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This work was conducted experimentally to investigate the material removal rate and its mechanisms during the single-pulse and double-pulse nanosecond laser ablation of a silicon wafer in distilled water. The laser ablation processes were performed under the same experimental conditions with the same total pulse energy (E single pulse = E double pulse ). The amount of ablated material was estimated for all of the processes based on measuring the dimensions (depths and widths) and volumes of the laser-induced craters on the silicon wafer.The results indicate that double-pulsed laser processing can result in a higher material removal rate compared to the more common single-pulse process, when the inter-pulse delay time is less than the pulse duration. The higher ablation yield in the double-pulse process can be due to the higher coupling efficiency of the second laser pulse with the melted target induced by the laser pre-pulse, leading to the more efficient laser energy absorption and deposition within the irradiated region. The double-pulse nanosecond laser processing with delay time of ~5 ns not only results in a higher material removal rate, but also leads to preparation of silicon nanoparticles with a greater mean particle size compared to that of the more common singlepulse laser ablation process.

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In this paper we introduce a comparative approach for studying the emission properties of silicon nanocrystal (Si-nc) colloids prepared by single pulse and double pulse laser ablation processes of a silicon wafer in distilled water. Experiments were conducted to investigate the luminescence properties of the colloids considering the size distributions and surface characteristics of the synthesized Si-ncs. The results indicated that single pulse and double pulse laser ablation processes under similar experimental conditions can lead to the preparation of Si-nc colloids with almost the same size distributions and different surface chemistry. The results show that double pulse laser processing with an inter-pulse delay time of ~5 ns can produce Si-nc colloids with a much greater emission intensity (about fivefold to tenfold) in the orange-red region (550-700 nm) of the visible spectrum. Based on the detailed analysis of the Si-nc size distribution and surface characteristics, the observed prominent orange-red emission is possibly due to a different type of Si-OH surface termination that protects the nanocrystal core upon inward oxidation.