This paper reviews a new field of direct femtosecond laser surface nano/microstructuring and its applications. Over the past few years, direct femtosecond laser surface processing has distinguished itself from other conventional laser ablation methods and become one of the best ways to create surface structures at nano‐ and micro‐scales on metals and semiconductors due to its flexibility, simplicity, and controllability in creating various types of nano/microstructures that are suitable for a wide range of applications. Significant advancements were made recently in applying this technique to altering optical properties of metals and semiconductors. As a result, highly absorptive metals and semiconductors were created, dubbed as the “black metals” and “black silicon”. Furthermore, various colors other than black have been created through structural coloring on metals. Direct femtosecond laser processing is also capable of producing novel materials with wetting properties ranging from superhydrophilic to superhydrophobic. In the extreme case, superwicking materials were created that can make liquids run vertically uphill against the gravity over an extended surface area. Though impressive scientific achievements have been made so far, direct femtosecond laser processing is still a young research field and many exciting findings are expected to emerge on its horizon.

For centuries, it had been the dream of alchemists to turn inexpensive metals into gold. Certainly, it is not enough from an alchemist’s point of view to transfer only the appearance of a metal to gold. However, the possibility of rendering a certain metal to a completely different color without coating can be very interesting in its own right. In this work, we demonstrate a femtosecond laser processing technique that allows us to create a variety of colors on a metal that ultimately leads us to control its optical properties from UV to terahertz.

In contrast to the common belief for femtosecond laser ablation that the thermal energy remaining in the ablated sample should be negligible, we recently found that a significant amount of residual thermal energy is deposited in metal samples following multishot femtosecond laser ablation. This suggests that there might be a significant enhancement in laser light absorption following ablation. To understand the physical mechanisms of laser energy absorption, we perform a direct measurement of the change in absorptance of gold due to structural modification following multishot femtosecond laser ablation. We show that besides the known mechanisms of absorption enhancement via microstructuring and macrostructuring, there is also a significant absorption enhancement due to nanostructuring. It is found that nanostructuring alone can enhance the absorptance by a factor of about three. The physical mechanism of the total enhanced absorption is due to a combined effect of nanostructural, microstructural, and macrostructural surface modifications induced by femtosecond laser ablation. Virtually, at a sufficiently high fluence and with a large number of applied pulses, the absorptance of gold surface can reach a value close to 100%.

In this paper, we performed a detailed study of the formation of femtosecond laser-induced periodic surface structures (LIPSSs) on platinum and gold at near-damage threshold fluences. We find a unique type of LIPSS entirely covered with nanostructures. A distinctive feature of the nanostructure-covered LIPSS is that its period is appreciably less than that of the regular LIPSS. We show that the reduced period is caused by an increase of the real part of the effective refractive index of the air-metal interface when nanostructures develop and affect the propagation of surface plasmons.

In this work, viscoelastic, isothermal extrusion film casting modeling utilizing 1D membrane model and modified Leonov model was performed in order to understand the role of viscoelastic stress state at the die exit on the polymer melt film stretching in the post die area. Experimental data for LDPE and theoretical predictions based on the eXtended Pom-Pom (XPP) model taken from the open literature were used for the validation purposes. It was found that predicting capabilities of 1D membrane model utilizing XPP and modified Leonov model are comparable for the given processing conditions and material. Consequent theoretical parametric study revealed that increase in the viscoelastic stress state at the die exit, characterized as the ratio of second and first normal stress differences, -N 2 /N 1 , leads to increase in neck-in phenomenon. This suggests that specific attention should be paid to optimization of the extrusion die design in order to stabilize polymer melt film stretching in the post die area. respectively, for the long chain branched (LDPE, PP) and the linear (HDPE, PP) polymers. Even if many useful findings and conclusions regarding to neck-in phenomenon can be found in the open literature [29][30][31][32][33], the effect of polymer melt flow history, generated inside the extrusion die, on the neck-in phenomenon occurring in post die area still remains unclear. In order to extend the knowledge in this field, the 1D membrane model proposed by Silagy et al. [14] together with viscoelastic modified Leonov model [34] was utilized in this work for the EFC modeling. MODELING Modified Leonov ModelThis constitutive equation is based on heuristic thermodynamic arguments resulting from the theory of rubber elasticity [35][36][37]. Mathematically it is relating the stress and elastic strain stored in the material as: