Research Of Improvement In Contrast Of Ultrashort Laser Pulse By Plasma Mirror

The contrast ratio of laser pulse is one of the key parameters for ultra-intense laser beam. Laser pulse with high contrast ratio can prevent significant plasma formation on solid targets prior to the arrival of the main pulse. So how to improve the contrast of the beam pulses, as well as keeping high intensity become the key technologies in laser technology and laser-plasma interaction.In this thesis, plasma mirror method to improve the contrast ratio of ultra-short laser pulses is discussed. The population-propagation model is used to simulate the dependence of specular reflectivity with laser intensity for a S-polarized laser with the wavelength of 800 nm and pulse width of 30fs on the quartz surface. The simulation shows that the reflectivity is lower than 1% when the intensity is below 1015 W/cm², while the reflectivity is increasing at the intensity above 1015 W/cm², and the maximum reflectivity of almost 70% is at the intensity of 3×10¹⁵W/cm².The principle experiment for plasma mirror to improve the contrast ratio is performed on the SILEX laser system with the wavelength of 800nm, pulse width of 30fs and the energies up to 670 mJ. The laser beam is focused onto a AR-coated quarts substrate. The maximum reflectivity of 53.6% is measured at the intensity of 2.58×10¹⁵W/cm². The contrast ratio of the pulse is measured with a fast photoelectric diode and oscilloscope, and the contrast ratio of 80.3 is observed. This work lays a foundation for further study in the improvement of contrast ratio of ultrashort laser pulses with plasma mirror.