# Fabrication of Smart Materials Using Laser Processing: Analysis and Prospects
## 4 Laser Surface Modification by Direct Laser Interference Patterning (DLIP)  
DLIP for surface modification of materials utilizes coherent laser beam interference to create functional periodic microstructures, offering advantages over alternative methods. It stands out for its capacity to directly create large-scale microstructures, varying between centimeters to tens of nanometers, showcasing high speed and versatility. The distinguishing feature of DLIP is that it does not require focused laser beams, which enables high-performance processing with a substantial depth of focus and expanding structuring possibilities through laser parameter variations. This makes DLIP a potent and efficient technique for modifying surface properties across a spectrum of technological applications, encompassing electrical, optical, and tribological domains. The studies [123, 124] provided a comprehensive discussion of direct laser interference patterning (DLIP), a high-throughput surface structuring method using overlapping laser beams, with applications in industries like photonics, optoelectronics, nanotechnology, and biomedicine. Figure 8 showcases the DLIP treatment of \(\sim\)500 nm-thick indium tin oxide (ITO) thin films on glass substrates using a DLIP workstation equipped with a solid-state ultrashort-pulse laser with a maximum output power of 100 W and a pulse frequency of 1 MHz. The laser operated at a wavelength of 1030 \(\pm\) 10 nm, with a frequency of 50 kHz, and a pulse duration of 900 \(\pm\) 100 fs. SEM images were taken at varying fluence levels to assess the induced microgroove morphology. In Figure 8a, the SEM image displays the untreated ITO surface, while Figure 8b,c depict the DLIP-treated surface at \(F\) = 250 mJ cm\({}^{-2}\). The DLIP-treated sample exhibits a well-defined periodic array of grooves, and on the valleys of the texture, laser-induced periodic surface structures (LIPSS) are evident. LIPSS features align perpendicular to the laser polarization, extending on both the bottom and sidewalls of the texture. Spatial frequencies of DLIP and LIPSS are identified, with a spatial period of \(\Lambda_{\text{DLIP}}\) = 5.0 \(\pm\) 0.1 \(\upmu\)m for DLIP and \(\Lambda_{\text{LIPSS}}\) = 230 \(\pm\) 20 nm for LIPSS [124]. Figure 7: TEM images of cross-sections of 22 nm-thick silicon-on-insulator structures: (**a**) asphosphorous-implanted and after sub-melt multi-pulse UV-NLA at 0.58 J/cm\({}^{2}\) with (**b**) 30 or (**c**) 100 cumulative pulses [122]. The process of microstructuring in DLIP can engage either photothermal, photochemical, or a combination of these mechanisms, depending on the interaction between laser light and the utilized materials [125]. In [126], the DLIP process scheme was presented (Figure 9). The influence of pulse duration and pulse-to-pulse overlap was investigated. The study provides insights into surface phenomena and demonstrates efficient structuring of challenging, low-absorbing polymeric substrates using ultrashort pulsed lasers (266 fs-15 ps) operating with a frequency of 1 kHz and an emitting IR wavelength of 1030 nm. Here, the surface modification of polyether ether ketones, advanced polymeric materials replacing metals and ceramics, was demonstrated using femtosecond pulses. A laser beam with an elongated shape and a periodic intensity profile, created by the interference of two laser beams, was utilized to remove material from the surface. In the connector industry, the reduction in friction during the insertion in detachable electrical connections is crucial, commonly achieved through connector design and lubricants. This study utilizes DLIP to create asymmetric saw-tooth structures on Sn-coated Cu plates, showcasing improved performance with lower insertion and higher removal forces, while maintaining electrical behavior [127]. This study utilized a picolaser system, incorporating a laser with a pulse duration of 12 ps and a wavelength of 532 nm.