Comment on Coulomb explosion in femtosecond laser ablation of Si(111)

• In a recent letter Roeterdink 1 report on the occurrence of an electrostatic form of material removal from solid silicon samples irradiated with high-intensity ultrashort laser pulses. The arguments are essentially derived from time-of-flight observations of the emitted single- and double-ionized silicon atoms and considerations related to momentum conservation during the excitation and expansion phase. A linear scaling of the velocity of species with different charge states is considered as an argument for Coulomb explosion being responsible for ion emission from the irradiated silicon sample in a high fluence regime. A similar effect derived directly from momentum conservation upon particle ejection has been determined before as a proof for the occurrence of Coulomb explosion from laser irradiated dielectrics,2,3 but a similar process for semiconductors was not observed at intensities just above the ablation threshold.3 The absence of Coulomb explosion from semiconductors within the range of investigated fluences (up to 1 J/cm2) was related to efficient electronic transport able to counterbalance the laser-induced charge deficiency. The "discrepancy" discussed in the letter, involving a threshold criterion for the surface Coulombic explosion, is based on a misinterpretation of the data presented by Stoian 3 Usually the mechanism of Coulomb explosion is explained as being driven by positively charged superficial layers that will electrostatically repel each other, assisted in some cases by the pulling force exercised by the photo-emitted charge cloud close to the ionized surface. Recent calculations4 regarding the mechanisms for surface electrostatic disruption have shown that Coulomb explosion may occur even without the additional electron pull. Provided that substantial photoelectron emission occurs and carrier mobility is intrinsically low or lowered during the excitation, the electron transport from the excited bulk region cannot compensate the photoelectric flow, and, therefore, the surface neutrality will be broken. A significant uncompensated remnant positive charge will be localized within the first surface layers and the ions within this region will be mutually repelled. The authors have overlooked, when referring to the results of Stoian ,3 the fact that in subsurface regions where photoemission is less effective one can also obtain a high density of carriers. The discussion and arguments in Ref. 3 refer to the net charge (the absolute difference between the ion and electron density) in the surface layers, with no a priori restrictions for the absolute carrier density to reach supercritical values at the irradiation wavelength. According to the study of Roeterdink , the high carrier density induced by the ultrafast laser excitation, in excess of 1022 cm–3 will destabilize the lattice, leading to the surface electrostatic disruption. A fractional charge of more than 0.3 excited electrons per silicon atom is calculated to generate electric fields matching the observed momentum transfer, but the local neutrality of the sample appears not to be disturbed. Lattice destabilization at high carrier density is well documented in the literature5 but it does not involve any neutrality breakdown that may cause electrostatic material ejection. It is thus unclear how high carrier densities alone, even close to the solid density, may lead to high electrostatic fields and Coulomb explosion of the region, without any deviation from the electric neutrality. One may invoke either strong photoelectron emission or charge separation caused by nonequilibrium transport, but this is not clearly discussed in the letter. If the authors imply that high excitation simultaneously means high photoelectron yields, a net, uncompensated charge of more than 0.3 missing electrons per atom can be reached in the present case only if one assumes that all the excited electrons have been removed from the superficial layers, without any other forms of electronic supply during the emission time. If one considers that the excitation depth in silicon is several hundreds of nanometers and the electrons can be removed with a certain probability only from a narrow region beneath the surface, the uncompensated region will be rapidly neutralized by bulk electronic transport, keeping the net charge below the critical value for Coulomb explosion. Though the possibility to efficiently charge the Si surface layers beyond the threshold for bond-breaking and macroscopic rupture of the surface may conceivably appear under extreme irradiation conditions involving high intensities where high carrier densities are generated on the leading edge of the pulse and electronic transport is strongly perturbed in the destablized lattice, the experimental results are usually embedded in a series of additional effects and artifacts in the laser-generated plume that may obscure a clear interpretation of the ejection mechanisms. Moreover, one has to carefully consider the changes that occur in the irradiation geometry once high fluences and high irradiation doses are used. Here, one of the consequences is indicated by the authors in the appearance of a "second ejection channel." We suggest a possible alternative explanation for the experimental facts observed by the authors. Under the experimental conditions reported, i.e., fluence in the range of 3–10 J/cm2, several times higher than the ablation threshold, the material removal rates are considerable. An immediate effect, accentuated by the production of a crater at the surface (as acknowledged by the authors) after several laser pulses, is a highly collisional, dense plume, initially confined in the crater before expanding. Consequently charge separation may occur in the gas phase,6 leading to a similar experimental signature, namely a linear scale of the velocity of different ionized species with their intrinsic charge. More explicitly, the appearance of the so-called double layer will accelerate multiple charges and the same momentum regulations occur.6

Ämnesord

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

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laser ablation

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