fs Laser surface nano-structuring of high refractory ceramics to enhance solar radiation absorbance

The surface and structural modification of titanium (Ti) has been explored after the interaction of ultrashort laser pulses with the surface target. The targets were exposed by femtosecond Ti: Sapphire laser pulses in liquid (ethanol) and dry (air) environment. In order to explore the effect of pulse energy, the targets were exposed to 1,000 succeeding pulses for various pulse energies ranging from 200 to 500 μJ for pulse duration of 25 fs. SEM analyses were performed for central as well as the peripheral ablated areas of the target. It was found that in the case of ethanol (both for central and peripheral ablated areas) there is a grain growth along with nanoscale pores and dots when the target was irradiated for 200 μJ. For intermediate energies (300–400 μJ), grains of 1–2 μm with distinct boundaries are formed in the central ablated area. Whereas in the peripheral ablated area, laser-induced periodic surface structures (LIPSS) and globules are grown. For the highest pulse energy (500 μJ), distinct grains are observed for both regions. However, in the peripheral area the grains are of bigger size with cracks along the boundaries. In case of ablation in air, in the center of ablated areas, island-like structures with multiple ablative layer or LIPSS and nanoscale spheres are observed both for lower and intermediate pulse energies. For the highest pulse energy only nanoscale LIPSS could be observed. For ablation in air at the peripheral areas, well-defined, laser-induced periodic surface structures are observed for all pulse energies. Raman spectroscopy reveals that the liquid (ethanol) environment forms the carbonyl compounds with the metal and induces C–C stretching vibration, whereas in case of air, hydroxo complexes are formed. It has been found that surface treatment of Ti with ultrashort (25 fs) laser radiation in ethanol environment allows the growth of particular surface structures in the form of grains and simultaneously induces changes in its chemical composition.     

The growth of nanoscale periodic and dot-like structures on the surface of stainless steel with femtosecond laser pulses in the dry and wet ambient environment

The present work deals with growth of nanoscale periodic and dot-like structures on the surface of stainless steel (SS) by the irradiation of femtosecond laser pulses. For this purpose Ti: Sapphire femtosecond laser pulses (wavelength of 800 nm, pulse length of 25 fs and pulse repetition rate of 1 kHz) were employed in a dry (air) and liquid confined (deionized water and ethanol) environments. The targets were exposed to 1000 succeeding pulses for various fluences ranging from 50 to 150 mJ?cm^?2. Nanoscale structures including ripples, and dots were observed by SEM analysis. The growth and dependence of structure-formation on the ambient environment and laser fluence in both central as well as peripheral ablated areas is systematically investigated. The development of nanostructures and nanoripples is correlated with structural analysis carried out by micro Raman spectroscopy.     

SEM and Raman spectroscopy analyses of laser-induced periodic surface structures grown by ethanol-assisted femtosecond laser ablation of chromium

The effect of fluence and pulse duration on the growth of nanostructures on chromium (Cr) surfaces has been investigated upon irradiation of femtosecond (fs) laser pulses in a liquid confined environment of ethanol. In order to explore the effect of fluence, targets were exposed to 1000 pulses at various peak fluences ranging from 4.7 to 11.8?J?cm^–2 for pulse duration of ～25?fs. In order to explore the effect of pulse duration, targets were exposed to fs laser pulses of various pulse durations ranging from 25 to 100?fs, for a constant fluence of 11.8?J?cm^–2. Surface morphology and structural transformations have been analyzed by scanning electron microscopy and Raman spectroscopy, respectively. After laser irradiation, disordered sputtered surface with intense melting and cracking is obtained at the central ablated areas, which are augmented with increasing laser fluence due to enhanced thermal effects. At the peripheral ablated areas, where local fluence is approximately in the range of 1.4–4?mJ?cm^–2, very well-defined laser-induced periodic surface structures (LIPSS) with periodicity ranging from 270 to 370?nm along with dot-like structures are formed. As far as the pulse duration is concerned, a significant effect on the surface modification of Cr has been revealed. In the central ablated areas, for the shortest pulse duration (25?fs), only melting has been observed. However, LIPSS with dot-like structures and droplets have been grown for longer pulse durations. The periodicity of LIPSS increases and density of dot-like structures decreases with increasing pulse duration. The chemical and structural modifications of irradiated Cr have been revealed by Raman spectroscopy. It confirms the formation of new bands of chromium oxides and enol complexes or Cr-carbonyl compounds. The peak intensities of identified bands are dependent upon laser fluence and pulse duration.     

Surface modifications of materials by repetitive laser pulses

Laser-induced modifications on platinum (Pt) and silicon (Si) are compared by considering the development of various features on the irradiated surface. The experiments were carried out both in air and under vacuum. The interaction of 50 pulses of 1064 nm Nd:YAG laser with both targets in air resulted in non-linear phenomena. The periphery of the irradiated spot on the Pt surface exhibits wave-like patterns with a featureless central portion. A non-uniform distribution of cones of different sizes is also observed on the irradiated surface. In the case of silicon, the laser-induced periodic surface structures along with the formation of micro-column, rectangular blocks and grid are prominently observed features. However, when both the targets were irradiated with the same number of shots under vacuum (～10^?3 Torr), the surface morphologies of both the targets exhibited the hydrodynamic sputtering but with more explosive expulsion in Pt when compared with silicon. In platinum, there is a periodic variation in the distance between adjacent cones formed in various ablated zones. The Gaussian beam mode TEM₀₀ provided the evidences for melt pool formation in silicon when irradiated under vacuum. Additionally, we observed other mechanisms including splashing, sputtering, burning, re-solidification and redeposition on the surface of irradiated targets.     

Femtosecond laser-induced nanostructure-covered large-scale waves on metals

Through femtosecond (fs) laser pulse irradiation (pulse duration: 65 fs, central wavelength: 800 nm, and repetition rate: 250 Hz), we investigate the morphological evolution of fs laser-induced periodic surface structure on Au and Pt, called a nanostructure-covered large-scale wave (NC-LSW) with a period of tens of microns, densely covered by iterating stripe patterns of nanostructures and microstructures. We show that the surface morphology of NC-LSW crucially depends on the fluence of the laser, the number of irradiating pulses, and the incident beam angle. Our experimental observations allow us to establish a three-step model for the NC-LSW formation: the formation of laser-induced surface unevenness, inhomogeneous energy deposition due to the interference between the incident light and the scattered field, and nonuniform energy deposition due to shielding by the peaks of LSW.     

Dynamics of the formation of laser-induced periodic surface structures on dielectrics and semiconductors upon femtosecond laser pulse irradiation sequences

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica and silicon with multiple (N _DPS) irradiation sequences consisting of linearly polarized femtosecond laser pulse pairs (pulse duration ～150 fs, central wavelength ～800 nm) is studied experimentally. Nearly equal-energy double-pulse sequences are generated allowing the temporal pulse delay Δt between the cross-polarized individual fs-laser pulses to be varied from ?40 ps to +40 ps with a resolution of ～0.2 ps. The surface morphologies of the irradiated surface areas are characterized by means of scanning electron and scanning force microscopy. Particularly for dielectrics in the sub-ps delay range striking differences in the orientation and spatial characteristics of the LIPSS can be observed. For fused silica, a significant decrease of the LIPSS spatial periods from ～790 nm towards ～550 nm is demonstrated for delay changes of less than ～2 ps. In contrast, for silicon under similar irradiation conditions, the LIPSS periods remain constant (～760 nm) for delays up to 40 ps. The results prove the impact of laser-induced electrons in the conduction band of the solid and associated transient changes of the optical properties on fs-LIPSS formation.     

Two-dimensional measurements of laser-induced breakdown in air by high-speed two-frame shadowgraphy

We present two-dimensional measurements of the laser-induced plasma development and shock wave evolution in air. The breakdown is induced by a Q-switched Nd:YAG laser (λ=1064 nm) with a pulse duration of 4 ns. To study these fast laser-induced phenomena, we have developed a high-speed, two frame shadowgraph method. It enables 2D visualization of a laser-induced event in two time instances, which are delayed by an arbitrary time interval in the range from 300 ps to 30 ns. The established method is based on 30 ps, green (λ=532 nm), and linearly polarized laser pulse, which is split into two orthogonally polarized illumination pulses for direct and delayed illumination of the breakdown area. Exploiting polarization of the probe pulses, we capture two temporally and spatially separated frames with two CCD cameras. Special attention is given to the subsequent data processing, especially to the minimization of the systematic error due to alignment of both images, and to the determination of 2D velocity distribution from the captured image pairs.     

Crater geometry characterization of Al targets irradiated by single pulse and pulse trains of Nd:YAG laser in ambient air and water

High intensities laser pulses are capable to generate a crater when irradiating metal targets. In such condition, after each irradiation significant ablation occurs on the target surface and as a result a crater is formed. The crater characterization is very important specifically for some applications such as micromachining. In this paper, the crater formation in metal targets was studied experimentally. The planar aluminum 5052 targets were irradiated by frequency doubled (532 nm), Q-switched Nd:YAG (∼6 ns) laser beam in ambient air and distilled water. A crater was produced after each irradiation and it was characterized by an optical microscope. Different laser intensities as well as pulse trains were applied for crater formation. The effects of laser characteristics in crater geometry were examined. The depth of the craters was measured by optical microscope and the diameter (width) was characterized by processing of the crater image. The results were explained in terms of ablation threshold and plasma shielding. The results show that the crater geometry extremely depends on the laser pulse intensity, the number of laser pulses, and ambient.     

Identification of non-thermal and thermal processes in femtosecond laser-ablated aluminum

Non-thermal and thermal processes due to femtosecond laser ablation of aluminum (Al) at low, moderate, and high-fluence regimes are identified by Atomic Force Microscope (AFM) surface topography investigations. For this purpose, surface modifications of Al by employing 25 fs Ti: sapphire laser pulses at the central wavelength of 800 nm have been performed to explore different nano- and microscale features such as hillocks, bumps, pores, and craters. The mechanism for the formation of these diverse kinds of structures is discussed in the scenario of three ablation regimes. Ultrafast electronic and non-thermal processes are dominant in the lower fluence regime, whereas slow thermal processes are dominant at the higher fluence regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen: a lower fluence regime (0.06–0.5 J cm^?2 single-shot irradiation under ultrahigh vacuum condition and 0.25–2.5 J cm^?2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25–1.5 J cm^?2 multiple-shot irradiation), and a high-fluence regime 2.5–3.5 J cm^?2 multiple-shot irradiation. For the lower fluence (gentle ablation) regime, around the ablation threshold, the unique appearance of individual, localized Nano hillocks typically a few nanometers in height and less than 100 nm in diameter are identified. These Nano hillock-like features can be regarded as a nonthermal, electronically induced phase transition process due to localized energy deposition as a result of Coulomb explosion or field ion emission by surface optical rectification. At a moderate-fluence regime, slightly higher than ablation threshold multiple-pulse irradiation produces bump-formation and is attributed to ultrafast melting (plasma formation). The high-fluence regime results in greater rates of material removal with highly disturbed and chaotic surface of Al with an appearance of larger protrusions at laser fluence well above the ablation threshold. These nonsymmetrical shapes due to inhomogeneous nucleation, cluster formation, and resolidification of a metallic surface after melting are attributable to slow thermal processes (ps time scale).     

XRD and SEM analysis of a laser-irradiated cadmium

A pulsed Nd:YAG laser (10 mJ, 12 ns, 1064 nm) was employed to study the IR irradiation effects on metallic samples of cadmium. The laser was irradiated for 100, 200, and 300 shots under a vacuum ～10^?3 Torr. The results were investigated using a Hi Tech S3000H Scanning Electron Microscope (SEM) and X’pert Pro PANalytical X-ray Diffractometer (XRD). The micrographs obtained from SEM reveal that the surface morphological changes have occurred in the form of a crater. The forward expansion of plasma into an ambient gas coupled with the recondensation of the target surface results in the formation of debris. Large temperature gradients produce variations in the thermal resistance that leads to the distributed shape of the heat-affected zone. The hydrodynamic effects are apparent with a liquid flow to form the recast material around the periphery of the laser focal area. The turbulent resolidified material is formed when surface asperities are accelerated away from the liquid surface during each laser pulse due to melting followed by the thermal expansion of the liquid. The positive feed back of the repeated pulses resulted in the form of ripples. Grains appear on the surface as evidence of heterogeneous nucleation. The confirmation of the formation of these structures has been done by X-ray Diffractometer (XRD).     

Circular ripple formation on the silicon wafer surface after interaction with linearly polarized femtosecond laser pulses in air and water environments

Ultrashort laser pulse interaction with the surface of silicon wafer in air and water environments is investigated. Ti:sapphire laser with 40 femtosecond laser pulses at 790 nm and 10 Hz repetition rate was used. The ablation threshold of the silicon surface in the air was determined to be about 0.28 J cm^?2. The surface morphology was studied by using scanning electron microscope images. The size of the regular ripples formed in the air environment is a little smaller than the laser wavelength. Due to the nonlinear interaction and self-focusing before the target, the ripples size reduced to nearly a half of the laser wavelength in the water. Moreover, the spikes’ structure formation and their diameter in air and water were studied. Two regimes for spike formation in water are proposed that can explain the anomalous decrease of the spikes’ diameter in higher fluence. During the interaction of single linearly polarized femtosecond laser pulse with the surface, an irregular ripple formation that called circular ripple is observed. This structure which is a result of radiation pressure implies to the surface by the end of the pulse. A new physical model for interpretation of the circular ripples formation based on the ponderomotive force of an ultrashort pulse laser is proposed which can predict the size of the circular ripples. The calculated results are in accordance with our experimental findings.     

Sub-100-nm laser-induced periodic surface structures upon irradiation of titanium by Ti:sapphire femtosecond laser pulses in air

The formation of laser-induced periodic surface structures (LIPSS) on titanium upon irradiation with linearly polarized femtosecond (fs) laser pulses (τ=30 fs, λ=790 nm) in an air environment is studied experimentally and theoretically. In the experiments, the dependence on the laser fluence and the number of laser pulses per irradiation spot has been analyzed. For a moderate number of laser pulses (N<1000) and at fluences between ～0.09 and ～0.35 J/cm², predominantly low-spatial-frequency-LIPSS with periods between 400 nm and 800 nm are observed perpendicular to the polarization. In a narrow fluence range between 0.05 and 0.09 J/cm², high-spatial-frequency-LIPSS with sub-100-nm spatial periods (～λ/10) can be generated with an orientation parallel to the polarization (N=50). These experimental results are complemented by calculations based on a theoretical LIPSS model and compared to the present literature.     

Preparation of C/Ni–NiO composite nanofibers for anode materials in lithium-ion batteries

We report here on a systematic study about the formation of laser-induced periodic surface structures (LIPSS) on biopolymers. Self-standing films of the biopolymers chitosan, starch and the blend of chitosan with the synthetic polymer poly (vinyl pyrrolidone), PVP, were irradiated in air with linearly polarized laser beams at 193, 213 and 266 nm, with pulse durations in the range of 6–17 ns. The laser-induced periodic surface structures were topographically characterized by atomic force microscopy and the chemical modifications induced by laser irradiation were inspected via Raman spectroscopy. Formation of LIPSS parallel to the laser polarization direction, with periods similar to the laser wavelength, was observed at efficiently absorbed wavelengths in the case of the amorphous biopolymer chitosan and its blend with PVP, while formation of LIPSS is prevented in the crystalline starch biopolymer.     

Influence of picosecond laser pulses on the microstructure of austenitic materials

We investigate samples of austenitic materials ?C stainless steel and iron-based superalloy ?C which are widely used at high temperatures and pressures. The samples were exposed to Nd³⁺:YAG laser pulses with a wavelength of 1064 nm and a pulse duration of 170 ps. We employ different pulse energies and number of pulses. The spots appearing after the laser irradiation were examined by optical and scanning electron microscopy and analyzed by energy-dispersive spectroscopy. We also perform Vickers microhardness tests. We discuss the microstructure changes caused by different energy and number of pulses in air and in a He-enriched atmosphere with the aim to determine optimum laser parameters in surface-treatment technology.     

Surface and structural investigations on laser irradiated P-type silicon

P-type silicon (100) is irradiated with Nd:YAG laser (532 nm, 500 mJ and 6–7 ns) in air at room temperature for two different laser incident angles 0° and 45° with the surface normal of the target for 10–50 laser shots with a step of 10 shots. The surface morphological changes of exposed samples are explored by making use of the scanning electron microscope. The variations in crystal structure and crystallinity along with laser annealing effects are examined by employing Raman spectroscopy. The thermal sputtering, hydrodynamic sputtering, exfoliation sputtering, whisker formation and bubble bursting are the salient features found on the irradiated surfaces. An increase in laser shots causes incremental changes in area of melted and heat-affected zones and Raman shifts.     

Shot-to-shot correlation of residual energy and optical absorptance in femtosecond laser ablation

In this paper, we perform a shot-to-shot detailed study of how residual thermal energy correlates to the optical absorptance change due to laser-induced surface structural modifications in multi-shot femtosecond laser ablation. We observe an overall enhancement for residual thermal coupling and absorptance in air. Surprisingly, residual thermal coupling in air shows a non-monotonic dependence on pulse number and reaches a minimum value after a certain number of pulses, while these behaviors are not seen in absorptance. In vacuum, however, both suppression and enhancement are seen in residual energy coupling although absorptance is always enhanced. To explain these observations, we suggest that air plasma plays a dominant role in thermal coupling at a relatively low number of applied pulses, while the formation of a cavity plays a dominant role at a high number of pulses. PACS 78.20.Ci; 81.05.Bx     

Identification of ultra-fast electronic and thermal processes during femtosecond laser ablation of Si

Ultra-fast electronic and thermal processes for the energy deposition mechanism during femtosecond laser ablation of Si have been identified by means of atomic force microscopy and Raman scattering techniques. For this purpose, Si targets were exposed with 800-nm, 25-fs Ti:sapphire laser pulses for different laser fluencies in air and under UHV (ultra high vacuum) conditions. Various nano- and microstructures on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Ultra-fast electronic processes are dominant in the lower-fluence regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen: a lower-fluence regime (0.06–0.5 J?cm^?2 single-shot irradiation under UHV condition and 0.25–2.5 J?cm^?2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25–1.5 J?cm^?2 multiple-shot irradiation), and a higher-fluence regime (2.5–3.5 J?cm^?2 multiple-shot irradiation). Around the ablation threshold fluence, most significant features identified at the Si surface are nanohillock-like structures. The appearance of these nanohillocks is regarded as typical features for fast electronic processes (correlated with existence of hot electrons) and is explained on the basis of Coulomb explosion. The growth of these typical features (nanohillocks) by femtosecond laser irradiation is an element of novelty. At moderate irradiation fluence, a ring-shaped ablation with larger bumps and periodic surface structures is observed and is considered as a footprint of ultra-fast melting. Further increase in the laser fluence, i.e. a higher-fluence regime, resulted in strong enhancement of the thermal process with the appearance of larger islands. The change in surface topography provides an innovative clue to differentiate between ultra-fast electronic processes, i.e. Coulomb explosion (sub-100 fs) at a lower-fluence regime and ultra-fast melting (hundreds of fs) at a moderate-fluence regime, and slow thermal processes (ps time scale) at a higher-fluence regime. These fast electronic and thermal processes are well correlated to structural and crystallographic alterations, inferred from Raman spectroscopy.     

Surface topography of ultrashort laser-irradiated CaF2

A single-crystal CaF₂ (111) was irradiated with single and multiple laser (Ti:sapphire, 800 nm, 25 fs) shots at fluences ranging from 0.25 to 1.5 J cm^?2. In this fluence regime, a single laser pulse usually leads to typical bump-like features ranging from 200 nm to 1.5 μm in diameter and 10–50 nm in height. These bumps are related to compressive stresses due to a pressure build-up induced by fast laser heating and their subsequent relaxation. When CaF₂ is irradiated with successive (in our case 20) shots at a laser fluence of 1.5 J cm^?2, nanocavities at the top of the microbumps are observed. The formation of these nanocavities is regarded as an explosion and is attributed to the explosive expansion generated by shock waves due to laser-induced plasma after the nonlinear absorption of the laser energy by the material. Such kinds of surface structures at the nanometre scale could be attractive for nanolithography.     

Progressive formation of fine and coarse ripples on SiC surface by repeated irradiation of femtosecond laser pulses

In this work, we report the progressive formation of first nanoparticles, next fine ripples, and eventually coarse ripples during the irradiation of single-crystal 6H-SiC surfaces with increasing number of femtosecond laser pulses (λ = 515 nm, τ = 250 fs, repetition rate = 100 kHz). At laser fluences greater than the single-pulse ablation threshold, nanoparticles were produced on the surface by the first few pulses over which fine ripple patterns overlapped at increased pulse numbers. As the pulse number was further increased over ten, the surface was gradually transformed into a coarse ripple–covered one. At laser fluence below the threshold, however, only fine ripples were formed nonuniformly.     

Influence of consecutive picosecond pulses at 532 nm wavelength on laser ablation of human teeth

The interaction of 40 ps pulse duration laser emitting at 532 nm wavelength with human dental tissue (enamel, dentin, and dentin–enamel junction) has been investigated. The crater profile and the surface morphology have been studied by using a confocal auto-fluorescence microscope (working in reflection mode) and a scanning electron microscope. Crater profile and crater morphology were studied after applying consecutive laser pulses and it was found that the ablation depth increases with the number of consecutive pulses, leaving the crater diameter unchanged. We found that the thermal damage is reduced by using short duration laser pulses, which implies an increased retention of restorative material. We observe carbonization of the irradiated samples, which does not imply changes in the chemical composition. Finally, the use of 40 ps pulse duration laser may become a state of art in conservative dentistry.