Laser-induced periodic surface structures (LIPSS, ripples) are a universal phenomenon that can be observed on almost any material after the irradiation by linearly polarized laser beams, particularly when using ultrashort laser pulses with durations in the picosecond to femtosecond range. During the past few years significantly increasing research activities have been reported in the field of LIPSS, since their generation in a single-step process provides a simple way of nanostructuring and surface functionalization towards the control of optical, mechanical or chemical properties. In this contribution current applications of LIPSS are reviewed, including the colorization of technical surfaces, the control of surface wetting, the tailoring of surface colonization by bacterial biofilms, and the improvement of the tribological performance of nanostructured metal surfaces.
Over the past years, many applications based on laser-induced refractive index changes in the volume of transparent materials have been demonstrated. Ultrashort pulse lasers offer the possibility to process bulky transparent materials in three dimensions, suggesting that direct laser writing will play a decisive role in the development of integrated micro-optics. At the present time, applications such as 3D long term data storage or embedded laser marking are already into the phase of industrial development. However, a quantitative estimate of the laser-induced refractive index change is still very challenging to obtain. On another hand, several microscopy techniques have been recently developed to characterize bulk refractive index changes in-situ. They have been mostly applied to biological purposes. Among those, spatial light interference microscopy (SLIM), offers a very good robustness with minimal post acquisition data processing. In this paper, we report on using SLIM to measure fs-laser induced refractive index changes in different common glassy materials, such as fused silica and borofloat glass (B33). The advantages of SLIM over classical phase-contrast microscopy are discussed.
Ultrashort pulse lasers offer the possibility to structure the bulk of transparent materials on a microscale. As a
result, the optical properties of the irradiated material are locally modified in a permanent fashion. Depending
on the irradiation parameters, different types of laser-induced phase objects can be expected, from uniform
voxels (that can exhibit higher or lower refractive index than the bulk) to self-organized nanoplanes. We study
the physical mechanisms that lead to material restructuring, with a particular emphasis on events taking place
on a sub picosecond to a microsecond timescale following laser excitation. Those timescales are particularly
interesting as they correspond to the temporal distances between two consecutive laser pulses when performing
multiple pulse irradiation: burst microprocessing usually involves picosecond separation times and high repetition
rate systems operate in the MHz range. We employ a time-resolved microscopy technique based on a phase-contrast
microscope setup extended into a pump-probe scheme. This methods enables a dynamic observation
of the complex refractive index in the interaction region with a time resolution better than 300 fs. In optical
transmission mode, the transient absorption coefficient can be measured for different illumination wavelengths
(400 nm and 800 nm). The phase-contrast mode provides qualitative information about the real part of the
transient refractive index. Based on the study of those transient optical properties, we observe the onset and
relaxation of the laser-generated plasma into different channels such as defect creation, sample heating, and
shockwave generation. The majority of our experiments were carried out with amorphous silica, but our method
can be applied to the study of all transparent media.
The nonlinear absorption character determines a high potential of ultrafast laser pulses for 3D processing of transparent
materials, particularly for optical functions. This is based on refractive index engineering involving thermo-mechanical,
and structural rearrangements of the dielectric matrix. Challenges are related to the time-effectiveness of irradiation,
correct beam delivery, and the influence of material properties on the exposure results. Particularly for light-guiding
applications it is suitable to master positive refractive index changes in a time-efficient manner, considering that the
result depends on the deposited energy and its relaxation paths. To address these challenges several irradiation concepts
based on adaptive optics in spatial and temporal domains were developed. We review here some of the applications from
various perspectives. A physical aspect is related to temporal pulse shaping and time-synchronized energy delivery tuned
to material transient reactions, enabling thus a synergetic interaction between light and matter and, therefore, optimal
results. Examples will be given concerning refractive index flip in thermally expansive glasses by thermo-mechanical
regulation and energy confinement by nonlinear control. A second engineering aspect is related to processing efficiency.
We give insights into beam-delivery corrections and 3D parallel complex photoinscription techniques utilizing dynamic
wavefront engineering. Additionally, in energetic regimes, ultrafast laser radiation can generate an intriguing nanoscale
spontaneous arrangement, leading to form birefringence and modulated index patterns. Using the birefringence
properties and the deriving anisotropic optical character, polarization sensitive devices were designed and fabricated. The
polarization sensitivity allows particular light propagation and confinement properties in 3D structures.
The formation of laser induced periodic surface structures (LIPSS) is to a large extent of self-organizing nature and
in its early stages essentially influenced by optical scattering. The evolution of related mechanisms, however, has
still to be studied in detail and strongly depends on materials and laser parameters. Excitation with highly intense
ultrashort pulses leads to the creation of nanoripple structures with periods far below the fundamental wavelength
because of opening multiphoton excitation channels. Because of the drastically reduced spatial scale of such laser
induced periodic nanostructures (LIPNS), a particular influence of scattering is expected in this special case. Here
we report on first investigations of femtosecond-laser induced nanostructuring of sputtered titanium dioxide (TiO2)
layers in comparison to bulk material. The crucial role of the optical film quality for the morphology of the resulting
LIPNS was worked out. Typical periods of nanoripples were found to be within the range of 80-180 nm for an
excitation wavelength of 800 nm. Unlike our previously reported results on bulk TiO2, LIPNS in thin films appeared
preferentially at low pulse numbers (N=5-20). This observation was explained by a higher number of scattering
centers caused by the thin film structure and interfaces. The basic assumptions are further supported by
supplementary experiments with polished and unpolished surfaces of bulk TiO2 single crystals.
Ultrashort pulses lasers are tools of choice for functionalizing the bulk of transparent materials. In particular,
direct photoinscription of simple photonic functions have been demonstrated. Those elementary functions rely
on the local refractive index change induced when focusing an ultrashort pulse in the volume of a transparent
material. The range of possibilities offered by direct photoinscription is still under investigation. To help
understanding, optimizing and assessing the full potential of this method, we developed a time-resolved phase
contrast microscopy setup. The imaginary part (absorption) and the real part of the laser-induced complex
refractive index can be visualized in the irradiated region. The setup is based on a commercially available phase
contrast microscope extended into a pump-probe scheme. The originality of our approach is that the illumination
is performed by using a pulsed laser source (i.e. a probe beam). Speckle-related issues are solved by employing
adequate sets of diffusers. This laser-microscopy technique has a spatial resolution of 650 nm, and the impulse
response is about 300 fs. The laser-induced refractive index changes can be tracked up to milliseconds after the
energy deposition. The excitation beam (the pump) is focused with a microscope objective (numerical aperture
of 0.45) into the bulk of an a-SiO2 sample. The pump beam can be temporally shaped with a SLM-based pulse
shaping unit. This additional degree of flexibility allows for observing different interaction regimes. For instance,
bulk material processing with femtosecond and picosecond duration pulses will be studied.
The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of semiconductors and dielectrics by
linearly polarized high-intensity Ti:sapphire fs-laser pulses (τ ~100 fs, λ ~800 nm) is studied experimentally and
theoretically. In the experiments, two different types of LIPSS exhibiting very different spatial periods are observed (socalled
LSFL - low spatial frequency LIPSS, and HSFL - high spatial frequency LIPSS), both having a different
dependence on the incident laser fluence and pulse number per spot. The experimental results are analyzed by means of a
new theoretical approach, which combines the generally accepted LIPSS theory of J. E. Sipe and co-workers [Phys. Rev.
B 27, 1141-1154 (1983)] with a Drude model, in order to account for transient changes of the optical properties of the
irradiated materials. The joint Sipe-Drude model is capable of explaining numerous aspects of fs-LIPSS formation, i.e.,
the orientation of the LIPSS, their fluence dependence as well as their spatial periods. The latter aspect is specifically
demonstrated for silicon crystals, which show experimental LSFL periods Λ somewhat smaller than λ. This behaviour is
caused by the excitation of surface plasmon polaritons, SPP, (once the initially semiconducting material turns to a
metallic state upon formation of a dense free-electron-plasma in the material) and the subsequent interference between its
electrical fields with that of the incident laser beam, resulting in a spatially modulated energy deposition at the surface.
Upon multi-pulse irradiation, a feedback mechanism, caused by the redshift of the resonance in a grating-assisted SPP
excitation, is further reducing the LSFL spatial periods. The SPP-based mechanism of LSFL successfully explains the
remarkably large range of LSFL periods between ~0.6 λ and λ.
The combination of sample translation and line focusing by cylindrical optics is shown to be a convenient and highly
effective way of generating laser induced coherent periodic surface structures (LIPSS) in TiO2 over significantly
extended areas. Compared to known techniques based on a sample translation relative to a circular symmetric focus, the
approach is much less time consuming and requires only a single translation stage. The capability of the method to form
both high and low spatial frequency LIPSS (HSFL, LSFL) at the second harmonic wavelengths of a Ti:sapphire-laser
(around 400 nm) at properly chosen scanning velocity and laser pulse energies is demonstrated. Structured multi-mm2
areas with periods of 80 nm and 325 nm were obtained corresponding to distinct sets of optimized parameters.
Furthermore, the appearance of nano-bumps on 30 nm scale on the surface of the LSFL is reported. Basic technical
issues are discussed and potential applications of LIPSS in rutile-type TiO2 like superwetting, friction control, catalysis
and photovoltaic are proposed.
Femtosecond laser processing of bulk transparent materials can generate localized increase of the refractive
index. Thus, translation of the laser spot give potential access to three dimensionnal photowriting of waveguiding
structures. Increasing the number of machining foci can considerably reduce the processing efforts when complex
photonic structures are envisaged such as waveguide arrays. The present report presents a technique of dynamic
ultrafast laser beam spatial tailoring for parallel writing of photonic devices. The wavefront of the beam is
modulated by a periodical binary (0-π) phase mask of variable pattern to achieve dynamic multispot operation.
The parallel photoinscription of multiple waveguides is demonstrated in fused silica. Using this method, light
dividers in three dimensions relying on evanescent coupling are reported and wavelength-division demultiplexing
(WDD) devices were achieved in single sample scan.
Ultrafast lasers emerged as promising tools to process refractive index changes in band-gap materials, resulting in
waveguiding functions. Positive refractive index changes were often reported in fused silica matrices. However, in
glasses characterized by slow electronic relaxation and high thermal expansion, the refractive index change is usually
negative, detrimental for waveguide writing. This relates to the formation of hot regions, where, due to thermal
expansion, material is quenched in low-density phases. We discuss control mechanisms related to spatio-temporal heat-source
design which may be tuned by temporally shaped laser radiation. Programmable temporal tailoring of pulse
envelopes triggers transitions from thermal expansion to directional inelastic flow. Consequently, material compaction
leads to a positive refractive index change and guiding structures may thus be created. From an application perspective,
the structuring quality degrades with the focusing depth due to wavefront distortions generated at the air-dielectric
interface inducing spatial energy dispersion. Spatial beam tailoring corrects beam propagation distortion, improving the
structuring accuracy. The corrective process is becoming important when laser energy has to be transported without
losses at arbitrary depths, with the purpose of triggering mechanisms of positive index change.
Tight focusing of ultrashort near infrared laser pulses in the bulk of various transparent materials induces significant modifications of the optical properties by locally changing the material refractive index. Such laser-induced phase objects are of major technological interest, notably for direct writing of embedded optical functions. While extensive studies have been reported on ultrashort pulsed laser induced modifications in several materials, especially with regard to focusing conditions, incubation effects, or the influence of the energy content of the pulse, we emphasize here the role of the temporal design of the excitation sequence. We present phase-contrast microscopy investigations of the resultant morphology and discuss the refractive index topological map induced by different temporal pulse intensity envelopes in various transparent materials. The consequences of temporal profiles generated by a pulse shaping apparatus on the morphology of the interaction zone are illustrated, emphasizing the benefits of the synchronization between the excitation temporal profile and the material response.
Following advances in ultrafast laser technology as a reliable tool for material probing and processing, we discuss various options for control and optimization. The possibility to tailor the temporal shape of ultrashort laser pulses enables extended opportunities for material processing. The concept of optimizing laser interactions is based on the possibility to regulate the energy delivery so that control of laser-induced phenomena can be achieved and quality structures can be realized. An experimental demonstration of the possibility to design excitation sequences tailored with respect to the material response is described, laying the groundwork for adaptive optimization in materials structuring. We show that under particular irradiation conditions involving modulated excitation, the energy flow can be controlled and the material response can be guided to improve processing results. This is particularly important for processing brittle materials. Further examples are given to illuminate the possibility to optimize the kinetic properties of ions emitted from laser-irradiated semiconductors, using excitation sequences synchronized with the solid-to-liquid transformation time. Versatile sub-kilo-electron-volt ion beams are obtained, exploiting transitions to supercritical fluid states with minimal energetic expenses. Temporally selective irradiation can thus open up efficient thermodynamic paths, unfolding interesting perspectives for "intelligent," feedback-assisted processing of materials.
In a context where ultrafast lasers have become ideal tools for material probing and processing we present various concepts for process control and optimization. Temporal tailoring of ultrashort laser pulses enables synergies between radiation and material and, therefore, new opportunities for optimal processing of materials. The concept of optimizing laser interactions is based on the possibility to adjust energy delivery so that control of laser-induced processes can be achieved and particular states of matter can be accessed. We present recent results related to the implementation of adaptive feedback loops based on temporal shaping of ultrafast laser pulses to control laser-induced phenomena for practical applications. The chosen example indicates the possibility to manipulate the kinetic properties of ions emitted from ultrafast laser irradiated semiconducting samples, using excitation sequences synchronized with the phase-transformation characteristic times. Versatile sub-keV ion beams are obtained exploiting transitions to supercritical fluid states with minimal energetic expenses, while achieving very efficient energy coupling and thermodynamic paths towards highly volatile states. Temporally selective irradiation can thus open up efficient thermodynamic paths towards critical points, delivering at the same time an extended degree of control in material processing.
Dynamics of electronic excitation, heating and charge-carrier transport in different materials (metals, semiconductors, and dielectrics) under femtosecond pulsed laser irradiation is studied based on a unified continuum model. A simplified drift-diffusion approach is used to model the energy flow into the sample in the first hundreds of femtoseconds of the interaction. The laser-induced charging of the targets is investigated at laser intensities slightly above the material removal threshold. It is demonstrated that, under near-infrared femtosecond irradiation regimes, charging of dielectric surfaces causes a sub-picosecond electrostatic rupture of the superficial layers, alternatively called Coulomb explosion (CE), while this effect is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties. Various related aspects concerning the possibility of CE for different irradiation parameters (fluence, wavelength and pulse duration) as well as the limitations of the model are discussed. These include the temporal and spatial dynamics of charge-carrier generation in non-metallic targets and evolution of the optical (reflection and absorption) characteristics. A controversial topic concerning CE probability in laser irradiated semiconductor targets is also a subject of this work.
Ultrafast lasers have become reliable tools for material processing on reduced scales, with extended options for process control and optimization. The additional possibility to shape the temporal envelope of ultrashort laser pulses by Fourier synthesis of spectral components enables completely new opportunities for optimal processing of materials. The concept of optimizing laser interactions is based on the possibility to regulate the energy delivery rate and to achieve control of laser-induced phenomena. An experimental demonstration of the technique showing the possibility to design excitation sequences tailored with respect to the material response will be described, laying the groundwork for adaptive optimization in materials structuring. We present recent results related to the implementation of self-learning loops based on temporal shaping of the ultrafast laser pulses to control laser-induced phenomena for practical applications. Besides the fundamental interest, it will be shown that under particular excitation conditions involving modulated excitation, the energy flow can be controlled and the material response can be guided to improve processing results. Examples will be given illuminating the possibility to optimize the kinetic properties of ions emitted from laser-irradiated semiconductor samples, uisng excitation sequences synchronized with the phase-transformation characteristic times, unfolding also interesting perspectives for "intelligent" processing of materials.
Ultrafast lasers promise to become attractive and reliable tools for material processing on micro- and nanoscale. The additional possibility to temporally tailor ultrashort laser pulses by Fourier synthesis of spectral components enables extended opportunities for optimal processing of materials. An experimental demonstration of the technique showing the possibility to design particular excitation sequences tailored with respect to the individual material response will be described, laying the groundwork for adaptive optimization in materials structuring. We report recent results related to the implementation of self-learning, adaptive loops based on temporal shaping of the ultrafast laser pulses to control laser-induced phenomena for practical applications. Besides the fundamental interest, it is shown that under particular excitation conditions involving modulated excitation, the energy flow can be controlled and the material response can be guided to improve processing results. Examples are given illuminating the possibility to control and manipulate the kinetic properties of ions emitted from laser irradiated semiconductor samples using excitation sequences synchronized with the phase transformation characteristic times.
We present a continuum model, based on a drift-diffusion approach, aimed to describe the dynamics of electronic excitation, heating and charge-carrier transport in different materials (metals, semiconductors, and dielectrics) under femtosecond and nanosecond pulsed laser irradiation. The laser-induced charging of the targets is investigated at laser intensities above the material removal threshold. It is demonstrated that, under near-infrared femtosecond irradiation regimes, charging of dielectric surfaces causes a sub-picosecond electrostatic rupture of the superficial layers, alternatively called Coulomb explosion (CE), while this effect is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties. On the other hand, simulations of UV nanosecond pulsed laser interaction with bulk silicon have pointed out the possibility of Coulomb explosion in semiconductors. For such regimes a simple analytical theory for the threshold laser fluence of CE has been developed, showing results in agreement with the experimental observations. Various related aspects concerning the possibility of CE depending on different irradiation parameters (fluence, wavelength and pulse duration) are discussed. This includes the temporal and spatial dynamics of charge-carrier generation in non-metallic targets and evolution of the reflection and absorption characteristics.
The high peak powers of ultra short (ps and sub-ps) pulsed lasers available at relatively low single pulse energies potentially allow for a precise localization of photon energy, either on the surface or inside (transparent) materials. Three dimensional micro structuring of bulk transparent media without any sign of mechanical cracking has shown the potential of ultra short laser processing. In this study, the micro structuring of bulk transparent media was used to modify fused silica and especially the cladding-core interface in normal fused silica wave guides. The idea behind this technique is to enforce a local mismatch for total reflection at the interface at minimal mechanic stress to overcome the barrier for enhanced optical out-coupling. The laser-induced modifications were studied in dependence of pulse width, focal alignment, single pulse energy and pulse overlap. Micro traces with a thickness between 3 and 8 μm were generated with a spacing of 10 μm in the subsurface region using sub-ps and ps laser pulses at a wavelength of 800 nm. The optical leakage enforced by a micro spiral pattern is significant and can be utilized for medical applications or potentially also for telecommunications and fiber laser technology.
The availability of ultra short (ps and sub-ps) pulsed lasers has stimulated a growing interest in exploiting the enhanced flexibility of femtosecond and/or picosecond laser technology for micro-machining. The high peak powers available at relatively low single pulse energies potentially allow for a precise localization of photon energy, either on the surface or inside (transparent) materials. Three dimensional micro structuring of bulk transparent media without any sign of mechanical cracking has been demonstrated. In this study, the potential of ultra short laser processing was used to modify the cladding-core interface in normal fused silica wave guides. The idea behind this technique is to enforce a local mismatch for total reflection at the interface at minimal mechanic stress. The laser-induced modifications were studied in dependence of pulse width, focal alignment, single pulse energy and pulse overlap. Micro traces with a thickness between 3 and 8 μm were generated with a spacing of 10 μm in the sub-surface region using sub-ps and ps laser pulses at a wavelength of 800 nm. The optical leakage enforced by a micro spiral pattern is significant and can be utilized for medical applications or potentially also for telecommunications and fiber laser technology.
Phase manipulated ultrafast laser pulses and temporally tailored pulse trains with THz repetition rates are promising new tools for quality micromachining of brittle dielectric materials, allowing to adapt the laser light to the material properties for optimal processing quality. Different materials respond with specific reaction pathways to the sudden energy input depending on the efficiency of electron generation and on the ability to release the energy into the lattice. Loss and cooling mechanisms in the electron population, surface charging, as well as the strength of the electron-phonon interactions control the effectiveness of the energy deposition into the lattice. Knowledge of the response times of materials establishes a guideline for using temporally shaped pulses or pulse trains in order to optimize the structuring process with respect to efficient material removal and reduction of the residual damage. The sequential energy delivery with judiciously chosen pulse trains may induce softening of the material during the initial steps of excitation and change the energy coupling for the subsequent steps. We show, that this can result in lower stress, cleaner structures, and allow for a material-dependent optimization process.
A significant improvement in the quality of ultrafast laser micromachining of brittle dielectrics is demonstrated by using temporally shaped pulse trains with sub-ps separation, synchronized with the material specific relaxation times. The individual material response to laser radiation depends on the efficiency of electron generation and on the ability to release the energy into the lattice. Loss mechanisms in the electron population, surface charging, as well as the strength of the electron-phonon interactions control the effectiveness of the energy deposition into the lattice. Knowledge of the response times of materials establishes a guideline for using temporally shaped pulses or pulse trains in order to optimize the structuring process with respect to the efficiency of material removal and reduction of the residual damage. The sequential energy delivery induces a material softening during the initial steps of excitation changing the energy coupling for the subsequent steps. This leads to lower stress, cleaner structures, and provides a material-dependent optimization process.
Time resolved studies using femtosecond laser pulses at 800 nm illuminate the distinctions in the dynamics of ultrafast processing of dielectrics compared to semi-conductors and metals. Dielectric materials are strongly charged at the surface on the sub-ps time scale and undergo an impulsive Coulomb explosion prior to thermal ablation. Provided the laser pulse width remains in the ps or sub-ps time domain this effect can be exploited for processing. Otherwise, the high localization of energy accompanied by ultrafast laser micro structuring is of great advantage also for high quality processing of thin metallic or semi-conductive layers, where the surface charge is effectively quenched.
We performed ablation studies on multi-layer systems at different wavelength - pulse duration combinations. The multi-layer systems of interest, 150 nm thin indium tin oxide (ITO), 200 thin polyaniline (PANI) on 1 micrometers thick photo resist, and 280 nm PPV/pedot layer-combination on 150 nm thin ITO are optically transparent and used for a variety of industrial applications. One important goal of the study was to determine the possible process window for a complete removal of only the top layer, leaving the remaining layer basically unharmed. The investigations were conducted with the following wavelength - pulse duration combinations: 800 nm and 180 fs, 800 nm and 5 ps, 266 nm and 150 fs, 266 nm and 5 ns, 532 nm and 5 ns. We generated micro dots, lines and areas to determine the damage threshold, the processing quality and the processing speed for the specified application of selective layer removal. The structures were analyzed by means of optical and atomic force microscopy. In some cases, we observed a strong pulse duration dependence in the ablation threshold, an indication for the observed difficulties using laser pulse in the ns range. Comparative studies at different wavelengths demonstrate that laser pulses in the UV are not necessarily always a first choice to achieve a precise removal of the optically transparent top layer.
We performed surface and bulk processing experiments on different transparent materials with ultra short laser pulses. The investigations were performed mainly at 800 nm and at pulse widths ranging from 0.2 to 5 ps. We focused our attention on fluence and shot number dependencies to analyze possible incubation effects in the different materials and determine the damage threshold. In the multi- shot experiments we determined strong incubation effects which we attribute to laser-induced defect formation and accumulation. Inside the bulk we were able to generate dots and lines even in sub-micrometers sizes. The structures were analyzed by means of optical microscopy. Laser pulses at a pulse width above ca. 1 ps demonstrate strong self focusing which can be utilized for bulk and rear surface micro structuring. Below a certain pulse width other effects counteract self focusing and beam diffraction and fillamentation seem to dominate. Depending on focusing optics we observe strong differences in the possibility to process the bulk of transparent materials with fs laser pulses which we attribute to the effects in Kerr non- linearity. Also, the consequences of incubation effects on the structuring inside the bulk seem to depend strongly on the pulse width. We discuss the results based on possible technological relevance and the ablation mechanism involved.
Dielectric materials exposed to ultrashort laser radiation have evidenced individualized paths to deposit the energy into the lattice. Electronic and thermal mechanisms competing in the process of material removal depend on the efficiency of the electrostatic energy accumulation on the surface due to photoionization, as well as on the lattice heating which follows the electron-phonon coupling. The electrostatic surface break-up is a fast, sub-picosecond process, while thermal mechanisms start to dominate on a longer, picosecond time scale given by the electron-lattice equilibration and phase transformation time. The Coulomb- explosion induced ion ejection due to surplus charge accumulated on the surface during the photoionization process is significant only in dielectrics while in semiconductors and metals an efficient neutralization occurs. The significance of the different channels in dielectric materials can be reduced or enhanced by using laser pulses which are modulated on a time scale characteristic for the above mentioned mechanisms. Thus, amplified temporally-shaped pulses, double peaks, or pulse trains with a separation below 1 ps can have a significant effect on the quality of micromachining of transparent materials. The energy deposition can be modulated in such a way that the first pulse of properly chosen energy leads to a softening of the material associated with the onset of heating, thus changing the coupling conditions for the next pulses. This leads to less residual stress accumulation, cleaner structures, and opens the way for a material dependent optimization process.
Ultrashort pulsed laser ablation of dielectrics has been investigated using ex-situ morphological examinations in combination with in-situ time-of-flight mass spectrometry of the ablated species. Analysis of the energy spectrum of the ablation products provides a wealth of information on the processes occurring during femtosecond laser ablation of materials. The presentation will focus on the case of sapphire (Al2O3) and discuss the fundamental processes in ultrashort pulsed laser sputtering. Two different ablation phases have been identified, a gentle phase with low ablation rates and a strong etch phase with higher ablation rates, but with limitation in structure quality. A comparison of the energy and momentum distributions of ejected ions, neutrals and electrons allows one to distinguish between non-thermal and thermal processes that lead to the macroscopic material removal. Fast positive ions with equal momenta are resulting from Coulomb explosion of the upper layers at low fluence and low number of irradiating laser pulses (gentle etch phase). Pump-probe studies with fs laser pulses reveal the dynamics of excitation and electron mediated energy transfer to the lattice. At higher laser fluences or after longer incubation, evidence for phase explosion can be derived from both the morphology of the surface and the results of the in-situ experiments.
Ultrashort laser pulses provide high power at low energy which is very promising to obtain precise microstructures inside and on the rear side of the transparent materials. Decreasing the pulse duration from several ps into the sub-ps range, we noticed strong differences in the processing and non-linear behavior. We performed experimental investigations on laser beam propagation through wide-band gap materials at high peak powers. We present recent results on different types of bulk modifications generated inside several different transparent materials and focus our attention on pulse duration dependent observations in a range between 0.2 and 4 ps.
We present the results of our investigations on the surface damage/ablation threshold and processing morphology for sapphire after single and multiple laser pulse irradiation at 800 nm in the picosecond and sub-picosecond duration range. The threshold for ablation drops sharply for multiple laser shot irradiation, due to material dependent incubation effects. We observe two distinctively etch phases: `gentle' and `strong'. Monitoring the mechanism and dynamics of the ion expulsion using combination of time-of-flight mass spectroscopy and femtosecond pump-probe technique, we identified Coulomb explosion as the dominant mechanism for ion emission in the `gentle' etch phase on a time scale of 1 ps. The momenta of the emitted ions are equal under these conditions. After sufficient incubation the damage threshold decreases and the ablation is shifted towards the `strong' phase. The velocity distributions shift to lower values, evidence for `phase explosion' is seen and the ions tend here to equal kinetic energies.
Ultrashort laser pulses have considerable potential for micron and sub-micron structuring of several materials. The lower energy impact, the reduction of thermal damage, the elimination of laser-plume interaction, and the exploitation of nonlinear optical effects all contribute to a strong improvement when compared to results using pulse widths in the nanosecond range. Depending on the choice of fluence compared to the damage threshold, with ultra-short laser pulses one is able to generate different types of structures, minimizing the heat affected zone. The damage threshold drops dramatically during the first laser shots, due to defect incubation. This has important consequences for applications, such as laser machining and for the lifetime of optical components. At a fluence below surface damage threshold we were also able to generate bulk modifications of different size and location in a controllable fashion by variation of laser pulse width, energy and number of shots, utilizing the beam narrowing effects during self focusing. A study of the dependence of the structure depth on the square root of the laser power for a given pulse length provides a straightforward method for determining the non-linear index of refraction.
We have investigated the material processing of oxides and fluorides using ultrashort laser pulses and have demonstrated a strong improvement when compared to results using longer pulse widths in the nanosecond range. High laser fluences (well above the damage threshold) at 800 nm and 248 nm are used to generate channels with high aspect ratios. Careful beam alignment can eliminate any remaining stress-induced damage outside the channel. At intermediate fluences just above the front surface processing threshold we observe a low ablation rate. In this 'gentle etch' phase it is possible to generate well-defined, smooth pockets and periodic patterns or ripples. The ripples appear when the laser pulse width is shorter than the lifetime of the electrons excited into the conduction band. In the low fluence regime (below the surface damage threshold) the self-focusing of laser pulses in the ps and sub-ps range can be utilized to obtain microstructures inside and on the rear side of the transparent materials.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.