Fabrication of Ge2Sb2Te5 crystal micro/nanostructures through single-shot Gaussian-shape femtosecond laser pulse irradiation.

Femtosecond (fs) laser-thin film interaction is one of the most practical methods for fabricating functional nanostructures. However, the details of the interaction mechanism remain unclear. In this study, we demonstrate an abnormal ablation effect on nanofilms by using a tightly focused single fs laser pulse. After the irradiation of a single Gaussian-shaped femtosecond laser pulse, a molten micro/nanopatch at the irradiated central high-power zone is isolated from the surrounding film. The confined localized threshold effect is proposed as the main mechanism for the phase isolation. With this effect, the high refractive index dielectric Ge2Sb2Te5 crystal nanostructures can be fabricated by directed dewetting of the isolated molten micro/nanopatch on Si substrates. After the laser irradiation, the central isolated liquid through an amorphous GST film is transformed into a crystalline state after resolidification. The isolated central micro/nanopatch size can be controlled by the focused spot size and pulse energy, so that the morphologies (size, geometrical morphology, and distribution) of GST nanostructures can be flexibly modulated. Furthermore, separated solid and liquid phase states detected using spatial-temporal-resolved microscopy validates the crucial role of the confined-localized threshold effect in the dewetting effect based on the separated liquid phase.

Direct observation of structure-assisted filament splitting during ultrafast multiple-pulse laser ablation.

Laser-induced plasma evolution in fused silica through multipulse laser ablation was studied using pump-probe technology. Filament splitting was observed in the early stage of plasma evolution (before ~300 fs). This phenomenon can be attributed to competition between laser divergent propagation induced by a pre-pulse-induced crater and the nonlinear self-focusing effect. This effect was validated through simulation results. With the increasing pulse number, the appearance of filament peak electron density was postponed. Furthermore, a second peak in the filament and peak position separation were observed because of an optical path difference between the lasers propagating from the crater center and edge. The experimental results revealed the influence of a prepulse-induced structure on the energy distribution of subsequent pulses, which are essential for understanding the mechanism of laser-material interactions, particularly in ultrafast multiple-pulse laser ablation.

One-Step Fabrication Method of GaN Films for Internal Quantum Efficiency Enhancement and Their Ultrafast Mechanism Investigation.

The third-generation semiconductors are the cornerstone of the power semiconductor leap forward and have attracted much attention because of their excellent properties and wide applications. Meanwhile, femtosecond laser processing as a convenient method further improves the performance of the related devices and expands the application prospect. In this work, an approximate 3 times improvement of the internal quantum efficiency (IQE) and a 5.5 times enhancement of the photoluminescence (PL) intensity were achieved in the GaN film prepared using a one-step femtosecond laser fabrication method. Three types of final micro/nanostructures were found with different femtosecond laser fluences, which could be attributed to the decomposition, melting, bubble nucleation, and phase explosion of GaN. The mechanisms of the microbump structure formation and enhancement of IQE were studied experimentally by the time-resolved reflection pump-probe technique, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Simulations for the laser-GaN interaction have also been performed to ascertain the micro/nanostructure formation principle. These results promote the potential applications of femtosecond lasers on GaN and other wide band gap semiconductors, such as UV-light-emitting diodes (LEDs), photodetectors, and random lasers for use in sensing and full-field imaging.

Asymmetric Response Optoelectronic Device Based on Femtosecond-Laser-Irradiated Perovskite.

We have explored an asymmetric optoelectronic response of an FAPb(I0.8Br0.2)3 (FA = formamidine) perovskite device irradiated by a femtosecond (fs) laser at different laser-fluence values. Photoluminescence (PL) spectra indicated a blue shift from 772 nm (1.606 eV) to 745 nm (1.664 eV) and more than 80% quenching of the irradiated perovskite. The blue shift of the PL spectra can be attributed to compositional variation, which was confirmed through elemental analysis and X-ray diffraction. Two distinct characteristic time constants 193-46 ps and 1.9-0.61 ns were obtained by using fs transient absorption spectroscopy. The fast one represents recombination at the interface, whereas the slow one represents band-to-band recombination in the interior of the grain. Interestingly, after the perovskite was irradiated by a femtosecond laser with an appropriate laser fluence (0.135 J/cm2), an asymmetric I-V characteristic was achieved, which should result from irreversible electric domain deflection. Due to the electron-phonon scattering induced by defects, the degree of asymmetry was sensitive to the illumination power. As the photosensitive asymmetric I-V characteristics have a bearing on its photoelectric properties, the findings would be of value in photodiode, memory, and other photoelectric devices.

Ultrafast optical response and ablation mechanisms of molybdenum disulfide under intense femtosecond laser irradiation.

Numerous valuable studies on electron dynamics have focussed on the extraordinary properties of molybdenum disulfide (MoS2); however, most of them were confined to the level below the damage threshold. Here the electron dynamics of MoS2 under intense ultrafast laser irradiation was investigated by experiments and simulations. Two kinds of ablation mechanisms were revealed, which led to two distinct types of electron dynamics and final ablation morphology. At a higher fluence, the emergence of superheated liquid induced a dramatic change in the transient reflectivity and micro-honeycomb structures. At a lower fluence, the material was just removed by sublimation, and the ablation structure was relatively flat. X-ray photoelectron spectroscopic (XPS) measurements demonstrated that thermal decomposition only occurred at the higher fluence. Furthermore, a theoretical model was developed to deeply reveal the ultrafast dynamics of MoS2 ablation. The simulation results were in good agreement with the temporal and spatial reflectivity distribution obtained from the experiment. The electron and lattice temperature evolution was also obtained to prove the ablation mechanism. Our results revealed ultrafast dynamics of MoS2 above the damage threshold and are helpful for understanding the interaction mechanism between MoS2 and intense ultrafast lasers, as well as for MoS2 processing applications.

Structure-Mediated Excitation of Air Plasma and Silicon Plasma Expansion in Femtosecond Laser Pulses Ablation.

Femtosecond laser-induced surface structures upon multiple pulses irradiation are strongly correlated with the pulse number, which in turn significantly affects successive laser-material interactions. By recording the dynamics of femtosecond laser ablation of silicon using time-resolved shadowgraphy, here we present direct visualization of the excitation of air plasma induced by the reflected laser during the second pulse irradiation. The interaction of the air plasma and silicon plasma is found to enhance the shockwave expansion induced by silicon ablation in the longitudinal direction, showing anisotropic expansion dynamics in different directions. We further demonstrate the vanishing of air plasma as the pulse number increases because of the generation of a rough surface without light focusing ability. In the scenario, the interaction of air plasma and silicon plasma disappears; the expansion of the silicon plasma and shockwave restores its original characteristic that is dominated by the laser-material coupling. The results show that the excitation of air plasma and the laser-material coupling involved in laser-induced plasma and shockwave expansion are structure mediated and dependent on the pulse number, which is of fundamental importance for deep insight into the nature of laser-material interactions during multiple pulses ablation.