New methods for probing the opacity and optical properties of dense low‐temperature plasmas

We have demonstrated new techniques by which well characterized dense (1019−1020 cm−3), cold (1−15 eV), and strongly coupled (Γ∼1) plasmas are produced by laser vaporization and ionization of thin metallic films. By limiting the plasmas to small (r∼100–500μm) but unconfined volumes it is possible to create fully accessible plasmas for diagnostic inquiry. The plasmas are very absorptive but, as a result of their small size, their optical depths are typically less than one. Laser interferometry, absorption probing, and spectroscopy are used to characterize the plasmas. Detailed measurements of the plasma density, temperature, opacity and spectra are compared to theory in the regime were the photon energies are of the same order as the average inter‐particle energies of the plasma.