Impact of the lasr wavelength and fluence on the ablation rate of aluminium

The dependence of the ablation rate of aluminium on the fluence of nanosecond laser pulses with wavelengths of 532 nm and respectively 1064 nm is investigated in atmospheric air. The fluence of the pulses is varied by changing the diameter of the irradiated area at the target surface, and the wavelength is varied by using the fundamental and the second harmonic of a Q-switched Nd-YAG laser system. The results indicate an approximately logarithmic increase of the ablation rate with the fluence for ablation rates smaller than ∼6 μm/pulse at 532 nm, and 0.3 μm/pulse at 1064 nm wavelength. The significantly smaller ablation rate at 1064 nm is due to the small optical absorptivity, the strong oxidation of the aluminium target, and to the strong attenuation of the pulses into the plasma plume at this wavelength. A jump of the ablation rate is observed at the fluence threshold value, which is ∼50 J/cm² for the second harmonic, and ∼15 J/cm² for the fundamental pulses. Further increasing the fluence leads to a steep increase of the ablation rate at both wavelengths, the increase of the ablation rate being approximately exponential in the case of visible pulses. The jump of the ablation rate at the threshold fluence value is due to the transition from a normal vaporization regime to a phase explosion regime, and to the change of the dimensionality of the hydrodynamics of the plasma-plume.      

Femtosecond laser ablation characteristics of nickel-based superalloy C263

Femtosecond laser (180 fs, 775 nm, 1 kHz) ablation characteristics of the nickel-based superalloy C263 are investigated. The single pulse ablation threshold is measured to be 0.26±0.03 J/cm² and the incubation parameter ξ=0.72±0.03 by also measuring the dependence of ablation threshold on the number of laser pulses. The ablation rate exhibits two logarithmic dependencies on fluence corresponding to ablation determined by the optical penetration depth at fluences below ∼5 J/cm² (for single pulse) and by the electron thermal diffusion length above that fluence. The central surface morphology of ablated craters (dimples) with laser fluence and number of laser pulses shows the development of several kinds of periodic structures (ripples) with different periodicities as well as the formation of resolidified material and holes at the centre of the ablated crater at high fluences. The debris produced during ablation consists of crystalline C263 oxidized nanoparticles with diameters of ∼2–20 nm (for F=9.6 J/cm²). The mechanisms involved in femtosecond laser microprocessing of the superalloy C263 as well as in the synthesis of C263 nanoparticles are elucidated and discussed in terms of the properties of the material.     

Selective ablation of thin SiO<Subscript>2</Subscript> layers on silicon substrates by femto- and picosecond laser pulses

The selective ablation of thin (∼100 nm) SiO₂ layers from silicon wafers has been investigated by applying ultra-short laser pulses at a wavelength of 800 nm with pulse durations in the range from 50 to 2000 fs. We found a strong, monotonic decrease of the laser fluence needed for complete ablation of the dielectric layer with decreasing pulse duration. The threshold fluence for 100% ablation probability decreased from 750 mJ/cm² at 2 ps to 480 mJ/cm² at 50 fs. Significant corruption of the opened Si surface has been observed above ∼1200 mJ/cm², independent of pulse duration. By a detailed analysis of the experimental series the values for melting and breaking thresholds are obtained; the physical mechanisms responsible for the significant dependence on the laser pulse duration are discussed.     

A comparative study of the ionic keV X-ray line emission from plasma produced by the femtosecond,picosecond and nanosecond duration laser pulses

We report here an experimental study of the ionic keV X-ray line emission from magnesium plasma produced by laser pulses of three widely different pulse durations (FWHM) of 45 fs, 25 ps and 3 ns, at a constant laser fluence of ∼1.5 × 10⁴ J cm^− 2. It is observed that the X-ray yield of the resonance lines from the higher ionization states such as H- and He-like ions decreases on decreasing the laser pulse duration, even though the peak laser intensities of 3.5 × 10¹⁷ W cm^− 2 for the 45 fs pulses and 6.2 × 10¹⁴ W cm^− 2 for the 25 ps pulses are much higher than 5 × 10¹² W cm^− 2 for the 3 ns laser pulse. The results were explained in terms of the ionization equilibrium time for different ionization states in the heated plasma. The study can be useful to make optimum choice of the laser pulse duration to produce short pulse intense X-ray line emission from the plasma and to get the knowledge of the degree of ionization in the plasma.     

Nonlinear optical and optical limiting studies of alkoxy phthalocyanines in solutions studied at 532 nm with nanosecond pulse excitation

We report our results on the nonlinear optical and optical limiting properties of two alkoxy phthalocyanines namely 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine and 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine zinc(II) studied at a wavelength of 532 nm using 6 ns pulses. Using the standard Z-scan technique we observed that both the phthalocyanines exhibited negative nonlinearity as revealed by the signature of closed aperture data. The magnitude of the nonlinear refractive index n₂ evaluated from the closed aperture data was ∼ 1.61×10^-11 cm²/W for the free-base phthalocyanine and ∼ 1.56×10^-11 cm²/W for the metallic phthalocyanine. Open aperture Z-scan data indicates strong nonlinear absorption in both the phthalocyanines with measured nonlinear coefficients of ∼ 1650 cm/GW and ∼ 1850 cm/GW respectively. We also report optical limiting properties of these phthalocyanines with limiting thresholds (I_1/2) of ∼ 0.5 J/cm². Our studies suggest that these phthalocyanines are one of the best molecules for nonlinear optical applications studied recently. PACS 42.65.-k; 42.70.Jk, 42.65.Jx     

Mechanism of backside etching of transparent materials with nanosecond UV-lasers

In consequence of high interest in micro- and nanomachining of transparent materials by laser irradiation, studies on the mechanism of laser-induced backside wet etching (LIBWE) are presented. To reveal the role of the surface modification due to LIBWE the backside ablation (BSA) of LIBWE-modified fused silica (mFS) surfaces at 248 nm was investigated. The threshold fluence and the etch rate of BSA are similar to that of LIBWE and amount ∼250 mJ/cm² and 30 nm for 1 J/cm², respectively. The sample transmission after backside ablation of mFS increases and proves the decreasing thickness of the absorbing layer. Time-resolved reflection studies at LIBWE and BSA of mFS show similar patterns in the backside reflection that can be assigned an ablation process as the comparison to thin polymer films demonstrates. By fitting the BSA data to an exponential decay absorption model a modification depth and a surface absorption of about 38 nm and α ^S ∼1.3×10⁷ m⁻¹ were calculated, respectively. In conclusion of the results a new model for LIBWE is proposed.     

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     

MAPLE deposition of PEG:PLGA thin films

We report on MAPLE deposition of thin films of PEG:PLGA blends. The films were analyzed in terms of morphology, chemical composition, wettability, and optical constants. These properties were particularly discussed in correlation with film thickness. The film thickness was increased by increasing the deposition rate (i.e., laser fluence). This method was effective for fluences up to 1 J/cm², above which the efficiency of the deposition leveled off. Moreover, with increasing fluence above 1 J/cm², important changes in the polymeric films were noticed: the surface roughness increased abruptly (up to ∼200 nm), the polymers lost their chemical integrity and their optical constants underwent significant changes. In addition, surface wettability decreased considerably, water contact angle reaching ∼90°; this was attributed to increased surface roughness and to orientation of the hydrophobic groups toward the surfaces of the films. An alternative method for obtaining thicker films was employed, by prolonging the deposition time while maintaining a constant, relatively low, deposition rate (i.e., fluence). In this case, the properties of the films were significantly less affected.     

Magnetoelectric effect in amorphous FeNiSiC ferromagnet-piezoelectric planar structures

The magnetoelectric (ME) effect is studied in composite two- and three-layer disk structures containing magnetostriction layers of an amorphous FeNiSiC ferromagnet and a lead zirconate titanate piezoelectric layer. Due to a high magnetostriction (∼33 × 10⁻⁶) and a low saturation field (∼200 Oe), an FeNiSiC layer has a high piezomagentic coefficient, which results in an effective ME coupling in low fields (∼25 Oe). The ME effect is ∼0.2 V cm⁻¹ Oe⁻¹ at a low frequency and increases to 11.9 and 13.2 V cm⁻¹ Oe⁻¹ when bending and in-plane mechanical vibrations are excited in a resonance manner in the structures at frequencies of ∼8.2 and ∼170.0 kHz, respectively. Structures containing amorphous FeNiSiC layers are promising for magnetic field transducers and electric energy generators and converters.     

Ultrashort-pulse laser ablation of indium phosphide in air

Ablation of indium phosphide wafers in air was performed with low repetition rate ultrashort laser pulses (130 fs, 10 Hz) of 800 nm wavelength. The relationships between the dimensions of the craters and the ablation parameters were analyzed. The ablation threshold fluence depends on the number of pulses applied to the same spot. The single-pulse ablation threshold value was estimated to be φ_th(1)=0.16 J/cm². The dependence of the threshold fluence on the number of laser pulses indicates an incubation effect. Morphological and chemical changes of the ablated regions were characterized by means of scanning electron microscopy and Auger electron spectroscopy. Received: 30 May 2000 / Accepted: 31 May 2000 / Published online: 23 August 2000     

Laser-assisted generation of self-assembled microstructures on stainless steel

Utilising a Nd:YVO₄ laser (wavelength of 532 nm, pulse duration of 8 ns, repetition rate of 30 kHz) and a Nd:YAG laser (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz), it was found that during the pulsed laser ablation of metal targets, such as stainless steel, periodic nodular microstructures (microcones) with average periods ranging from ∼30 to ∼50 μm were formed. This period depends on the number of accumulated laser pulses and is independent of the laser wavelength. It was found that the formation of microcones could occur after as little as 1500 pulses/spot (a lower number than previously reported) are fired onto a target surface location at laser fluence of ∼12 J/cm², intensity of ∼1.5 GW/cm². The initial feedback mechanism required for the formation of structures is attributed to the hydrodynamic instabilities of the melt. In addition to this, it has been shown that the structures grow along the optical axis of the incoming laser radiation. We demonstrate that highly regular structures can be produced at various angles, something not satisfactorily presented on metallic surfaces previously. The affecting factors such as incident angle of the laser beam and the structures that can be formed when varying the manner in which the laser beam is scanned over the target surface have also been investigated.     

Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF _thres = 172 mJ/cm². The ablation rate per pulse is measured as a function of intensity in the range of 5 × 10⁹ to 2 × 10¹¹ W/cm² and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.     

II–VI semiconductor nanoparticles synthesized by laser ablation

Nanoparticles of the II–VI semiconductors CdTe, CdSe and ZnTe were synthesized by laser ablation (387 nm, 180 fs, 1 kHz, pulse energy of 7 μJ (fluence of 2 J/cm²)) of the target materials in methanol, de-ionized water and acetone. The nanoparticles size distributions follow log-normal functions with median diameters between about 6 and 11 nm for the several materials. The nanoparticles have the same crystalline structure as that of the corresponding bulk material and under the present conditions of ablation are rich in the higher volatility element of the two in the binary alloy and oxidized. Photoluminescence emission in the green-yellow (∼570 nm) was detected from CdSe nanoparticles.     

Deep drilling on a silicon plate with a femtosecond laser: experiment and model analysis

The ablation rate when drilling fine holes having large aspect ratios in silicon substrates with femtosecond laser pulses was estimated from mechanically ground cross sections of the ablated holes. The ablation rate shows a dramatic change at the depth at which the laser pulse reaches a certain fluence, which is nearly constant when the initial laser fluence was varied from 14.5 to 59.4 J/cm². The ablation rate, threshold fluence, in three fluence domains, and the transition fluences at which the ablation rate shows a dramatic change, were derived. However, when a pulse energy of 200 μJ was used a much greater ablation rate was obtained, suggesting that another fluence domain for larger ablation rates exists. The experimentally obtained hole depths as a function of shot numbers were reproduced by a theoretical model, which incorporates laser pulse attenuation in the holes that is the same as that in waveguides for some attenuation coefficient and ablation rates for three fluence domains. PACS 42.62.-b; 42.65.Re; 78.40.Fy; 78.47.+p; 81.20.Wk     

Millimeter-long nanofibers of PMMA spun at super-high speed by ablation with a single pulse of a KrF excimer laser

A new form of matter removal in laser ablation is reported. Polymethylmethacrylate (PMMA) nanofibers are obtained when a PMMA target is irradiated with a single pulse of a KrF excimer laser, whose beam is sharply imaged on a square of side the order of 140 μm, so that a strong intensity gradient is produced. The fluence threshold at which fibers appear, 3 J/cm², is much larger than the ablation threshold, approximately 0.8–1 J/cm². Above this fluence, the melt depth is then large enough and the temperature profile is such that explosive boiling is obtained. The model suggests an expulsion of energetic droplets from the intense pressure of the plume to the exterior of the spot. For the transient melt of a polymeric viscoelastic liquid resulting from UV-laser excitation, such droplets provide the heads of the jets pulled from the melt bath, giving rise, after solidification, to nanofibers. The speed of fiber spinning is extremely high (∼800 m/s) and unusual properties of the laser-produced nanofibers may be expected. Received: 16 April 2002 / Accepted: 17 April 2002 / Published online: 19 July 2002     

Ablation of silicon suboxide thin layers

We investigate the ablation of SiO _x thin films on fused silica substrates using single-pulse exposures at 193 nm and 248 nm. Two ablation modes are considered: front side (the surface of a film is irradiated from above) and rear side (a film is irradiated through its supporting substrate). Fluence is varied from below 200 mJ/cm² to above 3 J/cm². SiO _x films of thickness 200 nm, 400 nm, and 600 nm are ablated. In the case of rear-side illumination, at moderate fluences (around 0.5 mJ/cm²) the ablation depth corresponds roughly to the film thickness, above 1 J/cm² part of the substrate is ablated as well. In the case of front-side ablation the single-pulse ablation depth is limited for all film thicknesses to less than 200 nm even at fluences up to 4 J/cm². Experimental results are discussed in relation to film thickness, fluence, and ablation mode. Simple numerical calculations are performed to clarify the influence of heat transport on the ablation process.     

Ultrafast laser ablation of indium tin oxide thin films for organic light-emitting diode application

Ultrafast laser ablation of ITO thin film coated on the glass has been investigated as a function of laser fluence as well as the number of laser pulses. The ablation threshold of ITO thin film was found to be 0.07 J/cm² that is much lower than that of glass substrate (about 1.2–1.6 J/cm²), which leads to a selective ablation of ITO film without damage on glass substrate. The changes in the electrical resistance and morphology of ablated trench of ITO electrode were found to be strongly dependent on the processing conditions. We present the performance of organic light-emitting diodes (OLED) fabricated with ITO electrode patterned by ultrafast laser ablation.     

High-vacuum time-resolved laser-induced incandescence of flame-generated soot

We have measured time-resolved laser-induced incandescence of flame-generated soot under high-vacuum conditions (4.1×10⁻⁶ mbar) at an excitation wavelength of 532 nm with laser fluences spanning 0.06–0.5 J/cm². We generated soot in an ethylene/air diffusion flame, introduced it into the vacuum system with an aerodynamic lens, heated it using a pulsed laser with a spatially homogeneous and temporally smooth laser profile, and recorded LII temporal profiles at 685 nm. At low laser fluences LII signal decay rates are slow, and LII signals persist beyond the residence time of the soot particles in the detection region. At these fluences, the temporal maximum of the LII signal increases nearly linearly with increasing laser fluence until reaching a plateau at ∼0.18 J/cm². At higher fluences, the LII signal maximum is independent of laser fluence within experimental uncertainty. At these fluences, the LII signal decays rapidly during the laser pulse. The fluence dependence of the vacuum LII signal is qualitatively similar to that observed under similar laser conditions in an atmospheric flame but requires higher fluences (by ∼0.03 J/cm²) for initiation. These data demonstrate the feasibility of recording vacuum LII temporal profiles of flame-generated soot under well-characterized conditions for model validation.     

SnO2 thin films grown by pulsed Nd:YAG laser deposition

SnO₂ thin films have been deposited on glass substrates by pulsed Nd:YAG laser at different oxygen pressures, and the effects of oxygen pressure on the physical properties of SnO₂ films have been investigated. The films were deposited at substrate temperature of 500°C in oxygen partial pressure between 5.0 and 125 mTorr. The thin films deposited between 5.0 to 50 mTorr showed evidence of diffraction peaks, but increasing the oxygen pressure up to 100 mTorr, three diffraction peaks (110), (101) and (211) were observed containing the SnO₂ tetragonal structure. The electrical resistivity was very sensitive to the oxygen pressure. At 100 mTorr the films showed electrical resistivity of 4×10⁻² Ω cm, free carrier density of 1.03×10¹⁹ cm⁻³, mobility of 10.26 cm² V⁻¹ s⁻¹ with average visible transmittance of ∼87%, and optical band gap of 3.6 eV.