Laser Fabrication: A Solid Present for the Brightest Future

“A solution seeking a problem”: this is how the laser was famously defined by its own developer upon its first appearance on the scientific and technological stage. Despite this, since their invention, lasers have found their place in countless manufacturing processes, thanks to their ability to process virtually any material, from metals and semiconductors to glass and crystals, including biological and biocompatible materials.

One of the first examples of technological applications of lasers in the production line is laser welding. Here, laser irradiation generates a localized temperature profile which leads to controlled autogenous melting, thus allowing accurate welding with reduced residual stresses. Several types of weld geometries have been investigated, i.e., butt, lap, and fillet. Among them, though very efficient in terms of welding strength and speed, the first one is certainly the most critical in terms of joint gap tolerance, which should be at most the same as the focused beam size [1]. This is why novel strategies for laser beam shaping are being investigated, to modify its spatial power distribution, e.g., through the use of Diffractive Optical Elements (DOEs) [2] which confer novel power distribution to the laser beam (line, ring, multi-foci spots), thus enhancing the bridging capability [3]. Moreover, in recent years, deformable mirror adaptive optics have been proposed as possible tools for the spatial beam shaping of high-power laser beams [4,5], e.g., to modify a Gaussian beam into elliptical, to optimize the coupling with the materials to be welded and generate wider joint widths.

In the past, another industrial sector where lasers have had the most impact is additive manufacturing (AM). This technology is based on the general concept of building components by adding material layer by layer, instead of subtracting it, to obtain the final desired geometry, starting from its digital model. It was reported that using lasers in this field allows a direct energy transfer to the material in a focused and precise position, thus ensuring high accuracy and throughput in the construction of 3D objects [6]. Usually, conventional continuous wave (cw) or long-pulsed lasers in the IR region are used to achieve this aim. Nonetheless, recent research has also reported about the use of ultrafast lasers to fully exploit the potential of such a technology and its application to materials with unusual properties, such as high melting temperature, high heat conductivity, and low absorption coefficient [7,8]. Such aspects highlight the extreme flexibility and reconfigurability of such a technology, which allow effectively fabricating components with very complex structures, e.g., elaborated bio-inspired structures, such as butterfly, beetle, and water spider configurations [9].

Continuous innovations related to ultrafast lasers, e.g., the temporal evolution of the average laser power following a trend like Moore’s law for the number of transistors in integrated circuits, thus doubling every two years during the past 20 years [10,11], have opened up the opportunity for numerous diverse applications. As an example, thanks to their peculiar timescales, ultrafast lasers are particularly suitable for the micromachining of every kind of material, as they enable generating reproducible and very precise micro- and nano-features to confer new functionalities, such as anti-wear, reduced friction, controlled wettability for anti-fogging, water capture and anti-icing, and tailored optical properties [12,13,14,15,16,17,18,19]. Moreover, ultrafast laser micromachining is also a highly flexible solution for the rapid prototyping of polymeric microfluidic devices, thus allowing the fabrication of precise, low-cost, and biocompatible microdevices for the sensing and manipulation of particles and cells [20].

The frontiers of ultrafast micromachining are also further broadening thanks to the introduction of spatial and temporal laser beam shaping. Exploiting optical diffractive elements, e.g., axicons, to modify the spatial distribution of energy along the optical axis enables the generation of buried structures in transparent materials, with possible applications in integrated optics and microfluidics [21,22]. This is because such special optics allow for replicating the properties of a Bessel beam, consisting of a focal line that, unlike the Gaussian one, maintains its transversal distribution along the optical axis. Furthermore, the advent of burst generators in GHz and THz regimes for the temporal shaping of laser pulses [23,24,25] paved the way to significant improvements in the micromachining process in terms of ablation rate [23,26,27], quality of the ablated structures [24,28], and possibility of generating novel surface structures [29,30].