Spatial Writing of Ultrafast All-Optical Switching.

Writing spatial information on ultrafast all-optical switching is essential for constructing ultrafast processing units in photonic applications, such as optical communication and computing networks. However, most methods ignore the fabrication and imaging of controllable switching area, limiting its spatial information and the further design in ultrafast devices. Here, we propose a method to spatially write in ultrafast all-optical switching based on MAPbI3 perovskite with nanocone structure and visualize the switching effect in arbitrary designed area. Due to the light confinement effect of nanocone fabrication using a fs laser, the light is strongly absorbed by perovskite and reach saturable absorption. It leads to ultrafast broadband transmittance change with 25 fs switching time and 10% modulation depth in nanocone perovskite area. Our preparation method offers high efficiency, performance, and flexibility for the spatial writing of ultrafast all-optical switching, which is promising for developing ultrafast all-optical networks and the next generation of communication technology.

Femtosecond Laser-Induced Anisotropic Structure and Nonlinear Optical Response of Yttria-Stabilized Zirconia Single Crystals with Different Planes.

Nonlinear optical properties have been extensively studied due to their promising nonlinear effects and various applications. With ultrashort duration and ultrahigh intensity, a femtosecond laser can fabricate various superior-quality micro-/nanostructures to improve the nonlinearity of materials, which are promising for stable and high-performance nonlinear devices. In this contribution, yttria-stabilized zirconia (YSZ) with fs laser-induced micro-/nanostructures is demonstrated to exhibit unique anisotropic light-material interaction and nonlinear optical response on [100], [110], and [111] planes. Time-resolved reflectivity of YSZ after fs laser excitation is investigated by a pump-probe experiment, which is consistent with simulations through the plasma model combined with a two-temperature model. These results indicate two early ablation mechanisms: Coulomb explosion and melting. Anisotropic crack structures are formed due to thermal stress, which is always ignored in fs laser fabrication and is verified by Raman mapping and analysis of slip systems on different crystal planes. Through the z-scan measurement, the nonlinear absorption (NLA) of crack structures is effectively improved, and a nearly 18 times enhancement of the NLA coefficient is acquired in [100] samples, while a 2 times enhancement in [110] and [111] samples. Such great enhancement of NLA is mainly due to the abundant presence of crack structures and the increase of fs laser-induced oxygen vacancies in [100] YSZ. These results provide a potential way of utilizing fs laser to improve the nonlinearity for the technological development in nonlinear devices.

Ultrafast Shaped Laser Induced Synthesis of MXene Quantum Dots/Graphene for Transparent Supercapacitors.

Ultratransparent electrodes have attracted considerable attention in optoelectronics and energy technology. However, balancing energy storage capability and transparency remains challenging. Herein, an in situ strategy employing a temporally and spatially shaped femtosecond laser is reported for photochemically synthesizing of MXene quantum dots (MQDs) uniformly attached to laser reduced graphene oxide (LRGO) with exceptional electrochemical capacitance and ultrahigh transparency. The mechanism and plasma dynamics of the synthesis process are analyzed and observed at the same time. The unique MQDs loaded on LRGO greatly improve the specific surface area of the electrode due to the nanoscale size and additional edge states. The MQD/LRGO supercapacitor has high flexibility and durability, ultrahigh energy density (2.04 × 10-3  mWh cm-2 ), long cycle life (97.6% after 12 000 cycles), and excellent capacitance (10.42 mF cm-2 ) with both high transparency (transmittance over 90%) and high performance. Furthermore, this method provides a means of preparing nanostructured composite electrode materials and exploiting quantum capacitance effects for energy storage.

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.