non- equilibrium ion-acoustic oscillations lead to an anomalous increase of the effective electron collision frequency and fast melting of metals.
Three different experimental approaches for the generation of coherent VUV and XUV radiation are studied and discussed. With a 400 fs KrF excimer laser system tunable VUV radiation around 155 nm with a peak power of approximately 0.5 GW has been generated by near-resonant four-wave difference-frequency mixing in xenon. A 150 fs, 0.7 TW Ti:Sapphire laser system has been used for investigations on high-order frequency mixing with linear and circular polarized laser fields. With the same system soft x-ray lasing in low charged oxygen ions has been demonstrated.
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