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 signifcantly afects 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 refected laser during the second pulse irradiation.Te 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 diferent 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.Te 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.

Controlling the light wavefront through a scattering medium based on direct digital frequency synthesis technology

Phase modulation is a crucial step when the frequency-based wavefront optimization technique is exploited to measure the optical transmission matrix(TM) of a scattering medium. We report a simple but powerful method, direct digital frequency synthesis(DDS) technology to modulate the phase front of the laser and measure the TM. By judiciously modulating the phase front of a He–Ne laser beam, we experimentally generate a high quality focus at any targeted location through a 2 mm thick 120 grit ground glass diffuser, which is commercially used in laser display and laser holographic display for improving brightness uniformity and reducing speckle. The signal to noise ratio(SNR) of the clear round focus is 50 and the size is about 44 μm. Our study will open up new avenues for enhancing light energy delivery to the optical engine in laser TV to lower the power consumption, phase compensation to reduce the speckle noise, and controlling the lasing threshold in random lasers.