藉由奈米碳管黑體作為電漿鏡來提升強場鈦藍寶石雷射系統106對比度

Abstract

過去的電漿鏡（plasma mirror）實驗會在靶材表面鍍上抗反射膜（antireflection coating），其目的是用來提升自身雷射之對比度。當高強度雷射脈衝經 過靶材時，其預脈衝（pre-pulse）被抗反射膜過濾，雷射主脈衝（main pulse）與 靶材作用形成電漿鏡反射，反射之雷射脈衝與入射脈衝相較下對比度大幅提升， 其數值為電漿鏡反射率和抗反射鍍膜（AR coating）反射率的比值。但由於抗反 射鍍膜之反射率通常不會低於千分之一，因此電漿鏡反射後雷射光對比度最好 能提升約 102。本論文選用散射率極低且幾乎完全吸收之靶材，利用極低散射之 特性過濾預脈衝，而主脈衝到達時形成電漿鏡反射，目標是讓電漿鏡反射後對 比度提升能突破 102 之限制。靶材為 Vantablack，吸收率高達 99.996% 且全積 分散射（Total Integrated Scatter）約 0.01%，初步估算對比度提升可高達 106。 而 Vantablack 電漿鏡反射率估算方式使用居量傳播模型（population-propagation model），根據理論模型來評估高強度鈦藍寶石雷射（Ti:Sapphire laser）和 CO2 雷射打 Vantablack 後形成電漿鏡之反射率，再從反射率和預脈衝之散射率推算對比度能提升多少，以做為實驗執行前的評估。

In the past plasma mirror experiment, anti-reflection coating on the surface of the target is used to improve the temporal contrast of laser pulses. The mechanism of contrast enhancement is that anti-reflection coating was applied to filter the pre-pulse of the laser, and the main ultraintense laser pulse interacts with the target to form plasma mirrors which can reflect the main laser pulse. The contrast of reflected beam is improved compared to the incident laser pulse, and the contrast enhancement depends on the ratio of plasma mirror reflectivity to anti-reflection coating reflectivity. However, anti-reflection coating reflectivity is not less than onethousandth and therefore the maximum contrast improvement is about 102 . In this paper, a target with ultralow scattering is selected to achieve the goal to exceed the 102 limitation of contrast enhancement. The pre-pulse is filtered due to ultralow scattering and plasma mirrors are formed as the main pulse arrives the surface of target. The target is Vantablack, absorbing up to 99.996% and total integrated scatter of Vantablack is about 0.01%. It is estimated the contrast improvement of 106 is achieved with Vantablack. We use population-propagation model to estimate Vantablack plasma reflectivity with ultraintense Ti: Sapphire laser or CO2 laser. With plasma reflectivity and scattering of pre-pulse, we can estimate contrast improvement for experimental evaluation.