Measurement of dynamics of plasma excited species on dielectric surface using waveguide type optical near field

在最近的放电应用中，等离子体和介质表面产生的激发态之间的相互作用是非常重要的。提出了一种利用波导型近场激光光谱定量观测等离子体介质表面激发态的新技术。采用离子交换法在LiNbO_3晶体上制备了光波导。用H_3PO_4液体在高温下进行离子交换，在氧气中退火可以降低传输损耗。通过与波导理论分析的比较，确定了最佳波导深度为1.8 μm， TM0模式下的激光传输适合本实验2。采用带波导的LiNbO_3材料，在真空室中构建了平行平板阻挡放电装置。放电是用纯氩气体产生的。激光从耦合prizm入射到波导上，在波导表面附近与Ar亚稳原子相互作用形成光近场。被吸收的激光在波导中传输后被检测。由于光学系统的改进，获得了高信噪比的亚稳吸收信号。利用吸收信号测量了介电表面氩亚稳密度随氩气压力（50torr）的变化，结果表明介电表面氩亚稳密度对压力的依赖性与体等离子体中亚稳密度的依赖性有很大不同。估计了表面亚稳态的通量。在实验中发现了一种新的现象，即在放电开始后，透射的激光强度呈指数型明显下降。这种现象应该加以分析。为了观察受激物质在表面上的动态行为，这种实验技术必须与交叉光束型光谱方法混合使用。

In recent discharge applications, an interaction between excited species produced in plasma and dielectric surface is fundamentally important. A new technique using an waveguide type optical near field laser spectroscopy is developed to quantitatively observe the excited species on the dielectric surface in plasma.1.An optical waveguide on LiNbO_3 crystal was produced by ion-exchanging method. Ion-exchange was performed with H_3PO_4 liquid in high temperature and an annealing in oxygen gas made it possible to reduce a transmission loss. From the comparison with theoretical analysis of waveguide, the optimum waveguide depth was determined to be 1.8 μm and the laser transmission in TM0 mode was suit for this experiment2.Using LiNbO_3 with waveguide, a parallel plate barrier discharge device was constructed in a vacuum chamber. The discharge was produced with a pure Ar gas. Laser was incident to the waveguide from a coupling prizm and the optical near field appeared on the waveguide interacted with Ar metastable atoms in the vicinity of surface. After transmitting in the waveguide, the absorbed laser was detected. As a result of improvement of the optical system, the absorption signal due to metastables was obtained with a high S/N.3.Ar metastable density on the dielectric surface was measured from the absorption signal as a function of Ar gas pressure up to 50 Torr and it was clear that the dependence on pressure was much different from the dependence of metastables in bulk plasma. The flux of metastables on the surface was estimated.4.A new phenomenon such that the transmitted laser intensity considerably decreased in exponentially just after the discharge started was found experimentally. This phenomenon should be analyzed. This experimental technique must be mixed with a cross-beam type spectroscopic method in order to observe a dynamic behavior of excited species on the surface.