Silicon clusters produced by femtosecond laser ablation: non-thermal emission and gas-phase condensation

Abstract

Neutral silicon clusters Sin (up to n = 7) and their cations Si + n (up to n = 10) have been produced by femtosecond laser ablation of bulk silicon in vacuum and investigated using time-offlight mass spectrometry. Two populations of the Si + n clusters with different velocity and abundance distributions in the ablation plume have been clearly distinguished. Possible mechanisms of cluster formation (Coulomb explosion, gas-phase condensation, phase explosion) are discussed.PACS: 52.38. MF; 61.46.+w; 79.20.Ds Femtosecond laser ablation is a rapidly developing technique offering new possibilities in various applications. The fundamental mechanisms that lead to material removal are, however, still poorly understood. The interaction of fs laser pulses with silicon is an example of a complex interplay of thermal and ultrafast, nonthermal processes involved to ablation [1,2,3]. Studies of composition and expansion dynamics of the laser-induced plume can provide a considerable insight into the ablation mechanisms. Several recent experiments on this subject [2,4,5,6] were mainly devoted to the atomic component of the plume though an observation of small silicon clusters was also mentioned [2]. Little is known about silicon cluster formation under ablation with short laser pulses. With ns pulses, low-fluence desorption of Si dimers [7] and small neutral Si n clusters [8] were observed. Desorption of small clusters from both crystalline and nanostructured Si surfaces was induced by using high-energy (6.4 eV) photons [9]. Multiple-charged cluster ions were formed in ps-laser stimulated field evaporation [10]. Recently we reported the first results on the observation of neutral and cationic clusters under femtosecond laser ablation of silicon [11]. In this work, we analyze mechanisms of clusters formation based on measurements of the abundance and velocity distributions. I. EXPERIMENTThe experiments were performed with Si[100] surface under ultrahigh vacuum conditions (∼ 10 −10 mbar). The Si target was irradiated at a 45˚incidence angle using a Ti:sapphire laser (Mai-Tai coupled with a TSA amplifier, Spectra Physics, 80 fs pulse duration, 10 Hz rep- * Fax: +7 3832 343480, Tel.: +7 3832 391045; Electronic address: bulgakov@itp.nsc.ru † Electronic address: ozerov@crmcn.univ-mrs.fr ‡ Electronic address: marine@crmcn.univ-mrs.fr etition rate, up to 30 mJ energy per pulse) operating at 800 nm. A part of the laser beam was selected by an aperture to provide a nearly uniform intensity distribution over the irradiated spot. The target was rotated/translated during measurements to avoid considerable cratering. Some experiments were performed with the fixed target in order to investigate the effect of accumulation of laser pulses at the same spot. The fluence on the target was varied in the range 80-800 mJ/cm 2 . The abundance distributions of neutral and positively charged particles in the laser-ablation plume were studied using a reflectron time-of-flight mass spectrometer. The neutral particles were analyzed using electron impact...

Imaging optical near-fields of nanostructures

“…For a more quantitative analysis of the ablation patterns, one will have to take the different coupling of normal and tangential field components into account. An improved understanding of the ablation processes induced by femtosecond pulses, which is still incomplete, 14 will also be required. On the other hand, the very local material removal taking place in the near-field of nanostructures might help to shed some light on the ablation mechanisms themselves.…”

Insight into electronic mechanisms of nanosecond-laser ablation of silicon

“…There are still debates about ultrafast melting, 1-5 resolidification dynamics, [6][7][8] surface structure modification, 9,10 thermal and nonthermal mechanisms of ablation, 6,[11][12][13][14][15][16][17][18] and direct cluster emission. 16,[19][20][21] This stimulates extensive studies, both experimental and theoretical, of the dynamics of laser heating, melting, resolidification, and ablation of silicon under laser irradiation with different pulse durations and laser wavelengths. It has been shown that the silicon ablation mechanisms can be controlled by tailored laser pulses via governing the thermodynamic pathways of material evolution.…”

Secondary ion emission from Si bombarded with large Ar cluster ions under UHV conditions

“…In such cases, only the thermal energy translated from the friction force generated between surface atoms and surface neighboring atoms is delivered to the surface atoms. On the other hand, Si clusters are well known to evaporate by thermal processes such as laser irradiation [12,13] and simple annealing [14]. In these cases, the probability for emission of Si clusters is found to be extremely high, and the enhancement of the secondary cluster ion yields under the incidence of large cluster ions may be due to increased Si cluster emission probabilities through thermal processes.…”