Plasma Characteristics Of "Black Silicon" Material For Laser-Driven Technology

Laser-induced plasma has a wide range of applications in research fields such as high-voltage drive,micro-propulsion,and laser plasma ignition.Relevant research on this technology has made significant progress,but there are still many problems,among which the more prominent is the low energy coupling efficiency between laser and material."Black silicon"refers to silicon material with ultra-high light absorption coefficient in the visible and near-infrared bands.Based on this characteristic,this article selects black silicon as the research object.The laser anti-reflection performance of different silicon wafers were studied.The laser-induced plasma performances of silicon wafers were studied.Meanwhile,the mechanism of laser absorption by different silicon wafers were analyzed from a microscopic perspective.The research results are as follows:Two-step metal-assisted chemical etching method was used to prepare pyramidal silicon and"pyramid-nanowire"micro-nano composite structure"black silicon"materials.The field emission scanning electron microscope,X-ray diffraction（XRD）and Raman spectra were used to characterize the microscopic morphology,crystal form and structural properties of the samples.The results show that the surface of the silicon wafer is covered with a pyramid structure with a height between 3 and 4μm.The pyramid structure is uniformly covered by a layer of nanowires with a diameter of less than 50 nm and a length of about 400 nm,and presents a periodic distribution.The crystal form and composition properties of pyramidal silicon and"black silicon"have not changed,and are consistent with single crystal silicon wafers.But,the intensity of Raman scattered light of"black silicon"has increased significantly.The near-infrared spectrometer was used to test the reflectance of polished silicon wafers,pyramidal silicon,and"pyramid-nanowire"micro-nano composite structure"black silicon"materials in the wavelength range of 900 nm to 1700 nm.The results show that the average reflectivity of the three silicon wafers is 52.64%,21.32%and 11.94%,respectively.Compared with polished silicon wafers,the low reflectivity of pyramidal silicon and"black silicon"were caused by multiple reflections of incident light inside within its own structure and various optical effects caused by the periodic structure of the surface.In addition,the internal interference of the equivalent multilayer film structure of"black silicon"makes the adjacent layer n _N-n _N-1 small,which reduces the reflection caused by the sudden change in refractive index to a certain extent.The laser-induced plasma characteristics of pyramidal silicon and"black silicon"were studied by atomic emission spectrometry and ultra-high-speed photography,and compared with polished silicon wafers.The results show that under the same laser energy,the emission spectrum of"black silicon"had the highest line intensity.The plasma duration of laser-induced"black silicon"became longer.The plasma electron temperature and electron density of laser-induced"black silicon"both increased.The plasma duration of laser-induced"black silicon"became longer.Among them,the laser-induced plasma electron temperature of"black silicon"was about 5000 K and 7000 K higher than pyramid silicon and polished silicon wafers,respectively.Under 12.05m J laser energy,the area and volume of ablation pits of"black silicon"were the largest,followed by pyramidal silicon,and polished silicon wafers are the smallest.The research results have important reference value for improving the practical application of"black silicon"materials in the fields of laser plasma drive,laser plasma propulsion,and laser plasma ignition.