1：Experimental Design and Method Selection:

The study utilized a one-box ultrafast Ti:sapphire amplifier (Coherent Libra) with a wavelength of 800 nm, pulse width of 50 fs, and repetition rate of 10 Hz. The laser polarization was adjusted using a quarter-wave plate to achieve circular and linear polarizations.

2：Sample Selection and Data Sources:

A pure Cu target (99.9%) was used. The produced plasma spectra were guided to a spectrometer (PI Action, 1200 lines per mm) through a fiber and detected using an ICCD (PI-MAX4).

3：9%) was used. The produced plasma spectra were guided to a spectrometer (PI Action, 1200 lines per mm) through a fiber and detected using an ICCD (PI-MAX4). List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: Equipment included a femtosecond laser system, half-wave plate, Glan laser polarizer, quarter-wave plate, focusing lens, three-dimensional translation stage, spectrometer, and ICCD.

4：Experimental Procedures and Operational Workflow:

The laser was focused perpendicularly to the Cu target. The plasma spectra were detected with 0 μs delay time and 10 μs gate width, averaging 100 shots per spectrum.

5：Data Analysis Methods:

The plasma temperature and electron density were calculated using the Boltzmann plot method and Stark broadening, respectively.