Recently, spontaneous production of laser-induced periodic surface structures using femtoseconds laser pulses has drawn much attention since the phenomenon bears enormous potential for application in precise machining of a wide variety of materials in air. Low-spatial-frequency laser-induced periodic surface structures are formed with spacing close to or less than the laser wavelength and high-spatial-frequency laser-induced periodic surface structures are formed with a significantly smaller spacing than laser wavelength. In recent studies, a laser fluence below the ablation threshold has been reported to form high-spatial-frequency laser-induced periodic surface structures with sub-100-nanometer spacing on titanium oriented parallel to the laser beam polarization direction. However, how this ultrafine parallel-oriented laser-induced periodic surface structures form on such metal surfaces has not yet been clarified.
Naoki Yasumaru and colleagues from the National Institute of Technology at Fukui College in Japan developed a new femtosecond-laser processing technique. They hoped to unearth the chemical, physical and morphological properties of the femtosecond- laser-induced periodic surface structures formed on titanium using surface analysis equipment particularly focusing on clarifying the difference in nature of the two laser-induced periodic surface structures. Their work is now published in the peer reviewed journal, Applied Surface Science.
The researchers adopted the available pure titanium plates with a Vickers hardness of 160 which they mechanically polished to a mirror-finish and a specific roughness. They then irradiated the specimens in air with linearly-polarized, 800nm, 180fs laser pulses from a titanium-sapphire chirped-pulse amplification system operation at a unique frequency. The laser pulses were focused at a specific spot using a parabolic mirror. The research team then analyzed the morphological changes in the laser irradiated specimens by optical microscopy, field-emission scanning electron microscopy and scanning probe microscopy. Eventually, the mean spacing D of the laser-induced periodic surface structures formed on the specimen surface was determined from the spectrum obtained using the Fourier transform of scanning electron microscopy image.
The research team also observed that for the obtained laser-induced periodic surface structures, the D was 66nm which was noted to be less than one tenth of the laser wavelength used in the study. They also noted that the surface roughness of the ultrafine laser-induced periodic surface structures was almost the same as that of the non-irradiated surface, whereas the roughness of the perpendicular-oriented laser-induced periodic surface structures was approximately one order of magnitude higher. Of great importance, using X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, they realized that although the ultrafine laser-induced periodic surface structures were covered with an organic substance that was present in the atmosphere, the perpendicular oriented laser-induced periodic surface structures were covered with a substance similar to a cellulose derivative, which is difficult to synthesize artificially.
In totality, the main difference in the characteristics of these two types of laser-induced periodic surface structures indicates that they have dissimilar formation mechanisms, and the organic substance spotted on the nanostructured surface may lead to a novel application of femtosecond-lasers. Consequently, the femtosecond-laser processing technique may become a new technology for the artificial synthesis of cellulose derivatives.
Naoki Yasumaru, EisukeSentoku and JunsukeKiuchi. Formation of organic layer on femtosecond laser-induced periodic surface structures. Applied Surface Science, volume 405 (2017) pages 267–272.Go To Applied Surface Science