Role of focusing distance in picosecond laser-induced Cu plasma spectra

To study the effects of focusing distance on the characteristics of copper plasma, a picosecond laser was utilized to ablate a pure copper plate to generate a plasma spectrum. Following numerous experiments on the subject, three significant factors have been determined: lens focal length, pulse energy and the lens-to-sample distance. These factors were employed to analyze the spectral intensity, plasma temperature and electron density in the local thermodynamic equilibrium (LTE) and optically thin condition. Due to the shielding effects of mixed plasma, the strongest spectral intensity can be obtained in the pre-focused case rather than on the focus, no matter how much beam irradiance was employed. The more intensive the beam irradiance is, the more the optimal position is distant from the focal point. Similarly, the evolution of plasma temperature and electron density was shown a peak in the pre-focused case, which is consistent with the trend of spectral intensity. For the case of extremely high irradiance (on the focus), the shielding effects become more apparent and the resultant above three factors decreased sharply. When a longer-focal-length lens was employed, the spectral intensity exhibited an obvious bimodal trend. In the pre-focused case, a longer-focal-length lens is helpful to eliminate the effects of the roughness of the target surface compared with a shorter one. Finally, the assumed LTE was validated by McWhirter relation, plasma relaxation time and diffusion length, and the optically thin condition also validated by spectral intensity ratio. We hope this work could be an important reference for the future design of highly optimized experiments for Calibration-Free Laser-Induced Breakdown Spectroscopy (CF-LIBS).