Theoretical Study On The Enhancement Of Laser-induced Plasma Acoustics By Terahertz Wave

The process for laser to form the plasma filament in air, for its connection with theinteraction between laser and materiel and with the complex course of plasma and nonlinearPhysics, draws great intention from researchers. When the femtosecond laser propagates inthe air and meets some certain conditions, it will travel long distances by forming plasmafilaments, along with some nonlinear optical phenomena. The laser intensity and the electrondistribution are worth studying and are of great importance. However, it`s difficult tomeasure the parameters of plasma without breaking the structural integrity of the filamentbecause the intensity is too high. The acoustic signal given out by the filament is influencedby the parameters of plasma and provides a new non-contact measurement method.Recently the enhancement of the plasma acoustic signal by the terahertz wave (THz) isconfirmed by experiments, which enlarges the application range of plasma acoustic signal. Theterahertz wave accelerates the electron motion in the plasma filament just like it heats the plasmaand enhances the acoustic pressure. By analyzing the acoustic signal, the information of theterahertz wave will be regained. This provides a new method of THz measurement.The main works of my essay are as follows:(1) Introducing the physical model of the enhancement of plasma acoustics by THzand analyzing the effect of the angle between the polarization of THz and laser and theapplied voltage. New acoustic equation with angle and voltage is gained according to therelationship that acoustic pressure is proportional to the energy that THz transfers to electron.Discussing how the angle affects the measuring error and giving the theory of using theapplied voltage to coherently detect THz.(2) Making several simulations. Discussing the influence of the relaxation time on thecurve of acoustic pressure. When the relaxation time changes from far small than the THzpulse width to bigger than the THz pulse width, the acoustic curves change to newlow-pressure-model equation.(3) Introducing the physical model of acoustic diagnostic of plasma and the s ionizationrate model. By analyzing the propagation process of THz in plasma and using the equationof acoustics enhanced by THz, we gain the theory of diagnosing plasma using TEA(Terahertz-Enhanced-Acoustics) phenomenon and find the differences between acousticdiagnostic and TEA diagnostic.