Fig. 1. Schematic diagram of the experimental system

Fig. 2. Distributions of the spectral line intensity with the laser energy and delay time
(a): Fe Ⅰ 374.55 nm without confinement; (b): Mn Ⅰ 403.08 nm nm without confinement;(c): Fe Ⅰ 374.55 nm with confinement; (d): Mn Ⅰ 403.08 nm with confinement

Fig. 3. Distribution of the enhanced factors as a function of the laser energy and delay time
(a): Fe Ⅰ 374.55 nm; (b): Mn Ⅰ 403.08 nm

Fig. 4. Optimal enhanced factor as a function of LTSD
(a): The first enhancement; (b): The second enhancement

Fig. 5. Optimal delay time as a function of LTSD
(a): The first enhancement; (b): The second enhancement