## 1 Introduction  
Composite materials, in particular carbon fiber reinforced polymers (CFRP), consisting of epoxy resin and carbon fibers (CFs) as reinforcement, are dominating the production of lightweight components for the aeronautics [1], marine [2], automotive [3], and wind energy [4] markets. This gradual domination of CFRP over metallic structures in the above industries, is mainly driven by the requirement for reduced body-mass parts, which leads to decreased fuel consumptions and, in turn, fuel cost and CO\({}_{2}\) emissions. A consequence of this persistent requirement for larger weight reductions, leads to the need for expanding CFRP's multifunctionality, to achieve improved system designs. For instance, external functions such as structural health monitoring [5], energy storage [6], strike light protection [7], electromagnetic shielding [8], and actuation [9] can now be successfully integrated into composite structures. A strong fiber-matrix interface is an important aspect for maximizing the structural performance of CFRP. Introducing nanostructures on the fiber, such as oriented carbon nanotubes (CNTs) [10; 11; 12; 13; 14; 15; 16],graphene nanoflakes (GNFs) [17; 18; 19], or metal oxide nanowires [20; 21; 22; 23; 24; 25] has proven an effective technique for simultaneously enhancing the interfacial properties of CFRP composites and imparting additional functionalities (e.g., enhanced electrical conductivity, piezoelectricity, etc.) by creating a hierarchical structure at the fiber-matrix interface region. Laboratory-scale procedures have fabricated hierarchical interfaces on fibers with remarkable multifunctional properties; however, these processes are not amenable to high-volume, continuous fiber manufacturing, which is essential for the commercialization of nano-engineered CFRP. The challenge lies in successfully integrating these hierarchical reinforcing structures, to offer scalability and economic sustainability to advance towards commercialization. One potential route for tackling this challenge is the laser-induced modification of bare CFs, which is rather unexplored so far. The utilization of pulsed lasers for the surface modification of CFRP composites in joining applications has gained increasing attention in the last decade [26; 27; 28; 29]. Being a non-contact technique, laser treatment diminishes mechanical failure and evades tool wear and surface contamination, which are common problems encountered with mechanical approaches like abrading [30] and blasting processes [31]. However, reducing the heat-affected zone (HAZ) in CFs induced by the thermal nature of the laser/material interactions remains an obstacle. Along these lines, ultrafast lasers operating in the picosecond or femtosecond (fs) pulse duration regimes are the most efficient for CFRP surface texturing and improving adhesive bonding between CFRP parts in terms of Mode I and II fracture energy [32; 33; 34], since the HAZ and mechanical degradation on the CFs are minimal. The direct writing of fs-laser-induced periodic surface structures (LIPSSs) on CFs is a single step process using linearly polarized irradiation and has gained increased attention. It is well-known that their formation mechanism is attributed to a spatially modulated energy pattern, formed by the interference of the incident laser irradiation with surface electromagnetic waves [35]. Tensile tests on single carbon fibers (10 \(\upmu\)m diameter) decorated with LIPSS of approximately 150 nm revealed that fs irradiation preserved the tensile strength of the exposed fibers [36]. Even-though fs-laser processing seems promising, its adoption in commercial CFRP manufacturing is prohibited by their high cost and reduced availability for roll to roll processing. On the other hand, continuous wave (CW) infrared lasers, such as CO\({}_{2}\), are widely commercially for micromachining applications at moderate prices. However, material-laser interactions are dominated by heating effects.