Short-pulse-laser-induced damage and ablation of thin films of amorphous, diamond-like carbon have been investigated. Material removal and damage are caused by fracture of the film and ejection of large fragments. The fragments exhibit a delayed, intense and broadband emission of microsecond duration. Both fracture and emission are attributed to the laser-initiated relaxation of the high internal stresses of the pulse laser deposition-grown films. Ablation of solids by ultrafast lasers attracts

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... increasing interest, in particular with respect to potential technological applications for high precision material processing. 1 In a number of experiments advantages of ultrashort laser pulses have been demonstrated. However, the understanding of the fundamental physical processes leading to material removal is still incomplete. For the case of linearly absorbing semiconductors and metals we have demonstrated 1 that nearthreshold ablation with single ultrashort laser pulses exhibits a material-independent, universal behavior. Removal of material is brought about by hydrodynamic expansion of the laser-generated hot, pressurized matter followed by its decomposition into a two-phase, liquid-gas mixture. 2, 3 However, for thin films laser-induced ablation can be distinctively different as compared to bulk materials. Often the primary mechanism of material removal is not the transformation of the irradiated solid material to a volatile phase, but for example spallation or adhesion failure to the substrate caused by high tensile stresses after fast laser heating of the film. 4,5 These effects have important technological ramifications since they influence the optical damage resistance of coatings 6 but may also open new possibilities for the controlled structuring or removal of thin films without damaging the substrate. 7, 8 In this letter we report on the distinct ablation behavior of thin films of amorphous, diamond-like carbon after irradiation with single femtosecond laser pulses. We find that the lowest threshold damage mechanism corresponds to fracture of the film followed by ejection of large ͑Ͼ10 m͒ fragments. These fragments exhibit a strong and broadband fracto-emission 9 of microsecond duration, which is attributed to the relaxation of high internal stresses characteristic for the as-deposited films. Amorphous diamond-like carbon films of 60 nm thickness have been grown by pulsed laser deposition ͑PLD͒. Carbon was ablated from a pyrolytic graphite target at fluences of about 25-50 J / cm 2 with a 10 Hz KrF-excimer laser ͑17 ns, 248 nm͒ and deposited onto single crystalline silicon and fused silica substrates at room temperature. 10 Detailed characterization of these films has been carried out using a variety of techniques indicating a high degree of fourfold coordination and a density of about 90% of that of diamond. These films were irradiated in ambient atmosphere with 120 fs laser pulses at 620 nm ͑p-polarized, angle of incidence 45°͒ delivered by a 10 Hz amplified colliding-pulse mode-locked ͑rhodamine 6G/ DODCI͒ dye laser. 11 To follow the evolution of the surface morphology we employed optical microscopy, either by using a second probe laser pulse as illumination, or in a configuration where we imaged the irradiated surface area using light emission originating from the ablating material ͑see below͒. For the imaging a highresolution long-distance microscope objective was used. The optical micrographs were recorded with the help of a charge coupled device ͑CCD͒ camera in conjunction with a computer controlled video digitizer. Typical experimental results about the final surface morphology are shown in the micrographs in the left panel of Fig. 1 . The visual impression of the damage morphology indicates ͑mechanical͒ fracture as the damage mechanism. In the vicinity of the well-defined damage threshold of 0.25 J / cm 2 the material seems only slightly elevated from the substrate and forms a periodic surface structure. For higher fluences the film is completely removed and large fragments, tens of micrometers in size, are ejected and deposited in and around the ablation region. Most surprisingly we found that these fragments are the source of a strong light emission, as can be seen in the right panel of Fig. 1 . The emission was easily visible to the bare eye and appeared bright white like a spark. Preliminary measurements with different color filters show that the emitted light covers a broad spectral range ͑from the blue to the near infrared͒. To investigate the temporal evolution of the emission we have carried out explicit time-dependent, but spatially integrating measurements using in the imaging setup a photodiode in conjunction with an oscilloscope as a detector instead a͒