Nanotechnology Now - Press Release: Efficiently processing high-quality periodic nanostructures with ultrafast laser

High efficiency or high precision, like fish or bear's paw, cannot have both in ultrafast laser nanoprocessing. In order to improve the efficiency, parallel processing methods such as spatial light modulation and microlens arrays have been developed, and the processing efficiency can be improved by a factor of 100-1000. However, due to the limitations of devices and/or methods, it is still difficult to achieve nanofabrication in a large-area (square meter).

In 1965, Birnbaum reported periodic ripples with a period smaller than laser wavelength on the ablation spot on germanium irradiated by single beam of Ruby laser. However, the periodic ripples induced by the long-pulse (>nanosecond) laser were usually very shallow and irregular, and the applications were premature.

Femtosecond laser processing has the advantages of minimal thermal effect and no material selectivity because of the ultrashort pulse durations and high peak powers. Compared with long-pulse lasers, femtosecond laser-induced periodic structures (femtosecond-LIPSs) are more regular and deeper, with periods ranging from 0.1-1 ?. The formation mechanism of femtosecond-LIPSs remains a very complex and challenging problem. Nevertheless, laser direct writing by femtosecond-LIPSs can efficiently process nanostructures on the scale of tens of micrometers or even millimeters, and has been demonstrated on different types of materials such as metals, semiconductors, dielectrics, and polymers. Femtosecond-LIPSs have become a useful method for laser nanofabrication, with broad prospects in structural colors, birefringence and data storage, enhanced light absorption and luminescence, wetting properties, anisotropic electrical properties, and tribological applications.

The research group of Prof. Jia from State Key Laboratory of Precision Spectroscopy, East China Normal University, review the formation mechanisms of femtosecond-LIPSs, the efficient processing techniques of high-quality LIPSs and the applications.

In order to study the formation mechanisms of femtosecond-LIPSs, scanning electron microscopy (SEM) and atomic force microscopy (AFM) are typically used to observe the LIPS morphology. The resolutions of these methods are very high, however, they cannot be used to study the dynamics of LIPS formation. Prof. Jia?s group reported a collinear pump-probe imaging method to study the transient processes of LIPS formation. The spatial resolution is 300 nm and the temporal resolution is 0.6 ps. Figure 1(a) shows the evolution of the LIPSs formed on an Au film irradiated by single pulse of 800 nm, 50 fs laser. Between 400 and 600 ps, transient LIPSs appear to be very distinct and regular. The ripples are perpendicular to the laser polarization with a period of 740 ? 10 nm. However, the ripples disappear after the solidification of the ablation spot.

Figure 1(b) shows that the LIPS periods increased from 685 nm to 770 nm with the laser fluences, which has a great deviation between the theoretical results of surface plasmon polaritons (SPP). Prof. Jia?s group proposed and studied in detail the effects of hot electron localization and d-band transitions on the dielectric constant at the highly excited states during femtosecond laser irradiation of noble metal of gold and silver, and common metal of Ni. The developed SPP model accorded well with the experimental results, indicating that SPP excitation was the key during the formation of LIPSs on different types of metal surfaces, and the strong thermal effects can reduce the LIPS depth, and even cause them to disappear.

To efficiently fabricate uniform, regular, and deep LIPSs, three main challenges should be addressed: enhancement of periodic deposition of laser energy, reduction of residual heat, and avoidance of debris deposited on the ablation spots. Prof. Jia?s group developed a 4f zero-dispersion pulse-shaping system, which could generate pulse trains with an interval of 0.1-16.2 ps by using periodic ?-phase step modulation. Regular and deep LIPSs were induced on the Si surface using a shaped pulse with an interval of 16.2 ps, as shown in Fig. 2.

The transient LIPSs started appearing on Si surface at a delay time of 4 ps after femtosecond laser irradiation, which was shorter than the interval between adjacent sub-pulses. Thus, the transient LIPSs have started to appear under the illumination of the two strongest subpulses. When the subsequent subpulse reached the sample surface, the transient LIPSs induced by the previous subpulses enhanced the excitation of the SPPs, as well as the periodic distribution of the laser field. When the subsequent small subpulses reached, the surface layer 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 subpulses, and the debris was further ionized and vaporized, resulting in fewer deposited particles.

The fabrication efficiency, depth, and regularity of the LIPSs by using the shaped pulses were significantly better than those using Gaussian femtosecond laser pulses. The scan velocity for fabricating regular LIPSs was 2.3 times faster, and the LIPSs depth was two times deeper, and the diffraction efficiency of LIPSs was three times higher.

The laser focal spot of a cylindrical lens is only a few to tens of micrometers wide, while its length is on the order of 1-10 mm, or even 100 mm for high-power (100-watt) femtosecond lasers. By using temporally shaped high-power femtosecond laser direct writing focused with a cylindrical lens, regular and deep LIPSs can be processed in an area of 1 m2 in several hours, which has important implications for the industrial application of femtosecond-LIPSs in the future.

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The corresponding Author of this review is Prof. Tianqing Jia, and the co-first Author Yuchan Zhang and Qilin Jiang are doctoral student of Prof. Jia. Prof. Tianqing Jia received his PHD degree at Tongji University in 2000. He did a postdoc at Shanghai Institute of Optics and Fine Mechanics, the Chinese Academy of Sciences. In 2001 he became an associate research fellow in Shanghai Institute of Optics and Fine Mechanics. In 2005 he became a research fellow in the Institute for Solid State Physics, the University of Tokyo. He has been a full professor at East China Normal University since 2007.

Prof. Tianqing Jia's research group focuses on laser precision machining, such as femtosecond laser-induced periodic nanostructures, and the ultrafast dynamics of femtosecond laser ablation. In the past ten years, His group has researched key issues and technologies related to the laser processing of cooling holes both in the turbine blades and flame tube of commercial aero-engine, and has developed laser-processing equipment with functions of on-line detection, 3D positions, et al.

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