Researchers optimise nanostructuring rates using ultrafast lasers

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(a) SEM pictures of LIPSs on silicon surface induced by the shaped pulse trains with an interval of 16.2 ps. (b) The cross-section of LIPSs. (c) Structural colours of a 'Chinese knot' pattern made using LIPSs. (Image: OEA)

Researchers have been studying the fundamental formation of laser-induced periodic structures (LIPSs) to better understand how they can be more efficiently produced over large surface areas.

The work, published in Opto-Electronic Science, has led to the scientists being able to form LIPs over a 1m2 area in several hours, which they say has important implications for the industrial application of femtosecond-LIPSs in the future.

LIPSs are periodic nanostructures that can be formed when irradiating different types of materials such as metals, semiconductors, dielectrics, and polymers with an ultrafast laser. The structures can alter material behaviour, for example their electrical, reflective, hydrophobic/philic and tribological properties.

Since the arrival of this promising laser materials processing application however, the rate at which LIPs can be produced has been regarded as far too low for practical implementation in industry. Therefore, in recent years, efforts have been made to increase the average power of ultrafast lasers, whose higher-energy pulses can then be split over a larger area using technologies such as spatial light modulators and microlens arrays. This enables parallel processing and thus a dramatically higher rate of LIPs formation, thus making the process more suited to industrial implementation.

In the announcement of their new paper however, the research group of Professor Jia from the State Key Laboratory of Precision Spectroscopy at East China Normal University claim there to be limitations of such devices and/or methods that will still make it difficult to achieve LIPS production over large areas, such as a square metre.

The group has therefore deployed a collinear pump-probe imaging method in order to better study and optimise the transient processes of LIPS formation.

More specifically, the group studied in detail the effects of hot electron localisation and d-band transitions on the dielectric constant at the highly excited states during femtosecond laser irradiation of gold, silver, nickel. The study revealed that surface plasmon polaritons (SPP) excitation was a key factor in the formation of LIPSs on different types of metal surfaces, and that strong thermal effects can reduce LIPS depth, and even cause them to disappear.

The group then identified three main challenges to be addressed in order to efficiently fabricate uniform, regular, and deep LIPSs: enhancement of periodic deposition of laser energy, reduction of residual heat, and avoidance of debris deposited on the ablation spots. 

To address these challenges, the researchers developed a ‘4f zero-dispersion pulse-shaping system’ that could generate pulse trains with an interval of 0.1-16.2ps using a periodic π-phase step modulation. The system was then used to produce regular and deep LIPSs on a silicon surface using a shaped pulse with an interval of 16.2ps (see lead image).

‘The transient LIPSs started appearing on the silicon surface at a delay time of 4ps after femtosecond laser irradiation, which was shorter than the interval between adjacent sub-pulses,’ the researchers explain. ‘Thus, the transient LIPSs have started to appear under the illumination of the two strongest sub-pulses. When the subsequent sub-pulse reached the sample surface, the transient LIPSs induced by the previous sub-pulses enhanced the excitation of the SPPs, as well as the periodic distribution of the laser field. When the subsequent small sub-pulses reached the surface layer, it remained at a very high temperature. It was further excited and partially ablated, taking away some of the remaining heat (ablative cooling effect). Moreover, the ablated plume was further excited by the subsequent sub-pulses, and the debris was further ionised and vaporised, resulting in fewer deposited particles.’

By using shaped pulses from their newly developed system, the researchers found that the fabrication efficiency, depth, and regularity of the LIPSs were significantly better than those produced using Gaussian femtosecond laser pulses. The scan velocity for fabricating regular LIPSs was 2.3 times faster, the LIPSs depth was two times deeper, and the diffraction efficiency of the LIPSs was three times higher. As a result, regular and deep LIPSs could be processed in an area of 1m2 in several hours, according to the researchers, which they say has important implications for the industrial application of femtosecond-LIPSs in the future.

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