Solitary Nanostructures Produced by Ultrashort Laser Pulse

Abstract

which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. AbstractLaser-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.

Quantitative evaluation of LAL productivity of colloidal nanomaterials: Which laser pulse width is more productive, ergonomic, and economic?

“…The possible reasons are: (i) nonlinear optical absorption of higher-intensity fs-laser pulses, [55,56] (ii) partial acoustic unloading of the warm dense matter during ps-laser pulses, reducing their expulsive kinetic energy, [57] (iii) deeper laser energy deposition via hot-carrier transport. [5,58] Besides this uncertainty, ultrashort-pulse LAL is affected by laser selffocusing and filamentation in the liquid ambient, [33][34][35]59] the confinement liquid effect on even shorter material removal step, [60][61][62] transient bubble formation, [6,36] etc. As a result, optimal conditions (fluences, pulsewidths) for ultrashort-pulse LAL were not identified yet, despite a number of focused studies.…”

“…[1,2,8,[15][16][17][18][19][20][21][22][23][24][25] Moreover, there were no so far systematic studies of NP yield in ultrashort-pulse LAL, apparently, because of the many above mentioned incoming circumstances. Furthermore, despite the impressive progress in modeling ultrashort-pulse LAL, there is still intense ongoing theoretical research, involving molecular dynamics [60][61][62] and molecular hydrodynamics [63] simulations, to catch main features of ultrashort-pulse LAL. Meanwhile, basic qualitative understanding and related quantitative description in ultrashortpulse LAL are still missing, making necessary novel profound enlightening insights in both experimental studies and theoretical modeling.…”

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.…”

Direct laser printing of high-resolution physically unclonable anti-counterfeit labels

“…upon their exposure by focused fs laser pulses. In particular, such pulsed laser exposure launches a sequence of rather unique physical processes: (i) local melting of the section of the metal film, (ii) subsequent detachment of the molten shell from the underlying substrate via acoustic relaxation, (iii) accumulation of the molten material at the top of the shell in the form of so-called nanojet, (iv) rearrangement and decay of the nanojet height via Rayleigh-Plateau hydrodynamic instability as well as (v) material resolidification that "frozens" the resulting shape of the metal film 17,18 . For the fixed size D opt of the laser spot defined by numerical aperture (NA) of the focusing lens, the course of the mentioned processes as well as resulting shape of the imprinted morphology are governed by the applied pulse energy E .…”