Solitary Nanostructures Produced by Ultrashort Laser Pulse

Abstract

Laser-produced surface nanostructures show considerable promise for many applications while fundamental questions concerning the corresponding mechanisms of structuring are still debated. Here, we present a simple physical model describing those mechanisms happened in a thin metal film on dielectric substrate irradiated by a tightly focused ultrashort laser pulse. The main ingredients included into the model are (i) the film–substrate hydrodynamic interaction, melting and separation of the film from substrate with velocity increasing with increase of absorbed fluence; (ii) the capillary forces decelerating expansion of the expanding flying film; and (iii) rapid freezing into a solid state if the rate of solidification is comparable or larger than hydrodynamic velocities. The developed model and performed simulations explain appearance of microbump inside the focal spot on the film surface. The model follows experimental findings about gradual transformation of the bump from small parabolic to a conical shape and to the bump with a jet on its tip with increasing fluence. Disruption of the bump as a result of thinning down the liquid film to a few interatomic distances or due to mechanical break-off of solid film is described together with the jetting and formation of one or many droplets. Developed theory opens door for optimizing laser parameters for intended nanostructuring in applications.

Light‐Induced Tuning and Reconfiguration of Nanophotonic Structures

“…Under spatially confined ultra‐short excitation, irradiated material undergoes solid–liquid–solid phase transitions, typically on the nanosecond time scales, resulting in a local reshaping and formation of different nanoscale surface features exhibiting properties of optical nanoantennas. Under femtosecond‐pulse irradiation, the local molten part of the noble‐metal thin film, characterized by weaker adhesion to the supporting substrate, detaches through ultra‐fast optical heating and local high‐pressure generation, producing a parabola‐shaped hollow bump or nanovoid (I–IV in Figure a) whose geometric parameters can be tuned precisely by applied fluence . Such nanostructures were produced on surfaces of Au and Ag films, and they demonstrate tunable size‐dependent resonant light scattering in the visible spectral range, supporting excitation and interference of axial and transverse surface plasmon modes in nanovoid shells (see Figure b and c).…”

“…For a certain critical fluence, a huge amount of molten metal material is ejected via the nanojet formation mechanism leaving a nano‐ or micro‐sized through hole in the irradiated part of the metal film . This regime of light–matter interaction corresponds to laser ablation and, thus, further increase of laser fluences is not applicable for precise tuning and reconfiguration of advanced photonic nanostructures.…”

Laser ablation of metal into liquid: Near critical point phenomena and hydrodynamic instability

“…There are two type of experiments with different lateral size of an irradiated spot (radius R L ) at a surface. People use diffraction limited tight focusing when R L ∼ λ in the first type of experiments [17][18][19][20][21][22]; for optical lasers λ ∼ 1µm. While in the other type the large spots (R L is many microns) are necessary.…”

Mathematical Modeling of the Concentrated Energy Flow Effect on Metallic Materials

“…We assume that the energy flow is homogenous in the cross-section, therefore, one-dimensional statement of problem is selected, that is, all functions are dependent on the only one spatial variable, where we consider the one-axial stress-strain state with non-zero components of the strain tensors. To state the mathematical problem we use the equations of motion and thermal conductivity, as well as the Duhamel-Neumann relation, the so called dynamic problem of the disconnected thermo-elastisity [31]:…”

“…Numerous publications emphasize the interest of researchers in development of mathematical models describing nano-structures under the action of concentrated energy flows [29][30][31][32][33][34][35][36][37][38][39]. These models are based mainly on computer simulation of processes, going with the development of nano-structural states.…”