Periodic patterning of silicon by direct nanosecond laser interference ablation

The production of periodic structures in silicon wafers by four-beam is presented. Because laser interference ablation is a single-step and cost-effective process, there is a great technological interest in the fabrication of these structures for their use as antireflection surfaces. Three different laser fluences are used to modify the silicon surface (0.8 J cm⁻², 1.3 J cm⁻², 2.0 J cm⁻²) creating bumps in the rim of the irradiated area. Laser induced periodic surface structures (LIPSS), in particular micro and nano-ripples, are also observed. Measurements of the reflectivity show a decrease in the reflectance for the samples processed with a laser fluence of 2.0 J cm⁻², probably caused by the appearance of the nano-ripples in the structured area, while bumps start to deteriorate.     

Clusterization of vacancy defects in ZnO irradiated with 2 MeV O

Slow positrons have been used to study ZnO layers grown on a-axis sapphire and irradiated by 2 MeV O⁺ ions to fluences from 10¹² cm⁻² to 10¹⁷ cm⁻². At low fluences Zn vacancies are observed, and their introduction rate is estimated as 2000 cm⁻¹. At the highest fluences of 10¹⁶-10¹⁷ cm⁻² vacancy clusters are formed. The extent of the primary damage and its recovery is discussed.     

Pulsed laser deposition of polyhydroxybutyrate biodegradable polymer thin films using ArF excimer laser

We demonstrated the pulsed laser deposition (PLD) of high quality films of a biodegradable polymer, the polyhydroxybutyrate (PHB). Thin films of PHB were deposited on KBr substrates and fused silica plates using an ArF (λ = 193 nm, FWHM = 30 ns) excimer laser with fluences between 0.05 and 1.5 J cm⁻². FTIR spectroscopic measurements proved that at the appropriate fluence (0.05, 0.09 and 0.12 J cm⁻²), the films exhibited similar functional groups with no significant laser-produced modifications present. Optical microscopic images showed that the layers were contiguous with embedded micrometer-sized grains. Ellipsometric results determined the wavelength dependence (λ ∼ 245-1000 nm) of the refractive index and absorption coefficient which were new information about the material and were not published in the scientific literature. We believe that our deposited PHB thin films would have more possible applications. For example to our supposal the thin layers would be applicable in laser induced forward transfer (LIFT) of biological materials using them as absorbing thin films.     

Laser ablation synthesis of indium oxide nanoparticles in water

Colloidal solutions of Indium oxide nanoparticles have been produced by means of laser ablation in liquids (LALs) technique by simply irradiating with a second harmonic (532 nm) Nd:YAG laser beam a metallic indium target immersed in distilled water and varying the laser fluence up to 10 J cm⁻² and the ablation time up to 120 min. At all the investigated fluences the vaporization process of the indium target is the dominant one. It produces a majority (>80%) of small size (<6 nm) nanoparticles, with a very limited content of larger ones (size between 10 and 20 nm). The amount of particles increases regularly with the ablation time, supporting the scalability of the production technique. The deposited nanoparticles stoichiometry has been verified by both X-ray photoelectron spectroscopy (XPS) and Energy Dispersive X-ray (EDX) analysis. Optical bandgap values of 3.70 eV were determined by UV-vis absorption measurements. All these results confirm the complete oxidation of the ablated material.     

Femtosecond laser-induced damage of gold films

Single- and multi-shot ablation thresholds of gold films in the thickness range of 31-1400 nm were determined employing a Ti:sapphire laser delivering pulses of 28 fs duration, 793 nm center wavelength at 1 kHz repetition rate. The gold layers were deposited on BK7 glass by an electron beam evaporation process and characterized by atomic force microscopy and ellipsometry. A linear dependence of the ablation threshold fluence F_th on the layer thickness d was found for d ≤ 180 nm. If a film thickness of about 180 nm was reached, the damage threshold remained constant at its bulk value. For different numbers of pulses per spot (N-on-1), bulk damage thresholds of ∼0.7 J cm⁻² (1-on-1), 0.5 J cm⁻² (10-on-1), 0.4 J cm⁻² (100-on-1), 0.25 J cm⁻² (1000-on-1), and 0.2 J cm⁻² (10000-on-1) were obtained experimentally indicating an incubation behavior. A characteristic layer thickness of L_c ≈ 180 nm can be defined which is a measure for the heat penetration depth within the electron gas before electron-phonon relaxation occurs. L_c is by more than an order of magnitude larger than the optical absorption length of α⁻¹ ≈ 12 nm at 793 nm wavelength.     

Periodic surface structures on crystalline silicon created by 532 nm picosecond Nd:YAG laser pulses

Creation of laser-induced morphology features, particularly laser-induced periodic surface structures (LIPSS), by a 532 nm picosecond Nd:YAG laser on crystalline silicon is reported. The LIPSS, often termed ripples, were produced at average laser irradiation fluences of 0.7, 1.6, and 7.9 J cm⁻². Two types of ripples were registered: micro-ripples (at micrometer scale) in the form of straight parallel lines extending over the entire irradiated spot, and nano-ripples (at nanometer scale), apparently concentric, registered only at the rim of the spot, with the periodicity dependent on laser fluence. There are indications that the parallel ripples are a consequence of the partial periodicity contained in the diffraction modulated laser beam, and the nano-ripples are very likely frozen capillary waves. The damage threshold fluence was estimated at 0.6 J cm⁻².     

Small size TiO2 nanoparticles prepared by laser ablation in water

Titanium dioxide nanoparticles in distilled H₂O solvent were prepared by laser ablation. The experiments were performed irradiating a Ti target with a second harmonic (532 nm) output of a Nd:YAG laser varying the operative fluence between 1 and 10 J cm⁻² and for an ablation time ranging from 10 to 30 min. Electron microscopy measurements have evidenced the predominant presence of nanoparticles with diameter smaller than 10 nm together with agglomerations of 100-200 nm whose content increases with the laser fluence. At low laser fluence the particles’ size distribution shows that more than 85% of the nanoparticles have a size smaller than 5 nm while at mid and high fluences the presence of 5-7 nm nanoparticles is predominant. XPS analysis has revealed the presence of different titanium suboxide phases with the prevalence of Ti-O bonds from TiO₂ species. The optical bandgap values, determined by UV-vis absorption measurements, are compatible with the anatase phase.     

Characterization of Ti6Al4V implant surface treated by Nd:YAG laser and emery paper for orthopaedic applications

A more noble and biocompatible Ti alloy was achieved at fluence of 140 J cm⁻² where the implant indicated a higher degree of hardness (825HV), higher corrosion resistance (−0.21 V) and highest hydrophilicity (i.e. θ_c = 37°) compared with 70° of the control sample. These values corresponded to 58 and 39 mN m⁻¹ of surface tension respectively. The laser treated samples at 140 J cm⁻² showed higher wettability characteristics than mechanically roughened surface. Cell growth and their spreading condition in a specific area were analyzed by SEM and Image J Program software. Clearly, more cells were attached (1.2 × 10⁵) to and spread (488 μm²) over the surface at 140 J cm⁻² than in any other condition. Pathologically, the treated samples indicated no sign of infection.     

The effects of xenon ion irradiation on the photoluminesce behavior of poly(p-cresolformaldeyde)/diazonaphtoquinone thin films

In the present paper, we investigate the origin of photoluminescence (PL) and the changes in the optical properties: refractive index and absorption coefficient, in poly(p-cresolformaldeyde) and diazonaphtoquinone thin films irradiated with Xe ions. Films 400 nm thick have been irradiated with 800 keV Xe²⁺ ions in a fluence range from 10¹³ to 6 × 10¹⁵ Xe cm⁻². The structural modifications were followed by the techniques of nuclear reaction analysis, elastic recoil detection analysis, Rutherford backscattering, Fourier transform infrared and Raman spectroscopies. The PL behavior was characterised with 488 nm excitation wavelength. The pristine films show emission with maxima of the main bands located at 635, 720 and 830 nm. For fluences up to 10¹⁴ Xe cm⁻², the photoluminescence intensity increases with the irradiation fluence. The chain mobility lowering, characterized by the crosslinked structure, explains this behavior in organic systems. Other possible contribution for increasing of PL intensity, at these fluences, is the presence of oxygen trapped in the polymer chains by the dangling bonds. At intermediate and higher fluences, the photoluminescence starts to decrease. At fluences higher than 10¹⁴ Xe cm⁻², irreversible changes of the organic structure occur and they are characterized by large losses of oxygen and hydrogen, transforming the material into amorphous carbon films. The loss of photoluminescent behavior is associated with the light absorption characteristics of the amorphous carbon structure. This conclusion is supported by the observed increase of the refractive indexes and absorption coefficients, obtained in the infrared region, as well as by the Raman results. Also, the effect of irradiation modifying the refractive index in the infrared region suggests the application of these films as waveguide in this region of wavelength.     

Property improvement of pulsed laser deposited boron carbide films by pulse shortening

Growth characteristics and surface morphology of boron carbide films fabricated by ablating a B₄C target in high vacuum with a traditional KrF excimer laser and a high brightness hybrid dye/excimer laser system emitting at the same wavelength while delivering 700 fs pulses are compared. The ultrashort pulse processing is highly effective. Energy densities between 0.25 and 2 J cm⁻² result in apparent growth rates ranging from 0.017 to 0.085 nm/pulse. Ablation with nanosecond pulses of one order of magnitude higher energy densities yields smaller growth rates, the figures increase from 0.002 to 0.016 nm/pulse within the 2-14.3 J cm⁻² fluence window. 2D thickness maps derived from variable angle spectroscopic ellipsometry reveal that, when ablating with sub-ps pulses, the spot size rather than the energy density determines both the deposition rate and the angular distribution of film material. Pulse shortening leads to significant improvement in surface morphology, as well. While droplets with number densities ranging from 1 × 10⁴ to 7 × 10⁴ mm⁻² deteriorate the surface of the films deposited by the KrF excimer laser, sub-ps pulses produce practically droplet-free films. The absence of droplets has also a beneficial effect on the stoichiometry and homogeneity of the films fabricated by ultrashort pulses.     

Time resolved magnetic ordering and photo-induced gyrotropy at room temperature in CdMgTe-CdMnTe semiconductor micro-cavities

Using a picosecond pump and probe time-resolved technique we evidence a single pump pulse photo-induced magnetic ordering in a Mn-doped semiconductor magneto-photonic microcavity operating in the strong coupling regime at room temperature. This nanosecond duration magnetization is attributed to a magnetic ordering of the Mn-impurities mediated through photo-generated holes and enhanced through the confinement. It is distinct from the preceding short lived photo-induced spin orientation of carriers also evidenced by our technique for circularly polarized pump beams. The photo-generated magnetic flux density amounts to a 1 kG for beam fluences of few tens μJ cm⁻² and effective Mn concentrations of 5 nm⁻³; large photo-induced magneto-optic Kerr rotations are also evidenced.     

Nitrogen plasma-based ion implantation of poly(tetrafluoroethylene): Effect of the main parameters on the surface properties

The surface of poly(tetrafluoroethylene) (PTFE or Teflon) was treated by nitrogen plasma-based ion implantation. Accelerating voltages between 15 and 30 kV, fluences between 1 × 10¹⁷ and 3 × 10¹⁷ cm⁻² and fluence rates between 3 × 10¹³ and 7 × 10¹³ cm⁻² s⁻¹ were applied. The effects of these main parameters were examined on the evolution of surface chemical composition, mean roughness, abrasive wear, wettability and surface electrical resistance. The aim was to obtain relationships, enabling to control the surface properties examined.The F/C atomic ratio determined by XPS strongly decreased, correlating inversely with voltage. The mean surface roughness characterized by topography measurements, increased, correlating directly with fluence and inversely with voltage. The wear volume obtained by multipass scratch tests did not show clear relationship with the main process parameters, however, it increased upon treatment with the increase of surface roughness and O/C atomic ratio. The water contact angle increased at low voltages and high fluences, due to preferential increase of roughness, and decreased at high voltages and low fluences, owing to intense defluorination and incorporation of N and O. The electrical resistance of the PBII-treated surfaces decreased by several orders of magnitude, showing a significant inverse correlation with fluence. It continued to decrease for samples exposed to air, primarily after treatments performed with low fluences, due to post-treatment type oxidation.     

Damage formation in Ge during Ar and He implantation at 15 K

Ar⁺ and He⁺ ions were implanted into Ge samples with (1 0 0), (1 1 0), (1 1 1) and (1 1 2) orientations at 15 K with fluences ranging from 1×10¹¹ to 1×10¹⁴ cm⁻² for the Ar⁺ ions and fluences ranging from 1×10¹² to 6×10¹⁵ cm⁻² for the He⁺ ions. The Rutherford backscattering (RBS) technique in the channelling orientation was used to study the damage built-up in situ. Implantation and RBS measurements were performed without changing the target temperature. The samples were mounted on a four axis goniometer cooled by a close cycle He cryostat. The implantations were performed with the surface being tilt 7° off the ion beam direction to prevent channelling effects. After each 300 keV Ar⁺ and 40 keV He⁺ implantation, RBS analysis was performed with 1.4 MeV He⁺ ions.For both the implantation ions, there is about no difference between the values found for the damage efficiency per ion for the four different orientations. This together with the high value (around 5 times higher than that found in Si), gives rise to the assumption of amorphous pocket formation per incident ion, i.e. direct impact amorphization, already at low implantation fluences. At higher fluences, when collision cascades overlap, there is a growth of the already amorphized regions.     

Study of the interplay of vaporisation, melt displacement and melt ejection mechanisms under multiple pulse irradiation of metals

A threefold study combining profilometry, high speed imaging and recoil momentum measurements is used to deconvolve the relative contributions to material removal attributable to vaporisation, melt displacement and explosive melt ejection. The interplay of these three mechanisms is studied as a function of the number of laser pulses incident on an aluminium target and pulse repetition frequency. This study shows cumulative heating affects matter removed as both vapour and liquid melt, and highlights the influence of the vapour plume and ablation crater morphology on the proportions of material removed as melt displacement and melt ejection.     

Surface modifications of crystalline silicon created by high intensity 1064 nm picosecond Nd:YAG laser pulses

A study of silicon modification induced by a high intensity picosecond Nd:YAG laser, emitting at 1064 nm, is presented. It is shown that laser intensities in the range of 5 × 10¹⁰-0.7 × 10¹² W cm⁻² drastically modified the silicon surface. The main modifications and effects can be considered as the appearance of a crater, hydrodynamic/deposition features, plasma, etc. The highest intensity of ∼0.7 × 10¹² W cm⁻² leads to the burning through a 500 μm thick sample. At these intensities, the surface morphology exhibits the transpiring of the explosive boiling/phase explosion (EB) in the interaction area. The picosecond Nd:YAG laser-silicon interaction was typically accompanied by massive ejection of target material in the surrounding environment. The threshold for the explosive boiling/phase explosion (TEB) was estimated to be in the interval 1.0 × 10¹⁰ W cm⁻² < TEB ≤ 3.8 × 10¹⁰ W cm⁻².     

Spin-glass behavior and magnetocaloric effect in Tb-based bulk metallic glass

A noncollinear-ferromagnetic spin-glass-like state was observed in Tb₅₅Co₂₀Al₂₅ bulk metallic glass due to the strong random magnetic anisotropy. Associated with this behavior, we observed a comparatively large magnetic entropy change (ΔS_m is 9.75 J K⁻¹ kg⁻¹) in a field change of 7 T and a correspondingly high value of the magnetic refrigeration capacity (RC is 540 J kg⁻¹) with almost no hysteresis loss in the vicinity of the so-called Curie temperature. This opens the possibility of using this material for magnetic cooling purposes.     

Vacancy profiles and clustering in light-ion-implanted GaN and ZnO

We present experimental results obtained in H-implanted GaN and He- and Li-implanted ZnO. The ion energies were varied in the range 100-850 keV, and the implantation fluences in the range 5 × 10¹³ to 1 × 10¹⁸ cm⁻². In addition, conventional and flash anneals at temperatures 500-1400 °C were performed on the ZnO samples. The data obtained with a slow positron beam show that vacancy clusters are formed in as-implanted samples with fluences above 1 × 10¹⁷ cm⁻². Below this value only single vacancies are detected after implantation, but vacancy clusters can be formed and subsequently dissociated by thermal annealings.     

Femtosecond laser ablation of silver foil with single and double pulses

The average ablation depth per pulse of silver foil by 130 fs laser pulses has been measured in vacuum over a range of three orders of magnitude of pulse fluence up to 900 J cm⁻². In addition, double pulses with separations up to 3.4 ns have been used to probe time scales of relevance for femtosecond ablation. The double pulse ablation depth, when each pulse fluence is 0.7 J cm⁻², falls to that of a single pulse as the pulse separation is increased from 0 ps to 700 ps. This time scale decreases to only 4 ps as the fluence is increased to 11 J cm⁻². It then jumps to 500 ps across a transition fluence where the slope of the ablation depth versus logarithmic fluence characteristic changes abruptly to a higher value. In addition, for pulse separations near 1000 ps, the second pulse can cause re-deposition of ejecta from the first pulse resulting in a double pulse ablation depth only 40% that of the first pulse alone. This has important implications for the interpretation of double pulse femto-LIBS intensities. Our results suggest that the optical properties of nano or mesoparticles play a significant role in double pulse ablation with large pulse separations.     

Effect of swift (∼100 MeV) heavy ion irradiation on surface morphology and electronic transport in Fe film on Si substrate

Fe film (∼50 nm) have been deposited on pSi substrate by electron beam evaporation technique. The bilayers have been irradiated by 100 MeV Fe⁷⁺ ions having fluences of 1 × 10¹³, 1 × 10¹⁴ and 5 × 10¹⁴ ions cm⁻². SEM study of the unirradiated devices show surface modifications having a annular structures. From XRD study of the bilayer, it is observed that grain size has reduced from 70 to 25 nm after the irradiation for a fluence of 1 × 10¹⁴ ions cm⁻². Moreover electronic transport data of the bilayer show practically no effect on the current flow for a fluence of 1 × 10¹³ ions cm⁻² irradiation whereas for 1 × 10¹⁴ ions cm⁻² fluence, there is very significant change in current flow (by two orders in magnitude) across the bilayer. However, for a higher fluence of irradiation 5 × 10¹⁴ ions cm⁻², the bilayer becomes highly resistive. It has been found from the above observations that the fluence of 1 × 10¹⁴ ions cm⁻² of swift heavy ion irradiation is a optimum fluence.     

In situ monitoring the pulse CO2 laser interaction with 316-L stainless steel using acoustical signals and plasma analysis

In most laser material processing, material removal by different mechanisms is involved. Here, application of acoustic signals with thermoelastic (below threshold) and breakdown origin (above threshold) together with plasma plume analysis as a simple monitoring system of interaction process is suggested. In this research the interaction of pulse CO₂ laser with 200 ns duration and maximum energy of 1.3 J operating at 1 Hz with austenitic stainless steel (316-L) is reported. The results showed that the non-linear point of the curve can serve as a useful indicator of melting fluence threshold (in this case ≈830 J cm⁻²) with corresponding temperature calculated using plasma plume analysis. Higher acoustic amplitudes and larger plasma plume volume indicates more intense interaction. Also, analysis showed that a phase explosion process with material removal (ejecta) in the form of non-adiabatic (i.e., d_t ? α⁻¹) is at play after laser pulse is ended. Also, SEM photographs show different surface quality medication at different laser intensities, which indicates the importance of recoil momentum pressure and possibly electrons and ions densities in heat transfer. Finally, electrochemical test indicate an improved corrosion resistance for laser treated samples compared to untreated ones.