Abstract
Under certain exposure conditions, a femtosecond laser beam focused in the bulk of fused silica leads to the formation of self-organized structures consisting of a series of ``nanolayers,'' parallel to one another. Remarkably, this laser-induced nanoscale anisotropy offers the possibility to locally engineer macroscopic properties of a given substrate by selectively exposing it in arbitrarily chosen locations to a laser beam with designed polarization states. Although various physical properties are affected by the laser, this paper specifically discusses in-plane elastic properties of these nanostructures. Using a method based on monitoring resonant properties of vibrating cantilevers combined with a mechanical model of the nanostructures, the Young's moduli of individual nanolayers are calculated and used to define the stiffness matrix of the composite structure. The model shows a good agreement with measured mechanical properties of arbitrarily oriented nanostructures. This work demonstrates the predictability and controllability of laser-induced nanoscale mechanical properties and offers a framework for engineering arbitrary elastic properties through 3D laser writing.
Highlights
Femtosecond laser bulk exposure can lead to the formation of self-organized nanolayers parallel to one another that are commonly referred to as nanogratings [1]
Since their discovery at the beginning of the 2000s, their formation and properties have been extensively studied [2,3,4]. These nanogratings are characterized by a self-organized modulation of nanoporous layers [5], roughly spaced by half the wavelength (λ/2n) [6] and perpendicularly aligned with the laser beam electrostatic field that defines its polarization state [2]
Postmortem experimental studies show a clear evolution of the self-organization with the increasing number of pulses [8,9], with a temporal pulse separation far longer than the lifetime of the induced plasma and self-trapped excitons [10], suggesting that the premodified material triggers the self-organization process [11]
Summary
Femtosecond laser bulk exposure can lead to the formation of self-organized nanolayers parallel to one another that are commonly referred to as nanogratings [1]. The exact mechanism leading to the creation of these nanofeatures [2,10,11,12,13] is to date not established This type of modification has been used in various photonics applications, such as polarization converters [14] or optical memory devices [15], and for controlling the stress state in materials [16,17], inducing controlled displacements [18,19,20], or for tuning properties, such as the coefficient of thermal expansion [21]. From an application point of view, this precise knowledge of the mechanical properties is essential for thermal expansion compensating schemes and thermomechanical devices, as well for stress-induced functionalities, like, for instance, used in photonics devices [16,17], mechanical positioners [19,20], and for tuning the resonance frequency of mechanical resonators
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