Femtosecond laser-induced periodic subwavelength and deep-subwavelength structures (SWS; DSWS) have attracted attention due to their subdiffraction resolution of surface and inner volume patterning. Understanding of the richness of laser-matter interaction during formation of SWS and DSWS is another quest which can help to find control for nanoscale fabrication. Lack of control over SWS and DSWS formation has impacted their wider use and calls for a deeper insight into the relationship between them. Herein we present a systematic study defining a criterion for imprinting either SWS or DSWS, which is based on a competition and their mutual incompatibility discriminated by the laser fluence and pulse accumulation. Structure evolution of SWS and DSWS is highly dependent on the localized effective laser fluence, which determines the instantaneous optical permittivity by the laser-excited electrons creating an active plasma layer. The proposed universal SWS and DSWS competition mechanism involving the laser-induced plasma wave at the plasma-substrate interface ties together many previous observations and unifies the discussed mechanisms of surface nanoripple formation.

This paper presents a theoretical and experimental investigation into the ablation threshold of nickel template by femtosecond laser in air at atmospheric pressure. The laser pulses used for the study are 800 nm in wavelength, 100fs in pulse duration, and 1KHz in repetition rate. The two-temperature model is used to predict the single-pulse ablation threshold for nickel theoretically. Micro-hole ablation experiments are carried out in air by focusing the femtosecond laser beam on the nickel target surface at normal incidence with the long-focus objective lens of enlargement factor 50 and NA=0.7 to determine the single-pulse and multi-pulse ablation thresholds for nickel by setting up the relationship between the measured hole diameters and the pulse energies. The single pulse ablation threshold of 4132.98 Jm-2obtained experimentally is very close to that of 3907.99 Jm-2predicted by two-temperature model. The incubation factorξ, which describes the changes of the multi-pulse ablation thresholds with the number of pulses, is determined to be 0.812 for nickel.

Rolling bearings are vital elements in rotating machinery and vibration signal is a kind of effective mean to characterize the status of rolling bearing fault. This paper presents a novel intelligent method for fault diagnosis based on empirical mode decomposition, fractal feature parameter extracting and orthogonal quadratic discriminant function classifier. The new method consists of three steps. Firstly, with investigating the feature of impact fault in vibration signals, the raw vibration signals are decomposed into intrinsic mode functions by empirical mode decomposition. Secondly, using the method of time sequences fractal dimension calculating, fractal feature parameters are extracted from intrinsic mode functions. Then, each raw signal sample has a feature set. Finally, training set and testing set are inputted into the orthogonal quadratic discriminant function model in the classification phase to identify different abnormal cases. The proposed method is applied to the fault diagnosis of rolling element bearing, and the test results indicate that the novel intelligent diagnosis method is sensitive to fault severity and capable of fault detection and fault diagnosis.