Theoretical And Experimental Study On The Propagation Characteristics Of Laser Induced Ultrasonic Waves In The Copper Film

The mechanism research of femtosecond laser interaction with matter is very important to understand the influence of ultrasound in various elastic material parameters. People can also represent defects on the surface of the material according to the detection of ultrasonic surface wave. In many theoretical mechanism research, scholars firstly consider that laser makes the material surface has a certain temperature field and then temperature field causes thermal stress, at last the thermal stress brings about deformation. The choices of many scholars tend to parabolic heat conduction equation in temperature field model, but for ultrashort pulse laser, two temperature model can better describe the temperature field.Responding to the trait of model of the short interaction time between ultrashort pulse laser and metal film, based on double temperature model of electronic and lattice, we have constructed metal film thermal deposition model of laser heating, and have given out the numerical solution of the thermal field in time evolution,at last we have got the the thermal equilibrium time between electron and lattice.Based on thermo elastic effect, we have constructed the space time evolution equation of the stress field, and have presented numerical solution of the stress field in the space-time evolution, at last have given out the speed of stress wave which is deiven by laser pulse in the copper film.Based on the conversion relationship between the stress and deformation, we have numerically simulated the distribution of displacement field under femtosecond laser in the copper film, also have analysed the production and propagation of the ultrasonic longitudinal wave in the copper film.In the experimental part of this paper, we ha ve set up a platform for pump detection ultrafast photography, using that the stress wave propagation in the material can cause birefringence effect, we can add a set of extinction polaroid before and after the copper film samples as super fast shutter. We have studied that the propagation process of ultrasonic which is excited by femtosecond laser in the surface of the copper film in the form of image, and have given the speed of ultrasound which spreads on the copper film surface.