Enhanced drilling using a dual-pulse Nd:YAG laser

A new method for improving the efficiency of laser drilling has been developed. Two synchronized free-running laser pulses from a tandem-head Nd:YAG laser are capable of drilling through 1/8-in-thick stainless-steel targets at a stand-off distance of 1 m without gas-assist. The combination of a high-energy laser pulse for melting with a properly tailored high-intensity laser pulse for liquid expulsion results in the efficient drilling of metal targets. We argue that the improvement in drilling is due to the recoil pressure generated by rapid evaporation of the molten material by the second laser pulse. Received: 29 August 2000 / Accepted: 18 December 2000 / Published online: 3 April 2001     

Ultrafast solid–liquid–vapor phase change of a gold film induced by pico- to femtosecond lasers

Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature, deeper melting and ablation depths.     

A Kinetic Theory approach for laser pulse heating process

Pulsed lasers are being widely used in industry for their high precision and low cost in material processing. In order to understand the laser machining mechanism, a thermal analysis of the process is of utmost importance. In the present study, heat transfer mechanisms relevant to pulsed laser heating is solved numerically using the Kinetic Theory approach for different types of laser pulses. A comparison between the temperature profiles developed due to different pulses is also carried out. In the heat transfer model, conduction and convection due to melting and evaporation are considered. The laser pulse profiles introduced in the analysis are selected as appropriate to laser machining processes. Stainless steel is selected as the heat transfer medium.     

Phase explosion induced by high-repetition rate pulsed laser

The characteristics and mechanisms of the damage to absorbing glass with high-repetition laser pulses (several kHz) are discussed. The results show that: (1) in the range of comparatively low-repetition rate, the damage is characterized by material melting and a small crater on the surface of substrate; (2) with the increase in repetition rate, a bigger and deeper crater is surrounded by re-deposition and crystalline granules originating from the cooling of vapor; and (3) the crater, surrounded by evaporation and an large number of solid particulates which is obviously the characters of phase explosion, becomes even bigger and deeper when the repetition rate is further increased. We modeled the temperature distribution in different repetition rate regime and found that heat accumulation plays a significant role in damage process. Because of the temperature dependence of damage mechanism, the temperature of the area irradiated by laser beam will ramp up with increasing the repetition rate, which triggers the melting and evaporation of dielectrics and phase explosion successively. Our results may benefit the understanding of laser-induced damage in optical materials.     

Self-organization in a chromium thin film under laser irradiation

Self-organization of chromium on glass was observed during laser ablation of the metal film with partially overlapping laser pulses. The beam of a nanosecond pulse laser tightly focused to a line was applied to the back-side ablation of the chromium thin film on a glass substrate. While the line ablated with a single laser pulse had sharp edges on both sides with ridges of the melted metal, the use of partially overlapping pulses formed a complicated structure made of the metal remaining from the ridges. Regular structures of ripples were developed in a certain range of laser fluence and pulse overlap. The ripple period could be controlled from 2.5 to 4 μm by variation of the processing parameters. Various experimental techniques were applied to test the structures, and different models of the ripple formation in the thin metal film were considered. The initial quasi-periodical formation started because of dewetting of thin liquid metal films on the glass substrate after its melting. Similar to the evaporation of liquid films, the small perturbation in the ridge thickness was able to induce instability in evaporation of the thin melted metal film. Freezing of the nonequilibrium state between laser pulses was one of the stabilizing factors in self-organization of the metal.     

Photoacoustic effect upon material melting and evaporation by laser pulses

Photoacoustic signals in laser-irradiated samples were mathematically modeled for the cases when melting and evaporation occur. In particular, it was shown that rapid melting processes induced by nanosecond laser pulses result in a rather narrow pressure peak or dip in photoacoustic pressure signals due to density changes in the moving melting front. Amplitudes of these peaks or dips depend on the melting front velocity, as well as on the magnitude of density change. Experimental detection of this effect using a piezoelectric transducer requires a uniform laser intensity distribution over the irradiation spot.     

Laser induced melting and superheating in Te and in films for optical data storage

Thin solid tellurium and indium films on a substrate used for optical data storage show superheating during exposure to laser pulses shorter than 10 μs. The times for melting of both Te and In indicate that the speed of the melting front is limited by the availability of vacancies in the solid film material.     

Design and application of a femtosecond optical parametric oscillator for time-resolved spectroscopy of semiconductor heterostructures

Femtosecond pulses of a collinearly pumped Optical Parametric Oscillator (OPO) are applied for investigations of the carrier dynamics in ternary and quaternary semiconductor quantum wells. The design and the specifications of the OPO are given in detail. We show that no measurable jitter exists between the pump pulses and the output pulses of the OPO. Therefore, it is possible to use the OPO and its pump laser for two-color experiments with a time resolution limited by the pulse lengths. We present and discuss results of transient four-wave mixing experiments on (InGa) As/InP quantum wells, and find a new kind of polarization-dependent quantum beat phenomenon. In addition, non-degenerate experiments on quantum wells from the quaternary (InGaAl) As material system, using two pulses at different wavelengths (one from the OPO and one from the pump laser), are discussed as a novel experimental technique to study carrier trapping into quantum wells.     

3D atom probe assisted by femtosecond laser pulses

The 3DAP allows to image a material in 3D on a nearly atomic scale. It is based on the field evaporation occurring at the surface of a biased tip like shape specimen with an end radius of 50 nm. Surface atoms are removed one by one from the tip by means of fs laser pulses so that the physical process involved in this laser enhanced field evaporation might correspond to the very early stages of the ablation process. This technique makes possible to distinguish between different regimes of material removal such as thermal evaporation or in the case of metals or semiconductors an evaporation assisted by the rectification of the optical field at the surface. In this paper the principle of the 3DAP is presented and the underlying physics involved in the field evaporation assisted by femtosecond laser pulses is discussed.     

脉冲强激光破坏Hg0.8Cd0.2Te晶片材料的机理分析

 利用光学金相显微镜对TEA-CO₂脉冲强激光辐照的Hg_0.8Cd_0.2Te晶片表面进行了观察。在单脉冲能量为37.5 J，能量密度为937.5 J/cm²的强激光辐照下，晶片表面呈现出熔融迹象和大量的微裂纹，微裂纹密度从激光辐照区中心向外逐渐减少，裂纹沿晶体的(111)面扩展。随着脉冲连续作用次数的增加，晶片表面熔融更加剧烈，裂纹数目、裂纹深度和宽度都有所增加。分析认为：HgCdTe晶片的破坏与激光辐照能量、脉冲连续作用次数、激光场强分布、激光热应力、激光支持的燃烧波和物质的蒸发波等冲击波有关。     

Influence of adsorbates on UV-pulsed laser melting of Si in ultra-high vacuum

The dynamics of laser melting of atomically clean Si is investigated in ultra-high-vacuum (UHV) by transient reflectivity with single-pulse sensitivity in the presence of monitored amounts of chlorine, oxygen or propene. Adsorption of one monolayer (1 ML) leads to measurable variations of the melting dynamics, which are strongly adsorbate-dependent. The variations differ qualitatively and quantitatively from those observed with heavy exposures to gases. The melting dynamics returns to that of clean Si upon subsequent irradiation by laser pulses without readsorption. The required number of pulses for return to clean Si dynamics depends strongly on the type of adsorbate. Adsorbate-induced changes of absorption and reflectivity, and/or incorporation of adsorbates into the substrate, do not explain the results. By contrast, the variations of the melting dynamics are correlated to the photoemitted electron yield, suggesting that laser melting is sensitive to the presence of electrons in the conduction band. These results show that accurate modelling of laser melting of Si interacting with gases should take into account the presence of the gases. Received: 12 September 2000 / Accepted: 9 January 2001 / Published online: 27 June 2001     

Fast simulation code for heating, phase changes and dopant diffusion in silicon laser processing using the alternating direction explicit (ADE) method

Enhancements to our existing finite-differences code for the simulation of laser heating, melting and evaporation of silicon are presented. The Knudsen evaporation model has been added to the previously used enthalpy-based model in order to simulate laser pulses with pulse lengths down to a few nanoseconds. Fick’s diffusion law has also been incorporated allowing laser doping by dopant diffusion in silicon melt to be described. Finally, the basic equations for the alternating direction explicit method (ADE) have been adapted to consider nonlinear temperature-enthalpy relations, thus including affects due to phase changes. This improved the simulation speed by up to factor of 100 compared to standard explicit and implicit time integration methods. Details of the ADE algorithm and numerical stability issues are presented in this paper. Validation of the code is presented by comparing to different integration methods and to experimental results. The final code successfully simulates melting, evaporation and dopant diffusion by multiple laser pulses in three dimensions in an acceptable computing time.     

UV-laser ablation of ductile and brittle metal films

Single-shot laser damage of Ni and Cr films on fused silica substrates has been studied as a function of film thickness, utilizing 248 nm/14 ns pulses and detection by probe beam deflection. Threshold fluences for visible damage and vaporization are compared to predictions of the heat diffusion model. The model fits thresholds for visible damage well and identifies their origin, which is melting for Ni films and brittle-to-ductile phase transition for Cr films. When predicting thresholds for vaporization, the diffusion model is of limited success in case of Ni films but fails completely for Cr films, indicating that transient thermal properties of the material should be taken into account. Microscopic inspection shows that Cr films rupture at low fluences before entering the common sequence of melting and vaporizing with increasing fluence. 17 December 1996/Accepted: 17 December 1996     

Elimination of cracking during UV laser ablation of SrTiO3 single crystals by employing a femtosecond laser

We have performed a comparative study of UV laser ablation of SrTiO₃ with nanosecond- and sub-picosecond sources, respectively. The experiments were performed with lasers at a wavelength of 248 nm and pulse durations of 34 ns and 500 fs. Femtosecond ablation turns out to be more efficient by one order of magnitude and eliminated the known problem of cracking of SrTiO₃ during laser machining with longer pulses. In addition, the cavities ablated with femtosecond pulses display a smoother surface with no indication of melting and well-defined, sharp edges. These effects can be explained by the reduced thermal shock effect on the material by using ultrashort pulses.     

Simulation of laser ablation in aluminum: the effectivity of double pulses

Lasers are becoming a more and more important tool in cutting and shaping materials. Improving precision and effectivity is an ongoing demand in science and industry. One possibility is double pulses. Here, we study laser ablation of aluminum by the two-temperature model. There the laser is modeled as a source in a continuum heat conduction equation for the electrons, whose temperature then is transferred to a molecular dynamics particle model by an electron–phonon coupling term. The melting and ablation effectivity is investigated depending on the relative intensity and the time delay between two Gaussian shaped laser pulses. It turns out that at least for aluminum the optimal pulse shapes are standard Gaussian pulses. For double pulses with delay times up to 200 ps, we find a behavior as observed in experiment: The ablation depth decreases beyond a delay of 10 ps even if one does not account for the weakening at the second pulse due to laser–plasma interaction.     

Effect of melting on the excitation of surface acoustic wave pulses by UV nanosecond laser pulses in silicon

The influence of melting on the excitation of Surface Acoustic Wave (SAW) pulses in silicon is studied both theoretically and experimentally. The developed theory of Rayleigh-type SAW laser-induced thermoelastic excitation in a structure composed of a liquid layer on a solid substrate predicts that the SAW is predominantly generated in the solid phase due to the absence of shear rigidity in a liquid. The characteristic changes in the SAW pulse shape as well as the saturation and even the decrease of the SAW pulse amplitude observed above the melting threshold are explained theoretically to be a result of the decrease of the heat flux into the solid phase as well as due to the decrease of the volume of the solid phase caused by melting. Although the heat flux into the solid phase is decreased both as a consequence of the reflectivity increase and the additional energy losses (latent heat of melting) at the phase transition, it is demonstrated that the influence of reflectivity changes on the SAW pulse is negligible in comparison with the effect of melt-front motion. For laser pulses of 7 ns duration at 355 nm, the threshold value of laser fluence for meltingF _m=0.23±0.04 J/cm² and for the ablationF _a=1.3±0.2 J/cm² were determined experimentally as the points of characteristic changes in the observed SAW pulses.     

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     

Study of temperature dependence and angular distribution of poly(9,9-dioctylfluorene) polymer films deposited by matrix-assisted pulsed laser evaporation (MAPLE)

Poly(9,9-dioctylfluorene) (PFO) polymer films were deposited by matrix-assisted pulsed laser evaporation (MAPLE) technique. The polymer was diluted (0.5 wt%) in tetrahydrofuran and, once cooled to liquid nitrogen temperature, it was irradiated with a KrF excimer laser. 10,000 laser pulses were used to deposit PFO films on 〈1 0 0〉 Si substrates at different temperatures (−16, 30, 50 and 70 °C). One PFO film was deposited with 16,000 laser pulses at a substrate temperature of 50 °C. The morphology, optical and structural properties of the films were investigated by SEM, AFM, PL and FTIR spectroscopy. SEM inspection showed different characteristic features on the film surface, like deflated balloons, droplets and entangled polymer filaments. The roughness of the films was, at least partially, controlled by substrate heating, which however had the effect to reduce the deposition rate. The increase of the laser pulse number modified the target composition and increased the surface roughness. The angular distribution of the material ejected from the target confirmed the forward ejection of the target material. PFO films presented negligible modification of the chemical structure respect to the bulk material.     

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