Electron emission and ultrafast low-fluence plasma formation during single-shot femtosecond laser surface ablation of various materials

Abstract

Emission of erosive plasma has been observed during electric probe and optical emission spectral measure ments of plumes produced by single shot femtosecond laser ablation of optical quality surfaces of various materials-copper, titanium, and silicon-at laser fluences well below the corresponding thermal ablation thresholds, replacing presumably electron emission at lower fluences. The onset of erosive plasma correlates on the fluence scale with saturation of dependences of self reflectivity of the pumping femtosecond laser pulses, reflecting the "freezing" of electron dynamics (variation of electron density or temperature) during the pumping pulses, despite the monotonically increasing laser fluences.

Applying Ultrashort Pulsed Direct Laser Interference Patterning for Functional Surfaces

“…In comparison to single pulse processing, the ablation of material is significantly enhanced in dual pulsing. In fact, it was shown that consecutive pulsing alters the absorptivity of metals by oxidation as well as surface www.nature.com/scientificreports www.nature.com/scientificreports/ roughening, leading to multiple reflections and absorption of the irradiating laser beam 20,42 . Surface roughness in the nanometre-scale has been proven to effectively enhance the absorptivity up to two-fold 37 , which might also be the reason for enhanced thermal interaction at scratches on copper and CuZn37 surfaces, leading to the evaporation induced craters visible in Fig.…”

Nanoscale Imaging of Ultrafast Light Coupling to Self-Organized Nanostructures

“…A focused laser beam exhibits nontrivial field distribution in the focal region, depending on the beam and nanostructure features. − Surface irradiation in the subablative regime can trigger a self-organized arrangement of nanostructures, referred to as LIPSS, with periods ranging from λ down to λ/5 n , where λ is the incident laser wavelength and n is the refractive index of the medium. − , The role of optical feedback is central in LIPSS formation, but difficult to access by optical means . The periodic behavior of the topography is assigned to different mechanisms, such as the coherent superposition of incident with scattered waves, the surface plasmon wave excitation, the near-field enhancement, and the corrugation-mediated feedback effects. ,, In the multipulse regime, it is generally accepted that LIPSS periodicity exhibits a decrease when the number of laser pulses impinging the target increases. , The reasons were attributed to surface plasmon resonance affected by surface roughness, supported by macroscopic approaches such as effective medium theory , and analytical expressions, taking account for roughness-induced modifications of surface plasmon vector. , Recently, the question of roughness influence on the absorbed energy distribution was revisited from a microscopic point of view, considering singular and collective response of nanocavities-dipoles on a metal surface . However, the influence of surface roughness and nanocavity concentration on LIPSS periods has never been experimentally demonstrated; therefore, the proposed scenarios still require careful examination.…”

“…22,24 The reasons were attributed to surface plasmon resonance affected by surface roughness, supported by macroscopic approaches such as effective medium theory 25,26 and analytical expressions, taking account for roughnessinduced modifications of surface plasmon vector. 27,28 Recently, the question of roughness influence on the absorbed energy distribution was revisited from a microscopic point of view, considering singular and collective response of nanocavitiesdipoles on a metal surface. 13 However, the influence of surface roughness and nanocavity concentration on LIPSS periods has never been experimentally demonstrated; therefore, the proposed scenarios still require careful examination.…”

Fluence-Dependent Transient Reflectance of Stainless Steel Investigated by Ultrafast Imaging Pump–Probe Reflectometry

“…The irradiation of stainless steel at large fluences might also induce the emission of thermionic electrons, − as well as the generation of a laser-induced plasma. − Both processes feature electrons, whose equation of motion and optical properties are approximately describable by the Drude theory . The reflectance R (λ probe ) of such a free electron gas depends on its electron density n e and the time between two collisions τ c ( T e ) of an electron (Figure , left).…”