JLMN-Journal of Laser Micro/Nanoengineering Vol. 7, No. 2, 2012 164 Formation Mechanism of Nanostructures on the Stainless Steel Surface by Femtosecond Laser Pulses Md. Shamim Ahsan *1,*2 , Yeong Gyu Kim *1 , and Man Seop Lee *1 *1 Photonics Application Lab, Korea Advanced Institute of Science and Technology (KAIST), 335 Gwahak-ro, Yuseong-gu, Daejeon 305-701, South Korea *2 Electronics and Communication Engineering Discipline, School of Science, Engineering and Technology, Khulna University, Khulna-9208, Bangladesh E-mail: shamim@kaist.ac.kr In this paper, we report on the formation of a variety of micro/nano-metric structures on the stainless steel surfaces by single-beam direct femtosecond laser writing. Periodic microgratings of different line widths and period are fabricated on the sample surfaces. Random nanostructures are observed inside the microgratings, when laser pulses with low laser energies are applied on the samples. Unlike random nanostructures, periodic nanostructures are produced inside the mi- crogratings, when laser pulses with high average pulse energy are applied. Besides, self-organized nanogratings, with variable periods, are formed all over the micro-machined area on the stainless steel surfaces due to the overlap of the microgratings, where the orientation of the nanogratings is parallel to the incident light polarization. Finally, we illustrate the formation mechanism of various kinds of nanostructures, produced on the stainless steel surfaces during our experiment. Keywords: Femtosecond Laser, single beam direct laser writing technique, microgratings, self- organized nanogratings, nanostructures 1. Introduction In the last few decades, femtosecond laser based pro- cessing of various materials has gained the attention of many researchers. In recent time, femtosecond lasers have been considered as a promising tool in micro/nano- machining of diverse metals [1–10]. Researchers have al- ready reported the micro/nano-structuring of various metals using several femtosecond laser based technologies: holo- graphic technology [11] and near-field enhancement in- duced by self-assembled Particle Lens Array (PLA) [12]. However, holographic technology is complicated due to the requirement of the obligatory coincidence of multiple laser beams. On the counterpart, PLA based technology is lim- ited to surface micro/nano-machining only, because this technology can’t produce structures inside the samples. Due to the limitations of other available technologies, Sin- gle Beam Direct Laser Writing (SBDLW) technology comes to the forefront of the research field. Using SBDLW technology, researchers reported the formation of a variety of micro/nano-metric structures on different material surfaces. Interaction of femtosecond laser pulses with various metals and the formation of Laser In- duced Periodic Surface Structures (LIPSS) or Nanostruc- ture Covered Laser Induced Periodic Surface Structures (NC-LIPSS) on various metal surfaces have been studied comprehensively in some recent studies [1–5]. Researchers have also reported the absorption [6] and the enhanced en- ergy coupling [7–9] on diverse metal surfaces by producing a variety of micro/nano-scale features. Colorizing stainless steel surfaces by femtosecond laser induced self-organized nanogratings (LIPSS) or periodic microholes have also been reported [10]. Due to the complexity involved in the formation of nanostructures (LIPSS/NC-LIPSS) on metal surfaces by femtosecond lasers, researchers have proposed quite a few different models in explaining the formation mechanism of LIPSS/NC-LIPSS: interference of the high- intensity laser beam with the laser induced plasma waves [13] and Coulomb explosion [14]. Although a lot of studies have been reported so far on femtosecond laser-metal inter- actions and the formation of LIPSS/NC-LIPSS on different metals, the information regarding the interactions of femto- second laser pulses with stainless steel surfaces is still in- adequate. In addition, the formation mechanism of the laser induced nanostructures on stainless steel surfaces needed more clarification. Thus, it is important to examine the formation of various laser induced nanostructures on stain- less steel surfaces and their formation mechanism. In this paper, using SBDLW technology, we produced long distance (we fabricated 1 mm long) horizontal mi- crogratings on the stainless steel surfaces by controlling the irradiation conditions of a femtosecond laser beam. The period of the microgratings are varied from 10 µm to 100 µm. Self-organized nanogratings are produced all over the laser micromachined area, when two or more horizontal microlines are overlapped. The period of the sub- wavelength self-organized nanogratings are ranging from 474 nm to 596 nm. Periodic nanostructures, other than self- organized nanogratings, are evident inside the mi- crogratings at high average pulse energy of the laser pulses (1.3 µJ) with the scanning speed of 1 mm/s. At low laser energies (varied from 437 nJ to 538 nJ), random nanostruc- tures are formed inside the microgratings. The experi- mental results clearly suggest the certain dependence of the nanostructures’ patterns on various irradiation conditions of the femtosecond laser beam such as the average pulse en- ergy, the scanning speed, and the scanning step. Above all,