High power single-shot laser ablation of silicon with nanosecond 355 nm

We report on high intensity single-shot laser ablation of monocrystalline silicon with a nanosecond Nd:YAG at 355 nm. It is shown that for incident laser intensities exceeding ∼11.5 GW/cm² on the silicon surface, unusually high etch depths can be achieved reaching values up to 60 μm. The results support previous observations of dramatic increase in etch rates in single-shot laser ablation at 266 nm. A laser-induced explosive boiling mechanism together with secondary plasma heating is believed to be associated with this effect.     

Tailored ablation processing of advanced biomedical hydroxyapatite by femtosecond laser pulses

The micromachining of hydroxyapatite (HAp) is highly important for orthopedics and dentistry, since human bone and teeth consist mainly of HAp. We demonstrate ultrashort Ti:sapphire laser ablation of HAp, using pulse-widths of 50 fs, 500 fs, and 2 ps at a wavelength of 820 nm and at 1 kpps. The crucial medical issue is to preserve the chemical properties of the machined (ablated) surface. If the chemical properties of HAp change, the human bone or tooth cannot re-grow after laser processing. Using X-ray photoelectron spectroscopy, we observe chemical properties of HAp ablated in air. The HAp is ablated at laser fluences of 3.2 J/cm² (6.4×10¹³ W/cm² at 50 fs), 3.3 J/cm² (6.6×10¹² W/cm² at 500 fs), and 9.6 J/cm² (4.8×10¹² W/cm² at 2 ps), respectively. As a result it is found that the ablated surface is unchanged after laser ablation over the pulse-width range used in this experiment. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-45/566-1533, E-mail: obara@obara.elec.keio.ac.jp     

Solitary surface acoustic waves and bulk solitons in nanosecond and picosecond laser ultrasonics

Recent achievements of nonlinear acoustics concerning the realization of solitons and solitary waves in crystals and their surfaces attained by nanosecond and picosecond laser ultrasonics are discussed and compared. The corresponding pump-probe setups are described, which allow an all-optical contact-free excitation and detection of short strain pulses in the broad frequency range between 10 MHz and about 300 GHz. The formation of solitons in the propagating longitudinal strain pulses is investigated for nonlinear media with intrinsic lattice-based dispersion. The excitation of solitary surface acoustic waves is realized by a geometric film-based dispersion effect. Future developments and potential applications of nonlinear nanosecond and picosecond ultrasonics are discussed.     

Phase diffractive optical gratings on glass substrates by laser ablation

A novel and rapid laser ablation method for the fabrication of phase diffractive gratings in low-cost glass substrates is presented. The structures are characterized in terms of their shape and physical parameters by confocal microscopy and SEM microscopy. The first-order diffraction efficiency at normal incidence under 632.8 nm wavelength is shown. The influence of the overlap factor on the period grating is studied.     

Mechanisms of small clusters production by short and ultra-short laser ablation

The mechanisms involved into the formation of clusters by pulsed laser ablation are studied both numerically and experimentally. To facilitate the model validation by comparison with experimental results, the time and length scales of the simulation are considerably increased. This increase is achieved by using a combination of molecular dynamics (MD) and the direct simulation Monte Carlo (DSMC) methods. The combined MD-DSMC model is then used to compare the relative contribution of the two channels of the cluster production by laser ablation: (i) direct cluster ejection upon the laser-material interaction, and (ii) collisional sticking and aggregation in the ablated gas flow. Calculation results demonstrate that both of these mechanisms play a role. The initial cluster ejection provides cluster precursors thus eliminating the three-body collision bottleneck in the cluster growth process. The presence of clusters thus facilitates the following collisional condensation and evaporation processes. The rates of these processes become considerable, leading to the modification of not only the plume cluster composition, but also the dynamics of the plume expansion. Calculation results explain several recent experimental findings.     

Self-organization of thin metal films by irradiation with nanosecond laser pulses

Modifications in thin metal films under intensive laser irradiation were studied. Gold, silver, copper, chromium and aluminum films with the thickness of 100 nm were deposited on the glass substrate. Back-side irradiation through the substrate with a burst of nanosecond pulses tightly focused to a line was applied. The film removal threshold with a single pulse F_th was estimated for every material and laser fluence was kept above it in the range of 1.5-3 F_th during experiments. Diverse behavior of the films depending on the metal, the shift between pulses and laser fluence was observed. In chromium, the regular structures were developed in a quite wide range of processing parameters. In gold, three kinds of ripples were observed: transverse (similar to ripples in chromium), longitudinal and a structure of ripples oriented at 60° to each other. The combination of physical properties facilitated the regular assembly of the molten metal in chromium and to some extent in gold.     

Tungsten microcone arrays grown using nanosecond pulsed-Nd:YAG laser in a low-pressure He-gas atmosphere

Tungsten microcone arrays with a high aspect ratio are formed by the cumulative nanosecond pulsed-Nd:YAG laser irradiation of single-crystal tungsten under low pressure in an inert atmosphere. The morphology of the microcones and their density were strongly affected by the number of laser pulses. The microcones grew to a length of 20 μm with a diameter of about 1.5 μm at the tip after irradiation with more than 1200 pulses under our experimental conditions. They may have potential applications for emission cathodes in a field-emission display (FED) and in microelectronic devices. Received: 8 January 2001 / Accepted: 13 June 2001 / Published online: 2 October 2001     

Effect of a small increase in the Ni content on the properties of a laser surface clad Fe-based alloy

Two Fe-based alloys with a small variation in the Ni content, Fe-15.2Cr-5.1Ni and Fe-15.7Cr-7.1Ni (wt.%), were fabricated on a martensitic stainless steel 1Cr13 substrate by laser surface cladding (LSC) using a CO₂ laser and Ar shielding gas that was blown into a molten pool. Both LSC alloys exhibited typical rapid directional solidification structures. However, 2 wt.% Ni increase led to ∼9% increase in the weight fraction of austenite, and ∼5% increase in the area proportion of interdendritic regions, which contained the higher Cr contents. These microstructural changes caused a great reduction in the microhardness and great improvements in the resistance to electrochemical corrosion in 0.5 M H₂SO₄ solution and high temperature oxidation in air at 900 °C. The reasons for these differences are discussed in detail.     

Machining of transparent materials using an IR and UV nanosecond pulsed laser

Channels are traditionally machined in materials by drilling from the front side into the bulk. The processing rate can be increased by two orders of magnitude for transparent materials by growing the channel from the rear side. The process is demonstrated using nanosecond laser pulses to drill millimeter-sized channels through thick silica windows. Absorbing defects are introduced onto the rear surface to initiate the coupling of energy into the material. Laser drilling then takes place when the fluence exceeds a threshold. The drilling rate increases linearly with fluence above this threshold. While UV light drills about four times faster than IR light, the pulse length (in the nanosecond regime) and the pulse repetition rate (in the 0.1–10 Hz range) do not greatly influence the drilling rate per pulse. Drilling rates in excess of 100 μm per pulse are achieved by taking advantage of the propagation characteristics of the plasma created at the drilling front. The plasma during rear-side drilling generates a laser-supported detonation wave into the bulk material. The geometry also seems to increase the efficiency of the laser-induced plasma combustion and shock wave during the pulse by confining it in front of the channel tip. Received: 1 July 1999 / Accepted: 17 April 2000 / Published online: 20 September 2000     

The mechanism of nanostructuring upon nanosecond laser irradiation of a STM tip

Received: 20 April 1998/Accepted: 2 June 1998     

Femtosecond laser ablation of carbon reinforced polymers

Interaction of intense ultrashort laser pulses (120 fs at 795 nm) with polymer based composites has been investigated. We have found that carbon filled polymers exhibit different ultrafast ablation behaviour depending on whether the filling material is carbon black or carbon fiber and on the polymer matrix itself. The shape and dimensions of the filling material are responsible for some geometrical bad quality effects in the entrance and inner surfaces of drilled microholes. We give an explanation for these non-quality effects in terms of fundamentals of ultrafast ablation process, specifically threshold laser fluences and material removal paths. Since carbon fiber reinforced polymers seemed particularly concerned, this could prevent the use of ultrafast ablation for microprocessing purposes of some of these materials.     

Excimer laser polymer ablation: twenty years on

Research and development in excimer laser polymer ablation has been actively pursued for some twenty years, driven by interest in the basic science as well as by numerous applications that have emerged for this high-resolution technique for material removal. This paper reviews some of the basic mechanistic aspects of the UV laser–polymer interaction as a prelude to dealing with practical matters related to polymer processing by ablation. Applications in micro-machining and potential areas for future research are briefly covered. Received: 7 October 2002 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +44-1482/465-606, E-mail: p.e.dyer@hull.ac.uk     

Non-thermal ablation of neural tissue with femtosecond laser pulses

2 ; for 100 fs pulses from the same laser the experimental threshold was at 1.5 J/cm². Histopathological examinations and scanning electron micrographs confirm the high quality of the excisions. No sign of significant thermal damage was observed. Received: 29 January 1997/Revised version: 14 July 1997     

Drilling enhancement by nanosecond-nanosecond collinear dual-pulse laser ablation of titanium in vacuum

Laser ablation of titanium in vacuum was performed using single- and dual-pulse regime in order to study crater formation. Crater profiles were analyzed by optical microscopy. It was found that the repetition-rate plays an important role in a process of laser ablation. The drilling is most effective for the highest repetition-rate. For the same total number of laser pulses clear drilling enhancement was achieved by dual-pulse regime of ablation in comparison to single-pulse regime. The strongest ablation rate in dual-pulse regime was achieved for the delay time between the pulses τ = 370 ns. Results are discussed in terms of decreased ablation threshold due to continuous heating of the target during the experiment.     

Short-pulse UV laser ablation of solid and liquid metals: indium

2 to 2.5 mJ/cm² when a 0.5 ps pulse is used instead of a 15 ns laser pulse. Measurements on liquid indium show a different behavior. With 15 ns laser pulses the threshold fluence is lowered by a factor of ∼3 from 100 mJ/cm² for solid indium to 30 mJ/cm² for liquid indium. In contrast, measurements with 0.5 ps laser pulses do not show any change in the ablation threshold and are independent of the phase of the metal at 2.5 mJ/cm². This behavior could be explained by thermal diffusion and heat conduction during the laser pulse and demonstrates in an independent way the energy lost into the material when long laser pulses are applied. Time-of-flight measurements to investigate the underlying ablation mechanism show thermal behavior of the ablated indium atoms for both ps and ns ablation and can be fitted to Maxwell-Boltzmann distributions. Received: 2 December 1996/Accepted: 11 December 1996     

Photochemical effects in the UV laser ablation of polymers: Implications for laser restoration of painted artworks

0 ∝t^1/2. The best results are expected for a circular top-hat beam shape. Received: 15 January 1999 / Accepted: 18 January 1999     

KrF excimer laser ablation of thin Cr film on glass substrate

Received: 5 October 1998 / Accepted: 14 December 1998 / Published online: 24 February 1999     

Ripple formation in the chromium thin film during laser ablation

The beam of a nanosecond pulse laser tightly focused to a line was applied for the back-side ablation of the chromium thin film on a glass substrate. The stripe ablated with a single laser pulse had sharp edges on both sides and ridges of the melted metal around it. The partially overlapping pulses formed a wide cleaned area with a complicated structure made of the metal remaining from the ridges. Regular structures, ripples, were developed when laser fluence was slightly above the single-pulse removal threshold and the shift between pulses was less than half width of the line ablated with a single laser pulse. The ripples were located periodically (∼4 μm) and were orientated perpendicularly to the long axis of the beam spot. Their orientation did not depend on the laser beam polarization. Different models of the ripple formation in the thin metal film were considered, and instability of the moving vapor-liquid-solid contact line during evaporation of thin liquid films appears to be the most probable process responsible for the observed phenomena. Formation of regular gratings with the unlimited line length was experimentally implemented by using the above-mentioned technique.     

Modelling of the solid–liquid interface during laser processing using an inverse methodology

This study presents a methodology for estimating the melt depth during laser processing of solid materials. The determination of the melt depth is treated as an inverse heat conduction problem, which includes the solid and liquid phases. The conjugate gradient method is applied to treat the inverse problem using the available temperature measurements. Without the inverse methodology the melt depth is very difficult to obtain with precision. The proposed method can also be applied during microthermal machining to determine the location of the solid–liquid interface and the temperature distributions of the two phases by using scanning thermal microscopy.     

Low-threshold field electron emission of Si micro-tip arrays produced by laser ablation

Low-threshold field electron emission (FEE) is reported for periodic arrays of micro-tips produced by laser ablation of Si wafers. The best samples show emission at threshold fields as low as 4–5 V/μm for n-type Si substrates and of 1–2 V/μm for p-doped Si substrates, as measured with a flat-screen technique. Auger electron spectroscopy and X-ray electron spectroscopy reveal island-like deviation of the SiO₂ stoichiometry on the tip surfaces, with lateral dimensions of less than 100 nm. Microscopic studies using a special field-emission STM show that the emission originates from well-conducting regions of sub-micron size. The experimental data suggest FEE from the tip arrays by a geometric field enhancement of both the individual micro-tip and the narrow conducting channels in the tip body. Received: 3 May 2002 / Accepted: 1 July 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/135-82-34, E-mail: shafeev@kapella.gpi.ru